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

摘要

Knowledge of the time span of arc activity, essentialfor correct tectonic reconstructions, has been lacking for theKermadec arc system, but is supplied in this paper through study ofmicrofossils contained in dredge samples, and K-Ar ages ondredged basalt clasts. The Kermadec system at south latitudes 30to 32° in the southwest Pacific comprises from west to east theColville Ridge (remnant arc), Havre Trough (backarc basin),Kermadec Ridge (active arc) and Kermadec Trench (site of west-dipping subduction of Pacific plate lithosphere beneath theAustralian plate). Data are presented from two traverses (dredge,magnetic, single-channel seismic) across the whole system. Animportant transverse tectonic boundary, the 32°S Boundary, liesbetween the two traverse lines and separates distinct northern (32-25°S) and southern (32-36°S) sectors. The northern sector issector is shallower and well sedimented with broad ridges and adiffuse backarc basin. The southern sector is deeper with narrowridges and steep escarpments facing inwards to a little-sedimented,rifted backarc basin. The Kermadec Ridge slopes smoothlytrenchward to a mid-slope terrace (forearc basin) with minorsediment fill at 5-6 km water depth. A steeper (10-24°) and morerugged lower trench slope is mantled with New Zealand-sourcedrhyolitic vitric mud diamictons containing locally derived maficvolcanic clasts; one clast is of late Miocene age (K-Ar age 7.84Ma). The arc (Kermadec Ridge) is capped by active volcanoes; veryyoung K-Ar ages (<150 ka) from baslts dredged from the 40-kmwide transition zone between the Kermadec Ridge and the HavreTrough, north of the 32°S Boundary, support the concept of arcretreat to the between the Kermadec Ridge and the Havre Trough,north of the 32°S Boundary, support the concept of arc retreat tothe southeast. South of the 32°S Boundary the Kermadec Ridgedeepens and narrows, makes a left-step of 10 km, and presents a2.8-km scarp face to the Havre Trough; K-Ar ages from dredgedbasalt clasts range from 1.25 to 2.04 Ma and indicate exposure foolder arc rocks. The deepest and most sedimented protion of thebackarc basin lies on the western side, both north and south of the32°S Boundary, and the center of basin opening is inferred to lieon the eastern side. There is foundered arc material and formerhydriothermal activity in the center of the rifted basin. The remnantarc (Colville Ridge) has subsided approximately 700m. On thenorthern profile it is broad and has a perched sedimentary basin at1km water depth on the eastern flank. On the southern profile it isnarrow and presents a 2.5-km scarp face to the Havre Trough. Arcsubstrate rocks are exposed on both ridges. Derived microfossils,sedimentary clasts with fossil-based depositional ages in the lateMiocene and Pliocene, clasts of hypabyssal and plutonic rocks, anddated basalt clasts as old as 2Ma, together indicate continuingcollapse and surficial reworking on both ridges. Derivedmicrofossils establish that the Colville and Kermadec Ridges haveexisted (initially as one ridge) since at least the earliest Miocene;by inference, ridge volcanism has been active since the same time,about 25Ma. Rare older microfossils may indicate earlier existenceof the ridge. A 25-Ma inception of arc volcanism is synchronouswith contiguous arc sectors to the north (Tonga) and south (NewZealand). We have no new data on the age of the age of the HavreTrough, which is generally considered to be less than 5 Ma. Aseamount entering the Kermadec Trench is early Eocene or older,and a ridge/seamount in the South Fiji Basin (west of the ColvilleRidge) is middle Miocene or older.