Spatial and Temporal Variance in Rock Dome Exfoliation and Weathering Near Twain Harte, California, USA

摘要

Large-scale exfoliation cracks 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, providing a unique opportunity to study the short (101 yr.)- and long (105 yr.)-term evolution of a rock dome. To do so, detailed mapping and morphologic and weathering characterization of exfoliation slabs was conducted at Twain Harte and 15 other nearby sites. In addition, previously collected data was analyzed at the Twain Harte site including the monitoring of cracking at the Twain Harte dome for 7 months using acoustic emission (AE) sensors, near-surface temperatures and light intensities at the same locations, and crack meters at the Twain Harte dome to measure post-event deformation. 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 weathering characteristics including compressive strength, crack length, and spalling height, with older slabs generally exhibiting greater degrees of weathering. Observed chronofunctions of weathering features provide evidence of a recurrence interval of slab formation that may be similar through time. Ongoing macroscale cracking appears limited to summer months suggesting a thermal trigger for observed events. Together, these data provide evidence of both spatial and temporal continuity in exfoliation processes, and could be used to test hypotheses of exfoliation slab and dome formation mechanisms.