Geomorphological variations at hydrothermal sites in the southern Mariana Trough: Relationship between hydrothermal activity and topographic characteristics
Shuro Yoshikawa, Kyoko Okino, Miho Asada

This study presents the first detailed geomorphological characterization of field-scale geological features associated with hydrothermal systems in the southern Mariana Trough, using near-bottom swath mapping data collected by the autonomous underwater vehicle (AUV) Urashima during cruise YK09-08 and dive observation data acquired by the submersible Shinkai6500 during cruise YK10-11. The motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity, and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area. Two of the hydrothermal sites in the study area (near 12°57’N, 143°37’E) are located on the active backarc spreading axis (the Snail and Yamanaka sites), one is located at the eastern foot of the axial high (the Archean site), and two are located on an off-axis knoll about 5 km from the spreading axis (the Pika and Urashima sites). The on-axis area is divided into tectonically dominant and volcanically dominant zones; volcanically dominant zones are characterized by mounds (height, 5–30 m; diameter, 250–320 m) cut by fissures. The Snail and Yamanaka sites are located adjacent to these fissures, and are possibly represented local activity associated with a 4th order segment-scale diking event (on the basis of comparisons with previously studied cases on the East Pacific Rise with similar on-axis geological characteristics). In contrast to the on-axis sites, the off-axis sites show no evidence of faulting. The Archean site at the foot of the axial high is characterized by a single mound (height, 50–100; diameter, 250–300 m), pronounced off-axis lava flows, and the presence of high-amplitude rugged seafloor features; the site is located at the top of the mound. Numerous ridge lines (height, mainly 2–6 m) extend radially from the top of the mound, and several chimney-like structures (up to approximately 6 m high) occur on the top and slopes of the mound. The Pika site is located on the western peak of an off-axis knoll, and the newly discovered Urashima site is located at the northern foot of the western peak of the same knoll. The western peak is characterized by bumpy seabed textures formed by numerous smaller-scale mounds and ridge lines; however, the eastern peak has a very smooth top and slope, and shows no signs of hydrothermal activity. Numerous mounds (heights, 5–75 m; diameters, 50–350 m) are developed on the comparatively gentle slope of the knoll, in contrast to the numerous ridge lines (height, mainly 1–6 m) developed on the relatively steep slopes of the knoll. On the basis of the associated geomorphological features, the three off-axis sites (Archean, Pika, and Urashima) were identified as localities created by relatively long-term large-scale hydrothermal activity, as compared with sites in the on-axis area. The sustained activity at off-axis sites appears closely related to an off-axis upwelling magma system. The three off-axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting. These areas are characterized by numerous ridge lines, conical mounds, and bumpy seabed texture, whereas the on-axis sites are characterized by the absence of ridge lines, and the presence of white smoker and shimmering observed on dome-shaped pillow mounds. Hence, the distribution of ridge lines, mound morphology, and bumpy seabed texture is likely to correlate with hydrothermal activity.