Two million years of cracks: Co-evolution of subcritical fracturing and rock thermal properties at Mullins Glacier, Antarctica

Abstract:

Recent studies have revealed the potential importance of climate dependent subcritical cracking as a mechanism for mechanical weathering of rocks at the surface. Diurnal solar heating imposes directional thermal stresses on rocks that lead to the 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, we have collected a suite of cobbles from Mullins Glacier, McMurdo Dry Valleys, Antarctica. The Dry Valleys are some of the oldest exposed surfaces on the planet, and our preliminary ¹⁰Be and ²⁶Al exposure ages on some of the cobbles exceed two million years. Our samples form a chronosequence from relatively young exposure ages at the headwall of the glacier on. Because of the extremely slow weathering rates aided by the consistently cold, dry climate, these rocks may preserve rare information about the mechanical weathering evolution of individual cobbles. We have 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. Since we hypothesize that thermal stresses are the main driver of these fractures, we also have examined the thermal properties of these rocks, with special focus on thermal conductivity. Thermal conductivity has an important role in climate-dependent subcritical cracking, as the ability of a rock to conduct heat helps to determine how steep a thermal gradient will be in response to a heat input, in turn governing the magnitude of thermal stress in the rock, and possibly its potential to retain liquid-water in fractures during warmer temperatures. Thermal conductivity is expected to change as fractures progress through the rock and new void space is introduced. In this work, we begin to disentangle these complex feedbacks by examining the co-evolution of fractures and rock thermal properties over geologic timescales in cold-dry end-member climate conditions.