MORPHOLOGY OF A LARGE MEANDERING SUBMARINE-CANYON SYSTEM ON THE PERU CHILE FORE-ARC

A SeaMARC II side-scan sonar, single-channel seismic reflection, and b ottom sampling survey across the Peru-Chile forearc mapped a large sub marine 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 c anyon is intricately meandering and has an overall sinuosity of 1.95, comparable to highly meandering subaerial rivers. Meandering is appare ntly controlled by the basin slope and the characteristics of the turb idity currents that form the canyons. The upper threshold of basin slo pe for meandering in this canyon is about 15 m/km, which is approximat ely 10 times greater than that observed for subaerial rivers of compar able size. Bordering terraces at several levels above the canyon floor are remnants of meander belts that developed during early stages of c anyon formation. Several cutoff meander loops are preserved on these a rcuate terraces. Other sedimentary structures observed along the canyo ns 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 s udden, nearly right-angle bend to the west and continues across the lo wer forearc slope to the trench axis. This lower stretch of canyon may have formed by headward erosion that pirated the upslope portion of t he southward flowing canyon. The lower slope canyon, south of the sudd en bend in the East Arequipa Basin canyon, may be a remnant of the low er portion of this ancestral canyon.