Oceanic detachment faults generate compression in extension

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geology 45 (2017): 923-926, doi:10.1130/G39232.1.In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates 244,000 events detected along 25 km of the ridge axis, to depths of ∼10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3–7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.This research was funded by Natural Environment Research Council (NERC) grants NE/J02029X/1, NE/ J021741/1, and NE/J022551/1, and by U.S. National Science Foundation grant OCE-1458084