PERSISTENCE OF DEFORMATIONAL FEATURES ON A LARGE, ACTIVE LANDSLIDE

Long-term observations of the 3.9-km-long, continuously active Slumgullion landslide in southwestern Colorado have shown that the major deformational features on the slide persist through time. Moreover, episodic detailed mapping between 1992 and 2013 has shown that the position and character of many of the major structures in the main body of the landslide remain spatially stationary, relative to nonmoving points of reference, as the slide continues to move. Flanks of the landslide constitute strike-slip boundaries and deformational features near the flanks, including pull-apart basins, flank ridges, and associated zones of distributed shortening, extension, and shear exist in the same locations year after year, despite translational movement as great as 7 m/year in the narrowest section of the slide. Details of the local topography, scarps, fractures and internal toes change from year to year as landslide material of variable thickness and texture translates through these structures. Nevertheless, the position and shape of the strike-slip boundaries, and associated major structures in the main body of the slide, remain nearly constant. Observed stationarity of the internal structures is consistent with long-term stability of the geometry of basal and lateral boundaries and indicates that these boundaries control internal landslide deformation.

The toe of the landslide continues to advance about 1 – 2 m/year and the zone of longitudinal shortening that occupies the lower 600 m of the active landslide continues to enlarge downslope. At the head of the landslide, the slide is becoming thinner and movement is slowing relative to the main body of the landslide. Consequently major zones of extension in the head appear to be migrating downslope, rather than staying stationary. If the current trend of declining movement in the head of the landslide continues, it appears likely that the active head will migrate to a position at least 500 m downslope. Continued thinning of the head may eventually cause long-active movement and structures to gradually become inactive regardless of long-term increases or decreases in available moisture.