Influence of Fluid Flow and Alteration on the Seismic Style of Oceanic Transform Faults

Abstract:

Oceanic transform faults experience smaller (rarely ≥ Mw 7) and less frequent earthquakes than expected for their length. Although the seismic-aseismic transition in mafic rocks is thought to occur at ~600°C, oceanic transforms commonly creep at T <600°C. Previous studies, based on remote seismic observations, propose that along-strike heterogeneity in seismicity is controlled by fault zone damage. In their interpretations, volumes of reduced seismic velocity are inferred to be highly damaged and host creep and microseismicity, whilst relatively intact rocks host larger earthquakes.

We constrain the effects of fault zone damage and fluids on deformation in the exhumed Southern Troodos Transform Fault Zone, Cyprus. Here, we document brittle and ductile deformation, localised in up to 100s m thick zones of steeply dipping, E-W striking, anastomosing fault strands within oceanic crust and upper mantle rocks. Extensive alteration of the mafic crust is consistent with abundant hydrothermal fluid circulation within the transform system at greenschist facies conditions (~300°C). Geological mapping reveals a series of fault rocks derived from dolerite dykes, including fractured dolerite, indurated breccias and foliated clay-rich gouges interpreted to reflect progressive increase in strain. Fracturing and brecciation of dolerite dykes increased permeability and localised fluid flow. This fluid flow aided syn-kinematic alteration of dolerite to clay-rich fault rocks by dissolution-precipitation processes. Clay growth acted to reduce the fault permeability and cause a transition in deformation style to sliding along foliation planes. We infer that the combination of progressive brittle fracture and alteration, and the absence of fault healing, was responsible for gradual weakening of individual fault strands along this oceanic transform.

In single-direct shear deformation experiments, intact dolerite is strong and velocity-weakening, whereas the clay-rich fault rocks are weak and velocity-strengthening. The progressive weakening characterised from geological observations is therefore matched by a transition from seismic to aseismic slip. Hence, we suggest that variable fault zone damage and hydration play a critical role in the different seismic behaviours observed along oceanic transforms.