Abstract
Determining the composition and physical properties of shallow-dipping, active normal faults (dips < 35° with respect to the horizontal) is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere. Seismic reflection images1 and earthquake source parameters2 show that a magnitude 6.2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25–30° located ahead of a propagating spreading centre in the Woodlark basin. Here we present results from a genetic algorithm inversion of seismic reflection data, which shows that the fault at 4–5 km depth contains a 33-m-thick layer with seismic velocities of about 4.3 km s-1, which we interpret to be composed of serpentinite fault gouge. Isolated zones exhibit velocities as low as ∼1.7 km s-1 with high porosities, which we suggest are maintained by high fluid pressures. We propose that hydrothermal fluid flow, possibly driven by a deep magmatic heat source, and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low-angle normal fault.
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Acknowledgements
We thank W. Menke, C. Scholz, S. Carbotte, A. Lerner-Lam and D. Goldberg for helpful discussions and R. Detrick and W. Bosworth for comments that improved this Letter. J.F. and J.M. were supported under a grant from JOI/USSAC. We thank the captain and crew of the RV Maurice Ewing for MCS data acquired on EW9510. ODP Leg 180 drilling results used in this study were obtained thanks to the efforts of the captain and crew of the DV JOIDES Resolution; A. Klaus and the TAMU scientific support staff; co-chiefs P. Houchon and B. Taylor and the ODP Leg 180 Scientific Party.
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Floyd, J., Mutter, J., Goodliffe, A. et al. Evidence for fault weakness and fluid flow within an active low-angle normal fault. Nature 411, 779–783 (2001). https://doi.org/10.1038/35081040
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DOI: https://doi.org/10.1038/35081040
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