Tomography

Seismic tomography provides a powerful way to investigate the deep structure of the mantle beneath Southeast Asia and the southwest Pacific and to test tectonic reconstructions of this exceptionally complex region. Much of the SE Asia Research Group’s work in this area has been driven by the integration of mantle tomography led by Wim Spakman with plate tectonic reconstructions developed by Robert Hall, combining deep Earth imaging with regional tectonic modelling.

Plate tectonic reconstructions predict where lithosphere has been subducted through geological time and how much lithosphere has been consumed at convergent margins. Tomographic methods provide an independent way to test these predictions. Subducted lithosphere is colder and denser than the surrounding mantle and produces strong temperature anomalies, which appear in seismic tomographic models as regions with relatively high seismic wave speeds. These anomalies can therefore be interpreted as preserved slabs within the mantle.

In Southeast Asia and the southwest Pacific, where subduction and collision have destroyed much of the geological evidence at the surface, tomography allows parts of the tectonic history to be reconstructed from the mantle record itself.

Integrating Tomography and Plate Reconstructions

A major aim of this work is to integrate seismic tomographic imaging of the mantle with plate tectonic reconstructions of Southeast Asia and the southwest Pacific.

Robert Hall’s plate reconstructions predict where lithosphere should have been subducted through time. Tomographic models developed by Wim Spakman provide images of mantle structure that may represent those subducted slabs. Comparing the predicted positions of slabs with the imaged anomalies allows tectonic models to be tested and refined.

This combined approach has been applied to a large 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.

Imaging the Mantle Beneath Southeast Asia

Recent developments in tomographic methods have produced a new generation of global mantle models capable of resolving much finer details of mantle structure. In particular, the P-wave velocity model developed by Wim Spakman and colleagues provides high-resolution images of slab morphology beneath Southeast Asia and the southwest Pacific.

These models allow researchers to visualise the preserved record of former subduction zones within the mantle. In many cases the geometry, depth and orientation of slab anomalies can be compared directly with predictions from tectonic reconstructions.

Because Southeast Asia has experienced rapid plate motions, multiple subduction zones, and repeated changes in subduction polarity, this approach is especially valuable for distinguishing between competing tectonic interpretations.

Slab Geometry, Rollback and Ancient Subduction Systems

Tomographic images of the mantle beneath Southeast Asia and the southwest Pacific reveal a number of large positive velocity anomalies interpreted as subducted slabs.

Some of these anomalies correspond closely to present-day or Neogene subduction systems, including those beneath the Tonga, New Hebrides, New Britain and Halmahera arcs. The geometry of these slabs suggests rapid rollback of subduction hinges since approximately 10 million years ago.

Other anomalies occur at greater depths and are not associated with modern subduction zones. These are interpreted as remnants of older subduction systems that operated before about 25 million years ago.

For example:

Beneath the Bird’s Head and Arafura Sea an anomaly is interpreted as the result of north-dipping subduction beneath the Philippines–Halmahera arc between about 45 and 25 Ma.
A very large anomaly extending from the Papuan Peninsula to New Hebrides, and from the Solomon Islands to the eastern Australian margin, is interpreted as the result of south-dipping subduction beneath the Melanesian arc during the same interval.
A large lower-mantle anomaly beneath northern Australia may represent a much older slab, possibly related to Cretaceous or early Cenozoic subduction north of Australia.

These interpretations suggest that flat-lying slabs can survive for tens of millions of years at the base of the upper mantle, preserving a long-term record of plate convergence.

Interestingly, one subduction system predicted by tectonic reconstructions—the Marumuni arc of Papua New Guinea—is not clearly visible in the tomographic images. This absence highlights the importance of integrating mantle imaging with tectonic modelling, since not all predicted structures are necessarily preserved or resolvable in the mantle record.

Regional Significance

The integration of mantle tomography and plate tectonic reconstruction, driven by the collaboration between Wim Spakman and Robert Hall, has provided new insights into the tectonic evolution of Southeast Asia and the southwest Pacific.

By linking deep mantle structure with surface geology, this work helps to:

identify the locations and geometries of ancient subduction zones
test competing tectonic models for the region
reconstruct the Cenozoic evolution of the northern Australian plate margin
understand the long-term interaction between slabs, mantle flow and plate motion

Tomography therefore provides a crucial additional perspective on the geological evolution of Southeast Asia, revealing parts of the tectonic record that are no longer preserved at the surface.