A STRUCTURAL PERSPECTIVE ON UNDERSTANDING INTRAPLATE MAGMATISM: AN EXAMPLE FROM THE EASTERN NORTH AMERICAN MARGIN

Recent work has shown that a variety of processes can generate intraplate magmatism. Difficulties arise in distinguishing magmatism from mantle plumes versus shallow mantle processes, largely because of the influence of lithospheric structure on the location of plume-related magmatism in continental lithosphere. Our work on the Early Cretaceous stress fields in New England seeks to improve our understanding of the crustal expression of intraplate magmatism driven by edge-driven convection, a common shallow mantle process at passive continental margins.

The eastern North American margin has undergone multiple episodes of postrift magmatism. In New England, Early Cretaceous magmatism geographically overlaps with the track of the Great Meteor hotspot and with the footprint of the Northern Appalachian Anomaly, which has been interpreted as the product of modern edge-driven convection. In westernmost New England, our geochronological work has shown that magmatism significantly predates and postdates hotspot-related magmatism and is consistent with generation by edge-driven convection.

By combining structural data with our previously obtained geochronological data, we identified local shifts in the Early Cretaceous extensional stress field that coincide with the episodes of postrift magmatism interpreted to be related to edge-driven convection. The ~140–130 Ma episode is associated with a N–S extensional stress field while the ~110–100 Ma episode is associated with a NW–SE extensional stress field. Ongoing work is focused to the east in the vicinity of the ~122 Ma Ascutney plutonic complex that is temporally and geographically consistent with the Great Meteor hotspot track. The Ascutney plutonic complex is crosscut by dikes with similar strikes to those of the dominant dike set within the geographic footprint of the ~110–100 Ma episode of magmatism. We analyzed mapped mafic dike orientations in the vicinity of Mount Ascutney using the mixed Bingham distribution cluster analysis of Yamaji and Sato (2011), and N–S and NW–SE extensional stress fields were identified. We interpret N–S extension as occurring prior to pluton emplacement and NW–SE extension after pluton emplacement. Mesoscale normal faults and extension fractures in the region also support NW–SE extension after pluton emplacement.