Gaku Kimura

Oblique subduction and collision: Forearc tectonics of the Kuril arc

The tectonics of the southwestern Kuril arc are a result of the oblique subduction of the Pacific plate at the Kuril Trench. In association with forearc sliver migration caused by the oblique subduction, collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin. As a result of the collision, a deep crustal section of island arc is observed at the leading margin of the forearc sliver. Tectonics of the Kuril arc related to oblique subduction are different from those of the western Sunda arc, where backarc spreading occurs at the leading margin. This difference is due to margin morphology of the oblique subduction zone.

Underplated units in an accretionary complex: Melange of the Shimanto Belt of eastern Shikoku, southwest Japan

Imbricated thrust stacks composed of melange and oceanic slabs occur in the Shimanto Belt, eastern Shikoku, SW Japan. The melanges are divided into two types: Melange I consists of lenticular sandstone blocks surrounded by scaly shale matrix, and Melange II is composed of blocks of oceanic material, such as basalts, cherts and red shale, in a scaly matrix of shale. Melange II is observed only near the boundaries between Melange I and oceanic slabs. The fabric of Melange I indicates progressive deformation during underthrusting of originally coherent trench-fill turbidites. Layer-parallel extension characterizes early deformation and is divided into two stages: an early stage formed during normal faulting, and a subseguent stage formed perpendicular to the early extension. The early extension is interpreted in terms of Riedel shear associated with layer-parallel shearing and tectonically induced loading normal to the layering. The second extension is due to sticking-thrust movement. Late deformation of Melange I is represented by folding in association with layer-parallel shearing. This appears to be related to compression just before or during underplating of Melange I beneath the accretionary prism. Layer-parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments. Subsequently, the shear zone penetrates pelagic sediments and, finally, oceanic basement. At this time, Melange II is formed by the shearing. An imbricated stack of Melanges I, II and oceanic slabs forms duplexes during the underplating process. The weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin.

Plumes in central Panthalassa? Deductions from accreted oceanic fragments in Japan

Accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of Japan. Major accretion occurred twice; a late Early Carboniferous seamount chain accreted in latest Permian to Middle Jurassic time, and a Late Jurassic oceanic plateau accreted in the Early Cretaceous. The Late Jurassic plateau now in northern Japan and Sakhalin is here named the “Sorachi Plateau” and a Tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones. Petrochemical characteristics of basalts suggest two kinds of sources; depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling. All the accreted fragments in Japan can be traced back to the central part of the Panthalassa by using paleomagnetic data and plate “trajectory analysis”. The timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change. Their place of origin, in the mid-Pan thalassa, retains a positive residual geoid anomaly and slow Vp in the lower mantle, which suggest a past upwelling of hot material from the core-mantle boundary.

Mesozoic collision—extrusion tectonics in eastern Asia

Abstract The eastern part of the Asian continent was intensively deformed in early Cretaceous time. Deformation has been interpreted to be related to subduction tectonics along the eastern continental margin. We propose an alternative tectonic framework of collision—extrusion tectonics resulting from collision of the Indochina Block with the Eurasian continent. This event controlled the Jurassic to Cretaceous tectonic history in eastern Asia and the evolution of the subduction zone along the eastern margin of the continent.

Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block

Paleomagnetic studies were performed on Jurassic, Cretaceous and Tertiary rocks sampled from the Qitaihe area in Heilongjiang and Benxi area in Liaoning Provinces, northeast China. Both locations are near the Tancheng-Lujiang (Tan-Lu) fault system; Benxi is close to but on the eastern side of the fault while Qitahe lies between two major branches of the northwestern extension of this fault. In Mesozoic rocks, secondary magnetization in the present field direction was observed, but it was possible to retrieve the primary components by taking the high-temperature portion of the demagnetizing spectra. The Mesozoic poles thus obtained, especially those for the Cretaceous, deviate from the paleomagnetic poles of similar ages from the central part of the North China Block (NCB), Siberian Block or South China Block (SCB). Although the distances to the poles (flattening) are quite similar, the Benxi pole suggests a small clockwise rotation with respect to the central NCB poles, while the the Qitaihe poles indicate a much larger rotation in the opposite direction. It is shown that the deviation of the Benxi pole is similar to that observed for the Korean Peninsula and Shangdong Province, which all lie to the east of the Tan-Lu fault in the NCB. The Qitaihe pole position is quite different from the poles either west or east of the Tan-Lu fault. From these observations, it is concluded that a left-lateral strike-slip movement at the Tan-Lu fault system since the Cretaceous is the cause of systematic deviation in the position of the poles obtained from east of the fault including the Benxi area, while anomalous direction of Qitaihe rocks may represent a small scale rotation within the Tan-Lu fault system. The estimation of the movement along the Tan-Lu fault depends on which branch of the fault system is considered most active. If the main branch is assumed to be the place of slip, the movement can be represented by an Euler pole which lies to the south of Honshu Island (20°N, 150°E), with an estimated total displacement of 800 km since the Cretaceous.

Mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex: Example from southern Sakhalin

Abstract Four modes of mixture of oceanic materials such as cherts, limestones and basalts, and terrigenous trench fill sediments are recognized in the Cretaceous accretionary complexes in southern Sakhalin, Far eastern Pacific margin of the U.S.S.R. The first mode is a thin-skinned stacking of pelagic bedded cherts with black shale, due to duplex formation in association with shallow underplating or off-scraping. The second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation. Reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt, with overlying pelagic sediments of chert and limestone, olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench, and finally trenchfill sediments of turbidite. This sequence is imbricated by thrusting, which appears to have occurred at the time of underplating. The third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate. A reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust. The fourth mode is in-situ basalt volcanism at the trench, where thick baked margins of terrigenous shale are formed around the injected basalts. This mode appears to represent a ridge-trench encounter. These four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts.

Deformed soft sediments at the junction between the Mariana and Yap Trenches

Abstract Highly deformed unconsolidated hemipelagic sediments were found below the sea bed surface at anomalously shallow depths near the junction between the Mariana and Yap arc-trench systems. The order of deformation is (1) sigmoidal veins and spaced foliations, (2) layer-parallel shear planes, (3) reverse faults and kink bands and (4) more veins. All the deformation structures are considered to have been developed progressively during shear parallel to bedding, accompanied by volume decrease due to pore reduction and fluid expulsion during consolidation, while the sediments are not yet lithified.