THE NEOPROTEROZOIC-PALEOZOIC TECTONIC HISTORY OF THE QILIAN SHAN AND ITS CONTROL ON DEVELOPMENT OF THE TIBETAN PLATEAU’S NORTHERN MARGIN

The northern margin of the Tibetan Plateau, located more than 1500 km from the Himalaya, is defined by the Cenozoic Qilian Shan thrust and Haiyuan left-slip fault systems. This actively deforming zone has marked the northern extent of Earth’s highest and largest plateau since deformation initiated here in the Eocene. Cenozoic structures are constructed upon a complex region that experienced repeated ocean closure and continental collision since the Proterozoic. The pre-Cenozoic evolution of the Qilian Shan and its controls on Cenozoic deformation remain enigmatic. Our integrated geologic mapping and geochronologic studies across the central Qilian Shan and Longshou Shan reveal that Cenozoic deformation and plateau construction are closely tied to the Paleozoic and older structural framework. Here we first restore Cenozoic N-S deformation across the region—30-40% strain within the Qilian Shan and ~50% strain along the northern range thrusts—to examine the pre-Cenozoic architecture of northern Tibet. Detrital zircon geochronology suggests that Tarim, Qaidam, and North China shared a contiguous margin in the Mesoproterozoic. This continuous continent was later intruded by 1.0-0.9 Ga arc plutons as part of a south-facing (present-day coordinates) arc-subduction system. Neoproterozoic rifting opened the Qilian Ocean either as (1) an embayed marginal sea or (2) a thoroughgoing ocean. The first model implies that Qaidam and Tarim were connected in the Neoproterozoic whereas the second requires a Phanerozoic suture between them, which is not observed. Ordovician-Silurian southward subduction of this ocean beneath Qaidam established the Qilian arc, which is evidenced by the widespread occurrence of arc-plutons (475-420 Ma). Magmatism temporally and spatially overlaps with UHP metamorphism in North Qaidam, suggesting that the Qilian arc involved protracted southward subduction that accommodated the convergence and collision of Qaidam and North China. If this is true, a subduction-mélange channel likely stretched from the Qilian Shan to North Qaidam, which was later disrupted by Mesozoic extension and Cenozoic shortening. The hanging walls of Cenozoic thrusts expose ophiolitic material, indicating that this mélange zone may have focused Cenozoic deformation in the Qilian Shan and along the Haiyuan fault.