Tomography

Tectonic reconstructions predict where lithosphere has been subducted through geological time and consequently how much lithosphere has been consumed in plate convergence zones. Tomographic methods can provide new information which can help to test and improve tectonic models. In general terms, subducted lithosphere produces a strong temperature anomaly in the upper mantle which causes slabs to be seismologically detectable as regions with relatively fast seismic wave speeds. Thus, tomographic models provide an additional and independent source of information on tectonic evolution which can be exploited to test tectonic reconstructions while plate tectonic models of a region can help in developing new interpretations of imaged mantle structure.

The region of SE Asia and the SW Pacific is a complex and actively deforming part of the globe which includes some of the fastest relative plate motions on Earth. Present-day plate motions offer some insight into the recent history of the region but even a cursory study of the region shows that present motions provide only a snapshot of the tectonic history. Plate motion models indicate that Australia has moved rapidly north since the Eocene and that this movement has been accompanied by subduction and collision events. To reconstruct the region in the past requires an interpretation of the evidence remaining, but the nature of subduction and collision means that some of the evidence is destroyed. Thus, there are many different tectonic models and distinguishing between them can be difficult. With seismic tomography the preserved record of former plate convergence can be made visible, within spatial resolution limits.

Recent developments in data analysis and tomographic techniques have led to a new generation of global models which are capable of resolving smaller details of mantle structure. In particular, the P-wave speed model of Wim Spakman�s group at theVening Meinesz Research School of Geodynamics Utrecht shows details of slab morphology comparable to those previously seen only in studies of smaller regions of Australia and the SW Pacific.

So far, Wim Spakman and Robert Hall have been comparing the tomographic model and tectonic interpretations of a region extending from the Molucca Sea eastward to Tonga, and from the Australian craton north into the Pacific, in order to understand the evolution of the northern Australian plate margin during the Cenozoic. This allows a qualitative comparison of predictions of subduction derived from the tectonic reconstruction to imaged mantle structure for the region extending from the Molucca Sea eastwards to Tonga, and from the Australian craton north into the Pacific. It has been possible to interpret several enigmatic, spatially large, wave speed anomalies at great depth in the mantle as remnants of old (pre-25 Ma) subduction systems.

The tomographic images of the mantle beneath the region extending from the Molucca Sea eastward to Tonga, and from the Australian craton north into the Pacific, reveal a number of distinctive positive anomalies. The anomalies can be interpreted as subducted slabs and the positions of the slabs can be compared to predictions made by tectonic models for the region. Several strong anomalies are due to present-day subduction and the slab lengths and positions are consistent with Neogene subduction at the Tonga and the New Hebrides trenches, where the anomalies suggest rapid rollback of subduction hinges since about 10 Ma, and beneath the New Britain and Halmahera arcs. There are several generally flat-lying deeper anomalies which are not related to present subduction. Beneath the Bird�s Head and Arafura Sea is an anomaly which we interpret to be the result of north-dipping subduction beneath the Philippines-Halmahera arc between 45 and 25 Ma. A very large anomaly, which extends from the Papuan Peninsula to the New Hebrides and from the Solomon Islands to the east Australian margin, is interpreted as the result of south-dipping subduction beneath the Melanesian arc between 45 and 25 Ma. The interpretation implies that a flat-lying slab can survive for many tens of millions of years at the bottom of the upper mantle. There is a huge anomaly in the lower mantle which extends from beneath the Gulf of Carpentaria to Papua. This is suggested to be a slab subducted before 45 Ma which may be correlated with a Cretaceous slab beneath the Australian-Antarctic discordance or an early Cenozoic slab subducted north of Australia. The anomaly is located above the position where there must have been a change in polarity in subduction at the boundary between the north and south-dipping subduction zones north of Australia between 45 and 25 Ma. All of these have been overridden by Australia since 25 Ma. One subduction system predicted by the tectonic models, the Marumuni arc of Papua New Guinea, is not seen on the tomographic images.