[1] New geophysical data collected at the Aden-Owen-Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia-Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 kmlong ridge portion located immediately west of the Owen fracture zone. A second 500 kmlong ridge portion developed westward up to the Alula-Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km-long ridge portion was emplaced between the Alula-Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr −1. The ridge propagation resulted from the Arabia-Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia-India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.

[1] The main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp, the development with the construction of the relief, and the arrest because of the onset of another thrusting event. A simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the Coulomb criterion. The evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem. The development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force. The approach is first validated by proving that the critical slope angle a c for the classical triangular wedge is properly captured. It is shown that a perturbation, in the form of an extra relief in this perfectly triangular geometry, leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation, respectively, for a range of slope dip close to the critical angle a c . The study of normal thrust sequences (from the rear to the front in the wedge toe) reveals that weakening of the ramp, accounted for by changing its friction angle from an initial to a smaller final value, is necessary for each thrust to have a finite life span. This life span is longer with a larger relief buildup for more pronounced weakening. Decreasing the décollement friction angle results in an increase in the number of thrusts in the sequence, each thrust creating milder relief. The normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp. The proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of Nankai's accretionary wedge. The inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp, from the geometry of the corresponding thrusts, and the topography. It is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp, justifying a key hypothesis needed to predict discrete sequences of thrusting.

No abstract

The Arabia-India plate boundary-also called the Owen fracture zone (OFZ)-is perhaps the least known boundary among large tectonic plates 1-6. Although it has early been identified by Wilson 7 as a type example of transform fault converting the divergent motion along the Carlsberg Ridge into convergent motion in the Himalayas, its structure and rate of motion are still poorly defined. We here present the first direct evidence for active dextral strike-slip motion along this fault, based on seafloor multibeam mapping of the Arabia-India-Somalia triple junction in the NW Indian Ocean. The mapped segment of the OFZ trends N10°E, displays a finite offset of ~12 km, and terminates to the south into a 50 km-wide pull-apart basin bounded by active faults. Combining these new constraints with the ITRF2005 global geodetic solution, we determine a robust Arabia-India relative angular velocity implying a 2-4 mm yr-1 dextral motion along the OFZ. This transform fault probably initiated