X. Le Pichon

The active Main Marmara Fault

Abstract This paper presents selections from and a synthesis of a high resolution bathymetric, sparker and deep-towed seismic reflection data set recently acquired by the French Ifremer R.V. Le Suroit in an E–W deep trough that forms the northern half of the Sea of Marmara in NW Turkey. It includes the first high resolution complete bathymetric map of this area. A single, throughgoing dextral strike–slip fault system, which is the western continuation of the northern branch of the North Anatolian Fault, cuts this trough lengthwise and joins the 1999.8.17 Kocaeli earthquake fault with the 1912.8.09 Şarkoy–Murefte earthquake fault, both of which display strike–slip offset. In its eastern one fourth, the structure follows closely the northern margin of the deep trough, whereas in the west it hugs its southern margin. The eastern one fourth of the structure has a minor component of its displacement distributed across the deep trough owing to a possible original bend in the course of the dextral structure. The present course of the North Anatolian Fault in the Sea of Marmara originated some 2×105 a ago, by cutting across the older basin fabric generated by a dominant NNE–SSW extension before it began taking up major motion in the Pliocene.

Velocity field in Asia inferred from Quaternary fault slip rates and Global Positioning System observations

We perform a joint inversion of Quaternary strain rates and 238 Global Positioning System (GPS) velocities in Asia for a self-consistent velocity field. The reference frames for all geodetic velocity observations are determined in our inversion procedure. India (IN) moves relative to Eurasia (EU) about a pole of rotation at (29.78°N, 7.51°E, 0.353° Myr−1), which yields a velocity along the Himalaya within India that is ∼73–76% of the magnitude of the IN-EU NUVEL-1A velocity and a vector azimuth that is 8–10° clockwise of NUVEL-1A IN-EU vector azimuth. Relative to Eurasia, south China moves at 9–11 mm/yr in the direction 110–120° with a pole position (64.84°N, 156.74°E, 0.12° Myr−1). Amurian block motion has a pole position in a similar location but at a slower rate (64.61°N, 158.23°E, 0.077° Myr−1) and most of the Amurian-Eurasia motion is accommodated by extension across Lake Baikal. Tarim Basin moves relative to Eurasia about a pole of rotation at (39.24°N, 98.2°E, −0.539° Myr−1) and ∼16–18 mm/yr of shortening is accommodated across the west central Tien Shan. There is distributed E-W extension throughout both southern and north central Tibet. Within southern Tibet, between the longitudes of 77°E to 92°E, the deformation field accommodates ∼16–19 mm/yr of E-W extension. We compare predicted seismic moment rates with those observed in this century in Asia. Total observed seismic moment rates within the entire area of central and east Asia (2.2×107 km2) in this century are 2.26±0.7×1020 N m yr−1 as compared with a predicted total rate of 2.03±0.066×1020 N m yr−1. Comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long-term seismic moment rates from a catalog of limited duration (94 years). An observation period of ∼10,000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates, inferred from the joint inversion of GPS and Quaternary rates of strain. We show that in general, a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude (mb ≥ 5.0, for the period January 1, 1965, to January 1, 1999).

The rotation of Arabia and the Levant fault system

Abstract We establish the kinematics of the rotation of Arabia with respect to Africa and show that the rotation rate of Africa increased by a factor of about four in late Serravallian time, about 12–13 Ma ago. We discuss evidence that oceanic accretion started simultaneously in the Gulf of Aden and in the Red Sea at this time. It is also at this time that the Levant and Anatolian fault systems were initiated. Our initial reconstruction of the Red Sea Rift in Late Oligocene-Early Miocene time approximately superposes the present shore lines which coincide with the mid-point of the zones of rapid crustal thinning, the future continental slopes. The amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension, and by checking these estimates through closure around the Sinai triple junction. In the Late Miocene, the Southern evant fault zone was a zone of distributed shear deformation, through counterclockwise block rotation, extending soul h ward within the northernmost Red Sea. Since the Early Pliocene, the shear motion has been localized within the Gulf of Aqaba, changing the tectonic pattern in the northern Red Sea.

THE RED RIVER FAULT SYSTEM IN THE TONKIN GULF, VIETNAM

Abstract The Red River fault system in the Tonkin Gulf offshore Haiphong was studied using seismic profiles calibrated by deep wells. Well characterised left-lateral strike-slip occurred continuously within a narrow 30-km-wide zone southwest of the Vinh Minh fault between 30 Ma and 5.5 Ma. However, the corresponding amount of offset probably does not exceed a few tens of kilometres. No sign of post-5.5 Ma right-lateral motion can be detected. Prior to 30 Ma, there is a widespread extension in a wider 100-km-wide zone which could be related to a significant amount of left-lateral motion. The motion of the fault splays to the northeast of the Vinh Minh fault prior to 30 Ma was absorbed in the rifting of the Gulf of Beibu. A 15.5-Ma unconformity separates the transtensional regime from a later transpresional regime. This 15.5-Ma date coincides with the cessation of sea-floor spreading in the South China Sea.

Seismic study of the crust of the northern Red Sea and Gulf of Suez

We report the results of fifteen Expanding Spread Profiles (ESPs), and a seismic wide angle reflection-refraction Une, performed during March–April 1986, in the Gulf of Suez and the Egyptian part of the northern Red Sea area (north of 25°N). Four 16.4 air guns were used as a sound source on board R.V. “Le Suroit” and a 96-channel 2.4-km long streamer was towed by a supply vessel, the “Whity Tide”. Most of the profiles show good crustal reflection and refraction arrivals and often good Moho arrivals obtained for a distance of 80 to 100 km. We present the results of X-T and τ-p analysis, obtained by a velocity inversion performed in the τ-p plane and by ray-tracing modeling of both the τ-p and the X-T sections. The velocity models are computed for planar and linear gradient velocity layers. The northernmost part of the Red Sea appears to be characterized by a continental type crust, extremely thinned (β ≥ 4), lying at a mean depth of 7–8 km, whereas the Moho is at a mean 14–15 km depth. The southern part shows a seismic velocity structure of an oceanic type, except in the 40 km closest to the coastline. In both parts, seismic waves progressively get more attenuated with distance from the shore to the axial zone, which is presently tectonically active. The difference between the northern continental and southern oceanic zone is related to the termination of the Levant Fault. The northern continental area appears to represent the termination of the Levant Fault as a zone of distributed deformation.

Indochina Peninsula and the collision of India and Eurasia

The collision of India with Asia results in a rapidly changing stress pattern around both Himalayan syntaxes. The maximum compressive stress, which is north- south north of the indenter, fans radially through nearly 180° around the syntaxes to accommodate the necessary strike-slip motion on the edges of the indenter. Thus, the whole Indochina Peninsula was swept by the migrating stress and associated strain pattern as the eastern syntaxis moved to the north. As a result, the maximum horizontal component of the stress field is predicted to have been first oriented east-west, then north-south. We propose that this changing stress field controlled the opening of the sedimentary basins and of the South China Sea as well as the direction of motion of the major strike-slip faults.

CENOZOIC DEFORMATION OF CENTRAL AND SOUTH VIETNAM

Abstract The pre-Tertiary basement of central and south Vietnam is affected by pervasive strike-slip and normal faulting, which appears to control the shape of the present continental margin of eastern Indochina. Using remote sensing and field studies, we show the existence of two superposed strike-slip fault systems which were probably active during the Paleogene and early Neogene, respectively. The older system consists of large NW-SE left-lateral strike-slip faults, parallel to the Red River Fault, compatible with an E-W maximum shortening axis. In south Vietnam, conjugate N50°E right-lateral faults are also present, reactivating pre-existing Paleozoic and Mesozoic faults. The younger fault system consists of dominant N160°E to N-S right-lateral faults, compatible with a N10–30°E maximum shortening axis. These N-S-trending dextral strike-slip faults are parallel to the escarpment limiting the continental margin of Vietnam, south of Da Nang. Some of the N-S and N50°E faults have been reactivated locally as normal faults, especially during the uplift of central and south Vietnam, which was associated with voluminous late Neogene-Quaternary basaltic volcanism. These new field data show that eastern Indochina was affected by the collision of India with Eurasia, first through pervasive NW-SE left-lateral strike-slip faulting compatible with the extrusion of Indochina, and then through N160°E to N-S right-lateral faulting. Thus, a 90° rotation of the strain pattern occurred over Indochina as the collision proceeded and as the eastern syntaxis of the Himalayas migrated northward. We propose that the younger strain pattern is compatible with the existence of a large right-lateral sub-meridian shear zone over the eastern margin of Indochina as the South China Sea basin was opening.

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.

Study provides data on active plate tectonics in southeast Asia region

A major geodynamic study has provided significant new information about the location of active plate boundaries in and around Southeast Asia, as well as deformation processes in the Sulawesi region of Indonesia and tectonic activity in the Philippine archipelago. Results also have confirmed the existence of the so-called Sunda Block, which appears to be rotating with respect to adjacent plates. The study, known as the Geodynamics of South and South-East Asia (GEODYSSEA) project, has been a joint venture of the European Commission and the Association of South- East Asian Nations. It began in 1991 and involved a large team of European and Asian scientists and technicians studying the complex geodynamic processes and natural hazards of the region from the Southeast Asia mainland to the Philippines to northern Australia. Earthquakes, volcanic eruptions, tsunamis, and tectonically induced landslides endanger the lives of millions of people in the region, and the tectonic activity behind these natural hazards results from the convergence and collision of the Eurasian, Philippine, and Indo-Australian Plates at relative velocities of up to 10 cm per year.