Seismic stratigraphy of the central Tonga Ridge

Abstract

Seismic stratigraphic analyses of single- and multichannel seismic reflection profiles from the central part of the Tonga arc system, SW Pacific, are used to interpret its geological evolution. On the central Tonga Ridge (a high-standing part of the frontal arc, as defined by Karig (1970)) and its flanks between 18°S and 22°S, we identify and map six seismic sequences overlying inferred lower(?)—middle Eocene volcanic (acoustic) basement. Available island sections and boreholes, along with seismic facies, suggest that the upper Eocene to upper Miocene stratigraphic succession on the central Tonga Ridge consists of volcaniclastic turbidites and hemipelagic sediments. In contrast, carbonate deposition predominates from the early Pliocene to the present. We interpret major hiatuses during the early Oligocene, early Miocene and early Pliocene. Based upon the available seismic data and previous results, we infer that the extreme eastern Tofua Trough, a partially filled basin that now separates the central Tonga Ridge from the Tofua Ridge (the modern volcanic arc), may be the site of latest Eocene or Oligocene extension which may be related to rifting and back-arc spreading in the South Fiji Basin to the west. Another late Miocene or early Pliocene phase of rifting deformed strata deposited in the Palaeogene basin, and led to creation of at least part of the modern Tofua Trough. Available evidence suggests that the Neogene extension is related to initiation of sea floor spreading in the Lau Basin. Widespread carbonate deposition beginning in the early Pliocene suggests a decrease of volcanic activity in the vicinity of the central Tonga Ridge. An apparent N-S migration of Miocene(?) and younger depocentres may be related to sequential uplift of parts of the central Tonga Ridge in response to progressive southward subduction of the Louisville Ridge at the Tonga Trench.