Morphology and history of the Kermadec trench–arc–backarc basin–remnant arc system at 30 to 32°S: geophysical profile, microfossil and K–Ar data

Abstract

Knowledge of the time span of arc activity, essential for correct tectonic reconstructions, has been lacking for the Kermadec arc system, but is supplied in this paper through study of microfossils contained in dredge samples, and K–Ar ages on dredged basalt clasts. The Kermadec system at south latitudes 30 to 32° in the southwest Pacific comprises from west to east the Colville Ridge (remnant arc), Havre Trough (backarc basin), Kermadec Ridge (active arc) and Kermadec Trench (site of west-dipping subduction of Pacific plate lithosphere beneath the Australian plate). Data are presented from two traverses (dredge, magnetic, single-channel seismic) across the whole system. An important transverse tectonic boundary, the 32°S Boundary, lies between the two traverse lines and separates distinct northern (32–25°S) and southern (32–36°S) sectors. The northern sector is shallower and well sedimented with broad ridges and a diffuse backarc basin. The southern sector is deeper with narrow ridges and steep escarpments facing inwards to a little-sedimented, rifted backarc basin. The Kermadec Ridge slopes smoothly trenchward to a mid-slope terrace (forearc basin) with minor sediment fill at 5–6 km water depth. A steeper (10–24°) and more rugged lower trench slope is mantled with New Zealand-sourced rhyolitic vitric mud diamictons containing locally derived mafic volcanic clasts; one clast is of late Miocene age (K–Ar age 7.84 Ma). The arc (Kermadec Ridge) is capped by active volcanoes; very young K–Ar ages (<150 ka) from basalts dredged from the 40-km wide transition zone between the Kermadec Ridge and the Havre Trough, north of the 32°S Boundary, support the concept of arc retreat to the southeast. South of the 32°S Boundary the Kermadec Ridge deepens and narrows, makes a left-step of 10 km, and presents a 2.8-km scarp face to the Havre Trough; K–Ar ages from dredged basalt clasts range from 1.25 to 2.04 Ma and indicate exposure of older arc rocks. The deepest and most sedimented portion of the backarc basin lies on the western side, both north and south of the 32°S Boundary, and the centre of basin opening is inferred to lie on the eastern side. There is foundered arc material and former hydrothermal activity in the centre of the rifted basin. The remnant arc (Colville Ridge) has subsided approximately 700 m. On the northern profile it is broad and has a perched sedimentary basin at 1 km water depth on the eastern flank. On the southern profile it is narrow and presents a 2.5-km scarp face to the Havre Trough. Arc substrate rocks are exposed on both ridges. Derived microfossils, sedimentary clasts with fossil-based depositional ages in the late Miocene and Pliocene, clasts of hypabyssal and plutonic rocks, and dated basalt clasts as old as 2 Ma, together indicate continuing collapse and surficial reworking on both ridges. Derived microfossils establish that the Colville and Kermadec Ridges have existed (initially as one ridge) since at least the earliest Miocene; by inference, ridge volcanism has been active since the same time, about 25 Ma. Rare older microfossils may indicate earlier existence of the ridge. A 25-Ma inception of arc volcanism is synchronous with contiguous arc sectors to the north (Tonga) and south (New Zealand). We have no new data on the age of the age of the Havre Trough, which is generally considered to be less than 5 Ma. A seamount entering the Kermadec Trench is early Eocene or older, and a ridge/seamount in the South Fiji Basin (west of the Colville Ridge) is middle Miocene or older.