Architecture and thermochronology of the base Adelaidean unconformity at Arkaroola.

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2022

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Imbrogno, D. N.

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The Arkaroola unconformity represents a gap of almost 750 Ma, yet as well as being demonstrably erosive, the basement Mount Neill Granite is strongly foliated in a style similar to that reported from extensional core complexes. The architecture and strain along this lithological boundary varies in geometry and intensity, with past research providing limited constraints on the structural relationship between the basement and overlying basin rocks. This thesis maps this strain seen in the basement, investigates the relative timing of the shear strain with respect to macroscopic fold development, examines microstructural record, and dates cooling after strain. Samples were collected over a sequence of the unconformity with an increase of shear strain in the basement approaching this boundary, and into the overlying Paralana Quartzite. Normal shearing kinematic indicators at multiple boundary locations suggest basement foliation pre-dates large scale folding during the Delamerian Orogeny. Orientation of c-axes in quartz grains in the protomylonitic basement proximal to the boundary show evidence for non-coaxial strain, with the quartzite directly above producing a girdle representing pure shear, plane strain deformation. This thesis examines previous thoughts on coeval intrusion and deformation of the Mount Neill Granite, with new evidence for basement foliation developing shortly before deposition and rifting sequences of the Adelaidean Superbasin. Presence of prism [c] slip and dynamic recrystallisation in quartz grains indicates temperatures reached at least 500 °C. In-situ Rb–Sr muscovite ages from this study correlate to cooling after the Delamerian Orogeny, the intrusion of the British Empire Granite, hydrothermal deformation, and localised pegmatite intrusions. This study presents a tectonostratigraphic timeline for the evolution of the boundary, with evidence for a mid-crustal extensional shear zone in early formation of the Adelaide Rift Complex. Future work on additional boundary locations is needed to better understand regional architecture resulting from early basin formation.

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School of Physical Sciences

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Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, YEAR

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