shallow portions of major continental strike - slip faults are exposed to brittle deformation conditions . at crustal levels
34  km , kakirites ( heitzmann , 1985 ) dominate such strike - slip faults , the porosity is therefore high , and vertical fluid flow is dominant resulting in fluid  rock interaction and formation of fault gouge ( caine et  al . , 1996 ; wibberley et  al . , 2008 ) .
in terms of fluid motion , this crustal section is considered to represent the hydrothermal convection zone ( saishu et  al . , 2014 ) .
ductile fabrics respectively plastically formed fabrics are generally missing within quartzofeldspathic fault rocks at these shallow crustal levels .
in contrast , evaporitic rocks show plastic behavior even at low temperatures and pressures ( rutter , 1986 ) .
consequently , the presence of viscously behaving rocks like evaporites ( halite , gypsum ) at such shallow levels may significantly change the fault zone architecture and related porosity and permeability structure .
evaporitic layers frequently act as detachment horizons ( e.g. de paola et  al . , 2008 and references therein ) .
however , evaporitic strike - slip fault zones are rare ( schorn and neubauer , 2014 ) and detailed descriptions of the internal structure of such shear zones are hampered by intense and quick surface weathering of friable mylonitic gypsum .
furthermore , structures and fabrics of secondary tectonic origin within evaporitic sequences are often misinterpreted as primary , depositional features , as outlined by schreiber and helman ( 2005 ) . here
we present structural observations from the comeglians - paularo strike - slip shear zone in the southern alps , which propagated entirely through the subvertically tilted gypsum - dominated member of the lower bellerophon formation .
in such a case the steeply dipping trend of a rheologically weak layer entirely controls propagation and the structural trend of the shear zone , whereas in strike - slip fault systems within gently to moderately dipping sedimentary successions , the lithology virtually does not play a major role .
there , fault location in the cover rocks is primarily controlled by the strength of the basement rocks ( mandl , 1987 ; wilcox et  al . , 1973 ) .
a change in trend of the bellerophon formation also seems to control the position of fault termination .
such terminations are geometrically and kinematically complex and a number of distinct brittle structures such as extensive and compressive horsetails are often associated with fault tips ( sylvester , 1988 and references therein ) .
we demonstrate how the ductilely behaving shear zone accommodates n  s shortening and dextral displacement by two mechanisms , ( 1 ) non - coaxial and coaxial deformation and ( 2 ) a compressive horse - tail at the western fault termination .
this paper first gives an overview of the geological and tectonic setting of the comeglians - paularo shear zone .
then we describe detailed field observations of the internal fault structures and then assess the microscale deformation structures . in the discussion , we focus on the low - temperature fabrics and assess the role of viscously behaving evaporites within the otherwise brittle shallow crust for the hydrothermal convection zone . the fella - sava fault , and its western extension , the comeglians - paularo shear zone apparently form the northern boundary of a seismogenic zone , which is affected by intense seismicity with dominant n  s thrusting . on a large scale ,
the active dextral comeglians - paularo shear zone / fella - sava fault represents the pliocene - recent southern boundary of the east - directed lateral extruding block .
the study area is located in the ( eastern ) southern alps , which are bordered in the north by the periadriatic fault , the major strike - slip fault of the alpine orogen ( handy et  al . , 2010 ) .
the n - ward moving adriatic microplate and the southern alps at its front act as a rigid indenter . north of the periadriatic fault , part of the shortening is accommodated by lateral extrusion and tectonic escape to the east along e - trending strike - slip faults and n - trending extensional detachments ( fig .  1 ) ( ratschbacher et  al .
, 1991a , 1991b ) . the dextral periadriatic fault in the south and the sinistral innsbruck - salzburg - amstetten ( isam ) , salzach - enns - mariazell - puchberg ( semp ) and mur - mrz faults in the north ( fig . 
eastward extrusion starts in the oligocene mainly on the isam and periadriatic faults ( wlfler et  al . , 2011 ) .
in the early miocene activity migrates to the semp fault , which then defines the northern border of the extrusion block .
a successive rejuvenation of fault activity from the northernmost faults ( e. g. isam fault ) to the more in the se located faults ( e. g. mur - mrz fault ) has been suggested by wlfler et  al .
the eastern sector of the periadriatic fault is transected by the dextral ca . nw  se striking mll valley and lavanttal faults , which were active during the miocene and pliocene ( pischinger et  al . , 2008 ) .
based on the opening of the gorenjska basin since late miocene times a displacement shift to the fella - sava fault is evident ( fodor et  al . , 1998 ;
the comeglians - paularo shear zone is the westernmost segment of the fella - sava fault ( e.g. venturini et  al . , 2009a ) .
lateral offsets of 3060  km ( fodor et  al . , 1998 ; pinter et  al . , 2006 ; vrabec et  al . ,
2006 ) or up to 6570  km ( placer , 1996 ) along the fella - sava fault have been inferred .
the long - term slip rate for the last 20 ma has been reported to range from 1 to 5  mm / a ( jamek rupnik et  al . , 2012 ) .
ongoing dextral offset of 1.2  mm / a was reported for the central fella - sava fault in slovenia , whereas no movement was observed along its eastern part and along the periadriatic fault within the scope of the same study ( vrabec et  al . , 2006 ) .
se contraction and large e  w oriented shear strain with strain rates of 20  ppb / a ( grenerczy , 2013 ) .
the steeply southward dipping fella - sava fault can be traced to the ne corner of the gorenjska basin ( poljak , 2007 ) , where it disappears beneath neogene sediments .
an eastward continuation of the fault to celje and to the croatian border has been assumed ( fig .  1 , placer , 1996 ; plenicar , 2009 ) . between the periadriatic and the fella - sava fault systems en echelon sedimentary basins formed along left - stepping oversteps ( vrabec et  al .
the fella - sava fault separates mainly the ladinian schlern formation in the south from the permian grden formation and part of the bellerophon formation in the north ( selli , 1963 ) .
its westernmost segment was introduced by gortani ( 1926 ) as the comeglians - paularo line and later mapped in detail by braga et  al .
the comeglians - paularo shear zone developed within the gypsiferous member of the bellerophon formation ( venturini et  al .
, 2009a ) and approximately juxtaposes the older pre - variscan basement with its pennsylvanian cover in the north with permian to mesozoic post - variscan sedimentary strata in the south ( venturini et  al .
the entire sedimentary succession is steeply tilted to the south ( merlini et  al . , 2002 ) .
reconstruction of the evolution of the main river system of friuli ( tagliamento river ) indicates a watershed between the fella - sava fault and the friuli foreland up to messinian times ( monegato and stefani , 2010 ; monegato et  al . ,
the supposed river along the fella - sava fault probably drained eastwards into the gorenjska and similar slovenian basins ( dunkl et  al .
, 2005 ; monegato and stefani , 2010 ) . in zanclean times , the watershed migrated northwards and the e  w drainage along the fella - sava fault was captured by the southward dewatering tagliamento river moneg ( monegato and stefani , 2010 ) .
the southern alps are tilted southward and are divided into a basement , the carnic alps , which are exposed along the periadriatic fault , and the southalpine cover unit ( fig . 
both parts represent sedimentary mega - sequences : the carnic alps show the evolution of an early paleozoic passive continental margin of the pre - variscan cycle ( rantitsch , 1997 ) , whereas the southalpine cover unit represents the upper post - variscan cycle ( schnlaub and heinisch , 1993 ) .
the basement of the carnic alps contains a nearly continuous geological section ranging from late ordovician to early pennsylvanian , which is overprinted by both the variscan and the alpine orogenies ( e.g. brime et  al . ,
upper ordovician clastic sediments are overlain by thick devonian shelf margin carbonates . during the earliest visan ,
the extensional regime shifted to a contractional regime and mississippian synorogenic flysch was deposited ( mader et  al . , 2007 ) .
a pennsylvanian unconformity separates the folded and faulted variscan basement from the post - variscan cover .
the deposition of the cover started with intracontinental siliciclastic molasse sediments ( leppig et  al . ,
2005 ) the relative sea level changed as a result of gondwana glaciation and shelf and later massive limestone were deposited during stages of high sea - level ( krainer , 2007 ) .
a hiatus indicates regression at the end of the lower permian during which the carbonate platform was partly destructed ( schnlaub , 1992 ) . the semiarid grden formation , with conglomerates , sandstones and fine - clastic layers
subsequently , a west to east progressing transgression began ( cassinis et  al . , 2011 ) and
the grden formation locally interfingered with the marine bellerophon formation ( venturini , 1990 ) . the upper permian bellerophon formation ( stratigraphic log in fig . 
2b ) was originally a gently to the se dipping carbonate ramp located between sabkhas in the west and an open marine regime in the east ( schnlaub , 1992 and references therein ) .
the bellerophon formation was deposited in the shallow tropical  bellerophon sea  , an embayment of the paleo - tethys ( posenato , 2010 ) .
the bellerophon formation is divided into two parts : the lower part contains evaporites and cataclastic dolomite at the base ( massari and neri , 1997 ) .
the gypsiferous member acted as a major dcollement horizon in the southalpine unit ( schnborn , 1999 ) .
the upper part of the bellerophon formation consists of shoal - water deposits of dolomite , often tectonically brecciated , while the uppermost part is made up of bituminous limestone ( massari and neri , 1997 ) deposited during a regression ( scholger et  al . ,
2000 and references therein ) . in the study area , the permian bellerophon formation is approximately 260  m thick ( discenza and venturini , 2002 ) and is concordantly overlain by the lower triassic werfen formation . during the opening of the alpine tethys in jurassic times , the southalpine cover unit represents part of the passive continental margin of
the adriatic plate and sedimentation continued until the eocene ( bertotti et  al . , 1993 ; carminati et  al . ,
the southalpine unit shows no or only a very low - grade metamorphic overprint ( e.g. brime et  al . , 2008 ; rantitsch , 1997 and references therein ) . in the variscan basement , the metamorphic grade increases from east to west and from south to north ( sassi et  al . , 2004 and references therein ) , and a cretaceous age ( eo - alpine ) for the metamorphism
the width of the comeglians - paularo shear zone varies from 100 to 500  m and depends on the thickness of the gypsum - bearing member of the bellerophon formation .
the high erodibility of gypsum within the shear zone led to the formation of e  w striking linear valleys and mountain passes in between ( fig . 
approximately 11  km west of comeglians the valley changes its strike from e  w to ene
n  s draining confluents of the tagliamento river cross the shear zone perpendicularly . in the area around paluzza and comeglians , the rivers degano and bt are dextrally deflected along the shear zone and lateral offsets of 0.5  km respectively 1  km of the drainages are evident ( fig . 
in addition to detailed field observations we performed map interpretation in order to characterize the brittle , large - scale structures associated with the comeglians - paularo shear zone . a schematic overview of the whole shear - zone ( fig . 
5b  d ) of the rio turiea valley in the east ( fig .  5b , after venturini , 1990 ) , the area around comeglians in the west ( fig .  5c , after venturini et  al . , 2002 ) and the western termination of the shear zone ( fig
5d , after cantelli et  al . , 1960 ) are presented . along the gypsiferous comeglians - paularo shear zone
5 ) dextral nw  se striking riedel shears ( b in fig .  5 ) and sinistral nnw 
5 ) developed . in the area around the rio turiea valley venturini ( 1990 ) described sw- vergent folds with axes plunging to the se parallel to the riedel shears ( b in fig . 
5 ) . in the area near comeglians , a restraining bend ( d in fig .  5c ,
2 ) with ene  wsw striking thrusts and folds with sse - vergency is consistent with the dextral movement of the main shear zone .
5d ) parallel to the trend of the valley the e  w gypsum - bearing strike - slip shear zone passes into a ene  wsw striking thrust fault ( g in fig . 
the thrust fault transported sandstones of the permian grden formation over triassic carbonates and gypsum is lacking along the thrust ( fig . 
the thrust accommodates , therefore , dextral displacement along the comeglians - paularo shear zone .
further structures were formed during a different deformation event : the structures e and f are interpreted as conjugate strike - slip fault system indicating n  s shortening .
we performed detailed structural studies , including measurements of the penetrative foliation s1 and stretching lineations l1 , fold - axes ( f2 ) and shear bands between paularo and the valley west of comeglians ( fig . 
outcrop locations and results of orientation data , including composite diagrams , are given in fig . 
structural investigations along the shear zone revealed that the entire gypsiferous part of the lower bellerophon formation is strongly deformed in a ductile respectively plastic manner representing the shear zone .
brittle behavior is restricted to boudinaged dolomitic layers within gypsum and more competent lithologies of the wall rocks .
although poorly exposed , a sharp contact between foliated gypsum and brittely deformed bedded dolomites in the hangingwall respectively footwall was observed in a few outcrops ( e.g. , locations 1 , 2 and 9 of fig . 
these rocks represent the damage zone adjacent to the gypsum - bearing fault core ( according to caine et  al . , 1996 ) .
the subvertical mylonitic foliation s1 and the stretching lineation l1 are the most prominent micro- and meso - scale structure ( fig . 
the foliation of gypsum - rich rocks generally dips steeply to the s to sse ( fig .  4 ) .
the foliation dips steeply to the ese consistent with its location within the restraining bend .
gypsum attains its maximum thickness and is mined commercially in an opencast ( photo ii in fig .  2 ) .
l1 dominantly trends wsw  ene and plunges mostly eastward with an angle between 10 and 25. only at locations 13 ( fig . 
the stretching lineation l1 is defined by stretched and/or elongated millimeter - sized gypsum grains and rigid dolomite clasts .
the latter indicate a non - coaxial deformation component and show asymmetric pressure shadows clearly indicating dextral sense of shear ( fig . 
no significant variations of type and strain of fault rocks were observed along - strike of the shear zone .
shear bands dip moderately to steeply to the ssw , the associated fault striae plunge gently to the w and show dextral or normal movement ( fig .  4 ) . in the western part of the comeglians - paularo shear zone , subvertical shear bands with subhorizontal lineation ( fig . 
, 1979a , 1979b ) also indicate dextral movement of the northern block to the sw . in the trail of the shear bands ,
the dark dolomite formed pinch - and - swell shaped boudins with visible dextral displacement . additionally ,
6c ) were found . gently plunging fold axes show a wide scatter with a preferential trend to the e to se ( fig . 
the folds are isoclinal , containing a cm - spaced axial plane foliation s1 and a stretching lineation l1 .
the foliation s1 and the stretching lineation l1 are refolded during the younger event during which open folds ( f2 ) with moderately s - dipping axial plane foliation s2 were formed . the same e 
all thin sections were cut parallel to the lineation and perpendicular to the foliation and are composed of mylonitic gypsum .
0.050.2  mm ) and matrix supported , with gypsum as matrix ( figs .  7 and 8) .
the gypsum grains are mostly subhedral , subrounded , inequigranular and elongated parallel to the stretching lineation .
this is consistent with the preservation of micro - shear zones as shown in fig . 
8a  c ) shows one generation of gypsum whereas in type 2 ( fig . 
only the older generation is deformed , in type 3 both gypsum generations are deformed and recrystallization is common .
macroscopically , the mylonitic foliation and the stretching lineation are visible in all microfabric types .
the grain size of gypsum in type 1 varies from 10 to 50  m , in type 2 from 30 to 220  m and in type 3 from 30 to 150  m . in type 1 ( fig . 
8c ) the matrix is inequigranular , the grains are elongated and undulatory extinction is visible , the grain boundaries are diffuse and interlobate .
8f ) has an inequigranular matrix and subhedral grains , the grain boundaries are polygonal and triple points are visible .
two generations of gypsum are present in type 2 , the older grains are fine - grained with sutured grain boundaries , the younger grains are large , elongated and show undulatory extinction . fig . 
8h illustrates a disintegrated grain next to grains of the older generation with polygonal grain boundaries and triple points .
8i ) is foliated , the matrix is fine - grained and inequigranular , the grain boundaries are interlobate and few triple points were found .
the majority of the porphyroclasts is symmetric , consisting of carbonate and showing eye - shaped patterns with tails consistent to the principal matrix lineation . in some porphyroclasts ( fig . 
9e ) with stair stepping tails indicate dextral shear sense consistent with observations at outcrop - scale . in fringes around many porphyroclasts , two stages of crystal growth
are visible ; the fringe at the strain cap ( passchier and trouw , 2005 ) is even - grained and blocky and fibrous at the tails ( fig . 
the fibers are straight , parallel to the lineation and represent the x - direction of the finite strain .
the existence of fibrous structures indicates conditions with a high fluid pressure and solution transfer during deformation ( passchier and trouw , 2005 ) .
we interpret this as a result of extension parallel to the foliation s1 and lineation l1 .
the new data on the comeglians - paularo shear zone show ductile fabrics along the entire length of the subvertical to steeply ca .
the country rocks in the north and south are less affected by strike - slip deformation .
there mainly riedel - type brittle fault zones , oversteps and bends can be observed . for simplification ,
the gypsum - bearing part of the comeglians - paularo shear zone can be considered as the fault core and the adjacent brittely deformed country rocks as damage zones applying the model of caine et  al .
the above mentioned riedel - type brittle fault zones are considered as separate systems , each with a separate fault core ( without gypsum ) and a separate damage zone . in the following discussion we first focus on the low - temperature ductile deformation of the gypsum rocks and its potential implications on fluid flow .
finally we address the role of the fella - sava fault and comeglians - paularo shear zone for earthquake distribution and propagation .
the microfabric study of gypsum mylonites reveals extremely small grain sizes and the lack of significant annealing and recrystallization structures .
most gypsum grains of microfabrics type 1 and 2 are undulose and amoeboid grain boundaries indicate bulging as the main deformation mechanism . even in the partly recrystallized gypsum grains of microfabrics type 3
amoeboid / interlobate deformation microstructures prevail and grain sizes remained small in contrast to what is observed in many other gypsum - dominated evaporitic deformation zones ( de paola et  al . , 2008 ) .
the absence of annealing indicates that no thermal overprint following deformation occurred and that no hydrous , hot fluids invaded the comeglians - paularo shear zone .
we expected both plastic and cataclastic behavior of gypsum as described by de paola et  al .
within the comeglians - paularo shear zone with dominant gypsum , low - temperature ductile respectively plastic deformation occurred while in the adjacent damage zone brittle deformation prevailed . in all investigated outcrops ,
gypsum was found as the sole sulfate mineral , and no bassanite or anhydrite was observed . in experiments , gypsum dehydrates at 100  c to bassanite , at temperatures of 140  c a further alteration to anhydrite takes place ( brantut et  al . , 2011 ) .
the absence of evidence for dehydration and annealing structures in thin - sections indicates deformation temperatures well below 100  c . we did not find any indication ( e.g. coarse , undeformed gypsum grains , loss of foliation and lineation ) for rehydration processes as observed within many other deformed gypsum - dominated deformation zones ( de paola et  al . , 2008 ; de paola et  al .
this implies dry country rocks and limited fluid flow also in the damage zones adjacent to the shear zone .
the presence of gypsum results in low porosity and low permeability at shallow crustal levels .
the core of the shear zone acts as a barrier and impedes fluid flow as indicated by the absence of rehydration products .
thus it potentially separates two different convection zones of thermal waters within the adjacent damage zones , which dissolve gypsum from shear zone boundaries .
sulfur - bearing thermal waters are known at comeglians and at the fella - sava fault in the east ( fig . 
4 ) ( e.g. italiano et  al . , 2009 ; petrini et  al . ,
2011 ) . applying the model of caine et  al . ( 1996 ) , the shear zone is a barrier for fluid flow , whereas the adjacent damage zones represents a conduit .
the missing annealing is in contrast to gypsum - rich shear zones observed elsewhere , e.g. , in the appennines ( de paola et  al . , 2008 ; de paola et  al . , 2007 ) or northern calcareous alps ( leitner and neubauer , 2011 ; schorn and neubauer , 2011 ) , where higher temperature conditions are assumed and access of hydrous fluids resulted in pervasive recrystallization of gypsum rocks .
the parallel strike of adjacent units and the resulting lack of convincing piercing points renders determination of the amount of structural offset  along the fella - sava fault difficult . based on structurally offset oligocene and triassic rocks , fodor et  al .
( 1998 ) and references therein inferred a displacement of 3040  km for the easternmost part .
20  km as indicated by the northern margin of the gorenjska ( ljubljana ) basin .
( 2006 ) suggest a transfer of parts of the displacement to the pull - apart gorenjska basin ( fig . 
( placer , 1999 ) , presenting a transition zone toward the slovenian basin and the bosnian basin in the n and e ( plenicar , 2009 ) .
the comeglians - paularo shear zone has been assumed to be the western extension of the fella - sava fault , ( placer , 1999 ; plenicar , 2009 ) .
a contact of the comeglians - paularo shear zone with the valsugana thrust has also been discussed ( venturini et  al . , 2009a and references therein ) .
as a matter of fact displacement along a fault decreases towards its tips ( e.g. barnett et  al . , 1987 ) .
no movement will be observed at the tip and fault internal deformation will accommodate strain ( e.g. froitzheim et  al . , 2006 ) .
further to the west of comeglians , the gypsum member of the bellerophon formation changes its strike from e  w to ene  wsw and is thrusted onto the triassic werfen formation ( g in fig . 
5c ) , which accommodated part of the dextral displacement and consider it to be the western tip of the shear zone .
if this is correct , dextral splay faults close to the western end around comeglians ( b in fig . 
5b ) could be considered as extensive horsetails , which partly accommodated the lateral displacement , too .
such shortening has been found by kinematic and paleostress analyses by peresson and decker ( 1997 ) who ascribed a late miocene age to this compressional event . along the comeglians - paularo shear zone
we observe accommodation of n  s shortening and dextral displacement by a major pure shear and a subordinate simple shear component .
the subsimple shear zone and shows evidence for a combination of two stretching faults ( means , 1989 , 1990 ) as illustrated by an interpretative sketch in fig . 
symmetric boudins , - and -porphyroclasts and additional extension veins within the clasts ( fig . 
a subordinate simple - shear component is evidenced by asymmetric clasts , shear bands , micro - shear zones , en chelon structures and block rotation with internal antithetic shear . at the surface ,
the comeglians - paularo shear zone delimits a zone of widespread seismic activity in the south from a zone of insignificant seismicity in the north ( fig . 
present - day or historical seismicity is restricted to a few minor earthquakes ( e.g. paluzza ml 4.8 , 1956 ; de panfilis , 1959 ; feliziani and marcelli , 1965 , 1966 ; reinecker and lenhardt , 1999 ) . along its eastern extension , the fella - sava fault , the large historical earthquake of 1348 a. d. ( intensity of ca .
10 ) was recently placed on the fault ( tiberi et  al . , 2012 ) .
in 1895 , ljubljana was destroyed by a major earthquake ( mm 6.1 earthquake ; ribari , 1982 ) , which was rather located within the gorenjska basin than on the fella - sava fault itself .
ongoing n  s thrusting activity within the gorenjska basin has recently been documented by folded quaternary sediments and the active vodice reverse thrust fault
n of ljubljana ( jamek rupnik et  al . , 2013 and references therein ) .
we speculate that the occurrence of gypsum along the western fault trace ( comeglians - paularo shear zone ) may have absorbed stresses by viscous behavior and thus may have prevented earthquake nucleation .
in contrast , the predominance of carbonate - dominated sequences along the eastern segment of the fault may have favored earthquake nucleation . south of the comeglians - paularo shear zone along a parallel fault in the area of tolmezzo few larger earthquakes ( international seismological centre , 2011 ) with dextral focal mechanisms ( bressan et  al . , 1998 ; willingshofer and cloetingh , 2003 )
this observation supports the theory of further southward migration of the displacement due to the younging of fault activity from the periadriatic fault to southern parallel faults ( vrabec et  al . , 2006 ) .
thus the fella - sava fault represents the southernmost and the semp and mur - mrz faults the northern boundaries of the extruding blocks during late miocene / pliocene reactivation and ongoing lateral extrusion ( caporali et  al . , 2013 ; grenerczy , 2013 ) .
structural observations along the comeglians - paularo shear zone allow the following major conclusions:we corroborate that the comeglians - paularo shear zone represents the westernmost extension of the fella - sava fault .
it terminates west of comeglians with a horse - tail type thrust.the rheologically weak , gypsum - dominated lithology determines shear zone propagation and termination .
fault rocks are gypsum - mylonites with a consistently developed steep e  w striking foliation and a subhorizontal stretching lineation.missing
dehydration products of gypsum and the lack of annealing indicate unusually low temperatures < 100  c during mylonitic deformation of the gypsum . resulting low porosity and low permeability
may lead to the core of the shear zone acting as a barrier , impeding fluid flow.the structural and fabric elements are generally consistent in style and orientation at all scale and suggest pure shear e 
w extension during n  s shortening and subordinate dextral displacement along the shear zone.the subsimple shear zone is confined by two stretching faults with stretching occurring within the rheologically weak gypsum layer in between.both internal stretching within the gypsum and brittle structures like the horsetail west of comeglians accommodated portions of the displacement at the comeglians - paularo shear zone .
the displacement migrated from along the periadriatic fault southward to the fella - sava fault and as earthquake data suggests , even further to the south during recent times.the comeglians - paularo shear zone impedes fluid flow and possibly earthquake propagation .
we corroborate that the comeglians - paularo shear zone represents the westernmost extension of the fella - sava fault .
fault rocks are gypsum - mylonites with a consistently developed steep e  w striking foliation and a subhorizontal stretching lineation . missing dehydration products of gypsum and the lack of annealing indicate unusually low temperatures < 100  c during mylonitic deformation of the gypsum . resulting low porosity and low permeability
may lead to the core of the shear zone acting as a barrier , impeding fluid flow .
the structural and fabric elements are generally consistent in style and orientation at all scale and suggest pure shear e  w extension during n  s shortening and subordinate dextral displacement along the shear zone .
the subsimple shear zone is confined by two stretching faults with stretching occurring within the rheologically weak gypsum layer in between . both internal stretching within the gypsum and brittle structures like the horsetail west of comeglians accommodated portions of the displacement at the comeglians - paularo shear zone .
the displacement migrated from along the periadriatic fault southward to the fella - sava fault and as earthquake data suggests , even further to the south during recent times .
 based on structural and fabric analyses at variable scales we investigate the evaporitic gypsum - dominated comeglians - paularo shear zone in the southern alps ( friuli ) . 
 it represents the lateral western termination of the brittle fella - sava fault . missing dehydration products of gypsum and the lack of annealing indicate temperatures below 100  c during development of the shear zone . despite of such low temperatures 
 the shear zone clearly exhibits mylonitic flow , thus evidencing laterally coeval activity of brittle and viscous deformation.the dominant structures within the gypsum rocks of the lower bellerophon formation are a steeply to gently s - dipping foliation , a subhorizontal stretching lineation and pure shear - dominated porphyroclast systems . a subordinate simple shear component with dextral displacement 
 is indicated by scattered -clasts . 
 both meso- and microscale structures are characteristic of a subsimple shear type of deformation with components of both coaxial and non - coaxial strain . shortening in a transpressive regime 
 was accommodated by right - lateral displacement and internal pure shear deformation within the comeglians - paularo shear zone . 
 the shear zone shows evidence for a combination of two stretching faults , where stretching occurred in the rheologically weaker gypsum member and brittle behavior in enveloping lithologies . 
Introduction
Geological setting
Results
Discussion
Conclusions