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 26ø35'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 longlived 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 lowamplitude 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. IntroductionMid-ocean ridges that have limited magma supply are strongly affected by normal faulting that creates rough abyssal hill topography. The largest fault scarps occur toward the ends of spreading segments [Shaw, 1992] Megamullions have two prominent characteristics: (1) a gently domed, overall turtleback shape (i.e., megamullion) and (2) a surface that is interpreted to be a single and extensive fault plane that is distinguished by the presence of prominent corrugations (mullion structures) that parallel the fault slip direction (e.g., Figures 1 and 2). In the direction of fault slip the updip limit of a megamullion is a breakaway zone where the fault initially nucleated. The downdip limit, or termination, normally is marked by contact with a hanging wall. ...

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[1] A continuous, domed detachment surface (FUJI Dome) has been imaged on the very slow-spreading southwest Indian Ridge using deep-towed side-scan sonar, and has been investigated by manned submersible and sea-surface geophysics. The Dome is morphologically similar to other oceanic detachments, core complexes or mega-mullions. In addition to bathymetric mullions observed in shipborne bathymetry, finer scale spreading-parallel striations were imaged with the side scan. On the detachment surface, metabasalt crops out near the termination, probably as part of a thin fault sliver. Gabbro and troctolite probably crop out near the summit of the dome. The rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite. Most of the detachment surface dips toward the ridge axis at 10°-20°, but near the breakaway it is strongly rotated outward, and dips away from the axis at up to 40°. Normal, undeformed volcanic seafloor crops out adjacent to the detachment. Modeling of sea surface magnetic data suggest the detachment was active from 1.95 Ma for about 1 Ma during a period of reduced and asymmetric magmatic accretion. Modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the Dome, and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the Dome.

Peridotite samples from a backarc basin setting will help better understand global mid‐oceanic ridge processes. Here we report detailed petrological data of serpentinized peridotite and gabbro from the extinct Parece Vela Basin in the Philippine Sea. Despite its relatively fast spreading rate (8.8–7.0 cm/y full‐rate), the Parece Vela Basin spreading ridge (the Parece Vela Rift) has the distinct morpho‐tectonic characteristics that indicate a small degree of mantle melting, including the presence of a huge mullion structure (the Godzilla Mullion). Peridotites in the Parece Vela Rift are exposed on the Godzilla Mullion as well as at a segment midpoint. The most notable characteristic of Parece Vela Rift peridotites is small‐scale juxtaposition (i.e., a single‐dredge‐haul scale) of fertile peridotite and depleted peridotite (dunite and plagioclase‐bearing peridotite). We interpret that the fertile peridotite (F‐type) is the residue of a small degree of mantle melting (∼4% near‐fractional melting of a MORB‐type mantle), whereas dunite (D‐type) and plagioclase‐bearing peridotite (P‐type) are products of melt‐mantle interaction. The associated evolved gabbros may represent the shallow level fractionated melt intruded into P‐type. The distinct morpho‐tectonic characteristics, peridotite exposure at a segment midpoint, and the presence of fertile peridotite may result from an extreme transform fault effect caused by the ridge‐transform geometry of short first‐order segments sandwiched by closely spaced fracture zones (“transform sandwich effect”).

Eruption of 1-million-year-old tholeiitic basalt >1800 meters below sea level (>18 megapascals) in a backarc rift behind the Bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions. Explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column. The resulting 135-meter-thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of <2.5 meters. High magmatic water content, which is about 1.3 percent by weight after vesiculation, contributed to the explosivity.

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