A. M. Goodliffe; B Taylor; F. Martinez; R. N. Hey (SOEST, University of Hawai'i, 2525 Correa Rd., Honolulu, HI 96822; ph. 808-956-5238; e-mail: andrew@soest.hawaii.edu)
HAWAII-MR1 swath bathymetry and acoustic imagery, Hydrosweep bathymetry, and a dense coverage of magnetic data, have allowed us to unambiguously define the entire 500 km length of the Woodlark basin spreading center. These data show that the spreading center has undergone a 15 degree reorientation within the Brunhes chron. Reorientation took place geologically simultaneously along the 500 km length of the spreading center. There is no evidence for a propagation event, and the new spreading segments cross cut the older fabric with disregard for any pre-existing weaknesses.The westward propagating Woodlark spreading center can be subdivided into five first-order segments, with an abrupt westward shoaling of the sea-floor across the north-south trending Moresby transform between segments 2 and 3. As part of the reorientation, three segments (1, 3 and 4) subdivided, segments 1 and 3 into two parts, segment 4 into three parts. There is an overlapping relationship between each of the new subsegments. The 150 km long segment 2, as well as the short (40 km) segment 5 remained intact. Segments 2 and 3A completely transect the paleo-spreading axis.In the case of all the first-order segments a sharp boundary is seen between the post- and pre-reorientation fabric. This allows us to date the reorientation, with the help of sea-floor magnetization data, at about 0.1 Ma and to limit the rotation duration to less than 0.01 Ma.This type of instantaneous reorientation challenges some of the accepted ideas for the mechanisms of crustal fracture, i.e. that the only way to fracture crust is by stress focusing in the case of rift propagation, or by exploiting pre-existing weaknesses.