ORIGIN AND ACTIVITY OF ANTISLOPE SCARPS IN THE MOUNTAINS OF SOUTHWESTERN BRITISH COLUMBIA

Antislope scarps are widespread at moderate to high elevations in mountain ranges throughout southwestern British Columbia. They occur in a variety of rock types, including granitic, volcanic, and foliated metasedimentary rocks. Some scarps are orthogonal to the slope of the mountainside on which they occur, but others are continuous across ridge crests and thus appear to be independent of topography. Two theories have been proposed to explain the origin of the scarps; 1) they are surface traces of active or recently active faults that have formed as a result of large earthquakes during Holocene time, 2) they are the surface manifestation of slow, deep-seated gravitational movements (“sackung” or slope sagging). These opposing hypotheses carry important implications for hazard assessment for civil engineering projects located in the region. Geomorphic evidence strongly suggests, however, that most of the antislope scarps are gravitational. This includes the presence of a series of scarps on slopes that follow contour lines, classical “doppelgrat” features, the restriction of the scarps to the higher portions of mountains slopes, and slickensides on scarps with trends consistent with downslope movement of the rock mass. Geodetic surveys of some slopes that are crossed by antislope scarps have demonstrated that they are slowly moving in a manner consistent with gravitational creep. At three locations, exploratory trenches were dug through sediment fills in troughs formed upslope of antislope scarps that other researchers had identified as earthquake-produced landforms. Trenching revealed that the scarps occur along the surface traces of ancient, gouge-filled faults. Deformation of the sediments at the three sites decreases upward through the fill. The pattern of deformation is consistent with slow continuous or possibly episodic downslope movement from latest Pleistocene time, when the region became deglaciated, to the late Holocene; in at least two of the sites, movement appears still to be occurring. Some deep-seated slope sags have released as rockslides or rock avalanches, but it is thought that most will continue to move slowly indefinitely, with little or no potential for catastrophic failure. The conditions under which sackung-style slope deformation is transformed into catastrophic mountain slope failure remains to be determined.