Fault weakening across the frictional‐viscous transition zone, Karakoram Fault Zone, NW Himalaya

Abstract

[1] Exhumed fault rocks formed in the frictional-viscous transition zone (FVTZ) provide test material that can be used to assess the strength of natural fault zones. In the Karakoram Fault Zone (KFZ), such rocks contain evidence of several long-term weakening mechanisms associated with reduced coefficients of friction (<0.4). The Nubra, Tangtse, and Arganglas strands of the KFZ are focused along metavolcano-sedimentary formations indicating weakness relative to the bounding granitoids. Synkinematic retrogression suggests that reaction softening has weakened the margins of granitoids along the Nubra and Tangtse strands and the Nubra Formation within the Nubra strand. The resultant phyllosilicates have formed well-developed interconnected weak layers within phyllonites and granitic mylonites. Micaceous foliae with increased proportions of opaque minerals in granitic mylonites suggest that fluid-assisted diffusive mass transfer aided deformation within the Nubra and Tangtse strands. Microstructures within Nubra strand phyllonites suggest that frictional-viscous flow accommodated deformation at low shear stresses in the FVTZ. Multiple generations of veining within each strand indicate overpressured pore fluids within the fault zone across a range of depths. Active springs and travertines along the fault indicate ongoing suprahydrostatic fluid flow within the KFZ. Despite such evidence for weakening mechanisms, the KFZ is currently locked and most likely generates moment magnitude 7.5+ earthquakes. Evidence for multiple fault weakening mechanisms reduces potential for shear heating within the KFZ and suggests that the long-term strength of the lithosphere must reside below the depth of penetration of the fault.

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

“…The Nubra Formation consists of greenschist facies metavolcanics and metapelitic lithologies of greenschist to mid-amphibolite grade, including slates, phyllites, kyanite-garnet-mica schist, andalusitegarnet-mica schist and local occurrences of marble and serpentinite pods (Fig. 3) (Wallis et al, 2013). Metavolcanics are found in the SW portion of the Nubra Formation and metapelites generally increase in metamorphic grade towards the NE.…”

“…The Nubra Formation records a range of deformational fabrics (mylonitic, phyllonitic and cataclastic) formed under varied conditions within the Nubra strand of the KFZ (Phillips and Searle, 2007;Wallis et al, 2013), all of which strike approximately parallel to the regional trend of the fault (c. 140 , Fig. 3).…”

“…2a). The presently exposed phyllonite that is the focus of this study was estimated by Wallis et al (2013) to have formed at or since c. 14 Ma. However, as the KFZ likely cuts similar metapelitic lithologies at both higher (now eroded) and lower (still buried) structural levels, the deformation processes that it records are potentially analogous to those operating at depth, both earlier in its history and at the present day.…”

“…Geological constraints on the shear strength of the KFZ are therefore required to test the validity of the strain partitioning models. Wallis et al (2013) described greenschist facies phyllonites within the KFZ and interpreted them to play a significant role in weakening this major strike-slip fault of the India-Asia collision zone. In this contribution we describe the microstructure of these phyllonites in detail and determine the temperature of their formation using geothermometry of synkinematic chlorites.…”

Metamorphic P – T conditions and CO 2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India

“…Earlier studies suggest this metamorphism as a result of shear heating under fault-guided magmatic advection within the KFZ (Rolland and Pêcher, 2001;Rolland et al, 2009;Valli et al, 2008;Thanh et al, 2009Thanh et al, , 2011. However an alternative explanation states that the metamorphism was the result of ongoing crustal thickening (Searle et al, , 2011Streule et al, 2009;Wallis et al, 2013Wallis et al, , 2014. Further, the isotopic studies ( 3 He/ 4 He) of the geothermal springs along the Karakorum fault seem to indicate that the KFZ might have evolved from mantle depth (Klemperer et al, 2013).…”