Seismic Structure and Seismicity in the Southern Mariana Trough and Their Relation to Hydrothermal Activity

Abstract

The Southern Mariana Trough is an active back-arc basin with hydrothermal activity. We investigated relations between the back-arc spreading system and the hydrothermal system in this area by conducting a seismic reflection/refraction survey and a three-month campaign of seismic observations using ocean bottom seismometers. From a 3D seismic velocity structure analysis, we mapped a low-velocity structure just beneath the spreading axis, a high-velocity structure with convex upward beneath an off-axis knoll, and a thickening of layer 2 (to about 3 km) over the refraction survey area compared with normal mid-ocean ridges. We found very low seismicity in the hydrothermal area and high seismicity in areas of high topographic relief that probably represent arc volcanoes. The low-velocity structure at the axis suggests that there is some magmatic activity beneath the axis in the form of sheetlike mantle upwellings. These may constitute the hydrothermal heat source at this site. The high-velocity structure with convex upward at the off-axis knoll suggests the presence of off-axis volcanism there. The very low seismicity suggests that this volcanism may have ceased, thus residual heat of this off-axis volcanism may contribute the heat for hydrothermal activity at this site. A comparison of the velocity structure with other back-arc spreading zones and mid-ocean ridges shows that the Southern Mariana Trough has a relatively thick layer 2 with lower seismic velocities, suggesting that the crust was formed by magmas with high volatile contents, consistent with upwelling mantle influenced by subduction. The very low seismicity at the hydrothermal sites indicates that there are no faults or fractures related to the hydrothermal activity. This suggests that the activity is not related to tectonic stresses there.