Non-Radial Dikes of the Central Atlantic Magmatic Province Reveal Tectonic Source and Evolution of the Break-Up of Pangaea

Erin K. Beutel

Dept. Geology & Environmental Geosciences, College of Charleston, beutele@cofc.edu

This webpage is a synopsis of the paper: Beutel, E.K., Magmatic rifting of Pangaea linked to onset of South American plate motion, Tectonophysics, 468, 149-157, 2009.

Background

In 1971, P.R. May published a paper suggesting that the diabase dike swarm along the south and central eastern margin of North America was radial. This was based on aeromagnetic data showing NW-SE, N-S, and NE-SW trending dikes geographically separated along the coast. These dikes are associated with the break-up of Pangaea and the emplacement of the Central Atlantic Magmatic Province (CAMP) and have been proposed to be the result of a plume (e.g., Ernst & Buchanan, 1997; Wilson, 1997; Marzoli et al., 1999; Dalziel et al., 2000; Storey et al., 2001; Janey & Castillo, 2001) despite numerous papers to the contrary (e.g., King & Anderson, 1998; McHone, 2000; van Wijk et al., 2001; Beutel et al., 2005; Nomade et al., 2007).

However, despite much reiteration that the dike swarm is radial, field observations and State Survey maps from the Carolinas suggest that the swarm is not radial, and in fact overlaps to a considerable degree in South and North Carolina along the SE margin of North America (Figure 1). Furthermore, these dikes have been shown to have been emplaced in a regular order, not simultaneously, within a 2-Ma time window around 200 Ma (Hames et al., 2000; Salters et al., 2003; Beutel et al., 2005; Nomade et al., 2007). Cross-cutting relationships consistently show that the NW-trending dikes were emplaced first over 230-Ma NE-trending rifts, followed by the N-trending and finally by the NE-trending dikes (Ragland, 1983; McHone et al., 1987; McHone et al., 1988; Schlishe 2002; Schlishe et al., 2003). There appears to be a geographic and probably orientation (and therefore timing) relationship between the dikes (Ragland, 1983; de Boer & Snider, 1997; Salters et al., 2003; Beutel et al., 2005; Nomade et al., 2007). New, more detailed studies are currently underway (A. Marzoli, personal communication).

Figure 1: Dikes of eastern North America modified from Beutel (2009). This map shows more complicated dike patterns than previously depicted. Rose diagram shows dike directions from State Survey Geologic Maps and represents confirmed true dike orientations.

Given the non-radial nature of the dikes, the rapidly changing stress field necessary to create three distinct dike-swarm orientations within 2 Ma became the focus of my research. The rift- and dike-orientations suggest that at ~ 230 Ma the stress field was predominantly NW-SE extensional, but that at ~ 200 Ma it rotated by 90° to become NE-SW extensional (Figure 2). Then within 2 Ma it changed again and became E-W extensional, and finally NW-SE extensional again (Figure 2). While local and even regional stress fields may vary widely due to faulting and strain partitioning, the continental scale of the emplaced dikes and required stress field suggests that major tectonic forces must have controlled the stress field changes within the continent.

Figure 2: Stress evolution based on observed extensional features from Beutel (2009). Left column shows the likely orientation of extensional stress based on the observed features in the right column. Right column shows the age and orientation of extensional features observed in the field. Top box in right column shows normal faults while the middle and lower boxes show the overlying dikes.

Modeling Stress Fields

To determine the plausibility of tectonic changes being able to cause rapid, 90° changes in the stress field orientation throughout a continent, an evolving model of Pangaea break-up was created using the finite-element program FElt (Gobat & Atkinson, 1996). The models constructed were based on the most likely tectonic scenarios. For example, it was deemed unlikely that a single, large continent would rotate in one direction for millions of years and then switch and rotate through 90° in 2 Ma while attached to at least two other large continents. Consideration was also given to application of the stresses as well as the physical properties of the continents and their connections. Continental stresses, those assumed to move large continents, were applied to a generalized area associated with the continental keel such that the motion driving the continents would be best distributed over the entire continent. Some models were run with only marginal stresses applied, but these were to be insufficient to move the entire continent and would have resulted in the formation of extensional margins along the outside edges of the continents. The interiors of the continents were modeled as thicker than the margins and the suture zones between the continents were modeled as weaker than the surrounding lithosphere. Models were run with the sutures the same strength as the surrounding lithosphere, but while similar, continent-scale stress fields were successfully modeled, the lack of a defined suture zone created a stress field that would likely not have followed the actual breaks in the continents.

Model Results

Numerous models with variations in plate motions and suture locations and strengths were studied (Figure 3). What follows is the most likely history of stress evolution that explains how the continents move today and moved in the past, and the 90° rotation in stress field orientation within 2 Ma shown by the diabase dikes associated with continental break-up.

At about 230 Ma North America began to move to the NW. This created NE-trending extensional basins along the SE margin of North America. Resistance to this motion appears necessary to cause large-magnitude stresses (compared to those applied). This resistance was simulated by fixing Africa. It could also have been accomplished by moving Africa to the SW and by having it resistant to movement as a result of its massive keel. The resulting stress field in SE North America is NW extensional, which would result in NE-trending normal faults. These are indeed seen in the geologic record (Figure 3A).

At ~ 200 Ma, when volcanism onset in SE North America, South America began to move to the SW. This changed the stress field orientation along the SE margin of North America from NW extensional to NE extensional. The new stress field results in NW-trending extensional features, consistent with the NW-trending dike swarm along the SE margin of North America. This also created an E-W trending extensional regime north of the present-day Gulf of Mexico, which resulted in the N-trending dikes seen along that margin. The stress field varies close to the meandering suture zone. This implies that pre-existing variability in lithospheric strength affected the stress field and can account for regional features on a scale smaller than continental (Figure 3B).

At ~ 199 Ma South America separated from North America. This is best modeled as the formation of oceanic crust and a mid-ocean ridge. Fast- to medium-spreading ridges are weak features and do not effectively transmit stress across themselves. This results in the stress field along the eastern margin of North America rapidly reverting to NW extensional, successfully explaining the observed NE-trending diabase dikes. The presence of N-trending dikes between the two end-members is likely as the stress field rotated between NE extensional and NW extensional. Of note is the sudden decrease in extensional stress along the southern margin of North America and the increase in stress along the mid-southeast portion of the margin. This change in the stress field agrees with the lack of multiple dike orientations along the southern margin and perhaps the sudden northward propagation of the dikes and rifts into what is now the New England margin of eastern North America (Figure 3C).

Figure 3: Left column images are the results of the finite-element model. Background colors indicate maximum stress magnitude and type (blue - compressional, red - extensional), bars are the maximum and minimum stress directions (black - extensional, white - compressional). Middle column images show the projected field-observable extensional features created by this stress field. Right column images show the applied stress fields. Black dots are elements and nodes held fixed in space (modified from Beutel, 2009).

Conclusions

The non-radial CAMP dike swarm associated with the break-up of Pangaea requires a rapidly evolving orientation of stress to account for the change from NE trending normal faults at 230 Ma to NW trending diabase dikes at ~200 Ma, followed by N-trending and then NE-trending diabase dikes within 2 Ma, all of which overlap along the southeast margin of North America. A plume scenario cannot account for such an evolving stress pattern, but tectonic events that are known to have occurred can.

Finite-element models demonstrate that the observed extensional features can be explained by ongoing motion of North America to the NW in the presence of a stationary Africa, followed by SW-motion  of  South America, and then detachment and final separation from North America at a mid-ocean ridge. The driving force behind the magmatism is tectonic rather than plume in origin, and the dikes cannot be used to prove a plume source for the break-up of Pangaea.

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