SUBDUCTION, COLLISION AND DEVELOPMENT OF OROGENIC BELTS: A SE ASIAN PERSPECTIVE ON THE TECTONIC CROSSROADS OF EURASIA–AFRICA–ARABIA
Because subduction continues to the present-day in and around SE Asia we have some advantages when trying to reconstruct its history. Many features of the surface geology are forming today, or were produced very recently, and therefore can be related to observed motions at the surface, seismicity, and mantle structure. On the other hand, in most places we do not see the deep crust because it is buried beneath young volcanic products and thick sediments. Nevertheless, it does seem that SE Asia is an unusual region. What features may be of importance for understanding the orogenic crossroads of Eurasia–Africa–Arabia?
Around SE Asia the Pacific, Indian/Australian and Eurasian plates are converging and plate tectonics provides a first order explanation of its history. As oceans closed the region has grown principally by addition of continental fragments but, despite a long subduction history, relatively little by magmatism. However, plate tectonics is only part of the story. Today, in many areas the major plates have no clear-cut boundaries and many lines drawn on maps solve an apparent kinematic problem but do not reflect reality. The region cannot be understood simply as multiple small rigid microplates bounded by lithospheric faults.
In addition to major plate movements three important factors have influenced what we see today. First, as result of a long subduction history the region has a weak thin lithosphere, highly responsive to plate boundary forces. Second, the region’s history means it has a complex basement structure, with strong and weak parts. It is not a rigid plate and most of the region is really a wide plate boundary zone between a broadly weak continent and surrounding strong lithosphere. Third, the rifting history and consequent shape of continental margins influenced the way in which different parts of the region responded to plate tectonic forces during convergence and collision.
Collisions of continental blocks formed a Sundaland promontory to the Eurasian plate in the Early Mesozoic and other fragments were added during the Cretaceous. These rifted from Australia during the Late Jurassic and Early Cretaceous and are now in Borneo, Java and Sulawesi, not West Burma or Sumatra. Their arrival terminated subduction beneath Sundaland at about 90 Ma, although not north of India, and contributed to emergence of an extensive landmass. At 45 Ma subduction resumed as Australia began to move rapidly north. The shape of the Australian margin resulted in a complex collision history which has caused much controversy and confusion. The first impact of Australia on SE Asia was in the Early Miocene, at about 23 Ma, with collision of the Sula Spur promontory. From 15 Ma there was subduction rollback into the Banda oceanic embayment and major extension. Collision of the Banda volcanic arc with the southern margin of the embayment began at about 4 Ma. After more than 20 million years the Australian–SE Asia collision is far from complete.
Collision and extension have obviously contributed to vertical movements but deformation of the upper crust reflects several processes other than plate movements. Within SE Asia there are remarkable contrasts between strong areas, which include oceanic and continental lithosphere, and adjacent weaker areas. Subduction rollback of strong oceanic lithosphere led to extension and subsidence, but also to continental delamination contributing to rapid uplift. Strong continental areas are little deformed and record only small vertical movements relative to sea level. In contrast, in weaker areas the upper crust appears to be deforming almost independently of the deeper lithosphere and there have been exceptionally high rates of vertical movements, both up and down. A common feature of the region is synchronous rapid uplift of mountains close to areas of major subsidence. These movements have been accompanied by high rates of erosion, and very great thicknesses of sediment have been deposited offshore. It appears that a hot weak deeper crust has flowed in response to tectonic forces, topographic forces and sedimentary loading so that in many areas GPS observations do not record motions of major- or micro- plates but local movements of the upper crust. Models developed in completely different tectonic settings are likely to mislead in predicting features such as heatflow, and rates of uplift, subsidence, erosion and sedimentation. However, the contrasts between weak and strong areas, and the rates and scales of processes observed in SE Asia, may be relevant to understanding older orogenic belts such as those of the Middle East.