Quantitative constraint on footwall rotations at the 15°45'N oceanic core complex, Mid-Atlantic Ridge: implications for oceanic detachment fault processes

The subsurface geometry of detachment faults at slow spreading mid-ocean ridges is debated: are they planar features that form and slip at low angles, as often inferred for their continental equivalents, or do they initiate at steep angles and then flatten in response to flexural unloading as displacement proceeds, as predicted in “rolling hinge” conceptual models? An essential difference is that significant rotation of the footwall should occur in the rolling hinge but not the planar fault model. This can be tested using paleomagnetism. Previous attempts to address this question have relied upon data from azimuthally unoriented drill cores. Although results are consistent with large rotations having occurred, these interpretations are very nonunique, and other solutions that require minimal rotations are equally permissible. We here present a rigorous analysis of paleomagnetic and structural data from a unique set of azimuthally oriented cores, collected using a seabed rock drill, from the 15°45′N oceanic core complex on the Mid-Atlantic Ridge. By considering the full paleomagnetic remanence vector in combination with kinematic data from the detachment fault shear zone we are able to quantitatively constrain the geometrically permissible axes and magnitudes of rotation of the detachment fault footwall, for the first time without having to make a priori assumptions about the orientation of the axis. We show that significant rotations (64° ± 16°) have indeed occurred, about a gently plunging, near-ridge-parallel axis, robustly supporting the rolling hinge models. We further discuss the geological implications of this result for oceanic detachment fault processes