Morphology of a large meandering submarine canyon system on the Peru-Chile forearc

Abstract

A SeaMARC II side-scan sonar, single-channel seismic reflection, and bottom sampling survey across the Peru-Chile forearc mapped a large submarine canyon system offshore of southern Peru and northern Chile. The main branch of this canyon system extends over 160 km from the shelf break off southern Peru to its termination in the trench off northern Chile. The canyon is 350 to 1100 m wide, and has relief of 150 to 250 m through most of this distance. Within the East Arequipa Basin, the canyon is intricately meandering and has an overall sinuosity of 1.95, comparable to highly meandering subaerial rivers. Meandering is apparently controlled by the basin slope and the characteristics of the turbidity currents that form the canyons. The upper threshold of basin slope for meandering in this canyon is about 15 m/km, which is approximately 10 times greater than that observed for subaerial rivers of comparable size. Bordering terraces at several levels above the canyon floor are remnants of meander belts that developed during early stages of canyon formation. Several cutoff meander loops are preserved on these arcuate terraces. Other sedimentary structures observed along the canyons include levees, overbank deposits, and crevasse splays. The canyon exits the East Arequipa Basin by breaching the forearc structural high along a fault. Downslope of the structural high, the canyon makes a sudden, nearly right-angle bend to the west and continues across the lower forearc slope to the trench axis. This lower stretch of canyon may have formed by headward erosion that pirated the upslope portion of the southward flowing canyon. The lower slope canyon, south of the sudden bend in the East Arequipa Basin canyon, may be a remnant of the lower portion of this ancestral canyon.