Origin of sackung uphill-facing scarps in the Saint Elias orogen, Alaska: LIDAR data visualization and stress modeling

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 superfi cial 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 fl exural 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 fi nite-element model of the mountain is used to investigate variations in stress fi eld and kine matics of bedding-plane failure by fl exural toppling. The results demonstrate the strong infl uence 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 fl exural 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 fl anks following retreat of glaciers also en-hance the probability of failure by fl exural toppling, especially in the upper parts of mountain slopes, where earthquake ground motion is amplifi ed. The fi nite-element model does not incorporate these latter two processes, although we discuss their infl uence in qualitative terms.

Gravitational reactivation of a pre-existing post-Caledonian fault system: the deep-seated gravitational slope deformation at Middagstinden, western Norway

“…Moreover, at the transition where the dip of the sliding surface becomes steeper, higher tensional stresses increase brittle failure and rock fragmentation along pre-existing structures, similar to the proposal by Li et al (2010) for bending moment faults caused by folding at the hinge of an anticline. Preexisting structures, such as here JS and J1, will favour those processes.…”

“…Spreading of fractures due to tensional stresses and brittle failure during block sliding and subsequent erosion of the uphill side can cause counterscarps even in the case of a shallow-dipping sliding plane (Gutiérrez-Santolalla et al, 2005;Li et al, 2010). Moreover, at the transition where the dip of the sliding surface becomes steeper, higher tensional stresses increase brittle failure and rock fragmentation along pre-existing structures, similar to the proposal by Li et al (2010) for bending moment faults caused by folding at the hinge of an anticline.…”

“…This causes deep-seated, internal deformation of Gravitational flexural toppling has been suggested by several authors to account for the origin of counterscarps (Hippolyte et al, 2006;Reitner & Linner, 2009;Li et al, 2010). Li et al (2010) also suggested that this flexural toppling could contribute to the development of a basal sliding surface or sliding zone in complex largescale slope deformation. However, those examples of large-scale flexural toppling are limited to sedimentary rocks or well-foliated metamorphic rocks with penetrative, subvertical bedding or foliation structures.…”

“…Especially several kilometre-long linear features that expose either an uphill-or a downhill-facing vertical step along the slope, such as a counterscarp, have been interpreted to be related either to tectonic faulting or to gravitational slope deformation. This has sometimes led to a reinterpretation of these features (Thompson et al, 1997;Hippolyte et al, 2006;Li et al, 2010) because some of them can show characteristic features of both origins (Thompson et al, 1997). However, various mechanisms, including gravitational deformation, folding and faulting are described by Li et al (2010) who have also highlighted the influence of mountain topography on kinematics of deformation.…”

“…This has sometimes led to a reinterpretation of these features (Thompson et al, 1997;Hippolyte et al, 2006;Li et al, 2010) because some of them can show characteristic features of both origins (Thompson et al, 1997). However, various mechanisms, including gravitational deformation, folding and faulting are described by Li et al (2010) who have also highlighted the influence of mountain topography on kinematics of deformation. Therein, flexural toppling along subvertical bedding planes is suggested to be the primary origin for a swarm of counterscarps, similar to Hippolyte et al (2006) who additionally considered the gravitational reactivation of inherited fault structures.…”