Jura, Alps and the boundary of the Adria subplate
Abstract The thin-skinned Jura fold/thrust belt is the frontal part of a thick-skinned late Miocene Alpine nappe. Important late Miocene structures in the Alps that help define this nappe are the External Massifs in the north, the Orobic thrusts and the Piemontese Southern Alps in the south, the Giudicarie–Brenner Line in the east and, more vaguely, the south end of the Belledonne Massif in the west. A bottom boundary compatible with the data is the Brittle–Ductile Transition (BDT). This Jura nappe is separated from the (latest Oligocene)–early Miocene Helvetic nappe system most obviously by the late Miocene Orobic–Giudicarie–Brenner fault complex. In contrast, the distinctive boundary of the early Miocene thick-skinned Helvetic nappe was the Iorio–Tonale Line (IT) in the south, merging into the Piemontese Southern Alps at what may be termed the “Locarno Singularity”–the point of minimum width of the thick-skinned Alps; the northern boundary passed through the Gotthard–and Montblanc Massifs. Motion along the southern boundary was dextrally transpressive, associated with complex strain partitioning patterns. One part of these patterns is the confusing entity comprising the Piemontese Southern Alps and the Lepontine “dome”. The slivers composing the Piemontese Southern Alps are dragged dextrally by the Iorio–Tonale strike–slip at the “Locarno Singularity” and thereby exhibit an important characteristic trait of “exotic terranes” — crustal slivers dragged along strike–slip faults. The Lepontine “dome” around the Locarno Singularity on the north side of the Iorio–Tonale Line displays the characteristics of a backstop in thrust systems as modeled in sandbox experiments. The special boundary functioning as a backstop, compatible with the transpressive kinematics, was the Ivrea body, a strong mafic–ultramafic–granulitic complex which in the early Miocene migrated dextrally along the Iorio–Tonale Line, forcing detachment to descend into the hot middle crust and producing an upsurge of the hot masses. In the late Miocene Jura phase this particular impediment had moved to the west, enabling decollement at the Brittle–Ductile Transition to propagate across the Insubric Line into the Lombardic domain. Large-scale strain partitioning manifestations resemble those of the equally dextrally transpressive southern boundary of the Caribbean plate, in spite of a difference in scales. The foremost of these manifestations are regional right-slip faults slicing through a broad band of contraction, accompanied by collapse basins.