The Crust in the Pamir: Insights From Receiver Functions

Abstract

The 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.

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 2. Timing of Basin Inversion, Tian Shan Mountain Building, and Relation to Pamir‐Plateau Advance and Deep India‐Asia Indentation

“…Jurassic‐Oligocene pretectonic, shallow‐marine to continental strata evolved into Neogene synorogenic, continental clastic deposits derived from the Tian Shan in the north and the Pamir and Hindu Kush in the east and south. Inversion of the basin into the Tajik fold‐thrust belt (FTB) occurred during the Neogene, but the structural evolution, timing, and relationship to the Pamir and Tian Shan are under discussion (e.g., Bourgeois et al, 1997; Chapman et al, 2017; Stübner, Ratschbacher, Rutte, et al, 2013; Jepson, Glorie, Konopelko, Gillespie, et al, 2018; Käßner et al, 2016; Kufner et al, 2018; Schneider et al, 2019; Schurr et al, 2014;Thomas, Chauvon, et al, 1994).…”

“…Intense intermediate‐depth (~70–300 km) seismicity attests to subcrustal processes in the Hindu Kush and the Pamir (e.g., Kufner et al, 2017, 2016; Mechie et al, 2012;Pegler & Das, 1998; Schneider et al, 2013, 2019; Sippl, Schurr, Tympel, et al, 2013; Sippl, Schurr, Yuan, et al, 2013). Beneath the Hindu Kush, subduction of marginal Indian continental crust was inferred.…”

“…Thus, the seismicity cluster and the subducted Tajik middle crust approximate the down‐dip extension of the detachment(s) of the Tajik FTB and its basement. This geometry defines the curved upper interface of the rolling back Tajik‐basin lithosphere and is mimicked by the Tajik Moho down to ~90 km (Schneider et al, 2019). The ≥175‐km shortening estimates in the Tian Shan foreland and the Tajik FTB (Gągała et al, 2020) and the yet unquantified amounts of accretion and deep subduction (Sippl, Schurr, Yuan, et al, 2013) of Tajik (Asian) upper and middle crust in and beneath the North Pamir are considered a proxy for the amount of convergence since ~12 Ma.…”

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 3. Preorogenic to Synorogenic Retro‐foreland Basin Evolution in the Eastern Tajik Depression and Linkage to the Pamir Hinterland

“…The seismicity cluster in the Tajik lower crust and the subducted Tajik middle crust (Sippl, Schurr, Yuan, et al, ; Sippl, Schurr, Tympel, et al, ) approximate the down‐dip extension of the detachments of the Tajik foreland. This geometry defines the curved upper interface of the rolling back Tajik‐basin lithosphere; it is mirrored by the Tajik Moho, traced down to ~90 km (Schneider et al, ). The ≥175‐km shortening estimates in the Tian Shan foreland and the inverted Tajik basin—the Tajik fold‐thrust belt (Gągała et al, )—and the yet unquantified amounts of accretion and deep subduction (Sippl, Schurr, Tympel, et al, ) of Tajik upper‐middle crust beneath the North Pamir are proxies for the amount of convergence since ~12 Ma.…”

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 1. Structure, Kinematics, and Salt Tectonics in the Tajik Fold‐and‐Thrust Belt of the Western Foreland of the Pamir

“…The base‐salt map (Figure 4), indicating ≤10‐km‐thick Tajik‐basin deposits, and the ~35‐ to 45‐km crustal thickness of the Tajik basin derived from receiver functions (Schneider et al, 2019) allow an estimate of the thickness of crystalline crust (25–35 km) under the Tajik basin. The Tajik‐basin crust may serve as a proxy of the original crust connecting the Tajik basin with Tarim, now overthrusted by the Pamir.…”