The crystalline units of the High Himalayas in the Lahul–Zanskar region (northwest India): metamorphic–tectonic history and geochronology of the collided and imbricated Indian plate

Abstract

-In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as Ml and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle" structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb-Sr and U-Th-Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the 'cooler' Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the latemetamorphic Zanskar Fault.

Petrographic features as an effective indicator for the variation in strength of granites

“…Numerous Rb-Sr and U-Pb studies (e.g. Pognante et al, 1990;Hodges et al, 1996) confirm the Cambro-Ordovician age of these gneisses. Several leuco-granitic bodies have also been identified in GHS (LeFort et al, 1987;Guillot et al, 1993;Hodges et al, 1996) with younger ages ranging from <4 Ma in Nanga Parbat region (Zeitler et al, 1993;Burg et al, 1998) to 23 Ma in the central Nepal (Harrison et al, 1995;Searle et al, 1999).…”

“…Both formations are separated by sharp and conformable contact (Colchen et al, 1986). The third formation consists homogenous augen orthogneisses majorly noticed in central to eastern Nepal (LeFort et al, 1986) and extend up to Bhutan towards east (Gansser, 1983) and as far west as Zanskar, NW India (Pognante et al, 1990). Numerous Rb-Sr and U-Pb studies (e.g.…”

Late Holocene Glacier Dynamics in the Miyar Basin, Lahaul Himalaya, India

“…85 [26][27][28][29][30]. Climatically, Lahaul is sandwiched between the monsoon dominated areas of the Pir Panjal to the south and the mid-latitude Westerlies dominated arid areas of Ladakh to the north of the Great Himalayan Range.…”

A workflow to facilitate three-dimensional geometrical modelling of complex poly-deformed geological units

“…This is especially true in mid-crustal levels where several successive ductile deformation phases entailing km-scale isoclinal recumbent folding and regional-scale thrusting may occur. Developing an internally consistent model of such areas can be very difficult (compare examples in Milnes, 1974;Steck, 1984;Brunel, 1986;Pognante et al, 1990;Spring and Crespo, 1992;Grujic and Mancktelow, 1996). Moreover, mid-and lower-crustal tectonic levels are generally dominated by magmatic or metamorphic crystalline basement rocks with typically rather low contrasts in their geophysical properties.…”