British Geological Survey, Keyworth, United Kingdom

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent Tonga Platform, the shallowest, flattest part of the forearc region. The greatest sediment thicknesses accumulate in debris aprons close to the volcanic front. Collision of seamounts, notably the Capricorn Seamount and the Louisville Ridge, with the forearc radically shortens and steepens the adjacent modern trench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during collision generates angular unconformities, while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the Tonga Platform. Collision also reactivated major across-strike fault zones on the forearc; the zones are subsequently exploited by canyons depositing sediment into the trench. Collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins, measuring approximately 5 km by 15 km in the midslope area. This morphology is especially developed at 18 degrees 30'S to 20 degrees S, implying a 2-3 m.y. interval for their formation following Louisville Ridge collision. Trenchward of these depocenters sedimentation is slow, resulting in manganese crust formation and localized mass wasting along fault scarps. Over longer periods of time (>5 m.y.) tectonic erosion reestablishes a wide, gently sloping forearc into which canyons incise the shallow Tonga Platform by headwall erosion and collapse.