Oblique boundary systems: nature Vs theory

Abstract:

Most plate boundaries are activated obliquely with respect to the direction of far field stresses, and purely orthogonal systems are exceptional. The obliquity of these systems is controlled by (i) lateral rheological variations within the lithosphere and (ii) the geodynamics of the global plate circuit. Plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influences strain localization. Geodynamical evolution controls large-scale mantle convection and plate re-organization, thus triggering plate kinematics variations, and the adjustment and re-orientation of far field stresses. These geological processes explain the localization of the strain along structures that are not perpendicular to the far field stresses direction. Here we present two natural examples of oblique boundary systems: (i) the Main Ethiopian Rift, which is a transtensive plate boundary between the Nubian and Somalian plates and (ii) the Sunda Subduction Zone, which is a transpressive boundary between the overriding Sunda plate and the subducting Australian plate.

In extensional oblique boundary systems, field data shows dip-slip faults indicating pure extension, irrespective of the fault strike with respect to the stress direction. This observation is in total disagreement with the commonly admitted theory, which states that during oblique extension, structures that are orthogonal to the stress direction show dip-slip kinematics, whereas the ones that are oblique to the stress direction show strike-slip kinematics.

In compressional oblique boundary systems, field investigations along plate-scale strike slip faults allow the identification of two domains, compressional and extensional, of diffuse strain. Here, the offset between theory and nature is also relevant, with the former stating that oblique convergence activates pure dip-slip in the subduction thrust and the development of a plate-scale strike-slip fault, and the latter showing a complex distributed strain pattern.

We should therefore aim at deepening our understanding on the mechanics of strain distribution and its controlling factors in oblique boundary systems, through detailed analyses of their inherited conditions, their kinematics and geodynamic evolution.