GEOLOGIC MAP OF THE TSO MORARI UHP NAPPE, NORTHWESTERN INDIAN HIMALAYA: IMPLICATIONS FOR CONSTRUCTION AS A LARGE-SCALE, COHERENT UHP SHEET

Diverse models compete to explain how ultrahigh pressure (UHP) nappes are created and exhumed from >100 km depths. Geologic mapping of the tectonostratigraphy and deformation geometry of UHP nappes allows testing endmember genetic models that UHP nappes are either constructed as large-scale, structurally coherent sheets or as chaotic mixtures of km-scale rock masses. Here, we present a 1:150,000-scale geologic map of the Tso Morari UHP nappe (TMN) in the NW Himalaya, India, to test these models. We We mapped six 11-72 km-long transects and combined them with published mapping. The Indus suture lies to the northeast of the TMN and consists of the Paleogene Indus Molasse and a series of ophiolitic and mélange packages that are deformed into a NW-striking fold-thrust system. To the southwest, the NW-striking North Himalayan nappes include the TMN and the successively overlying Tetraogal and Mata nappes; these consist of Neoproterozoic-Mesozoic Tethyan Himalayan (Indian affinity) metasedimentary rocks that have been intruded by Cambrian-Ordovician granites. The TMN has a NW-SE exposed length of 115 km and a NE-SW exposed width up to 35 km, and is deformed into an elongate dome. The TMN is composed of eclogite- and amphibolite-facies Neoproterozoic-Cambrian schist, metapsammite, and marble intruded by ~510-480 Ma granites (now sheared into orthogneiss). The orthogneiss contains eclogitized boudins of mafic dikes that record UHP conditions. Rocks in the TMN exhibit E-trending (095° average) mineral stretching lineations. We define the structural top of the TMN at the upper limit of E-trending lineations, which contrast with the NW-, N- and NE-trending lineations in the overlying Tetraogal and Mata nappes. The Tetraogal and Mata nappes are composed of greenschist-facies Neoproterozoic-Mesozoic sedimentary rocks that have been intruded by ~480-460 Ma granites. The TMN exhibits a uniform tectonostratigraphy, consisting of a structurally higher ‘mantle’ of metasedimentary rocks that is mappable across the full extent of the nappe, which has been intruded by a ‘core’ of orthogneiss that is 85 km-long and up to 30 km-wide. The consistent large-scale tectonostratigraphy and deformation geometry of the TMN rules out the km-scale structural interleaving predicted by models of chaotic mixing and supports the formation of UHP nappes as large-scale, structurally coherent sheets.