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