Tectonic Erosion and Slope Instabilities At The Ecuador Convergent Margin : Effects of Thecarnegie Ridge Subduction ?

Multichannel seismic reflection data, swath mapping morphology and stratigraphic data from an offshore drill site were used to derive a geological model of the Ecuador margin and to examine its along-strike structural variations in relation to the Carnegie Ridge subduction. MCS data were recorded using a 45-L airgun seismic source and a 360-channel streamer. Shots were fired every 50-m, providing a 45-fold coverage. Swath bathymetry was collected using a 162 beams EM12D Simrad system. These data show that the Carnegie Ridge is blanketed by 0-450 m of sediment overlaying a basement that returns strong acoustic reflections over ~3 std. Near the trench, the ridge is cut by east-facing normal faults, which delineate left-stepping segments of the deformation front and structural troughs containing a thin sediment fill. The plate in- terface has been imaged as strong, discontinuous reflectors dipping landward from the trench, over a distance of 40-50 km. This interface bounds the top of the subduction channel, which may contain pelagites and deforming ridge rocks. The margin, which is mostly devoid of an accretionary wedge, consists of six acoustic units. The correlation of the upper fifth units with the drill site and island outcrops allows to interpret, from bottom to top, the lower Cretaceous Piñón formation, the upper Cretaceous volcano- sedimentary Calentura and Cayo s.s. units, the middle Eocene volcano-sedimentary Ancón unit, and Recent slope deposits. A sixth unit overlaying locally the plate inter- face suggests under-plating, Morphologically the margin divides in two areas. North of 140S, the slope is smooth and fronted by a narrow bench, whereas south of 140S the slope shows breaks, indentations, and is fronted by a wide bench. The overall structure of the margin reveals the brunt of vigorous tectonic erosion. The downward truncation at the plate interface of the acoustic basement and seaward dipping acous- tic units supports basal tectonic erosion. Based on paleobathymetry, the Ancón unit, which includes near shore and beach deposits and extends to near the trench, would have subsided by more than 3000 m since the middle Eocene. In addition to basal ero- sion, which affects the entire margin, frontal erosion characterized by collapses, debris avalanches and large rotational slumps, developed mainly south of latitude 1 40S, in relationship with subduction of major seamounts studding the Carnegie Ridge south-

1 ern flank. Mass wasting products and slump blocks underlay the lower slope benches, and are locally underthrust or incorporated to the imbricated margin front

2