Abstract

Quaternary fault scarps occur in mountain blocks throughout the Saint Elias orogen of southern Alaska. Mechanisms proposed for formation of these scarps include deformation caused by active folding, downhill creep and landsliding on steep, previously glaciated slopes, and superficial faulting caused by strong ground motion during earthquakes. Field observations and interpretation of high-resolution topographic models constructed from a light detection and ranging (LIDAR) survey indicate that failure by flexural toppling creates the uphill-facing scarps in the mountain that we selected for detailed analysis. Toppling failure occurs by shearing and outward rotation of sedimentary bedding that dips steeply into the mountainside. The scarps are therefore created primarily by gravitational stresses, and are not tectonic faults. A three-dimensional finite-element model of the mountain is used to investigate variations in stress field and kinematics of bedding-plane failure by flexural toppling. The results demonstrate the strong influence of mountain morphology on stress orientations and kinematics of shearing along bedding planes. Addition of horizontal compressive tectonic stress may reorient principal stresses caused by gravitational loading, and either enhance or restrict flexural toppling depending upon the angle between bedding surfaces and mountainside. However, modulation of gravity loading by tectonic stress requires the basal sliding surfaces beneath the toppled strata to be either locked or only partly developed. Horizontal acceleration caused by earthquake ground motion and lateral relaxation of mountain flanks following retreat of glaciers also enhance the probability of failure by flexural toppling, especially in the upper parts of mountain slopes, where earthquake ground motion is amplified. The finite-element model does not incorporate these latter two processes, although we discuss their influence in qualitative terms.

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