Formation of a dome-and-basin fold interference pattern by granite intrusions

Abstract

Folding of layered sequences is influenced by the presence of igneous intrusions, which form competent bodies that control fold distribution. Here, we show an example from the Broken River Province (northeastern Australia), where macroscopic folding affected sedimentary rocks and felsic intrusions, forming a dome-and-basin pattern. Field relations indicate synkinematic emplacement of the granitic intrusions, which occur as folded sill-like microgranitic bodies, mildly deformed stocks at structural domes, and undeformed granite that cuts across the folded sedimentary sequence. U-Pb zircon ages from the various intrusions yielded an age range of 335−320 Ma, thus constraining the timing of folding. The deformed zone is restricted to an ∼30-km-wide corridor bounded by two crustal-scale shear zones. Kinematic indicators show that folding was generated by dextral transpression, giving rise to the dominant orientation of folds. Domes and basins are generally concentrated in the proximity of the granitic intrusions, indicating that their development was likely controlled by the synkinematic emplacement of these plutons. The volume of granite added to the exposed section could have altered the local strain regime, switching the horizontal stretching direction during transpression to a shortening direction. We therefore suggest that forced intrusion of synkinematic plutons into a confined transpressional zone (dominated by flattening strain) can create local constrictional strain where shortening occurs in two broadly orthogonal horizontal directions. The results demonstrate a link between Carboniferous granitic dome-and-basin patterns and syn-transpressional magmatism. It is plausible that similar processes were responsible for the development of granitic dome-and-basin patterns in Archean terranes.