Vincent Renard

Submersible observations at the axis of the ultra-fast-spreading East Pacific Rise (17°30′ to 21°30′S)

Abstract Manned submersible observations conducted with the diving saucer “Cyana” at the axis of the East Pacific Rise at four sites near 21°30′, 21°10′, 18°30′ and 17°30′S demonstrate that the ridge evolves between two end-members. The volcanic end-member is characterized by a non-rifted or only slightly rifted topographic ridge with widespread but diffuse water venting. The tectonic end-member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition. At one site, a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall. The dives illustrate the discontinuous nature of volcanic, tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean.

Magnetic anomalies in the Shikoku Basin: a new interpretation

Kaiko surveys over the Nankai Trough made available new magnetic and structural data for the northern Shikoku Basin. A survey of the oceanic lithosphere subducting below Southwest Japan along the central Nankai Trough revealed the existence of several north-south basement troughs. They are probably transform faults related to a north-south spreading system. We exanune the possibility of a late phase of north-south spreading limited to the axial northernmost Shikoku Basin, active between 14 and 12 Ma. If this system was already active before that time, i.e. during the N55 ° opening of the southeastern basin, then a triple junction should be found between both areas. Based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin. From early east-west rifting to late north-south spreading, opening of the Shikoku Basin proceeded through multiple episodes of spreading. The analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space. Two successive counter-clockwise rotations of the spreading direction are postulated, at anomaly 6 (19 Ma) and at anomaly 5B (14 Ma), involving segmentation and rotation of the spreading ridge.

Strain distribution over the east Mediterranean ridge: A synthesis incorporating new Sea-Beam data

Abstract We present the results of several multi-narrow-beam (Sea-Beam) traverses of the Mediterranean ridge on board R.V. “Jean Charcot”. These results can be considered to be complementary, in resolution and coverage, to the previously published long-range side-scan sonar (Gloria) results. A characteristic of the Mediterranean ridge is the presence over about half of its surface of an extensive fold system affecting the Messinian and Plio-Quaternary cover, which is dissected by conjugate strike-slip faults. The resolution of Sea-Beam was used to map the main characteristics of folding and faulting. This analysis, together with previously available data, enables us to define the strain pattern over the Mediterranean ridge. We thus now have an overall view of the strain pattern from a marginal basin (Aegea) through the Hellenic trench and adjacent sea-floor to the opposite continent. The bottom of the Hellenic trench is a boundary between an extensional domain to the north and a compressional one to the south. The southern domain is strongly affected by the presence of the Lybian promontary. It is related to the formation of an accretionary ridge due to the inability of the upper 3–4 km of sediments to subduct. This ridge is similar to the accretionary prism but it is much wider with respect to its height, probably because of the existence of a decollement over a level of overpressure. The Hellenic trench then is in the situation of a fore-arc basin. Its origin is due to the fact that the Aegean continental slope is not vertical but rather has a small 4° dip. As a result, a sedimentary wedge, symmetric but shorter than the main accretionary wedge develops over the buried portion of the slope. It is characterized by landward thrusts. Such an origin probably applies to fore-arc basins in general.

Tectonics of the hellenic trench: A synthesis of sea-beam and submersible observations

Abstract Detailed bathymetric investigations were performed in four zones of the Hellenic trench system in 1978, using the multi-narrow beam echo-sounder “Sea-Beam” on R.V. “Jean Charcot”. In 1979, a field-study using the 3000 m submersible “Cyana” was conducted in three of the four sites covered by Sea-Beam. We summarize here the principal tectonic results of these studies and publish the corresponding Sea-Beam maps. It is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall. In addition, there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the Mediterranean ridge. We suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall. We show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m.y. ago.

Detailed geological mapping by submersible of the East Pacific Rise axial graben near 13°N

An intensive exploration of the East Pacific Rise spreading center was conducted in two areas of restricted size near 13°N: site A (0.75 km2) and site B (0.15 km2). The two sites were chosen for their different general appearance based on data acquired previously. A large number of submersible dives (“Cyana”) at site A (21) and site B (6), has permitted the construction of morpho-structural maps at a scale of 1/1000. This paper compares the two areas on the basis of their structure, morphology, lava types, hydrothermal activity and mineral associations. The observed variations are interpreted as the expression of different stages of a cyclic evolution of the axial zone of mid-ocean ridges in contrast to the steady-state model. In this evolution, a tectonic phase leads to the creation of the axial graben by progressive collapse of slabs separated by fissuring of the rise. After extension of the graben, the tectonic phase is followed by a volcanic phase which partially or locally fills in the central graben with volcanic effusions. Ultimately, a dome replaces the central graben with lava flowing down the flanks of the rise. Hydrothermal activity does not appear to be preferentially linked to a given stage of this cyclic evolution. This evolution of alternate tectonic and volcanic cycles best explains observations at sites A and B, which are only 5.5 km apart. Possible magmatic processes responsible for these cycles are discussed.

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.

From subduction to transform motion: a seabeam survey of the Hellenic trench system

Abstract Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.

Nankai Trough and the fossil Shikoku Ridge: results of Box 6 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the Nankai Trough where the fossil Shikoku Ridge is subducted below southwest Japan. The geometry of the oceanic lithosphere bending under the margin as well as the three-dimensional structure of the accretionary prism have thus been determined in detail. Three 350° trending, probably transform faults have been identified in the area of the survey. They do not extend further south and appear to be limited to the last phase of spreading within the Shikoku Basin, probably between 15 and 12 Ma; this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east-west to N 350°. The two eastern transform faults limit a zone of reduced Nankai trench fill of turbidites opposite to the Tosa Bae Embayment. This observation suggests that the Tosa Bae Embayment actually results from this reduced supply of trench fill to the imbricate thrusting process. The accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts, the Lower and Upper Main Thrusts. Surface shortening is limited to the lower accretionary prism south of the Upper Main Thrust (UMT) whereas uplift with possible extension characterizes the prism above the UMT. Deformation, due to the relative plate motion, mostly affects the lower accretionary prism south of the UMT.

Oblique and near collision subduction, Sagami and Suruga Troughs —preliminary results of the French-Japanese 1984 Kaiko cruise, Leg 2

Abstract Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago.

SINGLE-BUBBLE MARINE SOURCE OFFERS NEW PERSPECTIVES FOR LITHOSPHERIC EXPLORATION

Abstract We present deep penetration seismic time sections obtained in the last two years with the “single-bubble” pulse generating method using marine seismic airguns. Whole crustal penetration and resolution have been obtained on the Ionian quasi oceanic basin and its margin, on the continental crust of the Aegean Sea, on the continental margins, and oceanic basin of the Gulf of Biscay. We show that this method is more efficient than others in radiating the lower frequencies returned by deep structure, without loosing resolution. Hence it allows deep crustal imaging from relatively low-power academic ships, and if the method is applied to powerful industrial vessels, it may open the subcrustal lithosphere to imaging.