B. Taylor, F. Martinez, A. Goodliffe (SOEST, University of Hawaii, Honolulu HI 96822; 808-956-6649, taylor@soest.hawaii.edu)
Extensional transform zones (ETZs) are accreting plate boundary segments of order 100 km long that strike at angles between 15 and 45 degrees to the extension direction. They are characterized by neovolcanic/tectonic zones comprising overlapping en echelon volcanic systems and/or faults that trend 30 to 75 degrees to the extension direction, sometimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate boundary is probably continuous. Taylor et al. 1994 (JGR 99, 19707) described the Manus Basin, Reykjanes Peninsula, Tjornes, Mak'Arrasou, Brawley and Cerro-Prieta ETZs. They also postulated that in the Lau back-arc basin an ETZ may link the Peggy Ridge Transform Fault (PRTF) and the Central Lau Spreading Center (CLSC).We report results from a 1996 marine geophysical and HAWAII MR1 sidescan survey of the Lau Basin (16.5-18.6S) that confirm the existence, and characterize the nature, of the Lau ETZ (LETZ). About 240 km long and 20-25 km wide, the LETZ is the largest ETZ discovered to date. In the north, the LETZ overlaps the south side of the PRTF (a sharp ridge which trends 129) to its intersection with the NW Lau Spreading Center. It includes volcanic highs shoaler than 1 km as well as pull-apart basins 3 km deep and a 135-trending strike-slip fault. The southern LETZ trends 158 and comprises en echelon, sigmoidal volcanic systems with fissure ridges trending 174-200. A strike-slip fault trending 149 cuts the inside corner between the northern and southern LETZ. The LETZ intersection with the CLSC (trend 025) is an overlapping spreading center. The LETZ developed from an originally orthogonal intersection between the CLSC and the PRTF and has cross cut and overprinted previous crustal fabric and magnetic anomalies. The CLSC has asymmetrically propagated southward, lengthening the LETZ. The total opening rates on the LETZ, averaged over the Brunhes chron, are up to 100 mm/yr.