ANATOMY OF A 10-KM-SCALE SHEATH FOLD AND IMPLICATIONS FOR FLOW AT GRANULITE FACIES CONDITIONS IN THE DEEP CRUST

Mt. Hay, in the Arunta Inlier of central Australia, comprises interlayered gabbroic, charnockitic, and metasedimentary granulites that were metamorphosed and deformed in the deep crust during the Paleoproterozoic Strangways orogeny (1740-1690 Ma). Using lithologic, structural, and fabric-type mapping, we demonstrate that Mt. Hay is a 10-km-scale, antiformal sheath fold with a moderately dipping, gently folded axial plane and slightly overturned southern limb. The current orientation and exposure of the sheath fold, which are a result of exhumation during the Devonian Alice Springs orogeny, yield an oblique cross-section from near the domal nose of the fold (NW) downward toward its base (ESE). Compositional layering defines the map-scale eye structure, whereas grain and mineralogical domain shape preferred orientation defines an axial planar tectonic foliation and pervasive, steeply plunging lineation. The down-plunge asymmetry of mesoscale folds reverses across major hinge zones and minor “culmination” surfaces as observed in other natural sheath folds. Finite strain is quantified in a widely distributed gabbroic granulite unit, which shows little variation in total finite strain magnitude but reveals a distinct pattern of flattening or plane strain on fold limbs grading into constrictional strain in the fold core. Primary magmatic features of the layered intrusive protolith are preserved locally in constriction-dominated hinge zones. These results agree well with strain distribution patterns documented in other natural sheath folds and indicate a common mode of sheath fold formation, perhaps related to the geometry of early buckling instabilities at mechanically active compositional boundaries. The uniformity of foliation, lineation, mesoscale fold orientation, and strain magnitude (if not strain type) across Mt. Hay suggests that the sheath fold and its subsidiary structures formed during a single progressive deformational event. The Mt. Hay sheath fold therefore provides evidence of large-magnitude flow in the deep crust during tectonism.