Tectonic setting of serpentinite exposures on the western median valley wall of the MARK area in the vicinity of Site 920

Abstract

Mantle-derived serpentinized peridotites crop out in a belt approximately 2 km wide and 20 km long along the western median valley wall of the Mid-Atlantic Ridge just south of the Kane Transform in the MARK area. Serpentinites extend southward from extensive exposures of gabbroic rocks near the Kane Transform. The belt crops out along approximately half the length of a well-defined ridge segment parallel to a prominent neovolcanic ridge. It terminates in a segment boundary zone to the south marked by a bathymetric depression whose trace extends obliquely off axis northwestward into older crust where serpentinites have also been dredged. The seΦentinites are considered to have been exposed by extreme lithospheric extension along a major crustal detachment, as suggested for mafic plutonic rocks that crop out as an oceanic "core complex" to the north. Gently dipping metamoΦhic fabrics and fault surfaces in the seΦentinites suggest a similar structural history for these two adjacent areas. Consistently oriented high-and low-temperature fabrics along the length of the belt of seΦentinized peridotites do not support diapiric uplift as a mechanism for the exposure of these exotic rocks. Gabbroic to diabasic intrusions and overlying basaltic lavas suggest that the seΦentinites were exposed by uplift and faulting of upper mantle material that did not develop an extensive overlying magmatic crust as it rose beneath the ridge axis and spread laterally. These types of exposures of hydrated upper mantle material appear to be common elements of oceanic crust formed at slow-spreading ridges with low magma budgets.

Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)

“…Along steep fault scarps, foliated serpentinites are directly overlain by coarse, clast-supported breccia consisting of angular cobbles of foliated serpentinite in a matrix of consolidated carbonate. Furthermore, in the median valley wall of the MARK area, mass transport processes have produced rock deposits with angular shapes (see Karson and Lawrence, 1997), similar to those found in our CCU. In our model, the serpentinite breccia of BrFm1 could have been reactivated by normal faults, providing the source material delivered to clastic material and blocks of the SedMé.…”

“…Similar serpentinite breccias have been observed in the Atlantic Ocean, along the western median valley wall of the MARK (Mid Atlantic Ridge-Kane Fracture Zone) area. Submersible diving on Alvin (Karson et al, 1987) and Nautile (Mével et al, 1991) reveals that this region is characterized by active faulting and mass wasting dominated by extensive debris-slide deposits (Karson and Lawrence, 1997). Along steep fault scarps, foliated serpentinites are directly overlain by coarse, clast-supported breccia consisting of angular cobbles of foliated serpentinite in a matrix of consolidated carbonate.…”

Fossil mantle-sediments interface recognized in the Western Alps metaophiolites: a key to unravel the accretion mechanism of the Jurassic Tethys ocean

“…), whereas larger blocks of serpentinite randomly distributed within foliated impure marbles and calcschist may be interpreted as coarse, poorly sorted blocks deposited by mass wasting. Similar deposits have been extensively observed and sampled in the Atlantic Ocean (e.g., Karson & Lawrence, 1997). Ophicarbonate rocks resting upon the serpentinite basement indicate that the upper mantle peridotites were already exhumed on the seafloor prior to the onset of subduction zone tectonics within the ocean basin.…”

Laboratory Constraints on the Rheology of the Upper Mantle

“…Geological and geophysical constraints indicate that serpentinites are an important component of the oceanic lithosphere at slow-spreading ridges (e.g., Bonatti and Michael 1989;Dick 1989;Cannat et al 1995;Karson and Lawrence 1997). Similarly, structural and geochemical observations from ophiolites indicate that serpentinization occurs at active ridge axes (e.g., Coulton et al 1995).…”