The effects of folding on the C-axis fabrics of a quartz mylonite

Abstract

The progressive c-axis fabric development in quartz-mylonites from the Cap de Creus (NE Spain) shear zones, is briefly summarized and the subsequent effects of fold development on these fabrics is analyzed in more detail. A stable fabric pattern consisting of a girdle lying in, or almost in, a plane perpendicular to both the mylonitic foliation and the lineation and with two maxima slightly inclined to the mylonitic foliation plane develops at relatively low shear strains. Folds in the mylonite belts have their axes disposed at variable angles to the stretching lineation. Generally the inclination of the axial planes to the plane of foliation decreases as the fold axes become parallel to the lineation. Similar folds develop as the axial planes become parallel to the foliation. The modification of the stable c-axis fabric pattern by folding can be achieved by means of two processes: bulk pseudo-passive rotation or intragranular deformation. The stable fabric pattern rotates about the Y direction of the incremental strain ellipsoid if only pseudo-passive rotation occurs. Where grain elongation is marked, a new girdle symmetrically arranged about the axial plane of the fold appears. Where both occur and if grain elongation is slight, a crossed-girdle pattern appears consisting of two elements; a weakened rotated old pattern and a stronger new one related to the axial plane. Their relative intensity depends on the degree of intragranular deformation indicated by grain elongation. Transmission electron microscopy studies were made of folded specimens and also unfolded mylonites which exhibited the stable c-axis microfabric. The dislocation sub-structures in most grains in all specimens were indicative of recovery. However in some grains high densities of dislocations which did not show any effects of recovery were present. These are thought to reflect slight straining during uplift whereas the former are more likely to be associated with the deformation which produced the microfabrics. Burgers vectors were determined by invisibility criteria, and slip planes mainly by the trace analysis of loops. The recovered dislocations indicated slip in an a direction along {101̄0}, (0001) and {101̄2} planes and in a c direction along {101̄0} and {112̄0}planes in both folded and unfolded specimens. The unrecovered dislocations indicated that the post-tectonic slip occured mainly in an a direction along the (0001) plane.