TECTONIC CONTROLS ON SEDIMENTATION AND DIAGENESIS IN THE TONGA TRENCHAND FORE-ARC, SOUTHWEST PACIFIC

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the are 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, w ith the forearc radically shortens and steepens the adjacent modern tr ench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during c ollision 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 reactivat ed major across-strike fault zones on the forearc; the zones are subse quently exploited by canyons depositing sediment into the trench. Coll apse and renewed extension of the forearc in the wake of collision res ult in the development of small perched basins, measuring approximatel y 5 km by 15 km in the midslope area. This morphology is especially de veloped at 18 degrees 30'S to 20 degrees S, implying a 2-3 m.y. interv al for their formation following Louisville Ridge collision. Trenchwar d of these depocenters sedimentation is slow, resulting in manganese c rust formation and localized mass wasting along fault scarps. Over lon ger periods of time (>5 m.y.) tectonic erosion reestablishes a wide, g ently sloping forearc into which canyons incise the shallow Tonga Plat form by headwall erosion and collapse.