An Yin

An indentation model for the North and South China collision and the development of the Tan-Lu and Honam Fault Systems, eastern Asia

Passive continental margins are geometrically irregular as a consequence of either triple-junction evolution or the development of transfer zones in detachment fault systems, whereas active continental margins are smoothly arc-shaped due to subduction of plates on the Earths spherical surface. We propose that this basic difference in boundary geometry has played an important role in the latest Paleozoic-early Mesozoic collision of North and South China. In particular, we suggest that prior to collision, the active southern margin of the North China Block (NCB) was contiguous across the Qilian Shan, Qinling, Dabie Shan, Shandong peninsula of east central China to the Imjingang area of central Korea. The passive northern margin of the South China Block (SCB), in contrast, had a more irregular shape, such that its northeastern segment in northern Jiangsu and eastern Anhui provinces of China extended some 500 km farther north than its western counterparts in northern Sichuan, southern Shaanxi, and northern Hubei provinces. Collision of the NCB and the SCB began by indentation of the northeastern SCB into the eastern NCB in the late Early Permian and lasted until the Late Triassic-Early Jurassic. The indentation produced the left-slip Tan-Lu fault in northeastern China and the right-slip Honam shear zone in southeastern Korea and caused the northward displacement of the Shandong and the Imjingang metamorphic belts. This model predicts that collision along the Dabie and Qinling metamorphic belt occurred significantly later than along the Shandong belt, which is consistent with radiometric and depositional constraints on the time of collision. The proposed model accounts for the abrupt termination of the Tan-Lu fault at its south end and the drastic decrease in slip along the Tan-Lu fault north of the Shandong metamorphic belt. The model also predicts the distribution and ages of metamorphism along the suture and the observed local but intense Triassic deformation (=Indosinian orogeny) in northeastern China and northern Korea, which was previously an enigmatic feature in this region.

Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-km-wide east-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing melange and upper Paleozoic strata in the core and mainly Triassic–Jurassic strata along the limbs. In the western Qiangtang terrane (∼84°E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32°N, 84°E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic melange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ∼150 km at 84°E, decreasing eastward to ∼70 km at 87°E. Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene–Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post–mid-Cretaceous shortening. The Tertiary south-dipping Gaize–Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet. Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust melange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision.

Did the Indo-Asian collision alone create the Tibetan plateau?

It is widely believed that the Tibetan plateau is a late Cenozoic feature produced by the Indo-Asian collision. However, because Tibet was the locus of continental accretion and subduction throughout the Mesozoic, crustal thickening during that time may also have contributed to growth of the plateau. This portion of the geologic history was investigated in a traverse through the central Lhasa block, southern Tibet. Together with earlier studies, our mapping and geochronological results show that the Lhasa block underwent little north-south shortening during the Cenozoic. Rather, our mapping shows that ∼60% crustal shortening, perhaps due to the collision between the Lhasa and Qiangtang blocks, occurred during the Early Cretaceous. This observation implies that a significant portion of southern Tibet was raised to perhaps 3–4 km elevation prior to the Indo-Asian collision.

Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation

The active left-slip Altyn Tagh fault defines the northern edge of the Tibetan plateau. To determine its deformation history we conducted integrated research on Cenozoic stratigraphic sections in the southern part of the Tarim Basin. Fission-track ages of detrital apatites, existing biostratigraphic data, and magnetostratigraphic analysis were used to establish chronostratigraphy, whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region. Much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults, implying a temporal link between sedimentation and deformation. The results of our studies, together with existing stratigraphic data from the Qaidam Basin and the Hexi Corridor, suggest that crustal thickening in northern Tibet began prior to 46 Ma for the western Kunlun Shan thrust belt, at ca. 49 Ma for the Qimen Tagh and North Qaidam thrust systems bounding the north and south margins of the Qaidam Basin, and prior to ca. 33 Ma for the Nan Shan thrust belt. These ages suggest that deformation front reached northern Tibet only ∼10 ± 5 m.y. after the initial collision of India with Asia at 65–55 Ma. Because the aforementioned thrust systems are either termination structures or branching faults of the Altyn Tagh left-slip system, the Altyn Tagh fault must have been active since ca. 49 Ma. The Altyn Tagh Range between the Tarim Basin and the Altyn Tagh fault has been a long-lived topographic high since at least the early Oligocene or possibly late Eocene. This range has shed sediments into both the Tarim and Qaidam Basins while being offset by the Altyn Tagh fault. Its continuous motion has made the range act as a sliding door, which eventually closed the outlets of westward-flowing drainages in the Qaidam Basin. This process has caused large amounts of Oligocene–Miocene sediments to be trapped in the Qaidam Basin. The estimated total slip of 470 ± 70 km and the initiation age of 49 Ma yield an average slip rate along the Altyn Tagh fault of 9 ± 2 mm/yr, remarkably similar to the rates determined by GPS (Global Positioning System) surveys. This result implies that geologic deformation rates are steady state over millions of years during continental collision.

Late Cenozoic tectonic evolution of the southern Chinese Tian Shan

Structural, sedimentological, magnetostratigraphic, and 40Ar/39Ar thermochronological investigations were conducted in the southern Chinese Tian Shan. On the basis of our own mapping and earlier investigations in the area, the Late Cenozoic southern Tian Shan thrust belt may be divided into four segments based on their style of deformation. From west to east, they are (1) Kashi-Aksu imbricate thrust system, (2) the Baicheng-Kuche fold and thrust system, (3) the Korla right-slip transfer system, and (4) the Lop-Nor thrust system. The westernmost Kashi-Aksu system is characterized by the occurrence of evenly spaced (12–15 km) imbricate thrusts. The Baicheng-Kuche and Korla systems are expressed by a major north dipping thrust (the Kuche thrust) that changes its strike eastward to become a NW striking oblique thrust ramp (the Korla transfer zone). The Lop Nor system in the eastern-most part of the southern Chinese Tian Shan consists of widely spaced thrusts, all involved with basement rocks. Geologic mapping and cross-section construction suggest that at least 20–40 km of crustal shortening with a horizontal shortening strain of 20–30% has occurred in the southern Chinese Tian Shan during the late Cenozoic. These estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections. The timing of initial thrusting is best constrained in the Kuche basin where crustal shortening may have occurred at 21–24 Ma, the time of a major facies transition between lacustrine and braided-fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy. This estimate represents only a minimum age, as development of thrusts in the southern Chinese Tian Shan may have propagated southward toward the foreland. Thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting. If our estimate of timing for the thrust initiation (21–24 Ma) is correct, using the estimated magnitude of shortening (20–40 km) and shortening strain (20–30%), the averaged rates of late Cenozoic horizontal slip and shortening strain are 1–1.9 mm yr−1 and 2.9–4.5 × 10−16 s−1, respectively. Our reconnaissance 40Ar/39Ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the Chinese Tian Shan suggests that the magnitude of Cenozoic denudation is no more than 10 km, most likely less than 5 km. We demonstrate via a simple Airy-isostasy model that when the thermal effect on changes in surface elevation is negligible, determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the Tian Shan. Using this simple model, the loosely constrained magnitude of crustal-shortening strain and denudation in the southern Chinese Tian Shan implies that it may have been elevated 1.0–2.0 km since the onset of Cenozoic thrusting.

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

For providing a square end on a dye spring centre of the type comprising a helical main spring having a wire lacing the turns of which extend between the adjacent helices of the main spring, the invention provides an end adapter defining an annular channel which fits on the terminal helix of the dye spring centre and in which are protrusions of different heights to engage the terminal helix and hold the adapter square.

A Late Miocene-Pliocene origin for the Central Himalayan inverted metamorphism

Abstract Perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the Himalayan Main Central Thrust (MCT), generally thought to have been active during the Early Miocene. However, in situ 208 Pb/ 232 Th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the MCT footwall occurred in this portion of the central Himalaya at only ca. 6 Ma. The apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the MCT which tectonically telescoped the young metamorphic sequence. This explanation may resolve some outstanding problems in Himalayan tectonics, such why the MCT and not the more recently initiated thrusts marks the break in slope of the present day mountain range. It also renders unnecessary the need for exceptional physical conditions (e.g., high shear stress) to explain the generation of the Himalayan leucogranites.

Detrital zircon geochronology of pre-Tertiary strata in the Tibetan-Himalayan orogen

Detrital zircon data have recently become available from many different portions of the Tibetan-Himalayan orogen. This study uses 13,441 new or existing U-Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan-Qilian Shan-Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet-Himalaya region during Neoproterozoic to Mesozoic time. First-order conclusions are as follows: (1) Most ages from these crustal fragments are <1.4 Ga, which suggests formation in accretionary orogens involving little pre-mid-Proterozoic cratonal material; (2) all fragments south of the Jinsa suture evolved along the northern margin of India as part of a circum-Gondwana convergent margin system; (3) these Gondwana-margin assemblages were blanketed by glaciogenic sediment during Carboniferous-Permian time; (4) terranes north of the Jinsa suture formed along the southern margin of the Tarim-North China craton; (5) the northern (Tarim-North China) terranes and Gondwana-margin assemblages may have been juxtaposed during mid-Paleozoic time, followed by rifting that formed the Paleo-Tethys and Meso-Tethys ocean basins; (6) the abundance of Permian-Triassic arc-derived detritus in the Lhasa and Qiangtang terranes is interpreted to record their northward migration across the Paleo- and Meso-Tethys ocean basins; and (7) the arrival of India juxtaposed the Tethyan assemblage on its northern margin against the Lhasa terrane, and is the latest in a long history of collisional tectonism. Copyright 2011 by the American Geophysical Union.

A tectonic model for Cenozoic igneous activities in the eastern Indo-Asian collision zone

Geochronologic dating and compilation of existing age data suggest that Cenozoic activities in the eastern Indo^ Asian collision zone of southeast China and Indochina occurred in two episodes, each with distinctive geochemical signatures, at 42^24 Myr and 16^0 Myr. The older rocks are localized along major strike^slip faults such as the Red River fault system and erupted synchronously with transpression. The younger rocks are widely distributed in rift basins and are coeval with east^west extension of Tibet and eastern Asia. Geochemical data suggest that the early igneous phase was generated by continental subduction while the late episode was caused by decompression melting of a metasomatically altered, depleted mantle. The magmatic gap between the two magmatic sequences represents an important geodynamic transition in the evolution of the eastern Indo^Asian collision zone, from processes controlled mainly by crustal deformation to that largely dominated by mantle tectonics. fl 2001 Elsevier Science B.V. All rights reserved.

Tectonic evolution of the early Mesozoic blueschist‐bearing Qiangtang metamorphic belt, central Tibet

This is the published version. Copyright 2003 American Geophysical Union. All Rights Reserved.