The big crunch: Physical and chemical expressions of arc/continent collision in the Western Bismarck arc
Introduction
The Bismarck volcanic arc is an intra-oceanic subduction system formed at the southern margin of the Bismarck Sea in the southwest Pacific Ocean. The arc can be divided into two sectors, east and west of longitude 148° E. The eastern part—the island of New Britain—continues to involve northward subduction of the small Solomon Sea plate, while the western arc now involves convergence between the Australian and South Bismarck plates in a zone of arc–continent collision. Magma genesis in the New Britain sector has been documented in some detail in previous publications (e.g. Johnson, 1977, Woodhead and Johnson, 1993, Woodhead et al., 1998, Woodhead et al., 2001); here we concentrate on the chemistry of magmas from the western collision zone, drawing upon previous work on New Britain for comparative purposes. The Western Bismarck arc constitutes a unique setting in which to study the process of arc–continent collision, since oblique convergence has shifted the collision front progressively eastward, allowing an unparalleled opportunity to study its temporal expression.
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Geological setting and previous studies
Papua New Guinea is one of the most complex tectonic regions in the world characterised by multiple interactions along a variety of plate boundaries (e.g.,Tregoning et al., 2000, Wallace et al., 2004). The broad tectonic context is determined by oblique convergence between the Australian and Pacific plates, with complexity introduced by the partitioning of deformation among three smaller plates—the Woodlark, South Bismarck and Solomon Sea plates—and a host of diffusely deforming zones, located
Samples studied and analytical methods
Our sample suite consists of a subset of 27 samples representative of the 169 X-ray fluorescence analyses presented by Johnson and Chappell (1979) for the Bismarck arc, encompassing fifteen volcanic centres of the Western Bismarck arc between Cape Gloucester on New Britain island and the Schoutens. For these samples we present inductively coupled plasma mass spectrometry (ICPMS) trace element and Sr-, Nd-, Pb-, and Hf-isotope analyses (Table 1). XRF major element and selected trace element data
Geophysical responses to arc–continent collision
Since the benchmark study of Pegler et al. (1995), the number of focal mechanism solutions in the Harvard Central Moment Tensor (CMT) database has almost doubled making it timely to reassess the seismic structure of this region. Here we briefly review the sub-lithospheric structure of the Western Bismarck collision zone as revealed by focal mechanisms, showing that these newly available data lend strong support for the main conclusions of Pegler et al. (1995), who postulated the existence of a
Conclusions
The great tectonic complexity of the Papua New Guinea region has resulted in a remarkable natural laboratory for the study of plate convergence and subduction. The Western Bismarck arc, situated off the north coast of Papua New Guinea preserves a valuable record of oblique and gradual arc–continent collision with the Australian plate spanning several million years.
Available seismic data reveal a steeply dipping and well-defined subducting slab in the east which appears decoupled from the
Acknowledgements
Samples were collected as part of a Geoscience Australia (formerly Bureau of Mineral Resources) mapping program in the 1970s. Bruce Chappell produced the XRF data for the original Papua New Guinea studies and Steve Eggins is thanked for his assistance with the ICPMS analyses at ANU. Detailed reviews from Richard Price and one anonymous reviewer helped to improve the manuscript significantly. Finally, it is a pleasure to acknowledge the many and varied contributions made by John Gamble over the
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