Ocean Drilling Program, United States

The lateral variation from active continental rifting to seafloor spreading within a small region makes the western Woodlark Basin an attractive area to investigate the mechanics of lithospheric extension. Earthquake source parameters and seismic reflection data indicate that low-angle ( approximately 25 degrees to 30 degrees ) normal faulting is active in the region of incipient continental separation. A low-angle normal fault emerges along the northern flank of Moresby Seamount, a continental crustal block with greenschist metamorphic basement. Asymmetric basement fault blocks overlain by only minor ponded sediments characterize the margin to the south, whereas the margin to the north has a down flexed prerift sedimentary basin and basement sequence unconformably onlapped by synrift sediments. Leg 180 will drill a transect of three sites just ahead of the spreading tip: ACE-9a on the down flexed northern margin; ACE-8a through the rift basin sediments, the low-angle normal fault zone, and into the footwall; and ACE-3c near the crest of the footwall fault block (Moresby Seamount). The primary objectives at these sites are (1) to characterize the composition and in situ properties (stress, permeability, temperature, pressure, physical properties, and fluid pressure) of an active low-angle normal fault zone to understand how such faults slip, and (2) to determine the vertical motion history of both the downflexed hanging wall and the unloaded footwall as local groundtruth for input into regional models to determine the timing and amount of extension prior to spreading initiation.