Bathymetric segmentation and faulting on the Mid-Atlantic Ridge, 24 ~ to 24~

Abstract: This investigation f valley-wall faulting and its relationship to bathymetry focuses on the first two segments north of the Kane Fracture Zone. The valley-wall faults imaged by deep-towed sidescan sonar show aspects common to continental normal faults, and a similar erosional pattern, from a fresh fault surface, through eroded, talus covered scarps, to scarps draped with pelagic sediments. The first, southernmost egment is a symmetric graben at the segment centre, and an asymmetric half-graben at the northern end. The second, northern segment isan asymmetric half-graben for its entire length. The gross morphology is reflected in the faulting style, with multiple small faults at he symmetric graben, and one or two large faults on the eastern wall, defining the asymmetic half-graben. This difference in faulting style may reflect differences in depth to the brittle-ductile transition, with asymmetry occurring when the brittle-ductile transition is deep and brittle faults can cross the axial valley, locking faults on the other side. A gross estimate of horizontal strain accommodated by the valley-wall faults correlates well with valley-floor bathymetry, suggesting that bathymetric segmentation is partially due to tectonic thinning at segment ends. Bathymetric segmentation (e.g. Schouten et al. 1985) correlates with variation in the gravity along the ridge axis, expressed as the mantle Bouguer anomaly. Mantle Bouguer anomaly lows at the segment centres may either reflect hot, low density crust or mantle, thicker crust, or some combination of both (Lin et al. 1990). Recent refraction studies in the southern Atlan-tic (Tolstoy et al. 1993) indicate that crust thins at segment ends. Magma chambers at slow spread-ing ridges have not been imaged on seismic reflection surveys (Detrick et al. 1990), and it is believed that they are relatively small, tempo-rary features. The magma supply is considered to strongly affect local tectonics (Tapponier &amp