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Anomalous Arching During Rifting, Breakup, and Drifting: Evidence on the Passive Margin of Eastern North America for Distal Magmatic Underplating


On many passive margins, an episode of uplift, not predicted by simple lithospheric stretching, developed during the transition from rifting to drifting. The resultant unconformity, termed the breakup or post-rift unconformity, separates the syn-rift and post-rift strata. Likely causes include asthenospheric upwelling, magmatic underplating, phase transitions, and/or lithospheric detachment near the site of breakup. In this study, we have integrated geologic and geophysical data (seismic, field, core, borehole, sonic-log, and vitrinite-reflectance) from the central segment of the passive margin of eastern North America (ENAM) to better define the extent, magnitude, and timing of uplift during the rift-drift transition. We have identified and defined a distinctive type of uplift on the ENAM margin, consisting of a broad arch 100 to 200 km landward of the continent-ocean boundary. The axis of the arch parallels the syn-rift crustal fabric, not the continent-ocean boundary. Restorations suggest that the rift basins in the study area (i.e., the Newark, New York Bight, Connecticut Valley basins) were deep, broad, and interconnected during the Late Triassic. Arching and intrabasin normal faulting began during the eruption of basaltic lava flows and intrusion of diabase sheets associated with the Central Atlantic Magmatic Province (CAMP, ~201 Ma) during the final stages of rifting in latest Triassic/earliest Jurassic time. Arching and erosion continued after breakup throughout much of the Jurassic. The rift basins in the study area underwent significant exhumation, especially along the axis of the arch where exhumation exceeded 5 km. The eroded material contributed to the infilling of the adjacent, subsiding post-rift Baltimore Canyon trough, which contains up to 13 km of sediments, 9 km of which is Jurassic in age. Based on the synchronous timing of arching and CAMP-related magmatism, the prolonged duration of exhumation, and the parallelism of the arch with the syn-rift crustal fabric, we propose that distal magmatic underplating during the final stages of rifting produced the arching observed on the central segment of the ENAM margin. Unlike the underplating observed beneath other passive margins, this underplating was focused beneath rift basins considerably offset from the site of breakup. Similar arches likely exist on other parts of the ENAM margin, significantly affecting the petroleum potential of the ENAM rift basins and post-rift basins.