Deciphering the coupled Paleozoic and Cenozoic tectonic history of the Qilian Shan, northeastern Tibetan Plateau
Tuesday, 16 December 2014
The Cenozoic Qilian Shan--the widest thrust belt on the Tibetan Plateau--exposes a record of early Paleozoic subduction-accretion associated with closure of the Qilian Ocean as the Qaidam microcontinent converged with North China. Despite decades of intense research, there is little consensus regarding the nature of the Qilian orogen (e.g., subduction polarity or number of arcs). For example, are the scattered ophiolite-bearing mélange complexes in the Qilian Shan the result of multiple arcs colliding along several suture zones in the Paleozoic or Cenozoic thrust duplication of a single Paleozoic suture zone? A major problem is that existing hypotheses neglect Cenozoic reorganization of the earlier tectonic framework, and the coupling between Paleozoic and Cenozoic structures has yet to be systematically investigated. To address this issue, we examine the Paleozoic Qilian Shan in the context of Cenozoic deformation. We conducted detailed field mapping (~1:50,000), balanced cross-section construction and restoration, U-Pb-Th zircon geochronology, Th-Pb dating of monazite inclusions in garnet, thermobarometry, and whole-rock geochemistry across the central Qilian Shan and in the Hexi Corridor foreland near Jinchan, where the North China craton abuts directly against the Qilian orogen. Successions of juxtaposed amphibolite facies Proterozoic gneiss (T: 725 ± 53°C, P: 7.9 ± 0.9 kbar), Cambrian oceanic material (U-Pb zircon ages: 530-520 Ma), and Ordovician-Silurian arc-derived granite (U-Pb zircon ages: 475-445 Ma) are exposed in the hanging walls of south-directed Cenozoic thrusts that place this basement over younger strata. A regionally correlative unconformity at the base of Carboniferous-Triassic strata is duplicated by this deformation and is used as marker horizon in our restoration. Initial estimates indicate a minimum post-Triassic shortening strain of ~42-45% across the range. By removing this deformation on mapped faults and adhering to observed field relationships, we are able to restore the pre-Cenozoic configuration of the Qilian orogen. Our observations are consistent with a single south-dipping Cambrian-Silurian subduction-collision zone, while the present-day apparent multitude of sutures can be explained as the direct result of Cenozoic deformation and duplication.