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Stress configurations during and following emplacement of ENA basalts in the northern Appalachians

By
Jelle Zeilinga de Boer
Jelle Zeilinga de Boer
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Published:
January 01, 1992

The Jurassic tholeiitic magmas of the Eastern North American (ENA) igneous province probably originated at the mantle-crust interface below the extended segment of the passive margin. Large volumes appear to have accumulated in the lower crust resulting in basaltic “underplating.” From these sites magma may have migrated eastwards towards the more extended crustal areas where it accumulated in “megasills” or intruded upwards along subvertical fractures to form the ENA dike swarms. Magmas from the “megasills” intruded into the upper crust and gave rise locally to massive flood basalts (J reflector) and/or migrated westward along major décollement zones towards “Newark” type basins (Bell and others, 1988). Flows in the Hartford and Fundy basins were fed from multiple dikes that probably soled on these low-angle normal faults. The magmas, which rose directly from underplated regions to form ENA dike swarms, possess different chemistries (New England quartz-normative versus Carolinas olivine-normative tholeiites). These variations can be attributed to differences in upper mantle thermal regimes resulting from mantle plume activity.

Detailed studies of brittle failure in “Newark” type rift valleys indicate that the tensional stress fields present during emplacement of the tholeiites differed. Variations are probably due to differences in the attitudes of the principle décollement zones and/or sinusoidal inflections of the Hinge zone (Wernicke and Tilke, 1989). Besides extension more or less at right angles to the Appalachian grain, crustal stretching also occurred parallel to this orogenic belt. In certain regions (Carolinas) such extension may have dominated during tectonic periods in the Jurassic.

Stress configurations changed after emplacement of the ENA tholeiites and deformation was dominated by a grain-parallel compressive stress. This stress penetrated both the rift basins and adjacent basement complexes. The transformation from stretching to shortening led to expulsion of hot brines that had accumulated in the rift basins, resulting in the emplacement of mineralized veins. Associated thermal effects reset K-Ar ages and partially remagnetized igneous units and redbeds. The tectonic data suggest that the embryonic opening of the Atlantic was a complex process characterized by at least three tectonic phases: rifting, shifting, and drifting.

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GSA Special Papers

Eastern North American Mesozoic Magmatism

John H. Puffer
John H. Puffer
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Paul C. Ragland
Paul C. Ragland
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Geological Society of America
Volume
268
ISBN print:
9780813722689
Publication date:
January 01, 1992

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