A new perspective on Tertiary basin evolution in northeastern Tibet:Evidence for crustal extension during the middle Tertiary

New stratigraphic observations from the northern margin of the mid-Tertiary Longxi basin in northeastern Tibet reveal that basin subsidence and sediment accumulation was associated with crustal extension. The time-space patterns of development of high topography associated with the Indo-Asian collision are central to ongoing debates over the mechanics of intracontinental deformation and purported linkages between climate and tectonics. In northeastern Tibet, the widespread appearance of coarse terrestrial sediment during the Oligocene is commonly interpreted to reflect the initial development of a foreland basin that reflects crustal thickening along the present-day margin of the plateau. Basin development was temporally associated with clockwise vertical-axis rotations (Dupont-Nivet et al., 2004), exhumation of marginal ranges along the southern basin margin (Clark et al., 2010), and fault activity (Duvall et al., 2011). However, a lack of direct observations relating sediment accumulation to fault activity leaves this interpretation uncertain. Here we present new evidence from stratigraphic studies along the northernmost margin of the Tertiary Longxi basin that challenge existing models for the kinematics of basin formation. Tertiary sediment along the northern margin of the Longxi basin initiated at 29 - 30 Ma (Wang et al., 2011) and is characterized by coarse-grained, proximal deposits of the Sikouzi Formation. Sedimentary provenance, paleocurrents, facies, and isopach distributions reveal that sediments were sourced from fault-bound blocks now exposed in the Liupan Shan and associated ranges that mark the northern margin of the basin. Fault systems presently strike E-W to NW-SE and dip steeply to the south; isopach distributions show that the Sikouzi Fm. is thickest on the hanging-wall blocks and thins away from the faults. These relationships suggest that extensional slip along the faults controlled development of accommodation space during the Oligocene-Early Miocene. Extension was followed by continued sediment accumulation over a broad region during the middle Miocene; sedimentary facies represent low-energy (lacustrine) environments. Miocene deposits onlap footwall ranges suggesting a cessation of faulting during this time. Reactivation of fault systems during active sinistral transpression along the Haiyuan fault initiated at ca.10 Ma and was marked by the appearance of locally-derived, coarse detritus (Wang et al., 2011). The association of initial basin subsidence in the Oligocene with normal faulting along basin-bounding faults implies a local kinematic regime characterized by N-NE directed extension. Similarities with kinematics of Tertiary deformation in the Ordos region (Zhang et al., 1999) and along the West Qinling orogen (Ratschbacher et al., 2003) imply that this event was regional in scale and affected a broad region of north China. We suggest that the onset of Tertiary sedimentation in the Longxi basin did not develop in the foreland of a nascent Tibetan Plateau, but rather is more likely tied to extensional/transtensional deformation, perhaps associated with the opening of east Asian marginal basins.