Geometry and controls on the development of igneous sill-related forced folds: A 2-D seismic reflection case study from offshore southern Australia

Abstract

Emplacement of magma in the shallow subsurface can result in the development of dome-shaped at the Earth's sur- face. These so-called folds have been described in the fi eld and in subsur- face data sets, although the exact geometry of the and the nature of their relation- ship to under lying sills remain unclear and, in some cases, controversial. In this study we use high-quality, two-dimensional (2-D) seismic refl ection and borehole data from the Ceduna sub-basin, offshore southern Australia, to describe the structure and infer the evolution of igneous sill-related forced in the Bight Basin igneous complex. Thirty-three igneous sills, which were em- placed 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 refl ections, which are 32-250 m thick and 7-19 km in diameter. We observe fi ve main types of intrusion: type 1, strata-concordant sills; type 2, weakly strata-discordant, trans- gressive sills; type 3, saucer-shaped sills; type 4, laccoliths; and type 5, hybrid intru- sions, 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 re- lief. The edges of these coincide with the margins of the underlying sills and the display the greatest relief where the underly- ing sills are thickest; the are therefore interpreted as forced that formed in response to emplacement of magma in the shallow subsurface. The are onlapped by Lutetian (middle Eocene) strata, indicat- ing they formed and the intrusions were em- placed during the latest Ypresian (ca. 48 Ma). We demonstrate that fold amplitude is typi- cally less than sill thickness even for sills with very large diameter-to-depth ratios, suggest- ing that pure elastic bending (forced folding) of the overburden is not the only process ac- commodating magma emplacement, and that supra-sill compaction may be important even at relatively shallow depths. Based on the ob- servation that the sills intruded a shallowly buried succession, the discrepancy between fold amplitude and sill thickness may refl ect loss of host rock volume by fland pore fl uid expulsion from poorly lithifi ed, water-rich beds. This study indicates that host rock composition, emplacement depth, and deformation mechanisms are important controls on the style of deformation that oc- curs during intrusive igneous activity, and that forced fold amplitude may not in all cases refl ect the thickness of an underlying igneous intrusion. In addition, the results of this study suggest that physical and numeri- cal 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 em- placement in the Earth's shallow subsurface.