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A 180-km-long, high-resolution seismic-refl ection survey that imaged the entire crust and the uppermost mantle lithosphere was conducted across the northeastern Tibetan Plateau. This work had three aims: (1) to examine whether the left-slip Haiyuan and Tianjing faults defi ning the margin of NE Tibet are crustal-or lithospheric-scale structures, (2) to determine whether seismic fabrics are consistent with middle-and/ or lower-crustal channel fl ow, and (3) to establish the minimum amount of Cenozoic shortening strain in the region. Analysis of our newly obtained seismic-refl ection data suggests that the left-slip Haiyuan and Tianjing faults have multiple strands and cut through the upper and middle crust. The faults likely terminate at a low-angle detachment shear zone in the lower crust, because the fl at Moho directly below the projected traces of the faults is continuous. The seismic image displays subvertical zones of highly refl ective sequences containing parallel and subhorizontal refl ectors that are truncated by seismically transparent regions with irregular shape. The transparent regions in the middle crust are traceable to the seismically transparent lower crust and are interpreted as early Paleozoic plutons emplaced during the construction of the Qilian arc in the region. The presence of the undisturbed subvertical contacts between zones of highly refl ective and seismically transparent regions rules out the occurrence of channel fl ow in the middle crust, as this process would require through-going subhorizontal refl ectors bounding the channel above and below. The lack of continuous refl ectors longer than a few kilometers in the lower crust makes a laminar mode of channel fl ow unfavorable, but lateral lower-crustal fl ow could have occurred via small-scale ductile deformation involving folding (less than a few kilometers in wavelength and amplitude). Integrating surface geology and the seismic data, we fi nd that the upper crust along a segment of the seismic surveying line experienced up to 46% crustal shortening postdating the Cretaceous and is thus interpreted as entirely accumulated in the Cenozoic. If the estimated shortening strain is representative across northeastern Tibet, its magnitude is suffi cient to explain the current elevation of the region without an appeal for additional contributing factors such as channel fl ow and/or a thermal event in the upper mantle.

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SUMMARY We present S receiver functions and SKS splitting measurements from the China Seismograph Network located in the Qinghai and Gansu provinces. Teleseismic data are used to interpret the structure of the lithosphere–asthenosphere boundary (LAB) and upper‐mantle deformation beneath the northeastern Tibetan Plateau (NETP) in regions north of the east Kunlun Fault. Based on our observations, the LAB lies at a depth of 125–135 km beneath the northeastern Songpan–Ganzi block and the west Qinling orogen, between 145 and 175 km beneath the Kunlun and Qilian orogen, and deepens below the Qaidam Basin (175–190 km), Ordos Craton (170 km) and Alashan platform (200 km). The NETP is characterized by a nearly uniform fast NW–SE S‐wave direction. These observations are different from those to the south of the Kunlun Fault where fast S directions are rotating clockwise from the inner plateau. The change in fast directions across the Kunlun Fault implies a sudden variation of upper‐mantle deformation. Shear wave splitting delay times vary from 0.8 to 1.9 s. Data from beneath regions north of the Kunlun–Ayimaqin suture showed that delay time was positively correlated with lithospheric thickness with an increase of 0.7 s per 100 km. This indicates that the anisotropy may develop in the uppermost mantle, such as the lithosphere, beneath the NETP.

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