Crack propagation by differential insolation on desert surface clasts

In the southwest U.S., cracks in alluvial fan surface clasts have a preferred orientation independent of rock fabric and shape. In this paper, we show that differential insolation of incipient cracks of random orientations predicts a distribution of crack orientations consistent with field observations. In this model, crack growth by hydration and/ or thermal weathering is primarily a function of local water content at the crack tip. Crack tips that experience minimal solar insolation maintain a greater average moisture and, hence, weather more rapidly than cracks that experience greater solar insolation. To show this, we used a numerical radiative transfer code to quantify the solar insolation of rectangular cracks at 35° N. latitude with a range of depths and orientations. The amount of solar energy reaching the bottom of each crack was calculated at 5-min intervals over the day for several days of the year to determine hourly, daily, seasonal, and annual energy deposition as a function of crack depth and orientation. By assuming that only crack orientations that effectively shield their interiors and minimize their water loss are able to grow, the pattern of cracks produced by the model is consistent with field observations. The annual average insolation, which controls water retention, is associated with the two primary modes of crack orientation. The effect of daily recharge by summer rains of the North American monsoon system is consistent with the observed deviations from these primary modes. Model results suggest that both the annual average insolation and the daily pattern of rainfall is recorded in the preferred crack orientations of surface clasts in the southwest U. The breakdown mechanism of surface clasts in arid environments has been a topic of debate for almost a century. A central question in this debate is whether thermal stresses or hydration weathering is the dominant processes of clast breakdown (Mabbutt, 1977). Recently, McFadden et al. (2005) documented a preferred N–S orientation in the cracks of surface clasts in deserts of the southwest U.S. with the effects of rock fabric and shape removed (Fig. 1). Because surface clasts are deposited randomly, this preferred orientation develops after clast deposi-tion. McFadden et al. (2005) argued that this pattern provides direct evidence for the predominant role of thermally induced mechanical stresses resulting from differential heating of clasts as the sun moves across the sky during the course of a day. Other breakdown mechanisms can also be influenced, however, …