Wonn Soh

Accretion of Japanese island arcs and implications for the origin of Archean greenstone belts

The present-day region of arc-arc collision between the Izu-Bonin Arc and mainland Japan (Honshu Arc) in the western Pacific (called the Izu Collision Zone or ICZ) provides a useful kinematic model for the development of many Archean greenstone belts. The compositional assemblage, thermal structure and structural style of the crust in the ICZ are very similar to those inferred for many Archean greenstone belts. In both cases, thick, young and flaky arc crust with a high heat flux is in collision with another crustal block. The ICZ has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms. Many Archean greenstone belts show similar geological features, and we suggest that the incremental imbrication of the crust was a common tectonic style in some Archean convergent margins.

Basin evolution in the arc-arc Izu Collision Zone, Mio-Pliocene Miura Group, central Japan

A comparative study of the present collision zone between the Izu–Bonin Ridge (island arc) with mainland Japan (Honshu Arc), and the Mio-Pliocene of onshore SE Japan, suggests that arc—arc collision processes and the resulting stratigraphic successions may be repetitious and predictable. Arc—arc collision has led to the incremental accretion of segments of delaminated Izu—Bonin Arc crust onto the Honshu Arc, associated with the sequential southward migration in jumps of the plate boundary and trench. Prior to accretion of a segment of Izu-Bonin Arc crust, the leading edge underwent uplift to generate an approximately trench-parallel topographic high, the Zenisu Ridge being the present example with the Hayama-Mineoka uplift zone as a Mio-Pliocene example. The ridge separated a northern trench or trough from a southern intra-oceanic arc basin. During collision-accretion, the trench received both Honshu Arc-derived, terrigenous, and Izu-Bonin Arc-derived volcaniclastic, sediments, whereas the arc basin tended to receive only arc deposits. During the final stages of accretion, the arc basin began to receive ever-increasing volumes of terrigenous, Honshu Arc-derived, detritus fed through basement-controlled canyons. The accretionary process was accompanied by intense deformation and the residual deep-marine basin was then infilled above an angular unconformity.

Rejuvenation of submarine canyon associated with ridge subduction, Tenryu Canyon, off Tokai, central Japan

Abstract The 130-km-long Tenryu Canyon is an active deep water meandering canyon transversely developed on the tectonically active Tokai (Eastern Nankai) accretionary prism, where the multiple intraplate deformation caused by the arc–arc Izu collision. Over 2000 km of EM 12 data, 3.5-kHz profiles and PASISAR profiles obtained during the KAIKO-TOKAI Project, 1996, IZANAGI data, the bathymetric chart, and the preexisting seismic profile data, enable us to better understand the geomorphology and development of the Tenryu Canyon. The Tenryu Canyon is a very steep entrenched channel with its lower stream steeper in both canyon slope and channel slope. Similarly, the relative height between the channel bottom and the levee surface tends to increase unusually toward the lower reach. A likely interpretation of such geomorphological features is that the Nankai Trough floor, as the termination of the Tenryu Canyon, has subsided, associated with subduction of the Paleo Zenisu ridge. Based on the result of a quantitative analysis, the total amount of relative subsidence of >800 m was estimated in the Nankai Trough floor. As a result, the Tenryu Canyon has rejuvenated, catching up the topographic gap between the subsided Nankai Trough floor and the continental slope.

VOLCANICLASTIC SEDIMENTS, PROCESS INTERACTION AND DEPOSITIONAL SETTING OF THE MIO-PLIOCENE MIURA GROUP, SE JAPAN

Abstract The Mid-Miocene to Early Pliocene (14-3 Ma) Miura Group exposed on the Miura and Boso peninsulas around Tokyo Bay in southern Honshu comprises at least a 2 km thick succession of volcaniclastic sediments. These accumulated in a forearc basin or basins on the eastern side of a proto Izu-Bonin arc prior to and during accretion of the Miura block onto the Honshu arc and its subsequent dextral transcurrent eduction. The basin fill shows a complex admixture of facies that result from multiple processes, which operated on a relatively shallow-slope basin. The main facies groups include dark-coloured, mostly coarse-grained scoriaceous beds; pale-coloured, muddy-silty pumiceous bioturbated sediment; yellowish and whitish-coloured tuffaceous horizons; and chaotic slump, debris flow and injection units. Many of the facies and beds in the older Misaki Formation can be readily interpreted as the result of: (a) direct pyroclastic fall through the air and water; (b) downslope resedimentation via turbidity currents and related processes, in some cases derived from subaqueous base-surge pyroclastic flow or vertical density currents; (c) hemipelagic settling, commonly under the influence of thermohaline bottom currents. In other cases, composite beds are believed to result from the interaction of these processes in more complex events. The younger Hasse Formation is dominated by more shallow-water, marine-currents and storm events and in parts, by fluvial or proximal fan-delta processes. The distribution of these different facies illustrates the evolution and shallowing of the Miura-Boso forearc basins during accretion and emplacement.

HIGH-RESOLUTION SEISMIC IMAGES OF THE FORMOSA RIDGE OFF SOUTHWESTERN TAIWAN WHERE “HYDROTHERMAL” CHEMOSYNTHETIC COMMUNITY IS PRESENT AT A COLD SEEP SITE

A high-resolution seismic reflection survey was conducted during the NT07-05 cruise over the Formosa Ridge offshore southwestern Taiwan where strong and continuous bottom simulating reflections (BSR) have been observed. Previous seafloor pictures taken from a deep-towed camera indicate that there are some chemosynthetic colonies. During the NT07-05 cruise, not only large and dense chemosynthetic communities were confirmed at the plume site, ROV Hyper Corresponding author: Phone: +886 2 33661387 Fax +886 2 23660881 E-mail: csliu@ntu.edu.tw. Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

Initiation of ophiolite emplacement: a modern example from Okushiri Ridge, Northeast Japan arc

Abstract Submersible dives by Shinkai 2000 discovered thickly piled back-arc basin basalts and an overlying sequence of siliceous hemipelagites and turbidites on the western fault escarpment of the Okushiri Ridge, off western Hokkaido. The Okushiri Ridge is an allochthonous tectonic block marked by an east-dipping main boundary thrust and a west-dipping back thrust. The block is interpreted to have formed as a result of the plate convergence between the Northeast Japan and Eurasian plates and all available data, including seismic profiles and geochemical analyses, suggest that the Okushiri Ridge constitutes a piece of oceanic crust of the Japan Basin. Ultimately, this piece of oceanic crust will be incorporated into the Japan magmatic arc. The Okushiri Ridge therefore demonstrates that tectonic inversion, i.e. the change from extension to compression, plays an important role in ophiolite emplacement.

Formation of the Japan and Kuril Basins in the Late Tertiary

Formation of the Japan and Kuril Basins is reconstructed based on geological and geophysical data, and the overall plate kinematics that operated in Northeast Asia from Eocene through Pliocene time (55-5 Ma). The initial positions of the Japanese islands and the West Sakhalin Block along the Siberian coast and the East Sakhalin Block along the Okhotsk Block, prior to opening of the Japan and Kuril Basins, are fixed based on onshore geology, offshore seismic profiles and paleomagnetic measurements. The manner by which the Japan and Kuril Basins opened during the Oligocene and Miocene differs from what is known of most backarc basins in East and Southeast Asia. From indistinct magnetic anomaly patterns in both basins, we deduce that opening commenced by extrusion of blocks along dextral shear zones, unlike the typical fan-shaped opening about an Euler pole seen in other backarc basins. Nevertheless, an essential factor in the opening is the presence of elevated heatflow in the backarc produced by subduction. The shear zones were formed between the southwestward-translating North American plate and the eastward-shifting Amur Block during the Late Tertiary. Three stages are distinguished in the development of the Japan Sea. In the first stage, the dextral shear zones formed, and the Tohoku and Sakhalin Blocks shifted southwards during the late Oligocene to early Miocene. The second stage involved the rotation of the southern Japanese and Kuril Blocks throughout the middle Miocene. In the Pliocene, the third and final phase was characterized by compression.