|
1 Structural Processes Group, Department of Geological Sciences, University of Vienna, Althanstrasse 14, Vienna A-1090, Austria michael.edwards{at}univie.ac.at
2 Institut für Geologie, TU-Bergakademie Freiberg, D-09596 Freiberg, Germany
Fault-zone surveying and microstructural analyses focus on a transtensionally strained section of a >100 km-long, 5–15 km-wide, active, sinistral strike-slip W-E fault — the Damxung-Jiali Shear Zone (DJSZ). Deformation fabric superposition, palaeostress and neotectonic measurements reveal a progressive dominance of the transtension coaxial component. Vertical flattening and broadening of the DJSZ results in a 10–20 km-wide pull-apart depression with reciprocal emergence of flanking highlands that expose abandoned fault-zone domains that are hundreds of metres by tens of km in size. Polylithological fault rocks preserve a suite of frictional and viscous deformation to c. 350 °C. Amongst the fault strands, intensity and distribution of grain size reduction from comminution, widespread solution transfer, intragranular plasticity and recrystallization is heterogeneous. A regular pattern of pseudotachylite-coated surfaces short-cutting strong volcanic mylonite domains between weak creeping calcite domains indicates seismic with aseismic strain, and is interpreted as velocity-weakening evidence. Re-brecciation of solution-transfer accommodation-assisted S-C-cataclasite domains document switches from frictional to viscous strain and back again. The co-location of brittle and plastic constituent behaviour for varied fault lithologies plus our interpretations on deformation temperatures suggests major overlap for the frictional and viscous depth ranges. This indicates a very broad and strongly velocity- and fluid-dependent ‘brittle-ductile’ transition.
