Lithospheric Delamination of the Pamir Driven by the Underthrusting Indian Plate as Inferred From Seismic Observations

Abstract

The tectonic evolution and deformation mechanisms of the Pamir remain controversial, partly because the crustal response to deep lithospheric geodynamic processes is insufficiently constrained. Two competing lithospheric tectonic models are under consideration: subduction of the Asian slab and delamination of Pamir lithosphere, each of which predicts distinct crustal anisotropic signatures. Here, we use shear-wave splitting of Moho P-to-S converted-phases from receiver functions recorded at 198 seismic stations to map crustal anisotropy across the Pamir and the surrounding regions, thereby providing direct constraints on crustal deformation styles. We further determine crustal thicknesses and Vp/Vs ratios using H-κ-c stacking. Crustal thickness correlates positively with surface elevation and negatively with Bouguer gravity anomalies, indicating that crustal thickening provides the dominant first-order support for the high topography. More importantly, Pms splitting measurements reveal strong crustal anisotropy, with a mean delay time of ∼0.66 s, and a systematic rotation of fast-polarization orientations across the intermediate-depth seismicity belt. In the southern Pamir, convergence-parallel fast directions record N-S-oriented basal shear imposed by the indented Indian lithospheric mantle, which has likely promoted Quaternary extension of southern Pamir. Conversely, to the north of this belt, orogen-parallel fast-directions signify pure-shear thickening of the Pamir lithosphere. Together with low crustal Vp/Vs ratios of 1.60–1.72 beneath the southern Pamir, which are consistent with a felsic bulk crust and removal of mafic lower crust, these observations favor Indian-lithosphere-induced delamination of the Pamir lithosphere rather than a model dominated by uniform southward subduction of Asian lithosphere.