GIANT RADIATING DYKE SWARMS ON EARTH AND VENUS

Concentrations of dykes of basic composition emplaced in the same igne ous episode or along similar trends are known as mafic dyke swarms and they occur in a wide variety of environments and over a wide range of scales on Earth. Recent radar mapping of Venus has revealed families of linear features interpreted to be the surface expression of near-su rface dyke swarms. The lack of significant erosion on Venus provides a view of the surface manifestation of dyke swarm emplacement, one whic h complements the terrestrial perspective of erosion to deeper levels. The goal of this review is to synthesize the information available on both planets in order to use the complementary and synergistic record of mafic dyke swarm emplacement to build toward a better understandin g of this important phenomenon in planetary history. We focus on the f ormation and evolution of giant dyke swarms which cover tens to hundre ds of thousands of square kilometres on both Earth and Venus. Mafic dy ke swarms on Earth occur in a wide range of modes and are observed in environments ranging from volcanic edifices (e.g., Hawaii), lo central complexes (e.g., Spanish Peaks Complex, USA; Ramon Swarm, Israel), sp reading centres and ophiolite complexes, compressional plate boundarie s in back-are settings (Columbia River Basalts, USA) and in continent- continent collisions. One of the most impressive modes of occurrence i s that linked to the formation and evolution of mantle plumes. Terrest rial examples include a giant radiating swarm covering 100 degrees of azimuth (the Mackenzie swarm, Canada), a 360 degrees giant radiating s warm (the Central Atlantic reconstructed swarm), deformed giant radiat ing swarms (the Matachewan swarm, Canada), rift-arm associated swarms (e.g., Grenville swarm, Canada; Yakutsk swarm, Siberia), and one consi sting of widely separated dykes (e.g., the Abitibi swarm, Canada). We summarize the geometric, chemical and isotopic characteristics of terr estrial dyke swarms, including their size and geometry, ages, presence and absence of subswarms, and the relation between swarms of differen t ages. We also summarize the characteristics of individual dykes, exa mining dyke length and continuity, en echelon offsets, dyke bifurcatio n, dyke height, width and depth; dyke intrusion and cooling history, a nd evidence for flow directions. On Venus at least 163 large radiating lineament systems (radius generally > 100 km) composed of graben, fis sure and fracture elements have been identified. On the basis of their structure, plan view geometry and volcanic associations, the radial e lements of more than 70% of these are interpreted to have formed prima rily through subsurface dyke swarm emplacement, with the remainder for ming through uplift or some combination of these two mechanisms. These systems are essentially uneroded and provide a view of the surface ch aracteristics of giant radial swarms prior to the erosion which common ly occurs on Earth. The individual graben, fissures and fractures of w hich the systems are composed are typically less than several kilometr es in width and cluster near the centre, with fissures grading smoothl y into fractures at greater distances to define the overall radial pat tern. While the largest systems, like those on Earth, are thousands of kilometres in radius, the population average is about 325 km, and the y generally do not extend to equal lengths in all directions. In their distal regions, however, the elements in 72% of the systems continue along a purely radial trend, while distal elements in the remaining 28 % curve gradually into unidirectional, sub-parallel geometries, genera lly interpreted to be related to regional stress patterns. The radial systems have a strong association with volcanism; all but seven displa y some form of volcanic signature. A review of models of the emplaceme nt of lateral dykes from magma chambers under constant (buffered) driv ing pressure conditions and declining (unbuffered) driving pressure co nditions indicates that the two pressure scenarios lead to distinctly different styles of dyke emplacement. Emplacement of lateral dykes in the constant driving pressure (buffered) case, however, can produce dy kes which have sizes and widths which are very large and independent o f chamber size. On Earth, the characteristics of giant mafic dyke swar ms such as the Mackenzie dyke swarm in Canada strongly suggest that th ey were emplaced in buffered conditions. On Earth, giant radiating dyk e swarms are usually preserved as fan-shaped fragments which have been dismembered and distorted by subsequent plate tectonic rifting events . The abundant intact giant radiating swarms on Venus provide criteria by which fragmented terrestrial swarms can be reconstructed.