FROM PROPAGATION TO ACCRETION - A SUBMERS IBLE STUDY OF THE NORTHERN WALL OF THE WESTERN BLANCO DEPRESSION (BIANCO FRACTURE-ZONE, NE PACIFIC)

The northern wall of the Western Blanco Depression (WED), at the inter section of the Blanco Fracture Zone with the Juan de Fuca Ridge (JdFR) , in the NE Pacific, exhibits a piece of young oceanic crust (from zer o to 2 Ma in age), over more than 2000 m in height and 60 km long. Twe nty geological cross-sections were done along this wall, using the Fre nch submersible Nautile, during the Blanconaute cruise (1991). We pres ent in this paper the main results of this diving programme. (1) The n orthern wall of the WED is tectonically active, and is the locus of th e present transform movement. (2) The wall exposes only the upper ocea nic crust, with a lithological section including: an upper volcanic un it (mainly basaltic pillow-lavas, about 800 m thick), a 300 to 500 m t hick transition zone, and a lower massive diabase unit (700 m visible) . (3) From east to west, the wall exposes a suite of picritic basalts, grading to variably differentiated basalts and ferrobasalts, themselv es grading progressively to moderately fractionated MORB-type basalts, similar to those presently produced by the JdFR. This remarkable evol ution in time - from 2 Ma to <<zero>> age - reflects the from rift pro pagation conditions to a standard accretion regime. (4) In these lavas , the alteration conditions vary vertically (from brownstone facies in the volcanic unit to zeolite and greenschists facies in the transitio n zone and diabase unit), and horizontally, with a progressive - but v ery irregular - hydration of the crust with time, along with the devel opment of a network of hydrothermal veins. (5) Vertical magnetic profi les carried out along the scarp face show a consistent pattern of stro ngly magnetized volcanic unit (pillow-lavas), underlain by a weakly ma gnetized diabase section. Such a stratigraphy supports the hypothesis that the pillow-lava section produces most of the oceanic magnetic ano malies, at least in this area.