Abstract
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.
Highlights
Oceanic transform fault plate boundaries may offset mid-ocean ridges by hundreds of kilometres
Most clearly displayed in the sheeted dyke sequence, the mafic crust deformed by a combination of discrete faults and distributed fracturing (Figure 4g), leading to local development of broad breccia zones (Figure 4e, f)
We suggest that the internal structure of modern oceanic transform faults will be broadly comparable to that of the STTFZ, and that the seismic style of modern oceanic transforms (Figure 1) is likely to result from similar geological complexity across a range of scales
Summary
Oceanic transform fault plate boundaries may offset mid-ocean ridges by hundreds of kilometres. These discrete serpentinite shear zones typically form E-W striking, vertical features; they may bound and juxtapose brittlely-deformed disrupted ocean crustal blocks inferred to have formed at the Anti-Troodos ridge axis and been incorporated into the broader transform-tectonised zone. Faults may either be transform parallel, with a normal component of displacement stepping down towards the AFB and northern LFC, or else dyke parallel On the latter, low-angle slickensides with minor sinistral offsets may be developed on individual dyke margins in relatively low-strain lenses, surrounded by discrete surfaces with gouge and foliated cataclasite passing into indurated breccia zones, together forming an anastomosing zone of deformation distributed over a tens to hundreds of metre scale (Figure 4e). Mineralisation is concentrated within the fault zones, demonstrating the strong link between fluid flow and deformation
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