Plutonic foundation of a slow-spreading ridge segment : oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q05014, doi:10.1029/2007GC001645.We mapped the Kane megamullion, an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of a ∼50 km long, second-order ridge segment. The complex was exhumed by long-lived slip on a normal-sense detachment fault at the base of the rift valley wall from ∼3.3 to 2.1 Ma (Williams, 2007). Mantle peridotites, gabbros, and diabase dikes are exposed in the detachment footwall and in outward facing high-angle normal fault scarps and slide-scar headwalls that cut through the detachment. These rocks directly constrain crustal architecture and the pattern of melt flow from the mantle to and within the lower crust. In addition, the volcanic carapace that originally overlay the complex is preserved intact on the conjugate African plate, so the complete internal and external architecture of the paleoridge segment can be studied. Seafloor spreading during formation of the core complex was highly asymmetric, and crustal accretion occurred largely in the footwall of the detachment fault exposing the core complex. Because additions to the footwall, both magmatic and amagmatic, are nonconservative, oceanic detachment faults are plutonic growth faults. A local volcano and fissure eruptions partially cover the northwestern quarter of the complex. This volcanism is associated with outward facing normal faults and possible, intersecting transform-parallel faults that formed during exhumation of the megamullion, suggesting the volcanics erupted off-axis. We find a zone of late-stage vertical melt transport through the mantle to the crust in the southern part of the segment marked by a ∼10 km wide zone of dunites that likely fed a large gabbro and troctolite intrusion intercalated with dikes. This zone correlates with the midpoint of a lineated axial volcanic high of the same age on the conjugate African plate. In the central region of the segment, however, primitive gabbro is rare, massive depleted peridotite tectonites abundant, and dunites nearly absent, which indicate that little melt crossed the crust-mantle boundary there. Greenschist facies diabase and pillow basalt hanging wall debris are scattered over the detachment surface. The diabase indicates lateral melt transport in dikes that fed the volcanic carapace away from the magmatic centers. At the northern edge of the complex (southern wall of the Kane transform) is a second magmatic center marked by olivine gabbro and minor troctolite intruded into mantle peridotite tectonite. This center varied substantially in size with time, consistent with waxing and waning volcanism near the transform as is also inferred from volcanic abyssal-hill relief on the conjugate African plate. Our results indicate that melt flow from the mantle focuses to local magmatic centers and creates plutonic complexes within the ridge segment whose position varies in space and time rather than fixed at a single central point. Distal to and between these complexes there may not be continuous gabbroic crust, but only a thin carapace of pillow lavas overlying dike complexes laterally fed from the magmatic centers. This is consistent with plate-driven flow that engenders local, stochastically distributed transient instabilities at depth in the partially molten mantle that fed the magmatic centers. Fixed boundaries, such as large-offset fracture zones, or relatively short segment lengths, however, may help to focus episodes of repeated melt extraction in the same location. While no previous model for ocean crust is like that inferred here, our observations do not invalidate them but rather extend the known diversity of ridge architecture.NSF Grants OCE-0118445, OCE-0624408 and OCE-0621660 supported this research. B. Tucholke was also supported by the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

Interaction between transform faults and rift systems: A combined field and experimental approach

We present a detailed field structural survey of the area of interaction between the active NW-striking transform Husavik-Flatey Fault (HFF) and the N–S Theystareykir Fissure Swarm (TFS), in North Iceland, integrated by analog scaled models. Field data contribute to a better understanding of how transform faults work, at a much higher detail than classical marine geophysical studies. Analog experiments are conducted to analyse the fracture patterns resulting from different possible cases where transform faulting accompanies or postpones the rift motions. Different tectonic block configurations are also considered and results are compared with field data in order to study as thoroughly as possible the interaction between the HFF and the TFS as well as the possible prolongation of the HFF into the TFS. West of the intersection between the transform fault (HFF) and the rift zone (TFS), the former splays with a gradual bending giving rise to a leading extensional imbricate fan. The westernmost structure of the rift, the N–S Gudfinnugja Fault (GF), is divided into two segments: the southern segment makes a junction with the HFF and is part of the imbricate fan; north of the junction instead, the northern GF appears right-laterally offset by about 20 m. Southeast of the junction, along the possible prolongation of the HFF across the TFS, the strike of the rift faults rotates in an anticlockwise direction, attaining a NNW–SSE orientation. Moreover, the TFS faults north of the HFF prolongation are fewer and have smaller offsets than those located to the south. Through the comparison between the structural data collected in the field at the HFF–TFS connection zone and a set of scaled experiments, we confirm a prolongation of the HFF through the rift, although here the transform fault has a much lower slip-rate than west of the junction. Our data suggest that transform fault terminations may be more complex than previously known, and propagate across a rift through a modification of the rift pattern

Vema Fracture Zone (central Atlantic): Tectonic and Magmatic Evolution of the Median Ridge and the Eastern Ridge-Transform Intersection Domain

International audienceThe eastern Vema fracture zone intersection with the Mid‐Atlantic Ridge (MAR) axis was surveyed with the French submersible Nautile during the Vemanaute cruise in 1989. At this ridge‐transform intersection (RTI), an elongated, E‐W ridge, more than 50 km long, is present in the transform valley. This median ridge rises up to 1000 m above the surrounding seafloor. The crest of the median ridge is lower and presents an arcuate shape at the tip of the MAR axis. Dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation. This detrital cover consists of polymictic sedimentary breccias, sandstones and siltstones, composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle. Most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation. Disaggregation, mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls. This event could be related to the uplift of the southern wall of the fracture zone (the “transverse ridge”) which took place probably between 10 and 3 Ma ago. Since this uplift episode, the transverse ridge (which is now undergoing subsidence) and the detritus covered transform valley floor, separated by the transform fault zone, have migrated westward and eastward respectively. Vertical tectonics of the median ridge at the approach of the RTI can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors. A possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western RTI, is more than 1 km out of isostatic equilibrium. Recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the MAR axis have been recorded on the southern flank of the median ridge. Several stages in the very recent tectonic‐volcanic history of the eastern RTI, that is, roughly during the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shape of the median ridge east of 40°57′W are inferred to have resulted from tectonic extension during the creation of the nodal basin