Geology of the Tonga Forearc: A Supra-Subduction Zone Ophiolite

1996 American Geophysical Union Fall Meeting

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

Dept of Geosciences, Oregon State Univ

Corvallis, OR 97331-5506

541-737-1229

It has been hypothesized that the Tonga forearc comprises an ophiolitic basement, produced in the earliest stages of subduction. A similar history has been outlined for the Ogasawara-Mariana forearc. We have just completed an ODP site survey cruise (Boomerang Leg 8 aboard R/V Melville) in the Tonga forearc from 28 to 14 degrees S and now have a much more detailed understanding of the geology of the forearc.

Dredge hauls from the landward trench slope (4000-6000 m) identified exposures of all the major components of an ophiolite, including tectonized harzburgites and fresh dunite, transitional mantle lithologies (including gabbroic dikes in harzburgite), mafic cumulates, isotropic gabbros, diabases (including diabase-basalt intrusive contacts), a variety of volcanic rocks, and volcaniclastic sediments. The ages and tectonic affinities of these samples are not yet known. The gross distribution of rock types shows the trench slope to be roughly layered above the downgoing Pacific plate. Ultramafic rocks comprise the lowest reaches (7000-8000 m), gabbros and diabases are more common above that, and volcanic rocks are the principal lithology recovered shoaler than 5000 m. Rare serpentinite samples do occur at all levels in the trench slope, but do not appear to come from large diapiric structures. The lower slope exposures of ultramafic rock are likely fault-bounded; we suspect the shallower samples are thin slivers squeezed up along major faults. This layered stratigraphy is not apparent at 23o30'S, at Horizon Deep, where downfaulted Miocene volcaniclastic sediments (identical to those drilled at Site 841) were recovered all the way to the base of the slope. This appears consistent with accelerated tectonic erosion in these latitudes.

We also mapped the northern termination of the Tofua Arc and the northeast arm of the King's Triple Junction. The arc terminates abruptly, in a single large volcano of dacitic to andesitic composition. The boninite exposures in the northern forearc are clearly young, and are associated with a large NNE trending graben. The northeast arm of the King's Triple Junction is manifest by a well developed spreading center to within 30 miles of the trench axis; we have sampled the northernmost end of that ridge.

*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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