CHAPTER 2

KINEMATIC STUDIES OF THE SLUMGULLION LANDSLIDE, HINSDALE COUNTRY, COLORADO

by Rex L. Baum and Robert W. Fleming

Introduction

During the past decade, we have mapped surface structures and monitored movement of several different slow-moving landslides to identify landslide-movement processes (Fleming and Johnson, 1989; Baum and others, 1993). In 1990, we began a field study of the Slumgullion landslide to compare its surface features and movement processes with those of landslides in Utah and Hawaii.

Slumgullion Landslide Structure and Movement

The 3.7-km-long active part of the Slumgullion landslide moves 0.5 to 5.5 m annually, producing and destroying a variety of features on the landslide surface. The landslide boundaries define a sinuous, tabular slide mass, 120 m wide at the head and 420 m wide at the toe, that consists of two distinct landslides (fig. 1). A relatively small landslide that moves 0.2 to 0.5 m/yr extends south from the head of the slide at elevation 3,485 m to a poorly developed internal toe near elevation 3,430 m (shaded area, fig. 1). This 0.4-km-long landslide occupies the western part of a bowl-shaped scar at the south edge of Mesa Seco and apparently provides the driving force that triggered movement farther downslope. Near the internal toe, the slide begins to turn toward the southwest, and 0.2 km downslope from the internal toe, a zone of scarps near elevation 3,380 m forms the head of the remainder of the active landslide. The southwest-trending landslide is hour-glass-shaped and extends 3.1 km downslope to the active toe at elevation 2,960 m (fig. 1).

Strike-slip shear zones, cracks, scarps, and internal toes are the surface manifestations of structures that divide the landslide into a number of distinct elements; these elements are the product of irregular boundaries and rates of movement. Several structures facilitate the necessary convergence and acceleration in the top of the "hour glass, "where slide width decreases from 370 m at the zone of scarps to 160 m at the neck of the hour glass. Displacement increases from 1 m/yr at the scarps to 5.5 m/yr at the neck (fig. 1). Internal right- or left-lateral shear zones coupled with transverse scarps isolate wedges along the flanks from faster moving blocks, and a hopper-like structure folds material from the edges toward the axis of the slide (fig. 1). The hopper-like structure is a steep-sided topographic basin within the landslide; its lowest point is near the axis of the landslide, and material appears to move toward the low point as in a hopper. Structures that accommodate distributed shearing at the flanks are well expressed near the neck. Most of the shear occurs on a surface or in a thin shear zone at the flank, and oblique cracks or listric faults (expressed by downhill-facing scarps) next to the flank carry the remainder. Downslope from the neck, the slide widens gradually, which results in a series of steps at the north flank. Right-lateral shear zones stepping to the right create small pull-apart basins at points of widening along the flank and cause the center of the slide to be higher than the edge (fig. 2). Shortening in the downslope direction below the neck has produced several groups of internal toes. Splaying of the right-lateral shear zone along the north flank accommodates further widening to 420 m at the toe. The most prominent part of the toe moves about 2 m/yr, and shear-zone-bounded wedges in the north part of the toe move 0-1 m/yr (fig. 1). A curious feature of the main segment of the slide is a ubiquitously higher rate of movement along the left (south) flank than at the right (north) flank. A discontinuous strip of longitudinal scarps and cracks along the axis of the slide appears to be a right-lateral shear zone that separates the faster moving left half from the right. Flank ridges occur at several places on both flanks. Field evidence is consistent with screw-like (forward with upward and inward rotation) movement at the shear zone where these ridges are forming (fig. 3). Several flank ridges on other landslides are similar to the one shown in figure 3 (Fleming and Johnson, 1989); however, the processes of ridge formation might be more complicated than simple screw-like movement (Baum and others, 1993). Flank ridges and tilted fault blocks parallel to the flanks may be analogous to tectonic structures associated with strike-slip faults. Monitoring of displacement and deformation as well as detailed plane-table mapping in areas of distributed shearing, flank ridges, pull-apart basins, and tilted fault blocks are aimed at gaining a better understanding of these structures.

Conclusions

Our field studies have shown that the Slumgullion landslide has many features in common with other landslides. In addition, we have identified previously unrecognized mechanisms for changing width. The landslide becomes narrower by isolating wedges of material along its flanks and by transversely compressing material in a hopper-like basin. The landslide widens using a series of lateral steps in the flanks and by wedges that move obliquely from the main shear zone in the north flank.

References Cited

Baum, R.L., Fleming, R.W., and Johnson, A.M., 1993, Kinematics of the Aspen Grove landslide, Ephraim Canyon, central Utah: U.S. Geological Survey Bulletin 1842-F, p. F1-F34.

Fleming, R.W., and Johnson, A.M., 1989, Structures associated with strike-slip faults that bound landslide elements: Engineering Geology, v. 27, p. 39-114.

Smith, W.K., 1993, Photogrammetric determination of movement on the Slumgullion Slide, Hinsdale County, Colorado 1985-1990: U.S. Geological Survey Open-File Report 93-597, 17 p., 1 pl.

Varnes, D.J., Smith, W.K., Savage, W.Z., and Varnes, K.L., 1993, Control and deformation surveys at the Slumgullion Slide, Hinsdale County, Colorado--A progress report: U.S. Geological Survey Open-File Report 93-577, 15 p. 1 pl.