Evolution of uphill-facing scarps by flexural toppling of slate with high-angle faults

Abstract

Uphill-facing scarps are typical surface expressions of deep-seated gravitational slope deformations and are commonly created by flexural toppling. However, their evolution mechanisms are not well-understood partly because of the difficult in observing their internal structures. This study therefore performed high-resolution geomorphic analyses and thorough geological mapping in a slate-rich area where flexural toppling has occurred, and uphill-facing scarps have developed. Following slope failures was crucial in exposing the inside of the toppled slate amid the toppled area. Uphill-facing scarps developed when slate contained a crush zone of a high-angle fault as a weak layer. Uphill-facing scarps developed with the rotation of a wedge, which was defined by the high-angle fault and cleavage that passed through the intersection of the fault and basal hinge surface of the toppling. The wedge developed up- or downslope of the fault, depending on whether the fault dipped into the slope or valleyward. During wedge rotation, displacement occurred along the fault, and an uphill-facing scarp developed along the fault. The uphill-facing scarp became higher when the cleavage dipped into the slope and the fault dipped valleyward. When the fault dipped into the slope, the uphill-facing scarp did not grow significantly. When a high-angle fault was not involved, flexural toppling of slate continuously deformed the slope and was unlikely to form an uphill-facing scarp. When flexural toppling extended to the ridge top, it generally formed a ridge-top depression, regardless of whether a fault was involved. Flexural toppling of rock mass created many openings, making the rock highly permeable. However, when a high-angle fault with a thick crush zone was involved, groundwater was stored between the fault crush zone and flexural toppling hinge surface, which could predispose the flexural toppling to catastrophic failure during a rainstorm and/or earthquake.