Eastern Margin Of Asia Research Articles

A Late Cretaceous paleomagnetic pole from Koto rhyolite, southwest Japan: Implications for eastern margin deformation of Asia

A paleomagnetic pole was obtained from Late Cretaceous rhyolitic ash flow tuffs and associated ring dikes, which are collectively called the Koto rhyolite in the eastern part of southwest Japan. Characteristic remanent magnetizations were isolated from 33 sites through thermal demagnetization. Normal and reversed polarity magnetizations were found and are mutually antipodal. Although the magnetization directions become more dispersed after tilt correction, the clearly defined polarity sequence and the positive contact test between ash flow tuffs and ring dikes ensure that the characteristic magnetizations of ash flow tuffs predate the intrusion of associated ring dikes. Since no regional tilting is expected, we regard the in situ magnetization directions to represent the Late Cretaceous (70 Ma) paleomagnetic field (D = 84.6°, I = 57.5°, α95 = 5.5°) in eastern southwest Japan. The paleomagnetic pole is not significantly different from the coeval poles of western southwest Japan. This result gives a further support to a microplate model for the Miocene clockwise rotation of southwest Japan. By canceling out the effect of the Miocene rotation, we were able to obtain a paleomagnetic pole of 70 Ma (219.7°E, 63.0°N, α95 = 12.2°) for southwest Japan when the Japan Arc was incorporated in the eastern margin of Asia. Paleomagnetic poles of Late Cretaceous age from Korea, south China coast, and restored southwest Japan are statistically indistinguishable from each other but significantly differ from that of the continental interior of east Asia. The observed rotation of about 20° probably results from shear deformation exerted by northeast‐trending strike‐slip faults along the eastern continental margin of Asia.

Cretaceous deformation history of the middle Tan-Lu fault zone in Shandong Province, eastern China

Based on field analysis of fault-slip data from different rock units of the Cretaceous basins along the middle part of the Tan-Lu fault zone (Shandong Province, eastern China), we document polyphase tectonic stress fields and address the changes in sense of motion of the Tan-Lu fault zone during the Cretaceous. The Cretaceous deformation history of the Tan-Lu fault zone can be divided into four main stages. The first stage, during the earliest Cretaceous, was dominated by N-S extension responsible for the formation of the Jiaolai basin. We interpret this extension to be related to dextral strike-slip pull-apart opening guided by the Tan-Lu fault zone. The second stage, during the middle Early Cretaceous, was overwhelmingly rift-dominated and characterized by widespread silicic to intermediate volcanism, normal faulting and basin subsidence. It was at this stage that the Tan-Lu-parallel Yi-Shu Rift was initiated by E-W to WNW-ESE extension. The tectonic regime then changed during the late Early Cretaceous to NW-SE-oriented transpression, causing inversion of the Early Cretaceous rift basin and sinistral slip along the Tan-Lu fault zone. During the Late Cretaceous, dextral activation of the Tan-Lu fault zone resulted in pull-apart opening of the Zhucheng basin, which was subsequently deformed by NE-SW compression. This deformation chronology of the Tan-Lu fault zone and the associated Cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of Asia, or related to collision in the Tibet region.

Mesozoic contractional deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold–thrust belt, China

Intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concentrations into weak zones. These weak zones may be pre-existing crustal heterogeneities such as faults, thermally weakened zones due to magmatism and/or the effects of a thick sedimentary cover, or mechanical contrasts between adjacent crustal blocks. Intraplate deformation is not exclusively related to continent–continent collision; it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries. Asia has many examples of active intraplate deformation that are thought to be related to the Cenozoic Indo-Asian collision. Although much attention is placed on active examples, discerning controls on pre-Cenozoic Asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of Asia. Situated on either side of the Yellow River along the western margin of the Ordos Plateau, the intraplate Western Ordos fold–thrust belt involves units as old as Archean basement and as young as Late Jurassic(?). The fold–thrust belt is spectacularly exposed in two ranges, the Helan Shan and the Zhuozi Shan. Cross-sections drawn through the north-trending fold–thrust belt suggest 30% minimum shortening. A paleocurrent reversal within the Lower–Middle Jurassic section is interpreted to mark the onset of contraction and mountain building. A Late Jurassic(?) synorogenic boulder conglomerate records the final deformation in the fold–thrust belt. Synchronous with contraction, and cutting across the fold–thrust belt, is an ∼E–W-striking strike-slip fault that displays right-lateral drag and sub-horizontal striae. We interpret this fault as an accommodation structure in the thrust belt. Following ∼E–W contraction, the thrust belt was dismembered by an ∼N15°W-trending left-lateral strike-slip fault. The left-lateral fault displaces the frontal portion of the thrust belt (the Zhuozi Shan) 62 km to the north, relative to a more internal portion (the Helan Shan). We propose that the anomalous orientation and intraplate location of the Western Ordos fold–thrust belt is a function of mechanical contrasts between a previously deformed area and a stable crustal block (Ordos) to the east. Pre-existing crustal weakness or anisotropy along the western margin of the Ordos block is most likely related to either a Late Proterozoic–Early Paleozoic aulacogen, a Triassic fault system, or to both. Although its intraplate location and the complexities of Mesozoic Asian plate interactions make it difficult to relate deformation in the Western Ordos fold–thrust belt to a specific contemporaneous plate boundary, its N–S trend may suggest a link with paleo-Pacific subduction along the eastern margin of Asia.

Paleomagnetism of mid-Cretaceous red beds in west-central Kyushu Island, southwest Japan: paleoposition of Cretaceous sedimentary basins along the eastern margin of Asia

A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240–480°C), and high (480–680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α 95=5°, which yields a paleomagnetic pole at 39°N, 186°E and A 95=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.

Low‐latitude and Southern Hemisphere origin of Anisian (Triassic) bedded chert in the Inuyama area, Mino terrane, central Japan

Paleomagnetic and paleontologic studies of Middle Triassic to earliest Jurassic radiolarian bedded chert in the Inuyama area, southern Mino terrane of central Japan, have shown that the chert was deposited in an equatorial region with a paleolatitude between 10°N and 10°S, and in particular, the lower to middle Anisian chert depositional basin was situated in the Southern Hemisphere. The chert has four characteristic components of remanent magnetization. The highest blocking temperature component, carried by hematite, shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition. The other three components are interpreted as a present field overprint and two postfolding remagnetizations. The absolute, tilt‐corrected inclinations of the primary component are less than 20° for all the samples of Middle Triassic to earliest Jurassic age, with the mean inclination of the lower to middle Anisian chert being 11.0°±4.3°. Paleomagnetic polarity reversals observed for the lower to middle Anisian chert are well correlated with those of the European Triassic sections, under paleontologic age constraints using radiolarians and conodonts, provided that the Anisian chert in Inuyama was deposited in the Southern Hemisphere. This conclusion illuminates the paleogeographic evolution of the chert‐bearing accretionary complexes not only in Japan but along the eastern margin of Asia as well.

Ring-morphostructures of the Amur-Songhua-Huanghe lineament (eastern margin of Asia)

The “ring-morphostructures” (RM) are the object of this study. They are one of the most significant element of lineament’s structure and unknown for the majority of investigations. RM are numerous to lineament’s zone: they have-different size (from 10–20 km to 500–800km in diameter), structure, age, geological history and have been divided into several classes on the most important signs. They have a spatial-genetic connection Amur-Songhua-Huanghe lineament (ASHL) and had been developed simultaneously with it. The RM exerted an essential influence on the geologic-geomorphological structure, evolution and geodynamics of lineament. The main factor of evolution of lineament is a long geological process of extension of the earth’s crust which is a result of the expanding earth.

An imbrication model for the Rajang Accretionary Complex in Sarawak, Borneo

The structural framework of the Rajang Accretionary Complex is interpreted on the basis of field surveys conducted around the Lupar Fault Zone in southwestern Sarawak. Bedding dips in the Rajang Accretionary Complex are generally southward, while the complex as a whole becomes younger northward. The complex is interpreted as a series of thrust slices formed by accretion at a subduction trench. Each slice forms a unit 10 to 15 km in width. Thrust slices of this order of this order of magnitude are consistent with those formed by accretion processes at modern subduction trenches. It is suggested that accretion of Late Jurassic to Cretaceous oceanic crust occurred initially from the Pacific (eastern) side of Borneo in the Late Cretaceous, forming part of an arc system extending all along the eastern margin of Asia from Japan to Kalimantan. Subsequently, in the Early Tertiary, due to bending of the southern end of the arc system in Borneo, the direction of subduction changed, so that accretion occurred from the north.

Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, northern China

The Bohai Basin is one of a family of early Cenozoic extensional basins that lie along the eastern margin of Asia from Russia to Vietnam. Initial extension was probably triggered by subduction roll-back of the oceanic Pacific Plate from the Asian continent. There were two phases in the Bohai Basin's rift history. The earlier, Paleocene-early Eocene phase resulted in the deposition of the Kongdian Formation and the fourth (lowest) member of the Shahejie Formation in a series of elongate half grabens. These half grabens have master faults with a NNE-SSW orientation. Secondary normal faults are typically clockwise oblique to the master faults, indicating a component of dextral transtension. Deposition was focused in the west and south of the present basin. These rocks are mainly alluvial fan and fluvial red beds. The architecture of the basin underwent an important change at ca. 43–45 Ma (middle Eocene), beginning with the deposition of the third member of the Shahejie Formation. In part, these sediments were deposited in the same half grabens as the Kongdian Formation, but the Bozhong Depression in the central part of the basin originated at this time, and became the major depocentre. The Bozhong Depression superficially resembles a pull-apart basin. It formed when continued transtension of the earlier Tertiary fault systems to the east and west created an extensional overlap between them. During this phase, the basin as a whole had a geometry with elements typical of both pull-apart and transtensional basins. Regional extension in many east Asian basins ended at the end of the Oligocene, probably because of the onset of transpression within eastern Asia, caused by the collision of Australia with the Philippine Sea Plate. Dextral transpression caused minor inversion of some of the earlier normal faults in Bohai, but as a whole the basin began to subside in a post-rift phase of thermal subsidence that has lasted until the present day.

Cenozoic basins in northern Taiwan and tectonic implications for the development of the eastern Asian continental margin

North of Taiwan, four offshore Tertiary basins extend seaward from the inner shelf of the East China Sea to the west of the Okinawa Trough. These basins are characterized by rifting features with NE-SW-trending half-grabens and fault blocks. Episodic rifting events have taken place from Paleocene to Early Miocene with the eastward shift of depositional centers. The Paleogene rift basins are filled primarily with lacustrine to shallow marine sediments, but also contain a slight amount of pyroclastics and basaltic flows. Regional subsidence has dominated during the late Tertiary and these Paleogene half-grabens have united into a large marginal sea basin overlain by a Neogene sedimentary wedge of shallow marine sediments. A fifth basin, the Paleogene Hsuehshan half-graben trough in present-day northern Taiwan has been deformed into mountain ranges by the collision of the Luzon Arc with the eastern Asian margin during the late Cenozoic Era. The tectonic development of eastern Asian margin in northern Taiwan region has evolved into two phases. First, the eastward retreat of the West Pacific subduction zone and accompanied rifting of continental crust occurred during Late Cretaceous to Eocene times. Second, as the intensity of rifting decreased, thermal subsidence of eastern Asian margin prevailed and subsequently caused a tilting seaward continental margin on which a large marginal sea basin rests. Collision of the Luzon Arc with the eastern Asian margin in Taiwan deformed the Paleogene Hseuhshan Trough into mountain ranges (Late Miocene to present)

Données nouvelles sur la distribution géographique d'espèces Américaines de Grands Foraminifères du Crétacé Terminal

Several genera of Larger Foraminifera from the Campanian-Maastrichtian of the wider Caribbean region have been regarded as endemic to it. Afterwards, some species of these genera were recorded on the islands of the Pacific Ocean and the eastern margin of Asia. But above all they were found in abundance in the holes of the Legs 33 and 61 of the D.S.D.P./O.D.P. program. Their lists have been published by J.P Beckmann and by Premoli Silva & Brusa, respectively. The latter two authors have distinguished species of both American and Eurasian affinities and consequently have proposed migrations in opposite direction, from the eastern and western margins of the Pacific Ocean, in the Late Senonian. the author of the present paper has revised the microfaunas, based on the material which the previous authors had sent him and also on samples received of the O.D.P. program. As a result of this study he regards all species as having American affinities and that therefore the migrations of the Larger Foraminifera in the Late Senonian of the Legs 33 and 61 took place from the American margin of the Pacific Ocean.

Right‐lateral shear along the Northwest Pacific Margin and the India‐Eurasia Collision

Right‐lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull‐apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right‐lateral shear is a consequence of indentation of Asia by India. As in small‐scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left‐lateral shear zones (Tien Shan‐Baikal‐Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re‐emphasize the role played in the opening of marginal basins by the Pacific subduction as a free boundary to the east.

Tectonic evolution of the continental lithosphere in eastern China and adjacent areas

The major tectonic units of eastern China and its adjacent areas include five Precambrian massifs and four Phanerozoic orogenic belts. The former were characterized by their relatively small size and great tectonic mobility except for the Siberian platform; the latter were composite systems made up of a series of small oceanic basins and microcontinents occurring as island chains except for the South China zone. The integration of the various massifs has involved a complex process of collision and welding. The related orogenic belts are thought to be polycyclic collision-imbrication orogenes rather than simple subduction type or collision type. The Yanshanian movement is the most important geological event since Phanerozoic in this region. It was due to the Yanshanian orogeny that the above belts eventually accomplished their long and complicated polycyclic orogenic processes, and the continental blocks were finally welded together as a unity, becoming a part of the giant unified Eurasian craton. Since the middle Cretaceous, the region has entered a completely new stage of tectonic development—the disintegration stage of eastern Asia. The eastern margin of Asia is not an accreted margin, but a destructed or disintegrated margin.

Estimation of the attenuation operator for multiple‐ScS waves using digitally recorded broadband data

Broadband multiple‐ScS waveforms have been obtained for several deep‐focus earthquakes in the Izu‐Okhotsk region from the intermediate‐period channels of the RSTN network. Differential travel times have been computed by waveform cross‐correlation. Estimates of the multiple‐ScS attenuation operator have been obtained using a phase‐equalization and stacking technique. For this mixed, but mostly tectonic path crossing the eastern margin of Asia and northwestern North America we obtain a median ScSn‐ ScSn−1 travel time residual of +0.6s with respect to the J‐B velocity model and a QScS of 182±21 over the frequency band 0.017‐0.117Hz. In this band there is no apparent frequency dependence. We find that the Aleutian back‐arc region and central Alaska is slower and has a higher attenuation than the rest of the path. Even though the data contain much higher frequencies, beyond ∼0.117 Hz the multiple‐ScS phases lose coherence and the results become unstable.

Crustal accretion and metamorphism in Taiwan, a post-Palaeozoic mobile belt

Taiwan consists chiefly of Permo-Mesozoic basement unconformably overlain by Cainozoic cover strata; on the east, this complex is juxtaposed along the Longitudinal Valley against the Neogene Coastal Range, the northern extension of the Luzon calc-alkaline arc and intervening fore-arc basin. Accumulation of well-ordered sandstones, shales, limestones and intercalated basaltic units, and outboard argillite melange along the eastern margin of Asia in Permian and early to mid-Mesozoic time was terminated by a major late Cretaceous (85—90 Ma) dynamothermal event that produced the composite basement complex of Taiwan, and also affected much of southeastern China. This basement consists of a westerly Tailuko belt, separated from the easterly Yuli belt by a major fault. The miogeoclinal Tailuko belt is characterized by chloride and biotitic greenschist facies low-pressure assemblages, except in the north where amphibolite facies assemblages are associated with emplacement of remobilized granitic rocks; metamorphic grade increases gradually eastward. The eugeoclinal Yuli belt lacks marble layers and granitic intrusions, and instead contains serpentinite bodies and associated rare highpressure epidote-bearing barroisitic amphibolite tectonic lenses. Tailuko quartzofeldspathic rocks contain mineralogic and geochronologic evidence of Palaeogene and Neogene reheating. Mafic tectonic blocks in the Yuli belt show partial conversion to glaucophanic assemblages; radiometric ages for the blueschist metamorphism are 8-14 Ma. The Cainozoic slate sequence was deposited on the basement complex following renewed Palaeogene rifting. It consists largely of sedimentary strata (and intercalated basalts) laid down on the Asiatic passive margin and seaward in the South China Sea as continental slope deposits. An accretionary wedge was constructed adjacent to the approaching Neogene Luzon arc, marking the non-subducted western margin of the Philippine Sea plate. During Plio-Pleistocene collision of the Luzon arc with the Chinese continental margin, the landward Cainozoic shelf and slope units were imbricated and thrust westward; increased pressure ( P ) and temperature ( T ) during this loading evidently promoted recrystallization of the basement and passive margin cover. Metamorphism ranged from diagenetic and zeolite facies in the Western Foothills to upper greenschist and locally amphibolite facies in the basement complex. No metamorphic hiatus between Mesozoic basement and Cainozoic cover is recognized. The accretionary terrane lying east of the Longitudinal Valley is virtually unmetamorphosed. However, the East Taiwan Ophiolite, occurring as clastic debris and slide blocks in the unmetamorphosed olistostromal, largely Pliocene, Lichi Melange of the Coastal Range fore-arc basin, carries the effects of a mid-Miocene oceanic-ridge recrystallization of actinolite hornfels facies, overprinted by a late Miocene ocean-floor zeolitization. Large portions of the sialic crust making up Taiwan evidently formed approximately in situ , then were deformed and thrust landward during subsequent tectonic events, but far-travelled terranes and allochthonous fragments of oceanic material played an important role in accretion. Recognized and suspected exotics include. (1) Lower or mid-Mesozoic meta-ophiolites, now greenstones, amphibolites and minor serpentinites, previously obducted into the Upper Mesozoic Tailuko belt; amphibolites also occur as enclaves in granitic intrusives 85—90 Ma old; (2) the entire Upper Mesozoic Yuli belt; (3) Mio-Pliocene tectonic blocks of blueschistic meta-ophiolite, emplaced in the east-central part of the Yuli terrane; (4) olistostromal ophiolitic debris (preexisting Miocene oceanic crust of the South China Sea) within the Pliocene Lichi Melange of the Coastal Range; and (5) the Neogene calc-alkaline Luzon arc, which began to collide with Asiatic continental crust in Plio-Pleistocene time. The Cainozoic slate series represents an additional, parautochthonous terrane, which was deposited as the Tertiary miogeoclinal cover along the Asiatic passive margin, and was thrust westward during Plio-Pleistocene arc collision. Among the suspected allochthonous units, Mesozoic amphibolites of both Tailuko and Yuli belts have been identified as of chiefly normal more affinites from their chemical and Nd and Sr isotopic compositions. The clearly oceanic East Taiwan Ophiolite apparently formed along a transform-interrupted spreading centre of the South China Sea marginal basin, based on mineralogic, chemical and isotopic evidence.

HAUTERIVIAN AND BARREMIAN IN THE FAR EAST

Valanginian marine sediments are widespread in the eastern margin of Asia from China northward over the Sikhote-Alin region, as far west as the Amur river, and as far north as the Amgun river. This was an interior type sea with abundant islands and a boreal fauna. It is surmised that Kamchatka, Sakhalin, Hokkaido, and the other Japanese islands were not invaded by the sea. A paralic coal basin is known on southwestern Honshu. Post-Valanginian folding and erosion preceded a Barremian-Albian marine transgression over the same general area. In these younger Cretaceous rocks southern faunas appear and the boreal fauna is present north of 45° latitude. Aucella and Aucellina species are important elements of the faunas. This review of faunas and correlations, including western North America, suggests that Hauterivian folding was a major event in North Pacific tectonic history. -- C. G. Tillman.