Evidence for fault weakness and fluid flow within an active low-angle normal fault

Determining the composition and physical properties of shallow-dipping, act ive normal faults (dips, <35<degrees> with respect to the horizontal) is im portant for understanding how such faults slip under low resolved shear str ess and accommodate significant extension of the crust and lithosphere. Sei smic reflection images(1) and earthquake source parameters(2) show that a m agnitude 6.2 earthquake occurred at about 5 km depth on or close to a norma l fault with a dip of 25-30 degrees located ahead of a propagating spreadin g centre in the Woodlark basin. Here we present results from a genetic algo rithm 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 similar to1.7 km s(-1) with hig h porosities, which we suggest are maintained by high fluid pressures. We p ropose that hydrothermal fluid flow, possibly driven by a deep magmatic hea t source, and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low-angle nor mal fault.