On seismicity and structural style of oceanic transform faults: A field geological perspective from the Troodos Ophiolite, Cyprus

Aseismic creep accommodates the majority of displacements along active oceanic transform faults, within their thermally defined seismogenic zones. The significant earthquakes that do occur are near periodic, and repeat in nearly constant locations. Neither of these observations is explained by the current models that infer an olivine-dominated rheology and a thermally controlled seismogenic zone. In this contribution, we review the geological observations from the exhumed Southern Troodos Transform Fault Zone of Cyprus, and discuss their implications for seismogenesis at modern oceanic transform faults. In crustal level rocks, displacement was accommodated on discrete faults and in broad breccia zones, whereas at mantle levels the dominant structures are serpentinite mélanges overprinting rare and volumetrically minor, ductilely deformed peridotites. We speculate that the seismic style of crustal level faults depends on whether slip is localized or distributed over a broad zone that must dilate during shear. At mantle levels, we highlight that the dominant deforming material is serpentinite, at least when—as in the case of Troodos—sufficient hydration has taken place. Our observations and inferences imply that transform fault seismicity depends on time-, strain-, and permeability-dependent processes, and is governed by the geological complexity at a range of scales