Tectonic window into gabbroic rocks of the middle oceanic crust in the MARK area near Sites 921–924

Abstract

Extensive exposures of gabbroic rocks, inferred to represent mid-crustal intervals of the oceanic crust, crop out on the western median valley wall of the spreading segment of the Mid-Atlantic Ridge immediately south of the Kane Transform. Surface exposures of the gabbroic rocks have been mapped by numerous submersible dives, deeply towed camera transects, and sidescan sonar surveys, making this region one of the most intensively studied areas of the mid-ocean ridge system. Recent drilling in this tectonic window at Sites 921-924 provides a new perspective on the architecture and mode of accretion of mid-crustal rocks of the oceanic crust.Surface studies show that the gabbroic rocks crop out in an area that extends at least 15 km along the ridge axis and 8-10 km normal to the ridge axis. The median valley wall of the Mid-Atlantic Ridge here is essentially a dip-slope on a major crustal detachment fault. This low-angle normal fault zone has been cut by numerous steeply dipping normal faults that impart a stairstep morphology to the terrain. Basaltic pillow lavas crop out at the top of the median valley wall and probably represent upper crustal rocks that formed structurally above the gabbroic units. To the east, the median valley floor of the Mid-Atlantic Ridge is dominated by relatively young pillow lavas and an extensive neovolcanic ridge. These lavas overlie the detachment footwall, and are probably the surface expression of dikes and plutonic bodies that cut the detachment at depth.Gabbroic and related rocks recovered from Sites 921 to 924 indicate that the mid-crustal terrain exposed in this tectonic window was assembled as a collage of relatively small (less than about 1 km across) plutons. Individual plutons have been variably deformed and metamorphosed, and subsequently intruded by later plutons and dikes. Thus, whereas surface exposures show evidence of major faulting over the past 0.5 m.y., subsurface data from Ocean Drilling Program cores indicate a more continuous style of synkinematic intrusion and crustal stretching. These processes may be broadly coeval expressions of slow seafloor spreading in this environment. Exposures of similar crustal levels in the adjacent Kane Transform suggest a similar pattern of accretion. This type of spreading may be common at ridge-transform intersections and other places on slow-spreading ridges with very low or episodic magma budgets. 1 Karson, J. A., Cannat, M., Miller, D.J., and Elthon, D. (Eds.), 1997. Proc. ODP, Sci. Results, 153: College Station, TX (Ocean Drilling Program). department

The architecture of an incipient oceanic basin: a tentative reconstruction of the Jurassic Liguria-Piemonte basin along the Northern Apennines–Alpine Corsica transect

“…However, the subcontinental mantle rocks record a history of two different magmatic cycles, an early magmatic, non-volcanic stage followed by a late volcanic one ). These characteristics are typical of the ophiolites derived from slow-and very slow-spreading ridges, as suggested by the data from the present-day Atlantic ocean (e.g., MARK area; Mé vel et al 1991;Karson and Lawrence 1997;Cannat et al 1997). Therefore, the ocean crust of the Liguria-Piemonte basin could possibly also be derived from a slow-to very-slow spreading ridge.…”

Oceanic Core Complex Development in Modern and Ancient Oceanic Lithosphere: Gabbro‐Localized versus Peridotite‐Localized Detachment Models

“…The ultramafic rocks recovered from these fault surfaces have been used to invoke a limited role of magmatism during core complex formation, leading to the development of many "amagmatic" models of oceanic detachment faulting (Tucholke and Lin 1994;Tucholke et al 1998;Escartín et al 2003). In contrast, the detachment-fault surfaces of the Atlantis Bank (SWIR) and Kane (MAR) core complexes are dominated by gabbroic and troctolitic rocks and fault schists (Karson and Lawrence 1997;MacLeod et al 1998;Arai et al 2000;Kinoshita et al 2001;Matsumoto et al 2002), leading to models of core complex development that emphasize moderate levels of magmatism during detachment faulting (Dick et al 1992Cannat et al 1997;Karson 1999;MacLeod et al 2003;Ildefonse et al 2007). However, recent deep drilling of two core complexes thought to have developed during reduced magma supply, the Atlantis Massif and the 15Њ45ЈN, has revealed long sections (∼1400 and ∼200 m, respectively) of gabbroic rocks in the footwalls of both massifs Kelemen et al 2007).…”

“…A few OCCs have been explored with both seafloor sampling and deep drilling via the Ocean Drilling Program (ODP) or the Integrated Ocean Drilling Program (IODP): the Atlantis Bank, Southwest Indian Ridge (SWIR); the Kane inside corner high, 23Њ30ЈN, Mid-Atlantic Ridge (MAR); the 15Њ45ЈN core complex, MAR; and the Atlantis Massif, MAR (e.g., Karson and Lawrence 1997;Blackman et al 2006;Kelemen et al 2007). Of these core complexes, the Atlantis Bank (SWIR) and Atlantis Massif (MAR) OCCs are the sites of the deepest ODP and IODP drill holes, respectively, and they have been sampled by more submersible dives than any other drilled core complexes ( fig.…”

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…Gabbros and serpentinized peridotites have been sampled from OCC 1 and OCC3 during submersible dives Karson and Lawrence, 1997] and from drill cores (Ocean Drilling Program (ODP) 670 [Detrick et al, 1990] and ODP 921-924 ). The westernmost massif, OCC4, has not been sampled to date.…”