Cenozoic Reconstructions of SE Asia and the SW Pacific: changing patterns of land and sea

The Cenozoic has seen the major tectonic events which have determined the present configuration of land and sea in SE Asia and the SW Pacific. Subduction throughout this period maintained volcanic arcs which formed discontinuously emergent island chains crossing the region. Early in the Cenozoic the major collision of India with SE Asia enlarged the area of land connected to Eurasia. Later, the continuing collision with Australia led to connections between Australia, Eurasia, and the Pacific. Despite long-term convergence of the major plates there have been important episodes of extension, forming ocean basins and causing subsidence within continental regions, which were probably driven by subduction. It is clear that very rapid changes in topography and distribution of land and sea have occurred. The geological and biogeographic interface of perhaps most interest is that between Sundaland and Australia, extending from Borneo to the Bird’s Head of New Guinea. The biogeographic divides of Wallace and later workers have all been drawn through this region. Since the early Miocene the original deep water barrier between Australia and Sundaland has been eliminated but the process of convergence has never produced a simple route for the mixing of Australian and Asian floras and faunas. It is clear from the plate tectonic model summarised here that there have been multiple opportunities for dispersal and vicariance caused by regional tectonic processes. There are also more subtle geologically-related forces which may have modified biogeographic patterns, such as links between tectonics and sea level, the rise of mountains and global/local climate, and closure of seaways and oceanic circulation, which are suggested by the tectonic model All these changes occurred within a framework of overall long-term cooling. Further, more extreme, changes in climate and sea level occurred during the Quaternary glacial and interglacial periods. A simple picture of convergence in which Australia and Sundaland collided, causing land to emerge, allowing colonisation by animals and plants from east and west is therefore probably too simple. Since the early Miocene Australia and Sundaland have moved closer together but as land emerged and mountains rose in some areas, new deep basins developed. The distribution of Australian and Asian plants and animals should therefore reflect this complexity, with further important modifications imposed by glacially-related sea level and climatic change in the Quaternary. In this picture, the zone of Wallace’s Line is partly an ancient deep water barrier, partly a dynamic boundary marking a migration front, but also a relic of Neogene patterns which have been tectonically disrupted and modified by Quaternary climate change.