Abstract. Earthquakes in the Woodlark-D'Entrecasteaux rift system reveal changes in fault geometry along strike, as extension progresses from continental to oceanic modes. In one segment, evidence is found for earthquakes on shallow-dipping normal faults. To examine these trends, locations of 147 earthquakes are redetermined, and focal mechanisms for 21 events are derived from waveform inversion. Where seafloor spreading occurs, east of 152øE, most earthquakes illuminate a NE-SW strike-slip fault north of the oceanic rift system along the Woodlark Rise. Elsewhere, earthquakes occur along a well-defined rift zone, and focal mechanisms reveal normal faulting with north-south T axes and depths less than 10 km. Most normal-faulting earthquakes show high-angle fault planes (dips 40ø-50ø), but several are consistent with slip on moderately low-angle planes dipping 15ø-35 ø. These low-angle nodal planes are only found in a region between 150.5øE and 152.5øE, where the transition occurs from seafloor spreading to continental rifting. A seismic reflection profile near 151 øE reveals a major normal fault that dips to the north at 25 ø -35 ø just landward of the oceanic rift tip. That fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied. Hence normal faults dipping -30 ø are likely to be seismically active in at least one part of this rift system. In this region just landward from the rift tip the highest extension rates are expected (--40 mm/yr) and strain appears localized on a single system of normal faults. Because shallow dips for normal faults are seen near the rift tip, but not elsewhere along strike, high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips. Perhaps high shear traction at the base of the upper crust, localized by nearby spreading, sufficiently alters the local stress regime.

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Seismic reflection data from the East Pacific Rise at lat 16°N, which is spreading at the high end of intermediate rates, suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate. Rather, magma-chamber depths form two distinct populations, each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate-spreading-rate range. Our data (1) show that the melt lens at high-intermediate-spreading ridges lies at a shallow level similar to lens depths at fasterspreading ridges, and (2) provide further support for the spreading-rate invariance of ridges with axial highs noted in other ridge properties. The axial morphology of the two ridge segments within the study area differs markedly, and a large contrast in magma supply is inferred. The ridge segment with greater magma supply is associated with a broader and more continuous melt lens, a wider region over which the extrusive crust accumulates, and a thicker extrusive layer off-axis where supply to the ridge segment appears to be centered. However, on-axis, the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed, consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens.