PALEOMAGNETISM AND TECTONICS OF MALAITA, SOLOMON-ISLANDS

Malaita, in the Solomon Islands, is the emergent expression of the Pacific Province, a geologically distinct suspect terrane which is commonly held to be the margin of the Ontong Java Plateau, a thickened portion of Pacific plate overthrust onto the rest of the Solomon Islands Arc during a late Miocene reversal of the polarity of the arc. Subduction of the Australian plate replaced Pacific plate subduction during this event, and the Solomon Islands Arc was transferred from the Australian plate to the Pacific plate. A paleomagnetic survey of the eastern Solomon Islands included sampling in southern Malaita. Magnetostratigraphic dating gives an age of 5.8 Ma for the Haruta Calcisiltite‐Hada Calcisiltite facies change on Malaita. This facies change resulted from renewed volcanic activity following the initiation of Australian plate subduction. Paleomagnetic poles from two pre‐Pliocene units do not lie near the Pacific apparent polar wander path (APWP), as the conventional model would suggest, but rather fall on the Australian APWP, suggesting that the Pacific Province developed as a part of the Australian plate. Correction for 6.5 Ma of Pacific‐Australia relative motion produces an improved, very close fit to the Australian APWP, consistent with the transfer of Malaita, along with the rest of the arc, to the Pacific plate at about that time. Magnetic paleolatitudes from a DSDP hole on the Ontong Java Plateau, which disagree with usually accepted Pacific Tertiary motion, also fail to match the Malaitan data; the possibility that Malaita could be a part of an Ontong Java sub‐plate, somehow detached from the rest of the Pacific plate, can be discounted. Malaita and the rest of the Pacific Province arose in the fore‐arc of the originally north‐east facing arc. Collision of the Ontong Java Plateau with the trench slowed subduction of the Pacific plate and began subduction of the Australian plate below the arc. Both subduction zones may have been active for a period, beginning at about 10 Ma and ending at about 3 Ma, until Pacific plate subduction broke down, and the Pacific Province was subsequently uplifted and deformed. Copyright 1990 by the American Geophysical Union.