Abstract

Oceanic core complexes are the footwalls of low-angle detachment fault systems that exhume lower crust and mantle rocks at slow-spreading mid-ocean ridges. However, relatively little is known about how they initiate or become inactive, and their lithological structure is uncertain. I shall present results from a recent study of active and recently defunct core complexes on the Mid-Atlantic Ridge near 13°N. High-resolution sonar images and limited sampling are consistent with a model in which the smooth, domed and striated part of the footwall represents the top of the mantle section and is connected to a rugged, deformed high massif representing the mafic crust. The detachments initiated as high-angle normal faults similar to other typical valley-wall normal faults. Like these, they rapidly flatten by >30° in response to flexural unloading. Local variations in magma supply appear to play a critical role in determining the fate of ridge-axis normal faults. When most plate separation is accommodated by axial intrusion of magma (classic sea-floor spreading), normal faults play a minor role. They take up less than 20% of plate extension, slipping for a few hundred thousand years until lithospheric thickening and strengthening make continued fault slip harder than the creation of a new fault. However, if magma supply to the ridge axis is significantly reduced, most plate extension must be taken up by faulting. In areas of reduced magma, peridotite is more accessible to normal faults and penetrating water, leading to creation of talc and serpentine. These extremely weak minerals allow continued slip and runaway displacement on pre-existing faults, leading to core complex formation. Now the effective plate boundary has jumped from the magmatic axis to an off-axis detachment fault. In this configuration plate accretion is highly asymmetric and unstable: the detachment is forced to migrate back towards the magmatic axis. Our imaging data confirm that ridge-axis volcanism is absent opposite actively extending detachment faults. It appears that renewed axial volcanism propagates along axis and eventually cuts off the migrating detachment footwall at depth, thus terminating its activity.

Reference: MacLeod, C.J., R.C. Searle, B.J. Murton, et al., Life cycle of oceanic core complexes, Earth and Planetary Science Letters, under review, 2009.