229 Crustal Petrology (Posters)

This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.

Tuesday, 21 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Metamorphic History and Detrital Provenance of the Torbashi Thrust Sheet in the Karakul–Mazar Terrane, Eastern Pamir, and Regional Implications for Closure of the Paleo-Tethys

Paper number: 229-16
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Torbashi thrust sheet lies along the Paleo-Tethys suture zone along the eastern margin of the Pamir.  However, its place within the suture zone and how its evolution relates to the closure of the Paleo-Tethys remains debated. Some previous studies suggested that much of the eastern Pamir consists of a structurally contiguous late Proterozoic–Cambrian terrane, the Bulunkuole Group, while others argued the Bulunkuole Group is only exposed in a relatively narrow structural block in the southeastern Pamir and the metasedimentary Torbashi thrust sheet together with its northern footwall are part of the Triassic Karakul-Mazar terrane. Further, the timing of high-grade metamorphism of the thrust sheet is not well documented with different studies suggesting peak metamorphism from ca. 250–220 Ma to ca. 200–180 Ma.

To better understand the tectonic evolution and affinity of the Torbashi thrust sheet and closure of the Paleo-Tethys in the eastern Pamir we conducted: 1) petrologic modeling based on thin section analysis and bulk rock chemical compositions to address the metamorphic evolution, and 2) zircon U–Pb dating of metamorphic rocks to address the timing of metamorphism and tectonic affinity of the thrust sheet.

Bulk composition-based phase diagrams and petrographic analysis of various metamorphic rocks (garnet-biotite,  garnet-staurolite, and garnet-sillimanite-staurolite schists) from the Torbashi thrust sheet show consistent amphibolite facies peak conditions with temperatures of ∼600–680°C and pressures of 4-6 kbar.

Unpolished zircon depth-profiling was conducted to date the timing of metamorphism. Results yield outermost rims of zircon grains with ages of ~200 Ma and Th/U ratios <0.1 interpreted to date timing of metamorphism in the Torbashi thrust sheet. Several igneous bodies from the Northwestern Torbashi yield ages of ~210 Ma, consistent with a Karakul-Mazar terrane affiliation. Detrital zircon analyses of the metasediments from the western portion of the thrust sheet yield populations at ∼330 Ma, 500–650 Ma and 700–1,000 Ma, similar to the Gondwanan Bulunkuole Group.

Based on these observations, we conclude that 1) the Torbashi thrust sheet reached amphibolite facies peak metamorphic conditions at ~200 Ma during the final collision between the Gondwanan terranes and Asia; 2) sediments of the Torbashi thrust sheet were sourced both from the Asian Karakul-Mazar terrane and the Gondwanan Bulunkuole terrane during the closure of the Paleo-Tethys.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413281

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.