We report new geochemical and geochronological data on igneous rocks of the little-studied Western Belt of the Vanuatu intra-oceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place formation of these rocks in the late Eocene to Miocene; hence, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general does match the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not be found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intra-oceanic arc provides an alternative source for the crustal trace element and isotopic signature of island-arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intra-oceanic arcs

The Bismarck Sea region of Papua New Guinea is marked by recent arc–continent collision giving rise to a highly dynamic tectonic environment, characterised by complex plate interactions that are yet to be fully understood. We present a new crustal and upper mantle crustal architecture model for northeastern Papua New Guinea and western New Britain that reveals complex tectonic geometries of overprinting slab subduction and partial continental subduction, resulting in a unique setting in which to investigate along-arc magmatic variation. Earthquake hypocentre databases are combined with detailed topography and seafloor structure together with geology and regional-scale gravity to unravel the sub-surface structure of northeastern Papua New Guinea. These data are used in conjunction with an updated 3-D slab map of the region to propose a new interpretation of the area whereby Australian continental crust extends as an underthrust block beneath the accreted Finisterre Terrane. The subducting continental crust combined with slab stagnation has resulted in a complex pattern of arc-related geochemical signatures from east to west along the Bismarck arc. In the east, where the Solomon Sea plate is subducting beneath New Britain, the sedimentary component is low, whereas in the west, the arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal sediments. As a result, a new plate reconstruction is provided for the region together with a forward-looking reconstruction of the Papuan peninsula, the Solomon Sea plate and New Britain that illustrates that the same process will likely be repeated in some 5–10 m.y.

An unusual group of more than 100 earthquakes is located ∼600 km beneath the North Fiji Basin in the southwest Pacific Ocean. These earthquakes are attributed to seismicity within detached segments of the subducted Australian plate. One of these detached slab segments has collided with, and now impinges on, the subducted Pacific plate at a depth of ∼500 km. The region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large-magnitude (Mw >7.0) earthquakes for this depth. The folded shape of the Pacific slab beneath Fiji, as well as the abundance of earthquakes, is interpreted to result from deformation and deformation-enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity. Detachment of the slab segments is interpreted to have occurred at ca. 5 Ma, but collision between the eastern segment and the west-dipping Pacific slab is interpreted to have occurred at ca. 4 Ma, coincident with initial opening of the overriding Lau Basin