Submersible study of an oceanic megamullion in the central North Atlantic

Abstract

Recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long-lived detachment faults operating near the ends of spreading segments in slow spreading crust. We conducted five submersible dives on one of these features just east of the rift valley in the Mid-Atlantic Ridge at 26°35′N and obtained visual, rock sample, gravity, and heat flow data along a transect from the breakaway zone (where the fault is interpreted to have first nucleated in ∼2.0–2.2 Ma crust) westward to near the termination (∼0.7 Ma). Our observations are consistent with the detachment fault hypothesis and show the following features. In the breakaway zone, faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone; the youngest normal faults in this sequence are interpreted to have evolved into the long-lived detachment fault. In younger crust the interpreted detachment surface rises as monotonously 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 mantle rises steeply and is exposed within ∼7 km west of the breakaway but that rocks with intermediate densities prevail farther west. Within ∼5 km of the termination, small volcanic cones appear on the detachment surface, indicating melt input into the footwall. We interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment, with mantle being exhumed by the detachment fault <0.5 m.y. after its initiation. Increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between ∼1.3 m.y. and 0.7 m.y. Identifiable but low-amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component. We infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes.