Siegfried Lallemant

Geodetic determination of the kinematics of central Greece with respect to Europe: Implications for eastern Mediterranean tectonics

We use a new satellite laser ranging/Global Positioning System (SLR/GPS) solution at seven sites in Anatolia and Aegea to obtain a better definition of the extrusion motion of the Anatolian-Aegean block with respect to Europe. We show that this motion can be described in a first approximation by a counterclockwise rotation which transfers most of the motion of Arabia to Anatolia. We combine 78 displacement vectors obtained at common points of two triangulation nets measured in central Greece in 1895 and 1975 with the SLR/GPS measurements to compute the velocity field over Greece with respect to Europe. These measurements indicate that central Greece is a zone of extension between the Anatolian-Aegean counterclockwise rotation to the south and the northern Greece clockwise rotation to the north. This extension is principally localized within the Gulf of Corinth to the east but is distributed to the west. We then extrapolate this velocity field to the whole Aegea and western Anatolia using recently published GPS results as well as the SLR results. The narrow dextral North Anatolian fault, which limits the velocity field to the north, progressively gives way to a much wider boundary zone where extension becomes dominant. We show that the collision between the Mediterranean Ridge and Africa began 3-6 Ma, and we describe the modifications that this collision has produced on the kinematic pattern both in Aegea and on the Mediterranean Ridge.

Sediment deformation and hydrogeology of the Nankai Trough accretionary prism: Synthesis of shipboard results of ODP Leg 131

The main objective of Leg 131 was to provide data on the deformational processes and associated hydrogeology of the Nankai prism toe. Drilling succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, reaching 1327 mbsf (metres below seafloor) with good core recovery (55%). Excellent correlation of the lithology and structure, including the frontal thrust and the decollement, with seismic reflection images was also determined. Bedding dips, faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to NW-SE shortening, as expected from the plate tectonic convergence vector. Below the decollement, no significant deformation features were observed, indicating that the decollement is a sharp discontinuity in stress transmission. Physical properties data show major discontinuities at the decollement, notably an increase in porosity below the later. This may indicate excess pore pressure in the subducted section and decollement zone. A less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, and some calibration of laboratory physical properties toin-situ conditions has been provided, andin-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flows is inconclusive. No sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found. Thermal measurements are not significantly different from those predicted by a purely conductive heat flow model. A signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events. We conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site.

Fluid flow in and around a mud volcano field seaward of the Barbados accretionary wedge: Results from Manon cruise

A field of mud diapirs and mud volcanoes situated in the Barbados trench at 13°50′N and extending along an old oceanic fracture zone (Mercurus) was investigated during the Manon cruise using both surface ship and Nautile submersible sampling and measurements. The entire zone from 13°50′N up to 14°20′N has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide. Two structures interpreted as diatremes, Atalante and Cyclops, expell large amounts of water and methane. We propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area. However they expell only a small fraction of the incoming fluid, implying that disperse flow is the dominant mode of expulsion in this area. The chemoautotrophic communities on the surface of the structures rely mostly on sulfides. Submersible observations, temperature measurements in the sediment, and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high-permeability channels, such as cemented carbonate conduits. We propose that this convection is driven by the density difference between the pore fluid and seawater, but fresh water released by the dissolution of shallow hydrates may also contribute. This shallow convection may be a frequent process in cold seep environments.

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Abstract Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.

Zenisu Ridge : A deep intraoceanic thrust related to subduction, off Southwest Japan

Abstract During the French-Japanese Kaiko project, Seabeam, seismic and submersible observations were made in the eastern part of the Nankai subduction zone, close to the area of collision between the Izu-Bonin island arc and the Japan margin. The most prominent feature is the Zenisu Ridge, an elongated relief of the Philippine Sea plate running parallel to the Trench. Magnetic anomalies indicate that the crust of the Zenisu Ridge is a part of the Shikoku oceanic basin formed in the Early Miocene, 23 Ma ago and presumably uplifted at a later stage. Structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics. Mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust, both segments being of equal magnitude (vertical offset about 1 to 1.5 km). Geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth. The overall structure of the surrounding area reveals a compressive deformation zone widening toward the east, the magnitude of the compressive deformation decreasing westward as well as southward of the Zenisu Ridge.

Spatial transition from compression to extension in the Western Mediterranean Ridge accretionary complex

The Mediterranean Ridge has been described since 1982 as an accretionary prism related to the Hellenic subduction zone. However, little attention has been paid to the actual mechanical boundary between the sedimentary wedge and the crustal backstop of the outer Hellenic arc. Using seismic velocity structures from recent ESPs (Pasiphae cruise), we propose a new interpretation of existing geological and geophysical data around the western Hellenic trenches. The velocity structure of the inner unit of the Mediterranean Ridge suggests that the Late Neogene to Quaternary sequences were deposited above a high-velocity rigid body. We propose to correlate the boundary of this rigid body (backstop of the accretionary wedge) with a series of oblique reflectors dipping to the southwest, near DSDP sites 126 and 377. The backstop extends further south than previously thought and decreases by one third the total width of the compressional wedge; it has thus major implications on the volume of accreted material. The short-wavelength folding of Plio-Quaternary sediments observed on the outer wall of the South Matapan trench is probably a consequence of gravity-gliding layers above the high velocity body, gently dipping (2°) to the northeast using the Messinian evaporite as a decollement. On the other hand, the Hellenic trenches appear to be narrow half-grabens affected by the widespread extensional deformation active on the outer Hellenic arc. They are situated on the landward boundary of a wider and poorly sedimented fore-arc basin. We conclude that extensional deformation extends at least over the width of the Hellenic trenches, on the continental side of the backstop, whereas compressional deformation stops near the edge of the backstop itself, 100 km to the southwest.

Submersible study of mud volcanoes seaward of the Barbados accretionary wedge: sedimentology, structure and rheology

Abstract In 1992, the Nautile went to a mud volcano field located east of the Barbados accretionary wedge near 13 ° 50N. Using nannofossil analysis on cores, we determined the sedimentation rate, and provided a new estimation of the age of the mud volcanoes (750,000 years for the oldest one). Six structures have been explored with the submersible Nautile, and manifestations of fluid venting (chimneys, carbonate cementation and chemosynthetic communities) were observed on all. Sedimentological analysis identifies two sources of diapiric mud. Most mud volcanoes expel mud containing Late Miocene to Quaternary faunae that have the same composition as sediments drilled above the Barbados wedge decollement. One volcano also contains older Oligocene taxa, with a mud composition corresponding to the sedimentary sequence below the decollement. We use diving observations to map the fine-scale morphology, the distribution of chemosynthetic fauna and define two end-member types of structures: mud-pies (flat topped mud volcanoes) and conical mounds. Mud-pies (Atalante and Cyclops) are characterised by the presence of a lake of highporosity mud (70% to 75%) in their central parts. Chemosynthetic benthic communities ( Calyptogena colonies and sponge bushes) are concentrated in the outer parts. Contrasting morphologies of the two mud-pies indicate different stages of activity: Cyclops is growing whereas Atalante is collapsing. Expulsion of water and methane occurs mostly through the mud lake and may be stronger during the collapse phase. On conical mounds there are no mud lakes, fluid venting concentrates near the summit and occurs through carbonate cemented chimneys which form within the sediment. Viscosity measurements have been carried out on mud samples from the two mud-pies and one conical mound. All mud samples have a plastic fluid behaviour, the plastic threshold decreases with porosity, and thixotropy is observed for a porosity of more than 70%. An analogue experiment shows that for this thixotropic mud, shearing in the feeding conduit liquefies the mud which then spreads to form a mud-pie. Conical mounds form when the mud remains plastic. We show that the dissociation of methane hydrate is the cause of the high porosity in mud-pies and confirm that these structures are a consequence of large-scale dissociation of methane hydrate at the base of its stability field. Dissociation of hydrates before and during ascent is only slightly contributing to the pore fluid in conical mounds, but solid hydrates still present in the mud may contribute to its buoyancy.

The Mediterranean Ridge backstop and the Hellenic nappes

The core of the Mediterranean Ridge backstop consists of a pile of Hellenic nappes that migrated outward from the Aegean continent within the adjacent Mediterranean basins during late middle Miocene, about 15 Ma, and the present Mediterranean Ridge has developed since that time by accretion of a new wedge. The arguments we use are: (1) a similarity in thickness, seismic velocity and structure between the backstop and the Hellenic nappes in southern Peloponnesus; (2) the geometry of the westward limit of the backstop that is the one expected from the amount of relative displacement of the nappes on land; (3) an agreement between the estimated ages of the present accretionary wedge and the period of activity of two major faults connecting the backstop to the adjacent Aegean continent. The seaward limit of the backstop is situated about 170 km southwest of western Crete where the displacement of the nappes is maximal and it is close to the margin near the Ionian islands where their displacement is small. We assume that the Hellenic subduction zone migrated southwestward relative to Eurasia during middle Miocene and that the corresponding gravity collapse of the Aegean continent was maximum at this time because the westward extrusion of Anatolia had not yet started.

Fluid venting along Japanese trenches: tectonic context and thermal modeling

Abstract Large benthic chemosynthetic communities have been observed at four main locations during the Kaiko submersible dives in the Japanese trenches. They appear to be associated with venting along fractures. The first site for our observation was along the Japan and Kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench. The benthic communities there seem to be related to tension gashes parallel to the subduction vector. The other communities were found on the toe of the Nankai accretionary prism, along the frontal thrust and tension gashes. The temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water. As the anomaly has a small spatial extent and as the peak thermal gradient is high, the best fitting model is to be found in a vertical upward flow at a velocity of 100 m/yr in a cylindrical conduit leading out of an underlying shallow thrust.

Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (Pakistan)

The Makran accretionary prism developed in the north-western part of the Indian Ocean as a consequence of the subduction of the Arabian Sea since Late Cretaceous times. It extends from southern Iran to the Baluchistan region of Pakistan where it joins the Chaman-Ornach-Nal left-lateral strike-slip fault systems to the north and the Owen Fracture Zone-Murray Ridge transtensional (right-lateral) system to the south in a complex triple junction near the city of Karachi. In September to October of 2004, we surveyed most of the accretionary complex off Pakistan with R/V Marion Dufresne. We achieved a nearly continuous bathymetric mapping of the prism and the subduction trench from 62°30′E to the triple junction near 62°30′E together with nearly 1000 km of seismic reflection (13 lines) and we took 18 piston cores in different geological settings. One of the main results is that the frontal part of the Makran accretionary prism is less two-dimensional than previously expected. We interpret the along-strike tectonic variation as a consequence of lateral variations in sediment deposition as well as a consequence of the under-thrusting of a series of basement highs and finally of the vicinity to the triple junction.