Strain localization in the oceanic lithosphere: The role of alteration products and fluid flow

Strain localization in the oceanic lithosphere is most extreme where ultramafic and mafic rocks are associated. An extreme manifestation of this strain localization is found along oceanic detachment faults, which can operate over periods of time of up to a few million years, accumulate displacements of 10s of km while accommodating >50% of the plate separation, and exhume deep-seated crustal and mantle rocks. These faults are systematically associated with fluid flow, as demonstrated by their association to submarine hydrothermal vents, or by fault rocks recording a complex interaction of deformation and coeval fluid flow. This fluid circulation within faults leads to concomitent serpentinisation and metasomatism of rock margins which pervasively form the fault rock, resulting in a peculiar assemblage of reaction products, composed of phyllosilicates (talc, chlorite, serpentine) and tremolite in different proportions and abundances. Mechanical properties and deformation mechanisms have been investigated experimentally for talc and serpentinite (P<1 GPa, T<700°C). They are weak relative to other rocks, and display low friction coefficients that decrease significantly with increasing T (from ~0.2 to <0.1 for talc at room T and 600°C). At the same time they exhibit an efficient strain localization at high T, with deformation taking place in the brittle or semibrittle regime; no full plasticity was achieved at experimental conditions. In naturally deformed rocks, talc growth is either static or syntectonic, displaying semibrittle deformation (intragranular kinking and grain sliding along weak bonding basal planes). When present, talc preferentially accommodates strain. Serpentine schists are scarce, suggesting that fluid flow and metasomatism along the fault is pervasive. In naturally deformed rocks tremolite displays both inter- and intragranular fracturing, or ductile deformation by syntectonic growth and later kinking. Fluid circulation between grains leads to progressive replacement of amphibole by chlorite oriented along shear planes and thus to a foliation development. The complex microstructural relationships suggests transient modes of deformation within a weak fault, probably controlled by the nature of fluid flow and the resulting mineral assemblage. Talc, which is stable at T<750°C and potentially present to the base of the lithosphere, and serpentinites, stable at T<600°C, have a velocity-strengthening behavior promoting aseismic slip. Seismicity associated with oceanic detachments extends to ~7 km bsf and displays constant rates. This suggests that talc and serpentinite may not be ubiquitous along the fault zone. Instead, their presence in small amounts and/or with a heterogeneous distribution may reduce significantly the overall strength of faults through the whole lithospheric thickness, limit the rupture size of earthquakes, and promote sustained deformation at detachments. Microstructural observations of naturally deformed rocks, together with the structural similarities that these sheeted minerals display, suggest that these phases in general may strongly favor strain localization in a wide range of oceanic environments (mid-ocean ridges, transform faults, subduction zones).