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

Knowledge of the time span of are activity, essential for correct tectonic reconstructions, has been lacking for the Kermadec are system, but is suppl ied in this paper through study of microfossils contained in dredge samples , and K-Ar ages on dredged basalt clasts. The Kermadec system at south lati tudes 30 to 32 degrees in the southwest Pacific comprises from west to east the Colville Ridge (remnant are), Havre Trough (backarc basin), Kermadec R idge (active are) and Kermadec Trench (site of west-dipping subduction of P acific plate lithosphere beneath the Australian plate). Data are presented from two traverses (dredge, magnetic, single-channel seismic) across the wh ole system. An important transverse tectonic boundary, the 32 degrees S Bou ndary, Lies between the two traverse lines and separates distinct northern (32-25 degrees S) and southern (32-36 degrees S) sectors. The northern sect or is shallower and well sedimented with broad ridges and a diffuse backarc basin. The southern sector is deeper with narrow ridges and steep escarpme nts facing inwards to a little-sedimented, rifted backarc basin. The Kermad ec 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 degrees) a nd more rugged lower trench slope is mantled with New Zealand-sourced rhyol itic vitric mud diamictons containing locally derived mafic volcanic clasts ; one clast is of late Miocene age (K-Ar age 7.84 Ma). The are (Kermadec Ri dge) is capped by active volcanoes; very young K-Ar ages (< 150 ka) from ba salts dredged from the 40-km wide transition zone between the Kermadec Ridg e and the Havre Trough, north of the 32 degrees S Boundary, support the con cept of are retreat to the southeast. South of the 32 degrees S Boundary th e Kermadec Ridge deepens and narrows, makes a left-step of 10 km, and prese nts 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 are rocks. The deepest and most sedimented portion of the backarc basin lies on the w estern side, both north and south of the 32 degrees S Boundary, and the cen tre of basin opening is inferred to lie on the eastern side. There is found ered are material and former hydrothermal activity in the centre of the rif ted basin. The remnant are (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 nar row and presents a 2.5-km setup face to the Havre Trough. Are substrate roc ks are exposed on both ridges. Derived microfossils, sedimentary clasts wit h fossil-based depositional ages in the late Miocene and Pliocene, clasts o f hypabyssal and plutonic rocks, and dated basalt clasts as old as 2 Ma, to gether 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 in ference, ridge volcanism has been active since the same time, about 25 Ma. Rare older microfossils may indicate earlier existence of the ridge. A 25-M a inception of are volcanism is synchronous with contiguous are sectors to the north (Tonga) and south (New Zealand). We have no new data on the age o f the age of the Havre Trough, which is generally considered to be less tha n 5 Ma. A seamount entering the Kermadec Trench is early Eocene or older, a nd a ridge/seamount in the South Fiji Basin (west of the Colville Ridge) is middle Miocene or older. (C) 1999 Elsevier Science B.V. All rights reserve d.