Development and Runout of a Detachment Slide, Bracebridge Inlet, Bathurst Island, Northwest Territories, Canada

Abstract

A detachment slide on Bathurst Island, Northwest Territories, Canada, occurred on an extensively vegetated, south-facing slope of approximately 5d. The slide is 260 m long, 20–30 m wide, generally less than 1 m thick, involves 4,800 m of material, and exhibits four distinct geomorphic zones. These zones are: (1) the source area, characterized by barren ground and depressed topography; (2) the extension zone, characterized by upwelled mud deposits and extension fractures, containing the upslope portion of the slide mass; (3) the compression zone which shows evidence of folding and thrusting of the tundra mat and occurs in the downslope portion of the slide mass; and (4) the disturbed zone, located to the sides and in front of the slide toe, exhibiting simple folding and tension gashes of the tundra mat. It is suggested that the detachment slide developed through a unique set of conditions which are: (1) wind-driven snow is trapped along the lee of the valley ridge, (2) summer melting of the snow provides a supply of water to the head of the slope, and (3) melt water accelerates melting of the permafrost and develops excess pore water pressures. The detachment slide then progresses as follows: (1) downhill creep (solifluction) at the head of the slope increases the tensile stress acting in the vegetative mat until the tundra mat fails, (2) the resulting slide overloads the slope below, leading to a series of progressive detachments, and (3) as the slide progresses downslope, the shear strength of the slope materials increases due to a decreasing water content leading to cessation of movement.