The Crust in the Pamir: Insights From Receiver Functions

F.M Schneider,S Orunbaev,V Minaev,C Sippl,L Ratschbacher,S Murodkulov,A Ischuk,S.‐K Kufner,X Yuan,F Tilmann,U Abdybachaev,M Gadoev,I Oimahmadov,J Mechie,B Schurr

doi:10.1029/2019jb017765

Abstract

AbstractThe Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (<1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system.

Highlights

The Pamir has been forming at the northwestern tip of the India‐Asia collision zone since ~50 Ma (e.g., Najman et al, 2010) and, together with the Tibet Plateau, contains the thickest crust on Earth (e.g., Reguzzoni et al, 2013; Figure 1)
Previous studies mostly based on satellite gravity observations showed that the Pamir has a continental crustal thickness of twice the global average, detailed information on the thickness and bulk composition based on local seismic data is missing
A map of the crustal Vp/Vs ratios was created by interpolating the individual measurements from the H‐κ algorithm at each station (Figure 4)

Summary

Introduction

The Pamir has been forming at the northwestern tip of the India‐Asia collision zone since ~50 Ma (e.g., Najman et al, 2010) and, together with the Tibet Plateau, contains the thickest crust on Earth (e.g., Reguzzoni et al, 2013; Figure 1). Robert et al (2017) obtained a crustal thickness map of Central Asia by combining elevation, geoid anomaly, and thermal data. Previous studies mostly based on satellite gravity observations showed that the Pamir has a continental crustal thickness of twice the global average, detailed information on the thickness and bulk composition based on local seismic data is missing. These studies imply a crustal thickness of up to 75 km in the Pamir and Tian Shan, but with low spatial resolution (>0.5°) and without detailed information on the crustal structure and composition

Results

Discussion

Conclusion