RECONCILING GEOLOGIC OBSERVATIONS WITH MODELS AND EXPERIMENTS FOR SLIP AND CREEP

Natural shear zones exhibit enormous heterogeneity and a wide range of metamorphic conditions and deformational time scales. Upper crustal faults contain both highly localized slip surfaces, and wide cataclasite- and gouge-filled zones. Middle-crustal shear zones commonly have mixtures of strong and weak materials that were stretched via crystal-plastic deformation mechanisms. Both upper crustal and middle crustal shear zones are in many places cut by syn-tectonic veins, testament to fluid-present fracturing. Despite this diversity, both upper and middle crustal shear zones are capable of producing: coseismic slip, aseismic creep, long-term strain transients, and slow-slip events. All these behaviors have different time scales and exhibit differing degrees of periodicity. Common experimental and model frameworks for exploring these slip and creep behaviors focus on dynamic frictional responses and effective stress changes. Yet, neither of these frameworks fully describes either the diversity of slip and creep phenomena, or the geological environments that produce them. An alternative approach is to view natural shear zones as granular media. Physical experiments using analog materials provide some insight into such an approach. Shear zones of dry, granular materials produce stick-slip events via jamming phenomena, and the nature of the granular materials and their packing influences the periodicity and duration of events. When viscous materials are added to such experiments, they enhance localization and smooth stick-slip events. Analytical and numerical solutions for fracture propagation into semi-brittle media can also produce strain transients with a wide range of durations and recurrence intervals. A particularly powerful example is in the description of creep behavior of shear zones that deformed via predominantly viscous, crystal plastic mechanisms. Fracture propagation into mixtures of strong and weak materials could explain the generation of strain transients, including deep episodic tremor and slip. Extending this analysis into the upper crust, however, will require a more complete framework for describing the strength of granular materials in the crust.