Crustal magnetism, tectonic inheritance, and continental rifting in the southeastern United States
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The Brunswick magnetic anomaly (BMA) in southern Georgia is coincident with seismic reflectivity marking the deep crustal suture between Laurentia and a crustal block of Gondwanan affinity. The source of the BMA remains enigmatic because of its apparent relationship with both the Permo-Carboniferous Alleghanian orogeny (ca. 315–270 Ma) and the emplacement of the Central Atlantic Magmatic Province (ca. 200 Ma). In this paper, the BMA is modeled using relatively weak (<0.5 A/m) reversed-polarity remanent magnetization in lower crustal rocks (16–24 km depth) outboard of the Laurentian margin. The acquisition of this magnetic signature is consistent with transpression and strike-slip motion along the margin during the initial stage of Alleghanian convergence, which overlaps with the Kiaman Reversed Superchron (ca. 320–263 Ma). Simple magnetic models show that the onshore segment of the BMA can be explained as an effect of continental collision rather than voluminous magmatism along the suture zone. If Central Atlantic Magmatic Province intrusions were not focused along the suture zone, then evidence for tectonic wedging at the crust-mantle boundary associated with Alleghanian convergence may be preserved along the onshore segment of the BMA, rather than over-printed by Mesozoic magmatism.
Manuscript received 4 June 2013; accepted 5 Sept. 2013.