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

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.