Abstract
Solidified remnants of major magma pathways, and localized clusters of smaller intrusive sheets, are exposed in the mountains of South Victoria Land, Antarctica. Together the intrusions represent the shallowest 2 km of the plumbing system for the Early Jurassic Ferrar Large Igneous Province. Three sites were investigated: Allan Hills exposes the shallowest portion (<1 km) of the Ferrar plumbing system, whereas Terra Cotta Mountain and Solitary Rocks represent a paleodepth of ≥2 km.
Ferrar Dolerite sills were emplaced into a highly stratified and vertically anisotropic sedimentary sequence. They evolved as interconnections of shallowly dipping Ferrar Dolerite sheets and sills, or ‘transgressive intrusions’. These intrusions represent the preferred mode of propagation of magma in the Ferrar LIP and are observed even at shallow depths.
The largest sills, such as the Basement Sill exposed at Solitary Rocks, represent major magma pathways that were continuously injected during the Ferrar magmatism, and which transported magma for long distances through rock now exposed in the Transantarctic Mountains. At Terra Cotta Mountain, and several other locations in South Victoria Land, clusters of igneous sheets formed thanks to high localized tensile and shear stresses between intrusions propagated along adjacent stratigraphic levels (bedding planes). Within 1 km of the surface (Allan Hills), sills propagated very close to, or even intersected, the surface. The injection of magma into fractures formed in a roof of country rock, which was buoyed up above such shallow-seated sills, resulted in clusters of segmented intrusions, with both irregular and transgressive geometries, and complex mutual relationships.
There are no significant petrological differences among intrusions exposed in these different parts of the Ferrar system. Magma flow paths inferred from rock magnetic fabrics indicate a strong connection between magmatic flow directions and complex intrusion geometries, and the general heterogeneity of both is inferred to indicate that magma created its own set of fractures both across the Beacon sedimentary rocks.
The model developed for the Ferrar plumbing system on the basis of the above observations and inferences is, at least between 0 and 2 km depth below the Jurassic paleosurface, propagation of sills and connecting sheets of the Jurassic Ferrar LIP was essentially selfdriven, with little or no control from significant stresses acting on the lengthscale of the sill network. The principal structural controls on magma emplacement were provided by structural anisotropies within the Beacon sedimentary sequence. Finally, during evolution of the province the lowest sill intrusions became thickened at depth as the result of repeated injections of magma; this phenomenon favoured the lateral propagation of magma for long distances across the Ferrar LIP.