REVISITING CLASSIC MESOSCALE FIELD STRUCTURES IN THE CONTEXT OF EXPERIMENTAL RHEOLOGY

Sharon Mosher has long been fascinated with mesoscale structures of strained rocks and how they connect to larger scale tectonic processes. Structures such as veins, foliations, cleavage, and similar, are now well-understood in terms of strain history, deformation mechanism, and tectonic environment. However, the mesoscale crustal rheology governing these structures’ evolution remains less explored. Recent experiments, begun at the University of Texas and now being conducted at the Oklahoma Geological Survey, use polymers to explore the rheology of flow. These experiments are not precisely scaled to earth processes, but approximately describe the weak regimes in a crustal strength profile. The experiments also capture how specific instabilities nucleate and lead to further structural evolution. For example, in an experiment that injects a low-viscosity fluid into a viscoelastoplastic gel, branches that form off the main injection result from instabilities governed by interfacial stresses. The mechanical explanations common in structural geology for such branching vein structures are that some combination of fluid-pressure fluctuations, bulk-shear, and/or crack-tip processes drive incremental propagation. In contrast, the experiments show that in some cases the interfacial stresses govern the nucleation, and a competition between viscous and elastic stresses govern the evolution. Monitoring the deformation surrounding the experimental injection also suggests that a fair amount of strain is accommodated around the injection, both during its propagation as well as through elastic recovery post-injection. In natural rocks, whether migmatitic rocks from the deep crust or carbonate veins in shallow orogenic flysch, one can find compelling examples that such instabilities and non-plastic stresses are also important in nature. These structures, in addition to tracking geologic strains across orogens, may therefore also form during runaway phenomena such as unusually large fluid (inc. melt) flux events, or geophysically detectable strain transients, illustrating that the structural geology that fascinated Sharon throughout her career remains relevant to the new era of geophysics and geodynamics that she facilitated at UT Austin.