Serge Lallemand

Japan Sea: a pull-apart basin?

Abstract Recent field work in the Hokkaido Central Belt and marine geology studies along the eastern margin of Japan Sea in addition to previously published data lead us to propose a new model of opening of the Japan Sea. The synthesis of both on-land and offshore structural data gives new constraints about the structural evolution of the system. The rhombohedral shape of the Japan Basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early Eo-Oligocene rifting of a pull-apart basin accommodated along two large right-lateral shear zones, east of Korea and west of northeast Japan and Sakhalin. It is followed, during Upper Oligocene/Lower Miocene, by the main opening of the Japan Basin as a mega pull-apart. Then a back-arc spreading probably related to the subduction process, induced the creation of the Yamato and Tsushima Basins at the end of Lower Miocene and in Middle Miocene. Clockwise rotation of southwest Japan larger than 20° or major bending of Honshu mainland deduced from paleomagnetic studies is unlikely at this time. Since 1 or 2 My B.P. to Present, compression prevails along the eastern margin of the Japan Sea. The generation of marginal basins as pull-apart basins along intracontinental strike-slip faults is a mechanism which has been proposed by other authors concerning the South China Sea, the question then is whether the fragmentation of the Asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one.

Subduction Zone Geodynamics

Subduction Zone Geodynamics.- A Review of the Role of Subduction Dynamics for Regional and Global Plate Motions.- Subduction with Variations in Slab Buoyancy: Models and Application to the Banda and Apennine Systems.- Continental Collision and the STEP-wise Evolution of Convergent Plate Boundaries: From Structure to Dynamics.- Seismic Tomography and Anisotropy.- Seismic Anisotropy of Subduction Zone Minerals-Contribution of Hydrous Phases.- Local Earthquake Tomography in the Southern Tyrrhenian Region of Italy: Geophysical and Petrological Inferences on the Subducting Lithosphere.- Great Subduction Zone Earthquakes.- Effect of Subducting Seafloor Topography on the Rupture Characteristics of Great Subduction Zone Earthquakes.- Great Earthquakes in Slow-Subduction, Low-Taper Margins.- Seismogenic Zone Characterization.- Convergent Margin Structure in High-Quality Geophysical Images and Current Kinematic and Dynamic Models.- Imaging Interseismic Locking at the Nankai Subduction Zone, Southwest Japan.- Continental and Ridge Subduction Processes.- Exhumation Processes in Oceanic and Continental Subduction Contexts: A Review.- Evolution of Subductions Indicated by Melanges in Taiwan.- Subduction of an Active Spreading Ridge Beneath Southern South America: A Review of the Cenozoic Geological Records from the Andean Foreland, Central Patagonia (46-47 S).- Configuration of the Colombian Caribbean Margin: Constraints from 2D Seismic Reflection data and Potential Fields Interpretation.

Arc-continent collision in Taiwan: New marine observations and tectonic evolution

Marine observations offshore of Taiwan indicate intense deformation of the Luzon arc-forearc complex, with episodic eastward migration of the active deformation front across the complex. The Philippine Sea Plate (PHS) began colliding with the Eurasian continental margin in Pliocene time. Because of the obliquity of plate convergence, the collision has propagated through time from north of Taiwan to the south with the more advanced stages being presently observed to the north, whereas the subduction of the oceanic lithosphere of the South China Sea beneath the PHS occurs to the south. Offshore, the collision zone is characterized by deformation of the arc including the forearc region to the south. This active tectonic domain absorbs a significant amount of shortening between the Eurasia margin and the PHS, which is moving towards N 310° E at about 8 cm/yr relative to Eurasia. Swath bathymetry and backscattering data, together with seismic reflection and geopotential data obtained during the ACT cruise onboard the R/V LAtalante, showed major north to south changes in the tectonic style in both the indenting arc and the host margin. In the southern domain, left-lateral transpression is recorded by deformed and folded series of the forearc domain that are unconformably overlain by collision-derived sediments of the Southern Longitudinal Trough (SLT). Today, the loci of deformation has jumped to the east and it is characterized by the growth of a sedimentary ridge (the Huatung ridge, rear portion of the former Manila oceanic accretionary wedge including forearc and intra-arc sequences), which overthrusts the basement of the island arc. In the northern domain, north of 22°30N, active westward thrusting of the Coastal Range (remnants of the island arc and forearc basins) over the Lichi melange develops onland along the Longitudinal Valley. Offshore, at the base of the eastern slope, prominent fault scarps suggest an active eastward thrusting of parts of the arc (volcanic edifices and intra-arc or forearc sediments) onto the oceanic crust of the Philippine Sea Plate. It accounts today for part of the convergence. The

Coulomb wedge model applied to the subduction of seamounts in the Japan Trench

Results from the French-Japanese Kaiko exploration program of the Japan Trench and complementary data are used to propose a model of subduction of seamounts. Examples in two seamount chains, one at the junction between the Japan and Kuril trenches and the other in the southern part of the Japan Trench off Inubo Cape, illustrate the stages of subduction. The Japan Trench margin evolved as a homogeneous wedge of deformable, noncohesive Coulomb material during seamount subduction. This is illustrated by the subduction of the Daiichi Kashima seamount in the southern Japan Trench. Our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge; this agrees with our observations in the Daiichi Kashima area. This thickening shifts landward across the wedge with the advancing flank of the seamount. Erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge. A reentrant of the toe of the wedge is thus created; it is largest when the oceanic flank of the seamount coincides with the base of the landward slope. At this time, the trench axis will probably have an abnormally large amount of sediments, as illustrated at the junction between the Japan and Kuril trenches. After the subduction of the seamount, the edge of the wedge should progressively move back to its initial location. Frontal accretion is absent or very limited.

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?

Subduction of the Daiichi Kashima Seamount in the Japan Trench

Lallemand, S., Culotta, R. and Von Huene, R., 1989 Subduction of the Daiichi Kashimi Seamount in the Japan Trench. In: J.P. Cadet and S. Uyeda (Editors), Subduction Zones: The Kaiko Project. Tectonophysics, 160: 231-247. In 1984-1985, the Kaiko consortium collected Seabeam, single-channel seismic and submersible sampling data in the vicinity of the Daiichi-Kashima seamount and the southern Japan trench. We performed a prestack migration of a Shell multichannel seismic profile, that crosses this area, and examined it in the light of this unusually diverse Kaiko dataset. Unlike the frontal structure of the northern Japan trench, where mass-wasting appears to be the dominant tectonic process, the margin in front of the Daiiclr-Kashima shows indentation, imbrication, uplift and erosion. Emplacement of the front one-third of the seamount beneath the margin front occurs without accretion. We conclude that the Daiichi-Kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin.

Deep scientific dives in the Japan and Kuril Trenches

Abstract In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible “Nautile”. The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2–3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.

High rates of arc consumption by subduction processes: Some consequences

New estimates of long-term consumption of Japanese and Peruvian continental fore-arc margins together with recent drilling data (Ocean Drilling Program) on some other erosional island arcs such as Izu-Bonin, Mariana, or Tonga allow the calculation of a mean rate of 7 ± 3 km/m.y. landward migration of the arc-trench system on both east and west sides of the Pacific basin during Cenozoic time. Such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the Atlantic Ocean. Retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the Pacific basin. Mass transfers in subduction zones must be reconsidered in the light of these new estimates, especially the balance between net growth and loss of continental crust in recent geologic time. A 50 m.y. period is long enough to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone, assuming a mean trench–volcanic-arc landward migration of 5 km/m.y. and a mean trench–volcanic-arc distance of 250 km. This circumstance thus raises the question of whether the Izu-Bonin-Mariana and Tonga arcs are older than their reported Eocene age.

AN INTRODUCTION TO ACTIVE COLLISION IN TAIWAN

This issue contains a collection of twelve papers on Active Collision in Taiwan (ACT) presented at the 3rd Sino-French symposium in Taipei, Taiwan, 22-23 March 1995. Previous Sino-French symposia on earth sciences have been held in Taipei (1984) and Paris (1988). Proceedings have been published in special issues of Tectonophysics in 1986 (Angelier et al., 1986) and 1990 (Angelier, 1990). Unlike the previous Sino-French symposia, which emphasized the eastern Taiwan margin, the 45 (oral and poster) papers presented at the 3rd meeting were focused on Taiwan geodynamics. The conference took place just before two recent major marine geology and geophysics cruises, aimed at improving our knowledge of the geology of Taiwan and surrounding seas. Thus, results of the geophysical cruises, conducted aboard R/V M. Ewing (deep-seismic reflection and OBS) in September 1995 and R/V LAtalante (swath-mapping and seismics) in June 1996, are not mentioned in this volume. A fourth Sino-French symposium will be organized in France in 1998 to present the main results from these oceanographic cruises, as well as the recent developments of onland geology. For many authors, the Taiwan mountain belt is the result of an extremely recent arc-continent collision

Geodynamic setting of Izu-Bonin-Mariana boninites

Abstract The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. The study of the Cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the Tonga and Fiji regions lead us to define three tectonic settings that favour the formation of boninites in back-arc basins in addition to previous settings that involve the presence of a mantle plume: (1) propagation at low angle between a spreading centre and the associated volcanic arc; (2) intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc; and (3) intersection at a right angle between an active spreading centre and a newly created subduction zone. A geodynamic model of the Philippine Sea Plate shows that boninites in the Bonin Islands are related to the second mechanism mentioned above, whereas Mariana forearc boninites are relevant to the third mechanism. In the early Eocene, the transform plate boundary bounding the eastern margin of the Philippine Sea Plate at the location of the present-day Mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the West Philippine Basin, somewhere around 43–47 Ma. The presence of a mantle plume in the vicinity of the subduction zone bounding the northern IBM arc explains boninites that erupted in its northern part, but only in early Eocene time.