Kinematics of footwall exhumation at oceanic detachment faults: solid-block rotation and apparent unbending

Abstract

Abstract Seafloor spreading at slow rates can be accommodated on large-offset oceanic detachment faults (ODFs), that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). Footwall rock experiences large rotation during exhumation, yet important aspects of the kinematics - particularly the relative roles of rigid block rotation and flexure - are not clearly understood. Using a high-resolution numerical model, we explore the exhumation kinematics in the footwall beneath an emergent ODF/OCC. A key feature of the models is that footwall motion is dominated by solid rotation, accommodated by the concave-down ODF. This is attributed to a system behaviour in which the accumulation of distributed plastic strain is minimized. A consequence of these kinematics is that curvature measured along the ODF is representative of a neutral stress configuration, rather than a 'bent' one. Instead, it is in the subsequent process of `apparent unbending’ that significant flexural stresses are developed in the model footwall. The brittle strain associated with apparent unbending is produced dominantly in extension, beneath the OCC, consistent with earthquake clustering observed in the Trans-Atlantic Geotraverse at the Mid-Atlantic Ridge.