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International audience; The Paleoproterozoic domain of Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of longlived Paleoproterozoic “weak type” orogen.

The Chi-Chi 1999 earthquake ruptured the out-of-sequence Chelungpu Thrust Fault (CTF) in the fold-and-thrust belt in Western Central Taiwan. An important feature of this rupture is that the calculated slip increases approximately linearly in the SE– NW convergence plate direction from very little at its deeper edge to a maximum near the surface. We propose here a new explanation for this co-seismic slip distribution based on the study of both stress and displacement over the long-term as well as over a seismic cycle. Over the last 0.5 My, the convergence rate in the mountain front belt is accommodated by the frontal Changhua Fault (Ch.F), the CTF and the Shuangtung Fault (Sh.F). Based on previously published balanced cross sections, we estimate that the long-term slip of the Ch.F and of the CTF accommodate 5–30% and 30–55% of the convergence rate, respectively. This long-term partitioning of the convergence rate and the modeling of inter-seismic and post-seismic displacements suggest that the peculiar linear co-seismic slip distribution is accounted for by a combination of the effect of the obliquity of the CTF to the direction of inter-seismic loading, and of increasing aseismic creep on the deeper part of the Ch.F and CTF. Many previous interpretations of this slip distribution have been done including the effects of material properties, lubrication, site effect, fault geometry and dynamic waves. The importance of these processes with respect to the effects proposed here is still unknown. Taking into account the dip angle of the CTF, asperity dynamic models have been proposed to explain the general features of coseismic slip distribution. In particular, recent works show the importance of heterogeneous spatial distribution of stress prior to the Chi-Chi earthquake. Our analysis of seismicity shows that previous large historic earthquakes cannot explain the amplitude of this heterogeneity. Based onour approach, we rather think that the highstress in the northern part ofthe CTF proposed by Oglesby and Day [Oglesby, D.D., Day, S.M., 2001. Fault geometry and the dynamics of the 1999 Chi-Chi (Taiwan) earthquake. Bull. Seismol. Soc. Am. 91, 1099–1111] reflects the latitudinal variation of inter-seismic coupling due to the obliquity of the CTF. D 2004 Elsevier B.V. All rights reserved.

15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc.

The maximum average elevation of the Alpine-Himalayan belt does not exceed 2.5 km from the Alps to the Zagros but abruptly increases to 5 km in the High Himalayas. The limitation of the elevation to 2.5 km can be explained if eclogitization occurs below 55 km from the Alps to the Zagros. The 5-km elevation of the High Himalayas is due to the presence of crustal-like densities to 75 km and this presence requires a mechanism to prevent the eclogitization to occur above 75 km there. Thermal models are constructed assuming a geometry based on the INDEPTH seismic profile, which shows underthrusting of the whole Indian crust below the crustal wedge of the High Himalayas, and possibly as far as the Indus-Tsangpo suture. Assuming a rate of underthrusting of about 15 mm/yr on a 10° dip fault plane, undeformed Indian crust had already reached the Indus-Tsangpo suture at the end of the main phase of thrusting along the Main Central Thrust (MCT) 20 myr ago. Consequently, it is possible to estimate the current isotherms in the Himalayan belt from a single thrust model. The high temperatures and partial melting observed in the Higher Himalayan Crystallines (HHC) imply that similar conditions should occur within the upper crustal wedge. Our models show that these conditions at the base of the upper crustal wedge can be obtained by the conjunction of a thick upper crustal pile (with radioactive heat production 2 × 10 −6 W/m 3 ) and a high erosion rate (1 mm/yr). The absence of eclogitization above 75 km within the underthrust Indian lower crust can be explained either if the temperature is higher than 800°C (in the granulite field) or lower than 500–550°C, within the blueschist field. Because the fast rate of underthrusting of the lower crust prevents its heating, the temperature there cannot reach values in excess of 800°C. On the other hand, with reasonable thermal parameters, temperatures of less than 500–550°C may prevail. In this case, the 5-km elevation can be reproduced if the transformation in blueschist is inhibited in the lower crust, presumably because of low water availability. We consequently propose that the combination of a relatively cold underthrust Precambrian crust and of a fast rate of underthrusting prevents eclogitization above 75 km and enables the mountain belt to reach altitudes of 5 km. Thus the main difference between the Zagros and the Himalayas would be the critically slower rate of underthrusting in the Zagros compared to the Himalayas.

Seismic and seismological studies as well as gravimetric models indicate that a slab of European lithospheric mantle and lower crust is currently underthrust below the Apulian crust. We assume a simple kinematic model in which the lower and upper subducted European crusts are decoupled along a decollement. The lower crust goes into subduction without deformation. The upper crust deforms by pure shear with a horizontal compressional axis. The total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry, different distributions of erosion being used. The computed temperature field is steady-state if the kinematic model applies during a minimum time of 40 Myr for a convergence rate of 8 mm/yr. Equilibrium mineral assemblages and densities are determined from the computed P, T conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust. Assuming local isostasy, the density model fits the average topographic profile across the Central Alps. The P-T-t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure Oligocene metamorphism in the Lepontine dome. The peak calculated temperature for the deepest non subducted crustal rocks is 600°C for a pressure of 0.8 GPa, near the lower limit of high-pressure amphibolites. We thus propose that the Lepontine metamorphism corresponds to the steady-state thermal regime. However, either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt. The computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km. We conclude that the Moho limiting the deepest part of the root may correspond to the eclogitisation phase change. Lower crust eclogites have a density comparable to or higher than that of the mantle, depending on their chemical composition (3.37 for andesitic eclogites, 3.56 for gabbroic eclogites). Thus, andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink.

Using a simplified petrogenetic grid and a one-dimensional thermal model to compute the evolution of the density of the crust with time, the contribution to the uplift of Tibet of metamorphic transitions in the lower crust is shown to be large. The computations are made for a lower crust of either mafic or intermediate composition. A mafic lower crust would have a mantle-like density, incompatible with seismological results; this conclusion implies that any pre-thickening gabbroic lower crust has been removed during the orogenic process. A lower crust of intermediate composition would be affected by about 3 km of post-thickening uplift, over a few tens of millions of years. This uplift is essentially the result of the eclogite to granulite transformation. The resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere. Such a process would also explain why the Neogene uplift of Tibet to an average elevation of 5 km appears to have occurred without crustal shortening. The same model may explain the uplift of the Altiplano-Puna Plateau, which also occurred without surface shortening since about 10 Ma. A comparison is made with the now collapsed Variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years.

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 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.

It is often stated that the spherical plate tectonic model was independently and simultaneously developed by MC Kenzie and Parker in their paper published in Nature in November 1967 and Morgan in his paper published in Journal of Geophysical Research in March 1968 (e.g. Cox, 1973). MC Kenzie and Parker thus have publication anteriority, although the first submission of Morgan’s paper occurred on August 30, 1967 whereas the paper of MC Kenzie and Parker was submitted about two weeks later. Yet, Jason Morgan had presented a first version of his paper entitled “Rises, trenches, great faults and crustal blocks” at the American Geophysical Union (AGU) meeting in Washington D.C. on April 17, 1967 to the packed audience of a “Sea Floor Spreading” special symposium presided by Fred Vine and H.W. Menard. This occurred two months before Dan MC Kenzie, then at Scripps Institution of Oceanography, began working on the same model (D.P. MC Kenzie, pers. commun., 1983). The reason why the presentation by Jason Morgan at the AGU meeting has not established his priority is that the talk he made did not correspond to the abstract he had sent. Consequently, there is no written record of this communication there. Actually, Jason Morgan had written an eleven page extended outline of his presentation, including the nine figures illustrating his talk. This short paper was sent to about ten people immediately after the meeting. I was among these ten people. In addition to myself, Morgan (pers. commun., July 1987) does not remember all the addresses of his paper. He writes “I am quite sure I gave copies to Bill Menard (at Scripps) and Tuzo Wilson (at Toronto) and, I am fairly sure, to Lynn Sykes (at Lamont), Carl Bowin (and/or Joe Philipps, at Woods Hole) and Fred Vine (at Princeton). I might have sent one to Jerry Van Andel as I used magnetic profiles from the Circe cruise”. Unfortunately, Morgan lost his own copy and none of those who received it have yet made it available to the scientific community. I thought I had also lost my own copy. Yet, during a recent move of my office, I found it. I publish it here with the agreement of J. Morgan, thus revealing the exact substance of his presentation at the AGU. I also found a preprint of the paper he submitted to Journal of Geophysical Research later on August 30, 1967, which was later revised and accepted on November 30, 1967. According to J. Morgan (pers. commun., July 1987) “this short description of the main ideas in plate tectonics was written the week before the AGU. The last two pages were written and reproduced the night before the meeting” (as indicated by the different quality of reproduction of these two pages). Morgan writes “I was in Guyot Hall (in Princeton University) until 2 or 3 A.M. and we were to leave for Washington at dawn.” Note that the extended outline has the same title as the paper later published in the Journal of

We discuss nine palinspastic geological maps (Plates 1–9), at 1 20,000,000 scale, which depict the evolution of the Tethys belt from the Pliensbachian (190 Ma) to the Tortonian (10 Ma). A Present structural map (Plate 10) is shown for comparison at the same scale with the same conventions. Our reconstructions are based on a kinematic synthesis (Savostin et al., 1986), a paleomagnetic synthesis (Westphal et al., 1986) and geological compilations and analyses concerning in particular the western domain (Ricou et al., 1986), the eastern passive margins (Kazmin et al., 1986a), the eastern active margins (Kazmin et al., 1986b), the Black Sea-Caspian Sea basins (Zonenshain and Le Pichon, 1986) and the ophiolites (Knipper et al., 1986).

We report the results of fifteen Expanding Spread Profiles (ESP's), 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.

The behaviour of the lithosphere under extensional strain is different whether the surface of the lithosphere is above or below the level that would be reached by the asthenosphere in its absence. Below this level, the continuity of the old lithosphere may be rapidly broken and oceanic accretion starts. Above this level, extensive thinning of the old lithosphere occurs until the level is reached, and then the asthenospheric material may break out to the surface. The thinning probably depends on the strain rate. Two extreme cases must be distinguished. For low strain rates: the lower lithosphere may be thinned by diapiric intrusions and lithospheric sinking; the upper one is much less affected, as occurs in the African Rifts or the Rhine Graben. For high strain rates, the whole lithosphere is thinned rather uniformly, as occurs in the Aegean region and probably on many continental margins. The greatest portion of the lithosphere is extended plastically, while the upper brittle portion, about 10 km thick, is extended by normal faulting. The pattern of faulting in these widely extended areas is compared to a pack of cards resting at an angle on a plane, with each card (tilted block) making a slight angle with the preceding one. Using this simple model, we discuss the case of the Pindos basin of the Hellenides. Similar intra-continental basins 150 to 400 km wide may be created before the subsidence increases sufficiently for oceanic accretion to begin. We point out that these basins are especially susceptible to compressional failure whenever the stress system changes from extensional to compressional. Consequently, many orogens are localized along such zones of earlier extension, as shown by geological studies of the Western Alpine system.

A new reassembly of the continents around the North Atlantic Ocean is presented. The first criterion used for this reassembly is the identification of the structural framework related to the opening which consists of marginal fracture zones generated by offsets of the Rift. The Africa—North America, Eurasia—Greenland, Greenland—North America and Eurasia—North America adjustments are successively discussed. It is argued that the adjustments are best made at the 3000-meters isobath between Africa and North America and at the 2000-meters isobath for the younger rifts. The difference is attributed to subsidence and modification of continental margins with time. The importance of the Late Paleozoic tectonic phase in determining the subsequent pattern of Mesozoic rifting is emphasized.

The Gulf of Suez is a Neogene rift which has evolved as one arm of the Sinai triple junction together with the Gulf of Aqaba and the Red Sea. Backstripping of well data in the central and southern Gulf of Suez shows three distinct phases of tectonic subsidence. During the initial stage of opening of the rift, the rate of subsidence was very low. In the late Burdigalian (Rudeis fm.) there is a rapid increase in subsidence. By the Middle Miocene, the subsidence had slowed again. Extending the backstripping to a two-dimensional cross-section of the Gulf of Suez allows better quantitative estimates of extension. Correcting for the large regional uplift (~ 1100 m on the Egyptian side) is critical for obtaining accurate values for the extension. Uplift of the rift decreases the net tectonic subsidence by over one fourth, relative to that predicted by uniform extension. The total extension at the latitude of Gebel Zeit (28° N) is ~ 30 km, which corresponds to 32–36 km at the triple junction. Approximately 13 to12 of the extension occurred during the rapid subsidence of the second opening phase. Slower extension continued for the rest of the Miocene and throughout the Plio-Pleistocene. Stress directions calculated from microstructures exhibit several directional phases to the opening of the Gulf of Suez. These data, together with constraints from the other arms of the Sinai triple junction, can be combined into a scenario for the kinematic evolution of the region. At first, the Gulf of Suez is the northward continuation of the Red Sea, and both open at ≈ N30°. The first subsidence phase in the Gulf of Suez represents an initial startup of the rifting with very low amounts of extension. This geometry continued through the main phase of Suez extension. Subsequently, the Gulf of Aqaba formed as a strike-slip boundary. As the Arabia-Africa motion transferred to the Gulf of Aqaba, the direction of extension in the Gulf of Suez rotated clockwise towards perpendicular extension and slowed. The post-Miocene shift to oblique opening in the Gulf of Aqaba and the Dead Sea transform is the result of a minor change in the pole of opening. This change is below the current resolution of the data for the Gulf of Suez, which is probably opening at < 1 mm/yr.

The concentration of extension in southern Aegea is a consequence of the velocities imposed along the boundaries of this region. This result is demonstrated by using the finite element method and velocities derived independently from the kinematics of the Hellenic subduction zone and that of the North-Anatolian fault. The thinning of the lithosphere resulting from this extension is in good agreement with that required to explain the Aegean subsidenceand the forces along the boundaries are approximately normal to the Aegean arc. Neotectonic data agree with these results.

We present an updated series of kinematic reconstructions of the major plates around the Tethys from the Atlantic Ocean to the Pamirs between the Early Jurassic and the Present. This set is used elsewhere as a basis for paleogeographic maps of the entire region. The problems related to the positions of the continents in the Lower Triassic are also discussed. No direct analyses of magnetic anomalies and fracture zones in the Atlantic have been made. Rather, all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors. The reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies, except for the Jurassic-Cretaceous boundary and the Cretaceous-Tertiary boundary which correspond to interpolated positions. Paleolatitudes have been drawn using the study of Westphal et al. (1986). An attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching. The resulting relative vector of motions along the northern boundary of the Tethys shows a significant change 80 m.y. ago. Left-lateral motion with compression dominates before whereas right-lateral motion with compression dominates after. To the east, rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the Late Jurassic, Late Cretaceous, Eocene and post Middle Miocene. To the west, north of Apulia, on the contrary, the motion rate has not changed significantly since the Early Cretaceous and is close to 1 cm/yr as an average. These rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the Africa-Eurasia pole of rotation and seem to be mostly governed by the tectonics of the Tethys plate boundary.

Seabeam, seismic and submersible surveys took place during the Kaiko Project and revealed significant compressive deformation at the northeastern end of the Philippine Sea plate, related to the recent collision of the Izu-Ogasawara Arc against Central Japan. Intraoceanic thrusting at the base of the Zenisu Ridge, a linear topographic high running a few tens of kilometers south of the Nankai Trough, is supported by tectonic, magnetic and gravimetric data. We investigate the formation of the Zenisu Ridge in terms of compressive mechanical failure of a thin elastic-perfectly plastic plate, subducting at a trench and subject to a regional compressive axial force. The rheological envelope concept is used throughout the numerical calculations. Based on a detailed study of flexure of the present-day bending far from the deformation zone, we evaluate the bending forces involved: the bulge is 120 to 150 m high and the compressive stress all along the Nankai Trough is about −100 MPa. In the Zenisu Ridge area, an additional compressive stress is superimposed due to the nearby collision at Izu-Peninsula. We compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20–25 km in the trench area, and again at the surface 60 to 100 km seaward, in the vicinity of the bulge. The development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed. The model predicts that the formation of the Zenisu Ridge did not occur before 4 Ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the Nankai Trough.

We review the geological and geophysical structural framework of the deep Black Sea and Caspian Sea basins. Based on seismic evidence and subsidence history, we conclude that the deep basins have an oceanic crust formed in a marginal sea environment. We propose that the present deep basins are remnants of a much greater marginal sea formed during three separate episodes during the Mesozoic: in the Middle Jurassic, Upper Jurassic and Late Cretaceous. A tentative sketch of the geologic evolution of the area is presented. The marginal sea reached its greatest extent in the Early Tertiary when it was about 900 km wide and 3000 km long. The central part of the marginal sea has since disappeared during the collision between the Arabian promontory and the Eurasian margin.

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.

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.

We use field analyses together with interpretation of aerial photographs, orbital imagery and Sea-Beam data to define more accurately the geometry of fault systems in the Hellenic arc. The tectonics of the area are dominated by extensional processes related to normal faulting. We then use Sea-Beam data, previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the Cretan Sea basin, which is found consistent with the land fault pattern. The mechanisms of the late Neogene and Quaternary deformation of the central and southern Aegean region are analysed in terms of stress trajectories, as revealed by field studies of fault populations. We then examine the subsidence of the Sea of Crete in terms of a model of uniform stretching of the lithosphere. The opening of the Sea of Crete was cylindrical and did not result in bending of Crete. This leads us to modify the southern portion of an earlier kinematic reconstruction of Aegea, in agreement with recent paleomagnetic results. The implications of the opening of the Sea of Crete on the tectonic evolution of the straits to the west and east of Crete are discussed. Finally, the uplift of the outer arc is explained by underplating due to subduction.

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.

International audience; Large benthic chemosynthetic communities have been observed at four main locations during the Kaiko submersi- ble 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 anomafy 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.

Results of a recent detailed survey of the axial trough of the Red Sea are used to obtain the present instantaneous kinematic pattern over the whole Red Sea—Gulf of Aden area. This pattern applies to the most recent 4–5 m.y. old phase of motion. The earlier Miocene phase of motion is discussed in light of these results. It is argued that the total amount of opening has been considerably less than the amount predicted by matching the coastlines.

Using the fault plane mechanisms of the shallow earthquakes occurring along the Hellenic arc and the extent of the intermediate seismic belt, we make a quantitative estimate of the relative motion occurring between the Hellenic arc and the adjacent sea floor. This estimate is then used to evaluate the deformation in the Aegean area and to reconstruct the pattern of motion over the Eastern Mediterranean region for the last 13 m.y. It is shown that this pattern is compatible with the neotectonic and seismicity studies in Aegea. We then discuss the dynamics of the area and propose that, since Serravallian-Tortonian time, Aegea has been spreading gravitationally in front of the southwestward advancing Turkey. The reasons for this gravitational spreading are discussed.

Geophysical measurements have mostly confirmed the validity of the standard structure of the oceanic crust as deduced from seismic refraction measurements in the early fifties. The relatively small degree of variability of the oceanic crust structure contrasts strongly with the comparatively large variability of the continental crust. Layer 1 consists of unconsolidated or semiconsolidated sediments no older than Lower Cretaceous. Layer 2 probably consists mainly of basalt, but may be locally covered by a layer of consolidated sediments of about Upper Jurassic age, or older. The results fit very well with the spreading floor hypothesis. There are no definitive arguments either for or against the serpentinite composition of layer 3 but an important fact is the progressive thickening of layer 3 away from the axial zone. The problem of the creation of the crust by the sea-floor spreading mechanism should be investigated within the framework of the mobile lithospheric plate concept.