Submersible study of an oceanic megamullion in the central North Atlantic

Abstract

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. ...

Three‐dimensional structure of oceanic core complexes: Effects on gravity signature and ridge flank morphology, Mid‐Atlantic Ridge, 30°N

“…[4] Prior processing of bathymetry and gravity data in this region [Blackman et al, 1998;Nooner et al, 2003] and at other OCCs [Cannat et al, 2006;Fujiwara et al, 2003;Okino et al, 2004;Searle et al, 2003;Tucholke et al, 2001] indicates that the shallow domes are not locally compensated, each being associated with a positive mantle Bouguer gravity anomaly. However, care should be taken in using mantle Bouguer anomalies to investigate subsurface density structure in the vicinity of OCCs.…”

Widespread active detachment faulting and core complex formation near 13° N on the Mid-Atlantic Ridge

“…The origin of the corrugations is still enigmatic. Direct observations have shown that they are not produced by faults parallel to the spreading direction 14,19 . One possibility is that they originate by continuous casting 20 of a ductile footwall by irregularities in a strong and brittle hanging wall.…”

Taxa de soerguimento atual do arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial

“…Essa cadeia foi interpretada como um megamullion, que foi deformado, posteriormente, por tectonismo recente Sichel et al, 2009). O megamullion, chamado também de mantle core complex, é a saliência morfológica abissal na forma de carapaça de tartaruga constituída por rochas ultramáficas do manto abissal (Figura 6A; Tucholke et al, 2001). Além das rochas ultramáficas, ocorre uma quantidade significativa de gabro.…”