Anagolay: the shape of the Philippines and the Luzon Syntaxis

The India-Asia collision can have only a limited role as an actualistic model for the closure of Western Tethys and the subsequent Alpine orogenies because the impacting margins appear to have been sub-parallel and rather regular and the intervening ocean seems to have contained few volcanic edifices or continental fragments. A better guide to possible pre-collision processes is provided by the incipient Australia – Southeast Asia collision, which has already proved its worth as a key area for the study of small extensional zones within overall compressional environments. Insights into the possible roles of ridge-related features during oceanic closure are now being obtained from studies in the northern part of the Philippines Archipelago, which was largely formed by post-Middle Cretaceous volcanic activity associated with subduction of oceanic crust from both east and west. Double-sided subduction inevitably produces geomorphological complexity, but not all the anomalous features of the Philippines can be attributed to this cause. A sharp bend in topographic trends involving most of the southern part of the island of Luzon is here interpreted as a consequence of the impact on the east-facing subduction zone of the Anagolay volcanic massif formed by hot-spot volcanism associated with the spreading ridge in the West Philippine Basin. This bend can be considered a small-scale analogue of the syntaxes that define the limits of the India-Asia collision and demonstrates the way in which the presence of even a relatively small region of thickened crust can influence the morphology of an entire collision zone. Similar processes must have operated in other Alpino-type orogenic belts but may be hard to recognise because the generative units are no longer observable and their effects may be partly concealed by later tectonic over-printing.