Bertrand Maillot

Arabia-Somalia plate kinematics, evolution of the Aden-Owen-Carlsberg triple junction, and opening of the Gulf of Aden

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.

The influence of fluid flow in fault zones on patterns of seismicity: A numerical investigation

We present a coupled two-dimensional model of the fluid flow within a tabular fault zone and frictional failure of the fault, incorporating the effects of compaction and dilatancy. The model fault zone is loaded externally, resulting in a constant shear traction along the fault and a constant normal stress τn across it. Nonuniform compaction of the fault material leads to fluid pressure gradients and fluid flow. Frictional failure of element i is triggered when the fluid pressure is sufficiently high that the shear stress exceeds a critical level τci = μi(τn - Pfi), where μi is the frictional coefficient and Pfi is the fluid pressure. Failure of a fault element results in an increase in element porosity and a decrease in fluid pressure. We model the diffusion of fluid pressure using a lattice Bhatnagar-Gross-Krook technique, and frictional failure is simulated by a simple cellular automaton type model. We show that the failure history of the fault is critically dependent on the ratio of the fluid diffusivity of the fault material to the compaction rate. For high diffusivities a power law distribution of failure sizes is observed, but at low diffusivities a non-power law distribution results. High diffusivities promote a cyclic failure history, whereas at low diffusivities the failure occurs at a constant level. Clusters of failed fault elements may be identified with seismic events, and therefore our results place bounds on the range of fault parameters which are permitted in a situation in which seismicity is observed to follow a Gutenberg-Richter distribution.

A lattice BGK model for the diffusion of pore fluid pressure, including anisotropy, heterogeneity, and gravity effects

Fluid flow in the Earths crust can be accurately described by a diffusion equation, with highly anisotropic, heterogeneous and time dependent diffusivities. The so-called Bhatnagar-Gross-Krook (BGK) models provide a simple and efficient way to solve the scalar diffusion equation with complex boundary conditions. We show that it can be adapted to include an anisotropic, space and time dependent diffusivity, and a gravity body force, hence providing a suitable numerical model for realistic simulations of incompressible pore fluid diffusion in heterogeneous, anisotropic rocks in the presence of a gravity field. A stability condition limits in some cases the magnitude of anisotropy that can be simulated. Numerical results are in very good agreement with a known analytical solution.

Tectonic and gravity extensional collapses in overpressured cohesive and frictional wedges

Two modes of extensional collapse in a cohesive and frictional wedge of arbitrary topography, finite extent, and resting on an inclined weak decollement are examined by analytical means. The first mode consists of the gravitational collapse by the action of a half-graben, rooting on the decollement and pushing seaward the frontal part of the wedge. The second mode results from the tectonics extension at the back wall with a similar half-graben kinematics and the landward sliding of the rear part of the wedge. The predictions of the maximum strength theorem, equivalent to the kinematic approach of limit analysis and based on these two collapse mechanisms, not only match exactly the solutions of the critical Coulomb wedge theory, once properly amended, but generalizes them in several aspects: wedge of finite size, composed of cohesive material and of arbitrary topography. This generalization is advantageous to progress in our understanding of many laboratory experiments and field cases. For example, it is claimed from analytical results validated by experiments that the stability transition for a cohesive, triangular wedge occurs with the activation of the maximum length of the decollement. It is shown that the details of the topography, for the particular example of the Mejillones peninsula (North Chile) is, however, responsible for the selection of a short length-scale, dynamic instability corresponding to a frontal gravitational instability. A reasonable amount of cohesion is sufficient for the pressures proposed in the literature to correspond to a stability transition and not with a dynamically unstable state.