Structural complexity resulting from pervasive ductile deformation in the Karakoram Shear Zone, Ladakh, NW India

Abstract

[1] The NW-SE trending, ∼800 km long, dextral Karakoram Shear Zone bounds southwest Tibet. We investigate an area immediately NW of the Pangong Lake where midcrustal rocks sections of the shear zone are exposed. Here, the dominant shear zone is characterized by dextral mylonitic rocks sheared at amphibolite facies and partly retrogressed to greenschist facies. These rocks define a kilometric, SE plunging inclinedto-recumbent fold in the footwall of a north directed, oblique dextral thrust that exhumed anatectic rocks of the Pangong Range. Argon cooling ages reveal that the Pangong Range cooled from ∼700°C to 300°C between ∼18-15 and ∼10 Ma, contemporaneously with overthrusting. Toward the SE, closer to the Pangong Lake, this kilometric fold is rotated from its NW-SE trend to an E-W trend, and the sense of shear changes to sinistral, with a broadly north directed thrusting component. The documented structures contradict predictions of normal movement in this area derived from considerations of rigid block behavior of the crust and support the interpretation that the system records dextral transpression. Thrusting and exhumation of the deeper and hot rocks exposed in the Pangong Range, folding of the thrust footwall, and the development of a sinistral E-W trending shear zone are all compatible with transpression caused by a northward push by a rigid indenter such as the Ladakh Batholith.

Synkinematic magmatism, heterogeneous deformation, and progressive strain localization in a strike‐slip shear zone: The case of the right‐lateral Karakorum fault

“…Two main mylonitic strands bracket the Pangong range: the Tangtse strand to the SW and the Muglib strand to the NE. The exhumation of granulitic rocks (800°C and 5.5 Kb) of the Pangong Range [ Rolland et al , 2009], and their rapid cooling, has been related to right‐lateral transpressive deformation between the two strands [ Dunlap et al , 1998; Mc Carthy and Weinberg , 2010; Rolland et al , 2009].…”

The dike swarm of the Karakoram shear zone, Ladakh, NW India: Linking granite source to batholith

“…Locally, mylonitic rocks of the Pangong shear zone record a greenschist-facies overprint where stretching lineations, defi ned by muscovite, epidote, and chlorite, plunge moderately SE, indi cating reactivation with a NE-side-down component (McCarthy and Weinberg, 2010;Rutter et al, 2007). Evidence for present-day movement in the area has been reported only in one locality from the Pangong shear zone, whereas the Tangtse shear zone seems to be inactive (Brown et al, 2002).…”

“…Geometric com patibil ity suggests that folding within the Pangong metamorphic complex was contemporaneous with the shearing that gave rise to the two main mylonitic Tangtse and Pangong shear zones (Weinberg and Mark, 2008). The Pangong metamorphic complex rocks were uplifted in relation to the surrounding rocks, in a pop-up structure, which resulted in exposure of upperamphibolite-facies anatectic rocks in contrast to lower-grade rocks to the NE and SW McCarthy and Weinberg, 2010). Ar-Ar data suggest that deformation continued from amphibolite to greenschist facies during cooling after the anatectic event as a result of transpression McCarthy and Weinberg, 2010;Rolland et al, 2009).…”

“…An ubiquitous mineral and stretching lineation plunges gently to moderately (0°-45°, generally between 20° and 45°) NW to NNW McCarthy and Weinberg, 2010;Weinberg et al, 2009), parallel to the axis of tight to isoclinal upright folds. Geometric com patibil ity suggests that folding within the Pangong metamorphic complex was contemporaneous with the shearing that gave rise to the two main mylonitic Tangtse and Pangong shear zones (Weinberg and Mark, 2008).…”

“…A possible source for these fl uids is the metasedimentary rocks of the Karakoram meta morphic complex, which were underthrusted during oblique movement of the shear zone (McCarthy and Weinberg, 2010). The shear zone most likely created local and regional pressure gradients driving water migration into it, and then played an active role in driving magma migration, forming the complex and interconnected magma network.…”