Abstract
Recent multibeam bathymetric and geophysical data recorded in the West Philippine Basin, east of Taiwan, revealnew information on the structure and the tectonic origin of the oceanic Gagua Ridge. This linear, 300 km-long,4 km-high, north-south-trending ridge, is being subducted beneath the Ryukyu Trench along 123◦ E. This basementhigh separates two basins of different ages. Its summit is marked by two crests and an axial valley. A map of thebasement top shows the region of the ridge to be composed of a set of linear and parallel ridges and troughs.All these elements suggest that the development of the ridge, and its surroundings, has been influenced by strikeslip deformation. Nevertheless, the height of the ridge indicates also an important compressive component in thedeformation. Gravity models across the ridge show local compensation with a crustal root, indicating that anoverthickening of the crust occurred when it was young and thus more easily deformable. This idea is strengthenedwith flexural modeling of the lithosphere that bends under the load of the ridge, indeed it indicates that the highprobably formed when the underlying lithosphere was young. We interpret the Gagua Ridge as a fracture zonetransverse ridge uplifted during a transpressive episode along a north-south -trending fracture zone in the middleEocene time, if we accept Hilde and Lee’s (1984) model of magnetic lineations. This tectonic event could becontemporaneous with a change of the pole of rotation of the West Philippine Basin which occurred about 43/45 Maago.