QUANTIFYING THE LAST STEPS IN THE CHRONOLOGY OF ROCK DEFORMATION

Fracturing is the most dominant deformation mechanism in Earth’s upper crust. Rock continues to fracture progressively in response to ongoing environmental, tectonic, and gravitational stresses when it is exhumed and exposed at or near Earth’s surface. Little data exists, however, quantifying the rates or nature of brittle deformation that comprise this final step in the transition of bedrock to clastic sediment. Here we employ a ‘chronosequence’ approach to better understand and characterize long-term (~0 to 10⁵ years) changes in rock fracturing as rock is exposed to environmental factors. We collected ten ~25 cm diameter granitic boulders with exposure ages ranging from ~0 - ~150 ka from two sites in the Eastern Sierra, California, USA where the boulders were deposited on the surface of alluvial terraces and fans. These geomorphic surfaces provide a natural laboratory in which rocks of consistent lithology have been exposed to similar environmental conditions for different lengths of time. Focusing only on similarly sized boulders removes any ambiguities in tectonic and exhumation history that might arise in outcrop samples, thus ensuring that rocks from each site have experienced similar stress conditions; namely those restricted to the environment.

We measured key rock mechanical properties (tensile strength, uniaxial compressive strength (UCS), and Young’s modulus (E)) as well as commonly employed proxies for crack damage (porosity, microcracks, compressional wave velocity (Vp), and shear wave velocity (Vs)). We find that all measured parameters evolve as a function of exposure age, with systematic increases in porosity and microcrack length and density and systematic decreases in Vp, Vs, tensile strength, UCS, and E. We interpret these changes as reflecting progressive subcritical crack growth that arises due to ubiquitous, but relatively low magnitude, environmental stresses continuously acting on the boulders, as opposed to differences inherited before their erosion from bedrock. These observations have significant implications for understanding overall processes of fracture in Earth’s crust as well as for the interpretation of any measurements made on rocks exposed at Earth’s surface, even if the age of exposure is relatively short compared to the age of the geologic deposit itself.