1 Introduction

The well-documented 2014–2016 exfoliation of Twain Harte Dome, located in the western foothills of California's Sierra Nevada Mesozoic granitic batholith, provides an unprecedented opportunity to study progressive fracture of exfoliation domes. The dome seemingly spontaneously fractured on five occasions during the late summer of 2014, with subsequent fracturing in the summers of 2015 and 2016. The warm-weather timing of all events strongly suggests that thermally-induced environmental conditions are responsible for triggering exfoliation. Some of the exfoliation events consisted of rock slabs being thrust ~40 cm into the air in a matter of seconds, with both audible and observable explosive-like energy release. We initiated a research project to investigate the triggering conditions and fracture mechanics of dome exfoliation under this active setting to serve as a proxy for understanding precursor signals that might occur prior to rock falls on near-vertical cliffs. Here we present the portion of our data that captures the collapse of the slab that was deformed during the 2014 events. The data suggest that the collapse and other ongoing deformation is occurring as a result of progressive fracture driven by diurnal heating and cooling.

2 Exfoliation sheet monitoring

Following the August 2014 fracture events, we installed three vibrating-wire-type strain gauge "crackmeters" (Geokon modified 4420; see Collins & Stock 2016) between a newly detached portion of the uppermost slab of the dome and the intact rock beneath (Fig. 1). Two crackmeters monitored open fractures and a third crackmeter acted as a control to differentiate between thermal strains in the instrumentation verses thermal strains in the granitic slab. We also deployed pendent-type temperature and light sensors (Onset Hobo UA-002–08) to act as a proxy for surface rock temperatures. Our instrumentation captured data at 5-minute intervals beginning on 22 August 2014 for 41 days until the upper slab detached and crushed the crackmeters.

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