A lithospheric-scale thrust-wedge model for the formation of the northern Tibetan plateau margin: Evidence from high-resolution seismic imaging

The Tibetan plateau is one of the best field laboratories to investigate mechanisms of continental-lithosphere deformation, which contrasts sharply with the typical rigid-plate behavior of the oceanic lithosphere. Several end-member processes have been proposed for the plateau formation involving vertically coherent pure-shear shortening, decoupled upper- and lower-crustal shortening, oblique continental subduction, and crustal channelized flow. In order to test the above models, we conducted an integrated broadband and dense-nodal seismic-array investigation along two traverses across the northern Tibetan plateau margin in the Qilian Shan and its foreland region. The integrated observation provides the first coherent structural image of the lithosphere in general and shallow crust correlative to surface geology in particular in northern Tibet with an updated resolution. Our seismic imaging shows that the surface-exposed south-dipping Qilian Shan frontal thrust zone is closely associated with a low-velocity zone with concentrated seismicity in the upper crust, and along an inclined convertor can be extended downward into the middle-lower crust near a >10-km Moho offset beneath the Haiyuan Fault zone. This dipping feature and an inferred lithospheric detachment zone with oriented anisotropic fabrics constitute a lithospheric-scale thrust zone that bounds crustal-scale thick-skinned thrusts above, coupled with the development of distributed thrust shear zones that cut across the lowermost crust and uppermost mantle. We interpret the bounding lithospheric-scale thrust zone as the basal thrust of a thrust-wedge system across the northern Tibetan plateau margin. A minimum ∼26% bulk shortening strain is accommodated in the thrust-wedge system, which is compatible with the surface-geology constraints. Together with magnetotelluric data, our seismic imaging reveals stark contrasts in the crustal structure and rheology across the left-slip Haiyuan Fault. The involvement of strain partitioning across this regional structure in the thrust wedge supports the earlier proposed transpressional continental underthrusting of North China beneath the Tibetan plateau. © 2021 Elsevier B.V.