Tectonic evolution of the Qilian Shan: An early Paleozoic orogen reactivated in the Cenozoic

Abstract

The Qilian Shan, located along the northeastern margin of the Tibetan Plateau, has experienced multiple episodes of tectonic deformation, including Neoproterozoic continental breakup, early Paleozoic subduction and continental collision, Mesozoic extension, and Cenozoic intracontinental orogenesis resulting from the India-Asia collision. In the central Qilian Shan, pre-Mesozoic ophiolite complexes, passive-continental margin sequences, and strongly deformed forearc strata were juxtaposed against arc plutonic/ volcanic rocks and ductilely deformed crystalline rocks during the early Paleozoic Qilian orogen. To better constrain this orogen and the resulting closure of the Neoproterozoic-Ordovician Qilian Ocean, we conducted an integrated investigation involving geologic mapping, U-Th-Pb zircon and monazite geochronology, whole-rock geochemistry, thermo barometry, and synthesis of existing data sets across northern Tibet. The central Qilian Shan experienced two phases of arc magmatism at 960-870 Ma and 475-445 Ma that were each followed by periods of protracted continental collision. Integrating our new data with previously published results, we propose the following tectonic model for the Proterozoic-Paleozoic history of northern Tibet. (1) Early Neoproterozoic subduction accommodated the convergence and collision between the South Tarim-Qaidam and North Tarim-North China continents. (2) Late Neoproterozoic rifting partially separated a peninsular Kunlun-Qaidam continent from the southern margin of the linked Tarim-North China craton and opened the Qilian Ocean as an embayed marginal sea; this separation broadly followed the trace of the earlier Neoproterozoic suture zone. (3) South-dipping subduction along the northern margin of the Kunlun-Qaidam continent initiated in the Cambrian, first developing as the Yushigou supra-subduction zone ophiolite and then transitioning into the continental Qilian arc. (4) South-dipping subduction, arc magmatism, and the convergence between Kunlun-Qaidam and North China continued throughout the Ordovician, with a trenchparallel intra-arc strike-slip fault system that is presently represented by high-grade metamorphic rocks that display a pervasive right-lateral shear sense. (5) Counterclockwise rotation of the peninsular Kunlun-Qaidam continent toward North China led to the closure of the Qilian Ocean, which is consistent with the right-lateral kinematics of intra-arc strike-slip faulting observed in the Qilian Shan and the westward tapering mapview geometry of Silurian flysch-basin strata. Continental collision at ca. 445-440 Ma led to widespread plutonism across the Qilian Shan and is recorded by recrystallized monazite (ca. 450-420 Ma) observed in this study. Our tectonic model implies the parallel closure of two oceans of different ages along the trace of the Qilian suture zone since ca. 1.0 Ga. In addition, the Qilian Ocean was neither the Proto-nor Paleo-Tethys (i.e., the earliest ocean separating Gondwana from Laurasia), as previously suggested, but was rather a relatively sm...

Tectonics of the Eastern Kunlun Range: Cenozoic Reactivation of a Paleozoic‐Early Mesozoic Orogen

“…A major age clustering between 860 and 1100 Ma is observed from the age spectrum of this sample. The younger component of this age range at circa 860–950 Ma is correlative with plutonic rocks along both the southern and northern margins of the Kunlun‐Qaidam terrane (i.e., Proto‐Kunlun arc; Wu et al, , ; He, Dong, Liu, et al, , He, Dong, Zhang, et al, , He et al, ; this study; Figure a), whereas the older component of the age clustering at circa 1.0–1.1 Ga is uniquely correlative to plutonic rocks from the Qilian Shan to the north (Zuza et al, ; Figure b). Late Neoproterozoic passive margin sedimentation here suggests that the early Neoproterozoic arc ultimately resulted in continental collision—the inferred Southern Qaidam suture of Wu et al ()—which then rifted in the late Neoproterozoic.…”

“…Both Cambrian samples in the Eastern Kunlun Range yield rather simple detrital zircon age spectra characterized by a large number of 1.1‐ to 0.8‐ and 0.6‐ to 0.52‐Ga zircon grains. The older ages can be related to the plutonic rocks along both the southern and northern margins of the Kunlun‐Qaidam terrane and/or within Central Qilian terrane, whereas the younger grains were probably derived from the Paleo‐Kunlun and Qilian arcs exposed in the Eastern Kunlun Range and central Qilian Shan (e.g.,Wu et al, ; Zuza et al, ; Figures a and b). The Archean, Paleoproterozoic, and Mesoproterozoic zircons of these samples may possibly be derived from the Tarim‐North China cratons (Figures c and d).…”

“…South China ages from Cawood et al () and Xu et al (). The Qilian Shan data are a compilation of Gehrels et al (), Wu et al (), and Zuza et al (). Tarim and North China data were compiled by Rojas‐Agramonte et al ().…”

“…Ordovician Time . The sustained northward subduction of Paleo‐Kunlun ocean and Ordovician volcanic arc magmatism began as the Qilian ocean was subducted beneath the Kunlun‐Qaidam continent (Zuza et al, ). We suggest that the Naijtal Group in the Eastern Kunlun Range was deposited between circa 447 Ma and circa 440 Ma based on our detrital zircon data (i.e., samples WC070514‐13 and WC010514‐8) and the intrusive relationships with late Ordovician‐early Silurian granitoid (i.e., sample WC071815‐1 in the Yeniugou mapping area of this study) (Figure k).…”

“…The original configuration of the exposed bedrock has been modified by early Mesozoic extension, resulting from the closure of the Meso‐ and Paleo‐Tethys Oceans (Cheng et al, ; Pullen et al, ; Wu et al, ; Wu et al, ; Zhang et al, ; Zuza et al, ), and Cenozoic intracontinental deformation associated with the India‐Asia collision (Vincent & Allen, ; Yin & Harrison, ; Chen et al, ; X. Cheng, Fu, et al, F. Cheng, Guo, et al, ; Zuza et al, , ; Allen et al, ). Below we briefly outline the important Phanerozoic tectonic events that affected Kunlun‐Qaidam terrane.…”