New SeaBeam 2000 Bathymetry and Sidescan Data on the Tonga Forearc and Trench: Results from Boomerang Leg 8, an ODP Site Survey

1996 American Geophysical Union Fall Meeting

D Wright (with S H Bloomer & Boomerang Leg 8 Shipboard Scientific Party*)

Dept of Geosciences, Oregon State Univ

Corvallis, OR 97331-5506

541-737-1229

Initial results from the Sea Beam 2000 wide swath multibeam sonar system are presented from Leg 8 of the Boomerang expedition aboard R/V Melville, May - June, 1996. Sea Beam data were collected from ~28° 00' to ~14° 00' S latitude, and ~176° 30' to 172° 10' W longitude, along the Tonga Forearc and Trench, including 10 proposed Ocean Drilling Program sites. Single-channel seismic, magnetic, and gravity profiles were also collected, to be reported on at a later date. In addition to the 121 beampoints of bathymetry, Sea Beam 2000 provided sidescan sonar coverage out to the complete swath width, approximately 10 km, with 10-m pixels correctly placed on the bathymetry in near real-time.

With the combined bathymetry and sidescan system we were able to detect major faults on the Tonga platform that can be traced well into the upper trench slope, corresponding to the major structural divisions of the platform. Horst-graben structures are clearly evident not only in the subducting Pacific Plate but at the base of lower trench slope. In addition, we have complete Sea Beam coverage of: (1) the landward trench slope at the westward "bend" in the Tonga Trench which reveals a striking picture of how the structural grain changes as the relative plate motion moves from convergence to strike-slip motion; (2) the northern arm of the King's Triple Junction which is manifested by a well-developed spreading center within ~50 km of the Tonga Trench axis. Preliminary results confirm the hypothesis that the Tonga convergent margin is undergoing active tectonic erosion and that the forearc is dominated by normal faulting and movement of material into the trench.

*Boomerang Leg 8 Shipboard Scientific Party
C.J. MacLeod (Dept. of Earth Sciences, University of Cardiff, PO Box 914,Cardiff CF1 3YE, UK), D. Tappin (British Geol. Surv., Keyworth, Nottinghamshire, NG12 5GG , UK), P. Clift (WHOI, Woods Hole, MA, 02543), T. Falloon (Dept. of Geology, Univ. Tasmania, GPO Box 252C, Hobart, Tasmania 7001, AUSTRALIA,) R.L. Fisher (Scripps Inst. of Ocean., UCSD, LaJolla, CA 92093-0225), K. Gillis (School of Earth and Ocean Sci., PO Box 1700, Univ. Victoria, Victoria, BC, V8W 2Y2, Canada), T. Ishii (ORI, Univ. of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164 Japan), M. Kelman, (Dept. of Geosciences, Oregon State Univ., Corvallis, OR), K. Mafi (Ministry of Lands and Resources, Kingdom of Tonga), H. Sato, (Program in Geoscience, Univ. Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305 JAPAN, and M. Winowitch (USGS, 600 4th St., St. Petersburg, FL, 33701)

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