Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope: The Kushiro Submarine Canyon, northern Japan

Submarine canyons on active forearc margins are major conduits for huge amounts of detritus from uplifting land areas to subduction zones. Comprehensive geological surveys have revealed the physiographical and sedimentological characteristics of the Kushiro Submarine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230~km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacific. The forearc slope has a convex-upward geometry that can be divided into upper and lower parts separated by an outer-arc high (3200--3500~m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimentary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250~m of thalweg water-depth) are developed on the upper slope, showing a weakly concave-upward longitudinal profile with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infill of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900~m thalweg water-depth) is inactive, the lower part of the upper reaches (900--3250~m thalweg water-depth) is considered to be an active conduit to the lower reaches, as determined from voluminous turbidites recovered in sediment cores (ca.~76-year intervals) and rockfalls observed in the canyon bottom by deep-sea camera. A number of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbidity currents, giving rise to a down-canyon increase in the frequency of flow events. The down-canyon increase in flow occurrence is related to a gradual decrease in gradient, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250--7000~m thalweg water-depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon indicates that the erosional force of turbidity currents decreases down-canyon. The gradient of the lower reaches largely reflects the morphology of the forearc slope along the canyon, which has been deformed by subduction- related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by subduction- related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and that by tectonic processes along the forearc slope are important in the development of the physiography of submarine canyons along active forearc margins.