Discrete proterozoic structural terranes associated with low- P, high- T metamorphism, Anmatjira Range, Arunta Inlier, central Australia: tectonic implications

Abstract

Structural overprinting relationships indicate that two discrete terranes, Mt. Stafford and Weldon, occur in the Anmatjira Range, northern Arunta Inlier, central Australia. In the Mt. Stafford terrane, early recumbent structures associated with D 1a, 1b deformation are restricted to areas of granulite facies metamorphism and are overprinted by upright, km-scale folds F 1c), which extend into areas of lower metamorphic grade. Structural relationships are simple in the low—grade rocks, but complex and variable in higher grade equivalents. The three deformation events in the Mt. Stafford terrane constitute the first tectonic cycle ( D 1- D 2) deformation in the Weldon terrane comprises the second tectonic cycle. The earliest foliation ( S 2 a ) was largely obliterated by the dominant reclined to recumbent mylonitic foliation ( S 2 b ), produced during progressive non-coaxial deformation, with local sheath folds and W- to SW-directed thrusts. Locally, ( D 2 d ) tectonites have been rotated by N—S-trending, upright ( F 2 c ) folds, but the regional upright fold event ( F 2 d ), also evident in the adjacent Reynolds Range, rotated earlier surfaces into shallow-plunging, NW—SE-trending folds that dominate the regional outcrop pattern. The terranes can be separated on structural, metamorphic and isotopic criteria. A high-strain D 2 mylonite zone, produced during W- to SW-directed thrusting, separates the Weldon and Mt. Stafford terranes. 1820 Ma megacrystic granites in the Mt. Stafford terrane intruded high-grade metamorphic rocks that had undergone D 1 a and D 1 b deformation, but in turn were deformed by S 1 c , which provides a minimum age limit for the first structural—metamorphic event. 1760 Ma charnockites in the Weldon terrane were emplaced post- D 2 a , and metamorphosed under granulite facies conditions during D 2 b , constraining the second tectonic cycle to this period. Each terrane is associated with low- P, high- T metamorphism, characterized by anticlockwise P— T— t paths, with the thermal peaks occurring before or very early in the tectonic cycle. These relations are not compatible with continental-style collision, nor with extensional tectonics as the deformation was compressional. The preferred model involves thickening of previously thinned lithosphere, at a stage significantly after (>50 Ma) the early extensional event. Compression was driven by external forces such as plate convergence, but deformation was largely confined to and around composite granitoid sheets in the mid-crust. The sheets comprise up to 80% of the terranes and induced low- P, high- T metamorphism, including migmatization, thereby markedly reducing the yield strength and accelerating deformation of the country rocks. Mid-crustal ductile shearing and reclined to recumbent folding resulted, followed by upright folding that extended beyond the thermal anomaly. Thus, thermal softening induced by heat-focusing is capable of generating discrete structural terranes characterized by subhorizontal ductile shear in the mid-crust, localized around large granitoid intrusions.