THE ROLE OF FABRIC AND SHEAR INVERSION on THE DEVELOPMENT OF FRACTURES IN BRITTLE SHEAR ZONES

Experiments on clay were conducted in an effort to understand the development of an unusual set of small-scale fractures affecting Archean metasedimentary rocks in northern Minnesota. The natural fractures occur in sets associated with thoroughgoing faults of overall right-lateral sense of displacement. The orientations of the fractures and displacements of opposite sense found locally on adjacent fault segments are not compatible with a simple history of fracture development in brittle shear zones. In an attempt to reproduce the fractures observed in outcrop, we performed experiments in clay subjected to localized strike-slip shear above a rigid ‘basement’ fault. In these experiments, we varied the rheological properties of the clay and the method of clay preparation in order to best match the natural fracture patterns. The details of the fracture development and propagation were analyzed using Particle Image Velocimetry (PIV), which allows us to measure the displacements of different particles throughout the deformation and with this calculate strain, rotation, and area change. By constructing detailed fracture maps we were able to quantify similarities and differences between the fractures produced in experiments and the fractures examined in outcrop.

We conclude that two factors are key to understanding the natural fracture pattern: 1) fabric - imparted during sample preparation in the clay and developed by a combination of sedimentary processes and an early ductile deformation in the host rock; and 2) and shear inversion, which explains the near symmetrical arrangement of the fracture sets about the faults and opposing sense of slip on adjacent fault segments.