Jeffrey A. Karson and Róisín M. Lawrence


Extensive exposures of gabbroic rocks, inferred to represent mid-crustal intervals of the oceanic crust, crop out on the western median valley wall of the spreading segment of the Mid-Atlantic Ridge immediately south of the Kane Transform. Surface exposures of the gabbroic rocks have been mapped by numerous submersible dives, deeply towed camera transects, and side-scan sonar surveys, making this region one of the most intensively studied areas of the mid-ocean ridge system. Recent drilling in this tectonic window at Sites 921-924 provides a new perspective on the architecture and mode of accretion of mid-crustal rocks of the oceanic crust.

Surface studies show that the gabbroic rocks crop out in an area that extends at least 15 km along the ridge axis and 8-10 km normal to the ridge axis. The median valley wall of the Mid-Atlantic Ridge here is essentially a dip-slope on a major crustal detachment fault. This low-angle normal fault zone has been cut by numerous steeply dipping normal faults that impart a stair-step morphology to the terrain. Basaltic pillow lavas crop out at the top of the median valley wall and probably represent upper crustal rocks that formed structurally above the gabbroic units. To the east, the median valley floor of the Mid-Atlantic Ridge is dominated by relatively young pillow lavas and an extensive neovolcanic ridge. These lavas overlie the detachment footwall, and are probably the surface expression of dikes and plutonic bodies that cut the detachment at depth.

Gabbroic and related rocks recovered from Sites 921 to 924 indicate that the mid-crustal terrain exposed in this tectonic window was assembled as a collage of relatively small (less than about 1 km across) plutons. Individual plutons have been variably deformed and metamorphosed, and subsequently intruded by later plutons and dikes. Thus, whereas surface exposures show evidence of major faulting over the past 0.5 m.y., subsurface data from Ocean Drilling Program cores indicate a more continuous style of synkinematic intrusion and crustal stretching. These processes may be broadly coeval expressions of slow seafloor spreading in this environment. Exposures of similar crustal levels in the adjacent Kane Transform suggest a similar pattern of accretion. This type of spreading may be common at ridge-transform intersections and other places on slow-spreading ridges with very low or episodic magma budgets.

Date of initial receipt: 19 July 1995
Date of acceptance: 5 March 1996

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