Showing papers by "Philippe Huchon published in 1994"

The Somalia plate and the East African Rift System: present-day kinematics

Abstract: SUMMARY The motion of the Somalia plate relative to the Nubia (Africa), Arabia and Antarctica platcs is re-evaluated using a new inversion method based on a Monte Carlo technique and a least absolute value misfit criterion. A subset of the NUVEL 1 data set, with additional data along the Levant Fault and in the Red Sea is used. The results confirm that the motion of Arabia with respect to Africa is significantly different from the motion relative to Somalia. It is further shown that the data along the SW Indian Ridge are compatible with a pole of relative motion between Africa and Somalia located close to the hypothetical diffuse triple junction between the ridge and thc East African Rift. The resulting Africa-Somalia motion is then compatible with the geological structures and seismological data along the East African Rift system. Assuming a separate Somalia plate thus solves kinematic and geological problems around the Afar triple junction and along the East African Rift.

Neogene strike-slip faulting in Sakhalin and the Japan Sea opening

Abstract: We describe structural data from a 2000 km N-S dextral strike-slip zone extending from northern Sakhalin to the southeast corner of the Japan Sea. Satellite images, field data, and focal mechanisms of earthquakes in Sakhalin are included in the interpretation. Since Miocene time the deformation in Sakhalin has been taken up by N-S dextral strike-slip faults with a reverse component and associated en e'chelon folds. Narrow en echelon Neogene basins were formed along strike-sup faults and were later folded in a second stage of deformation. We propose a model of basin formation along extension al faults delimitating dominos between two major strike-slip faults, and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime, basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening. The association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in Sakhalin. We present fault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin-East Japan Sea strike-slip zone. Early and middle Miocene formations recorded the same transtensional regime as observed along the west coast of NE Honshu. During the early and middle Miocene the strike-slip regime was transpressional to the north in Sakhalin and Hokkaido, and transtensional to the south along the west coast of NE Honshu as far as Noto Peninsula and Yatsuo basin. Dextral motion accommodated the opening of the Japan Sea as a pull-apart basin, with the Tsushima fault to the west. The opening of the Japan Sea ceased at the end of the middle Miocene when transtension started to change to E-W compression in the Japan arc. Subduction of the Japan Sea lithosphere under the Japan arc started 1.8 Ma ago. The evolution of the stress regime from transtensional to compressional in the southern part of the strike-slip zone is related to the inception of the subduction of the young Philippine Sea Plate lithosphere under the Japan arc during the late Miocene. Subduction related extension is a necessary condition for the opening of the Japan Sea. Two possible mechanisms can account for dextral shear in this area: (1) counterclockwise rotation of crustal blocks due to the collision of India with Asia, (2) extrusion of the Okhotsk Sea block squeezed between the North America and Eurasia plates.

Indochina Peninsula and the collision of India and Eurasia

Abstract: 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.

New Marine Data from Vietnam Margin Limit the Amount of Extrusion of Indochina During the Opening of the South China Sea: ABSTRACT

Abstract: A total of 9300 km of high resolution, wide coverage multibeam (Simrad EM12) bathymetric data have been acquired offshore Vietnam during the Ponaga cruise of the R/V L'Atalante in May 1993. Gravity and magnetic measurements, 6-channel seismic data, as well as 6 dredges also have been obtained. East of central Vietnam, the margin displays northeast-southwest tectonic structures typical of a passive margin. The depth of the basement of the Nha Trang basin suggests that it could be of oceanic nature, with a 20 to 30 Ma age compatible with the age of the South China Sea oceanic crust located further east. Southeast of South Vietnam, the authors identified the western tip of the fossil axis of the South China Sea. It constitutes a propagating ridge into a highly stretched continental crust, partly intruded by volcanics. East of 110[degrees]30[prime]E, tilted blocks are symmetric with respect to the oceanic axis, whereas west of 110[degrees]30[prime]E they are mostly tilted toward the south, which suggests the occurrence a large listric normal fault associated with a large amount of extension. The normal faults bend progressively to a more northerly direction when approaching the north-south scarp that bounds the Conson basin. This geometry is compatible withmore » a right-lateral motion, and the normal faults associated with the oceanic propagator suggest that the dextral motion is synchronous with at least the last phase of spreading in the South China Sea (23-16 Ma). Since recent offshore oil data have established that the prolongation of the Red River fault within the Gulf of Tonkin was affected by left-lateral motion from the Oligocene to the upper Miocene, the results suggest that the change from left-lateral motion in the Gulf of Tonkin to right-lateral motion along the Vietnam margin occurs because the South China Sea basin opens more rapidly than the extrusion of Indochina. Thus, the total amount of extrusion of Indochina probably does not exceed 100 or 200 km.« less