GEOPHYSICAL AND MORPHO-TECTONIC STUDY OF THE TRANSITION BETWEEN SEA-FLOOR SPREADING AND CONTINENTAL RIFTING, WESTERN WOODLARK BASIN, PAPUA-NEW-GUINEA

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and contin ental rifting in the western Woodlark Basin, off-shore eastern Papua N ew Guinea. The oceanic domain comprises new oceanic crust formed durin g the Bruhnes Epoch, older transitional crust and the rifted continent al margins. Two rift branches are recognized within the continental do main. The southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crus tal accretion. Based on magnetic data, seafloor spreading in the weste rn part of the Woodlark Basin commenced between 3.5 and 2.5 Ma along t he northern margin of the basin and maintained its position adjacent t o this margin up to 0.8 Ma (Jaramillo event). Just prior to the Bruhne s Epoch, the seafloor spreading axis jumped southward to its current p osition. Frequent jumps of seafloor spreading centers and relocation o f paleoaxes indicate the instability of the Woodlark extensional syste m. The transition between the two domains is characterized by differen tial localization of extensional strain. Variations in the localizatio n can result from progressive change of extensional mode when the init ial rifting evolves into seafloor spreading and/or from differences in lithospheric rheology as the rift propagates into the continental mar gin. A consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced acros s the transfer zone by broadly distributed deformation within continen tal lithosphere. (C) 1997 Elsevier Science B.V.