A large block slide affecting an area of approximately 1.0 square km was reported to have occurred on Cement Creek Y.T. (115G/5) between February 15 and March 15, 1983. Landsliding is confined to basaltic andesites in the lower unit of the Wrangell Lavas. Hydrothermal alteration of flow-top breccias and the tilting of strata towards an open face bordering Cement Creek predispose rock to slip along bedding planes. Surveys conducted during June to August 1986 detected displacements of 10-12 cm during an 8 week period. Seismic refraction and electrical resistivity surveys conducted on the slide mass suggest that most ground failure was caused by strong ground motion during an initial episode of rapid displacement and that the rupture surface lies beneath the lower limit of permafrost. Ground-water accumulation on and above the rupture surface followed by landslide initiation during a burst of low magnitude seismicity is suggested as a slide mechanism.

The Rackla River area is underlain by normal faulted and gently folded sedimentary strata of the Paleoproterozoic Wernecke Supergroup, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Gabbro dikes and sills that are likely age equivalent to the ca. 1380 Ma Hart River Sills cut the Wernecke Supergroup rocks. The presence of a mafic volcaniclastic horizon within the Bouvette allows its informal subdivision into a lower and upper member. These volcaniclastic rocks may the distal equivalent to volcanic rocks near the Tiger deposit, located ~20 km to the southwest. Three major angular unconformities are documented in the map area: at the base of the Rapitan Group, the base of the lower Bouvette, and the base of the upper Bouvette Formation.

A pre-historic pseudo-circular rock-slope failure at the northern edge of Dawson City, Yukon occurs in altered ultramafic rocks. The middle section of the landslide debris continues to move down-slope, as is evident from sheared trenches, stretched roots and split trees along its edges, and a steep snout exposing fresh material. Dendrochronological analysis demonstrated that the split trunk of one tree has displaced over the last 40 to 45 years at an average movement rate of 4.5 cm/year. This moving section of the debris could be characterized as a rock glacier or as an earth flow, although our present observations and measurements do not confirm either mechanism. A block upslope from the headscarp of the landslide exhibits signs of recent movement. To assess the movement rate of the different sections of the landslide in the future, a monitoring array was set up and an initial set of measurements was taken in July, 2006.

The following report describes the settings, causes and geological controls of landslides in the Alaska Highway corridor. Although diverse geologic, geomorphic and climatic environments exist in the region, most landslides are related to the presence of shallow bedrock or permafrost, unconsolidated sediment on steep slopes, weak bedrock, groundwater hydrology, river erosion or the degradation of ice-rich permafrost. Where geologic controls provide appropriate settings, intense rainfall, rapid snow melt and seismic events play important roles in triggering failures. Rainstorms that reach thresholds of combined intensity and duration have triggered abundant shallow landslides within the corridor. Debris flows have historically posed the highest risk to lowlying regions and are capable of damaging settlements and transportation routes. The Shakwak Valley has the highest concentration of landslides within the corridor due to the abundance of steep slopes, high relief and widespread discontinuous permafrost. In Wellesley Depression, shallow permafrost and its subsidence has an important influence on slope instabilities. Landslides in the Yukon Plateau primarily relate to the presence of silt- and ice-rich tills on steep valley sides as well as the incision of fine-grained lacustrine terraces in valley bottoms. Debris flows after intense rainfall events are the most common form of landslide in the Kaska Mountains. Finally, in Liard Lowland, failures associated with glacial meltwater and modern stream incision are the most common landslide events. Permafrost plays an important role in landslide processes in the corridor due to its influence on soil moisture, drainage and strength. Slopes composed of icy sediment that have been burned by forest fires are particularly vulnerable to rapid mass movements due to permafrost degradation. The consequences of the dramatic increase in landslide potential after fire should be considered in local fire management plans. The climate¿s local and regional influence on hydrology, fire frequency and permafrost distribution greatly affects landslide processes. Current climate change projections call for warmer temperatures and increased precipitation for the Yukon in the next half century. Among the anticipated effects of global warming in southern Yukon, increased incidents of intense snowmelt and/or precipitation events, river migration, permafrost degradation or forest fires may lead to an increase in landslide frequency and/or magnitude within the settings described in this report. The most significant impact of increased landslide activity may not be a direct impact. Rather, increased sediment input from landslides will likely increase stream channel instability and flooding. This would be particularly acute in the vicinity of alluvial and colluvial fan complexes along Kluane Lake where highway maintenance is already a challenge.

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The Evelyn Creek–Mount Grant area includes a large region of igneous and meta-igneous rocks; here termed the Mount Grant batholith. The batholith comprises mostly deformed Mississippian metatonalite of the Simpson Range suite and variably, but mostly non-deformed Cretaceous granitic rocks of the Cassiar suite. The metatonalite has a sheet-like geometry and is in contact with contrasting metasedimentary successions. Overlying the metatonalite is the Slate Mountain succession, which includes quartzite, psammite, phyllite and limestone. The metatonalite overlies rocks of the Evelyn Creek succession (new), which has a tripartite stratigraphy; its lower part is dominated by chert and siliceous argillite, the middle unit comprises chlorite-muscovite schist and quartzite, while the upper unit is a prominent interval of pale coloured marble/calcsilicate. The contact between the Evelyn Creek succession and the overlying Mississippian metatonalite is interpreted to be structural rather than intrusive. Rocks of the Cassiar suite include large volumes of non-deformed granite and quartz monzonite and lesser amounts of foliated equivalents. Low pressure metamorphism is recorded by andalusite and/or sillimanite bearing schists that are restricted to the eastern part of the area. This metamorphism and deformation did not affect the remainder of the area, where latest penetrative structures are crosscut by Early Jurassic rocks of the Lokken suite.