Active Margins, Part 2—Tonga Trench, Profiles 0-1200 and G-150
P. Lehner, H. Doust, G. Bakker, P. Allenbach, J. Gueneau, 1983. "Active Margins, Part 2—Tonga Trench, Profiles 0-1200 and G-150", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The Tonga arc is of particular interest in plate tectonics since it was in this area that the concept of a descending "cold slab" of oceanic lithosphere was first demonstrated by Oliver and Isacks (1967).
The subduction zone of the Tonga trench is not located at the margin of a continent but entirely within the oceanic domain of the Pacific. It probably originated in Late Cretaceous time along a north to south trending transform fault.Along the outer wall of the Tonga trench the oceanic basement of the Pacific plunges from a depth of around 6 km (3.7 mi) to a depth of nearly 10 km (6.2 mi). Seismic indicates that a thickness of 300 to 400 m (984 to 1,312 ft) of sediment overlies oceanic basement. JOIDES core hole 204 encountered pelagic clays and oozes of Eocene to Recent age overlying volcanoclastics.
Seismic resolution on the inner wall of the trench is poor. A discontinuous weak reflection band, interpreted as the top of the oceanic basement, can be traced below the poorly defined accretionary wedge to a depth of about 14 km (8.7 mi). The accretionary wedge probably consists of scrapings from the Pacific floor mixed with volcanoclastics, Eocene limestones, and basement material slumped down from the Tonga platform.
The Tonga platform is underlain by a strongly faulted basement, which (according to outcrop evidence) consists of gabbros, diorites and andesitic basalts. This basement is overlain unconformably by shallow marine limestones of middle Eocene age. The bulk of the sedimentary sequence on the Tonga ridge consists of Miocene volcanoclastics covered with a veneer of limestones, ranging in age from Pliocene to Quaternary.The western rim of the Tonga platform is formed by a faulted escarpment which forms the margin of the Lau basin. The volcanic cones visible on profile 150 form part of the Tonga volcanic trend. The Lau basin is a typical small ocean basin of Neogene age.