A detailed study of the Gagua Ridge: A fracture zone uplifted during a plate reorganisation in the Mid-Eocene

Recent multibeam bathymetric and geophysical data recorded in the West Phil ippine Basin, east of Taiwan, reveal new information on the structure and t he 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 degrees E. This basement high separates two basins of diff erent ages. Its summit is marked by two crests and an axial valley. A map o f the basement 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 th e development of the ridge, and its surroundings, has been influenced by st rike-slip deformation. Nevertheless, the height of the ridge indicates also an important compressive component in the deformation. Gravity models acro ss the ridge show local compensation with a crustal root, indicating that a n overthickening of the crust occurred when it was young and thus more easi ly deformable. This idea is strengthened with flexural modeling of the lith osphere that bends under the load of the ridge, indeed it indicates that th e high probably formed when the underlying lithosphere was young. We interp ret the Gagua Ridge as a fracture zone transverse ridge uplifted during a t ranspressive episode along a north-south -trending fracture zone in the mid dle Eocene time, if we accept Hilde and Lee's (1984) model of magnetic line ations. This tectonic event could be contemporaneous with a change of the p ole of rotation of the West Philippine Basin which occurred about 43/45 Ma ago.