DETAILED STRUCTURE OF AN OCEANIC NORMAL-FAULT - A MULTISCALAR APPROACH AT DSDP ODP-SITE-504/

Seismic data, downhole measurements, images and core obtained at DSDP/ ODP Site 504, in the eastern equatorial Pacific, reveal the presence o f a normal fault about 500 minto basement. This north-dipping structur e is subvertical in the sediment, and dips about 45 degrees toward the ridge in the hole. Similar faults are expected from seismic data to c ut this deep hole within the sheeted dikes and close to the base of th e hole, where the drilling was abruptly terminated. The combined analy sis of geophysical and geological data reveals a 300-m-thick fault zon e, with modified physical and mineralogical properties. The fault was initially identified from a 4 degrees step in magnetic field inclinati on at 800 mbsf, and the lowest values of electrical resistivity in bas ement. The fault also exhibits modified mechanical properties and a hi gh open porosity fraction. While fracturing mapped from electrical ima ges decreases slightly versus depth in basement, the fracture density increases with increasing depth within the fault zone. The basalt magn etic properties are strongly modified near the base of the fault zone (from 900 to 1050 mbsf), possibly in relation with early faulting and the circulation of high-temperature fluids. To the contrary, present r uptures appear to be located near the top of the interval and suggest a propagation of the deformation toward the hanging wall with time.