Evidence for fault weakness and ̄ uid ̄ ow within an active low - angle normal fault

Determining the composition and physical properties of shallowdipping, active normal faults (dips , 358 with respect to the horizontal) is important for understanding how such faults slip under low resolved shear stress and accommodate signi®cant extension of the crust and lithosphere. Seismic re ̄ection images and earthquake source parameters 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±308 located ahead of a propagating spreading centre in the Woodlark basin. Here we present results from a genetic algorithm inversion of seismic re ̄ection 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, which we interpret to be composed of serpentinite fault gouge. Isolated zones exhibit velocities as low as ,1.7 km s with high porosities, which we suggest are maintained by high ̄uid pressures. We propose that hydrothermal ̄uid ̄ow, 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. We use multichannel seismic (MCS) re ̄ection data acquired aboard the RV Maurice Ewing during a 1995 survey and drilling results from Ocean Drilling Program (ODP) Leg 180 (refs 3 and 4; Fig. 1). MCS pro®les reveal a normal fault that maintains a dip of 25±308 to about 9 km depth and an offset of 10±12 km between the sedimented hanging wall and the Moresby seamount footwall, which is composed of Palaeocene arc-ophiolite gabbro and dolerite. Faulting and uplift of Moresby seamount is estimated to have begun within the last 3.5 Myr, based on the ®rst occurrence of metamorphic talus found in the downdropped hanging wall at ODP Site 1108 and an abrupt increase in sedimentation rate that indicates rapid subsidence of the northern margin at this time. This fault is one of the major structures on which continental extension appears to be localized. The close proximity of the fault to the