Seismic data, downhole measurements, images and core obtained at DSDP/ODP Site 504, in the eastern equatorial Pacific, reveal the presence of a normal fault about 500 m into basement. This north-dipping structure is sub-vertical in the sediment, and dips about 45¿ toward the ridge in the hole. Similar faults are expected from seismic data to cut this deep hole within the sheeted dikes and close to the base of the hole, where the drilling was abruptly terminated. The combined analysis of geophysical and geological data reveals a 300-m-thick fault zone, with modified physical and mineralogical properties. The fault was initially identified from a 4¿ step in magnetic field inclination at 800 mbsf, and the lowest values of electrical resistivity in basement. The fault also exhibits modified mechanical properties and a high open porosity fraction. While fracturing mapped from electrical images decreases slightly versus depth in basement, the fracture density increase with increasing depth within the fault zone. The basalt magnetic 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 ruptures appear to be located near the top of the interval and suggest a propagation of the deformation toward the hanging wall with time.¿ 1997 American Geophysical Union