Emplacement of Silicic Lavas: Pervasive Brittle-Tensile Deformation Above a Ductile Core

Abstract:

Silicic domes and lavas are widespread in the geologic record but, extensive extrusions are infrequent compared to mafic lavas. Emplacement styles and the thermo-rheological architecture of silicic lavas are understood from large-scale morphology, small-scale structures and fabrics, and analog models. Large-scale studies infer coeval compressional (e.g., folds and thrusts) and extensional (e.g., crease structures) ductile-brittle deformation domains on the upper surfaces of lavas during emplacement. Small-scale structures record some combination of ductile shearing within the conduit and lateral-spreading on the surface. The disagreement between different observation scales results in incomplete constraints on the internal thermal structure, the architecture of outgassing pathways, mechanical strength of flow fronts, and the duration of emplacement events. We have investigated the nature of meso-scale (e.g., mapped outcrops) deformation on the upper surface of Obsidian Dome, CA, to better understand the relative timing and styles of deformation, and the rheological conditions. We infer that brittle-tensile deformation was overwhelming at the upper surface in a carapace of weakly pumiceous obsidian, and that significant ductile deformation was restricted to low-porosity obsidian beneath the carapace. We find no evidence of compressional deformation related to horizontal shortening (e.g., surface folds). Obsidian Dome and other silicic lavas advance by gravity-spreading of the ductile core beneath a stretching carapace of brittlely deformed pumice. The brittle nature of the carapace enhances volatile out-gassing and cooling of the lava, but does not retard advance.