IDENTIFICATION AND CHARACTERIZATION OF GIANT CIRCUMFERENTIAL AND RADIATING DYKE SWARMS ON EARTH AS A TOOL FOR UNDERSTANDING THE PLUMBING SYSTEMS OF LARGE IGNEOUS PROVINCES ON VENUS AND MARS

Large Igneous Provinces (LIPs) occur widely on Earth, Venus, Mars and some other planetary bodies. Mafic dyke swarms and sills form the plumbing systems of LIPs on Earth and, hence, are a key element in understanding the genesis of LIPs. It is generally assumed that most terrestrial LIPs are fed by giant radiating swarms that likely focus above mantle plumes. However, another class of giant dyke swarm, termed circumferential swarms, has been identified with quasi-circular geometries and diameters up to at least 1800 km (e.g., Buchan & Ernst 2016, LPSC 47, abstr. 1183). Most appear to be associated with LIPs and some are centred on or near the focus of giant radiating swarms.

Both radiating graben-fissure systems and coronae (with quasi-circular graben-fissure systems and topographic expression) are common on Venus, may be underlain by dykes, and have been, in some cases, linked to volcanic flow fields that reach the size of terrestrial LIPs. Coronae may be analogues of terrestrial circumferential dyke swarms (e.g., Bethell et al. 2016, LPSC 47, abstr. 1471). Similar features are also observed on Mars.

Here we describe essential characteristics of terrestrial giant circumferential and radiating swarms to help in their identification and their use for understanding the plumbing systems of terrestrial and planetary LIPs, with emphasis on a newly-identified circumferential system associated with the ca. 130 Ma Paraná-Etendeka LIP of South America and Africa, the most prominent centre of magmatism associated with opening of the south Atlantic Ocean. In Brazil, coast-perpendicular Ponta Grossa, and coast-parallel Santos-Rio de Janeiro and Florianópolis dyke swarms are thought to be three arms of a radiating system likely associated with rifting above the Tristan da Cuhna plume. Less dense dykes of similar or slightly younger age crosscut each of these swarms at high angles (most clearly in the case of the Ponta Grossa and Florianópolis swarms). We suggest these may form portions of a ~900 km diameter circumferential swarm centred near the focus of the radiating system. In northern Namibia, coast-parallel Etendeka dykes, likely linked to the Florianópolis swarm of Brazil, are crosscut at right angles by a less dense dyke set, the probable extension of the giant circumferential swarm into Africa.