Abstract
AbstractOceanic transform faults display fewer and smaller‐magnitude earthquakes than expected for their length. Several mechanisms have been inferred to explain this seismic slip deficit, including increased fault zone damage resulting in elevated fluid flow, and the alteration of olivine to serpentine. However, to date, these possible mechanisms are not supported by direct observation. We use micro to kilometer scale observations from an exhumed oceanic transform fault in the Troodos Ophiolite, Cyprus, to determine mineral‐scale deformation mechanisms and infer likely controls on seismic behavior of serpentinized lithospheric mantle in active oceanic transform faults. We document a range of deformation fabrics including massive, scaly and phyllonitic serpentinite, attesting to mixed brittle‐ductile deformation within serpentinite shear zones. The progressive development of a foliation, with cumulative strain, is an efficient weakening mechanism in scaly and phyllonitic serpentinite. Further weakening is promoted by a transition in the serpentine polytype from lizardite‐dominated massive and scaly serpentinites to chrysotile‐dominated phyllonitic serpentinite. The development of a foliation and polytype transition requires dissolution‐precipitation processes. Discrete faults and fractures locally crosscut, but are also deformed by, foliated serpentinites. These brittle structures can be explained by local and transient elevated strain rates, and play a crucial role in strain localization by providing positive feedback for dissolution‐precipitation by increasing permeability. We propose that the evolution in structure and deformation style documented within the serpentinized lithospheric mantle of the Southern Troodos Transform Fault Zone is a viable explanation for the dominantly creeping behavior and long‐term weakness of oceanic transform faults.
Highlights
Oceanic transform faults offset mid-ocean ridge segments for up to hundreds of kilometers
Macroscopic Field Observations Mapping of an ∼2 km2 area of the exposed mantle section within the Limassol Forest Complex reveals that deformation is heterogeneously distributed, with intensity varying on a meter to kilometer scale (Figure 2)
The particular shear zone mapped here displays an anastomosing tectonic fabric with an average moderate to steeply dipping foliation striking ∼E-W (083/67° S) and gently plunging lineation (
Summary
Oceanic transform faults offset mid-ocean ridge segments for up to hundreds of kilometers. Assuming a 25 km thick seismogenic zone (e.g., Prigent et al, 2020), rupture of a 200 km long, vertical transform fault would produce an Mw ∼8 earthquake Despite their length and the fact they crosscut the brittle crust, geophysical observations show they host fewer and smaller earthquakes (rarely ≥Mw 7.0) than expected, and globally about 85% of their displacement occurs by aseismic creep (Boettcher & Jordan, 2004). The base of the seismogenic zone, defined as the depth range where earthquakes can nucleate, has been correlated with the 600°C isotherm based on the comparison of numerical thermal models and the depth limit of earthquake focal depths (Abercrombie & Ekström, 2001; Roland et al, 2010) This thermal control coincides with a change to ductile, velocity-strengthening behavior in olivine as extrapolated from laboratory deformation data (Boettcher et al, 2007). Along the East Pacific Rise most oceanic transforms accommodate displacement almost entirely aseismically, with a χ < 0.2 (Boettcher & Jordan, 2004)
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