Structural Data and Thermometry Indicate That the Tso Morari UHP Nappe (Indian Himalaya) Was Emplaced as a Large-Scale, Structurally Coherent Sheet

Ultrahigh-pressure (UHP) nappes provide important opportunities to investigate the processes that exhume rocks from upper mantle depths. Here, we investigate the UHP Tso Morari nappe (TMN) in the Indian Himalaya to test competing models for the construction of UHP nappes via detachment of single, structurally coherent sheets, accretion of multiple sheets, or chaotic accretion of km-scale rock packages. We collected Raman spectroscopy of carbonaceous material (RSCM) temperatures and thin section-scale finite strain, quartz crystallographic preferred-orientation (CPO) and shear-sense data sets from three transects, which we combine with published data to quantify trends across the 120 km by 40 km extent of the TMN. A laterally continuous tectonostratigraphy of metasedimentary rocks overlying granitic orthogneiss, a lack of field evidence for internal shear zones, uniformly high strain (3.4 average lineation-parallel Rs, 65% average lineation-parallel stretching) and CPO intensity (0.64 average cylindricity, 3.80 average J_PF), dominant top-to-east kinematics, and overall similar temperature conditions vertically and laterally are all consistent with detachment of the TMN as a single, structurally coherent sheet. The TMN lacks field evidence for partial melting, which is consistent with RSCM and maximum thermobarometric temperatures generally ≤700°C, and limits the importance of diapiric rise as an exhumation mechanism. Distributed top-down-to-east, normal-sense ductile shearing was an important process that facilitated exhumation of the TMN from mid-crustal depths at ∼51–46 Ma to upper-crustal depths at ∼45–29 Ma. This extensional shearing was contemporaneous with regional, SW-NE-directed shortening in the Himalayan fold-thrust belt, which could be the consequence of strain partitioning during oblique convergence.