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Characterization and evolution of fractures in low-volume pahoehoe lava flows, eastern Snake River Plain, Idaho

Conrad J. Schaefer and Simon A. Kattenhorn^,2

² Department of Geological Sciences, University of Idaho, P.O. Box 443022, Moscow, Idaho 83844-3022, USA

We characterize fracture evolution in pahoehoe lava flows of the eastern Snake River Plain, Idaho, and highlight significant differences to flood-basalt sheet flows and implications for hydrologic models. There are four distinct fracture types in east ern Snake River Plain flows: (1) column- bounding; (2) column-normal; (3) entablature; and (4) inflation fractures. Types 1–3 are driven by thermal stress, whereas type 4 is induced by lava pressure from within the flow. Thermal stress distribution in a flow is dictated by its aspect ratio (width/ height), which controls the shape of isotherms. Isotherms control column-bounding fracture orientations, resulting in increasingly radial fracture patterns as the aspect ratio approaches unity. Column-normal fractures form in response to thermal stress and fracture-induced stress within basalt columns. Overlap in the timing of column-bounding and column-normal fracture growth has resulted in complex fracture relationships. Column-normal fracture growth is strongly influenced by vesicular layers, which act as mechanical heterogeneities, creating preferential pathways for fracture growth as well as causing jogs or terminations along column-bounding fractures. Eastern Snake River Plain entablatures, which preserve the shape of the central lava core during the final stages of cooling, have distinctly different origins and fracture styles compared to sheet flows. Entablatures formed by penetration of the edges of pressurized lava cores by inflation fractures, causing rapid convective cooling. In addition, inflation fractures significantly perturb isotherm shapes in lava flows, affecting flow-scale fracture patterns and densities. The overall effect of all these processes is a complex pattern of fracturing that attests to a strong impact by each fracture type on the growth behavior of all other fracture types.

Key Words: basalt • fracture • lava flow • pahoehoe • inflation • entablature