AN EXPERIMENTAL STUDY OF THE OVER-PRINTING OF DEFORMATION FABRICS

We are conducting an experimental study on the 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 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. In particular, the evolution of fabrics is important for faults in which deeper rocks are exhumed and the conditions of deformation change.

The starting material is a quartz mylonite (grain size of 25 um) 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 our experiments the mylonite is subjected one of two deformation geometries: 1) a sense of shear that is the same as the first episode of deformation; and 2) a sense of shear such that the a-axes maximum is normal to the new shear direction. In addition to creating over-printed fabrics, the experiments in the first group are designed to study the contribution of grain boundary migration to the CPO; whereas the second group is designed to investigate the effect of existing CPOs on the strength of the mylonite.