Active spreading and hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.     

Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor: insights from the Tadjouraden cruise in the western Gulf of Aden

A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small-scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5 kHz sub-bottom profiler shows how the faults cut distinct layers within the recent, up to 60 m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the 'Ghoubbet' volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal–Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400 yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3–1.4 mm yr⁻¹ and indicate a major decrease in sedimentation rate after about 6000 yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.     

Structure and evolution of the backstop in the eastern Nankai Trough area (Japan): Implications for the soon-to-come Tokai earthquake

Abstract We present data showing that the intra-oceanic shortening now occurring south of the eastern Nankai Trough that has produced the Zenisu Ridge has also been responsible for the formation of a previous ridge now buried below the continental margin. This ridge, that we refer to as Paleo-Zenisu, is presently adjacent to the backstop and its location coincides with the outer limit of the seismogenic decollement. The subduction of the paleo-Zenisu ridge below the wedge has led to its complete reorganization and has given its identity to the Great Tokai earthquake rupture zone. The formation of paleo-Zenisu and its consequent subduction has induced the tilting of the backstop toward the northwest since ca 2 Ma. This model suggests that the backstop and possibly the wedge are dextrally sheared because they are extruded southwestward in relation to the collision of the Izu-Bonin Ridge with Japan. We use the finite motion from Zenisu to paleo-Zenisu to derive both the subduction vectors along the Nankai Trough and the shortening vectors within Zenisu-Izu. The amount of shortening absorbed within Zenisu-Izu increases toward the northeast. The corresponding subduction vectors of the Zenisu platelet below the wedge decrease accordingly to the northeast from 50 to less than 20 mm/year and the Zenisu body rotates clockwise with a pole near 36° North, 139° East. This might explain the apparent longer repetition time of great earthquakes in the Tokai area. On the other hand, the 25-35 mm/year obtained for the rate of shortening along the Zenisu thrust indicates a high seismic potential there.     

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

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.     

Post-rift volcanism and high heat-flow at the ocean-continent transition of the eastern Gulf of Aden

A high heat-flow (∼900 mW m⁻²) has been observed over a volcanic structure at the Ocean-Continent Transition in the Eastern Gulf of Aden (Oman margin). The anomaly is superposed to a progressive increase of heat-flow across the margin and can be interpreted either by (1) heat refraction, (2) fluid discharge or (3) cooling magma. The two first explanations cannot be ruled out definitely by modelling analysis, but require unlikely thermal conductivity or permeability values. The third one implies that the latest activity of the volcano was about 100 000 years old and therefore continued c. 18 Ma after the break-up of Africa and Arabia. This potential mechanism is consistent with other lines of evidence of post-rifting activity in the Gulf of Aden and could invalidate the conventional assumption that rifted-margins become passive after the break-up of continents.     

Did Yemeni Tertiary granites intrude neck zones of a stretched continental upper crust?

We demonstrate that the Miocene alkali granites of Yemen which emplaced along the Great Escarpment are contemporaneous with the stretching of the southern Red Sea continental crust. There is a gradient of Miocene extension towards the granites as well as a change of the geometry of tilted blocks around them. Geometrically, most of these granites seem to be emplaced passively in the necked areas of a stretched continental crust. These necked areas correspond to previous syn-plume Oligocene NNW–SSE to N–S trending areas of localized extension marked by basaltic dyke swarms. These early extensional areas were associated with the emplacement of the flood basalt volcanic pile and were probably localized over large basement discontinuities.     

Tectonic setting of Western Pacific marginal basins

Global kinematics as well as magnetic anomalies of marginal basins, with continental geology and paleomagnetic data as additional constraints, are used to present a set of reconstructions of the Western Pacific marginal basins between 56 Ma and the Present at key periods (56, 43, 32, 20, 12 and 3 Ma). Our model accounts for the rapid motion of “exotic terranes” along the whole of the Western Pacific convergent zone.

Marginal basins appear to open in a great variety of tectonic settings, the two extreme examples being the Mariana trough where trench suction may be the predominant driving force and the South China Sea where intracontinental deformation appears to be the major mechanism. The study of marginal basins is a 3-D problem which must take into account the whole tectonic context (subduction related tectonics in cross-section and upper plate deformation in map view) and not only a 2-D problem (the classical trench-arc-back-arc problem).     

Geoid,Pangea and convection

We show that the present geoid has a simple low-order configuration with an axis of symmetry in the equatorial plane. We show further that it is a “tennis-ball” pattern, with an equatorial high belt and a polar low one, which is clearly controlled by the rotation of the Earth. Finally, we show that the outline of Pangea between at least 200 Ma and 125 Ma ago lay along a great circle passing through the paleo-poles of rotation. Thus, it also had an axis of symmetry in the equatorial plane. This hemispheric super-continent configuration ended in Middle Cretaceous time during a major geologic catastrophe which was accompanied by high rates of spreading, hotspot outbreaks and high sea-level stands. We interpret this evidence in terms of separate steady state lower mantle convection, responsible for the present geoid, weakly coupled to the upper mantle one. This weak coupling leads to the hemispheric continent configuration which ends when excessive heating of the upper mantle due to the insulating continental cap leads to continent dispersal. The complete cycle, from one supercontinent to the next, might be of the order of 400 Ma.