Le séisme du 21 septembre 1999 : influence de l'héritage structural et implication du socle au front de la chaı̂ne de Taiwan

The 21-September-1999 Chichi earthquake (Mw=7.6) occurred near the western thrust front of the Taiwan mountain belt. The characteristics of this major earthquake bring new constraints on the deep structure of the western foothills of Taiwan. Seismotectonic information is combined with subsurface data in order to establish new balanced geological sections across the thrust zone. The structural style of the Taiwan belt front is characterized by the superimposition of multiple ‘decollements’, not only in the shallow sedimentary cover but also in the basement. Interpretations of drillings and seismic-reflection data indicate that the thrust is inherited from a normal fault, reactivated and inverted when collision reached the Foothills belt. The Chelungpu–Sani thrust observed at the surface is connected in depth (∼10 km) with a thrust ramp that dips 25° to the east in the basement. The tectonic history of this front thrust implies that a ‘decollement’ level exists, probably located at the brittle-ductile transition in the crust, and results in decoupling of the cover and shallow basement relative to the deeper basement. Such a ‘decollement’ could explain the basin inversion observed in the foreland platform. The occurrence of a thick-skinned tectonic deformation at the Taiwan belt front had been predicted by analyses of structural geology; it is now confirmed by the study of the Chichi earthquake.