Extrusion kinematics of UHP terrane in a collisional orogen: EBSD and microstructure-based approach from the Tso Morari Crystallines (Ladakh Himalaya)

Thermometry and Microstructural Analysis Imply Protracted Extensional Exhumation of the Tso Morari UHP Nappe, Northwestern Himalaya: Implications for Models of UHP Exhumation

“…However, top‐to‐east indicators are dominant at all structural levels ( n = 45) and are 2.5 times more abundant than top‐to‐west indicators ( n = 18). This is similar to the observations of Dutta and Mukherjee (2020), who also documented an overall dominant top‐to‐east shear sense within the TMN, with local outcrops that exhibit both top‐to‐east and top‐to‐west shear senses. In the Tetraogal and Mata nappes ( n = 32), shear‐sense directions exhibit more variation, as a consequence of differing lineation orientations (Figure 2 inset).…”

“…In addition, six samples from the TMN exhibit subgrain rotation recrystallization locally overprinting grain boundary migration microstructures (Table S1). Collectively, our observations from the TMN are similar to those of Dutta and Mukherjee (2020), who also documented grain boundary migration and subgrain rotation recrystallization. Rocks in the Tetraogal nappe exhibit grain boundary migration on B‐B′ and subgrain rotation on C‐C′, but no recrystallization was observed on A‐A′.…”

“…B values from the structurally lowest four samples of the TMN range between 0.65 and 0.97, defining strong CPO development between −3,450 and −1,450 m (Figure 11a). These are similar to cylindricity values obtained by Dutta and Mukherjee (2020) from TMN samples collected along our A‐A′ and C‐C′ transects, which typically range between ~0.5 and ~0.8. Moving structurally upward, B values at the upper boundary of the TMN (0 m) and in the overlying Mata nappe (1,350 m) decrease significantly to 0.25–0.31, indicating a decrease in CPO strength.…”

“…In summary, the data define uniformly high strain (Rs [ X / Z ] = ~3.9) within the basal 4 km of the TMN, an upward‐decreasing strain gradient distributed between 1 km below and 2 km above the top of the TMN, and uniformly low strain (Rs [ X / Z ] = ~1.8) at all examined structural levels above this. The uniformly high strain distributed through the TMN that we document is also supported by Dutta and Mukherjee (2020), who argued for a lack of variation in deformation intensity with structural height within the TMN, on the basis of distributed high‐cylindricity quartz fabrics and quartz grain aspect ratio statistics.…”

“…Using the pressure‐independent calibration of Faleiros et al (2016) (their equation 3), these opening angles correspond to deformation temperatures of 535 ± 50°C, 332 ± 50°C, 440 ± 50°C, and 457 ± 50°C. These opening angle temperatures overlap with those obtained from the TMN by Dutta and Mukherjee (2020), which generally range between ~400 ± 50°C and ~650 ± 50°C.…”