Wenjiao Xiao

Tectonic models for accretion of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (c. 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.

Paleozoic multiple subduction-accretion processes of the southern Altaids

The formation and development of the southern Altaids is controversial with regard to its accretionary orogenesis and continental growth. The Altay-East Junggar orogenic collage of North Xinjiang, China, offers a special natural laboratory to resolve this puzzle. Three tectonic units were juxtaposed, roughly from North to South, in the study area. The northern part (Chinese Altay), composed of variably deformed and metamorphosed Paleozoic sedimentary, volcanic, and granitic rocks, is interpreted as a Japan-type island arc of Paleozoic to Carboniferous-Permian age. The central part (Erqis), which consists of ophiolitic mélanges and coherent assemblages, is a Paleozoic accretionary complex. The southern part (East Junggar), characterized by imbricated ophiolitic mélanges, Nb-enriched basalts, adakitic rocks and volcanic rocks, is regarded as a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites, superimposed by Permian arc volcanism. A plagiogranite from an imbricated ophiolitic mélange (Armantai) in the East Junggar yields a new SHRIMP zircon age of 503 ± 7 Ma. Using published age constraints, we propose the presence of multiple subduction systems in this part of the Paloasian Ocean in the Paleozoic. The intraoceanic arcs became accreted to the southern active margin of the Siberian craton in the middle Carboniferous-Permian. During the long accretionary processes, in addition to large-scale southward-directed thrusting, large-scale, orogen-parallel, strike-slip movements (for example, Erqis fault) in the Permian translated fragments of these intraoceanic arcs and associated accretionary wedges. This new tectonic model has broad implications for the architecture and crustal growth of Central Asia and for other ancient orogens.

Tectonic evolution of the Tancheng‐Lujiang (Tan‐Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data

The north-striking Tancheng-Lujiang (Tan-Lu) fault is a conspicuous and controversial feature of the eastern Asian landscape. Near the southeast extremity of the fault in Anhui Province, we collected paleomagnetic samples at 17 Middle Triassic (T2) and 10 Upper Cretaceous (K2) to lower Cenozoic (E1) sites. T2 remanent magnetizations are interpreted as primary in two of three areas. The three areas are rotated 37° to 137° counterclockwise with respect to the South China Block (SCB) reference direction. K2-E1 remanent magnetization directions pass regional fold and reversals tests and are not rotated with respect to surrounding areas. Counterclockwise rotation of T2 strata therefore ended before K2 and is attributed to left lateral shear acting along Tan-Lu during the North China Block (NCB)-SCB collision. In Shandong Province, 700 km north of the Anhui sites, four areas containing 33 Upper Jurassic (J3) and Cretaceous sites have negligible declination differences, except for one which has dispersed directions. The fold test is inconclusive for this latter area and positive for the other three. Regional concordance of the J3-E1 paleomagnetic data (including paleolatitudes) together with observed deformation patterns suggest that an extensional regime prevailed in the Late Cretaceous and Cenozoic. Euler pole positions that constrain the North-South China collision and account for Tan-Lu motion suggest at least 500 km of sinistral shear took place along the fault, and either (1) subduction and related ultrahigh pressure (UHP) metamorphism occurred near the present location of the Qinling-Dabieshan and Sulu UHP belts while Tan-Lu acted as a transform fault that connected the two subduction zones, or (2) Tan-Lu and Sulu were parts of the same transform fault system and no UHP rocks formed in situ at Sulu. In either case, UHP rocks originally exhumed near Dabieshan could have been transported by plate capture toward Sulu along Tan-Lu. After North and South China impacted near Dabieshan, the Tan-Lu fault grew within the SCB as the Dabieshan corner indented the SCB, causing folds in SCB cover rocks to conform to the NCB margin. Late Cretaceous to Cenozoic reactivation of Tan-Lu, with both right lateral strike-slip and normal fault motion, occurred as the SCB extruded east relative to the NCB under the influence of the India-Asia collision.

Paleozoic multiple accretionary and collisional processes of the Beishan orogenic collage

The Beishan orogenic collage is located in the southernmost part of the Altaids, and connects the Southern Tien Shan suture to the west with the Solonker suture to the east. The orogen was previously regarded as early Paleozoic in age in contrast to the surrounding southern Altaid collages, which are Late Paleozoic or even Early Mesozoic. This paper reviews the tectonic units of the Beishan orogen, which along a north-south traverse consists of several arcs and ophiolitic mélanges. These tectonic units were thrust imbricated and overprinted by strike-slip faulting during Permian-Triassic times, and the youngest strata involved in the deformation are Permian. Stitching plutons are Late Permian in age. Peaks of magmatic-metamorphic-tectonic activity, and paleomagnetic and paleogeographic data indicate that the Beishan orogenic collage evolved by development of several, Early to Mid-Paleozoic arcs in different parts of the Paleoasian Ocean. The Late Paleozoic collage is characterized by three active continental margins or island arcs that are separated by two ophiolitic mélanges. The northernmost active margin is represented by the Queershan arc, which may have lasted until the Permian. The Shibanshan unit is the southernmost, subduction-related continental arc along the northern margin of the Dunhuang block. In the Late Carboniferous to Permian the eastern end (promontory) of the Tarim Craton probably collided with the Chinese eastern Tien Shan arc, forming a new active continental margin, which interacted with the Beishan Late Paleozoic archipelago, generating a complicated subduction-accretionary orogen; this is suggested to be one of the last phases in the development of the long-lived Altaid accretionary orogenesis. The new model for this orogen bridges the gap between the western and eastern ends of the southern Altaids. The modern Timor-Australia collision zone with its many surrounding arcs is an appropriate analog for the Altaids in the Late Paleozoic.

Delamination/thinning of sub-continental lithospheric mantle under Eastern China: The role of water and multiple subduction

We present a new model to explain one of the biggest tectonic problems of Earth Sciences today, namely, how and why did the Archean sub-continental lithospheric mantle under the Eastern Block of the North China Craton (NCC) delaminate or thin so drastically in the Cretaceous? The eastern NCC is surrounded by several sutures: to the north the south-dipping Solonker (Permo-Triassic formation age) and Mongol-Okhotsk suture (Jurassic formation age), to the south the north-dipping Dabie Shan and Song Ma sutures (Permo-Triassic formation age), and to the east the Pacific oceanic plate (200-100 Ma). Water was carried down under the eastern NCC by the hydrated Pacific plate for at least 100 Ma, and by oceanic plates subducted along the Solonker, Dabie Shan and Song Ma sutures for at least 250 Ma from the early Paleozoic to the Permo-Triassic, and by the Mongol-Okhotsk suture for at least 200 Ma until the Jurassic. Tomographic images show that the Pacific plate has ponded along the top of the mantle transition zone under the eastern NCC for over ca. 2000 km from the Japan trench. An addition of 0.2 weight percent H2O lowered the solidus temperature of hydrous mantle peridotite by 150 °C, which led to extensive melting in the hydrous mantle transition zone, and to the rise of hydrous plumes into the overlying crust-mantle. By the time of formation of the Permo-Triassic sutures, the hydration caused by subduction of four oceanic plates had caused major garnetization of the Archean crustal root of the eastern NCC, and post-collisional thrusting in the Jurassic led to major crust/mantle thickening and this triggered collapse of the hydro-weakened garnet-enriched crustal root. During the extensional Cretaceous period, abundant mafic, adakitic and granitic intrusions and extensive gold mineralization were emplaced and metamorphic core complexes and sedimentary and foreland basins formed in and around the eastern NCC. Part of the root was chemically transformed and replaced by upwelling fertile asthenospheric material, which fed the extrusion of extensive alkali flood basalts in the Cenozoic.

Arc-ophiolite obduction in the Western Kunlun Range (China): implications for the Palaeozoic evolution of central Asia

The nature of the ‘Kudi ophiolite’ in the Western Kunlun Range is hotly debated. Our new structural–geochemical data reveal that it is actually an arc ophiolite comprising: (1) arc- or ophiolite-derived turbidites of two generations containing Late Ordovician–Silurian and Late Devonian–Early Carboniferous radiolarians; (2) a central intra-oceanic (Yixieke) arc with basalt–andesite–tuff–agglomerate; (3) lower (Buziwan) oceanic crust containing dunite–harzburgite–gabbro. We propose the following tectonic evolution. South-dipping subduction in Late Cambrian to earliest Ordovician time generated the Yixieke arc on top of the Buziwan oceanic crust–mantle. This subduction led to emplacement of the arc northwards onto the North Kunlun terrane (Tarim block), creating an active continental margin with northward subduction below it. The Kudi ophiolite was thrust southeastward over the incoming Kudi continental (gneiss) terrane in mid-Ordovician–mid-Devonian time. During a tectonic hiatus in the Kudi region Late Devonian–Carboniferous subduction further west led to development of the Oytag arc, formerly regarded as an equivalent of the Kudi ophiolite. Lower Permian arc lavas and Upper Triassic granites in the Xiananqiao arc south of Kudi mark the resumption of north-dipping subduction before final collision with the incoming Qiangtang block. Comparison with the Lapeiquan ophiolite in the Eastern Kunlun assists regional correlation along this Palaeozoic orogen and constrains Cenozoic displacement of the Altyn Tagh fault.

Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas

We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2σ) laser step-heating 40Ar/39Ar plateau ages for phengite from quartzite mylonites from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone. We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred.

Accretionary Tectonics of the Western Kunlun Orogen, China: A Paleozoic–Early Mesozoic, Long‐Lived Active Continental Margin with Implications for the Growth of Southern Eurasia

Our new SHRIMP U‐Pb zircon ages from the Western Kunlun Orogen allow us to constrain the history of an active continental margin developed on the southern boundary of the Tarim block from the Ordovician to the Triassic. A \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Early Mesozoic thrust tectonics of the northwest Zhejiang region (Southeast China)

492\pm 7