Abstract

Submarine fans are well developed in the southern Chile Trench, from 33°S to 41°S latitude. SeaMARC-II side-scan sonar and seismic reflection records image steep erosional escarpments, as much as 400 m in relief, extending seaward across the trench basin from the mouths of submarine canyons. The scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough. Fanlobes are depositional and constructional up-gradient (south) from the canyon mouths. They are composed of aggraded channel/levee complexes, smooth and conformable sediment drapes, and crescentic levees rimming the headwalls of erosional scarps. Fanlobes are carved and dissected by erosional processes down-gradient (north) from the canyon mouths. They exhibit amalgamated lag pavements, composite sediment lobes, longitudinal furrows, braided channels, and canyon-mouth bars. Thick, massive-to-laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes, whereas fine-grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes. The texture and composition of the sediment supply, the onshore climate, and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans.

Stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled, high-amplitude seismic facies: reflective lenticular bodies, truncation and scour surfaces, planar-amalgamation, and sigmoidaccretion structures. The severe erosional dissection of down-gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin. Extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum. The trench fans may have been deposited on progressively steeper trench gradients, because the buoyant Chile Rise migrates northward as it subducts beneath the Andean continental margin.

Fan distributary channels, axial channels, slump scars, and erosional gullies are largely localized along structural features. Normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high-velocity mainstreams of turbidity currents. Orthogonal fault sets within the deposits of the trench basin, which parallel the spreading and transform structures of the extinct Pacific-Farallon Rise and the Chile Rise, are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin. Uplifted thrust ridges, generally restricted to a narrow zone along the base of the deformation front, are dissected by distributary channels, and channel courses are locally deflected seaward of these propagating structures. Transform-oriented basement ridges, associated with strike displacements of the axial channel and vertical faults in the trench basin, may accommodate renewed strike-slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin.

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