Evolution of volcanism and faulting in a segment of the Mid‐Atlantic Ridge at 25°N

Abstract

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.