Magnetic overprinting of the Brachina Formation/Ulupa Siltstone, Southern Adelaide Foldbelt, prior to Delamerian deformation

Abstract

The Proterozoic sediments of the Adelaide Geosyncline were deformed and metamorphosed during the Ordovician Delamerian Orogeny. In much of the Flinders Ranges, the relatively unmetamorphosed sediments, such as those of the Brachina and Bunyeroo formations in the Central Flinders Ranges, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic and cause prominent linear magnetic anomalies with amplitudes of 100 to 500 nT that can be traced for more than 50 km. The higher metamorphic grades are accompanied by the growth of metamorphic magnetite within beds that are thought to have originally contained siderite. These magnetic markers have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with magnetisations in the direction of the present magnetic field implying that remanence causes a significant proportion of the anomaly. The rocks display a strong thermochemical component, mostly acquired prior to deformation due to the Delamerian Orogeny. Remagnetisation prior to deformation is consistent with the high-temperature low-pressure metamorphism that characterises the eastern Mt Lofty Ranges. This secondary natural remanent magnetisation (NRM) component is similar to that of the nearby post-orogenic Black Hill Norite, which is believed to have intruded during the waning phases of the Delamerian Orogeny and is of the same magnetic polarity. We therefore have magnetic signatures of the same polarity that bracket the Delamerian Orogeny, both pre- and post-folding. The tilt corrected mean direction for the Ulupa Siltstone from 44 samples is D = 231.4°, I = 41.1° (α95 = 5.6°), and a pole position at latitude λp = 46.0°S, longitude φp = 49.6°E (A 95 = 6.4°).