Dynamic accretion beneath a slow spreading ridge segment: IODP Hole U1743A & the Atlantis Bank Oceanic Core Complex
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809-mbsf IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1508-m Hole 735B and 1.4 from 158-m Hole 1105A. With mapping, it provides the 1st 3-D view of the upper levels of a 660-km2 lower crustal batholith. It is laterally and vertically zoned, representing a complex interplay of cyclic intrusion, and ongoing deformation, with km-scale upward and lateral migration of interstial melt. Transform wall dives over the gabbro-peridotite contact found only evolved gabbro intruded directly into the mantle near the transform. There was no high-level melt lens, rather the gabbros crystallized at depth, and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal-plastic deformation and faulting. The residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif – likely near the crust-mantle boundary. Thus, MORB’s erupted to the seafloor from >1,550-mbsf. The MAR lower crust drilled at 23°N and Atlantis Massif experienced little high-temperature deformation and limited late-stage melt transport. They contain primitive cumulates, and represent direct intrusion, storage, and crystallization of parental MORB in thinner crust below the dike-gabbro transition. The strong asymmetric spreading of the SWIR to the south was due to fault capture, with the northern rift valley wall faultscutoffbyadetachmentfaultthatextendedacrossmostofthezoneofintrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south.
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