Honoring the Reality of Blocky Hillslopes: Case Study of a Vertical Dike at Shiprock, New Mexico

Abstract:

Rocky hillslopes that exhibit large blocks of rock and thin, non-uniform cover of mobile regolith are common in both steep landscapes and arid environments, as well as on other planets. While the evolution of homogeneous, soil-mantled, convex hillslopes can be well modeled, the influence of lithology and geologic structure on hillslope form and evolution has yet to be properly addressed. As a first case, we study hillslopes developed during exhumation of a near-vertical several-meter wide mid-Tertiary basalt dike intruded into Cretaceous shale at Shiprock, New Mexico. The dike stands several to several tens of meters above the tops of adjacent hundred-meter long slopes that grade onto flats. Profiles of the hillslopes reveal steep 30° slopes adjacent to the dike, with a narrow (~5-10 m) convexity, beyond which the slopes become broadly concave. Field observations suggest that the dike deteriorates in two distinct ways: hard cm-scale flakes from both the dike and the adjacent rapidly heated shale peel off the wall, while meter-scale basalt blocks tumble onto the hillslope in rare events. Both of these products differ dramatically from the weak, fine-grained shale chips derived from weathering of the subjacent shale. The size and percent cover of both flakes and blocks decreases downslope away from the dike. We hypothesize that coverage of the surface by basalt clasts modulates both the velocity of the mobile regolith cover and the rate of weathering of the underlying shale. We employ a numerical model to explore the conditions required to capture the essence of the observed slopes. A continuum model tracks regolith production, regolith flux, and areal concentration of small surface clasts. Treated as discrete elements, large blocks are allowed to change size to mimic weathering decay, and their presence alters the flow of regolith. This acknowledgement of the roles of lithology and structure on hillslope evolution takes us one step toward models of other features such as hogbacks, cuestas, flatirons and escarpments.