Robert Hall & Wim Spakman
Some important aspects of the geological evolution of SE Asia are reflected in the structure of the lithosphere in the mantle and the mantle itself. The mantle also contains a record of some of the history of plate motions in and around the region. Understanding more about the mantle beneath SE Asia and interpreting the tectonic record it contains is part of our continuing research.
Subducted lithosphere produces a strong temperature anomaly in the upper mantle which causes slabs to be detectable as regions with relatively fast seismic wave speeds. P- and S-wave tomography can identify these anomalies which can then be interpreted in terms of tectonic models. The P-wave global tomographic model of Wim Spakman at Utrecht allows variation in cell dimensions as a function of local seismic data density which can show many details of mantle structure at a fine scale and thus reveal past subduction. The smallest cells have dimensions of 0.6 degrees laterally and 35 km in depth. SE Asia and the SW Pacific is a complex and actively deforming part of the globe which includes some of the fastest moving plates on Earth. Present-day plate motions offer some insight into the recent history of the region but provide only a snapshot of tectonic history. Reconstructions based on geological evidence interpret where lithosphere has been subducted and how much has been consumed but during subduction and collision evidence is destroyed. Thus, distinguishing between the many different tectonic models can be difficult. Tomography provides an independent insight into tectonic evolution which can be used to test tectonic reconstructions.