In slow spreading environments, oceanic crust is formed by a combination of magmatic and tectonic processes. Using full waveform inversion applied to active-source ocean bottom seismometer data, we reveal the presence of a strong lateral variability in the 40–48 Ma old oceanic crust formed at the slow-spreading Mid-Atlantic Ridge in the equatorial Atlantic Ocean. Over a 120 km-long section between the St Paul fracture zone (FZ) and the Romanche transform fault (TF), we observe four distinct 20–30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, an ∼2 km thick layer with a P-wave velocity <6 km/s, a 1.5 km thick middle crust with a velocity of 6–6.5 km/s, and an underlying layer where velocity is ∼7 km/s, representing the lower crust. The segment associated with an abyssal hill morphology contains a high velocity of ∼7 km/s at 2–2.5 km below the basement, indicating the presence of primitive gabbro or serpenized peridotite. The segment associated with a low basement morphology seems to have 5.5–6.5 km/s velocity starting near the basement extending down to ∼4 km depth, indicating chemically distinct crust. The segment close to the Romanche TF, a velocity 4.5–5 km/s near the seafloor increasing to 7 km/s at 4 km depth indicates a magmatic origin. The four distinct crustal segments have a good correlation with the overlying seafloor morphology. These observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by thermal and/or chemical variations in the mantle during their formation along the ridge segment.

中文翻译：

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来自海底地震仪数据的全波形反演，沿缓慢扩张的赤道中大西洋中脊形成的 ∼40 马 旧洋壳段速度不均匀的地震证据


在缓慢扩散的环境中，海洋地壳是由岩浆和构造过程共同形成的。使用应用于主动源海底地震仪数据的完整波形反演，我们揭示了赤道大西洋缓慢扩张的中大西洋海脊形成的 40-48 马 旧洋壳中存在强烈的横向变率。在圣保罗断裂带 （FZ） 和 Romanche 变换断层 （TF） 之间 120 公里长的一段中，我们观察到四个不同的 20-30 公里长的地壳段。受 St Paul FZ 影响的段由三层组成，一个 ∼2 km 厚的层，P 波速度为 <6 km/s，一个 1.5 km 厚的中间地壳，速度为 6-6.5 km/s，以及一个速度约为 ∼7 km/s 的下层，代表下部地壳。与深海丘陵形态相关的段在基底以下 2-2.5 公里处包含 ∼7 公里/秒的高速，表明存在原始辉长岩或蛇纹橄榄岩。与低基底形态相关的节段似乎具有 5.5-6.5 公里/秒的速度，从基底附近开始向下延伸至 ∼4 公里深，表明化学性质不同的地壳。靠近 Romanche TF 的段，靠近海底的速度为 4.5-5 公里/秒，在 4 公里深处增加到 7 公里/秒，表明岩浆起源。四个不同的地壳段与上覆的海底形态具有良好的相关性。这些观察到的强烈的地壳非均质性可能是由沿山脊轴线的交替构造和岩浆过程引起的，在沿山脊段形成过程中，可能受到地幔的热和/或化学变化的调节。