Two high grade schistosities at broken hill and their relation to major and minor structures

The regional schistosity at Broken Hill, previously regarded as a single fabric element, has been separated into three overprinting elements-one of high meta-morphic grade (S₁), one of variable grade (S₂), and one consistently retrograde (S₃). S₁and S₂make up by far the majority of occurrences mapped to date. A new descriptive terminology is introduced to take account of this complexity. Extensive metamorphic crystallization has taken place during the first two periods of schistosity formation (F₁, F₂), and the distribution of metamorphic assemblages is thus partly controlled by the extent to which F₁assemblages have been modified by F₂, and the variable grade of F₂. A regional retrograde schistosity can locally be shown to be S₂, although it is sometimes superposed on F₂structures and is then classified as S₃. A number of major fold-hinges have been mapped in the region, primarily using the extensive mesoscopic data available from one rock type-a layered sillimanite gneiss. Almost without exception, these fold hinges (F₂) have an axial plane schistosity (S₂) which overprints S₁to varying degrees. S₁is commonly parallel to lithological layering, but it may make any angle with it, although recognizable F₁folds are rare. Most of the previously mapped fold-hinges (e.g., the Broken Hill and Hanging Wall Basins) are F₂; some of the nearly isoclinal major folds inferred on the basis of granite gneiss and amphibolite outcrop patterns (e.g., the Corruga West and Corruga East anticlines) may be F₁, but mesoscopic data have been inadequate to confirm this. The orientations of minor fold axes and layering-schistosity intersections of F₂show strong maxima locally, but are regionally highly variable. This variation is systematically related to their position with respect to the large F₂folds, and is attributed to an angular discordance between the divergent S₂schistosity and the axial planes of the major structure. Application of these results to the mine area broadens the range of possible interpretations of previous structural observations, and allows greater compatability between these observations and the gross conformability of the orebody. Copyright.