A multibeam reconnaissance of the Tonga Trench axis and its intersection with the Louisville guyot chain

A Seabeam reconnaissance of 1200 km of the deep sediment-starved axis of Tonga Trench delineated the fine-scale relief at the outcrop of a subduction zone generally characterized by tectonic erosion rather than accretion. The commonest axial cross-section has a steep (12°) irregular inner slope intersecting the thinly sedimented surface of Mesozoic ocean crust, which dips under it at 5–6°. There is little or no intervening turbidite fill, but small lenses interpreted as debris deposits occur at the foot of parts of the inner slope that lack basins or benches which elsewhere obstruct downslope sediment transport. The oceanic slope is severely broken by parallel but slightly sinuous fractures induced by bending of the plate, and entry of outer-slope grabens into the subduction zone is confirmed to be a morphologically and tectonically important process. Arrival of oceanic seamounts and volcanic ridges at the trench outer slope and axis affects the fracture pattern of the oceanic plate, the depth of the temporarily plugged axis, and the relief of the lower inner slope. Subduction of the Louisville guyot chain, or of the extensive hotspot swell and thick sediment apron that surrounds it, has important regional effects as well, shoaling 400 km of trench axis and causing development of a small accretionary prism with trench-slope basins. Because the intersection point of the hot-spot chain has moved rapidly south along the trench, structural changes that occur in the wake of guyot-chain subduction can also be inferred: accretion at the inner slope is followed by rapid tectonic erosion, which unroofs a wider strip of downgoing lithosphere and thereby deepens the trench axis. The longitudinal profile of axial depths, made locally irregular by the collision of medium-scale volcanic and tectonic relief on the oceanic plate, also has a step near 18.5° S, where there is a regional depth difference in the oceanic crust entering the trench.