Internal structure of uppermost oceanic crust along the Western Blanco Transform Scarp: Implications for subaxial accretion and deformation at the Juan de Fuca Ridge

[1] The uppermost 2 km of oceanic crust created near the southern end of the Juan de Fuca Ridge ( 60 mm/yr, full spreading rate) is exposed in a ‘‘tectonic window’’ created along the north wall of the Blanco Transform Fault. A total of 27 submersible dives transect this escarpment over a distance of >30 km, corresponding to 1 m.y. of spreading history. In this exposure, weakly deformed basaltic lavas grade downward into much more intensely fractured lavas cut by basaltic dikes. Lava flow surfaces dip persistently to the northwest (toward the spreading center) and the dikes dip southeast (away from the spreading center). The underlying sheeted dike complex is composed of subparallel dikes that dip moderately to the southeast. Discrete faults and more distributed cataclastic deformation zones separate the panels of sheeted dikes from one another. Disruption of the lava and dike units are interpreted as a result of dramatic, subaxial, vertical mass transport that created the ‘‘accommodation space’’ for the accumulation of a thickness of 1000–1500 m of basaltic lava. The observed patterns of tilting of lavas and dikes imply subsidence beneath the region of active volcanism ( 3 km wide) that is most rapid near the axis and diminishes quickly off-axis. Deformation that accommodates this subsidence has generated extensive fracture porosity in the subaxial region that could strongly influence upper crustal hydrothermal pathways, alteration history, seismic properties, and the distribution of subsurface biological communities.