Low effective fault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone, NW India

Abstract

a b s t r a c tPhyllosilicate-rich fault rocks are common in large-scale fault zones and can dramatically impact fault rheology. Experimental evidence suggests that multi-mechanism frictional-viscous flow (FVF) may operate in such lithologies, potentially significantly weakening mature fault cores. We report microstructures indicative of FVF in exhumed phyllonites of the Karakoram Fault Zone (KFZ), NW India. These include interconnected muscovite foliae, lack of quartz/feldspar crystal preferred orientations, and sutured grains and overgrowths indicative of fluid-assisted diffusive mass transfer. FVF microphysical modelling, using microstructural observations from the natural fault rock and experimentally-derived friction and diffusion coefficients, predicts low peak shear strengths of <20 MPa within the frictionalviscous transition zone. Chlorite geothermometry indicates that synkinematic chlorites grew at 351 ± 34 C (c. 10 km depth) during FVF, immediately above the transition to quartz crystal plasticity. The deformation processes and interpreted low shear strength of the exhumed KFZ fault rocks provide analogues for processes operating currently at depth in active faults of similar scale. If similar lithologies are present at depth, then the Quaternary seismic characteristics of the KFZ support faults with phyllonitic cores being able to accommodate large seismic ruptures. The results also provide rare rheological constraints for mechanical models of the India-Asia collision zone.

The Late Paleozoic Tectonometamorphic Evolution of Patagonia Revisited: Insights From the Pressure‐Temperature‐Deformation‐Time (P‐T‐D‐t) Path of the Gondwanide Basement of the North Patagonian Cordillera (Argentina)

“…Kink bands and open folds are commonly associated with widespread crystallization of fine‐grained chlorite and white mica (section , Text S1 in the supporting information; García‐Sansegundo et al, ), for which mean temperature conditions of ~ 335 °C were calculated from chlorite thermometry (section ). In shear zones, mineral associations and microstructures suggest peak deformation conditions of ~ 350–300 °C, though evidence of late cataclasite and pseudotachylite development suggests further deformation below brittle‐ductile transition conditions up to ~ 200–150 °C (Fagereng & Toy, ; Wallis et al, ; Wehrens et al, ). Despite being previously interpreted as the result of the final stages of the Gondwanide orogeny (García‐Sansegundo et al, ), new evidence suggests a post‐Paleozoic age for all these younger structures (see below).…”

Spatial variation in exhumation rates across Ladakh and the Karakoram: New apatite fission track data from the Eastern Karakoram, NW India

“…While few studies have investigated the temperature‐time history of the Nubra area using geothermometry or thermochronology [ Bhutani et al ., ; Wallis et al ., , ], the medium‐high temperature cooling history of the Tangtse area is relatively well constrained [ Boutonnet et al ., ; Dunlap et al ., ; Mukherjee et al ., ; Searle et al ., ; Wallis et al ., ]. The new AFT data reveal the low‐temperature cooling history of the Tangtse area.…”

Erratum to: Crystal preferred orientations of olivine, orthopyroxene, serpentine, chlorite, and amphibole, and implications for seismic anisotropy in subduction zones: a review

“…Type-1 CPO of chlorite was also observed in natural rocks (Puelles et al, 2012;Morales et al, 2013;Padrón-Navarta et al, 2015;Kang and Jung, submitted). Type-2 CPO of chlorite was less commonly observed and reported in natural samples (Padrón-Navarta et al, 2015;Wallis et al, 2015). The reason for the occurrence of these two chlorite CPO types is currently unknown, and may be a topic of further research.…”