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

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Abstract
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). S1 and S2 make 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 F1 assemblages 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 F2 structures 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 S1 to varying degrees. S1 is commonly parallel to lithological layering, but it may make any angle with it, although recognizable F1 folds 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 F2 show strong maxima locally, but are regionally highly variable. This variation is systematically related to their position with respect to the large F2 folds, and is attributed to an angular discordance between the divergent S2 schistosity 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.