EARLY CRETACEOUS POSTRIFT MAGMATISM AND NORMAL FAULTING IN THE NORTHERN APPALACHIANS: IMPLICATIONS FOR THE EVOLUTION OF THE EASTERN NORTH AMERICAN MARGIN

Postrift magmatism occurred in a number of locations along the eastern North American margin and, in New England, the postrift magmatism coincides with the footprint of the Northern Appalachian Anomaly. The Early Cretaceous postrift magmatism of western Vermont and eastern New York occurred within the northern Appalachians and geographically overlaps with normal faults within the Champlain Valley belt and northern Taconic allochthon. Our ⁴⁰Ar/³⁹Ar and U–Pb geochronology for the magmatism of western Vermont and eastern New York indicates magmatism spanned at least 35 m.y. The magmatism is bimodal and trace element patterns display consistent intraplate signatures through time. Trace element patterns suggest a common source for the prolonged magmatism and, therefore, a common driving mechanism. The duration of magmatism is difficult to reconcile with a hotspot origin and is inconsistent with the Great Meteor hotspot track. Our results are readily interpreted within the context of edge-driven convection along the eastern North American margin.

We evaluated the Early Cretaceous postrift stress field within western Vermont and eastern New York by assessing the compatibility of the stress fields associated with mesoscale normal faults and dike emplacement. Paleostress inversion produced evidence for multiple stress fields associated with normal faulting. NE–SW extension is interpreted as the Early Cretaceous regional stress field in the study area. N–S extension occurred in association with magmatism and is supported by E–W dike trends of northwestern Vermont and fault-slip paleostress inversion results. Younger NW–SE extension occurred in association with magmatism and is supported by NE–SW dike trends of southwestern Vermont and fault-slip paleostress inversion results. Crosscutting relationships support an Early Cretaceous and/or younger timing of the normal faulting. We interpret the N–S and NW–SE extensional events as perturbations to the regional stress field in association with magmatism driven by edge-driven convection.