Recent mapping off eastern North America has revealed four areally extensive slide zones between the Blake-Bahama Outer Ridge and Long Island. These slide zones are recognized on the basis of 3.5-kHz records and by the structures present in piston and box cores. The distribution of piston cores containing slump and debris-flow structures indicates that mass movements have taken place in this region on a wide scale and that slumping and sliding of sediments from the slope and upper rise onto the middle and lower rise are ubiquitous in the region during recent geologic time.

The slides appear to originate on the continental slope and upper rise and appear to have occurred in multiphase events. In the area south of Baltimore Canyon a series of apparently disconnected 20 to 30° scarps are present within the stratified hemipelagic sediments of the upper rise and slope. Large volumes of sediment which now cover an area of at least 2,000 sq km moved down through the valleys onto the middle rise (~3,100 m). The sudden loading of material appears to have triggered a series of smaller slides in deeper water (3,100 to 3,600 m), and this sediment then moved down onto the lower rise to depths as great as 4,200 m, forming a narrow mudflow tongue 10 km wide and 100 km long which appears to be disconnected from the shallower flows.

Three very large slide zones, each in excess of 10,000 sq km, occur on the slope and rise: (1) north of the

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Blake Outer Ridge, (2) between Washington and Norfolk canyons, and (3) southeast of Long Island. Deposits resulting from these events extend across the rise to depths greater than 4,000 m. In the zone north of the Blake Outer Ridge, the deposits extend onto the Hatteras Abyssal Plain to a water depth of 5,400 m.

The distribution of most of the major slide zones appears to relate to the position of canyon systems and may be due to the rapid buildup of sediment over-flowing from the canyons during the last glacial stage. Occasional large intraplate earthquakes such as the 1886 Charleston or the 1929 Grand Banks events may be the direct triggering mechanism. The ages of the slides are as young as middle Holocene. In time, the slope and rise probably will reach an equilibrium point (this may already be the situation), and the frequency of large-scale slides will decrease drastically until the next glacial stage.

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