Hidekazu Tokuyama

Continental Crust, Crustal Underplating, and Low-Q Upper Mantle Beneath an Oceanic Island Arc

A detailed structural model of the crust, subducting slab, and underlying upper mantle across the northern Izu-Ogasawara (Bonin) island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion. The model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid-crust. A highly attenuative mantle (that is, one with low quality Q) seems to be confined mainly beneath the presently active rift zone. In contrast, high P-wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin, suggesting a large-scale magma input responsible for the arc formation.

New rates of western Pacific island arc magmatism from seismic and gravity data

Numerous studies have been conducted in order to look into the evolution of the continental crust. Some suggest that one of the mechanisms which contribute to the growth of continental crust is arc magmatism. It is in this context that Reymer and Schubert (Tectonics 3 (1984) 63) estimated arc magmatic addition rates to the continental crust. Their results suggest that island arc magmatism was producing material at an average rate of 20–40 km3/km/Myr (volume per unit width along the strike direction of arc). The present work utilizes the most recent worldwide marine gravity data, together with improved seismic data from some oceanic island arcs in the western Pacific region. The combined gravity and seismic data allow a more accurate image of the subsurface configuration beneath the oceanic island arcs and yield better estimates of crustal volumes created during arc magmatic processes. Oceanic island arcs investigated in this study show a crustal thickness ranging from 20 to 30 km. Utilizing this thickness, the relevant crustal volume for each island arc is then estimated. Dividing the crustal volume by the age of initiation of subduction of the arc gives arc magmatic addition rates of 30–95 km3/km/Myr. The estimates presented here are nearly twice as high as the previous estimates of arc magmatic addition rates.

Paleothermal structure of the Shimanto accretionary prism, Shikoku, Japan: Role of an out-of-sequence thrust

Vitrinite reflectance study on the Cretaceous Shimanto accretionary prism of Southwest Japan shows that the vitrinite reflectance varies from 0.6% to 3.0%, corresponding to the estimated maximum paleotemperature variation of 90–210 °C. The thermal structure shows a sharp discontinuity across an out-of-sequence thrust separating it into two thermal blocks. In each block, vitrinite reflectance values increase systematically toward the prism younging direction, and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units. This provided us with a set of data to calculate the geometry and displacement of the out-of-sequence thrust. The fission-track zircon partial-annealing ages indicate that this thrust was active about 43 Ma, probably in response to the accretion of Eocene rocks. This study shows that initial thickening of the accretionary prism, in an order of several kilometres, was mainly controlled by out-of-sequence thrust.

Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough

Abstract Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide us with the regional heat flow variations using pressure–temperature conditions for methane hydrate stability. Estimated heat flows generally show constant values about 50 mW/m2 shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concentration of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissociated hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estimated from BSR positions offer information about active processes occurring in accretionary prisms.

The eastern and western ends of Nankai Trough: results of box 5 and box 7 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the Nankai Trough. The eastern survey covers the transition between the large Izu-Bonin (Izu-Ogasawara) Ridge collision with Honshu and the Nankai Trough subduction. It includes a northeast trending basement ridge, the Zenisu Ridge, to the southeast of Nankai Trough as well as two large channel systems, one following the trench, the Nankai channel and the other coming from the north, the Tenryu Canyon. The Zenisu Ridge is a zone of recent intra-plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast. We interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the Izu-Bonin Ridge and the concentrated shortening along the Nankai Trough. The shortening axis appears to change from north-south along the Zenisu Ridge to northwest-southeast along the Nankai Trough and finally to east-west within Honshu. The western survey covers the junction between the Nankai and Ryukyu Trenches and the Kyushu-Palau Ridge which is subducted without major internal deformation. The ridge appears to act as an indenter upon the margin. The tip of the indenter is presently situated under the upper accretionary prism.

Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan

[1] We developed a theoretical method for predicting effective stress and pore pressure based on rock physics model. We applied the method to reveal the pore pressure distribution within the Nankai accretionary prism off southwestern Japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement. From the crack aspect ratio spectrum estimated from laboratory and well-log data, we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory. By iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3D tomographic inversion, we estimated in situ mean effective stress within the accretionary prism. Pore pressure is then the difference between the effective stress and the confining stress. When we calculated pore pressure, we considered compressive state of stress in the accretionary prism. Our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement; we determined a normalized pore pressure ratio (λ*) of 0.4-0.7. Abnormal pore pressures develop in the under-thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement. Overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward. The increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence, and the pore pressure (mean effective stress) contrast at the decollement becomes smaller landward of the deformation front.

Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system

Abstract The southern Ryukyu island arc system is located at a convergent plate margin where the Philippine Sea Plate is subducting under the Eurasia Plate. We have conducted multi-channel seismic reflection surveys to study tectonic evolution and backarc rifting of the southern Ryukyu island arc system using the R/V Tansei-maru of the Ocean Research Institute, University of Tokyo, in June 1993 and 1994. We describe systematically a complete cross-section from the East China Sea continental shelf to the Ryukyu trench from the viewpoint of seismic stratigraphy. Seven major seismic units and three stages in the tectonic evolution of the system are identified: (1) during stage 1 from Late Miocene to earliest Pleistocene, pre-rift deposits of the Shimajiri Group accumulated over a wide region from the East China Sea continental shelf to the forearc region; (2) stage 2 is defined by a series of tectonic processes involving crustal doming, erosion, subsidence, and sedimentation, in association with initial rifting of the southern Okinawa Trough during most of Early Pleistocene time; and (3) the backarc rifting is still in progress and syn-rift sedimentation has been under way since the Late Pleistocene (stage 3). A new significant observation lies in the fact that the Pliocene Shimajiri Group is not distributed in most of the present-day southern Okinawa Trough. This implies that the backarc rifting of the southern Okinawa Trough was probably initiated after the deposition of the Shimajiri Group, that is, during the Early Pleistocene. Crustal-scale simple shear allowing an asymmetrical half-graben structure, rather than pure shear, governed the initial rifting of the southern Okinawa Trough. The possibility, however, cannot be excluded that pure shear associated with symmetrical rifting may be predominant in the present-day rifting of the southern Okinawa Trough as a result of northwestward migration of the rifting axis. The average extension rate of the southern Okinawa Trough is estimated to be approximately 1–2 cm/year on the basis of fault geometry observed on the acoustic basement.

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

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.

An anomalously thick layering of the crust of the yamato basin, southeastern sea of japan: the final stage of back-arc spreading

Abstract The Yamato Basin is believed to have been formed by back-arc spreading of the Japanese Islands. A seismic velocity structure beneath the basin was modeled on the basis of seismic data acquired during the DELP-85 Wakashio Cruise conducted by the Japanese Research Group of the International Lithosphere Project (ILP). The dataset consists of multichannel reflection profiles, airgun-shot refraction profiles recorded by ocean bottom seismometers (OBSs), and explosion refraction profiles, also recorded by the OBSs. The model indicates that the crustal layering is twice as thick as in a normal oceanic basin but the velocity in each layer and that of the underlying uppermost mantle is very similar to that of oceanic basin crust. The crust is 14 km thick at the middle of the oblong basin along its strike and thinner towards the rim of the basin, tending toward 11 km. Stratigraphy of the sedimentary layer and the acoustic basement suggest that volcanism during the late stage of spreading is responsible for the thick layering and the seamount chain located along the spreading axis of the basin.