Guo Peng, Singh Satish C, Vaddineni Venkata A, Grevemeyer Ingo, Saygin Erdinc
Deep Earth Imaging Future Science Platform, The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Kensington 6151, Australia.
Laboratoire de Géosciences Marines, Institut de Physique du Globe de Paris, Université de Paris Cité, Paris 75005, France.
Nat Geosci. 2022 Jul;15(7):591-596. doi: 10.1038/s41561-022-00963-w. Epub 2022 Jun 13.
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 dikes 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-million-year-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 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, that cool and crystallise 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.
大洋地壳在大洋中脊扩张中心通过岩浆作用和构造作用的结合而形成，岩浆作用形成了两个不同的层：上地壳和下地壳。根据地球物理和钻探结果，已知上地壳由熔岩流和玄武岩脉形成，而辉长质下地壳的形成仍存在争议。在这里，我们对来自在缓慢扩张的大西洋中脊形成的相对年轻（700 - 1200万年）地壳的广角地震数据进行了全波形反演。地震速度模型揭示了在距大洋基底约2公里以下，有交替出现的、厚度为400 - 500米、速度变化为±200米/秒的高速层和低速层。最上面的低速层与热液蚀变一致，确定了脊轴附近广泛热液循环的底部。下面的分层表明下地壳是通过熔体以岩床形式在不同深度侵入而形成的，这些熔体在原地冷却并结晶。分层沿着地震剖面延伸达5 - 15公里的距离，覆盖30 - 80万年，这表明这种下地壳增生形式是一个稳定的过程。
