UNDERSTANDING A TTG STRUCTURAL DOME USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS): REVEALING SOLID STATE FLOW PATTERNS WITHIN THE 3.5-3.2 GA MT EDGAR DOME, EAST PILBARA

The 3.6-2.8 Ga East Pilbara craton is one of the best-preserved of Earth’s earliest cratons. Large (50 km in diameter) domes of Tonalite-Trondhjemite-Granodiorite (TTG) composition emplaced through and deformed a thick supracrustal sequence of volcanics and sediments. There are multiple models that address the formation of these domes. We can constrain these models by providing quantitative structural and magnetic fabric from within the Mt Edgar dome, an archetypal dome within the East Pilbara craton. Oriented samples were taken at 150 locations throughout the dome, resulting in >1,200 individual 1-inch cores. Anisotropy of Magnetic Susceptibility (AMS) analysis – a magnetic technique that characterizes the shape and distribution of magnetic grains in a small specimen as an ellipsoid – was conducted on these specimens. AMS-based magnetic fabrics are in good agreement with measured field foliations and lineations, as well as lineaments interpreted from available aeromagnetic data and structural mapping. Microstructural characterization was also done for each TTG unit, and indicated that solid-state deformation affected the majority of the Mt Edgar dome. Combining the microstructural and magnetic characterizations, two key observations are highlighted. First, the central part of the dome has a consistent AMS orientation, with a magnetic lineation that is sub vertical and a magnetic foliation that strikes NW-SE and dips ~90. Near boundaries between TTG units of differing ages, the AMS orientation swings parallel to those boundaries. Second, at least one flow channel has been identified from the center of the dome to the SW margin. This flow channel is defined by prolate fabrics and sub-horizontal lineations perpendicular to the dome margin. The solid-state flow patterns, and cross-cutting relationships identified within them, suggest that the Mt Edgar dome experienced at least two episodes of significant solid-state flow. The first episode could represent internal convection predicted by a diapiric model of dome formation.