SPATIAL AND TEMPORAL EVOLUTION OF ROCK DOME EXFOLIATION IN DIFFERENT CLIMATIC, GEOMORPHIC AND GEOLOGIC SETTINGS

Surface parallel sheeting joints – common on rock domes – are found worldwide and can influence numerous geologic processes, such as mass-wasting and regolith/soil production. Their formation is traditionally attributed to overburden erosion, however no singular existing hypothesis sufficiently explains the complicated differential scales of observed sheeting joint formation (Martel, 2016, 2006; Collins & Stock, 2016). The purpose of this study was to assemble one of the first systematically collected datasets for the morphology and weathering characteristics of sheeting joints on granite domes in different geologic settings. At 4 sites near the Sierra Nevada range in CA and the Blue Ridge Mountains in NC & SC, detailed field mapping revealed ~ 3 ‘generations’ of exfoliation sheeting joints at all sites manifested as stratigraphically stacked slabs. Cracking morphology and Schmidt hammer data – proxies for exposure age – were collected, along with thickness, for each slab. Along transects on each slab, for every linear void >2 cm long, we measured geometry, length, orientation and degree of weathering. Representative samples were collected to measure thermal diffusivity and assess the relationship between slab thickness and thermal penetration depth. They were also used to create thin sections for analysis of mineralogy and microporosity. Generally, all slabs at all sites exhibited similar thicknesses but distinctly different weathering characteristics, with older slabs exhibiting greater degrees of weathering. For microscale (< 10 cm thickness) exfoliation, CA sites exhibited statistically significant positive correlations between crack length and thickness and slab generation, whereas the sites in the Carolinas did not. At minimum, these data suggest that a spatially and temporally consistent mechanism is influencing the depth of shallow (<2 m) major (~ 20 - 30 cm) exfoliation, while the mechanism producing microscale exfoliation may differ regionally. Chronofunctions of observed weathering features provide evidence of a possibly consistent sheet jointing recurrence interval through time and space, despite different geomorphic settings and exhumation histories. This comprehensive data set will be used to test existing hypotheses of sheeting joint and exfoliation slab formation mechanisms.