A CHRONOSEQUENCE OF SUBCRITICAL FRACTURE PROPAGATION FROM THE MCMURDO DRY VALLEYS, ANTARCTICA

The importance of climate dependent subcritical cracking as a mechanism for mechanical weathering of rocks at the surface is increasingly recognized. Diurnal solar heating imposes directional thermal stresses on rocks that lead to slow propagation of tensile fractures, evidenced by fracture orientations at sites in a variety of climate zones around the world and on Mars. However, many fundamental questions about fracture initiation, propagation rates, and controlling parameters remain unanswered, and most studies have focused on centimeter-to-meter scale fractures. To further explore subcritical fracture processes at a micro scale, we analyzed thin sections taken from a chronosequence of 10Be and 26Al dated cobbles from Mullins Glacier, McMurdo Dry Valleys, Antarctica. The Dry Valleys are some of the oldest exposed surfaces on the planet, and because of the extremely slow weathering rates aided by cold-dry end member climate conditions, these rocks may preserve rare information about the mechanical weathering evolution of individual cobbles. We characterized the densities, lengths, and orientations of microfractures in thin sections of sandstones from this sequence, to explore the evolution of these fractures as a function of time exposed at the surface, identifying probable relationships between fracture and valley orientations, evidencing topographic control on diel thermal stresses. We also find intriguing trends in microscopic fracture development through time, where the total number of fracture traces per unit area increases linearly through time, while the sum of total fracture length has a nonlinear trend, increasing rapidly over the first few 100 kyr, and then slowing, suggesting that while optimally conditioned fractures might propagate rapidly, new fractures form at a relatively constant rate.