This field-based study examined the exhumation history of the Tso Morari nappe in NW India through detailed structural, microtextural, and thermochronological analyses. The manuscript was published in Tectonics on November 25, 2020. You can find the article here.

Figure: Cross sections of our three mapped transects. Areas that exhibit similar apparent dip of tectonic foliation were divided into dip domains, which are separated by kink axes that bisect the interlimb angle (e.g., Suppe, 1983).

Coauthors:

Acknowledgement

This work was supported by start-up funds awarded to S. Long from the Washington State University School of the Environment, as well as grants EAR1450507 and OIA1545903 awarded to MJK from the US National Science Foundation. We sincerely thank Dr. Talat Ahmad, Dr. Reyaz Ahmad Dar, Irfan Bhat, and Gulam Nabiley for assistance with travel logistics, permitting, field work, and sample shipping. We would like to thank associate editor Djordje Grujic and reviewers Richard Palin and Paris Xypolias for constructive and thoughtful reviews.

Open Research

All of the data used for this research are contained within the manuscript and supplementary material. The data used for this research have been placed in the online data repository Zenodo doi: 10.5281/zenodo.4287542.

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 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.