Abstract

Emplacement of magma in the shallow subsurface can result in the development of dome-shaped folds at the Earth’s surface. These so-called “forced folds” have been described in the field and in subsurface data sets, although the exact geometry of the folds and the nature of their relationship to underlying sills remain unclear and, in some cases, controversial. In this study we use high-quality, two-dimensional (2-D) seismic reflection and borehole data from the Ceduna sub-basin, offshore southern Australia, to describe the structure and infer the evolution of igneous sill–related forced folds in the Bight Basin igneous complex. Thirty-three igneous sills, which were emplaced 200–1500 m below the paleo-seabed in Upper Cretaceous rocks, are mapped in the Ceduna sub-basin. The intrusions are expressed as packages of high-amplitude reflections, which are 32–250 m thick and 7–19 km in diameter. We observe five main types of intrusion: type 1, strata-concordant sills; type 2, weakly strata-discordant, transgressive sills; type 3, saucer-shaped sills; type 4, laccoliths; and type 5, hybrid intrusions, which have geometric characteristics of intrusion types 1–3. These intrusions are overlain by dome-shaped folds, which are up to 17 km wide and display up to 210 m of relief. The edges of these folds coincide with the margins of the underlying sills and the folds display the greatest relief where the underlying sills are thickest; the folds are therefore interpreted as forced folds that formed in response to emplacement of magma in the shallow subsurface. The folds are onlapped by Lutetian (middle Eocene) strata, indicating they formed and the intrusions were emplaced during the latest Ypresian (ca. 48 Ma). We demonstrate that fold amplitude is typically less than sill thickness even for sills with very large diameter-to-depth ratios, suggesting that pure elastic bending (forced folding) of the overburden is not the only process accommodating magma emplacement, and that supra-sill compaction may be important even at relatively shallow depths. Based on the observation that the sills intruded a shallowly buried succession, the discrepancy between fold amplitude and sill thickness may reflect loss of host rock volume by fluidization and pore fluid expulsion from poorly lithified, water-rich beds. This study indicates that host rock composition, emplacement depth, and deformation mechanisms are important controls on the style of deformation that occurs during intrusive igneous activity, and that forced fold amplitude may not in all cases reflect the thickness of an underlying igneous intrusion. In addition, the results of this study suggest that physical and numerical models need to model more complex host rock stratigraphies and rheologies if they are to capture the full range of deformation mechanisms that occur during magma emplacement in the Earth’s shallow subsurface.

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