Journal of Southeast Asian Earth Sciences, Vol. 6, No. 3/4, pp. 209-220, 1991 0743-9547/91 $3.1)0 + 0.00 Printed in Great Britain Pergamon Press Ltd The Philippine Mobile Belt: a complex plate boundary CLAUDE RANGIN Ddpartement de Grotectonique, URA 1315--CNRS, Universit6 Pierre et Marie Curie, T. 26, E 1, 4, Place Jussieu, 75252 Paris Crdex 05, France (Received 31 August 1990; accepted for publication 5 May 1991) A~tract--The Philippine archipelago is regarded as the product of the Late Cenozoic oblique collision of the Philippine Plate with the thinned margin of Eurasia. The Philippine Mobile Belt is presented, mainly composed by the Philippine arc, a Paleogene volcanic arc belonging to the Philippine Sea Plate and crustal fragments belonging to the Eurasian Plate. Kinematic and geological data are presented for this collision history. INTRODUCTION THE NORTH trending Philippine archipelago represents one of the most complex tectonic zones of the West Pacific region, an area for convergence of the Eurasian Plate, the Philippine Sea Plate, and the Indo-Australian Plate (Fig. 1). The collision zone between these plates extends southward from the Philippines into the Molucca Sea and Eastern Indonesia. Physiographically, the north trending Philippine archipelago is bounded westward by the South China Sea-Sulu Sea-Celebes Sea marginal basins, and to the east by the intra-oceanic Philippine basins. The western side of the archipelago is fringed by deep trenches (Manila, Negros and Cotobato) along which the S.E. Asia marginal basins are presently being subducted. Seismicity along these eastward dipping subduction zones is weak (Hamburger et al. 1983), intepreted by Seno (1977) as indicating the waning of convergence along these trenches. In contrast along the eastern margin of the Philippine archipelago, the deep Philippine Trench, devoid of accreted sediments, propagates north- ward along the East Luzon Trough, a diffuse eastward verging thrust zone extending to Taiwan (Lewis and Hayes 1983). South of Mindanao, the Philippine Trench ends abruptly in Halmahera (Hall 1987). The presence of convergent zones on both sides of this north-trending archipelago indicating a decoupling from the surround- ing major plates, justifies the term of Philippine Mobile Belt (PMB) proposed by Gervaiso (1971). Intense deformation affects the PMB itself. The sinistral Philippine Fault appears to behave as a zone of decoupling where a component of the oblique con- vergence between the Philippine Sea Plate and the PMB is being accommodated. In such a case the other com- ponent would be accommodated perpendicularly along the Philippine Trench following the shear partitioning model of Fitch (1972). This geometry implies that the fault and the trench are coeval. Preliminary studies along the central segment of the fault (Aurelio et al. 1990, Aurelio et al. 1991), suggest that the fault is not older than Late Miocene. This age is supported by the earlier conclusion of Cardwell et al. (1983) who described a steep dip in the seismic zone along the southern Philippine Trench, attributing it to the recent origin of the trench. On the other hand, the paleo- magnetic results of Cole et al. (1989) suggest that displacement along the Philippine Fault is of small magnitude. KINEMATICS OF THE EURASIA AND PHILIPPINE SEA PLATES In the present global kinematic framework, the Australian Plate moves toward the north at a constant rate of 8 cm/a (Stock and Molnar 1982). The Caroline and Pacific plates, poorly decoupled along the Mussau Trench, move toward the Philippine Sea Plate and Eurasia at a rate of about 10 cm/a (Seno 1977, Huchon 1985). The Philippine Sea Plate also moves westward with respect to Eurasia about a remote pole located close to the Philippine Sea-Pacific-Eurasian plate triple junction. Since the Eocene the Australian and Caroline Sea plates have had a relatively constant motion with Fig. 1. Geodynamic setting of the Philippine archipelago. The shaded area indicates the wide tectonic boundary between major plates (Eurasian, Caroline, Philippine Sea and Indo-Australian plates). A = trace of Fig. 3, and B = trace of Fig. 4. 209