B. Taylor1; A. Goodliffe1; J. Mutter2; A. Lerner-Lam2; J. Floyd2; G. Abers3; A. Ferris3; M. Craig4; H. Davies4; R. Perembo4
1University of Hawaii, SOEST, 1680 East-West Rd., Honolulu, HI 96822
2Columbia University, LDEO, Rt. 9W, Palisades, NY 10964
3Boston University, Dept. Earth Sciences, 685 Commonwealth Ave., Boston, MA 02215
4University of Papua New Guinea, Box 414, University P.O., NCD, Papua New Guinea
The Woodlark Basin in the SW Pacific is a result of the rifting of continental lithosphere of the Papuan Peninsula and the westward propagation of seafloor spreading. It is conjectured that during rifting, low-angle normal faulting can accommodate large amounts of extensional strain. Although this hypothesis violates classical theories of frictional slip on faults, focal mechanisms for a number of moderate-sized earthquakes in the area do permit normal slip on planes dipping at low-angles (25-35 degrees). One of these earthquakes is located on or near a fault plane that has been imaged by seismic reflection data as a shallow north-dipping fault continuous with the northern flank of the continental Moresby Seamount. To investigate further the dynamics in the transition region between continental rifting and seafloor spreading, and to help resolve the low-angle normal faulting paradox, an onshore/offshore seismic experiment was conducted in 1999-2000 utilizing both passive and active sources. As a part of this experiment, an array of 20 OBS/Hs were deployed over an area of 30 km X 30 km just north of the Moresby Seamount and west of the spreading tip. This array recorded microseismicity for 6 months, but at the beginning of the deployment also recorded airgun blasts from an MCS survey. We present the results of a tomographic inversion of first arrival traveltimes from the airgun blasts recorded at the 14 functioning OBS/Hs. Our results represent the first detailed look at 3-D crustal velocity variations in the area. The data constrain velocities within a 80 km X 80 km region to maximum depths of ~12 km, although resolution degrades outside of the OBS/H array. A narrow high-velocity region at the SE corner of the array marks the westernmost spreading segment. Low velocities are associated with the fault-bounded rift basin north of Moresby Seamount. Further north, an ophiolitic crustal slice at least 10 km thick and with velocities up to 7.5 km/s dips 10-15 degrees north. There is little E-W velocity variation except in the poorly resolved region west of the array, where mantle-type velocities are present at 8 km depth, and beneath Moresby Seamount, where 7 km/s velocities occur only 3 km below sea level. Future work will focus on analyzing the microseismicity: locating the earthquakes within the 3-D velocity model to illuminate active structures, determining fault planes and focal mechanisms, and thereby deducing if active low-angle normal faulting is indeed occurring in the Woodlark Basin.