Underthrusting and duplexing beneath the northern Tibetan Plateau and the evolution of the Himalayan-Tibetan orogen

The Cenozoic Qilian Shan thrust belt is the northern margin of the Tibetan Plateau, which developed in part due to progressive India-Asia convergence during Himalayan-Tibetan orogeny. Available geologic observations suggest that this thrust belt started deforming shortly after initial India-Asia collision at 60-55 Ma, and thus its kinematic development is intrinsically related to the construction and evolution of the Tibetan Plateau. Here, we present new field observations from a geologic traverse across the Qilian Shan to elucidate the style of deformation across the active thrust belt. In particular, we infer protracted out-of-sequence deformation here that is consistent with this thrust system remaining a stationary northern boundary to the Tibetan Plateau since the early Cenozoic. We present a lithosphere-scale model for this region that highlights the following: (1) coupled distributed crustal shortening and underthrusting of the North China craton beneath Tibet, which explains the spatial and temporal distribution of observed crustal shortening and thickness, (2) this underthrusting exploited the south-dipping early Paleozoic Qilian suture paleo-subduction melange channel, and (3) development of a lower-crustal duplex at the lithospheric underthrusting ramp. This last inference can explain the relatively high elevation, low relief, and thickened crust of the central Qilian Shan, as well as the comparative aseismicity of the region, which experiences fewer earthquakes due to less upper-crustal faulting. Both the northern and southern margins of the Himalayan-Tibetan orogen appear to have developed similarly, with continental underthrusting and crustal-scale imbrication and duplexing, despite vastly different climatic and plate-velocity boundary conditions, which suggests that the orogen-scale architecture of the thrust belt is controlled by neither of these forcing mechanisms. Instead, strength anisotropies of the crust probably control the kinematics and style of deformation, including the development of northern Tibet, where thrust systems are concentrated along pre-Cenozoic suture zones.