Structures associated with strike-slip faults that bound landslide elements

Abstract Large landslides are bounded on their flanks and on elements within the landslides by structures analogous to strike-slip faults. We observed the formation of thwse strike-slip faults and associated structures at two large landslides in central Utah during 1983–1985. The strike-slip faults in landslides are nearly vertical but locally may dip a few degrees toward or away from the moving ground. Fault surfaces are slickensided, and striations are subparallel to the ground surface. Displacement along strike-slip faults commonly produces scarps; scarps occur where local relief of the failure surface or ground surface is displaced and becomes adjacent to higher or lower ground, or where the landslide is thickening or thinning as a result of internal deformation. Several types of structures are formed at the ground surface as a strike-slip fault, which is fully developed at some depth below the ground surface, propagates upward in response to displacement. The simplest structure is a tension crack oriented at 45δ clockwise or counterclockwise from the trend of an underlying right- or left-lateral strike-slip fault, respectively. The tension cracks are typically arranged en echelon with the row of cracks parallel to the trace of the underlying strike-slip fault. Another common structure that forms above a developing strike-slip fault is a fault segment. Fault segments are discontinuous strike-slip faults that contain the same sense of slip but are turned clockwise or counterclockwise from a few to perhaps 20° from the underlying strike-slip fault. The fault segments are slickensided and striated a few centimeters below the ground surface; continued displacement of the landslide causes the fault segments to open and a short tension crack propagates out of one or both ends of the fault segments. These structures, open fault segments containing a short tension crack, are termed compound cracks; and the short tension crack that propagates from the tip of the fault segment is typically oriented 45° to the trend of the underlying fault. Fault segments are also typically arranged en echelon above the upward-propagating strike-slip fault. Continued displacement of the landslide causes the ground to buckle between the tension crack portions of the compound cracks. Still more displacement produces a thrust fault on one or both limbs of the buckle fold. These compressional structures form at right angles to the short tension cracks at the tips of the fault segments. Thus, the compressional structures are bounded on their ends by one face of a tension crack and detached from underlying material by thrusting or buckling. The tension cracks, fault segments, compound cracks, folds, and thrusts are ephemeral; they are created and destroyed with continuing displacement of the landslide. Ultimately, the structures are replaced by a throughgoing strike-slip fault. At one landslide, we observed the creation and destruction of the ephemeral structures as the landslide enlarged. Displacement of a few centimeters to about a decimeter was sufficient to produce scattered tension cracks and fault segments. Sets of compound cracks with associated folds and thrusts were produced by displacements of up to 1 m, and 1 to 2 m of displacement was required to produce a throughgoing strike-slip fault. The type of first-formed structure above an upward-propagating strike-slip fault is apparently controlled by the rheology of the material. Brittle material such as dry topsoil or the compact surface of a gravel road produces echelon tension cracks and sets of tension cracks and compressional structures, wherein the cracks and compressional structures are normal to each other and 45° to the strike-slip fault at depth. First-formed structures in more ductile material such as moist cohesive soil are fault segments. In very ductile material such as soft clay and very wet soil in swampy areas, the first-formed structure is a throughgoing strike-slip fault. There are other structures associated with strike-slip faults that are not ephemeral and tend to persist after many meters of displacement. These are extensional and compressional features, which are associated with steps and curves in strike-slip faults, and flank ridges, which form parallel to the strike-slip fault. Extensional structures form where a left- or right-lateral strike-slip fault steps or curves left or right, respectively. We monitored the formation of a pull-apart basin at a right step in a right-lateral strike-slip fault. The first crack formed between and connecting the strike-slip faults is diagonal and trends 45° to the traces of the faults. The upper few decimeters of the crack surface are rough, irregular, vertical and consistent with formation in tension. Below the tension crack, the surface dips downslope at about 60° and is slickensided and striated. The azimuth of the striations is parallel to the trend of the strike-slip faults but oblique to the diagonal fault surface. Thus, the upper part of the diagonal crack is a tension crack with an orientation consistent with published models for stresses that produce such cracks, but the lower part is an oblique-slip fault produced by displacement along the strike-slip faults upslope and downslope from the step. With continuing displacement, the diagonal crack is replaced by newly formed normal faults that are at right angles to strike-slip faults. The pull-apart basin becomes bounded on the uphill and downhill ends by normal faults, on one side by a strike-slip fault surface, and on the side within the moving ground by a monoclinal flexure. The monoclinal flexure develops a thrust fault at its hinge line where landslide debris moves laterally toward the basin. Compressional structures such as thrusts, folds and domes form where, for example, a left-lateral strike-slip fault steps right. These structures are similar to the small folds and thrusts formed between the ends of compound cracks, but they are much larger and tend to persist after many meters of landslide displacement. They apparently occur where segments of the upward-propagating strike-slip fault are grossly misaligned. The domes and folds occur within moving landslide debris. The thrust faults may result in displacement of landslide debris over adjacent, unfailed material. Flank ridges, which form parallel to a strike-slip fault, are up to several meters high, 1 to 3 decameters wide, and several decameters long. We recognized two fundamentally different types of flank ridges. One type, a depositinal flank ridge, forms on the outside of the strike-slip fault as a result of material spilling or being thrust over unfailed material. The other type, a deformational flank ridge, forms within the landslide debris, adjacent and parallel to the strike-slip fault. Some of the deformational ridges we examined contained intrusions and extrusions of highly plastic clay from deep within the landslide. At one flank-ridge site, the clay had been injected into the strike-slip fault as a tabular wedge and into adjoining landslide debris as anastomosing veins. Measurement of the growth of a ridge revealed that the crest of the ridge rises relative to adjacent material that is moving downhill as a result of displacement. Strain measurements revealed tension at the crest of the ridge and compression on the flank of the ridge but virtually no net strain accumulated across the entire cross section of a growing ridge. Therefore, the ridge cannot be the result of simple compression necessary for buckle folding. Field evidence for actively growing ridges includes tension cracks at the ridge crest, which are oriented oblique to the trend of the ridge, and small folds, which form around the downslope end and lower flank of a growing ridge. Indirect evidence for active ridge growth includes rupture of the ridge crest with exposure of fresh landslide debris and tilted trees.