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## **Meet the editor**

Yasuto Itoh was born in Osaka. He received his PhD degree in Kyoto University. Now he is the professor of the Graduate School of Science of Osaka Prefecture University. He conducts research into tectonics, stratigraphy, structural geology, and paleo-/rock magnetism and has published more than 100 papers in the fields of deformation process of active plate margins, seismic

hazard assessment of active faults, paleoenvironment of East Asia, and mechanism of sedimentary basin formation/development. His professional memberships include the American Geophysical Union, American Association of Petroleum Geologists, Japan Geoscience Union, and Japanese Association for Petroleum Technology. He is also a member of the Comprehensive Research and Study on Active Fault Systems organized by the Research and Development Bureau of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

## Contents

#### **Preface XI**

**Prologue 51** Yasuto Itoh

Chapter 10 **Effectiveness for Determination of Depositional Age by Detrital Zircon U–Pb Age in the Cretaceous Shimanto Accretionary Complex of Japan 201** Tetsuya Tokiwa, Makoto Takeuchi, Yusuke Shimura, Kazuho Shobu, Akari Ota, Koshi Yamamoto and Hiroshi Mori

## Preface

Chapter 6 **Alteration Reaction and Mass Transfer via Fluids with Progress**

Chapter 5 **Fission Track Thermochronology of Late Cretaceous Sandstones**

**Section 3 Research Frontiers of Diverse Tectonic Events: Backarc Rifting,**

Chapter 8 **Paleomagnetic Studies on Miocene Sequences of Hokutan and**

**Tottori Groups in Southwest Japan: Implications for Middle Miocene Rotational Movement of Southwest Japan Block**

Daisuke Miura, Kiyoshi Toshida, Ken-ichi Arai, Takeshi Wachi and

**Zircon U–Pb Age in the Cretaceous Shimanto Accretionary**

Tetsuya Tokiwa, Makoto Takeuchi, Yusuke Shimura, Kazuho Shobu,

Chapter 7 **Structural Features Along the Median Tectonic Line in**

**Fault System in Southwest Japan 101**

**Arc Volcanism, and Frontal Accretion 157**

**Associated with the Japan Sea Opening 159** Naoto Ishikawa, Takashi Suzuki and Shiro Ishida

**Volcano-Tectonic Orientations 181**

Yutaka Wada

**Complex of Japan 201**

Chapter 9 **Oki-Dozen Dike Swarm: Effect of the Regional Stress Field on**

Chapter 10 **Effectiveness for Determination of Depositional Age by Detrital**

Akari Ota, Koshi Yamamoto and Hiroshi Mori

**Southwest Japan 121**

**VI** Contents

and Ko‐Ichiro Fujimoto

**an Arc‐Bisecting Fault 143** Yasuto Itoh and Tomotaka Iwata

**of Fracturing along the Median Tectonic Line, Mie Prefecture,**

Yasuto Itoh, Paul F. Green, Keiji Takemura and Tomotaka Iwata

**of the Izumi Group Adjacent to the Median Tectonic Line Active**

Yumi Kaneko, Toru Takeshita, Yuto Watanabe, Norio Shigematsu

**Southwest Japan: An Example of Multiphase Deformation on**

This book deals with recent developments in evolutionary models for convergent margins. Reflecting transient modes for oceanic plate convergence, such boundaries are sites of varied tectonic processes, which provoke vigorous material recycling and frequent natural disas‐ ters such as massive earthquakes and catastrophic volcanism. Therefore, the origin of their diversity has long been one of the most significant themes in Earth science.

Section 1 of this book focuses on the formative processes of forearc basins at convergent margins, which are significant buffers for material recycling in arc-trench systems. In Chap‐ ter 1, Noda and Miyakawa present a comprehensive review of well-known modern forearc basins and suggest a causal relationship between basin formation and accretionary wedge growth. Their results indicate that forearc basins can be classified into three types: a singlewedge model characterized by landward tilting of the basin strata, a two-wedge model marked by contractional deformation linked to symmetric doubly vergent wedge uplift, and an additional strike-slip model represented by transpressional/transtensional deformation due to oblique subduction. In Chapter 2, Arato describes the development process for a tec‐ tonically erosive margin based on an interpretation of extensive seismic data covering an entire basin range. He emphasizes the significance of massive rearrangement of forearc com‐ ponents related to transcurrent faulting, which is generally applicable to oblique subduction margins around the globe.

Section 2 is a special section dealing with the evolution of arc-bisecting faults, which are commonly found on obliquely converging plates. The objective of the researchers involved is to elucidate the formation timeline for the Median Tectonic Line (MTL) dividing the southwestern Japan arc since the late Cretaceous. This ancient lateral fault has become inter‐ mittently active in response to temporal shifts in the rate of convergence of the oceanic plates in the northwestern Pacific basin. Following a short prologue that provides the read‐ ers with a spatiotemporal perspective of the regional fault, in Chapter 3, Itoh and others de‐ scribe the 3D architecture of the MTL based on a dataset of reflection on seismic surveys and borehole stratigraphy. They identify a Quaternary half-graben overlain by a large wedge of Cretaceous rocks and interpret this feature as a retroactive crustal failure juxtaposed on an active dextral trace of the MTL. The discrepancy between the geophysical low-angle view and the high-angle view expected as a result of predominantly strike-slip motion on the fault is highlighted by Kusumoto and others in Chapter 4, based on the latest analysis of gravity anomalies. They performed numerical modeling of the active geomorphology and concluded that the thrust is not a mechanical boundary in the upper crust associated with a recent shear stress but is instead a dormant material boundary that developed during an older contraction phase. A study of the incipient evolution of the MTL is presented by Itoh and others in Chapter 5, based on a fission track (FT) thermochronological analysis of sandy

clastics burying a gigantic pull-apart basin formed during Cretaceous sinistral activity at the fault. Considering the bimodal distribution of FT ages determined for apatite and zircon grains extracted from sandstone samples, they argue for two distinct provenances in south‐ west Japan. Thermodynamic modeling of annealing of apatite FTs led them to distinguish three cooling (exhumation) events, the youngest of which is linked to the most recent Mio‐ cene inversion within the arc, together with intensive reverse faulting at the MTL. A note‐ worthy study on structural buildup and material circulation at the incipient MTL is presented by Kaneko and others in Chapter 6. Based on a detailed structural and geochemi‐ cal analysis of the eastern segment of the regional fault, they identified a normal faulting phase during the early Paleogene period and material migration resulting from vigorous fluid circulation within the fault-brecciated zone. Finally, in Chapter 7, Itoh and Iwata de‐ scribe complex structural features of the MTL that were formed during recurrent tectonic episodes beginning in the late Cretaceous era. Their phase stripping method was designed to identify a single deformation phase and can successfully extract the neotectonic structural features of the fault.

Section 3 is a collection of studies on the southwestern Japan arc, but the researchers' inter‐ ests are not restricted to local geology. In Chapter 8, Ishikawa and others present a chronicle of arc rotation processes related to Miocene back-arc rifting. The opening process of the Ja‐ pan Sea is known to have been characterized by an extraordinarily rapid spreading rate with a fan-shaped mode. Their important paleomagnetic data place constraints on the tim‐ ing of each step of the back-arc opening sequence. In Chapter 9, Miura and others discuss volcanic activity associated with rifting events and present significant tectonic results based on a dike swarm analysis. As a result of extensive field observations, they advocate a clear relation between the physicochemical properties of magmas and the morphology of dikes, which should be taken into account when estimating ancient stress fields. In Chapter 10, Tokiwa and others report on an investigation of frontal tectonic processes to determine the tectono-stratigraphic order of Cretaceous accretionary complexes along the Nankai Trough. They succeeded in determining the settlement ages of accreted bodies based on a quantita‐ tive cluster analysis of high-resolution U-Pb dating results for clastic rocks. The striking co‐ incidence of their radiometric age estimates with previous biostratigraphic ages endorses the versatility of their analytical method.

The important scientific results obtained by these prominent researchers pave the way for further in-depth studies on mobile belt frontiers, where harsh conditions hinder efforts to understand the Earth's spatiotemporal changes.

> **Yasuto Itoh** Graduate School of Science, Osaka Prefecture University, Osaka, Japan

**Pursuit of Formative Processes of Forearc Basins on Convergent Margins**

clastics burying a gigantic pull-apart basin formed during Cretaceous sinistral activity at the fault. Considering the bimodal distribution of FT ages determined for apatite and zircon grains extracted from sandstone samples, they argue for two distinct provenances in south‐ west Japan. Thermodynamic modeling of annealing of apatite FTs led them to distinguish three cooling (exhumation) events, the youngest of which is linked to the most recent Mio‐ cene inversion within the arc, together with intensive reverse faulting at the MTL. A note‐ worthy study on structural buildup and material circulation at the incipient MTL is presented by Kaneko and others in Chapter 6. Based on a detailed structural and geochemi‐ cal analysis of the eastern segment of the regional fault, they identified a normal faulting phase during the early Paleogene period and material migration resulting from vigorous fluid circulation within the fault-brecciated zone. Finally, in Chapter 7, Itoh and Iwata de‐ scribe complex structural features of the MTL that were formed during recurrent tectonic episodes beginning in the late Cretaceous era. Their phase stripping method was designed to identify a single deformation phase and can successfully extract the neotectonic structural

Section 3 is a collection of studies on the southwestern Japan arc, but the researchers' inter‐ ests are not restricted to local geology. In Chapter 8, Ishikawa and others present a chronicle of arc rotation processes related to Miocene back-arc rifting. The opening process of the Ja‐ pan Sea is known to have been characterized by an extraordinarily rapid spreading rate with a fan-shaped mode. Their important paleomagnetic data place constraints on the tim‐ ing of each step of the back-arc opening sequence. In Chapter 9, Miura and others discuss volcanic activity associated with rifting events and present significant tectonic results based on a dike swarm analysis. As a result of extensive field observations, they advocate a clear relation between the physicochemical properties of magmas and the morphology of dikes, which should be taken into account when estimating ancient stress fields. In Chapter 10, Tokiwa and others report on an investigation of frontal tectonic processes to determine the tectono-stratigraphic order of Cretaceous accretionary complexes along the Nankai Trough. They succeeded in determining the settlement ages of accreted bodies based on a quantita‐ tive cluster analysis of high-resolution U-Pb dating results for clastic rocks. The striking co‐ incidence of their radiometric age estimates with previous biostratigraphic ages endorses

The important scientific results obtained by these prominent researchers pave the way for further in-depth studies on mobile belt frontiers, where harsh conditions hinder efforts to

**Yasuto Itoh**

Osaka, Japan

Graduate School of Science, Osaka Prefecture University,

features of the fault.

VIII Preface

the versatility of their analytical method.

understand the Earth's spatiotemporal changes.

## **Deposition and Deformation of Modern Accretionary-Type Forearc Basins: Linking Basin Formation and Accretionary Wedge Growth**

Atsushi Noda and Ayumu Miyakawa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67559

#### **Abstract**

Since a comprehensive review of forearc basins was published by Dickinson more than 20 years ago, a significant amount of new data about them have been published. These recent studies revealed details of depositional and deformation styles in the forearc basins, suggesting the formation processes were not unique. In this chapter, we reviewed modern forearc basins to understand how is the basin stratigraphy related with growth of accretionary wedges. The results indicate forearc basin can be classified into two (single‐ and two‐wedge models) plus one (strike‐slip model): (1) the single‐wedge model which is characterized by landward tilting of the basin strata ascribed to asymmetrical doubly vergent (single‐vergent) uplift of the outer arc high with forethrusts (seaward‐vergent thrusts in the pro‐wedge); (2) the two‐wedge model which is marked by contractional deformation caused by symmetrical doubly vergent uplift of the wedge with forethrusts in the prowedge and back‐thrusts (landward‐vergent thrusts) in the retro‐wedge; and (3) the strike‐slip model which is an additional one being represented by transpressional and/or transtensional deformations due to oblique subduction. We speculate that these models spatially and temporally depend on material fluxes at the plate interfaces that affect geometry and mechanical strength of backstops.

**Keywords:** forearc basin, accretionary wedge, deposition, deformation, plate convergent margin

#### **1. Introduction**

Forearc basin is one of the major elements of plate convergent margins (e.g., [1, 2]). The formation is considered to be closely associated with accretion or erosion at frontal and

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

basal parts of accretionary wedge (e.g., [3–5]). Especially, the seaward margin of the basin is directly affected by uplift and subsidence of the outer arc high of accretionary wedge, which acts as topographic barrier of sediments deposited in the basin. Forearc basin gener‐ ally develops at which interplate coupling is strong [6, 7], meaning the location is underlain by an area where strain release during earthquakes is large. Therefore, styles of deposition and deformation of forearc basins may record long‐term average stresses on the plate inter‐ faces over millions of years.

However, the formation and deformation processes of forearc basins are poorly known. Such basin is incorporated into a dynamic system of subduction zone composed of backstop, basin sediments, accretionary wedge, trench fill deposits, subducting plate with conduit for the subducting materials. This system is spatially and temporally influenced by changes of plate configuration (slap dip, obliquity, or convergent rate), activity of volcanic arc (sediment pro‐ duction), topography of subducting slab, strength and geometry of backstop, and material flux at the plate boundary.

The purpose of our study is to develop a tool for reconstruction of paleostress fields by lighten‐ ing how forearc basin formations respond to subduction zone dynamics that include frontal/ basal accretion/erosion and mechanical relationships among the basin, backstop, accretion‐ ary wedge, and subducting plate. For the first step of this purpose, we collected modern examples of deposition and deformation of forearc basins in the world. The studied areas are Sunda (Sumatra‐Java), Japan, Aleutian‐Alaskan, Lesser Antilles, South American (Columbia‐ Ecuador‐Peru‐Chile), and Tonga‐Kermadec‐Hikurangi margins (**Figure 1**). Most of them are classified into the compressional accretionary‐type forearc basin [2], which is characterized by growth of accretionary wedge by frontal/basal accretion and compressional deformation in

**Figure 1.** Index map of the study areas. Plate boundaries are based on Ref. [8].

the basins. This paper can contribute a basic understanding about the formation processes of forearc basins along plate convergent margins.

#### **2. Sunda arc (Sumatra‐Java)**

#### **2.1. Plate configuration**

basal parts of accretionary wedge (e.g., [3–5]). Especially, the seaward margin of the basin is directly affected by uplift and subsidence of the outer arc high of accretionary wedge, which acts as topographic barrier of sediments deposited in the basin. Forearc basin gener‐ ally develops at which interplate coupling is strong [6, 7], meaning the location is underlain by an area where strain release during earthquakes is large. Therefore, styles of deposition and deformation of forearc basins may record long‐term average stresses on the plate inter‐

However, the formation and deformation processes of forearc basins are poorly known. Such basin is incorporated into a dynamic system of subduction zone composed of backstop, basin sediments, accretionary wedge, trench fill deposits, subducting plate with conduit for the subducting materials. This system is spatially and temporally influenced by changes of plate configuration (slap dip, obliquity, or convergent rate), activity of volcanic arc (sediment pro‐ duction), topography of subducting slab, strength and geometry of backstop, and material

The purpose of our study is to develop a tool for reconstruction of paleostress fields by lighten‐ ing how forearc basin formations respond to subduction zone dynamics that include frontal/ basal accretion/erosion and mechanical relationships among the basin, backstop, accretion‐ ary wedge, and subducting plate. For the first step of this purpose, we collected modern examples of deposition and deformation of forearc basins in the world. The studied areas are Sunda (Sumatra‐Java), Japan, Aleutian‐Alaskan, Lesser Antilles, South American (Columbia‐ Ecuador‐Peru‐Chile), and Tonga‐Kermadec‐Hikurangi margins (**Figure 1**). Most of them are classified into the compressional accretionary‐type forearc basin [2], which is characterized by growth of accretionary wedge by frontal/basal accretion and compressional deformation in

**Figure 1.** Index map of the study areas. Plate boundaries are based on Ref. [8].

faces over millions of years.

4 Evolutionary Models of Convergent Margins - Origin of Their Diversity

flux at the plate boundary.

Along the Sunda (Sumatra‐Java) arc, the India‐Capricorn‐Australian plate subducts beneath the Sunda microplate (Eurasian continental plate) with 5–7 cm/yr in the southeastern Asia (**Figure 2**). Under the present configuration, the angle of subduction direction gradually var‐ ies from nearly orthogonal off Java Island to almost parallel off Andaman Islands farther north, which leads to development of arc‐parallel strike‐slip fault systems [9]. Water depth of the trench floor also gradually decreases from east (6000 m water depth off eastern Java) to northwest (4000 m water depth off Sumatra), which is ascribed to an enormous sediment input from the Bay of Bengal [10]. Age of the subducting plate is younging from ca. 130 Ma of east to ca. 50 m.y. of northwest [11].

In the northern part of the forearc (off Sumatra), multiple subbasins separated by structural highs have been formed, indicating transtension and transpression regimes are repeated due to strike‐slip deformation along this margin [12]. The central Sunda margin (off western Java to southern Sumatra) is marked by a transition zone from orthogonal convergence in the east to oblique subduction to the northwest.

The subduction along this arc is active since Eocene [13]. The oceanic plate initially subducted beneath continental‐type basement rock off Sumatra and oceanic‐type one off Java [14, 15]. The convergence rate increased from 5 cm/yr (Eocene to Miocene) to 7 cm/yr during the last 10 My [16].

#### **2.2. Forearc basins**

#### *2.2.1. Simuelue basin*

The Simuelue basin (**Figure 3**) is a moderate‐sized (100 × 260 km) basin with sediments of up to 5 s two‐way travel time off Sumatra Island (e.g., [15, 17]). The basement of this basin is considered as the continental backstop of the Sumatran continental crust with a regional horst and graben structures which might be generated from the Late Eocene to Early Oligocene [21] (**Figure 3A**). The northern part of this basin (**Figure 3A**) shows depositional and defor‐ mational styles changed between Units 3 and 4. A contractional strain ascribed to landward‐ vergent thrusts in the retro‐wedge was recorded before the deposition of unit 4. On the other hand, the strata of Units 4 and 5 are characterized by seaward tilting and seaward migration of the depocenter, suggesting the seaward side of the basin subsided relative to the landward side during that time.

**Figure 2.** Index of the Sunda margin. Blue lines with labels A–E indicate the locations of survey lines shown in **Figure 3**. Forearc basins: 1—Aceh, 2—Simuelue, 3—Siberut, 4—Bengkulu, 5—Enggano, 6—Mentawai, 7—Western Java, 8— Eastern Java, 9—Lombok basins. Plate names: SU—Sunda, CP—Capricorn, AU—Australia plates. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the Sunda plate based on Ref. [8].

The depositional history of the central part (**Figure 3B**) was divided into four stages [17]. At the end of Paleogene time (ca. 40 Ma), a regional erosional surface was developed after slope sediments covered the topographic lows (Unit 1 in **Figure 3B**). An Early/Mid Miocene stage is marked by a rapid subsidence in the trenchward and thick deposition occurred along the seaward margin of the basin (Unit 2). During the Late Miocene/Pliocene, subsid‐ ence continued in the northern and the trenchward region of the Simeulue basin (Unit 3). The basin from Pleistocene to recent was under an influence of strike‐slip faults due to the oblique subduction evidenced by a wrench faulting, and subsidence expanded significantly landward (Unit 4).

**Figure 3.** (A) Northern part of the Simuelue basin [15]. (B) Central part of the Simuelue basin [17]. (C) Mentawai basin [18]. (D) Western Java basin [19]. (E) Lombok basin along the Java Trench [20]. Locations of the survey lines are shown in **Figure 2**.

#### *2.2.2. Mentawai basin*

The depositional history of the central part (**Figure 3B**) was divided into four stages [17]. At the end of Paleogene time (ca. 40 Ma), a regional erosional surface was developed after slope sediments covered the topographic lows (Unit 1 in **Figure 3B**). An Early/Mid Miocene stage is marked by a rapid subsidence in the trenchward and thick deposition occurred along the seaward margin of the basin (Unit 2). During the Late Miocene/Pliocene, subsid‐ ence continued in the northern and the trenchward region of the Simeulue basin (Unit 3). The basin from Pleistocene to recent was under an influence of strike‐slip faults due to the oblique subduction evidenced by a wrench faulting, and subsidence expanded significantly

**Figure 2.** Index of the Sunda margin. Blue lines with labels A–E indicate the locations of survey lines shown in **Figure 3**. Forearc basins: 1—Aceh, 2—Simuelue, 3—Siberut, 4—Bengkulu, 5—Enggano, 6—Mentawai, 7—Western Java, 8— Eastern Java, 9—Lombok basins. Plate names: SU—Sunda, CP—Capricorn, AU—Australia plates. Red arrows indicate

the direction and velocity [cm/yr] of plate motion relative to the Sunda plate based on Ref. [8].

6 Evolutionary Models of Convergent Margins - Origin of Their Diversity

landward (Unit 4).

Deposition and deformation of the Mentawai basin have been strongly influenced by imbri‐ cated seaward‐vergent fore‐thrusts in the accretionary wedge and landward‐vergent back‐ thrusts in the Mentawai fault zone [18, 22, 23] (**Figure 3C**). This fault zone has both characters of strike‐slip fault (e.g., [23]) and back‐thrust [18]. The fault zone may have initiated dur‐ ing Early‐Middle Miocene and indicates the arc‐parallel transpressional features along the boundary between the accretionary wedge and the continental backstop [18]. A continuous contraction developed the fold‐thrust belt during Pliocene time.

The oldest deposits in this basin are the Middle Oligocene‐Early Miocene, which thinly cover the forearc slope (Unit 1 in **Figure 3C**). Since the latest Early Miocene, uplift of the outer arc high began with imbricated fore‐thrusts in the accretionary wedge, which reduced the thick‐ ness of forearc basin sediments trenchward to the outer arc high (Units 2–4). At the same time, landward‐vergent back‐thrusts also developed near the boundary between the accretionary wedge and the continental backstop. Although the activity of these thrusts waned in the early Late Miocene (Unit 3), it reactivated in the late Late Miocene (Units 4 and 5). This intense deformation might be induced by accretion of thick Bengal Fan sediments and increase of plate convergence rate since the Late Miocene.

#### *2.2.3. Western Java basin*

The basement of the forearc basin is considered as an oceanic‐type crust underlain by the shallow mantle deduced from large seismic velocities [14, 24] (**Figure 3D**). The seaward mar‐ gin of the basin is bounded by the outer arc high with a sharp and steep scarp. Up to 4 km, sediments are accumulated along the outer arc high [19]. Depositional ages and sedimentary faces in the forearc basin are unknown, because of no drilling core in and around the basin. Growth of the accretionary wedge might cause a compressional deformation in association with landward‐vergent back‐thrusts in the early stages of the basin formation (Units 1 and 2), which folded and uplifted the basin strata. The deformation and thickness variations reduced during the following stages (Units 3 and 4). A total displacement is smaller than that in the Mentawai Basin. The steep landward slope of the outer arc high implies that the vertical growth might be contributed by basal accretion beyond the frontal wedge [19].

#### *2.2.4. Lombok basin*

The Lombok forearc basin is a large‐sized (600 × 120 km) forearc basin with 3.5–4.5 km sedi‐ ment thickness off Lombok Island [13, 16, 20]. The basement of this basin is also interpreted as an altered, heavily fractured piece of an older oceanic plate [25]. The basement is character‐ ized by horst and graben structures (**Figure 3E**).

A progressive landward retreat of the depocenter in this basin is an evidence of a strong influ‐ ence by growth of accretionary wedge (**Figure 3E**). Before the formation of the basin (Late Eocene), sediments filled topographic lows of horst and graben structures under an extensional tectonic regime (Unit 1 in **Figure 3E**). After the beginning of the uplift of the outer arc high (Unit 2 of the Early Oligocene), the basin subsided by a pull of the subducting oceanic plate (Unit 3 of the Late Oligocene), resulting in down lapping strata on the unconformity. From Late Oligocene to present (Units 4 and 5), the depocenter of the forearc basin continuously shifted landward without any compressional deformation or thrusting in the seaward margin of the basin.

### **3. Japan**

#### **3.1. Plate configuration**

Two contrasting oceanic plates of the Pacific plate (old, cold, and steeply dipping) along the northern Japan and the Philippe Sea plate (young, hot, and gently dipping) along the south‐ ern Japan are subducting beneath the Okhotsk (North American) and Eurasian plates, respec‐ tively (**Figure 4**). Subduction of the Pacific plate is at a rate of ∼9 cm/yr with almost orthogonal angle to the trench. The Japan Trench is one of the well‐known examples of tectonically ero‐ sive margin (e.g., [26]).

**Figure 4.** Index map of the Japan margin. Series of forearc basins are developed between volcanic fronts and trenches. Blue lines with labels A and B indicate locations of survey lines shown in **Figure 5**. Plate names: PA—Pacific, PS— Philippine Sea, OK—Okhotsk, NA—North American, EU—Eurasian plates. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the Okhotsk or Eurasian plate based on Ref. [8].

In the southwestern Japan margin, the Philippine Sea plate subducts at a rate of 5–7 cm/yr with moderate obliquity at present. In this margin, the subducting plate changed from the Pacific to the Philippine Sea plate as a migration of a triple junction [27]. Thick accretionary wedge (the Shimanto Complex) and segmented forearc basins have been built from Cretaceous to present between the volcanic front and the Nankai Trough (e.g., [28]).

#### **3.2. Enshu and Kumano basins**

Late Miocene (Unit 3), it reactivated in the late Late Miocene (Units 4 and 5). This intense deformation might be induced by accretion of thick Bengal Fan sediments and increase of

The basement of the forearc basin is considered as an oceanic‐type crust underlain by the shallow mantle deduced from large seismic velocities [14, 24] (**Figure 3D**). The seaward mar‐ gin of the basin is bounded by the outer arc high with a sharp and steep scarp. Up to 4 km, sediments are accumulated along the outer arc high [19]. Depositional ages and sedimentary faces in the forearc basin are unknown, because of no drilling core in and around the basin. Growth of the accretionary wedge might cause a compressional deformation in association with landward‐vergent back‐thrusts in the early stages of the basin formation (Units 1 and 2), which folded and uplifted the basin strata. The deformation and thickness variations reduced during the following stages (Units 3 and 4). A total displacement is smaller than that in the Mentawai Basin. The steep landward slope of the outer arc high implies that the vertical

The Lombok forearc basin is a large‐sized (600 × 120 km) forearc basin with 3.5–4.5 km sedi‐ ment thickness off Lombok Island [13, 16, 20]. The basement of this basin is also interpreted as an altered, heavily fractured piece of an older oceanic plate [25]. The basement is character‐

A progressive landward retreat of the depocenter in this basin is an evidence of a strong influ‐ ence by growth of accretionary wedge (**Figure 3E**). Before the formation of the basin (Late Eocene), sediments filled topographic lows of horst and graben structures under an extensional tectonic regime (Unit 1 in **Figure 3E**). After the beginning of the uplift of the outer arc high (Unit 2 of the Early Oligocene), the basin subsided by a pull of the subducting oceanic plate (Unit 3 of the Late Oligocene), resulting in down lapping strata on the unconformity. From Late Oligocene to present (Units 4 and 5), the depocenter of the forearc basin continuously shifted landward without any compressional deformation or thrusting in the seaward margin of the basin.

Two contrasting oceanic plates of the Pacific plate (old, cold, and steeply dipping) along the northern Japan and the Philippe Sea plate (young, hot, and gently dipping) along the south‐ ern Japan are subducting beneath the Okhotsk (North American) and Eurasian plates, respec‐ tively (**Figure 4**). Subduction of the Pacific plate is at a rate of ∼9 cm/yr with almost orthogonal angle to the trench. The Japan Trench is one of the well‐known examples of tectonically ero‐

growth might be contributed by basal accretion beyond the frontal wedge [19].

plate convergence rate since the Late Miocene.

8 Evolutionary Models of Convergent Margins - Origin of Their Diversity

ized by horst and graben structures (**Figure 3E**).

*2.2.3. Western Java basin*

*2.2.4. Lombok basin*

**3. Japan**

**3.1. Plate configuration**

sive margin (e.g., [26]).

Enshu and Kumano forearc basins have been developed along the landward side of prom‐ inent outer arc highs associated with regional megasplay faults along the Nankai Trough (**Figure 5**) [29, 30]. Huge amounts of studies about the basins in terms of seismic interpreta‐ tions (e.g., [31]) and sediment cores obtained from IODP expeditions (e.g., [32, 33] and refer‐ ences therein).

The Kumano forearc basin of the ∼3 km sediments has been evolved by combined influences of frontal accretion and megasplay faults (out‐of‐sequence thrusts). A brief history about the basin can be summarized: (1) small trench‐slope basins developed during 6–2 Ma on older consolidated inner accretionary wedge of the Miocene‐Pliocene (Units 1 and 2 in **Figure 5**). (2) Splay fault development caused by a shallowing of the decollement (2–1.3 Ma) uplifted the slope basins and merged them into one large forearc basins (Unit 3). The basin floor was draped by submarine‐fan turbidities derived from gravity flows through submarine canyons. (3) Diachronous depositional surfaces were formed by spatial and temporal variations of the fault system around the basin during 1.3–0.9 Ma (Unit 4). Hemipelagic sediments were derived from the outer arc high in the seaward side of the basin, while terrigenous sediments were

**Figure 5.** Interpretations of seismic profiles across (A) the Enshu and (B) the Kumano basins (modified from Ref. [37]). Seaward margins of the basins are characterized by landward tilting strata resulted from uplift of the outer arc highs. *Abbreviations*: NF—normal faults, SF—splay faults, WBSF—wedge boundary strike‐slip fault. Locations of the profiles are shown in **Figure 4**.

deposited in the landward side. (4) Continuous uplift of the outer arc high by the splay faults resulted in a stable environment as the depocenter migrated landward (Unit 5 of <0.9 Ma). Normal faults and strike‐slip faults overprinted the deformation in the seaward margins of the basins under the present condition [34–36]. No contractional deformation was recorded in the seaward side of the Kumano basin.

#### **4. Aleutian‐Alaskan forearc**

#### **4.1. Plate configuration**

The Aleutian‐Alaskan margin is a long (∼3800 km) and arcuate subduction zone. The pres‐ ent configuration is marked by subduction of the Pacific plate beneath the North American plate at a rate of 6–7 cm/yr with increasing the obliquity from east (Alaskan margin) to west (central Aleutian margin). This margin might be established as a transition from erosional to accretionary margin occurred at ca. 6–3 Ma, triggered by an increase in sediment supply from glacial erosion in Alaska [38, 39].

#### **4.2. Tugidak basin**

The eastern Aleutian‐western Alaskan margin is an example of double forearc basins where seaward (Sanak, Shumagin, Tugidak, Albatross, Stevenson) and landward (Cook Inlet and Shelikof Strait) basins are separated by the uplifted Kodiak Island and Kenai Peninsula (**Figure 6**). This separation of forearc basins into two zones were explained by vertical growth of the accretionary wedge resulted from underplating [40] or flexural subsidence of the inland induced by a flat slab subduction [41].

The Tugidak basin is situated between Kodiak and Semidi Islands and at the landward side of the Tugidak anticline (**Figures 6** and **7**). The basin has a broad width and contains as much as 5 km of sediments from the late Miocene to the recent. The strata show that the depocenter shifted landward as the basin was shallowed from slope to shelf [42, 43]. The basin strata are broadly synclinal, but are contracted and thrusted near the Tugidak anticline that uplifted rapidly in Pliocene. On the other hand, normal faults are extensively developed in the land‐ ward side of the basin.

Although the depositional age is poorly constrained, a history of the basin development is as follows. The first unit (Unit 1 in **Figure 7B**) is slope‐cover deposits unconformably covered on the lower Miocene or older rocks, before the Tugidak anticline uplifted. Deposition of Unit 2 corresponds to growth of the anticline in the Pliocene. The depocenter gradually shifted land‐ ward with onlap on both landward and seaward sides of the basin as the anticline uplifted. Nearly constant thickness of the Unit 3 indicates a seaward shift of the depocenter, suggesting that the sediment supply exceeded the rate of uplift or relative uplift of the forearc basement (inner wedge) to the anticline (outer wedge). Units 4 and 5 represented by a broad basin with

deposited in the landward side. (4) Continuous uplift of the outer arc high by the splay faults resulted in a stable environment as the depocenter migrated landward (Unit 5 of <0.9 Ma). Normal faults and strike‐slip faults overprinted the deformation in the seaward margins of the basins under the present condition [34–36]. No contractional deformation was recorded in the

**Figure 5.** Interpretations of seismic profiles across (A) the Enshu and (B) the Kumano basins (modified from Ref. [37]). Seaward margins of the basins are characterized by landward tilting strata resulted from uplift of the outer arc highs. *Abbreviations*: NF—normal faults, SF—splay faults, WBSF—wedge boundary strike‐slip fault. Locations of the profiles

The Aleutian‐Alaskan margin is a long (∼3800 km) and arcuate subduction zone. The pres‐ ent configuration is marked by subduction of the Pacific plate beneath the North American plate at a rate of 6–7 cm/yr with increasing the obliquity from east (Alaskan margin) to west (central Aleutian margin). This margin might be established as a transition from erosional to accretionary margin occurred at ca. 6–3 Ma, triggered by an increase in sediment supply from

The eastern Aleutian‐western Alaskan margin is an example of double forearc basins where seaward (Sanak, Shumagin, Tugidak, Albatross, Stevenson) and landward (Cook Inlet and Shelikof Strait) basins are separated by the uplifted Kodiak Island and Kenai Peninsula

seaward side of the Kumano basin.

10 Evolutionary Models of Convergent Margins - Origin of Their Diversity

**4. Aleutian‐Alaskan forearc**

glacial erosion in Alaska [38, 39].

**4.1. Plate configuration**

are shown in **Figure 4**.

**4.2. Tugidak basin**

**Figure 6.** Index map of the Aleutian‐Alaskan margin. Forearc basins: At—Atka, Una—Unalaska, Uni—Unimak, Sa— Sanak, Sh—Shumagin, Tu—Tugidak, Al—Albatross, St—Stevenson, Co—Cook Inlet basins. Plate names; PA—Pacific and NA is North American. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the North American plate based on Ref. [8].

**Figure 7.** Interpretations of seismic profiles across the Tugidak basin (modified from Ref. [42]). The seaward margin is cut and tilted by several reverse faults related with growth of the Tugidak anticline. On the other hand, central and landward sides of the basin are characterized by numerous normal faults. Locations of the profiles are shown in **Figure 6**.

a landward shift of the depocenter, implying renewed growth of the anticline and regional subsidence of the forearc basement with numerous normal faults.

#### **5. Lesser Antilles margin**

#### **5.1. Plate configuration**

Caribbean plate was generated in the eastern Pacific as a Late Cretaceous oceanic plateau and advanced eastward between the North American and South American plates since Eocene. Subduction magmatism has constructed the Lesser Antilles island arc on the Caribbean plate during the past 40 m.y. A chain of volcanic islands stretches 800 km, forming a magmatic‐arc platform (**Figure 8**). The present configuration in this region is characterized by the subduc‐ tion of the North American plate (the Late Cretaceous Atlantic oceanic crust) beneath the Caribbean plate at a rate of ∼2 cm/yr.

The Barbados ridge accretionary complex (**Figure 8**), which is the largest accretionary complex on Earth, has formed by frontal and basal accretion of sediments from the North American plate to the Caribbean plate since Eocene (e.g., [44]). It is <40 km wide and 7 km maximum thickness in the northern margin and is 300 km wide and up to 20 km thickness in the south‐ ern margin. Most of the sediments accreted are sourced from Orinoco River along the trench.

#### **5.2. Forearc basins**

The Tobago forearc basin (Tobago Trough) is one of the largest forearc basins in the world [44–47], which has 100 km wide, 200 km long, and more than 10 km‐thick sediments (**Figure 9A** and **B**). The basin has formed between the crystalline platform of the Lesser Antilles arc and the Barbados Ridge accretionary complex. Barbados Island is a part of the outer arc high bounding the seaward margin of the basin. The strata near the outer arc high are deformed by landward‐ vergent thrusts in the retro‐wedge (Unit 1 in **Figure 9B**). Development of these thrusts probably began in early Miocene or earlier, which imbricated forearc basin strata as a duplex between the

a landward shift of the depocenter, implying renewed growth of the anticline and regional

**Figure 7.** Interpretations of seismic profiles across the Tugidak basin (modified from Ref. [42]). The seaward margin is cut and tilted by several reverse faults related with growth of the Tugidak anticline. On the other hand, central and landward sides of the basin are characterized by numerous normal faults. Locations of the profiles are shown in **Figure 6**.

Caribbean plate was generated in the eastern Pacific as a Late Cretaceous oceanic plateau and advanced eastward between the North American and South American plates since Eocene. Subduction magmatism has constructed the Lesser Antilles island arc on the Caribbean plate during the past 40 m.y. A chain of volcanic islands stretches 800 km, forming a magmatic‐arc platform (**Figure 8**). The present configuration in this region is characterized by the subduc‐ tion of the North American plate (the Late Cretaceous Atlantic oceanic crust) beneath the

The Barbados ridge accretionary complex (**Figure 8**), which is the largest accretionary complex on Earth, has formed by frontal and basal accretion of sediments from the North American plate to the Caribbean plate since Eocene (e.g., [44]). It is <40 km wide and 7 km maximum thickness in the northern margin and is 300 km wide and up to 20 km thickness in the south‐ ern margin. Most of the sediments accreted are sourced from Orinoco River along the trench.

The Tobago forearc basin (Tobago Trough) is one of the largest forearc basins in the world [44–47], which has 100 km wide, 200 km long, and more than 10 km‐thick sediments (**Figure 9A** and **B**). The basin has formed between the crystalline platform of the Lesser Antilles arc and the Barbados Ridge accretionary complex. Barbados Island is a part of the outer arc high bounding the seaward margin of the basin. The strata near the outer arc high are deformed by landward‐ vergent thrusts in the retro‐wedge (Unit 1 in **Figure 9B**). Development of these thrusts probably began in early Miocene or earlier, which imbricated forearc basin strata as a duplex between the

subsidence of the forearc basement with numerous normal faults.

12 Evolutionary Models of Convergent Margins - Origin of Their Diversity

**5. Lesser Antilles margin**

Caribbean plate at a rate of ∼2 cm/yr.

**5.1. Plate configuration**

**5.2. Forearc basins**

**Figure 8.** Index map of the Lesser Antilles margin. Blue lines with labels A–C indicate locations of survey lines shown in **Figure 9**. Plate names: NA—North American, CP—Caribbean, SA—South American plates. Other abbreviations: PRT— Puerto Rico Trench, BR—Barracuda Ridge, TR—Tiburon Ridge, BA—Barbados Ridge accretionary complex, NCDB— Northern Caribbean deformed belt, SCDB—Southern Caribbean deformed belt, SSF—San Sebastian Fault, EPF—El Pilar Fault, CRFZ—Central Range fault zone, UF—Urica Fault, PF—Pirital Fault. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the Caribbean plate based on Ref. [8].

accretionary wedge and the early detachment in late Miocene or Pliocene time. The horizontal contraction of the basin was estimated to be between 4 and 45%, increasing southward [45]. Thickened outer arc high due to this contraction might increase tectonic loading, resulting in a large subsidence and trenchward shift of the depocenter in Pliocene and Pleistocene (Unit 3 in **Figure 9B**).

Forearc basins off Guadeloupe and Dominica have been developed in the northern part of this margin where a seismic Barracuda and Tiburon ridges are obliquely subducting (e.g., [48–50]) (**Figures 8** and **9C**). The buoyant crust of the Tiburon Ridge that accreted within the past 3.5 m.y. forms a seaward‐dipping backstop in contact with the lower half of the accretionary wedge [48]. As much as half of the sediments underthrust at the toe of the accretionary wedge

**Figure 9.** (A) Interpretations of the seismic profiles across the Tobago Trough in the Lesser Antilles forearc. (B) Details of seaward margin of the Tobago forearc basin. Modified from Ref. [45]. (C) A cross section across a forearc basin off Guadeloupe (modified from Ref. [48]). Seaward margins of both basins are deformed and uplifted by backthrusts. Locations of the profiles are shown in **Figure 8**.

appears to be subducted to the toe of the backstop, which were transported through a subduc‐ tion channel between the subducting oceanic crust and the accretionary wedge.

#### **6. South American margin (Columbia‐Ecuador‐Peru‐Chile)**

#### **6.1. Plate configuration**

The South American margin is a very long (∼8000 km) subduction zone where the oceanic Nazca (6–7 cm/yr) and Antarctic (<2 cm/yr) plates subducts beneath the continental South American plate (**Figure 10**). Due to its large length, a wide variety of styles of subduction is recognized, in terms of tectonic erosion/accretion [26, 51], oblique subduction [52, 53], and flat‐slab and ridge subduction [54]. The width between the shelf and the trench is relatively small, and there are no large accretionary complex that does not exist throughout the margin.

#### **6.2. Forearc basins along the Columbia‐Ecuador margin**

appears to be subducted to the toe of the backstop, which were transported through a subduc‐

**Figure 9.** (A) Interpretations of the seismic profiles across the Tobago Trough in the Lesser Antilles forearc. (B) Details of seaward margin of the Tobago forearc basin. Modified from Ref. [45]. (C) A cross section across a forearc basin off Guadeloupe (modified from Ref. [48]). Seaward margins of both basins are deformed and uplifted by backthrusts.

The South American margin is a very long (∼8000 km) subduction zone where the oceanic Nazca (6–7 cm/yr) and Antarctic (<2 cm/yr) plates subducts beneath the continental South

tion channel between the subducting oceanic crust and the accretionary wedge.

**6. South American margin (Columbia‐Ecuador‐Peru‐Chile)**

**6.1. Plate configuration**

Locations of the profiles are shown in **Figure 8**.

14 Evolutionary Models of Convergent Margins - Origin of Their Diversity

Along the Columbia margin, double forearc basins elongate parallel to the trench similar with the southwestern Alaskan margin. The inner basin is mainly onshore, being synclinal sags from Paleogene with very large (∼10 km) depth (e.g., Tumaco Basin [55]) (**Figure 11A**). On

**Figure 10.** Index map of the South American margin (Colombian‐Ecuador‐Peru‐Chile). Blue lines with labels A–E indicate locations of survey lines shown in **Figure 11**. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the South American plate based on [8].

**Figure 11.** (A) Schematic cross section across the Tumaco and Manglares basins along the north Ecuador and south Colombia margin (modified from Ref. [58]). (B) Enlargement of the box in A. (C) Interpretation of seismic profile of the Manglares Basin (modified from Ref. [56]). Seismic units; 1—Eocene, 2—Oligocene, 3—Early‐Middle Miocene, 4— Middle‐Late Miocene, 5—Pliocene‐Pleistocene. (D) A cross‐section across the Ecuador margin (modified from Ref. [57]). (E) Interpretation of seismic profile for the boxed area in (C). Flower‐structures are conspicuous. The depositional ages of the seismic units are not determined. Locations of the profiles are shown in **Figure 10**.

the other hand, offshore forearc basins are smaller and shallower than the inner basins (e.g., Manglares Basin [56]) (**Figure 11B** and **C**).

The Manglares basin is fronted by the Colombian accretionary wedge. The substrate of the basin is possibly accreted mass of an oceanic plateau. The basal unit (Unit 1 in **Figure 11C**) is composed of deep‐water turbidite of the middle to late Eocene. The constant thickness suggests that they are slope (bathyal) deposits. In the Oligocene to the middle Miocene (Units 2 and 3), the outer arc high began to uplift in response to the plate kinematic reorganization during 40–35 Ma, inferred from landward propagation and onlap of the deposition. Unit 4 (late Miocene) corresponds to the overfilled sediments deposited on the truncated outer arc high, resulting in seaward prograda‐ tion of the sediments. Sediments in Unit 5 fill the central part of the basin during a relax‐phase of subsidence. Seaward‐vergent reverse faults have tilted these forearc basin sediments landward.

On the other hand, a forearc basin along the Ecuador margin (southern part of the Esmeraldas‐ Caraquez Basin) is highly deformed by strike‐slip faults, where Carnegie ridge crest is being subducted [57]. Conspicuous flower‐structures popped up sediments in the basin. The strike‐ slip fault system terminates at depth against Carnegie ridge beneath the margin.

### **7. Tonga‐Kermadec‐Hikurangi margin**

#### **7.1. Margin configuration**

Tonga‐Kermadec‐Hikurangi margin is a more than 3000 km‐long boundary between the Pacific and Indo‐Australian plates. Along the Tonga‐Kermadec margin, a thin and less buoyant oceanic crust with deep water depth subducts beneath island arc crusts of Tonga‐ Kermadec ridges (**Figure 12A**). The Tonga‐Kermadec margin is a typical example of tec‐ tonically erosive margins [4]. Along the Hikurangi margin, a thick (10–15 km) and buoyant oceanic crust (Hikurangi Plateau composed of Cretaceous large igneous province) with shallow water depth underthrusts beneath North Island of New Zealand (**Figure 12**). The Hikurangi margin is strongly controlled by westward oblique subduction of the Pacific plate.

At the Hikurangi margin, the subduction began at 25 Ma, when the plate motion reorga‐ nized from transtension to convergent. The Axial and Coastal ranges are composed of Mid‐ Cretaceous‐Paleogene passive margin sedimentary rocks and Late Cretaceous accretionary

**Figure 11.** (A) Schematic cross section across the Tumaco and Manglares basins along the north Ecuador and south Colombia margin (modified from Ref. [58]). (B) Enlargement of the box in A. (C) Interpretation of seismic profile of the Manglares Basin (modified from Ref. [56]). Seismic units; 1—Eocene, 2—Oligocene, 3—Early‐Middle Miocene, 4— Middle‐Late Miocene, 5—Pliocene‐Pleistocene. (D) A cross‐section across the Ecuador margin (modified from Ref. [57]). (E) Interpretation of seismic profile for the boxed area in (C). Flower‐structures are conspicuous. The depositional ages

the other hand, offshore forearc basins are smaller and shallower than the inner basins (e.g.,

The Manglares basin is fronted by the Colombian accretionary wedge. The substrate of the basin is possibly accreted mass of an oceanic plateau. The basal unit (Unit 1 in **Figure 11C**) is composed of deep‐water turbidite of the middle to late Eocene. The constant thickness suggests that they are slope (bathyal) deposits. In the Oligocene to the middle Miocene (Units 2 and 3), the outer arc high began to uplift in response to the plate kinematic reorganization during 40–35 Ma, inferred from landward propagation and onlap of the deposition. Unit 4 (late Miocene) corresponds to the overfilled sediments deposited on the truncated outer arc high, resulting in seaward prograda‐ tion of the sediments. Sediments in Unit 5 fill the central part of the basin during a relax‐phase of subsidence. Seaward‐vergent reverse faults have tilted these forearc basin sediments landward. On the other hand, a forearc basin along the Ecuador margin (southern part of the Esmeraldas‐ Caraquez Basin) is highly deformed by strike‐slip faults, where Carnegie ridge crest is being subducted [57]. Conspicuous flower‐structures popped up sediments in the basin. The strike‐

slip fault system terminates at depth against Carnegie ridge beneath the margin.

Tonga‐Kermadec‐Hikurangi margin is a more than 3000 km‐long boundary between the Pacific and Indo‐Australian plates. Along the Tonga‐Kermadec margin, a thin and less

of the seismic units are not determined. Locations of the profiles are shown in **Figure 10**.

Manglares Basin [56]) (**Figure 11B** and **C**).

16 Evolutionary Models of Convergent Margins - Origin of Their Diversity

**7. Tonga‐Kermadec‐Hikurangi margin**

**7.1. Margin configuration**

**Figure 12.** (A) Index map around the Kermadec‐Hikurangi margin. Plate names; AU—Australian and PA—Pacific plates. Red arrows indicate the direction and velocity [cm/yr] of plate motion relative to the Australian plate based on Ref. [8]. (B) Tectonic map of the southern Kermadec‐Hikurangi margin which can be divided into four domains from the north; tectonic erosion (southern Kermadec), seamount subduction and tectonic erosion (northern Hikurangi), frontal accretion (central Hikurangi), strike‐slip (southern Hikurangi). Blue lines with labels A–B indicate locations of the profiles shown in **Figure 13**. Forearc basins; PB—Poverty Bay, HB—Hawke Bay, MB—Motu‐o‐Kura, AB—Akitio, WFB—Wairarapa‐ Flaxbourne basins. Abbreviations; RDA—Ruatoria debris avalanche, PI—Poverty Indentation, LAF—Lachlan‐Ariel faults, LR—Lachlan Ridge, MR—Motu‐o‐Kura Ridge, CR—Coastal Range, AR—Axial Range, and NIDFB—North Island Dextral fault belt.

rocks, which have been affected by trench‐parallel dextral strike‐slip fault (the North Island Dextral Fault Belt) due to oblique subduction and has role of backstops of forearc deformation.

The southern Kermadec‐Hikurangi margin can be separated into four sections (**Figure 13B**). The southern Kermadec section is a tectonically erosive (sediment starved) margin where a relatively thin subducting crust and thin trench fill sediments (<1 km) with large orthogonal convergent rate are dominated. The northern Hikurangi section is characterized by not only tectonic erosion but also submarine landslides and debris avalanches, such as Ruatoria debris avalanche and Poverty Indentation.

In the central Hikurangi section, subducting crust of the Hikurangi Plateau is thicker (more buoyant) with smoother surface topography than the north section. Frontal accretion formed a wide (>100 km) accretionary wedge represented by seaward‐verging imbricated thrusts and ridges (e.g., [59, 60]), because of thick (∼4 km) trench fill sediments supplied through the Hikurangi Trough from an area of high mountains in South Island [61]. The deformation front migrated seaward at 30–100 km/m.y. [59]. In the southern Hikurangi section, the orthogonal

**Figure 13.** (A) A generalized cross section across the Hikurangi margin (modified from Ref. [64]). *Abbreviations*; MR— Motu‐o‐Kura Ridge, LR—Lachlan Ridge, LB—Lachlan Basin, KR—Kidnappers Ridge. (B) Details of the sequence units and depositional faces of the Motu‐o‐Kura Basin (modified from Ref. [63]). This basin is one of the upper trench‐slope basins on the accretionary wedge, bounded by Motu‐o‐Kura Ridge at the seaward margin and Kidnappers Ridge at the landward margin.

convergent rate becomes slow (2.5–3.0 cm/yr). The plate boundary merges into the Alpine Fault strike‐slip fault systems at the southern end. Accretionary wedge becomes narrower from central to this section (<50 km).

#### **7.2. Motu‐o‐Kura basin**

rocks, which have been affected by trench‐parallel dextral strike‐slip fault (the North Island Dextral Fault Belt) due to oblique subduction and has role of backstops of forearc deformation. The southern Kermadec‐Hikurangi margin can be separated into four sections (**Figure 13B**). The southern Kermadec section is a tectonically erosive (sediment starved) margin where a relatively thin subducting crust and thin trench fill sediments (<1 km) with large orthogonal convergent rate are dominated. The northern Hikurangi section is characterized by not only tectonic erosion but also submarine landslides and debris avalanches, such as Ruatoria debris

In the central Hikurangi section, subducting crust of the Hikurangi Plateau is thicker (more buoyant) with smoother surface topography than the north section. Frontal accretion formed a wide (>100 km) accretionary wedge represented by seaward‐verging imbricated thrusts and ridges (e.g., [59, 60]), because of thick (∼4 km) trench fill sediments supplied through the Hikurangi Trough from an area of high mountains in South Island [61]. The deformation front migrated seaward at 30–100 km/m.y. [59]. In the southern Hikurangi section, the orthogonal

**Figure 13.** (A) A generalized cross section across the Hikurangi margin (modified from Ref. [64]). *Abbreviations*; MR— Motu‐o‐Kura Ridge, LR—Lachlan Ridge, LB—Lachlan Basin, KR—Kidnappers Ridge. (B) Details of the sequence units and depositional faces of the Motu‐o‐Kura Basin (modified from Ref. [63]). This basin is one of the upper trench‐slope basins on the accretionary wedge, bounded by Motu‐o‐Kura Ridge at the seaward margin and Kidnappers Ridge at the

avalanche and Poverty Indentation.

18 Evolutionary Models of Convergent Margins - Origin of Their Diversity

landward margin.

The main forearc basin is located inland in the central and southern Hikurangi section and off‐ shore in the northern Hikurangi section (Poverty Bay basin and Hawke Bay basin). Number of trench‐slope basins develop offshore (Akitio basin [62] and Motu‐o‐Kura basin [63]) on the actively growing accretionary wedge (**Figure 13A**). Because some of the basins were emerged several times during their evolutions, the deposition was highly affected by eustatic sea‐level changes in addition to tectonics. In addition, the basin depocenters migrated three dimension‐ ally with diachronous unconformities, due to the oblique subduction and segmented geom‐ etries of the trench‐slope basins.

The Motu‐o‐Kura basin [63] includes sediments from ca. 1.1 Ma. The lower sequences (Units 9–11 in **Figure 13B**) show constant thickness in the basin and even on the Motu‐o‐Kura Ridge, which were interpreted as slope deposits before the ridge activated. The middle and upper sequences (Units 0–8) represent a tectonic growth sequence reflecting sedimentation contem‐ poraneous with thrust faulting and uplift of the ridge. The strata in the basin exhibit gentle synclinal folding associated with normal faults in the center of the basin. The Motu‐o‐Kura Ridge thrust increased the displacement rate in the middle sequences (Units 5–8) are apparent at ca. 800 and ca. 430 ka as revealed by growth strata.

#### **8. Summary**

This review showed deposition and deformation styles of accretionary‐type forearc basins were much variable. Especially, deformations in seaward margins of forearc basins are highly influenced by evolutional histories of accretionary wedge composed of a prowedge developing on the subducting plate and retro‐wedges (e.g., [65–67]). Sandbox analog experiments showed two stages of the wedge evolutions [68]; Stage I was characterized by dominant retro‐wedge thrusting and conjugate prowedge kinking and folding and by rapid uplift of the axial zone (outer arc high), and Stage II was characterized by progressive stacking of thrust sheets in the prowedge, by slow retro‐wedge thrusting, and by slow to very slow uplift in the axial zone. Transition from stage I to stage II occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion. In addition, the ratio of thickness of backstop (*H*<sup>b</sup> ) to thickness of accreted sediments (*H*<sup>c</sup> ) was considered as a factor for vergence partitioning in the wedge [69], suggesting the high and low (<1) ratios of *H*b/*H*<sup>c</sup> could resulted in proward vergence and doubly (pro‐ and retro‐ward) vergence, respectively.

Deformation of accretionary‐type forearc basins can be similarly classified into two models. We propose the following models for accretionary‐type forearc basins (**Figure 14**), which are: (A) single‐wedge (asymmetrically doubly vergent), (B) two‐wedge (symmetrically doubly vergent), and (C) strike‐slip models.

*Single‐wedge model:* Forearc basins in this model are less contracted but tilted landward, be‐ cause the outer arc highs move seaward with displacements of the fore‐thrusts (splay faults) in the accretionary wedges. The basin widths are extended by this seaward movement of the outer‐arc high as well as landward migration of the depocenter. Subsidence of the basin may be ascribed to an isostatic subsidence due to thickened wedge or tectonic erosion beneath the basin. Examples of this model are Lombok Basin, Nankai Trough (Kumano and Enshu troughs), Hikurangi Trough (Motu‐o‐Kura basin), and Colombia margin (Manglares basin)*.*

*Two‐wedge model:* The seaward margins of forearc basins in this model are more contracted with folds and faults by landward‐vergent thrusts in the retro‐wedges. The basin widths en‐ large landward with migration of the depocenters. Basin subsidence can be caused by tectonic loading of thickened wedges or basal erosion. Examples of this model are Sunda (Western Java basins), Alaskan (Tugidak basin), and Lesser Antilles (Tobago basin) margins. Back‐ thrusts‐related deformations in these margins are restricted in the early stages of the basin formations. The basin strata in the recent stage are generally undeformed*.*

*Strike‐slip model:* Obliquity of subducting plates can be an additional factor to modify the basin formations. High obliquity causes a strain‐partitioning along the seaward margin of the ba‐ sins with strike‐slip faults. Transpression and transtension lead to compressional and exten‐ sional deformations, respectively. They may further overprint the preexisting deformations of single‐ and two‐wedge models. Examples of this model are Sumatra (Simuelue and Mentawai basins), Ecuador, and southern Chile margins*.*

**Figure 14.** Schematic models of three models of deformation styles in forearc basins. (A) Single‐wedge model with asymmetrical doubly vergent (singly vergent) uplift. (B) Two‐wedge model with symmetrical doubly vergent uplift. (C) Strike‐slip model.

These single‐ and two‐wedge models may be comparable with P‐C and P‐U‐C modes of [70]. The P‐C mode, where pro‐wedge (P) and conduit of subducting materials (C) are active, is characterized by landward‐dipping backstops. Whereas the P‐U‐C mode (U means uplifted plug) creates an apparent seaward‐dipping backstops. Amounts of materials added by under‐ plating (basal accretion) or removed by tectonic erosion (basal erosion) can influence degree of uplift or subside the axes of the wedges (outer arc highs), respectively; the former grows uplifted plug between the pro‐ and retro‐wedges. Therefore, material flux between the sub‐ ducting and overriding plates can be one of the major causes of the differences between the two models.

Furthermore, we speculate that the retro‐wedge is overthrusted on the mechanical backstop, when the cross‐sectional widths of the pro‐wedge are too small to absorb the total strain caused by underthrusting of the subducting plate. Internal strength (friction and cohesion) and total thickness (tectonic load) of the wedges may be other factors to determine the thrust polarity. For example, basins of the two‐wedge models, such as the Western Java and Tobago basins, were deformed by landward‐vergent thrusts on the retro‐wedges. However, these deformations are recognized only in the lower sequences, indicating the thrust activities waned as the accretionary wedges grew.

Changes of subduction direction could cause oblique subduction, which overwrite the preex‐ istence deformations in single‐wedge (e.g., Nankai Trough) and two‐wedge (e.g., Mentawai Basin) models by transpressional and transtensional deformations (strike‐slip model). Spatial and temporal variations of mechanical conditions at the subduction zones might be recorded as styles of deposition and deformation of forearc basins. Forearc basin is a potentially power‐ ful tool for unraveling histories and mechanisms of subduction zone processes.

### **Author details**

*Single‐wedge model:* Forearc basins in this model are less contracted but tilted landward, be‐ cause the outer arc highs move seaward with displacements of the fore‐thrusts (splay faults) in the accretionary wedges. The basin widths are extended by this seaward movement of the outer‐arc high as well as landward migration of the depocenter. Subsidence of the basin may be ascribed to an isostatic subsidence due to thickened wedge or tectonic erosion beneath the basin. Examples of this model are Lombok Basin, Nankai Trough (Kumano and Enshu troughs), Hikurangi Trough (Motu‐o‐Kura basin), and Colombia margin (Manglares basin)*.*

*Two‐wedge model:* The seaward margins of forearc basins in this model are more contracted with folds and faults by landward‐vergent thrusts in the retro‐wedges. The basin widths en‐ large landward with migration of the depocenters. Basin subsidence can be caused by tectonic loading of thickened wedges or basal erosion. Examples of this model are Sunda (Western Java basins), Alaskan (Tugidak basin), and Lesser Antilles (Tobago basin) margins. Back‐ thrusts‐related deformations in these margins are restricted in the early stages of the basin

*Strike‐slip model:* Obliquity of subducting plates can be an additional factor to modify the basin formations. High obliquity causes a strain‐partitioning along the seaward margin of the ba‐ sins with strike‐slip faults. Transpression and transtension lead to compressional and exten‐ sional deformations, respectively. They may further overprint the preexisting deformations of single‐ and two‐wedge models. Examples of this model are Sumatra (Simuelue and Mentawai

**Figure 14.** Schematic models of three models of deformation styles in forearc basins. (A) Single‐wedge model with asymmetrical doubly vergent (singly vergent) uplift. (B) Two‐wedge model with symmetrical doubly vergent uplift. (C)

formations. The basin strata in the recent stage are generally undeformed*.*

basins), Ecuador, and southern Chile margins*.*

20 Evolutionary Models of Convergent Margins - Origin of Their Diversity

Strike‐slip model.

Atsushi Noda\* and Ayumu Miyakawa

\*Address all correspondence to<EMAIL_ADDRESS>
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan

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**Cenozoic Fault Zone Activity and Geologic Evolution of the Offshore Regions of Fukushima and Miyagi Prefectures, Northeastern Japan, Based on Petroleum Exploration Data**

Hiroyuki Arato

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67391

#### **Abstract**

Three major fault zones were identified in the offshore regions of Fukushima and Miyagi prefectures, northeastern Japan, from petroleum exploration seismic and well data acquired by the Ministry of Economy, Trade and Industry (METI) and its predecessor: the Kesennuma tectonic line (KTL), the Ishinomaki tectonic line (ITL), and the Joban tectonic line (JTL). The stratigraphic relationships indicate that these tectonic lines were activated during the Middle Miocene, and that their activity was likely closely connected with the opening of the Japan Sea and the drifting of the northeast Japan arc to its current position. Parts of these tectonic lines were reactivated during the Late Pliocene and Quaternary in the Sendai Bay area.

**Keywords:** northeast Japan arc, tectonic lines, strike-slip fault, Fukushima, Miyagi

#### **1. Introduction**

The East Asian Japanese Islands are located on the convergent margin of the western Pacific Ocean. These islands are tectonically characterized by backarcs (e.g., the Japan Sea, Okinawa Trough, and Okhotsk Basin), island arcs (e.g., the Kuril, northeast Japan, Izu-Bonin, southwest Japan, and Ryukyu arcs), and trenches (e.g., the Japan, Izu-Ogasawara, Kuril, and Ryukyu trenches, and the Sagami and Nankai troughs). The various tectonic settings of the Japanese islands were determined by onshore geological research, as well as offshore oceanic geological surveys and oceanic geophysical measurements. The tectonic transformation around the Japanese islands is therefore thought to have occurred as follows:

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Geotectonic research has predominantly focused on tectonic structure, deep crust processes, and the interaction and internal structure of oceanic and overriding plates. In contrast, scientific drilling and seismic surveys are generally not planned and designed to observe the thick Cretaceous and Cenozoic sediments on the overriding plate of arc-trench systems. Consequently, causes of the formation and deformation processes of the Japanese offshore basins are rarely discussed. Counter to the general research trend, the Japanese government has been acquiring geological and geophysical data for petroleum exploration for many years. The Ministry of International Trade and Industry (MITI) and its successor the Ministry of Economy, Trade and Industry (METI) have been conducting *MITI/METI Exploratory Survey for Petroleum Resources* programs through several iterations of a *Five-Year Plan for Domestic Oil and Natural Gas Resources*. The database covers the sedimentary basins occupying the shelf around the Japanese islands and primarily consists of data collected from more than 30 offshore exploratory wells, two-dimensional (2D) seismic surveys from an area >100,000 km, and three-dimensional (3D) seismic surveys over approximately 50,000 km<sup>2</sup> . This database facilitates discussion of the formation/deformation of the sedimentary basins, because the surveys were originally designed to investigate thick Cretaceous and Cenozoic sediments for petroleum exploration [14]. This study focuses on the sedimentary basins in the eastern offshore regions of Fukushima and Miyagi prefectures, northeastern Japan and uses seismic structural interpretation to investigate the sedimentary and deformation patterns of the basins. These sedimentary and deformation patterns are compared with the already known tectonic framework of the northeast Japan arc and the arc-trench system. The results of this study will contribute to the on-going discussions of the active structures and/or long-term tectonic movement of northeastern Japan and may help elucidate the mechanisms of large earthquakes and disasters. The study results will also provide background knowledge for assessing global environmental variation.

#### **2. Database**

Data sets of the MITI/METI seismic survey results and geological information from three MITI exploratory wells were used to assess the tectonic framework of the forearc basins of the northeast Japan arc. The study area encompasses approximately 36,000 km<sup>2</sup> of the shelf adjacent to the Fukushima and Miyagi prefectures, Japan.

#### **2.1. MITI/METI seismic surveys**

(i) clockwise rotation of the southwest Japan arc (e.g., [1]),

30 Evolutionary Models of Convergent Margins - Origin of Their Diversity

(iii) ocean floor spreading in the northern Japan Sea (e.g., [6–8]),

(vi) rotation of the Philippine Sea Plate (e.g., [12, 13]).

seismic surveys over approximately 50,000 km<sup>2</sup>

(iv) extension of the continental crust in the southern Japan Sea (e.g., [7, 9, 10]),

[2–5]),

**2. Database**

Sea (e.g., [11]), and

(ii) deformation of the northeast Japan arc by NNW-SSE-trending strike-slip faults (e.g.,

(v) inversion tectonics at the plate boundary on the eastern margin of the northern Japan

Geotectonic research has predominantly focused on tectonic structure, deep crust processes, and the interaction and internal structure of oceanic and overriding plates. In contrast, scientific drilling and seismic surveys are generally not planned and designed to observe the thick Cretaceous and Cenozoic sediments on the overriding plate of arc-trench systems. Consequently, causes of the formation and deformation processes of the Japanese offshore basins are rarely discussed. Counter to the general research trend, the Japanese government has been acquiring geological and geophysical data for petroleum exploration for many years. The Ministry of International Trade and Industry (MITI) and its successor the Ministry of Economy, Trade and Industry (METI) have been conducting *MITI/METI Exploratory Survey for Petroleum Resources* programs through several iterations of a *Five-Year Plan for Domestic Oil and Natural Gas Resources*. The database covers the sedimentary basins occupying the shelf around the Japanese islands and primarily consists of data collected from more than 30 offshore exploratory wells, two-dimensional (2D) seismic surveys from an area >100,000 km, and three-dimensional (3D)

mation/deformation of the sedimentary basins, because the surveys were originally designed to investigate thick Cretaceous and Cenozoic sediments for petroleum exploration [14]. This study focuses on the sedimentary basins in the eastern offshore regions of Fukushima and Miyagi prefectures, northeastern Japan and uses seismic structural interpretation to investigate the sedimentary and deformation patterns of the basins. These sedimentary and deformation patterns are compared with the already known tectonic framework of the northeast Japan arc and the arc-trench system. The results of this study will contribute to the on-going discussions of the active structures and/or long-term tectonic movement of northeastern Japan and may help elucidate the mechanisms of large earthquakes and disasters. The study results will also

Data sets of the MITI/METI seismic survey results and geological information from three MITI exploratory wells were used to assess the tectonic framework of the forearc basins of

provide background knowledge for assessing global environmental variation.

the northeast Japan arc. The study area encompasses approximately 36,000 km<sup>2</sup>

adjacent to the Fukushima and Miyagi prefectures, Japan.

. This database facilitates discussion of the for-

of the shelf

The Reconnaissance Geophysical Survey "Minami Sanriku-Kashima Oki" (MITI Seismic-MSK) was conducted by MITI in 1986 using the survey vessel *Kaiyomaru*. A survey of approximately 2500 km of 2D seismic lines was performed in the area from Miyagi to the Fukushima Pacific coast using a 15–20 km grid [15].

The Reconnaissance Geophysical Survey "Abukuma Ridge Nanbu 3D" (METI 3D Seismic-ARN) was conducted by METI in 2009 using the 3D geophysical survey vessel *Shigen*.

The line locations for the 2D seismic survey and area covered by the 3D seismic survey are shown in **Figure 1**, with detailed specifications for those seismic surveys provided in **Table 1**.

**Figure 1.** Index map showing the seismic lines, area, and wellsite locations. The blue lines are line location of MITI Reconnaissance Geophysical Survey "Minami Sanriku-Kashima Oki" (MITI Seismic-MSK). The numbers and alphabets indicate line name. The green dashed open box indicates the area of METI Reconnaissance Geophysical Survey "Abukuma Ridge Nanbu 3D" (METI 3D Seismic-ARN). The open circles are the location of MITI Kesennuma-oki well (MITI Well-K), MITI Soma-oki well (MITI Well-S), and MITI Joban-oki well (MITI Well-J).

#### **2.2. MITI exploratory wells**

The exploratory well "MITI Kesennuma Oki" (MITI Well-K) was drilled by MITI in 1985, approximately 30 km ESE from Kesennuma City in Miyagi prefecture, Japan. A Cretaceous granitic rock body was observed at a depth of 1843 m below mean sea level (MSL), and drilling was stopped at a depth of 2027 m MSL [16].

The exploratory well "MITI Joban Oki" (MITI Well-J) was drilled by MITI in 1990 approximately 60 km ENE from Onahama City in Fukushima prefecture, Japan. Cretaceous sediments overlain by Quaternary and Neogene strata were observed at a depth of 1345 m MSL, and drilling was continued for approximately 3000 m through the Cretaceous succession to a total depth of 3170 m MSL [17].

The exploratory well "MITI Soma Oki" (MITI Well-S) was drilled by MITI in 1991 approximately 50 km east of Soma City in Fukushima Prefecture, Japan. A Paleogene unit overlain by Quaternary and Neogene successions was observed at a depth of 1625 m MSL, and Cretaceous clastics were observed at a depth of 2450 m MSL, with a total well depth of 3500 m MSL [18].

The locations of these three MITI exploratory wells are shown in **Figure 1**, and the stratigraphic details are listed in **Table 2**.

**Table 1.** Specifications of the MITI seismic surveys.

Cenozoic Fault Zone Activity and Geologic Evolution of the Offshore Regions of Fukushima and Miyagi Prefectures... http://dx.doi.org/10.5772/67391 33

**Table 2.** Specifications and stratigraphy of the MITI wells. All depth from mean sea level (MSL).

#### **3. Methods**

granitic rock body was observed at a depth of 1843 m below mean sea level (MSL), and drill-

The exploratory well "MITI Joban Oki" (MITI Well-J) was drilled by MITI in 1990 approximately 60 km ENE from Onahama City in Fukushima prefecture, Japan. Cretaceous sediments overlain by Quaternary and Neogene strata were observed at a depth of 1345 m MSL, and drilling was continued for approximately 3000 m through the Cretaceous succession to a

The exploratory well "MITI Soma Oki" (MITI Well-S) was drilled by MITI in 1991 approximately 50 km east of Soma City in Fukushima Prefecture, Japan. A Paleogene unit overlain by Quaternary and Neogene successions was observed at a depth of 1625 m MSL, and Cretaceous clastics were observed at a depth of 2450 m MSL, with a total well depth of 3500 m MSL [18]. The locations of these three MITI exploratory wells are shown in **Figure 1**, and the strati-

> **MITI Seismic-MSK METI 3D Seismic-ARN Two-dimensional (2D) Three-dimensional (3D)**

Geophysical Survey Geophysical Survey

"Minami Sanriku-Kashima Oki" "Abukuma Ridge Nanbu 3D"

(full fold)

Formal survey name MITI Reconnaissance METI Reconnaissance

Energy source (size) Airgun (3223 cu.in.) Airgun (3090 cu.in. × 2)

CMP bin size ― inline: 12.5 m, xline: 25 m

NS: 6 NS: 80 km

Streamer 2400 m 4800 m × 10

Group number 96 ― Fold 48 ―

Sample interval 4 ms 4 ms Record length 5000 ms 8000 ms Line number or area shape EW: 14 EW: 25 km

**Table 1.** Specifications of the MITI seismic surveys.

Year acquired 1986 2009 Vessel Kaiyo Maru Shigen Data volume 2500.50 km 1950 km<sup>2</sup>

ing was stopped at a depth of 2027 m MSL [16].

32 Evolutionary Models of Convergent Margins - Origin of Their Diversity

total depth of 3170 m MSL [17].

graphic details are listed in **Table 2**.

This study was performed as follows:

**Table 3.** Stratigraphic position of the seismic units and their bounding horizons detected on MITI seismic MSK by using the seismic stratigraphic method.

#### **4. Stratigraphic control of seismic units**

Unit H, recognized below 1843 m in measured depth (MD) of MITI Well-K, is granite [16]. An radiometric age determination on the cored granite indicates an Early Cretaceous age, and the rock body is inferred to be correlative to intrusive granites in the South Kitakami Belt consisting of Mesozoic and Paleozoic strata [21].

Unit G is recognized below 2450 mMD of MITI Well-S [18], the interval from 508 to 1843 mMD at MITI Well-K [16], and in the interval from 1345 mMD to the total depth of 3170 mMD at MITI Well-J [17]. Cretaceous calcareous nannnofossils, planktic foraminifer fossils, and radioralian fossils have been reported from the intervals.

Unit F, found in the interval from 1562 to 2450 mMD at MITI Well-S, is correlative to Paleogene based on pollen biostratigraphic analysis. Its coaly lithofacies suggest that Unit F is equivalent to the coal-bearing Paleogene strata outcropping at the adjoining onshore area [18, 22].

From Unit E, recognized in the interval from 625 to 1345 mMD at MITI Well-J and the interval from 82 to 1562 mMD at MITI Well-S, Early Miocene diatom fossils and calcareous nannofossils and radioralian fossils have been reported [17, 18, 22].

Unit D is confirmed in the interval from 582 to 825 mMD and the interval from 588 to 625 mMD at MITI Well-S and Well-J, respectively. The Middle Miocene diatom fossils have been reported from those successions [17, 18].

Unit C, found in the interval from 525 to 582 mMD at MITI Well-S, includes Late Miocene radioralian fossils [18].

Unit B is correlative to Pliocene based on calcareous nannofossil biostratigraphic analysis [17].

Unit A is correlative to Quaternary based on the calcareous nannofossil and diatom biostratigraphic analyses [17, 18].

#### **5. Results**

(iv) A discontinuity (an unconformity and its correlative conformity) was recognized in the northwestern part of the study area based on a seismic sequence interpretation method

(v) The relationship between these structural and stratigraphic features and the known

Unit H, recognized below 1843 m in measured depth (MD) of MITI Well-K, is granite [16]. An radiometric age determination on the cored granite indicates an Early Cretaceous age, and the rock body is inferred to be correlative to intrusive granites in the South Kitakami Belt consist-

**Table 3.** Stratigraphic position of the seismic units and their bounding horizons detected on MITI seismic MSK by using

H Horizon 7 Paleozoic/Mesozoic/Cretaceous Granite

Unit G is recognized below 2450 mMD of MITI Well-S [18], the interval from 508 to 1843 mMD at MITI Well-K [16], and in the interval from 1345 mMD to the total depth of 3170 mMD at MITI Well-J [17]. Cretaceous calcareous nannnofossils, planktic foraminifer fossils, and radio-

Unit F, found in the interval from 1562 to 2450 mMD at MITI Well-S, is correlative to Paleogene based on pollen biostratigraphic analysis. Its coaly lithofacies suggest that Unit F is equivalent to the coal-bearing Paleogene strata outcropping at the adjoining onshore

From Unit E, recognized in the interval from 625 to 1345 mMD at MITI Well-J and the interval from 82 to 1562 mMD at MITI Well-S, Early Miocene diatom fossils and calcareous nannofos-

Unit D is confirmed in the interval from 582 to 825 mMD and the interval from 588 to 625 mMD at MITI Well-S and Well-J, respectively. The Middle Miocene diatom fossils have been

characteristics of the adjacent onshore region was assessed.

34 Evolutionary Models of Convergent Margins - Origin of Their Diversity

**Seismic unit Horizon name of unit top Geological system** A - Quaternary B Horizon 1 Pliocene

C Horizon 2 Upper Miocene D Horizon 3 Middle Miocene E Horizon 4 Lower Miocene F Horizon 5 Paleogene G Horizon 6 Cretaceous

**4. Stratigraphic control of seismic units**

ralian fossils have been reported from the intervals.

sils and radioralian fossils have been reported [17, 18, 22].

ing of Mesozoic and Paleozoic strata [21].

the seismic stratigraphic method.

reported from those successions [17, 18].

area [18, 22].

[19, 20].

The positions of faults and their direction of extension were estimated using seismic interpretations of the heteromorphic and structural gaps of the traced horizons. Although several smaller fault zones were identified, three relatively large fault zones were designated as tectonic lines.

#### **5.1. The Kesennuma tectonic line**

This tectonic line is located in the northern part of the study area and is typically recognized at position 1 on Line 1 (p1-1) of MITI Seismic-MSK (**Figure 2**).

**Figure 2.** Map showing the positions of faults and deformed strata observed on the seismic sections (red solid circles), and traced lines of the tectonic lines (heavy black lines) and other fault zones (thinner black lines). In addition, the locations of seismic section in **Figures 3**, **4**, and **7** are shown (colored heavy lines).

Horizon 7, the upper limit of Unit H (Cretaceous granite), is recognized in MITI Well-K at a depth of 1843 m MSL (corresponding to two-way travel time = 1.43 s). An apparent westdipping normal fault and its derived faults are located west of the MITI Well-K location on the Line 1 section (**Figure 3**). A rock unit with a two-way travel time upper limit of 1.5–2.0 s is recognized west of the fault zone, and is characterized by irregularly shaped, low frequency reflectors. The seismic facies characteristics of this unit resemble those of Unit H east of the fault zone. Furthermore, this unit decreases in depth toward a western shoreline and changes into a seismically transparent facies. Paleozoic and Mesozoic strata and granitic rocks of the southern Kitakami Belt are exposed at the surface in the region of extension. Therefore, the upper limit of this rock unit is correlative with Horizon 7, which is the top of Unit H. These spatial relationships were also observed at position pD-1 and indicate an apparent south-dipping normal fault. Therefore, the fault zone consists of a southwest-dipping normal fault with a NNW-SSE trend. The author tentatively designates this fault zone the Kesennuma tectonic line (KTL).

**Figure 3.** Uninterpreted section of MITI Seismic-MSK Line 1 (below), and its structural and stratigraphic interpretation (above).

The KTL, bounding the eastern edge of the wedge-shaped sedimentary body above Horizon 7 on Line 1, is traceable to positions p2-1, p3-1, and pC-1 with similar fault characteristics. It can also tentatively be traced to positions p4-1, pA-1, and p5-2 because some evidence of deformation is recognized, although the fault characteristics are not an exact match.

Despite a lack of direct stratigraphic control, based on a comprehensive regional seismic stratigraphic interpretation the wedge-shaped sedimentary body may consist largely of Cretaceous, Paleogene, and Lower to Middle Miocene sediments. The Upper Miocene and older strata are deformed by the KTL; however, the deformation does not extend into the overlying Units B (Pliocene) and A (Quaternary).

#### **5.2. Ishinomaki tectonic line**

Horizon 7, the upper limit of Unit H (Cretaceous granite), is recognized in MITI Well-K at a depth of 1843 m MSL (corresponding to two-way travel time = 1.43 s). An apparent westdipping normal fault and its derived faults are located west of the MITI Well-K location on the Line 1 section (**Figure 3**). A rock unit with a two-way travel time upper limit of 1.5–2.0 s is recognized west of the fault zone, and is characterized by irregularly shaped, low frequency reflectors. The seismic facies characteristics of this unit resemble those of Unit H east of the fault zone. Furthermore, this unit decreases in depth toward a western shoreline and changes into a seismically transparent facies. Paleozoic and Mesozoic strata and granitic rocks of the southern Kitakami Belt are exposed at the surface in the region of extension. Therefore, the upper limit of this rock unit is correlative with Horizon 7, which is the top of Unit H. These spatial relationships were also observed at position pD-1 and indicate an apparent south-dipping normal fault. Therefore, the fault zone consists of a southwest-dipping normal fault with a NNW-SSE trend. The author tentatively designates this fault zone the Kesennuma tectonic line (KTL).

36 Evolutionary Models of Convergent Margins - Origin of Their Diversity

The KTL, bounding the eastern edge of the wedge-shaped sedimentary body above Horizon 7 on Line 1, is traceable to positions p2-1, p3-1, and pC-1 with similar fault characteristics. It can also tentatively be traced to positions p4-1, pA-1, and p5-2 because some evidence of deforma-

**Figure 3.** Uninterpreted section of MITI Seismic-MSK Line 1 (below), and its structural and stratigraphic interpretation

Despite a lack of direct stratigraphic control, based on a comprehensive regional seismic stratigraphic interpretation the wedge-shaped sedimentary body may consist largely of Cretaceous, Paleogene, and Lower to Middle Miocene sediments. The Upper Miocene and

tion is recognized, although the fault characteristics are not an exact match.

(above).

This fault zone is located in the middle and southern parts of the study area. An apparent south-dipping normal fault is recognized at position pE-1 on the north-south Line E (**Figure 2**). In addition, an apparent west-dipping normal fault is observed at position p5-1 crossing the east-west Line 5 (**Figure 4**). These are judged to be different appearances of a single southwestdipping and NW-SE trending normal fault zone. The author tentatively designates this fault zone the Ishinomaki tectonic line (ITL).

**Figure 4.** Uninterpreted section of MITI Seismic-MSK Line E (below), and its structural and stratigraphic interpretation (above).

The footwall block of the ITL primarily consists of a seismically transparent layer, estimated to be the seaward extension of the southern Kitakami Belt, composed of Unit H (Paleozoic and Mesozoic strata, and granitic rocks). Horizon 7, the upper limit of Unit H, increases in depth east of position p5-1, and is overlain by the wedge-shaped sedimentary body seen on Lines 1–4. Based on stratigraphic data from MITI Well-S, the hanging wall block of the ITL is interpreted to consist of Units B to G (Paleogene and Neogene, and likely also Cretaceous). The northwestern extension of the ITL is not found at the north end of Line F, although the units making up the hanging wall block are recognized. The southeastern extension of the ITL can be traced to positions pD-2, p6-3, pC-2, p7-3, and p8-3. It is characterized by deformed strata, although the vertical fault displacement is not clearly detected because it passes through the wedge-shaped sedimentary body consisting of Cretaceous and Cenozoic strata seen near MITI Well-K, and then extends southward. F12, a branched fault zone of the ITL, is also identified on the southwest side, avnd is observed at positions pE-2, p6-2, p7-3, p8-2, pC-3, and p9-1. The deformation caused by the ITL does not generally extend into the overlying Units B (Pliocene) and A (Quaternary); however, the northwest portion of the ITL slightly displaces the Pliocene and Quaternary units.

#### **5.3. Joban tectonic line**

This fault zone is located in the southwestern part of the study area. An apparent north-dipping normal fault is recognized at position pF-2 on the north-south trending Line F (**Figure 2**). A flexural structure of Unit E (Lower Miocene), differentiated from the overburden, is found at position p8-1. Based on the spatial relationships, this fault zone is an apparent northeast-dipping normal fault, with a left-lateral strike-slip sense. The southeastern extension of this fault zone is found at position pE-4. The author tentatively designates this fault zone the Joban tectonic line (JTL).

F11 is another fault zone similar to the JTL, and is recognized at positions p6-1, p7-1, and pF-1. Unit C (Upper Miocene) and older strata are deformed by the JTL; however, the deformation does not extend into the overlying Units B (Pliocene) and A (Quaternary). A set of the JTL and F11 faces the ITL across the graben that occupies Sendai Bay, and shows a NW to SE extensional trend.

#### **5.4. Other faults**

Fault F4, derived from the KTL, is found on the northeast side of the KTL. Faults F1, F2, F3, and F5 are recognized northeast of the KTL and F4, and are characterized by a direction and dip similar to the KTL and F4. Faults F6, F7, F8, and F9 are found in the area between the ITL and the KTL/F11. F6 and F7 are northeast-dipping normal faults. In contrast, F8 and F9 are southwestdipping similar to the ITL. F10 is found within the graben between the ITL and the JTL/F11. All of these fault zones and the three tectonic lines show a NW-SE to NNW-SSE extensional trend.

#### **5.5. Discontinuity**

As a distinctive feature of basin development, a discontinuity is recognized in the seismic sections in the northwestern and western near-coast part of the study area. The strata above the discontinuity are Units B (Pliocene) and A (Quaternary). This discontinuity is subdivided into three zones based on the ages of the underlying strata and seismic sequence configurations (**Figure 5**).

The zone where eroded Unit H (Paleozoic, Mesozoic, and granitic rocks) unconformably underlies Unit B (Pliocene) is designated D1 (**Figure 6**). D1 is discernible at the western ends of Lines 2, 3, 4, 6, and 7 by an irregular or rough surface and its undersurface truncation, which is a type of reflection termination (Vail [19]). The eroded top surfaces of horsts are unconformably overlain by Unit B (Pliocene) at the western end of Lines 5 and 6 and the northern end of Line F (**Figure 7**).

Cenozoic Fault Zone Activity and Geologic Evolution of the Offshore Regions of Fukushima and Miyagi Prefectures... http://dx.doi.org/10.5772/67391 39

positions pD-2, p6-3, pC-2, p7-3, and p8-3. It is characterized by deformed strata, although the vertical fault displacement is not clearly detected because it passes through the wedge-shaped sedimentary body consisting of Cretaceous and Cenozoic strata seen near MITI Well-K, and then extends southward. F12, a branched fault zone of the ITL, is also identified on the southwest side, avnd is observed at positions pE-2, p6-2, p7-3, p8-2, pC-3, and p9-1. The deformation caused by the ITL does not generally extend into the overlying Units B (Pliocene) and A (Quaternary); however, the northwest portion of the ITL slightly displaces the Pliocene and Quaternary units.

This fault zone is located in the southwestern part of the study area. An apparent north-dipping normal fault is recognized at position pF-2 on the north-south trending Line F (**Figure 2**). A flexural structure of Unit E (Lower Miocene), differentiated from the overburden, is found at position p8-1. Based on the spatial relationships, this fault zone is an apparent northeast-dipping normal fault, with a left-lateral strike-slip sense. The southeastern extension of this fault zone is found at position pE-4. The author tentatively designates this fault zone the Joban tectonic line (JTL).

F11 is another fault zone similar to the JTL, and is recognized at positions p6-1, p7-1, and pF-1. Unit C (Upper Miocene) and older strata are deformed by the JTL; however, the deformation does not extend into the overlying Units B (Pliocene) and A (Quaternary). A set of the JTL and F11 faces the ITL across the graben that occupies Sendai Bay, and shows a NW to SE

Fault F4, derived from the KTL, is found on the northeast side of the KTL. Faults F1, F2, F3, and F5 are recognized northeast of the KTL and F4, and are characterized by a direction and dip similar to the KTL and F4. Faults F6, F7, F8, and F9 are found in the area between the ITL and the KTL/F11. F6 and F7 are northeast-dipping normal faults. In contrast, F8 and F9 are southwestdipping similar to the ITL. F10 is found within the graben between the ITL and the JTL/F11. All of these fault zones and the three tectonic lines show a NW-SE to NNW-SSE extensional trend.

As a distinctive feature of basin development, a discontinuity is recognized in the seismic sections in the northwestern and western near-coast part of the study area. The strata above the discontinuity are Units B (Pliocene) and A (Quaternary). This discontinuity is subdivided into three zones based on the ages of the underlying strata and seismic sequence configurations (**Figure 5**). The zone where eroded Unit H (Paleozoic, Mesozoic, and granitic rocks) unconformably underlies Unit B (Pliocene) is designated D1 (**Figure 6**). D1 is discernible at the western ends of Lines 2, 3, 4, 6, and 7 by an irregular or rough surface and its undersurface truncation, which is a type of reflection termination (Vail [19]). The eroded top surfaces of horsts are unconformably overlain by Unit B (Pliocene) at the western end of Lines 5 and 6 and the

**5.3. Joban tectonic line**

38 Evolutionary Models of Convergent Margins - Origin of Their Diversity

extensional trend.

**5.4. Other faults**

**5.5. Discontinuity**

northern end of Line F (**Figure 7**).

**Figure 5.** Schematic dip sectional image of the discontinuity and its subdivision showing stratigraphic relationship between its undersurface and overlying strata.

**Figure 6.** Map showing the distribution of discontinuity and its subdivisional zones.

The zone where eroded units G (Cretaceous) and F (Paleogene) underlie Unit B (Pliocene) is designated D2. D2 is found in the areas east and southeast of D1, and can be detected by low angle onlaps on a relatively smooth surface, its undersurface truncation, and east- or southeast-dipping reflections.

**Figure 7.** Uninterpreted composite Section of MITI Seismic-MSK Lines 7, F, 6, and D (below), and its structural and stratigraphic interpretation (above).

The zone where the top surface of the prograding Unit C (Upper Miocene) is overlain by Unit B (Pliocene) is designated D3. There is no seismic stratigraphic evidence of erosion in D3, and undersurface toplap of east- or southeast-dipping reflections are observed.

#### **6. Discussion**

#### **6.1. Strike of the tectonic lines**

The KTL, ITL, JTL and other fault zones are observed in the 2D MITI Seismic-MSK data acquired using an approximately 15–20 km grid. This means that the existence of a fault or deformed strata cannot be confirmed within the grid boxes lacking seismic lines. This is common for regional 2D seismic data sets, and does not mean that they are not useful for the interpretation of fault strike. With sufficiently high intensity, length, and a relatively stable extension direction, a tectonic line identified on a single seismic line may extend to any position on three other seismic lines surrounding the grid box. Thus, the observed deformation points are located on the lines extending NW-SE to NNW-SSE. This interpretation of the strike direction of the tectonic lines is supported by marine geological surveys and an active fault survey as described below.

Marine geological maps covering the northwestern part of the study area were compiled using a high resolution and high density shallow 2D seismic data set acquired by the Geological Survey of Japan. A shallow part of the northwestern portion of the ITL is indicated on the map off "Kinkazan" as the Ishinomaki Bay Fault, which extends NNW-SSE [23]. A shallow, northwestern portion of KTL is shown on the map off "Kamaishi", which also extends NNW-SSE [24].

Tohoku Electric Power Co., Inc. conducted the active fault survey around the northern part of Sendai Bay using high resolution and high density shallow 2D seismic data set. The results of the survey are documented in handouts from a meeting held by the Nuclear Regulation Authority of Japan on April 16, 2014 [25]. The handout suggests that a NNW-SSE trending fault system identified by the survey may correspond to the Ishinomaki Bay Fault.

#### **6.2. Sense of the tectonic lines**

The tectonic lines are estimated to have a relatively straight extensional trend. However, the sense of displacement is not as constant. For example, a tectonic line that appears as a normal fault with relatively large vertical displacement in one area can be identified in other area as local deformation of strata without vertical displacement. Generally, a fault with a vertical component of displacement is relatively recognizable on a 2D seismic profile compared with strike-slip or wrench faults lacking vertical displacement, because seismic surveys use vertical elastic waves and measure their reflections.

The following model is proposed to further the discussion (**Figure 8 (1)**):

The zone where the top surface of the prograding Unit C (Upper Miocene) is overlain by Unit B (Pliocene) is designated D3. There is no seismic stratigraphic evidence of erosion in D3, and

**Figure 7.** Uninterpreted composite Section of MITI Seismic-MSK Lines 7, F, 6, and D (below), and its structural and

The KTL, ITL, JTL and other fault zones are observed in the 2D MITI Seismic-MSK data acquired using an approximately 15–20 km grid. This means that the existence of a fault or deformed strata cannot be confirmed within the grid boxes lacking seismic lines. This is common for regional 2D seismic data sets, and does not mean that they are not useful for the interpretation of fault strike. With sufficiently high intensity, length, and a relatively stable extension direction, a tectonic line identified on a single seismic line may extend to any position on three other seismic lines surrounding the grid box. Thus, the observed deformation points are located on the lines extending NW-SE to NNW-SSE. This interpretation of the strike direction of the tectonic lines is supported by marine geological surveys and an active

Marine geological maps covering the northwestern part of the study area were compiled using a high resolution and high density shallow 2D seismic data set acquired by the Geological Survey of Japan. A shallow part of the northwestern portion of the ITL is indicated on the map off "Kinkazan" as the Ishinomaki Bay Fault, which extends NNW-SSE [23]. A shallow, northwestern portion of KTL is shown on the map off "Kamaishi", which also extends NNW-SSE [24].

Tohoku Electric Power Co., Inc. conducted the active fault survey around the northern part of Sendai Bay using high resolution and high density shallow 2D seismic data set. The results

undersurface toplap of east- or southeast-dipping reflections are observed.

**6. Discussion**

**6.1. Strike of the tectonic lines**

stratigraphic interpretation (above).

40 Evolutionary Models of Convergent Margins - Origin of Their Diversity

fault survey as described below.

**Figure 8.** Conceptual model illustrating a north-south trending syncline and anticline set cut by a diagonal southwest dipping sinistral strike-slip fault.

Within this model, because the southwest block of the fault slips toward the southeast, the axis of the syncline (a) may adjoin the axis of the anticline (a′) on the northeast block (**Figure 8 (2)**). The fault will appear as a west-dipping normal fault with significant vertical displacement on the east-west seismic line at this position (**Figure 9**).

**Figure 9.** The fault plane view of the conceptual model in **Figure 8** showing the positional relationship between the northeastern block and southwestern block after sinistral strike-slip. [L] and [L′] represent the area with apparent significant vertical displacement, and [S] represents the area with less vertical displacement. Furthermore, [L] will appear apparent southwest-dipping normal fault and [L′] will be reverse fault.

In contrast, with the same fault activity, the western flank of the syncline (b) on the southwest block may be adjacent to the western flank of the anticline (b') on the northeast block. The less vertically displaced deformation will be observed in the seismic section at this position.

This model suggests that the strike-slip sense causes tectonic lines to appear as an apparently normal fault, with vertical displacement in one area and as deformed strata lacking vertical displacement in other areas. Flower-like faulting and local folding without significant vertical displacement are frequently observed on the lines of the MITI Seismic-MSK survey. With this evidence, it is conceivable that the tectonic lines in the study area may have a strike-slip sense. If it is possible to acquire enough data, based on the above-mentioned model the horizontal displacement and direction of movement should be discussed in detail. However, the 2D MITI Seismic-MSK survey, with its rough grid data, is not worthy of further sophisticated discussion.

In contrast, 3D seismic data can be used for such discussions if the conditions are suitable. For example, the so-called "Abukuma Ridge" [26], lying subsurface along the edge of the continental shelf in the study area, is cut by the southeastern extension of the JTL. Based on METI 3D Seismic-ARN data this deformation is caused by a sinistral strike-slip fault [27].

#### **6.3. Formation age of the discontinuity**

D2, part of the discontinuity, is characterized by an eroded flat surface of Cretaceous to Middle Miocene strata overlain by Pliocene units with a low angle onlap. Based on a microbiostratigraphic study using a company's exploratory well samples, this surface at position pE-3 is covered by Upper Pliocene sediments and lacks the Lower Pliocene units [28]. This suggests that the low angle onlap above D2 must be a coastal onlap at the base of the Upper Pliocene, indicating a landward shift of the coastline (transgression) in the Late Pliocene.

The landward portion of the discontinuity, D1, is characterized by an eroded undulating surface of Paleozoic to Mesozoic sediments or granitic rocks overlain by Upper Pliocene sediments. This surface is interpreted as a drowned coast caused by further transgression in the Late Pliocene. This interpretation is supported by the landward connection of D1 to the recent Sanriku coast ria.

The basinward portion of the discontinuity, D3, is characterized by toplap or asymptotic truncation of Upper Miocene strata overlain by low angle onlap of Upper Pliocene sediments. This surface is relatively flat, and in some areas, lowers stepwise basinward. This suggests that D3 was formed as a Late Miocene strand plain. In the eastern and southeastern basinward directions, D3 is correlative with Horizon 2 at the top of Upper Miocene. Farther southeast, the Lower Pliocene conformably overlies the Upper Miocene [29]. Therefore, the relative sea level fall that formed the discontinuity, which consists of D1, the D2 erosional surface, and the D3 strand plain is estimated to have occurred during the Middle Pliocene.

Upper Miocene strata with a vertical thickness of 57 m were detected in the interval between depths 525 and 582 m MSL below D2 at MITI Well-S, and are unconformably underlain by Middle Miocene units [18]. Based on the stratigraphic data from the well drilled near position pE-3 this interval corresponds to the lower portion of the Upper Miocene [22, 28]. The unconformity between the Middle Miocene and the lower part of the Upper Miocene is also truncated by the erosion that occurred on the D2 surface. This stratigraphic relationship indicates that the erosion began after the deposition of the lower part of the Upper Miocene strata. Therefore, the discontinuity is interpreted to have formed between the early Late Miocene and Late Pliocene.

#### **6.4. Active age of the tectonic lines**

In contrast, with the same fault activity, the western flank of the syncline (b) on the southwest block may be adjacent to the western flank of the anticline (b') on the northeast block. The less vertically displaced deformation will be observed in the seismic section at this position.

**Figure 9.** The fault plane view of the conceptual model in **Figure 8** showing the positional relationship between the northeastern block and southwestern block after sinistral strike-slip. [L] and [L′] represent the area with apparent significant vertical displacement, and [S] represents the area with less vertical displacement. Furthermore, [L] will

appear apparent southwest-dipping normal fault and [L′] will be reverse fault.

42 Evolutionary Models of Convergent Margins - Origin of Their Diversity

This model suggests that the strike-slip sense causes tectonic lines to appear as an apparently normal fault, with vertical displacement in one area and as deformed strata lacking vertical displacement in other areas. Flower-like faulting and local folding without significant vertical displacement are frequently observed on the lines of the MITI Seismic-MSK survey. With this evidence, it is conceivable that the tectonic lines in the study area may have a strike-slip sense. If it is possible to acquire enough data, based on the above-mentioned model the horizontal displacement and direction of movement should be discussed in detail. However, the 2D MITI Seismic-MSK survey, with its rough grid data, is not worthy of further sophisticated

In contrast, 3D seismic data can be used for such discussions if the conditions are suitable. For example, the so-called "Abukuma Ridge" [26], lying subsurface along the edge of the continental shelf in the study area, is cut by the southeastern extension of the JTL. Based on METI

D2, part of the discontinuity, is characterized by an eroded flat surface of Cretaceous to Middle Miocene strata overlain by Pliocene units with a low angle onlap. Based on a microbiostratigraphic study using a company's exploratory well samples, this surface at position pE-3 is covered by Upper Pliocene sediments and lacks the Lower Pliocene units [28]. This suggests that the low angle onlap above D2 must be a coastal onlap at the base of the Upper Pliocene, indicating a landward shift of the coastline (transgression) in the Late Pliocene.

The landward portion of the discontinuity, D1, is characterized by an eroded undulating surface of Paleozoic to Mesozoic sediments or granitic rocks overlain by Upper Pliocene sediments. This surface is interpreted as a drowned coast caused by further transgression in the Late Pliocene. This interpretation is supported by the landward connection of D1 to the recent

3D Seismic-ARN data this deformation is caused by a sinistral strike-slip fault [27].

discussion.

Sanriku coast ria.

**6.3. Formation age of the discontinuity**

The discontinuity is not folded or faulted by the KTL or the JTL/F11. In addition, the ITL does not deform the discontinuity and its overlying sediments except in the Sendai Bay area. The strata above the discontinuity are generally flat and stable despite the existence of tectonic lines like the KTL, JTL, and ITL, as well as the undulating topography of the discontinuity itself. This likely indicates that the most active period of the tectonic lines had ended before the formation of the discontinuity, terminating before the Late Miocene. Furthermore, based on the interpretation of the METI Seismic ARN data, the Lower Miocene strata are cut at the Abkukuma Ridge by the southeast portion of the JTL [27]. Therefore, it is inferred that the tectonic lines were most active at least during the early Middle Miocene.

Those tectonic lines resemble generally Tanakura Tectonic Line (TTL), Hatakawa Tectonic Line (HTL), or Futaba Fault (FF) in sense [3, 4], which are the most important tectonic elements of the southern part of northeastern Japan. The activity of TTL had been active until the Middle Miocene; however, the activity is devided into several stages after the Middle Cretaceous. TTL is considered to connect the east margin of the Japan Sea Transform (JET) bounding the eastern limit of spreading area of the northern Japan Sea [4]. The Middle Miocene is the age of final stage for the opening of Japan Basin and Yamato Basin [6, 7]. The characteristics and sense of KTL, ITL, and JTL should be considered from a wider viewpoint of large scale tectonic framework around the northeastern Japan and northern Japan Sea.

#### **6.5. Subsurface graben in Sendai Bay**

The ITL is a fault with an apparent southwest dip in the Sendai Bay area. In contrast, the JTL/ F11 is an apparent northeast-dipping fault. Therefore, the area bounded in the northeast by the ITL and in the southwest by the JTL/F11 is a graben that extends NNW, landward to the Kitakami Lowland that is characterized by a graben-like topographic feature [30]. Based on the active age of the boundary-limiting tectonic lines, this graben is inferred to have formed prior to the Middle Miocene.

The ITL does not generally deform the discontinuity and its overburden in the study area; however, it does displace shallow sediments within the Sendai Bay area. This deformation is possibly due to an active structure, because the deformed overburden consists of Upper Pliocene and Quaternary sediments. The footwall of the ITL in the deformed area is interpreted to consist of rigid rocks within the Southern Kitakami Belt, such as Mesozoic sediments, Paleozoic sediments, or granitic rocks. This suggests that the Sendai Bay portion of the ITL is reactivated, or that the relatively unrigid Cretaceous to Middle Miocene hanging wall strata are differentially compacted.

The MITI Seismic-MSK data show an upward swelling of the hanging wall block along the tectonic line that seems to indicate the occurrence of inversion tectonics during the Pliocene or Pleistocene. Although no displacement seems to reach the ocean bottom, because the Pliocene and Pleistocene deposits are thin their internal seismic facies and reflection termination patterns are not easily observed or recognized. The existence of several multiples beneath the ocean bottom further complicates the proper observation of seismic characters in this series of seismic data.

The Ishinomaki Bay Fault (the northwestern shallow portion of the ITL) is a reverse fault that cuts the discontinuity and reaches the overlying uAK supersequence that is roughly correlative with the Upper Pliocene; however, no evidence for the deformation of the Pleistocene AK2r subsequence has been noted [23]. Based on the interpretation of the Tohoku Electric Power survey results, some active faults displace not only the discontinuity and its overlying Pliocene strata, but also influence the lower part of the Pleistocene sediments.

Based on the evidence presented, the deformation of the discontinuity and its overlying sediments is inferred to reflect inversion tectonics, with a reactivation of the ITL.

### **7. Conclusion**

The tectonic deformation history of the offshore regions of the Fukushima and Miyagi prefectures, Japan, was studied using MITI Seismic-MSK, METI 3D Seismic-ARN, and MITI well data. Three major tectonic lines and several medium-scale fault zones were recognized in the study area.

Despite the limitations of the large grids of the 2D seismic configuration, based on a general interpretation method the strike direction of the tectonic lines is inferred to be NNW-SSE to NW-SE.

The sense of the tectonic lines is not constant for each line. This can occur in the case of oblique cuts of anticlinal-synclinal trends by a strike-slip fault. In this case a sinistral strike-slip fault displaces the axis of the Abukuma Ridge, as determined from the METI 3D Seismic-ARN data.

Based on this investigation, the most active period of the tectonic lines is estimated to be after the Early Miocene, during which time Abukuma Ridge was formed. The activity ceased prior to the formation of the discontinuity that eroded the strata deformed by the activity of the tectonic lines. The discontinuity is interpreted to have formed between the early Late Miocene to Late Pliocene. Therefore, the tectonic lines must have been active during the early Middle Miocene at least. Based on the stratigraphic interpretation of MITI Seismic-MSK sections, an angular unconformity between the Lower Miocene succession and its overlying Middle Miocene is recognized in the part of this study area. The sedimentary basin including this study area has a two-story layered configuration bounded by the top Lower Miocene unconformity. The second story is obviously the succession in the forearc basin of the island arc after drifted to its current position. The lower story, in contrast, may represent succession within a basin of a continental margin arc in front of the Eurasian Continent. Not only the top Lower Miocene unconformity but also the tectonic lines detected in this study must have close relationship with such major tectonic deformation process of East Asia.

The tectonic lines bounding the subsurface graben in the Sendai Bay Area affected the deformation of the discontinuity and its overlying strata. The deformation must therefore be related to Quaternary inversion tectonics with a reactivation of the ITL. This graben is inferred to extend landward to the Kitakami Lowland.

## **Acknowledgements**

the ITL and in the southwest by the JTL/F11 is a graben that extends NNW, landward to the Kitakami Lowland that is characterized by a graben-like topographic feature [30]. Based on the active age of the boundary-limiting tectonic lines, this graben is inferred to have formed