TWO CENOZOIC DYANAMIC SYSTEMS AND THEIR RELATIONS TO THE TECTONIC EVOLUTION OF SE ASIA AND TO EXTRUSTION PROCESSES

A common feature of orogenic belts is the lateral extrusion of crustal fragments during convergence. Such processes require beyond the region where extrusion occurs, and information on mantle processes, the latter accessible as new imaging technology is becoming available. An example of the complexities and variability of the extrusion process is slowly being developed in SE Asia where convergent activity occurs within the India/Eurasia system and extrusion may be enhanced by the Western Pacific/Indonesian subduction system. Extrusion begins with the early Cenozoic collision and continues during post-collisional convergence of India and Eurasia. Several crustal/lithospheric continental fragments have moved southeastward from SE Tibet towards a retreating West Pacific/Indonesian subduction system. The pattern of roll back within the West Pacific/Indonesian subduction system changes sharply from moderate to large north and south of the SE projection of the Red River fault towards the Indonesian subduction system. This pattern of extrusion changed during late Cenozoic time (at about ~20 Ma), by 1. Increased tighter clockwise rotation of crustal/lithospheric fragments around the eastern Himalayan syntaxis toward the Burma subduction zone, 2. Less SE-ward movement toward the Indonesian subduction system, and 3 A change of process in extrusion from central Tibet to the east and northeast where lower crustal flow becomes an important mechanism. The magnitude of extrusion during the Early Cenozoic is controversial, but can be partly quantified during the Late Cenozoic. The processes within the lithosphere appear to evolve over time as the properties of the lithosphere change and different areas are decoupled at different times, by different degrees and at different depths.