Jianhua Li

New insights into Phanerozoic tectonics of South China: Part 1, polyphase deformation in the Jiuling and Lianyunshan domains of the central Jiangnan Orogen

The central Jiangnan Orogen, genetically formed by the Proterozoic Yangtze-Cathaysia collision, presents as a composite structural feature in the Phanerozoic with multiple ductile and brittle fabrics whose geometries, kinematics, and ages are crucial to decipher the tectonic evolution of south China. New structural observations coupled with thermochronological and geochronological studies of these fabrics document four main stages of deformation. The earliest stage in early Paleozoic time (460–420u2009Ma) corresponds to combined E-trending dextral and northwest directed thrust shearing that was variably partitioned in anastomosing high-strain zones under greenschist-facies conditions (~400–500°C), related to the continued Yangtze-Cathaysia convergence externally driven by the suturing of south China with Australia. This event was heterogeneously overprinted by the second stage characterized by ~E-oriented folding in middle Triassic time, geodynamically resulting from the continental collision of south China with Indochina and North China. The third stage was locally developed by northwest and southeast vergent thrusts that truncated ~E-oriented folds in the Late Jurassic, due to northwestward subduction of the Paleo-Pacific plate. The latest stage involved normal faulting and tectonic unroofing in Cretaceous time, which resulted in basin opening and reset footwall 40Ar/39Ar ages in proximity to the Hengshan detachment fault, associated with roll-back of the subducting Paleo-Pacific plate.

Mesozoic tectonic evolution of the Daba Shan Thrust Belt in the southern Qinling orogen, central China: Constraints from surface geology and reflection seismology

The Daba Shan Thrust Belt is located along the southern margin of the Qinling orogen that separates the north China block in the north from the south China block in the south. Despite decades of research, the total magnitude of shortening accommodated by continent-continent convergence across the Qinling orogen after Triassic ocean closure between north and south China remains poorly constrained. The lack of knowledge on the shortening magnitude in turn limits our ability to test a wide array of tectonic models for the development of the Qinling orogen and thus the convergence history between north and south China. In order to address this issue, we construct a balanced cross section and develop a new kinematic model for the evolution of the Daba Shan Thrust Belt. This work was accomplished by integrating (1) surface geologic mapping, (2) detailed kinematic analysis of key structures, (3) existing geochronologic and thermochronological data, and (4) a recently obtained lithospheric-scale seismic reflection profile. Restoration of the cross section indicates that the minimum shortening strain increases northward from ~10% in the foreland to >45% in the thrust belt interior. The estimated amount of upper crustal shortening across the Daba Shan Thrust Belt is >130u2009km, which is sufficient to allow the inferred mafic lower crust of the subducted south China lithosphere to have experienced eclogite phase transition. Thus, our work supports that the development of the Daba Shan Thrust Belt may have been driven by slab pull of the subducted mafic lower crust at the leading edge of the down-plunging south China continental lithosphere.

New insights into Phanerozoic tectonics of South China: Early Paleozoic sinistral and Triassic dextral transpression in the east Wuyishan and Chencai domains, NE Cathaysia

The northeast Cathaysia area is characterized by an archetypical, transpressional system with widespread strike-slip shear zones whose geometries, kinematics, and ages are critical for deciphering the Phanerozoic tectonic evolution of South China. We present new structural, geochronological and thermochronological data from the shear zones in the east Wuyishan and Chencai domains, which record two phases of deformation. The first phase corresponds to sinistral oblique shearing along arrays of NNE oriented, steep-dipping zones under amphibolite facies conditions. The sinistral oblique shearing commenced at ~451xa0Ma, concurrently with regional NW/SE directed thrust shearing and folding; the coexistence of sinistral and thrust structures indicates NW-SE transpressive shortening deformation. Dating by 40Ar/39Ar shows that such deformation terminated before 400xa0Ma and was followed by cooling through ~450–350°C at ~400–370xa0Ma. Our results, merged with published data, aid in tracing an Early Paleozoic orogen that extends through the Jiangnan domain into the northeast Cathaysia, with the southeast Yangtze acting as a foreland belt. The synorogenic shortening was interpreted as resulting from underthrusting of the Cathaysia beneath the east Yangtze. The second phase involved dextral oblique shearing associated with NNE-SSW transpression under greenschist to amphibolite facies conditions at 245–228xa0Ma, which was followed by postkinematic magmatism and cooling at ~221–200xa0Ma. In the Cathaysia, similar Middle Triassic dextral shear zones were widespread and operated with approximately east striking thrusts as mutually complementary structures; their kinematic coupling can be explained by a contractional termination model. Geodynamically, we attributed Middle Triassic dextral transpression to the collisions of South China with North China and Indochina.

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

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 KunlunQaidam 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 Protonor Paleo-Tethys (i.e., the earliest ocean separating Gondwana from Laurasia), as previously suggested, but was rather a relatively small embayed sea along the southern margin of the Laurasian continent. We also document >200 km of Cenozoic north-south shortening across the study area. The observed shortening distribution supports models of Tibetan Plateau development that involve distributed crustal shortening and southward underthrusting of Eurasia beneath the plateau. This India-Asia convergence-related deformation is focused along the sites of repeated ocean closure. Major Cenozoic left-slip faults parallel these sutures, and preexisting subduction-mélange channels may have facilitated Cenozoic shortening and continental underthrusting.