Tectonic evolution of SE Asia: introduction

SE Asia is probably the finest natural geological laboratory in the world yet is still not geologically well known. It is a spectacular region in which the manifestations and processes of plate collision can be observed at present and in which their history is recorded. It is a region that must be understood if we are to understand mountain belts, arc development, marginal basin evolution and, more generally, the behaviour of the lithosphere in collision settings. Furthermore, the region is developing rapidly on the economic front, and a major part of this rapid development is built on natural resources. The geological reasons for the distribution of these resources are therefore of major importance for the inhabitants of the region and for attempts to discover and exploit them. These were some of the thoughts which stimulated the collection of papers (Fig. 1) in this volume. In order to understand the development of this complex region an essential first step is to identify the key features of the active tectonics and determine how plates and sub-plate lithospheric fragments are moving. How successfully can rigid plate tectonics be applied in describing present tectonics? Where are the boundaries between plates? What are the rates at which different parts of the region are moving? The first part of this volume includes a number of papers which deal with these questions, based upon the application of GPS (Global Positioning System) measurements to determining the nature and rates of plate movements and plate boundary zone deformation, results from the BIRPS deep seismic reflection experiment in the Banda arc, the first to cross a modern active margin, and other geophysical and geological studies. MeCaffrey provides a regional overview of recent GPS results and earthquake data bearing on the present plate tectonics. It is ironic that in such an active region the identification of several plates and determination of some important relative plate motions, critical to a full kinematic description, are still very uncertain. McCaffrey discusses the way in which motion is partitioned in obliquely convergent settings, almost the rule in SE Asia. Oblique convergence is commonly inferred in the past for SE Asia (as shown in many of the later papers in the volume) and is often used as an explanatory tool in orogenic belts elsewhere in the world. He draws attention to the deformation of the upper plate in these convergent settings and emphasizes the importance of a three-dimensional understanding of the process. In this rapidly evolving region, information from slip vectors and geodetic measurements will allow a fourth dimension of time to be included as data accumulate, which will raise the important problem of whether and for how long the present motions can be assumed to extend back into the past. Two well-known areas of oblique convergence, the Sumatra and Philippine Sea plate margins, illustrate the realities of present tectonics, first in identifying small plates or smaller tectonic elements, and second in providing a kinematic description. The increase in obliquity of convergence between Java and north Sumatra has long been considered to result in thrusting normal to the subduction trench and arc-parallel movement on the Sumatran fault. However, as McCaffrey points out, this simple model does not predict some important features, such as subduction west of the Andaman Sea and differences in amounts of extension between north and south Sumatra. Malod & Kemal discuss evidence from marine surveys off Sumatra and propose that this area can be understood as a number of plate slivers between the trench and the Sumatran fault, with variations in the partitioning of movement in different parts of the Sunda forearc. This is explained as the result of differences in coupling between the subducting and overriding plates, possibly reflecting the presence of major structures on the subducting slab, notably an extinct spreading centre on the Indian plate. Further east, the Philippine fault and trench are also considered as the joint expression of partitioning of oblique convergence, but once again applying this simple model presents problems: how does this paired trench-fault system link southwards into the arc-arc collision of the Molucca Sea? Rangin et al. tackle the first important problem of what is actually present in the zone of transition by mapping structures from the south Philippines into Indonesia and determining which features belong