No abstract

In the La Serre horst of the Alpine foreland, the pre-Triassic La Serre median fault zone separates a Late Devonian-Early Carboniferous granite from an ignimbrite of unknown age and from Permian deposits. Motion along this fault zone took place first in ductile conditions and then evolved in brittle conditions. Both ductile and brittle shear criteria indicate a top-NE normaldextral displacement. Similar motions are reported along faults bounding Late Palaeozoic intramontane coal basins located in the Massif Central and correspond to a widespread NE-SW Late to Post-Orogenic extension that affected the Variscan basement during Late Carboniferous to Early Permian times.

Between the Median Tectonic Line (MTL) and the Japan Sea, the western Chugoku region of SW Japan is cut by a series of N45~ first-order faults and oblique (N60~176 second-order faults. This fault network, probably formed during Late Cretaceous-Palaeocene times (70-60Ma), defines a regional block structure. PrePlio-Quaternary kinematical indicators suggest left-lateral motion along the first-order faults and right-lateral motion along some of the second-order faults. Geomorphological evidence and earthquake focal mechanisms indicate that Plio-Quaternary slip senses are opposite to Pre-Plio-Quatemary ones.The overall fault pattern is geometrically and kinematically similar to patterns obtained by experimental modelling of simple shear deformation distributed at the base of a brittle layer analogue over its entire width. This similarity suggests the possibility of a midcrustal, flat-lying partial attachment zone which could have controlled the formation of the western Chugoku fault network in Cretaceous to Palaeocene times. The zone, presently inactive, could correspond to the 'proto-MTL', a low-angle fault recently imaged by seismic reflection studies and whose trace approximately coincides with the present-day MTL. Reactivation of the system occurred twice after its formation: firstly in Miocene times, during the opening of the Japan Sea and concomitant clockwise rotation of the entire SW Japan arc; and secondly in Late Pliocene to Quaternary times, after a shift of the relative direction of convergence between the Philippine and Eurasia plates. Unlike the first reactivation, the second reactivation led to an inversion of the sense of slip along the faults.Wrench tectonics can be localized along a single strike-slip fault but can also be diffuse and encompass large areas whose widths can exceed several tens or hundreds of kilometres (e.g. Moore 1979). Diffuse wrench zones are composed of crustal blocks and slivers delimited by vertical faults. The mechanism of internal deformation of diffuse wrench zones typically involves vertical-axis rotations of blocks and concomitant strike-slip along the block-bounding faults, the so-called 'bookshelf' mechanism of Mandl (1987). Examples of analyses of internal deformation of diffuse wrench zones are numerous and often rely on palaeomagnetic investigations (Luyendyk et al.