Jean-Paul Cadet

Geodynamics of the Ogcheon Belt (South Korea)

Abstract The Ogcheon Belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early Palaeozoic rift which was affected by polyphase tectonics. Its pre-Cretaceous structure results from three main orogenic events: the “Ogcheon” (late Silurian-Devonian (?)), Indosinian (late Permian-Triassic) and Yenshanian (middle Jurassic-early Cretaceous) tectonic phases. The external, autochthonous or para-autochthonous units represent platform zones with shallow-water deposits whereas rift facies characterize the internal domain. The Yeongweol sequence, forming an outer shelf on the southeastern margin of the rift, displays affinities with the Jiangnan basin of South China. It is in wrench fault contact with the easternmost Duwibong sequence correlated with the North China-North Korea platform. In the internal metamorphic units, post-Cambrian-early Ordovician pelitic and turbiditic rift metasediments overlie a thin Cambrian or Cambrian-Ordovician early platform sequence. Two subdomains are distinguished: the upper units (Chung Ju and Pibaryeong units) representing the deep “axial” part of the basin contain distal flyshoids and large olistoliths derived from the northwestern “narrow” margin; the lower units (Turung San, Poeun and Iwharyeong) are related to the “wide” margin located between the deep rift domain and the stable southeastern platform. The latter transitional domain is characterized by thick submarine debris flows, and volcanics of alkali or transitional tholeiitic affinity. The volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting. Lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the Cambrian-Ordovician platform sequence in one single basin. The post-middle Silurian-pre-mid-Carboniferous “Ogcheon” ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area. It gave rise to an intracontinental fold-thrust belt without ophiolites. This tectonic event may have been contemporaneous with early Palaeozoic tectonism in the Quinling Belt in Central China and in the Southeast China fold belt. The definitive constitution of the Sino-Korean craton took place before the mid-Carboniferous and “molasse” sedimentation was initiated on a single platform located on the western edge of the Yakuno oceanic basin of Southwest Japan. In late Permian and early Triassic times, the closure of the oceanic area and the subsequent westward collision of the Honshu Block are marked in Korea by the intracratonic “Indosinian” tectonism. Since late Triassic times, predominantly transcurrent movements have occurred along NNE-SSW faults associated with syntectonic intermontane troughs and important granitization.

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?

Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3

Abstract A detailed topographic and geophysical survey of the Daiichi-Kashima Seamount area in the southern Japan Trench, northwestern Pacific margin, clearly defines a high-angle normal fault which splits the seamount into two halves. A fan-shaped zone was investigated along 2–4 km spaced, 100 km long subparallel tracks using narrow multi-beam (Seabeam) echo-sounder with simultaneous measurements of gravity, magnetic total field and single-channel seismic reflection records. Vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported. The northern and southern extensions of the normal fault beyond the flank of the seamount were delineated. Materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount. Canyons observed in the upper landward slope terminate at the mid-slope terrace which has been uplifted since start of subduction of the seamount. Most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern Japan Trench wall. This survey was conducted on R/V “Jean Charcot” as a part of the Kaiko I cruise, Leg 3, in July–August 1984 under the auspices of the French-Japanese scientific cooperative program.

Genetic relations between the central and southern Philippine Trench and the Sangihe Trench

We surveyed the junction between the central and southern Philippine Trench and the Sangihe Trench near 6oN using swath bathymetry, gravity, and magnetics. These data, along with seismicity, allow us to discuss the genetic relations between these trenches and the forces acting on converging plates. Our final model favors the northern extension of the Halmahera Arc up to 8oN, with three segments offset left-laterally along NW-SE transform faults. Accretion of the northern segment to Mindanao Island 4 to 5 m.y. ago resulted in the failure within the Philippine Sea Plate east of the arc. Initiation of the Philippine Trench between 7oN and 10oN agrees with the maximum recorded depth of the Philippine Trench floor (10,000 m below sea-level) and Philippine Sea slab (200 km). South of 6oN (trench junction), another segment of the arc is being subducted beneath the Sangihe margin, while south of 3oN, the southern segment of the Halmahera Arc is still active. The rapid southward shallowing of the trench floor along the southern Philippine Trench, the type of faulting affecting both sides of the trench, the lack of significant interplate seismicity, and the concentration of the seismicity beneath the Miangas- Talaud Ridge are interpreted as a slowing down of the subduction along this branch of the Philippine Trench compared with the rest of the subduction zone. The Sangihe deformation front has been recognized up to 7oN but seems active only south of 6oN.

Regional seismicity and on-land deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough

The stress field evolution and the kinematics of opening of the Okinawa Trough are investigated on the basis of earthquake focal mechanisms and structural data in the Ryukyu arc and the Okinawa Trough. Focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions: a regional arc-perpendicular extension and a local arc-parallel extension. Both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal σ2 and σ3 axes. Earthquake slip vectors reveal a southward motion of the Ryukyu arc with respect to the south China block. The current pole of opening of the Okinawa Trough is located around 16°N and 50°E. Fracture analysis in Okinawa island allows identification of three episodes of extension: a late Miocene N40°W to N20°E extension (episode I), a late Pliocene to early Pleistocene N20°E extension (episode II), and a latest Pleistocene to present-day N20°W extension (episode III). Episodes II and III are characterized by permutations between the two horizontal σ2 and σ3 axes. By synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the Okinawa Trough since the late Miocene. The direction of divergence of the Ryukyu arc has rotated clockwise from ∼N150°E in the late Miocene to nearly N-S today.

Seafloor manifestations of fluid seepage at the top of a 2000-metre-deep ridge in the eastern Nankai accretionary wedge: Long-lived venting and tectonic implications

Abstract The results of the 1989 Kaiko-Nankai diving cruise to the eastern Nankai accretionary wedge show that a narrow corridor on the top of a 2000 m deep ridge is characterized by concentrated surface manifestations of fluid seepage and chaotic outcrop orientations, suggesting active faulting along a right-lateral shear zone. Modern biological indicators of fluid venting include bivalve vesicomyid species and vestimentiferan tube worms. Ancient manifestations of fluid venting consist of similar vesicomyid species found in 20 and 150 ka concretions. This suggests relatively constant biotypes. Carbonate deposits including fossil cemented shells were observed near active sites, either as “paving stones” (diffuse seepage and cementation) cut by channels or as pinnacles (local seepage and cementation). Despite its proximity (less than 15 km), a nearby 3800 m deep site, where substantial fluid discharge occurs, seems to be completely separate from the 2000 m deep ridge site. The 3800 m deep site differs from its shallower counterpart in terms of its seafloor manifestations and the composition of its fluids. The sediment at the top of the ridge ranges in age around 500 ± 300 ka, which attests that the transition from a depositional to an erosional stage was younger than 800 ka but older than 200 Ka. This transition can be due to either the incorporation of this unit from the trench fill into the accretionary wedge front or the uplift of a slope basin. The first hypothesis leads to a 2.5 cm/yr minimum progradation rate associated with a 2.5 mm/yr minimum uplift rate from the paleotrench to the present ridge. The second hypothesis implies a recent uplift of a “piggy-back” basin at a minimum rate of 1 mm/yr.

The pre-Cretaceous deep-seated tectonics of the Abukuma massif and its place in the structural framework of Japan

Abstract The structures and microstructures of the Takanuki and Hitachi areas in the Abukuma massif, Northeast Japan are described. In the Takanuki area, the basic Gosaisho series thrusts the pelitic Takanuki ones in a HP metamorphic context. The nappe structure is afterwards refolded by a migmatitic dome: the Samegawa dome, in a HT metamorphic context. Microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation. Geometric features suggest that the Samegawa dome was emplaced by diapirism. The role of the thrust surface as an instable interface promoting the doming is emphasized. The Hitachi metamorphic rocks composed of basic schist, limestone and sandstone shist thrust the pelitic rocks of the western Hitachi gneisses. As for the Takanuki area, the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement. Owing to lithologic, petrologic, structural similitudes, the nappe of the Hitachi metamorphic rocks and that of the Gosaisho series are unified into a unique nappe with a northward motion. The emplacement occurred between late Permian and late Cretaceous likely in late Jurassic. The allochthonous units of the Abukuma massif are correlated with the Green Schist nappe described in Southwest Japan, since they are surrounded by the same zones, namely the Tanba zone and the Kurosegawa-Kitakami one. Moreover both in Southwest and Northeast Japan, the emplacement of the Green Schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport, that is eastward in Southwest Japan and northward in Northeast Japan, owing to the late bending of the Japanese Islands. The late Jurassic nappe structure is obliquely overprinted by a HT metamorphism, Ryoke in Southwest Japan, Abukuma in Northeast Japan, and afterwards cut by late faults as the Median Tectonic Line or the Tanakura fault, giving rise to the present complexity.

Project Kaiko—Introduction

The Kaiko project was a French-Japanese cooperative scientific effort to investigate the structures and geodynamic processes in the deep trenches surrounding the Japanese islands. Seabeam mapping, as well as detailed geophysical investigation of eight selected areas in the Nankai, Suruga and Sagami Troughs and in the Japan Trench, including the trench triple junction, were performed in 1984. Twenty-seven manned dives, to a maximum depth of 6000 m, were made over five of these areas at carefully selected sites in 1985. The main results of the project are presented in this special issue together with a set of coloured bathymetric maps and block diagrams. They illustrate a great diversity of tectonic styles, from the turbidite-filled trenches of the Shikoku Basin with a well developed accretionary prism, to the much deeper Japan Trench with widespread evidence for large-scale slumping and erosion over the inner wall. Special insight has been gained into the processes involved in the subduction of seamounts and of volcanic ridges. Benthic communities, associated with chemosynthetic processes, were discovered at several locations during manned dives in the Nankai Trough as well as in the Japan Trench. They thus appear to be a common feature of deep-sea trenches. Thermal, geochemical and biogeochemical measurements, coupled with tectonic observations, indicate that they are related to overpressure fluid advections along zones of active surface deformation.

Subduction in the Hellenic Trench: probable role of a thick evaporitic layer based on Seabeam and submersible studies

Summary A Seabeam survey of the Hellenic Trench in 1978 was followed by a submersible survey in 1980. Fifteen dives were completed between 1500 and 3000 m covering 58 km on the bottom and obtaining 48 samples. Thus, the Hellenic Trench is probably now the most intensively studied trench as previous work included seismic reflection, dredging, coring, bottom photography, side-scan sonar and drilling. Because of the high sedimentation rate and the relatively shallow depth, we did not expect outcrops except along faults and in canyons. Active tectonics with numerous faults and folds have been observed. But we have also discovered vertical cliffs with hard rock exposure over a depth range of 1000–1500 m. In this paper, we discuss the nature of the hard rock outcrops observed both on the inner wall and on two hills that we interpret as diapirs. We conclude that at least part of these hard rocks are probably Messinian evaporites and concentrate on the tectonic role of this evaporitic layer in the Hellenic subduction. It is likely that the evaporites do not subduct but tend to pile up in the trench, to form a floating evaporitic basin.

Shear partitioning in the eastern Nankai Trough: evidence from submersible dives

Abstract Submersible dives over the wedge of the easternmost Nankai Trough demonstrate the existence of a left-lateral strike-slip system parallel to the subduction zone. This system develops as the trench, turning northward toward land, becomes highly oblique to the motion. Although shear partitioning is well known in other subduction zones, the process here has the peculiarity of continuing northward into land.