RESULTS OF NEW HIGH-RESOLUTION BATHYMETRIC AND BACK-SCATTER SONAR IMAGING AND DREDGING OF THE MARIANA ARC AND BACK-ARC SYSTEM

We report new high-resolution bathymetric and back-scatter sonar images of the Mariana arc and back-arc system. The images were acquired using the HMR-1 tow vehicle along overlapping courses over parts of the Mariana arc axis, cross-chain seamount trends, and back-arc axis. We surveyed ca. 48,000 square km while aboard the R/V Melville during Leg 7 of the Cook Expedition, Mar. 4 - Apr. 16, 2001.

Whole-rock, glass, and sediment samples were recovered from 77 dredges and 8 wax cores, with sites identified using the HMR-1 imagery. We sampled 41 arc volcanoes, 35 of which were sampled for the first time. Dredge results indicate a good correlation between the lithology of recovered samples and expected dredge returns based on interpretation of textural variations of the seafloor observed in back-scatter images. Areas of high sonar backscatter suggest rough, irregular surfaces associated with recent lava flows and proximal volcaniclastic flows, whereas low sonar backscatter areas suggest either significant cover associated with prolonged pelagic sedimentation, fine tephra, or distal volcaniclastic flows. A good correlation also exists between volcanic activity and the appearance of volcanic edifices on back-scatter images. Edifices with flat summits at shallow depths and rounded bases in plan view are extinct volcanoes that subsided beneath wave base and the depth of coral growth. Edifices with more irregular bases defined by radial debris channels may still be active.

The volcanoes vary widely in volume, from 1/3 to 1,000 cubic km. They range in height from 200 to 2,700 m and are constructed on a base that ranges in depth from 1 to 3 km. These volcanoes are spaced at distances that range from 7 to 45 km. The average volcano along the southern Mariana magmatic front is spaced 20 km from its neighbor, is built on a platform 1,800 m below sea level, rises 1,000 m above this platform, and occupies a volume of 200 cubic km. Lavas collected from 24 edifices along the magmatic front include abundant basalt and dacite; we also collected several cumulate gabbroic xenoliths.

Some unexpected results of the sonar imaging include: 1) km-scale ripple-like features on the flanks of two volcanic cones; and 2) two submarine channels, ca. 60 km long, 2 km wide, and 60 m deep, lying at depths of 3.5 to 4.5 km and emanating from between two volcanic centers.