| Abstract: | In the Ormiston Nappe Complex, west of Alice Springs, central Australia, a deformed zone up to 0.7 km thick is developed in the sedimentary Heavitree Quartzite. The deformed zone is adjacent to a major thrust fault and is defined by mylonitic foliation, which is parallel to the thrust plane and by isoclinal folds. Recognition of original detrital quartz grains allows strain ellipsoids to be measured across the zone. The strain generally plots in the flattening field and many specimens show pure flattening strain. The mylonitic foliation is an axial-plane structure to the folds and is parallel to the XY-plane of the strain ellipsoid. A quartz elongation lineation may be present within the foliation and is parallel to the principal extension direction (the X-axis) of the strain ellipsoid.Strain is accommodated principally by intracrystalline plastic deformation of the quartz grains. In detail the strain is not homogeneous and may vary even between adjacent grains of the same specimen. Quartz optic axis fabrics reflect this strain inhomogeneity. If the strain ellipsoid is an oblate spheroid, c-axes lie in small-circle girdles about the principal shortening axis (the Z-axis). With general triaxial strain, the c-axes lie in a great-circle girdle or girdles which intersect the foliation parallel to the intermediate strain axis (the Y-axis) and lie symmetrically about the Z-axis. A random population of grains from a specimen often shows a composite c-axis pattern between these two types.With approach to the thrust there is an increase in the amount of strain within the specimens. The increasing strain correlates with an increase in the degree of c-axis preferred orientation of the deformed detrital grains and in the amount of new strain-free grains present in the deformed quartzite. Adjacent to the thrust the quartzite is completely composed of polygonal new grains. The new grains probably formed under syntectonic conditions caused by movement along the thrust. The bulk of the new grains developed by increasing misorientation between the subareas of an initially polygonized old grain. There is no evidence of any marked host control on new-grain orientation, but new grain c-axis plots are generally similar to the corresponding old-grain plots from the same specimen. |
