The big crunch: Physical and chemical expressions of arc/continent collision in the Western Bismarck arc

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Abstract

The Western Bismarck arc of Papua New Guinea provides a unique setting in which to study both the seismic and chemical expressions of arc–continent collisional processes. Here collision has been oblique, occurring progressively along the arc from west to east, and currently centred around ∼ 148°E. This progressive collision is reflected in sub-lithospheric seismic activity. Centroid Moment Tensor (CMT) focal mechanism data show that the north-dipping Solomon Sea slab subducting beneath New Britain can be traced westwards beneath the collision front. Beneath the Finisterre Ranges it defines an inverted “U”-shape with a steep northern limb extending to at least 250 km depth. Mechanical decoupling of the slab from the overlying lithosphere is indicted by a 90° rotation in T-axes at ∼ 100 km. Further east, the seismic expression is less coherent suggesting progressive dismemberment and foundering of the slab, with maximum centroid depths shallowing to less than 125 km east of 144°E. When coupled with the known history of convergence, major, trace element and Sr-, Nd-, Pb-, and Hf-isotope compositions of volcanic rocks, provide further insights. The most dramatic geochemical expression of the collisional process is preserved in Pb-isotope ratios which peak in the zone of maximum convergence, reflecting the greatest slab-derived influence. Volcanic rocks in zones where convergence is complete show minimal expression of the slab-derived component although the fact that volcanism is still occurring suggests that either slab dewatering is not essential for initiating volcanism or that the process of dewatering continues long after subduction has ceased. Limited data from similar collision zones elsewhere suggest that these may be general observations applicable on a global scale and thus may provide a prediction of what might be expected in other areas of future convergence such as the Molucca Sea.

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.

Section snippets

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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