Giant swarms (greater than 300 km long) have linear, radiating and arcuate distributions (Halls, 1982; Fahrig, 1987; Ernst et al., 1997; Ernst et al., 2001). Linear swarms may be related to rifting, and arcuate swarms may reflect regional variation in the stress field during emplacement, or subsequent deformation. I focus herein on radiating swarms. While some radiating swarms have a pattern in which dykes trend along the arms of a triple junction (type 3 radiating swarms; Ernst & Buchan, 1997), I discuss those having a continuous or semi-continuous fan of dykes (types 1 and 2). Some dramatic examples are shown in Figures 1 a and 1 b. Giant radiating dyke swarms are interpreted to represent the plumbing system part of large igneous provinces (Ernst & Buchan, 1997).
Figure 1a: 2215 Ma Ungava radiating swarm, Superior Province, Canada (after Buchan & Ernst, 2004).
Figure 1b: 2500-2450 Ma Matachewan and Mistassini radiating swarms, Superior Province, Canada (after Ernst & Buchan, 2001).
On Venus and Mars
Giant radiating dyke swarms are also important on other planets. The surfaces of Venus and Mars are interrupted by extensive systems of long narrow troughs having linear, arcuate and radiating geometries (e.g., Grosfils & Head, 1994; Mege & Masson, 1996; Ernst et al,. 2001; Wilson & Head, 2002; Ernst et al., 2003). Some of these graben-fissure systems have a purely tectonic (extensional) origin. However others, particularly the large radiating systems, are interpreted as radiating dyke swarms. Specifically, the individual grabens are interpreted to represent surface deformation above underlying dykes.
Giant radiating swarms are particularly abundant on Venus (e.g., Figure 2) and commonly show continuous patterns spanning an angular arc of > 270°. Since Venus lacks plate tectonics, these patterns are primary. This contrasts with Earth where plate tectonic processes are expected to have dismembered radiating swarms.
Figure 2: Giant radiating graben-fissure systems in the Guinevere Planitia/Beta Regio region of Venus. Topography is shown in the background (after Ernst et al., 2003).
Evidence for Lateral Flow
Lateral emplacement is important in the genesis of many radiating dyke swarms. This has been most clearly demonstrated for the Mackenzie radiating swarm, where magnetic fabric data indicate vertical flow within 500 km of the centre, but horizontal (lateral) flow out to distances of 2,300 km (Ernst & Baragar, 1992). On Venus and Mars, the continuity of individual grabens over distances of hundreds of kilometres implies lateral dyke emplacement. Theoretical arguments in support of long-distance lateral flow in dykes are well developed (e.g., Rubin, 1995).
Controls on Emplacement
The salient points related to the origin of giant radiating dyke swarms include their radiating geometry, large areal extent, and evidence for lateral emplacement from a centrally located magma source. Two processes appear important in their generation:
Origin of Giant Radiating Swarms
Radiating swarm geometry and evidence for a centrally located magma source are compatible with mantle plume or mantle diapir models. It is unclear how giant radiating swarms can be generated by non-plume models. Specifically, models of lithospheric fracturing, or “EDGE” convection should generate linear belt-like magmatism not the “point-like” sources required for giant radiating swarms.
The number of known giant radiating swarms on Earth is small, only about 25 (Ernst & Buchan, 1997), and represents a small proportion of the current inventory of 154 giant swarms (Ernst et al., 1996; Buchan & Ernst, 2004). However, the primary geometry of most giant swarms is poorly known since plate tectonics must fragment them. Until more sub-linear swarms have been precisely dated and reconstructed using paleomagnetism, the true number of giant radiating swarms on Earth will remain unknown. Venus is of similar size to the Earth but lacks plate tectonics. Its inventory of giant radiating dyke swarms is probably in the hundreds (Grosfils & Head, 1994; Ernst et al., 2003).
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