Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s⁻¹ velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.

洋壳通过岩浆和构造过程的结合在洋中扩张中心形成，岩浆过程形成两个不同的层：上地壳和下地壳。虽然根据地球物理和钻探结果已知上地壳是由熔岩流和玄武岩岩脉形成的，但辉长岩下地壳的形成仍然存在争议。在这里，我们对在缓慢扩张的大西洋中脊形成的相对年轻（7-12 密尔龄）地壳的广角地震数据进行了完整的波形反演。地震速度模型揭示了距海洋基底约2公里以下的交替的400-500 m厚的高速层和低速层，速度变化为±200 ms ^-1 。最上面的低速层与热液蚀变一致，定义了脊轴附近广泛的热液循环的基础。下面的层状结构支持下地壳是通过熔体侵入不同深度的基台而形成的，熔体在原位冷却和结晶。分层沿着地震剖面延伸了5-15公里的距离，覆盖了30万-80万年，表明这种形式的下地壳增生是一个稳定的过程。