WHAT HAPPENS TO MICROSTRUCTURES IN RE-ACTIVATED SHEAR ZONES: AN EXPERIMENTAL APPROACH

We are conducting an experimental study on over-printing of deformation fabric during subsequent deformation of quartz-rich, continental crustal rocks. Deformation fabric in a rock includes microstructures (e.g., foliation, grain size, grain shape) and crystallographic preferred orientations (CPOs). Over-printing of fabric occurs when a region is subjected to more than one deformation event, which complicates the interpretation of the deformation history. In particular, the evolution of fabrics is important for faults in which deeper rocks are exhumed and the conditions of deformation change. In this experimental study, deformed rocks with documented microstructures and CPO patterns are experimentally deformed under well-constrained conditions and strain paths to investigate the effects of pre-existing fabric on subsequent deformation, and to determine the resulting over-printed fabric.

The starting material is a quartz mylonite (grainsize of 25 µm) from the Moine thrust with a foliation parallel to the Moine thrust and an oblique foliation at ~45˚. The CPO is as follows: the c-axes form a single girdle inclined in the direction of shear with two maxima at ~30˚ to the Y-axis of the strain ellipse frame of reference. The a-axes form a maximum in the X-Z plane at about 40˚ from the X-axis. In our experiments the mylonite is subjected to a sense of shear that is the reverse of the first episode of deformation. The experiments are carried out in a solid-media deformation apparatus (a Griggs Rig) at pressures, temperatures, and strainrates that enable dislocation creep to be the dominant deformation mechanism. These experiments simulate reactivation of a shear zone with shear parallel to the original shear zone boundaries, but in the opposite sense. These results should help in the interpretation of reactivated shear zones, and may help quantify the amount of shear strain required to reset deformation fabrics in complexly deformed regions.