Dynamic Accretion Beneath a Slow‐Spreading Ridge Segment: IODP Hole 1473A and the Atlantis Bank Oceanic Core Complex

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.

西南印度洋脊亚特兰蒂斯浅滩的综合大洋钻探计划809号深孔U1473A距离1508米深的735B孔2.2千米，距离158米深的1105A孔1.4千米。通过测绘，它提供了一个660平方千米的下地壳岩基上部层级的首个三维视角。它在横向和纵向上都具有分带性，代表了周期性侵入和持续变形的复杂相互作用，以及间隙熔体的千米级向上和横向迁移。在辉长岩 - 橄榄岩接触带上的转换断层壁潜水发现，只有演化的辉长岩直接侵入到转换断层附近的地幔中。不存在高位熔体透镜体，相反，辉长岩在深部结晶，然后通过晶粥的底辟上升被置入岩墙区域，随后发生晶体塑性变形和断层作用。使地壳与母熔体成分达到质量平衡的残余物位于地块中心下方的深部——可能靠近地壳 - 幔边界。因此，玄武岩是从海底以下>1550米处喷发出来的。相比之下，在北纬23°和亚特兰蒂斯地块钻探的大西洋中脊下地壳几乎没有经历高温变形，晚期熔体运移也有限。它们包含原始堆晶，代表了母源大洋中脊玄武岩在岩墙 - 辉长岩转换带以下较薄地壳中的直接侵入、储存和结晶。西南印度洋脊向南的强烈不对称扩张是由于断层捕获，北部裂谷壁断层被一条延伸穿过大部分侵入区域的拆离断层截断。这导致板块边界迅速向北迁移，而下地壳的大部分向南扩张，使亚特兰蒂斯浅滩剥蚀并抬升进入裂谷山脉。