A SUBMERSIBLE STUDY IN THE WESTERN-BLANCO FRACTURE-ZONE, NE PACIFIC -STRUCTURE AND EVOLUTION DURING THE LAST 1.6 MA

During July and August 1991, the French-American Blanconaute dive prog ram used the French submersible Nautile to investigate the West Blanco Depression (WBD), a deep, elongate trough located at the intersection of the Blanco Transform Fault Zone with the southern Juan de Fuca Rid ge (JdFR). Twenty dives were carried out along the north wall of the W BD, which exposes the upper oceanic crust over a 65 km distance, from the JdFR axis (to the west) to the oblique trace of an ancient propaga tor (to the east, crustal age around 2 Ma). Thirteen of these dives we re precisely located within a 13 x 7 km zone of the north wall, covere d by a high-resolution sonar mapping operation during the Blancotrough cruise in 1987. This series of geological traverses, plus 4 dives acr oss the south wall of the WBD (one dive) and the adjacent Parks Platea u (3 dives), collected 242 rock samples. We report here the main resul ts of the dive program and preliminary laboratory studies: 1. Transfor m-related tectonic activity has recently abandoned the southern margin of Parks Plateau, and is presently located inside the WBD area, mainl y along its northern wall. The tectonic features observed are compatib le with a right-lateral strike-slip system, with a NE-SW extensional c omponent. 2. Three main lithological units are exposed along the north wall of the WBD. From top to bottom, they are: (1) a Volcanic Unit, f orming a steep upper cliff, made of massive and pillow flows and basal tic dikes, with an estimated average thickness of 800 m; (2) a less st eep Transition Zone, about 150 to 400 m thick, largely masked by rubbl e but exposing both diabase outcrops and pillow flows; and (3) a massi ve Diabase Unit, exposed over 700-800 m, with a dike complex structure visible from place to place, and cut by a net of hydrothermal veins. Deep crustal rocks such as gabbros were not observed. 3. Spectacular m ass-wasting features are visible all along the north wall of the WBD. About 60% of the face of the wall is masked by talus cones, rubble, ro ck avalanche deposits and slide blocks. Three main landslides, of appr oximately one km(3) in volume each, were tentatively identified. One o f them was mapped in detail and consists of an approximately 300 m thi ck (0.85 km(3)), coherent slide block detached from a zone where inten se hydrothermal alteration and faulting have obviously weakened the be drock, that is in places entirely altered to blue clays. 4. The basalt ic lavas of the WBD north wall show a remarkable evolution with lime, from east to west. Around the tip of the ancient propagator, they are restricted to primitive, olivine-rich picritic basalts. Proceeding wes tward, they exhibit a wide range of differentiation, including highly fractionated, FeTi-rich ferrobasalts at about 35-45 km from the JdFR a xis. When approaching the JdFR axis, the FeTi enrichment decreases gra dually, and the ferrobasalts evolve towards slightly differentiated MO RB-type basalts, typical of the southern JdFR. This magmatic evolution marks the transition from the end of a propagating rift regime to a s teady-state accretion regime. 5. The WBD north wall also permits the s tudy of weathering and hydrothermal alteration processes and their evo lution in space and time. Vertically, the alteration products evolve f rom oceanic weathering and zeolite facies (Volcanic Unit) to the green schist facies (Transition Zone and Diabase Unit). Horizontally, the ev olution with time is mainly a general hydration of the crust that is, however, very irregularly distributed. 6. Several vertical magnetic tr averses along the north wall of the WBD, using a bottom magnetometer a ttached to the basket of the submersible, have shown a sharp 5000 to 7 000 nT positive anomaly at about 3500 m depth. This anomaly correspond s exactly to the first appearance of extrusive pillow-lava outcrops, a nd confirms the dramatic decrease in magnetic anomaly amplitude below that depth, detected during the Blancotrough cruise in 1987. The verti cal magnetic profiles thus appear to have imaged the base of the magne tic source layer.