FROM PROPAGATION TO ACCRETION - A SUBMERSIBLE STUDY OF THE NORTHERN WALL OF THE WESTERN BLANCO DEPRESSION (BIANCO FRACTURE-ZONE, NE PACIFIC)
Additional Document Info
The northern wall of the Western Blanco Depression (WED), at the intersection of the Blanco Fracture Zone with the Juan de Fuca Ridge (JdFR), in the NE Pacific, exhibits a piece of young oceanic crust (from zero to 2 Ma in age), over more than 2000 m in height and 60 km long. Twenty geological cross-sections were done along this wall, using the French submersible Nautile, during the Blanconaute cruise (1991). We present in this paper the main results of this diving programme. (1) The northern wall of the WED is tectonically active, and is the locus of the present transform movement. (2) The wall exposes only the upper oceanic crust, with a lithological section including: an upper volcanic unit (mainly basaltic pillow-lavas, about 800 m thick), a 300 to 500 m thick 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, themselves grading progressively to moderately fractionated MORB-type basalts, similar to those presently produced by the JdFR. This remarkable evolution in time - from 2 Ma to <> age - reflects the from rift propagation 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 transition zone and diabase unit), and horizontally, with a progressive - but very irregular - hydration of the crust with time, along with the development of a network of hydrothermal veins. (5) Vertical magnetic profiles carried out along the scarp face show a consistent pattern of strongly magnetized volcanic unit (pillow-lavas), underlain by a weakly magnetized diabase section. Such a stratigraphy supports the hypothesis that the pillow-lava section produces most of the oceanic magnetic anomalies, at least in this area.