Thermally induced fracture of macroscale surficial granite sheets

Geologically diverse landforms around the world show indications of energetic macroscale fracture. These fractures are sometimes displayed dramatically as so-called “A-tents”, whereby relatively thin rock sheets push upwards and fracture, forming tent-like voids beneath the ruptured sheets. The origin and formation of such features has been a topic of considerable interest and analysis for over a century. Here we show that thermally-induced stresses, coincident with particularly hot days during particularly hot years, were responsible for recent (2014-2016) energetic ruptures of rock sheets forming a granitic exfoliation dome in California, USA. Through a three-year field effort, we found that subcritical fracture occurred due to diurnal and seasonal cumulative thermal stresses. However, our analyses also indicate that subsequent critical fracture could only have been reached if thermal stresses acted in concert with existing tectonic stresses. Thus, we offer a superposition triggering mechanism (background tectonic stresses with cyclic thermal stresses) to explain these rock fracture features.