Submersible study of an oceanic megamullion in the central North Atlantic

Recently discovered megamullions on the seafloor have been interpreted to b e the exhumed footwalls of long-lived detachment faults operating near the ends of spreading segments in slow spreading crust. We conducted five subme rsible dives on one of these features just east of the rift valley in the M id-Atlantic Ridge at 26 degrees 35'N and obtained visual, rock sample, grav ity, and heat flow data along a transect from the breakaway zone (where the fault is interpreted to have first nucleated in similar to2.0-2.2 Ma crust ) westward to near the termination (similar to0.7 Ma). Our observations are consistent with the detachment fault hypothesis and show the following fea tures. In the breakaway zone, faulted and steeply backtilted basaltic block s suggest rotation above a deeper shear zone; the youngest normal faults in this sequence are interpreted to have evolved into the long-lived detachme nt fault. In younger crust the interpreted detachment surface rises as mono tonously flat seafloor in a pair of broad, gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment. The detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall. The domes coincide with a gravity high that continues along isochrons within the spreading segment. Modeling of on -bottom gravity measurements and recovery of serpentinites imply that mantl e rises steeply and is exposed within similar to7 km west of the breakaway but that rocks with intermediate densities prevail farther west. Within sim ilar to5 km. of the termination, small volcanic cones appear on the detachm ent surface, indicating melt input into the footwall. We interpret the mega mullion to have developed during a phase of limited magmatism in the spread ing segment, with mantle being exhumed by the detachment fault <0.5 m.y. af ter its initiation. Increasing magmatism may eventually have weakened the l ithosphere and facilitated propagation of a rift that terminated slip on th e detachment fault progressively between similar to1.3 m.y. and 0.7 m.y. Id entifiable but low-amplitude magnetic anomalies over the megamullion indica te that it incorporates a magmatic component. We infer that much of the foo twall is composed of variably serpentinized peridotite intruded by plutons and dikes.