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

Abstract

Documenting the processes that facilitate exhumation of ultrahigh-pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure-temperature-time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5-km-thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear-dominated, top-down-to-east shearing. Strain in the TMN is recorded by high-strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport-parallel lengthening and 46% transport-normal shortening. The TMN attained peak temperatures of~500-600°C, which decrease in the overlying Tetraogal and Mata nappes to~150-300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport-parallel lengthening and transport-normal shortening decrease to 14% and 18%, respectively. When combined with published 40 Ar/ 39 Ar thermochronometry, quartz fabric deformation temperatures as low as~330°C indicate that the top-to-east shearing that exhumed the TMN continued until~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5-17.5 km; when combined with published fission-track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until~30 Ma. The long-duration, convergence-subnormal shearing that exhumed the TMN outlasted rapid India-Asia convergence by~15 Myr and may be the consequence of strain partitioning during oblique convergence.

Electron Backscatter Diffraction Study of Ultrahigh-Pressure Tso Morari Eclogites (Trans-Himalayan Collisional Zone): Implications for Strain Regime Transition from Constrictional to Plane Strain during Exhumation

“…Transition of this constrictional strain regime to plane strain regime, as shown by retrograded type-2 and type-3 eclogites, can be explained by a gradual decrease in the steepness of the channel angle at a shallower depth and layer parallel transport of the TMCC (Figure 12). This stage is similar to the "transport parallel lengthening" [47]. Our study also shows a transition in activation of dominant slip systems in omphacite during the exhumation history of the TMCC.…”

“…A tectonic model was also proposed in which the "extrusion" of the TMCC was explained by a combination of pure and simple shear [46]. This was supported by EBSD studies of quartz in the Puga Gneiss [47,48].…”

“…EBSD analyses for LPOs of quartz have been done on the Puga Gneiss, host of the TMCC metabasic rocks, in the recent past [47,48]. A prolonged top-to-the-east shearing was inferred to be responsible for exhumation of the TMCC to shallow crustal depths till 30 Ma [47].…”

“…EBSD analyses for LPOs of quartz have been done on the Puga Gneiss, host of the TMCC metabasic rocks, in the recent past [47,48]. A prolonged top-to-the-east shearing was inferred to be responsible for exhumation of the TMCC to shallow crustal depths till 30 Ma [47]. It may be noted that the Puga Gneiss is essentially a quartzofeldspathic gneiss devoid of any high-pressure mineral assemblage, and its quartz LPOs, contrary to inferences drawn from EBSD analyses of quartz from Puga Gneiss [48], are more likely to bear signatures of late-stage exhumation rather than subduction, peak (HP) metamorphism, and postpeak (HP) exhumation at ≥1.5 GPa.…”

“…They were subducted, underwent UHP metamorphism, and were subsequently exhumed. Several previous studies of these units were devoted to characterizing their metamorphism and discussing the mechanism of exhumation of the TMCC [7,[44][45][46][47][48]. However, geometry, kinematics, and dynamics of tectonic transport along the subduction channel that accommodated deep burial and exhumation received less attention.…”