| Comments on “Giant Dikes: Patterns and Plate Tectonics” by J. G. McHone, D.L. Anderson, Y.A. Fialko – Richard E. Ernst “A seemingly radial pattern of giant dike swarms is often attributed to uplift caused by mantle plume heads (Ernst et al. 1997), although the amplitude of the resulting uplift is insufficient to accommodate large dike swarms...” This problem of insufficient uplift to accommodate large radiating dyke swarms was noted and addressed for the Mackenzie swarm by Baragar et al. (1996) and was also revisited by Mege & Ernst (2001) and Sengor (2001). It was concluded that gravitational sliding off a domal uplift was the explanation, and would result in circum-plume deformational structures. Analogy was drawn with the wrinkle ridges that circumscribe the Tharsis uplift on Mars. Thermal erosion is unlikely to be the explanation since, since the margins of giant dykes typically have sharp planar contacts with the host rock, and melt-back textures are rarely observed. “Rose diagrams and maps of dikes are needed to see how truly radial any dike swarm may be. Some so-called radial dike swarms are actually sub-parallel to or sub-perpendicular to rifted margins.” Yes, there are swarms in which dykes are concentrated along three arms (and linked to triple junction rifting) (Fahrig 1987). These we have previously termed Type 3 radiating dyke swarms (e.g., Ernst & Buchan 1997). However, as seen in the Giant radiating dyke swarms webpage, there are also swarms exhibiting continuous (Type 1) and semi-continuous (Type 2) radiating patterns. “Where are they on the other sides of the focus points?” This is the reason that we are so keen to expand dating of Proterozoic dyke swarms – in order to find the complementary radiating patterns on other formerly adjacent continental blocks. Hundreds of swarms around the world remain poorly dated. As these unknown dyke sets are dated, and as continents are reconstructed using paleomagnetism, we expect to identify the missing portions for radiating (fanning) swarms. Ultimately, we expect to find many complete radiating patterns such as we characteristically observe on Venus, a planet lacking plate tectonics (Grosfils & Head 1994; Ernst et al. 2003). “And why are there not radiating dikes around all plume-uplift centers?... Giant dikes do not radiate around Iceland, Columbia River, Parana, Siberia, Deccan, Yellowstone, or even Hawaii...” Part of the problem for these young events is that flood basalts hide the underlying dyke swarms. However, radiating swarms are observed in association with the NAVP (predecessor of Iceland), Columbia River, Parana, Siberia and Deccan (Ernst & Buchan 1997; 2001). In most of these cases, these are Type 3 radiating swarms. More continuous fanning patterns are only recognized in the Columbia River and Siberian Traps cases. Typically the dykes that parallel a breakup margin are of two generations, pre-rift and post-rift. Figure 4 and associated discussion: Note that the fanning stress pattern of southern Asia is not associated wtih a radiating dyke swarm. Therefore, a fanning stress pattern is not sufficient by itself to produce a radiating dyke swarm. A central magma source (plume-generated?) would also appear to be required. The use of dyke swarms to map out regional paleo-stress patterns is addressed by Ernst & Buchan (1999). References
30th March, 2004 |