The Seismic Signature of the Atlantis Massif Oceanic Detachment Fault Determined From Crystallographic Preferred Orientation and Microstructure

Abstract At slow‐spreading mid‐ocean ridges large scale detachment faults partly accommodate the spreading process. The mafic and ultramafic rocks at the ridges are infiltrated by water and change their mineralogy. Subsequently or contemporaneously to these reactions, detachment faulting takes place. Alteration and deformation of the rocks lead to a different seismic signal providing a possibility to identify detachment faults in seismic studies. Here, we analyze how alteration and deformation change the seismic signature of the oceanic lithosphere around such a detachment fault at the Atlantis Massif Oceanic Core Complex, drilled during International Ocean Discovery Program Expedition 357. A set of 10 samples from the drill cores is analyzed with respect to mineralogy, microstructure and crystallographic preferred orientation using X‐ray powder diffraction, optical microscopy as well as synchrotron and neutron texture analysis. Seismic velocities, anisotropies and ratios are calculated from the results, compared to a shipboard derived, experimental sample set and interpreted with respect to the spreading ridge. The formation of deformed talc‐amphibole‐chlorite schists from mafic rocks and foliated serpentinites from ultramafic rocks generates a bulk seismic anisotropy with fault (sub)parallel directions of fast wave velocities. Our results show that P‐wave velocities (Vp) < 6.9 km/s concomitant with shear wave splitting (dVs) in a range of 0.1–0.5 km/s may represent a unique seismic signature for such fault to differentiate from fresh, undeformed lithologies.