Development and evolution of detachment faulting along 50 km of the Mid‐Atlantic Ridge near 16.5°N

Abstract

A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5 N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along 50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2-3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a lowangle (6 ) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

“…Mass wasting is thus expected to decrease and ultimately cease, which may subsequently favor the exposure of the actual corrugated fault slip plane observed at many OCCs. Recent observations along the MAR at 16°30′N [ Smith et al ., ] support this hypothesis. In this area, a ∼20 km long fault scarp, with a relief of ∼1 km, shows at its base an incipient corrugated detachment fault emerging from the rift valley [ R. Parnell‐Turner et al ., ; Smith et al ., ].…”

“…Recent observations along the MAR at 16°30′N [ Smith et al ., ] support this hypothesis. In this area, a ∼20 km long fault scarp, with a relief of ∼1 km, shows at its base an incipient corrugated detachment fault emerging from the rift valley [ R. Parnell‐Turner et al ., ; Smith et al ., ]. This corrugated detachment fault shows an extension of ∼1.5 km in the spreading direction, corresponding to ∼120 kyr of extension with a spreading half‐rate of 12.5 km/Myr.…”

A Jurassic oceanic core complex in the high-pressure Monviso ophiolite (western Alps, NW Italy)

“…Basaltic lavas and the Upper Jurassic clastic rocks in the hanging wall represent a synextensional (i.e., synrift) sequence resting on the peridotites and gabbros. These processes are reminiscent of those that produce oceanic core complexes, in which deformed older rocks are tectonically overlain along detachment faults by relatively unde- formed, younger syntectonic sediments and basaltic lava flows (e.g., for ancient orogenic belts- Miranda and Dilek, 2010;Manatschal et al, 2011;e.g., for present-day oceanic settings-Cann et al, 1997;Tucholke et al, 1998;Cannat et al, 2006;Smith et al, 2014).…”

“…We interpret the Baracun shear zone (up to tens of meters thick and several hundreds of meters long in outcrop) as the northern segment of a major shear zone, which is tens of kilometers in length and tens to hundreds of meters in thickness (i.e., the Lago Superiore shear zone of Balestro et al, 2013; lower shear zone of Angiboust et al, 2011). This major shear zone is reminiscent of, both in length and thickness, detachment faults associated with modern oceanic core complexes, which range from a few kilometers up to tens of kilometers in length and up to hundreds of meters in thickness (e.g., Karson et al, 2006;Smith et al, 2014). Our major shear zone, which was intensely folded and thickened during subduction and collisional stages (Fig.…”

“…These studies have revealed the occurrence on the seafloor of oceanic detachment faults and associated shear zones with deformed mafic-ultramafic rocks. Detachment faults along nonvolcanic rifted margins and in young oceanic lithosphere accommodate high-magnitude extension, causing the exhumation of lower-crustal gabbros and upper-mantle peridotites on the seafloor, forming oceanic core complexes (Cannat et al, 2006;Karson et al, 2006;Smith et al, 2014). These rocks display mineral assemblages and structural fabrics developed during the interplay of ductile and brittle deformation episodes, fluidrock interactions, and metasomatism associated with their exhumation (Boschi et al, 2006).…”