SPATIAL AND TEMPORAL VARIABILITY IN ROCK DOME EXFOLIATION AND WEATHERING

Large-scale exfoliation cracks (aka sheet joints; exfoliation joints) and associated domes can strongly influence regional landscape evolution, hydrology and hazards, but their formation, mechanism(s), and long-term evolution are poorly understood. Beginning in August 2014, in Twain Harte, California, several rare, highly-rapid, exfoliation cracking events were observed and filmed (e.g., https://youtu.be/yAZ1V_DJKV8), providing a unique opportunity to study the short (10¹ yr.)- and long (10⁵ yr.)-term evolution of a granitic dome characterized by large-scale exfoliation events whose timing and characteristics are exactly known. To do so we have: 1) conducted detailed mapping and characterization of exfoliation slabs at Twain Harte and 15 other nearby domes; 2) monitored cracking for 7 months following the 2014 event using six acoustic emission (AE) sensors; 3) monitored near-surface temperatures and light intensities, 4) deployed crack meters to measure post-event deformation, and 5) monitored existing subsurface borehole extensometer instrumentation installed under the 2014 slab. Preliminary analysis of instrumentation data demonstrates temperature dependent, diurnal and seasonal expansion and contraction of exfoliation joints that is sometimes, but not always, accompanied by peaks in AE. Mapping revealed 2-4 generations of exfoliation joints at all sites, manifested as stacked slabs with characteristic thicknesses of ~ 20 – 30 cm. Slabs exhibit statistically different (via Student’s T-test) weathering characteristics including compressive strength (as measured by Schmidt hammer), crack length, and spalling height, with older slabs generally exhibiting greater degrees of weathering (lower compressive strength, increased spalling height). Observed chronofunctions of weathering features provide evidence of a recurrence interval of slab formation that may be steady through time. Ongoing macroscale cracking appears limited to summer months, suggesting a thermal trigger for observed events and provoking a hypothesis that slab thickness is dictated by thermal penetration depth. Together, these data provide evidence of both spatial and temporal continuity in exfoliation processes, and can be used to test hypotheses of exfoliation slab and dome formation mechanisms.