Paleomagnetic-tectonic study of Mesozoic dike swarms in the Appalachians

Paleomagnetic evidence indicates that most of the extensive dike swarms cutting Triassic and older formations probably intruded in a time of regional tectonic and magmatic activity distinct from the late Triassic tectogenesis. The fossil magnetic directions of the dikes coincide neither with Late Triassic nor with Early Cretaceous paleomagnetic directions, which suggests a Jurassic age for the intrusions. The dikes were emplaced along tensional fractures that were the surficial expressions of deep-seated movements. The fan-shaped arrangement of these fractures indicates a southwestward decrease of the rotational component of the shear couple and a sinistral polarity of the shear movements. Periodic increases in tectonic and mafic magmatic activity, such as this one in the Jurassic, appear to be characteristic of Mesozoic deformations in the Appalachians.