Tectonic development of the northeastern Tibetan Plateau as constrained by high-resolution deep seismic-reflection data

Abstract

A 180-km-long, high-resolution seismic-reflection 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 defining 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 flow, and (3) to establish the minimum amount of Cenozoic shortening strain in the region. Analysis of our newly obtained seismic-reflection 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 flat Moho directly below the projected traces of the faults is continuous. The seismic image displays subvertical zones of highly reflective sequences containing parallel and subhorizontal reflectors 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 reflective and seismically transparent regions rules out the occurrence of channel flow in the middle crust, as this process would require through-going subhorizontal reflectors bounding the channel above and below. The lack of continuous reflectors longer than a few kilometers in the lower crust makes a laminar mode of channel flow unfavorable, but lateral lower-crustal flow 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 find 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 sufficient to explain the current elevation of the region without an appeal for additional contributing factors such as channel flow and/or a thermal event in the upper mantle.