Zanskar shear zone: Northeast-southwest extension within the Higher Himalayas (Ladakh, India)

Abstract

A complete geologic section through the northwestern Himalayas is well exposed in Ladakh, India. The collision between the Indian shield and the Tibetan platform caused large-scale intracrustal thrusting and piling up of the Himalayan nappes. This compressional event was accompanied by prograde regional metamorphism. The metamorphism ranges from low-grade (anchimetamorphism) in the Indus suture zone in the northeast to upper amphibolite grade in the Higher Himalaya tectonic unit in the southwest. In the Zanskar area a well-exposed and morphologically prominent shear zone, 2.25 to 6.75 km wide, can be followed from the northwest to the southeast for at least 80 km. In contrast to the main compressional tectonics, this shear zone indicates an extensional event within the Higher Himalaya tectonic unit. The Zanskar shear zone involves granitic rocks, leucogranites of Miocene age, and various metasediments and, in general, separates the late Precambrian–Early Cambrian sedimentary sequence (Phe Formation) from the underlying crystalline basement (Zanskar Crystalline unit). A normal sense of shear has been determined by using asymmetric macroscopic and microscopic structures such as pegmatite boudins, feldspar augen, and crystallographic fabrics. The metamorphic isograds are very close within this shear zone as a result of shear deformation; a gradual transition from upper amphibolite to lower greenschist facies occurs within 200 m. Movement in this zone occurred late in the metamorphic history under greenschist or lower metamorphic conditions. The minimum horizontal extension and vertical displacement are on the order of 16 and 19 km, respectively.