Evolution of volcanism and faulting in a segment of the Mid‐Atlantic Ridge at 25°N
Evolution of volcanism and faulting in a segment of the Mid‐Atlantic Ridge at 25°N
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北纬 25° 大西洋中脊一段火山活动和断层演化
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发表时间：
2005
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通讯作者：
Deborah K. Smith
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作者：
J. Cann;Deborah K. Smith
我们重建了以25°N为中心的中大西洋海岭扩张段中谷底过去25万年来的火山和构造演化。在该部分的中心，多波束测深和深拖侧扫图像显示了大面积的光滑结构的熔岩流，更像东太平洋隆起而不是大西洋中脊。大西洋中脊更典型的丘状流被发现朝向该部分的南端。丰富的平滑纹理流的存在使我们能够解释该段中的火山和构造关系。我们使用（1）多波束测深法来识别关键的火山结构和断层崖，以及（2）高分辨率TOBI侧扫声纳图像来构建地质图，以根据后向散射亮度显示的整体沉积物覆盖来解释特征之间的年龄关系。底部的照片在中央谷底的关键功能产生详细的火山特征和断层之间的地层关系的信息，使我们能够校准后向散射亮度的沉积物覆盖，因此年龄。通过这种方式，我们可以在分段中心的详细调查区域获得火山活动和变形的历史，最近的流动大约在5000年前爆发，最年轻的平滑流动大约在10,000年前，由断层分开。使用测深和侧扫测量，我们将其外推到整个中央谷底。在板块中心，产生平滑流动的火山活动已经持续了大约25万年。在同一时期，该段南端不断爆发丘状水流。疏浚玄武岩玻璃的电子探针分析表明，有一个系统的组成与位置的部分变化。段中心的玄武岩都比段南端的玄武岩更演化。然而，与熔岩类型没有化学关系。无论它们来自丘状流还是平滑流，来自段中心的玄武岩具有非常接近相同的成分。在过去的几千年里，平滑流和丘状流之间的边界随着时间的推移而波动，并迅速向北迁移，因此，平滑流的爆发可能很快就会停止。调查显示，在侧扫图像上平滑的流动不一定是片状流动。在这项研究中，它们一致地显示出枕形形态。我们的结论是，平滑流可能爆发速度比丘流。
We reconstruct the volcanic and tectonic evolution over the last 250,000 years of the median valley floor in the spreading segment of the Mid‐Atlantic Ridge centered at 25°N. In the center of the segment, multibeam bathymetry and deep‐towed side‐scan images show a large area of smooth‐textured lava flows more like those of the East Pacific Rise than those of the Mid‐Atlantic Ridge. Hummocky flows more typical of the Mid‐Atlantic Ridge are found toward the southern end of the segment. The presence of the abundant smooth‐textured flows allows us to interpret the volcanic and tectonic relationships in the segment. We construct a geological map using (1) multibeam bathymetry to identify the key volcanic structures and fault scarps and (2) high‐resolution TOBI side‐scan sonar images to interpret age relationships between features on the basis of overall sediment cover as shown by backscatter brightness. Bottom photographs across key features on the median valley floor yield detailed information on stratigraphic relationships between volcanic features and faults and allow us to calibrate backscatter brightness in terms of sediment cover and hence of age. In this way we derive a history of volcanic activity and deformation in a detailed survey area at the segment center, with the most recent flows erupted about 5000 years ago, and the youngest smooth flows about 10,000 years ago, separated by an episode of faulting. Using bathymetry and side‐scan surveys, we extrapolate this to the whole of the median valley floor. The volcanic activity giving rise to the smooth flows has been continuous for about a quarter of a million years at the segment center. Over the same period, hummocky flows have been continuously erupted at the southern end of the segment. Electron probe analyses of dredged basalt glasses show that there is a systematic variation in composition with position in the segment. Basalts from the segment center are all more evolved than those at the southern end of the segment. There is, however, no relation of chemistry with lava type. The basalts from the segment center have very nearly the same composition whether they come from hummocky flows or smooth flows. The boundary between the smooth flows and hummocky flows has fluctuated with time and migrated rapidly northward over the last few thousand years, so that shortly the eruption of smooth flows will probably have ceased. The survey shows that flows that are smooth on side‐scan images are not necessarily sheet flows. In this study they uniformly show pillow morphology. We conclude that smooth flows were probably erupted at faster eruption rates than hummocky flows.