Crust of the Earth: A Symposium
Tonga Trench *
Published:January 01, 1955
During the Capricorn Expedition of 1952-1953 the research vessels Horizon and Spencer F. Baird spent two weeks in, the Tonga Trench and Archipelago making echo-sounding, coring, seismic-refraction and towed-magnetometer studies.
The Tonga Trench is a slightly arcuate furrow convex to the east over most of its length. Between Tafahi and Samoa it bends sharply to the northwest, appearing to follow the andesite line. It is continuous at depths greater than 7000 m for 1100 km, including in this segment three elongate basins deeper than 9000 m and one deeper than 10,000 m. The deepest basin explored lies at about 23° 15′ S., near a sharp bend in the axial trend, where bomb sounding gave a depth of 10,800 ± 200 m. A typical west-to-east profile, 350 km long, across the central Tonga Islands, crosses a chain of active volcanic islands at Kao, crosses the narrow Tofua Trough of 1700 m depth to the coralline island Lifuka, then down the west flank of the trench to a maximum depth of 9000 m in a steep-walled gorge 1–5 km wide. The east flank rises more gently to a broad ridge rising about 350 m above the general level of the sea bottom eastward. At 26° 00′ S. Lat. and at 18° 45′ S. Lat. the trench is constricted by large seamounts on the east flank. The latter seamount caps a smooth rise from 7500 m to 410 m and appears to be a guyot tilted westward.
Scattered bottom samples indicate that, except for the calcareous deposits found on the main Tongan ridge, the sea floor in the area is largely volcanic sand and gravel, probably ejected from the near-by active volcanoes.
Seismic-refraction profiles, parallel to the axis of the trench, indicate that the sediment foundation throughout the area is a rock, probably volcanic, of compressional wave velocity 5.2 ± 0.2 km/sec. The sediment thickness is about 2 km in the Tofua Trough, very thin, <200 m on the average, in the inner gorge of the trench, and about 400 m on the east flank, 50 km from the trench axis. Beneath the 5.2 km/sec basement the crustal velocity is 7 km/sec in Tofua Trough, 6½ km/sec at the trench axis and also on the east flank.
The Mohorovičić discontinuity, characterized by a velocity of 8.1 ± 0.1 km/sec, is estimated to be at a depth of 20 km below sea level at the trench axis, and 12 km below sea level on the east flank. This velocity was not observed beneath Tofua Trough, where the highest velocity was 7.6 km/sec, estimated to occur at a depth of 12 km below sea level.
Two profiles of total magnetic field strength, recorded across the trench, have been used to estimate profiles of sediment thickness. When the effects of basement topography are eliminated, a deep-seated asymmetry remains that could indicate either that the Curie point isothermal surface is deeper or that the magnetic susceptibility is greater on the west side than on the east side of the trench.