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Contrasting Processes Controlling Basing Formation in Cenozoic Island Arcs, Tonga and Aleutian Ridges

David W. Scholl, Tracy L. Vallier, Andrew J. Stevenson, Holly F. Ryan

The Aleutian and Tonga Ridges formed in the early Eocene as volcanic island arcs along the northern and southwestern rims of the Pacific basin, respectively. During their early history, extensional processes formed summit basins along the shallow submerged crests of both ridges. The arc massif of the original Tonga Ridge was rifted during the initial formative stages of the South Fiji (Oligocene) and Lau basins (late Cenozoic). But after the older rifting event, large summit grabens did not continue to evolve in the fragment of forearc crust remaining under the present Tonga Ridge. The older basins, although extensionally faulted, were filled and then overlapped by 3 to 5-km thick forearc sequences of relatively shallow-water volcaniclastic and carbonate deposits. In cont ast, summit basins along the Aleutian Ridge, which was not rifted to form a back-arc basin, continued to form throughout the Cenozoic. Older basins filled with volcaniclastic debris prior to being uplifted, intruded, and eroded to supply detritus to newer ones.

The Aleutian Ridge is flanked by a large, deep-water forearc basin, the Aleutian Terrace, which has filled with as much as 4 km of volcaniclastic and pelagic debris during the Pliocene and Quaternary. Arc volcanic rocks are thought to have been compressively elevated along the seaward side of the terrace in response to a massive subduction complex of trench deposits being jammed beneath the flanking trench slope. No similar complex exists along the landward slope of the Tonga Trench, which lacks a thick sediment fill and a well-formed deep-water forearc basin.

The contrasting histories of basin formation for the two coeval ridges emphasize that structural depressions on volcanic arcs do not evolve in a set fashion. Instead, formation of summit and deep-water forearc basins are linked to: (1) mechanical and thermal processes controlling whether, and where, arcs split to form back-arc basins (Tonga), or remain effectively whole and in place (Aleutian); (2) stresses conditioned by the relative direction, speed, age, and colliding bathymetric relief of underthrusting oceanic crust; and (3) a thick section of sedimentary deposits beneath the trench floor that can dominate the structural evolution of landward trench slopes.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.