Résumé
Marine observations offshore of Taiwan indicate intense deformation of theLuzon arc-forearc complex, with episodic eastward migration of the active deformation front across the complex. This active tectonic domain absorbs a significantamount of shortening between the Eurasia margin and the Philippine Sea Plate,which is moving towards N 310° E at about 8 cm/yr relative to Eurasia. Swath bathymetry and back-scattering data, together with seismic reflection and geopotential data obtained during the ACT (Active Collision in Taiwan) cruise onboard theR/V L'Atalante, showed major north to south changes in the tectonic style in boththe indenting arc and the host margin.Structural observations show that the forearc basement of the Luzon arc nolonger exists north of 22°30'N. To the south, only a small part of the forearc domain may remain beneath the Huatung Ridge (rear portion of the former Manilatrench oceanic accretionary wedge including forearc and intra-arc sequences) andrear of the thrust wedge. A tectonic model involving the progressive underthrusting of large slices of the forearc basement may account for the contrasting stylesof deformation encountered from south to north across the collisional orogen andapparent missing of the forearc region. The progressive subduction of the continental margin of China induces: 1) to the south, major eastward backthrustingand shortening of the forearc domain between the former oceanic accretionarywedge and the Luzon Arc volcanic edifice, 2) to the north, accretion of parts of thearc domain to the collisional belt associated with westward thrusting, eastwardbackthrusting at the base of the slope and block rotation.