Abstract
AbstractDocumenting 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 40Ar/39Ar 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.
Highlights
Documenting the metamorphic conditions and structural processes associated with the exhumation of ultra-high pressure (UHP; >27 kbar) rocks can lend insights into the deformation processes that operate at upper mantle depths, and can provide critical information for testing and refining models for the geodynamic evolution of orogenic belts (e.g., Chemenda et al, 1995; Warren et al, 2008A; Hacker et al, 2013)
In northwestern India (Fig. 1), rocks exposed in the Tso Morari nappe (TMN) preserve evidence for attaining peak pressures of ~27 kbar by ~54-51 Ma, followed by exhumation to ~7-10 kbar by ~48-46 Ma, which has been interpreted to represent a record of subduction, detachment, and extrusion of rocks of the leading edge of the Indian plate, all within ~10 Myr of initial India-Asia collision
Specific questions that we address include: How high up in the crust do UHP rocks exhume via extensional shearing during their initial ascent? How consistent are inferred P-T-t paths with published models of UHP formation and exhumation? How do subsequent large-scale orogenic processes impact the post-UHP metamorphic and structural evolution? To address these questions, we refine models for the structural evolution and exhumation history of three transects through the TMN and the overlying North Himalayan nappes
Summary
Documenting the metamorphic conditions and structural processes associated with the exhumation of ultra-high pressure (UHP; >27 kbar) rocks can lend insights into the deformation processes that operate at upper mantle depths, and can provide critical information for testing and refining models for the geodynamic evolution of orogenic belts (e.g., Chemenda et al, 1995; Warren et al, 2008A; Hacker et al, 2013). Investigations of the metamorphic conditions and timing of eclogite-facies UHP metamorphism in the TMN, as well as the Barrovian-style amphibolite facies metamorphism that occurred at mid-crustal depths along its exhumation path (e.g., Guillot et al, 1997; de Sigoyer et al, 2000; Girard, 2001; Schlup et al, 2003; Leech et al, 2005, 2007; St.-Onge et al, 2013; Pan et al, 2020), have provided foundational P-T-t datapoints that have informed regional tectonic models for the geodynamic evolution of the nappe (most notably de Sigoyer et al, 2004, and Epard and Steck, 2008). These gaps in our knowledge limit our ability to use the structural and metamorphic history of the TMN to develop a better understanding of Himalayan tectonics and of the metamorphism and exhumation of UHP rocks in general
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