Submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the Mariana back‐arc basin from Wadatsumi side‐scan sonar images

Abstract

High‐resolution, deep‐tow side‐scan sonar data were collected over two distinct spreading segments in the central part of the Mariana back‐arc basin. These data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows. Using these data, we observe spatial variations in seafloor deformation and volcanic activity within each segment, and also significant differences in the degree of tectonic deformation between the two segments. One segment, characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes, is in fact currently dominated by intense deformation rather than volcanism. The other segment, which exhibits morphology (wide and deep linear axial valley) typical of magma‐starved segments, is subjected to very limited deformation and is covered by mostly unfaulted, recent flows. Each segment also displays along‐axis variations in the degree of tectonic deformation and in lava flows freshness. We observe a decrease of lava effusion rate from segment centers toward their ends. We also investigated the apparent asymmetry of the Mariana basin. On the southern segment of the study area, azimuths of tectonic structures are divided into two groups, one segment‐parallel and one ∼15° oblique to strike of the segment. These two trends of linear features developed synchronously with volcanic activity. Currently oblique structures seem to be the most active ones. The tectonic structures that are parallel to the overall valley trend are distributed over the entire valley, whereas the oblique structures are only located proximal to the eastern valley wall. They are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall. Asymmetric character, such as nonuniform spacing and throw of faults, was not observed. These observations suggest that the asymmetry of the basin is not due to asymmetric spreading, but rather to eastward ridge jumps of several kilometers. Such small‐amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin.