Guy Pautot

La Mediterranee occidentale depuis l'Oligocene Schema d'evolution

-The evolution of the Western Mediterranean basin during Tertiary time is discussed. It is proposed that it was created by a continental drift process during Middle Oligocene time, synchronously with the creation of the grabens over the continents. The significance of the Messinian evaporitic sedimentary episode is discussed in liaison with the problem of large scale subsidence which has been at least six kilometers since Oligocene time.

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.

Surficial structures of the northern Red Sea axial valley from 23° N to 28°N: time and space evolution of neo-oceanic structures

Abstract During recent French cruises in the northern Red Sea, the surficial structures of the axial valley were studied by means of bathymetry, magnetics and single channel reflection seismics. New compilation maps have been established and comparison of the various data allow different segments of the northern Red Sea axial valley to the distinguished where only intra-evaporitic deformations (24–25 °N), large volcanic outcrops (Mabahiss deep, 25°30′–26°N) or several recent isolated volcanic intrusions (26–27 ° N) characterize different stages of evolution. This new information allows reconstruction of the evolution of oceanization cells from a simple intra-evaporitic deep to a limited oceanic rift zone such as those observed in the central Red Sea. A major characteristic of this evolution, also depicted in other young oceanic rifts, is a succession of volcanic and tectonic phases. The major event in the northern Red Sea axial valley is a recent episode of voluminous but well localized magmatism. It would represent a new stage in the Red Sea evolution and thus may have some implications on earlier phases.

Seabeam and seismic reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S)

Abstract Marine geophysical surveys employing Seabeam, multi- and single-channel seismic reflection, gravity and magnetic instruments were conducted at two locations along the continental slope of the Peru Trench during the Seaperc cruise of the R/V “Jean Charcot” in July 1986. These areas are centered around 5°30′S and 9°30′S off the coastal towns of Paita and Chimbote respectively. These data indicate that (1) the continental slope off Peru consists of three distinct morpho-structural domains (from west to east are the lower, middle and upper slopes) instead of just two as previously reported; (2) the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope; (3) the upper half of the lower slope appears to represent the product of mass wasting; (4) thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass-wasting are overprinted by a compressional tectonic fabric; (5) a change in the tectonic regime from tensional to compressional occurs at the mid-slope-lower slope boundary, the accretionary prism being restricted to the very base of the lower slope in the Paita area. The Andean margin off Peru is an “extensional active margin” or a “collapsing active margin” developing a subordinated accretionary complex induced by massive collapse of the middle slope area.

Deep-sea submersible survey in the Suruga, Sagami and Japan Trenches: preliminary results of the 1985 Kaiko cruise, Leg 2

Abstract Nine submersible dives were made in three trenches off central Japan, between 2990 and 5900 m of water depth. Our observations confirm the interpretation that Daiichi-Kashima Seamount is a Cretaceous guyot formed on the Pacific plate that has traveled into the Japan Trench. We also confirmed the previous interpretation of a large normal fault that splits the seamount in two halves, the lower one being now subducting beneath the Japan margin. Compressional deformation was identified within the lower part of the inner slope in front of the seamount. The pattern of deformation that affects Quaternary sediments is in agreement with the present kinematics of the convergence between the Pacific plate and Japan. Deep-water (5700 m) clam colonies are associated with advection of fluids, driven by the subduction-related overpressures. In the northern slope of the Boso Canyon, along the Sagami Trough system (Philippine Sea plate-Japan boundary), the deformation affecting a thick upper Miocene to lower Pliocene sequence indicates two directions of shortening: a N175°E direction which is consistent with the present relative motion along the Sagami Trough (N285–N300°E) and a N30°E direction which could be related to a more northerly direction of convergence that occured during the early Quaternary and earlier.

Constraints of Sea Beam data on crustal fabrics and seafloor spreading in the South China Sea

Abstract The South China Sea is a mid-late Tertiary marginal basin. The magnetic anomaly lineations in the eastern part of the basin trend approximately east-west [1,2], suggesting a north-south direction of spreading. In the spring of 1985, two cruises on the French research vessel “Jean Charcot” provided Sea Beam coverage, seismic reflection, magnetic and gravity profiles. The Sea Beam data exhibit two major structural trends: scarps striking N50°E± 15° , interpreted as normal faults, and scarps striking N140°E± 15° , interpreted as fracture zones. This fabric implies a northwest-southeast direction of spreading, up to about 100 km north and south of the inferred spreading axis [3]. Dense Sea Beam coverage of a roughly 1° square area northwest of the Scarborough Seamounts chain shows that the emplacement of these seamounts was, at least in its initial stage, controlled by faulting in two orthogonal directions, N50°E and N140°E. Magnetic and gravimetric maps of the same area also reveal anomalies trending roughly N50°E, which are disrupted by transform zones striking N140°E. This detailed study indicates that the fracture zones may be closely spaced (less than 20–30 km) east of Macclesfield Bank. Furthermore, magnetic anomalies identified as 6 and 6a (20 Myr) along two north-south profiles located at both edges to the north of this detailed study area may be correlated with the N80°E trend characteristic of such anomalies in the eastern part of the South China Sea. The east-west trend of magnetic anomalies 6 and 6a, south of Scarborough seamount chain, recognised by Taylor and Hayes (1983) [1] is incompatible with the trend of the fault scarps observed on Sea Beam data. We infer that progressive, right-lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east-west average trend of some of the magnetic anomalies, and of the Scarborough seamount chain, which represents the location of the relict spreading axis, in spite of a N50°E spreading direction. The whole central part of the basin east of 115°E may have formed in this way, since N130–140°E striking fracture zones are observed on the Sea Beam swaths. This model may correspond to the second of two successive spreading phases, with N-S and NW-SE directions of extension respectively, as was presented by the authors in a previous paper (Pautot et al., 1986 [3]). Alternatively, we may assume that the direction of extension did not change radically during the opening history [3], implying that oblique spreading occurred along N80°E trending ridge segments, creating the N80°E trending scarps and magnetic anomalies observed to the north and to the south of the study area. Such a direction of extension is compatible with mid-late Tertiary left-lateral movements along large N130–150°E strike-slip faults, such as the Red River and Wang Chao faults in South China and Sundaland.

Subduction of the South China Sea axial ridge below Luzon (Philippines)

Abstract The Scarborough Seamount chain, present at the axis of the extinct South China Sea spreading center, is being subducted obliquely along the Manila Trench. A detailed Seabeam survey of this convergent zone reveals that the fabric of the ridge is characterized by N60°E trending normal faults and N130°E transform faults. This ridge can be traced into the forearc area. This could be interpreted as the result of underplating of ridge fragments which are accreted to the upper plate.

Morphology, Limits, Origin, and Age of Salt Layer along South Atlantic African Margin

The west African continental margin between Abidjan (Ivory Coast) and Walvis Bay (Southwest Africa) was surveyed in 1971 by the R/V Jean Charcot. Fifty-eight seismic-reflection (flexotir source), bathymetric (3.5 and 12 kc), gravimetric, and magnetic profiles were obtained. The seaward limit of an evaporitic zone outlined during the survey is at the boundary between continental slope and continental rise. Variation of depth of this limit as a function of latitude shows the presence of a large offset at 11°S which seems to be related to an east-east-southeast line of magnetic seamounts cutting into the continental slope. The Annobon-Cameroun volcanic axis separates the salt zones of the Nigerian basin and Congo-Angola basin. The northern limit of salt deposition in th Congo-Angola basin is marked near 1°N by a strong southwest-northeast magnetic trend. On the south it extends to near 14°S. The Walvis Ridge, located farther south, does not seem to have an effect on evaporite deposition.

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.