Palaeomagnetic data from a Mesozoic Philippine Sea Plate ophiolite on Obi Island, Eastern Indonesia

Palaeomagnetic data are presented from part of the Halmahera ophiolite expo sed on Obi Island, eastern Indonesia. Until the late Neogene, Obi formed pa rt of the southern Philippine Sea Plate; it is now isolated from that plate and is located between fault strands in the left-lateral Sorong Fault Zone . Two areas were sampled: the first area comprised two sites from a microga bbro and a third site in a thin intruding dyke, and the second area yielded one site from a sheeted dyke suite. The mean in situ direction for the two areas is D = 216.1 degrees, I = 23.3 degrees, where the angular separation is 34.7 degrees. Rotating the mean directions back to the palaeo-vertical clusters the vectors, so that D = 219.4 degrees, I = 12.1 degrees, where th e angular separation is 20.1 degrees This clustering, together with other l ines of palaeomagnetic evidence, suggests that the magnetisation is primary . The ophiolite is Mesozoic, and most likely formed in the Jurassic. This i nformation, together with recently published palaeomagnetic data from nearb y Upper Cretaceous Philippine Sea Plate formations, suggest that the oldest parts of the Philippine Sea Plate were situated close to the equator in th e western Pacific in the middle Mesozoic. (C) 2001 Elsevier science Ltd. Al l rights reserved.