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Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 124(12), (2019): 12631-12659, doi:10.1029/2018JB016858., 809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 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 kilometer‐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, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at 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 faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains., The first author wishes to also recognize grants OCE1434452 and OCE1637130 from The National Science Foundation (NSF) for synthesis of the Atlantis Bank site survey data and post‐cruise rock analysis and for analysis of Expedition 360 and 362T cores and data. Additional support was also gratefully received from The Investment in Science Fund at WHOI., 2020-05-07

809 deep IODP Hole U1473A at Atlantis Bank, SWIR, is 2.2 km from 1,508‐m Hole 735B and 1.4 from 158‐m Hole 1105A. With mapping, it provides the first 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 kilometer‐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, basalts erupted to the seafloor from >1,550 mbsf. By contrast, the Mid‐Atlantic Ridge lower crust drilled at 23°N and at 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 faults cutoff by a detachment fault that extended across most of the zone of intrusion. This caused rapid migration of the plate boundary to the north, while the large majority of the lower crust to spread south unroofing Atlantis Bank and uplifting it into the rift mountains.