Qing-Ren Meng

Geologic framework and tectonic evolution of the Qinling orogen, central China

Abstract The geologic framework of the Qinling orogen was built up through interplay of three blocks, the North China block (including the North Qinling), the South Qinling, and the South China block, separated by the Shangdan and Mianlue sutures. The Shangdan suture resulted from Middle Paleozoic collision of the North China block and the South Qinling. The Mianlue suture resulted from Late Triassic collision of the South Qinling and the South China block. Present upper crust of the Qinling is structured dominantly by thrust–fold systems. The North Qinling displays thick-skinned deformation with crystalline basement involved, whilst the South Qinling is characterized by thin-skinned thrusts and folds detached above the Lower Sinian. Two types of Precambrian basement, crystalline and transitional, are defined according to lithology and metamorphic grade and different in age. Stratigraphic and sedimentary architecture is characterized by distinct zonation. The Qinling orogen experienced a prolonged continental divergence and convergence between blocks. During the period from Late Neoproterozoic to Early Paleozoic times, the South Qinling was the northern margin of the South China block, and the North Qinling was the southern margin of the North China block, separated by a Proto-Tethyan Qinling Ocean. The North Qinling evolved into an active margin when the Proto-Tethyan Qinling Ocean subducted northward during Ordovician time. Collision of the South and North Qinling took place in Middle Paleozoic along the Shangdan suture. Synchronous with the collision, rifting occurred at the southern rim of the South Qinling and was followed by the opening of the Paleo-Tethyan Qinling Ocean during the Late Paleozoic, resulting in the splitting of the South China block from the South Qinling. Collision of the South Qinling and the South China block came about in the Late Triassic along the Mianlue suture. The Late Triassic collisional orogeny caused extensive fold-and-thrust deformation and granitoid intrusions throughout the Qinling, and led to final amalgamation of the North and South China blocks.

TIMING OF COLLISION OF THE NORTH AND SOUTH CHINA BLOCKS : CONTROVERSY AND RECONCILIATION

The Qinling orogen was formed by the joining of the North and South China blocks, but the timing of their integration has been debated for more than a decade. The controversies obviously stem from different approaches to reconstruction of the integration history. Two contrasting lines of evidence yield two different ages for collision of the North and South China blocks—middle Paleozoic and Late Triassic. The Shangdan suture within the Qinling was regarded in previous studies as the trace along which the North and South China blocks collided. Our studies, however, demonstrate that there are two sutures within the Qinling: the well-documented Shangdan suture and the newly discovered Mianlue suture. We show in this paper that the Late Proterozoic to early Mesozoic evolution of the Qinling involved interactions between the North China block, the North and South Qinling orogens, and the South China block. The middle Paleozoic collision along the Shangdan suture, as constrained by some evidence, accreted only the South Qinling orogen to the southern part (i.e., the North Qinling) of the North China block. Contemporaneous rifting of the South China block and subsequent drifting separated the South Qinling from the South China block during the middle to late Paleozoic. The separation of the South from the North China blocks is supported by other evidence, in particular, geomagnetic data. Evidently it was the Late Triassic collision of the South China block with the South Qinling orogen along the Mianlue suture that led to final integration of the North and South China blocks.

What drove late Mesozoic extension of the northern China-Mongolia tract?

The northern China–Mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late Mesozoic time. Late Middle to early Late Jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated North China–Mongolia block and the Siberian plate, but widespread late Late Jurassic–Early Cretaceous extension has not been satisfactorily explained by existing models. Some prominent features of the extensional tectonics of the northern China–Mongolia tract are: (1) Late Jurassic voluminous volcanism prior to Early Cretaceous large-magnitude rapid extension; (2) overlapping in time of contractional deformation in the Yinshan–Yanshan belt with development of extensionrelated basins in the interior of the northern China–Mongolia tract; and (3) widespread occurrence of alkali granitic plutonism, extensional basins and metamorphic core complexes in the Early Cretaceous. A new explanation is advanced in this study for this sequence of events. The collision of amalgamated North China–Mongolia with Siberia led to crustal overthickening of the northern China–Mongolia tract and formation of a high-standing plateau. Subsequent breakoff at depth of the north-dipping Mongol–Okhotsk oceanic slab is suggested as the main trigger for late Mesozoic lithospheric extension of that tract. Slab breakoff resulted in mantle lithospheric stretching of the adjacent northern China–Mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust. Collectively, these phenomena triggered gravitational collapse of the previously thickened crust, leading to late Late Jurassic–Early Cretaceous crustal extension, and importantly, coeval contraction along the southern margin of the plateau in the Yinshan–Yanshan belt. The proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena, such as the synchronous extension of northerly terranes during major contraction in the neighboring Yanshan–Yinshan belt. D 2003 Elsevier B.V. All rights reserved.

Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block

We present new sedimentary data integrated into a regional Mesozoic stratigraphic framework to provide a detailed picture of spatio-temporal variations in deposition and depocenter migration of the northwest Sichuan basin. The Mesozoic sedimentary evolution is utilized to interpret basin subsidence history and to unravel coeval basin-margin tectonics. The northwest Sichuan basin, together with the Songpan-Ganzi terrane, behaved as a passive margin south of the Qinling Paleo-Tethys from late Paleozoic to early Middle Triassic times and then evolved into a peripheral foreland basin in response to collision of the North and South China blocks since the late Middle Triassic. Coeval with strong north-south contraction of the Songpan-Ganzi terrane in the Late Triassic, sinistral transpressional deformation of the Longmen Shan belt led to flexural subsidence of the adjacent western Sichuan basin. Renewed basin-margin fold-thrust activity triggered recurrence of flexural subsidence of the northwest Sichuan basin since the Middle Jurassic, with the depocenter eventually shifting to the northwestern corner of the basin in the Early Cretaceous. Sedimentary evolution of the northwest Sichuan basin and the basin-margin deformation imply that the South China block had been rotating clockwise relative to the North China block throughout the Mesozoic with an interim period of Early Jurassic tectonic quiescence. A model is advanced that invokes clockwise rotation of the South China block as a driver for tectonic evolution of both the basin and adjoining structural belts and provides an explanation for several salient features that are otherwise puzzling.

Mesozoic large-scale lateral extrusion, rotation, and uplift of the Tongbai-Dabie Shan belt in east China

The tectonic convergence between the South China block and Qinling belt that occurred in Mesozoic time was not homogeneous along strike; it was mainly concentrated in the Shengnongjia and Hannan domes, which formed a pair of indentors, along which the South China block penetrated the Qinling belt. As a consequence, the Tongbai‐Dabie Shan belt, the eastern part of the Qinling orogenic belt, may have undergone large-scale eastward extrusion, clockwise rotation, and uplift, as indicated by its boundary deformation features. It was originally located beneath the narrowest part of the Qinling belt, where it underwent ultrahigh-pressure metamorphism in the Early Triassic.

Structural and sedimentary evolution of the southern Songliao Basin, northeast China, and implications for hydrocarbon prospectivity

The southern Songliao Basin manifests itself as a wide-rift system that developed in northeast China from the Late Jurassic to Early Cretaceous. Individual basins in the system experienced marked rift subsidence, but the postrift subsidence was insignificant in most of the basins, contrasting strikingly with the tectonic subsidence history of the northern Songliao Basin. Two types of rift basins are defined according to whether the basins underwent prominent postrift subsidence. Type 1 basins are characterized by thick postrift accumulations. Type 2 basins, although experiencing minor postrift subsidence, represent most of the southern Songliao Basin and can be subdivided in accordance with bounding-fault geometry and areal extent: basins bounded by high-angle faults (type 2a), basins bounded by low-angle faults (type 2b), and basins with limited spatial area (type 2c). Many rift basins are expressed as narrow belts in map view and are composed internally of several segments linked through different types of transfer zones. Depositional processes and facies architecture of the basins are controlled primarily by dips and migration of active bounding faults. Synrift depocenters occur close to high-angle bounding faults, and deep-lake deposition commonly persists through much of the synrift subsidence. Lacustrine deposition can be enhanced by the backward (or toward the footwall) stepping of active bounding faults. Depocenters controlled by low-angle bounding faults, in contrast, tend to shift basinward through time, and deep-lake facies commonly develop in the middle stage of rifting. At the end of the Cretaceous, basin inversion was evident in type 1 basins, such as the Shiwu Basin on the north, but other basins appear to have mostly escaped the contractional deformation. Gravitational collapse of the previously thickened crust is considered the cause for the generation of the rift basins, and lateral flow of the ductile lower crust may explain the significantly induced postrift subsidence of type 2 basins. Effective plays occur in basins bounded by high-angle faults, and therefore, type 1 and type 2a basins are suggested to be the main targets of future oil exploration. Synrift source rock maturation might be partially attributed to the heating of Early Cretaceous magmatism because shallower burial alone could not elevate temperatures high enough for petroleum generation.

Block rotation: Tectonic response of the Sichuan basin to the southeastward growth of the Tibetan Plateau along the Xianshuihe-Xiaojiang fault

GPS field and seismic data show that the southeastern margin of the Tibetan Plateau is tectonically and seismically active. This activity is due to the southeastward extrusion of the Chuandian fragment, a large crustal block rotating clockwise around the northeastern syntaxis of the Himalayas. The eastern boundary fault of this fragment is defined by the left-lateral Xianshuihe-Xiaojiang fault, which abruptly truncates the Sichuan basin of the Yangtze block. Our paper presents evidence indicating that the Sichuan basin experienced right-lateral shear along its margin, including the Longmen Shan fault belt, as shown by the presence of a large number of interference deformation features, including S-shaped and Z-shaped folds and faults, aligned in an en echelon pattern. This study hypothesizes that the Sichuan basin experienced counterclockwise rotation, dragged by the left-lateral movement along the Xianshuihe fault, and it is this rotation that was the underlying cause of the 12 May 2008 Wenchuan Ms 7.9 earthquake. During the rotation, the Sichuan basin decoupled along a subhorizontal decollement fault zone that developed along Triassic gypsum- and coal-bearing rocks, at a mean depth of ~5000 m, below which the Paleozoic rocks experienced much more intense deformation than the overlying Mesozoic rocks, suggesting that the lower part of the basin experienced a larger-scale rotation relative to the uppermost part of the basin. Based on thermal data from the western margin of the Sichuan basin and from along the Xianshuihe fault, the counterclockwise bending/rotation of the Sichuan basin initiated in late Cenozoic time (~13 Ma).

Mesozoic evolution of the Hefei basin in eastern China: Sedimentary response to deformations in the adjacent Dabieshan and along the Tanlu fault

This paper presents a study of Jurassic–Early Cretaceous sedimentary evolution of the Hefei basin in eastern China and explores the relationship between clastic sedimentation and coeval deformation of the Dabieshan to the south and the Tanlu fault to the east. The Hefei basin experienced a two-stage evolution. The basin was initiated in the Early Jurassic and expanded in the Middle and Late Jurassic. The synsedimentary Jinzhai normal fault is considered to be a border fault of the basin because it controlled Middle to Upper Jurassic proximal alluvial deposits. Persistent N- to NE-directed paleo-flows in the southern Hefei basin indicate that sediments came from the Dabieshan, and the presence of Triassic coesite-bearing detrital zircon in Lower Jurassic sediments documents exhumation of ultrahigh-pressure rocks of the Dabieshan to the surface as early as the Early Jurassic. Occurrences of eclogite clasts in an Upper Jurassic unit indicate continued denudation of the Dabieshan at that time. Thickening of Jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the Jinzhai normal fault on the south and the NE-striking left-lateral transtensional Tanlu fault on the east. Jurassic extensional subsidence of the Hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the Dabieshan, which combines to indicate contemporaneous extension and contraction on the north and south sides of the Dabieshan, respectively. Vigorous volcanism and uplift of the southern Hefei basin characterized the second stage of development of the Hefei basin in the Early Cretaceous, and this led to a synchronous shift of its main depocenter to the north. This younger depocenter is characterized by lacustrine and fluvial-deltaic sedimentation, where alluvial and fan-deltaic coarse-grained deposition mainly occurred along the eastern edge of the basin. Early Cretaceous subsidence is attributed to E-W extension across the middle segment of the Tanlu fault, and the Zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin. A model is accordingly advanced to account for how the Hefei basin developed in response to the tectonic exhumation of the Dabieshan and the deformation of the Tanlu fault in the Mesozoic. It illustrates that the Hefei basin initiated and evolved during Jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra-high-pressure rocks of the Dabieshan. Early Cretaceous development of the basin was controlled by magmatism-related uplift of the Dabieshan on the south and orthogonal normal faulting of the middle segment of the Tanlu fault on the east. This study provides an independent constraint upon the exhumation processes of ultrahigh-pressure rocks of the Dabieshan.

Detrital zircon evidence for the linkage of the South China block with Gondwanaland in early Palaeozoic time

LA-ICP-MS U–Pb dating of Lower Devonian detrital zircon samples from three representative sections in the South China block yields dominant Grenvillian and Pan-African populations, similar to the age distribution of early Palaeozoic samples from Gondwana, the Tethyan Himalaya and West Australia, in particular. Hf isotopic compositions indicate the contributions of juvenile crust at 1.6 Ga and 2.5 Ga, and bear a resemblance to their counterparts from SE Australia and West Antarctica, revealing the mixed origin of the Pan-African and Grenvillian grains from juvenile magmas and melting of pre-existing crustal rocks. These results suggest that the South China block should be considered an integral part of East Gondwana in early Palaeozoic time, rather than a discrete continental block in the Palaeo-Pacific or a fragment of Laurentia.

Multiple controls on rift basin sedimentation in volcanic settings: Insights from the anatomy of a small Early Cretaceous basin in the Yanshan belt, northern North China

This paper deals with a small continental rift basin in a volcanic setting and tries to decipher potential controls on its synrift sedimentation. The Luanping basin, which is a well-exposed Early Cretaceous rift basin in the Yanshan belt on the northern periphery of the North China block, was chosen as a case study. We show that extensive and vigorous volcanism took place just prior to the onset of basin formation and continued into a synrift period. Basin fills consist predominantly of lacustrine, fan-delta, braid-delta, and volcaniclastic deposits. Lacustrine facies are composed mostly of deep-lake deposits that appear to have been established soon after basin initiation. Fan-delta systems developed along two high-angle border faults, the master northern Hongqi fault and the secondary western Xiaobaiqi fault. Fan-delta fronts were built up primarily by gravelly and sandy debrites and turbidites. Braid-delta deposits are restricted to the eastern corner of the basin and are characterized by an aggregation of braid-plain conglomeratic deposits. Underwater extrusions of magma and pyroclastic flows are inferred from the presence of volcanic breccias, ignimbrites, andesitic pillows, and peperites, which are enclosed by host lacustrine deposits. Deep-lake and fan-delta systems persisted throughout the synrift period, indicating a rapid subsidence rate, with both adequate sediment and water supply. The high-angle border faults are thought to have played a critical role in creating large accommodation space, and simultaneous displacements of the Hongqi and Xiaobaiqi faults led to the localization of a major depocenter in the western part of the basin. One-dimensional backstripping analysis of three stratigraphic sections shows that the Luanping basin experienced marked synrift tectonic subsidence up to 2 km at a subsidence rate close to 0.2 mm yr −1 . We propose that the high rate of basin subsidence might have been caused mainly by left-lateral transtensional faulting and was enhanced by the superposition of basement downsag induced by underlying magma withdrawal. A predominance of volcanic particles in both debrites and sandstones suggests that source areas were erodible volcanic edifices. A humid paleoclimate is inferred, which not only provided enough water supply, but also promoted weathering of volcanic rocks in source areas, thus maintaining both the deep-lake environment and sediment influx. We conclude that relatively small and isolated rift basins in volcanic settings can experience both rapid subsidence and receive thick lacustrine deposits if they are under the combined control of favorable structure, climate, and magmatism. The model proposed here is believed to be applicable to other rift basins in similar volcano-tectonic settings.