The Mid-Atlantic Ridge at 13° N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P-wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130,000 first arrival travel times, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P-wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high-Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hanging-wall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13° N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity-depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust-mantle boundary is a transition zone throughout most of the 13° N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created.

中文翻译：

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大西洋中脊 13°N 海洋核心复合体的 3-D P 波速度结构

位于 13° N 的大西洋中脊被认为是海洋核心复合体 (OCC) 的类型位置，因为它在沿扩展轴约 70 公里范围内包含四个处于其生命周期不同阶段的区域。现有海床观测和采样的丰富性使其成为解决现有 OCC 开发模型之间矛盾的理想目标。在这里，我们描述了 60 × 60 km 足迹内 P 波地震层析成像的结果，其中包含多个 OCC、脊轴和两侧，这决定了 OCC 地壳结构、拆离几何形状和 OCC 沿轴的互连性。沿 17 个横断面采集了广角地震折射数据网格，其中部署了 46 个海底地震仪网络。大约 130,000 次首次到达旅行时间，与稀疏的莫霍面反射一起被建模，将地壳和最上层地幔限制在海平面以下约 10 公里的深度。通过这个 3-D 模型的深度切片显示了几个独立的结构，每个结构的 P 波速度 (Vp) 都比其周围的要高。在海底，这些特征对应于 13°20'N 和 13°30'N 处的轴向谷壁附近以及 13°25'N 处的离轴处的 OCC。这些高 Vp 特征显示出向更深地壳倾斜的趋势，与每个 OCC 拆离的表面表现一致，这意味着下盘内中下地壳和最上地幔的岩石与悬垂中的低 Vp 物质并列墙。脊轴的新火山带在所有深度系统地具有比周围地壳低的 Vp，并且在 OCC 之间更宽。一般，在整个 13° N 区域，地壳厚度约为 6 公里。然而，在轴向 OCC 之间的深熔岩底盆地之下，地壳更薄，并且在分层和速度-深度结构方面更具海洋特征。脊轴处较厚的地壳表明当前地壳形成的岩浆阶段更多，而稀疏莫霍面反射的建模表明壳幔边界是贯穿 13° N 段大部分时间的过渡带。我们的结果支持一个模型，其中 OCC 以独立的拆离断层为边界，这些断层的倾角随深度增加并且随着每个 OCC 周围的方位角而变化，这表明断层的几何形状和机制比以前认为的更复杂。13°20′N 拆离北翼的陡峭程度表明它代表了南北不同断层制度之间的转换带。我们建议单个拆离可能不会沿轴连接，并且 OCC 充当连接正常扩展和拆离断层区域的转换带。岩浆供应沿山脊的变化影响了这种拆离断层的性质。因此，岩浆供应不仅控制着分离体在最终变得不活跃之前如何旋转和离轴迁移，而且还控制着新 OCC 的产生方式、时间和地点。岩浆供应沿山脊的变化影响了这种拆离断层的性质。因此，岩浆供应不仅控制着分离体在最终变得不活跃之前如何旋转和离轴迁移，而且还控制着新 OCC 的产生方式、时间和地点。岩浆供应沿山脊的变化影响了这种拆离断层的性质。因此，岩浆供应不仅控制着分离体在最终变得不活跃之前如何旋转和离轴迁移，而且还控制着新 OCC 的产生方式、时间和地点。