Tectonic Evolution of the Northern Pamir from the Late Paleozoic to Cenozoic

Date

2020-12

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Abstract

The Pamir form a pronounced northward convex salient at the western end of the Tibetan orogeny. Despite its prominence, the evolution of the Pamir salient, and its history of crustal thickening and shortening, are still not well understood. To investigate these questions, I conducted three studies looking at various aspects of the Pamir. It has been broadly accepted that the northward convex Pamir salient is a Cenozoic feature resulting from significant northward translation of the Pamir terranes during the India-Asia collision. However, structural-geometric analysis of the northern Pamir indicates that the Pamir cannot have experienced ~300 km of northward translation. Moreover, the Northern Pamir show a shift in detrital zircon provenance from west to east, indicating the Paleozoic NW and NE Pamir terranes were fed from distinct cratons and tectonically not connected until the Permian. These results suggest that the Pamir salient is an inherited feature built along a Late Paleozoic embayment on the southern Asian margin. While significant Cretaceous crustal shortening has been documented in the northeastern Pamir, the structural evolution of the NW Pamir is not well documented. Geologic mapping, structural, and Ar-Ar thermochronological analyses in the Kurgovat-Vanch areas, NW Pamir, document a regionally extensive Early Cretaceous thrust sheet, the Poshkharv thrust, as well as several other Early Cretaceous thrust faults. These structures imbricate a Carboniferous arc system in the NW Pamir in the Early Cretaceous in a retroarc environment related to northward subduction of the Neotethys Ocean. Thrust faulting in the Kurgovat-Vanch region was broadly coeval with Cretaceous crustal shortening and thickening in the NE Pamir, but older than shortening in the Southern Pamir, indicating southward migration of Cretaceous shortening. To resolve the debate about the timing (Early Jurassic or Cenozoic) of peak metamorphism and better understand the peak conditions in the Muztaghata gneiss dome, Sm-Nd isochron and zircon U-Pb-REE geochronology, and P-T analyses were conducted. The results show that timing of peak metamorphism is late Oligocene-Miocene, not Jurassic. Peak conditions of 22 kbar/~800 °C indicate a minimum crustal thickness of ~75-80 km in the late Oligocene, and exhumation of lower crust in the Muztaghata dome.

Description

Keywords

Pamir, Tectonics

Citation

Portions of this document appear in: Li, Yi-Peng, Alexander C. Robinson, Mustafo Gadoev, and Ilhomjon Oimuhammadzoda. "Was the Pamir salient built along a Late Paleozoic embayment on the southern Asian margin?." Earth and Planetary Science Letters 550 (2020): 116554. And in: Li, Yi‐Peng, Alexander C. Robinson, Thomas J. Lapen, Minako Righter, and Michael K. Stevens. "Muztaghata dome Miocene Eclogite Facies metamorphism: A record of lower crustal evolution of the NE Pamir." Tectonics 39, no. 7 (2020): e2019TC005917.