Deep Crustal Contact Between the Pamir and Tarim Basin Deduced From Receiver Functions

Abstract

The deep crustal deformation in the east Pamir in response to the Cenozoic collision with the Tien Shan and the Tarim Basin is so far poorly constrained. We present new insights into the crustal structure of the east Pamir and its surrounding regions using P receiver functions from 40 temporary and permanent seismic stations. The crustal thickness reaches a maximum of 88 km beneath the central and southern east Pamir and decreases sharply to 50–60 km along the southern Tien Shan and to 41–50 km below the Tarim Basin. The most prominent crustal structures involve a double Moho, suggesting eastward underthrusting of the Pamir lower crust beneath southern east Pamir, and two Moho offsets, supporting delamination of Asian lower crust below the central east Pamir and pure shear shortening along the northeastern margin between the Pamir and Tarim Basin.

Structure and Stress Field of the Lithosphere Between Pamir and Tarim

“…The stress field of the earthquakes inside the underthrusting crust L3 indicates that it moves with the NNW-ward moving indenter and underthrusts the Tarim hanging wall at a highly oblique angle. As the receiver function and interpreted tomographic Moho both dip ∼WSW beneath the northwestern Kunlun east of L3 (Figure 3c; Xu et al, 2021), we infer that Tarim underthrusts the northwestern Kunlun as well, building a stack of (from top to bottom) Kunlun-Tarim-Pamir crust (Figure 4c). This excess crust may be responsible for a positive anomaly in the isostatic gravity residual (20-mGal-contour in Figure 2; Balmino et al, 2012) that flanks the northern edge of the Tibet plateau (Figure 2, inset), and was interpreted to represent thrusting of Tarim crust under the western and central Kunlun (Wittlinger et al, 2004).…”

“…The ENE-dipping Moho trough (Figure S17 in Supporting Information S1; Xu et al, 2021) and V P anomalies (L3 and H3) can, in this scenario, be interpreted as Pamir crust and indenter mantle lithosphere that underthrust the Asian (Tarim) mantle lithosphere (Figure 3c). The earthquakes may, as in the Asian slab, occur in thickened crust undergoing eclogitization (Incel et al, 2019;John et al, 2009).…”

Deep Crustal Structure Beneath the Pamir–Tibetan Plateau: Insights From the Moho Depth and Vp/Vs Ratio Variation

“…Lateral variation of Moho discontinuity, crustal thickness, and bulk composition preserve first-order information in plate tectonic evolution and provide important clues in understanding the structural evolution of the crust and upper mantle. In the past two decades, several international seismic arrays, including TIPAGE (Mechie et al, 2012;Schneider et al, 2013), FERGHANA (Schneider et al, 2013;Feld et al, 2015), TIPTIMON (Kufner et al, 2018), and the East Pamir seismic experiment (Xu et al, 2021), have been carried out within the Pamir and its adjacent region, which allowed to obtain an improved local seismic image of the crust and upper mantle in this region. The results of seismic refraction/wide-angle reflection and P and S receiver functions indicate that the crustal thickness varies from ~58 km beneath the southern Tien Shan to ~74 km under the North Pamir and ~66 km beneath the South Pamir, where it shallows to about ~40 km below the basins surrounding the Pamirs (Mechie et al, 2012;Xu et al, 2021).…”

“…In the past two decades, several international seismic arrays, including TIPAGE (Mechie et al, 2012;Schneider et al, 2013), FERGHANA (Schneider et al, 2013;Feld et al, 2015), TIPTIMON (Kufner et al, 2018), and the East Pamir seismic experiment (Xu et al, 2021), have been carried out within the Pamir and its adjacent region, which allowed to obtain an improved local seismic image of the crust and upper mantle in this region. The results of seismic refraction/wide-angle reflection and P and S receiver functions indicate that the crustal thickness varies from ~58 km beneath the southern Tien Shan to ~74 km under the North Pamir and ~66 km beneath the South Pamir, where it shallows to about ~40 km below the basins surrounding the Pamirs (Mechie et al, 2012;Xu et al, 2021). The CCP stacking images indicate the presence of a Moho offset at the eastern part of the Pamir, which may mark the boundary between the Pamir and the Tarim block, suggesting that pure shear shortening is responsible for the crustal thickening in the northeast of the Pamir (Xu et al, 2021).…”

“…The results of seismic refraction/wide-angle reflection and P and S receiver functions indicate that the crustal thickness varies from ~58 km beneath the southern Tien Shan to ~74 km under the North Pamir and ~66 km beneath the South Pamir, where it shallows to about ~40 km below the basins surrounding the Pamirs (Mechie et al, 2012;Xu et al, 2021). The CCP stacking images indicate the presence of a Moho offset at the eastern part of the Pamir, which may mark the boundary between the Pamir and the Tarim block, suggesting that pure shear shortening is responsible for the crustal thickening in the northeast of the Pamir (Xu et al, 2021). Furthermore, a Moho doublet with the deeper interface down to a depth of ~90 km has been highlighted beneath the Central Pamir, where the Asian continental lower crust delaminates and rolls back (Schneider et al, 2019).…”