Progressive fracturing in alluvial clasts

Abstract

Rock fracturing sets the pace for a range of geomorphic processes. While experimental studies and modeling have provided invaluable insights into the mechanisms and rates of rock fracturing as a function of stress, time, and environmental conditions, field-based observations of subaerial fracturing evolution over geologic time are scarce. To address this knowledge gap, we conducted a systematic study of fractures that developed subaerially and in situ within clasts perched on abandoned late Quaternary alluvial surfaces (ca. 0, ca. 14, and ca. 62 ka in age) in the hyperarid Dead Sea Rift Valley, Israel. Using quantitative field observations, petrographic, and scanning electron microscopy, and micron-scale laser scans of fracture surfaces we found that fractures exhibit a consistent pattern of three distinctive weathering zones: (1) an “Outer Zone,” where fracture surface morphology resembles the clast exterior; (2) an “Accumulation Zone,” where fractures are infilled by “loose” accumulated particles; and (3) an “Inner Zone” where fractures extend inward to the crack-tip and preferentially follow grain boundaries. Crack-tips are characterized as a distinct micro domain that consists of fracture-parallel microcracks, chemical alteration, and dissolution morphologies. Altogether, the laboratory results indicate chemically enhanced fracturing and infiltration of water ahead of traction-free, open crack-tips. Field measurements also revealed an increase in fracture number density over geologic time. Our results highlight new details regarding the progressive nature of mechanical weathering through geologic time and the role of moisture as a potential rate-setting factor in the fracturing that allows mechanical weathering.