AN: T12B-1310
TI: The Evolution of the Lau Basin: A New Perspective
AU: * Goodliffe, A M
EM: amg@hawaii.edu
AF: SOEST, University of Hawaii, 1680 East West Road, Honolulu, HI 96822
AU: Taylor, B
AF: SOEST, University of Hawaii, 1680 East West Road, Honolulu, HI 96822
AU: Martinez, F
AF: SOEST, University of Hawaii, 1680 East West Road, Honolulu, HI 96822
AB: Previous models for the evolution of the Lau back-arc basin, in the southwestern Pacific, have involved a protracted period of arc extension followed by the propagation southward from Peggy Ridge of organized seafloor spreading on the East Lau Spreading Center (ELSC) and the Central Lau Spreading Center (CLSC). The extensional origin of the Western Extensional Basin (WEB) was based largely on sparse single channel seismic data and wide beam bathymetry profiles. Following a HAWAII-MR1 sidescan and swath bathymetry survey of the Lau Basin between 18.75S and 17S it became evident that the seafloor fabric of the WEB has many of the characteristics typical of seafloor spreading, including failed and truncated propagating spreading centers. The recovery of MORB-like basalts at ODP site 834 in the WEB, a recent refraction study showing that there is no crustal thickness difference between the WEB and crust formed on the ELSC, and the lack of basement samples older than the opening age of the basin further suggest an accretionary origin. So far, we have been unable to date the well lineated magnetization anomalies in the WEB. Spreading on the WEB was replaced by the southward-propagating ELSC, resulting in an outer pseudofault extending from 17S to 21.5S. ODP Site 835, located in a basin along this pseudofault, yielded 3.5 Ma basalts of MORB affinity. The age predicted by the seafloor magnetization solution is 3.58 Ma. Seafloor produced on the ELSC adjacent to the WEB includes crust formed prior to magnetic chron 2A (2.58-3.58 Ma), the oldest identifiable magnetization anomaly in the Lau Basin. Identification of anomaly 2A on the east side of the Lau Basin adjacent to the active arc shows that the ELSC continued north of the Peggy Ridge transform fault. From at least 3.58 Ma to present, the ELSC has been progressively replaced from the north by the southward propagating Fonualei Rift Spreading Center (FRSC). Presently the southern tip of the FRSC is at 18S, adjacent to the active arc. The southward propagating CLSC is a relatively recent feature. Initiating at or near the Peggy Ridge transform fault, the CLSC, as opposed to the FRSC, is now replacing spreading on the ELSC. The CLSC comprises only limited pre-Brunhes chron (0.78 Ma) crust, and must be same age or younger than Peggy Ridge, which formed after the end of chron 2n (1.77 Ma) and before the start of the Brunhes chron (0.78 Ma).