The role of veining and dissolution in the evolution of fine-grained mylonites: the McConnell thrust, Alberta

Microstructural observations of the McConnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the McConnell thrust limestone mylonites. The protolith to the mylonite is a micritic limestone. The mylonite contains numerous, discontinuous, bedding-parallel calcite veins that range from relatively undeformed (except for twinning and undulose extinction) to highly sheared. The highly sheared veins show evidence for dynamic recrystallization and grain-size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately < 1–3 μm. The bulk of the calcite matrix within the McConnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins. The emplacement of relatively coarse-grained veins promoted the activation of dislocation creep within the fault rocks of the thrust. In the absence of the increase in grain size afforded by vein emplacement, it is anticipated that the fine-grained protolith would have deformed predominantly by solution transfer (with grain-boundary sliding) and localized, brittle shearing. The layer-parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite. These clay seams may have contributed to the build up of high pore fluid pressures which, if intermittently reaching lithostatic conditions during displacement, would result in transient brittle failure and thrust-surface-parallel vein emplacement in an otherwise ductilely deforming rock.