OVER-PRINTING SHEAR ZONE MICROSTRUCTURES BY EXPERIMENTAL DEFORMATION

Over-printing of deformation fabric occurs when a region is subjected to more than one deformation event, complicating the interpretation of the deformation history. Deformation fabric in a rock includes microstructures (e.g., foliation, grain size, grain shape) and crystallographic preferred orientations (CPOs). In particular, the evolution of fabrics is important for faults in which deeper rocks are exhumed and the conditions of deformation change. In such a case, temperatures would decrease and/or the direction of shear may change. In this experimental study, a quartz-rich mylonite with documented microstructures and CPO is 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 (grain size 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. 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. In these experiments the mylonite is subjected one of three deformation geometries: 1) a sense of shear in the opposite direction as the first episode; 2) a sense of shear such that the original a-axes maximum is normal to the new shear direction; and 3) a sense of shear that is the same as the first episode of deformation. The experiments in the first group are designed to simulate reactivation of a shear zone with shear parallel to the original shear zone boundaries, but in the opposite sense. The second group is designed to simulate reactivation of a shear zone in yet a different direction. 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. The third group is designed to study the contribution of grain boundary migration to the CPO.