INFLUENCE OF ROCK SHAPE ON CRACK CHARACTERISTICS IN ALLUVIAL FANS OF ARID REGIONS

Weathering related to diurnal insolation is a crucial part of alluvial fan desert pavement development in arid regions (McFadden et al., 2005, Eppes et al., 2010). These studies demonstrate that rock fracture patterns, particularly orientation, reflect and the daily heating and cooling phases of the rock, where most fractures in desert pavement rocks have preferred orientations that are roughly north‐south. It is not known, however, to what extent rock shape has in influencing the effects of insolation on rock fracturing. This study was designed to investigate how orientation and size of flat surfaces found in desert pavement clasts affect crack characteristics in said rocks. Every crack measuring 2 cm in length or greater was examined on 131 rocks (volcanic, plutonic, and limestone) on a 12ka alluvial fan surface in the Mojave Desert of southern California. For each rock, the orientation, roughness and surface area was determined for all flat faces observed on the rock. Other data gathered included clast size, rock type; and crack size, orientation, and distance from any parallel (+/- 15 ^o) clast face. Preliminary results show that the mean vector orientation of all cracks parallel to flat surfaces on the rocks measured are non-random with an orientation of ~ 65^o(right hand rule). Clast flat faces themselves also exhibit preferred orientations with vector means of 146^o & 68^o when only boulders are examined. The area of the flat surface appears to have little to no effect on the depth or amount of cracks present, but the orientation of the face does, where more cracks are present on SE facing surfaces. This data suggest that ideal conditions for cracking may occur during the morning heating phase of the day, when a rock face can achieve the highest temperature gradient and surfaces facing east-southeast can absorb the most heat from the surface inward. We observe, however, that many NE oriented cracks are not associated with flat faces suggesting that a feedback may develop where NE flat faces are formed through insolation cracking and subsequent separation of these cracks. This data may be able to provide insight into the weathering patterns of building materials and could help enlighten material choice and structural design.