SPATIAL PATTERNS OF PEAK TEMPERATURE, FINITE STRAIN, AND CPO INTENSITY IN THE TSO MORARI UHP NAPPE AND OVERLYING NAPPES, NORTHWESTERN INDIAN HIMALAYA: IMPLICATIONS FOR THE CONSTRUCTION OF UHP NAPPES

Competing models for construction of ultrahigh pressure (UHP) nappes infer either large-scale, structurally coherent sheets, or chaotic mixing of km-scale packages. Here, we use quantitative trends in peak temperature, finite strain, and crystallographic preferred orientation (CPO) intensity in UHP nappes to test the coherent vs. chaotic model predictions. We mapped three 11-45 km-long transects through the Tso Morari UHP nappe (TMN) and overlying Tetraogal nappe in the NW Indian Himalaya. We defined the structural top of the TMN at the upper limit of E-trending (095° mean trend) mineral stretching lineations, which contrast with the SW-, S- and SE-trending lineations in the Tetraogal nappe. We collected peak temperatures from metasedimentary rocks (n=20) using Raman spectroscopy of carbonaceous material (RSCM) thermometry, finite strain data from metasedimentary rocks (n=15) using the Rf-θ method on stretched detrital quartz clasts in lineation-parallel thin sections, and quartz CPO intensity (using the parameter cylindricity, B) from quartz veins (n=13) using an optical fabric analyzer. A transect in the central TMN spans 1.2-3.7 km structural distance below the top of the TMN and yielded temperatures of 449-654°C, Rs (tectonic strain) values of 2.9-3.7, and B values of 0.46-0.76. Two transects in the southeast TMN yielded comparable temperatures of 501-581°C, Rs values of 2.9-3.1, and B values of 0.42-0.75 in the upper 1.6 km of the TMN. However, all values decrease upward in the basal 1.5 km of the Tetraogal nappe to 424-479°C, 1.2-3.1, and 0.44-0.79, respectively. Our new data combined with published data (36 RSCM temperatures, 34 Rs_XZ values, 12 B values) from the TMN, Tetraogal nappe, and overlying Mata nappe, demonstrate a structurally upward decrease in mean peak temperature (519°C in the TMN, 456°C in the Tetraogal nappe, 354°C in the Mata nappe), mean Rs (3.5 in the TMN, 2.1 in the Tetraogal and Mata nappes), and mean B (0.65 in the TMN, 0.61 in the Tetraogal nappe, 0.27 in the Mata nappe). These data demonstrate 1^st-order spatial continuity in metamorphic temperature, finite strain, and CPO intensity in the TMN at a scale of 10’s of km along- and across-strike and at a km-scale vertically. This continuity supports construction of UHP nappes via large-scale, structurally coherent sheets rather than through chaotic mixing.