Ake Fagereng, Chris MacLeod

Aseismic creep accommodates the majority of displacement along active oceanic transform faults, also within their thermally defined seismogenic zone. The significant earthquakes that do occur are near periodic, and repeat in nearly constant locations. Neither of these observations is explained by current models that infer an olivine-dominated rheology and a thermally controlled seismogenic zone. In this contribution we review 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 localised, 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- and strain- and permeability-dependent processes, and is governed by geological complexity at a range of scales.