Western Pacific-type Research Articles

Precursory atmospheric teleconnection patterns for strong Siberian High events

Based on Japanese 55-year daily reanalysis data, the present study reveals that a strong Siberian high (SH) is preceded by three teleconnection patterns—namely, the Scandinavian (SCA) pattern, western Pacific (WP) pattern, and Polar/Eurasian (POL) pattern. Accordingly, strong SH events are classified into three types. Composite results show that there are evident differences among the three types in their typical circulation characteristics and impacts on surface air temperature (SAT) in East Asia. The SCA type is characterized by a wave-train anomaly over mid- and high-latitude Eurasia, while the WP type is characterized by a western expansion of circulation anomalies from the downstream western North Pacific/Far East. The significant signal of the POL type is a gradual southward movement of anticyclonic anomalies from the North Pole. In terms of the impact on SAT in East Asia, the SCA type and POL type are similar since they can cause a wide range of cold anomalies in East Asia. However, the cold anomalies caused by the POL type in northern China show significant signals earlier. The WP type causes only modest cold anomalies over northern and eastern China, but the persistence of SAT anomalies is obvious.摘要本文利用日本55年逐日再分析资料, 发现在SH异常增强前, 有三种不同的大气遥相关型前兆信号, 即斯堪的纳维亚 (SCA) 型, 西太平洋 (WP) 型, 极地-欧亚 (POL) 型. 据此, 本文将异常增强的SH分为对应的三类事件. 合成结果表明, 三类事件在典型环流特征和对东亚气温的影响上存在着明显的不同. 具体而言, SCA类事件主要表现为在欧亚中高纬度上自西向东的波列异常, WP类事件表现为环流异常自下游太平洋/俄罗斯远东地区向西发展的特征, POL型事件的显著信号则来源于自极区向南移动的反气旋式环流异常. 在对东亚地表气温的影响上, SCA类与POL类事件类似, 它们均可造成大范围的低温异常. 而WP类仅在我国北方和东部地区造成强度较弱的地表气温异常, 但该异常的持续性特征较明显.

Large-scale Permo-Triassic back-arc extensions of the Mongol- Okhotsk Ocean

The Late Paleozoic−Mesozoic Mongol-Okhotsk orogenic belt marks the final aggregation of East Asia. The geodynamics of the Mongol-Okhotsk oceanic plate subduction are still poorly understood due to its curved orogenic architecture, complex kinematics, and the protracted active continent margin that developed during oceanic subduction. Here, we report the discovery of an ophiolite within the southern paleo-active margin of the Mongol-Okhotsk orogenic belt. The ophiolitic slices are composed of serpentinites, metagabbros, and metabasalts, and interleaved with deformed volcaniclastic rocks. Using zircon U-Pb dating, we determined an age of 253 ± 2 Ma for the metagabbro, which suggests that the ophiolite formed during the Late Permian. Geochemical data and geologic relationships indicate that this ophiolite formed in a back-arc setting. The contemporaneous granitic dikes (ca. 255 Ma) intruding the basalts of the ophiolite were likely formed by crustal anatexis during the back-arc extension. Detrital zircon in sandstone associated with the ophiolite shows a prominent peak age of ca. 273 Ma that probably represents the sediments of the back-arc basin. Combining our discovery with the available data from the literature, we suggest that a >5000-km-long back-arc extension belt existed in the southwestern segment of the Mongol-Okhotsk belt, which indicates a probable Western Pacific-type active margin rather than the previously proposed Andean-type margin that formed during the Permo-Triassic period.

A Mesozoic Andean-type active continental margin along coastal South China: New geological records from the basement of the northern South China Sea

Integrated analyses of in situ zircon U-Pb ages and geochemistry were conducted on both granitic and metasedimentary borehole samples from immediately below the Cenozoic sedimentary cover of the northern South China Sea basin. Our work suggests that basement rocks of the South China Sea basin are not Proterozoic or Paleozoic rocks, as previously speculated based on gravity-magnetic inversion work. The analyzed metavolcaniclastic and metasedimentary rocks were likely deposited at or soon after ca. 145 Ma but prior to ca. 85 Ma. A nearby source in coastal South China is proposed based on the generally immature nature of the metasedimentary rocks and the lack of detritus from older basement rocks farther inland. The vast majority of the metavolcaniclastic samples have sparse ca. 250–190 Ma (particularly 200–190 Ma) zircons. This magmatic quiescence is consistent with the existence of a coastal magmatic gap to the east of Hainan Island during this time, previously interpreted as reflecting a period of flat-slab subduction. Metaclastic samples mostly exhibit a dominant ca. 180–120 Ma zircon population, indicating extensive magmatism along the coastal areas. This finding, together with previously reported ca. 190–180 Ma I-type coastal granites and our newly dated ca. 115–98 Ma volcanic and granitic rocks under the western part of the South China Sea basin can best be interpreted by the former presence of an Andean-type continental arc during the Middle Jurassic – Early Cretaceous. Magmatism along this arc ceased soon after 90 Ma or before ca. 85 Ma, with its last pulse represented by a small group of ca. 100–85 Ma granitic intrusions in the eastern part of the northern South China Sea basin. The arc appears to have jumped toward the Western Pacific Ocean at or soon after 90 Ma, starting the development of the Western Pacific-type plate margin.

The timeline of prolonged accretionary processes in eastern Central Asian Orogenic Belt: Insights from episodic Paleozoic intrusions in central Inner Mongolia, North China

Abstract Updating magmatic profile in crucial constituent terranes across the Central Asian Orogenic Belt presents a key to chronicling the timeline of prolonged accretionary processes and termination of the Paleo-Asian Ocean in the northern China–southern Mongolia tract. Here we performed a systematic geochronological and geochemical study on a spectrum of Paleozoic intrusions from the Erenhot region in central Inner Mongolia, North China, within the hinterland of the tract, with four distinct magmatic episodes unraveled. Combining these episodes with the previously documented events from contiguous regions defines two major tectono-magmatic cycles. The early Paleozoic cycle (500–450 Ma) evolved from initial fluid-fluxed tholeiitic and calc-alkaline granitoids to melt-fertilized mafic-intermediate magmatism. It appears to experience the initiation and maturation of a Western Pacific-type intra-oceanic arc system that culminated in ridge subduction. The late Paleozoic cycle expanded in magmatic expression from sporadic Late Devonian (373–365 Ma) calc-alkaline intermediate-felsic pulses through Early-Middle Carboniferous (356–320 Ma) medium to high-K calc-alkaline flare-up to a Late Carboniferous–Early Permian (310–277 Ma) province of diverse lithologies. These magmatic episodes seem to encompass a complete active continental arc–back-arc system that spanned from resuming oceanic plate subduction through slab rollback and backarc rifting to ridge-trench collision and backarc basin closure. Featuring a Rodinia-aged terrane affinity and a representative Paleozoic magmatic profile, the Erenhot region provides an optimal site for correlating the evolution of mosaic terranes in southern Mongolia and northern China, and for evaluating the coupled evolution of shifting tectonic regimes and plural crustal generation mechanisms within a retreating accretionary orogen.

Erosion and sedimentation in SE Tibet and Myanmar during the evolution of the Burmese continental margin from the Late Cretaceous to Early Neogene

The evolution of large fluvial drainages in SE Tibet and adjacent areas reflects the degree of coupling between tectonics, erosion, and climate. Reconstruction of their spatio-temporal development provides significant constraints for our understanding of the palaeogeographic evolution of numerous blocks in this domain. The Burmese continental margin drained by the Irrawaddy River and its tributaries includes blocks of the Burma Terrane and the Mount Victoria Land (Eastern Belt of the Indo-Burman Ranges). To investigate how was this margin developed from the amalgamation of the two blocks since the mid-Cretaceous, we present seismic profiles, sedimentary budget, sandstone petrography, bulk-rock geochemistry, and detrital apatite U-Pb ages from the margin. Our results show that provenance of the Burmese Forearc and Backarc depressions shifted from the proximal Western Myanmar Arc and equivalents within northern Burma Terrane to the northern Mogok Metamorphic Belt and Dianxi-Burman batholiths within SE Tibet during the late Paleogene. The provenance transition is synchronous with tectonic evolution of the Burma Terrane from an(a) oceanic/continental island arc setting to an active continental margin. We propose an evolved Western Pacific-type margin for the Burma Terrane, which is dominated by typical accretionary wedge, calc-alkaline island arc, and extensional forearc/backarc basins. An increase in sediment accumulation rates (SARs) in the Burmese Forearc indicates that the input from basement rocks within northern Burma Terrane (Tagaung–Myitkyina Belt) became more prominent and the Eastern Belt of the northern Indo-Burman Ranges were totally exposed in the Middle-Late Eocene. This result corroborates the Burma Terrane assembled with SE Asia (Sibumasu Terrane) before the Middle Eocene. The first presence of SE Tibet-like detritus and the diversification of SARs in the Burmese Forearc and Backarc in the Oligocene are compatible with dextral motion on the Sagaing Fault, which may have triggered the birth of the palaeo-Irrawaddy river and fluvial drainage reorganization in its upper reaches. Higher sedimentary fluxes recorded in the Burmese Forearc and Backarc depressions and the Andaman Sea during the Neogene are compatible with enhanced rock erosion within SE Tibet and thus, can be explained by rapid tectonic uplift and an intensified Asia monsoon in this region.

Mesozoic to Cenozoic tectonic transition process in Zhanhua Sag, Bohai Bay Basin, East China

The Zhanhua sag is part of the Bohai Bay intracontinental basin system that has developed since the Mesozoic in East China. The timing of this basin system coincides with the final assembly of East Asia and the development of Western Pacific-type plate margin. Here we use 3-D seismic and core log data to investigate the evolution of this basin and discuss its broad tectonic settings. Our new structural study of Zhanhua sag suggests that there are four major tectonic transitions occurred in the Bohai Bay Basin during Mesozoic and Cenozoic: (1) The first tectonic transition was from stable Craton to thrusting during the Triassic, mainly caused by the South China Block's subduction northward beneath the North China Block, which induced the formation of the NW-striking thrust faults. (2) The second tectonic transition was mainly characterized by a change from compression to extension, which can be further divided into two-stages. At the first stage, two episodes of NW-SE shortening occurred in East Asia during Early-Middle Jurassic and Late Jurassic-earliest Cretaceous, respectively. At the second stage, the extension and left-lateral shearing took place during Early Cretaceous while compression occurred during Late Cretaceous. The NW-striking thrust faults changed to normal faults and the NNE-striking left-lateral strike-slip faults started to influence the eastern part of the basin. (3) The third transition occurred when the NW-SE extension and NNE-striking right-lateral shearing started to form during Paleogene, and the peak deformation happen around 40 Ma due to the change of the subduction direction of Pacific Plate relative to Eurasia Plate. The NE-striking normal faults are the main structure, and the pre-existing NNE-striking strike-slip faults changed from left-lateral to right-lateral. (4) The fourth transition saw the regional subsidence during Neogene, which was probably caused by the India-Asia “Hard collision” between 25 and 20 Ma.

Composition and evolution of the Ancestral South Sandwich Arc: Implications for the flow of deep ocean water and mantle through the Drake Passage Gateway

The Ancestral South Sandwich Arc (ASSA) has a short life-span of c. 20m.y. (early Oligocene to middle–late Miocene) before slab retreat and subsequent ‘resurrection’ as the active South Sandwich Island Arc (SSIA). The ASSA is, however, significant because it straddled the eastern margin of the Drake Passage Gateway where it formed a potential barrier to deep ocean water and mantle flow from the Pacific to Atlantic. The ASSA may be divided into three parts, from north to south: the Central Scotia Sea (CSS), the Discovery segment, and the Jane segment. Published age data coupled with new geochemical data (major elements, trace elements, Hf–Nd–Sr–Pb isotopes) from the three ASSA segments place constraints on models for the evolution of the arc and hence gateway development. The CSS segment has two known periods of activity. The older, Oligocene, period produced basic–acidic, mostly calc-alkaline rocks, best explained in terms of subduction initiation volcanism of Andean-type (no slab rollback). The younger, middle–late Miocene period produced basic–acidic, high-K calc-alkaline rocks (lavas and pyroclastic rocks with abundant volcanigenic sediments) which, despite being erupted on oceanic crust, have continental arc characteristics best explained in terms of a large, hot subduction flux most typical of a syn- or post-collision arc setting. Early–middle Miocene volcanism in the Discovery and Jane arc segments is geochemically quite different, being typically tholeiitic and compositionally similar to many lavas from the active South Sandwich Island Arc front. There is indirect evidence for Western Pacific-type (slab rollback) subduction initiation in the southern part of the ASSA and for the back-arc basins (the Jane and Scan Basins) to have been active at the time of arc volcanism. Models for the death of the ASSA in the south following a series of ridge–trench collisions are not positively supported by any geochemical evidence of hot subduction, but cessation of subduction by approach of progressively more buoyant oceanic lithosphere is consistent with both geochemistry and geodynamics. In terms of deep ocean water flow the early stages of spreading at the East Scotia Ridge (starting at 17–15Ma) may have been important in breaking up the ASSA barrier while the subsequent establishment of a STEP (Subduction-Transform Edge Propagator) fault east of the South Georgia microcontinent (<11Ma) led to formation of the South Georgia Passage used by the Antarctic Circumpolar Current today. In terms of mantle flow, the subduction zone and arc root likely acted as a barrier to mantle flow in the CSS arc segment such that the ASSA itself became the Pacific–South Atlantic mantle domain boundary. This was not the case in the Discovery and Jane arc segments, however, because the northward flow of the South Atlantic mantle behind the southern part of the ASSA gave an Atlantic provenance to the whole southern ASSA.

Magmatic switch-on and switch-off along the South China continental margin since the Permian: Transition from an Andean-type to a Western Pacific-type plate boundary

Detrital zircon provenance data for the Tananao schist in eastern Taiwan is consistent with its protolith being deposited on the South China continental margin at around, or soon after, 150Ma, rather than being of an exotic origin and much older as previously suggested. The absence of ca. 200Ma zircons agrees with the presence of a magmatic gap in the region after the orogenic and magmatic front migrated to central South China, due to a flat-slab subduction. The characteristic lack of input from interior South China (i.e., the lack of 1100–750Ma and 470–420Ma populations), and the immature nature of some of the schist units, suggest that they were sourced from the nearby coastal regions. On the other hand, they exhibit a dominant 190–150Ma magmatic zircon population, suggesting the presence of abundant magmatic rocks of that age along the coastal regions. This, along with our newly discovered ca. 180Ma I-type granites from eastern Zhejiang and other ca. 190–180Ma magmatic rocks recently reported from the coastal regions, led us to propose that a new continental arc was initiated after ca. 190Ma along the coastal region after a magmatic gap due to flat-slab subduction. This newly initiated arc likely persisted until ca. 90Ma, and is represented by the I-type granitic rocks in eastern Taiwan. Slab roll-back likely caused the arc system to retreat towards the Pacific Ocean after 90Ma, and ca. 60–17Ma bimodal magmatism adjacent to the South China Sea signifies continental margin extension in the lead-up to, and during, the opening of the South China Sea. We thus argue that the continental margin of East Asia was transformed from an Andean-type plate margin at 280–90Ma, to the present-day Western Pacific-type plate margin soon after 90Ma.

Tectonics of the North Qilian orogen, NW China

The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a complete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as representing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥710Ma as a consequence of breakup of supercontinent Rodinia.Initiation of the subduction in the Qilian Ocean probably occurred at ~520Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~517–490Ma. In the beginning of Ordovician (~490Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system represented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time period of ~490–445Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~6–7°C/km.The Qilian Ocean was closed at the end of the Ordovician (~445Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~100–200km at about 435–420Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal materials.Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and formation of arc and back-arc system, and to the final continental collision/subduction and exhumation.

Comparative evolution of the Iapetus and Rheic Oceans: A North America perspective

Differences in the origin and evolution of the Iapetan and Rheic oceans profoundly affected the development of the Ouachitan–Appalachian–Variscan orogen. The Iapetus Ocean was initiated in the Late Neoproterozoic–Early Cambrian by the separation of large continental landmasses (Laurentia, Baltica, Gondwana), whereas the Rheic Ocean originated in the Late Cambrian–Early Ordovician as the result of the separation of several ribbon continents, collectively termed peri-Gondwanan terranes, from the northern margin of Gondwana. Compared to the classic passive margin developed along the Laurentian margin of Iapetus, passive margin successions to the Rheic Ocean are more limited in extent. During convergence, both margins of the Iapetus Ocean developed western Pacific-type arc–back-arc systems, whereas subduction of the Rheic Ocean was primarily directed beneath Laurentia and was Andean in style. Closure of Iapetus involved a number of arc–back-arc accretionary events and had occurred by the Middle Silurian as a result of Laurentia–Baltica collision (to form Laurussia) and the accretion of the peri-Gondwanan terranes. Rheic Ocean closure occurred in the Carboniferous by way of continent–continent collision between Laurussia and Gondwana, with Gondwana on the lower plate. Within Iapetus, the formation of ophiolites occurred early in the orogenic cycle, and was followed soon after by obduction, HP–LT metamorphism and foreland-directed thrusting that is intimately related to accretionary events. These features dominate the northern Appalachian orogen where HT–LP metamorphism is attributed to post-accretionary slab break-off events. In contrast, HP–LT metamorphism within the Rheic Ocean reflects A-type subduction during Laurussia–Gondwana collision, and is rapidly succeeded by HT–LP metamorphism attributed to orogenic collapse. Ophiolites are rare and were formed either very early (Early Ordovician) or very late (Devonian–Carboniferous) in the evolution of the ocean. The Early Ordovician ophiolites were not obducted until the Carboniferous, at least 100 million years after their protoliths were formed. Rheic Ocean closure resulted in the formation of large-scale bivergent structures within the orogen, with symmetry changing on either side of the Rheic suture. Comparison of the evolution of the Iapetus and Rheic oceans suggests that differences in rift configuration, passive margin development, and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the Ouachita–Appalachian–Variscan orogen.

Sm–Nd isotopic systematics as tectonic tracers: an example from West Avalonia in the Canadian Appalachians

The tectonothermal evolution of West Avalonia in mainland Nova Scotia, Appalachian orogen, provides an excellent example of the application of Sm–Nd isotopic analyses to tectonic studies. Regional syntheses indicate that West Avalonia originated in the Neoproterozoic as one of a number of terranes (collectively known as “peri-Gondwanan terranes”) along the Gondwanan margin. These terranes are characterized by Late Neoproterozoic voluminous arc-related magmatism. West Avalonia became detached from Gondwana in the Early Ordovician, was accreted to Laurentia by the Silurian and was deformed by post-accretionary strike-slip tectonics in the Late Devonian and Carboniferous, associated with the amalgamation of Pangea. All stages of this evolution can be recognized in the Sm–Nd isotopic signature in various Neoproterozoic and Paleozoic volcanic and sedimentary successions in mainland Nova Scotia. Crustally derived felsic volcanic rocks of Late Neoproterozoic (ca. 615 Ma), Early Cambrian and Early Silurian ages define an envelope that provides gross constraints on the Sm–Nd isotopic composition of the underlying basement source. ε Nd values range from +0.78 to 2.92 in the Neoproterozoic rocks, from +2.0 to +5.0 in the Cambrian rocks, and from −0.1 to +2.5 in the Silurian rocks. T DM model ages for each of these suites are remarkably similar (they typically range from 0.9 to 1.2 Ga), suggesting that the Sm–Nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source. This Sm–Nd envelope provides a reference by which other suites may be compared and interpreted. For example, turbidites in a Late Neoproterozoic volcanic arc basin have Sm–Nd compositions that lie within this envelope, suggesting that they are derived from the coeval volcanic rocks that flank the basin. The genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre-date the main arc phase. Two felsic complexes have compositions that lie within the envelope and have similar model ages. The presence of this signature in early Avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca. 1.0 to 1.2 Ga that formed juvenile “proto-Avalonian” crust. By implication, the main arc phase in Avalonia recycled this crust. Sm–Nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre-dated Avalonian arc activity. Instead, the data indicate derivation from two distinct sources (ancient Gondwanan basement and juvenile Avalonian arc), suggesting deposition in a western Pacific-type back arc environment. This re-interpretation has important implications for Neoproterozoic reconstructions. The Sm–Nd isotopic composition of West Avalonian basement contrasts with that of other peri-Gondwanan terranes. For example, similarly aged felsic rocks in the Cadomian terrane of NW France are characterized by more negative ε Nd values and older T DM model ages (1.0–1.9 Ga). These data, together with detrital U–Pb data, suggest that West Avalonia was developed along the Amazonian margin, whereas Cadomia was developed along the West African margin of Gondwana. Sm–Nd isotopic analyses of Early Silurian clastic rocks in mainland Nova Scotia show fundamental differences with unconformably underlying Neoproterozoic and Early Paleozoic Avalonian successions. These data have ε Nd values ranging from −5.7 to −6.1 with T DM model ages of about 1.4 to 1.7 Ga, indicating that the clastic rocks cannot have been derived from underlying successions. A variety of geologic data and regional constraints suggests that they may have been derived from the Caledonide orogen of western Europe, thereby providing a minimum age for the accretion of West Avalonia to Laurentia–Baltica. Sm–Nd isotopic analyses from Late Devonian–Early Carboniferous continental clastic rocks deposited along the southern flank of Avalonia in mainland Nova Scotia also show little influence from Avalonian rocks, and are compatible with derivation from the Meguma terrane immediately to the south. These data are consistent with regional synthesis, which indicate uplift of the Meguma terrane relative to West Avalonia during dextral transpression.

830-820 Ma Mafic to Felsic Igneous Activity in South China and the Breakup of Rodinia

Detrital zircon provenance data for the Tananao schist in eastern Taiwan is consistent with its protolith being deposited on the South China continental margin at around, or soon after, 150 Ma, rather than being of an exotic origin and much older as previously suggested. The absence of ca. 200 Ma zircons agrees with the presence of a magmatic gap in the region after the orogenic and magmatic front migrated to central South China, due to a flat-slab subduction. The characteristic lack of input from interior South China (i.e., the lack of 1100–750 Ma and 470–420 Ma populations), and the immature nature of some of the schist units, suggest that they were sourced from the nearby coastal regions. On the other hand, they exhibit a dominant 190–150 Ma magmatic zircon population, suggesting the presence of abundant magmatic rocks of that age along the coastal regions. This, along with our newly discovered ca. 180 Ma I-type granites from eastern Zhejiang and other ca. 190–180 Ma magmatic rocks recently reported from the coastal regions, led us to propose that a new continental arc was initiated after ca. 190 Ma along the coastal region after a magmatic gap due to flat-slab subduction. This newly initiated arc likely persisted until ca. 90 Ma, and is represented by the I-type granitic rocks in eastern Taiwan. Slab roll-back likely caused the arc system to retreat towards the Pacific Ocean after 90 Ma, and ca. 60–17 Ma bimodal magmatism adjacent to the South China Sea signifies continental margin extension in the lead-up to, and during, the opening of the South China Sea. We thus argue that the continental margin of East Asia was transformed from an Andean-type plate margin at 280–90 Ma, to the present-day Western Pacific-type plate margin soon after 90 Ma.