20 Mesozoic Magmatism, Postrift Tectonics, and Passive Margin Development of Northeastern North America

The northeastern region experienced protracted Mesozoic (Triassic-Cretaceous) magmatism associated with continental rifting and subsequent passive margin development. We welcome contributions related to the occurrence, tectonics, or driving mechanisms of these syn- and postrift events.

Monday, 23 March 2026
8:00 AM - 12:00 PM
CCC, Room 27

The end of rift-related continental magmatism in the Northern Appalachians and its relationship to the early seafloor spreading history of the Central Atlantic Ocean

Paper number: 20-5
Program: Technical Sessions
Time: 9:25 AM – 9:45 AM

Authors

Abstract

The persistence of post-orogenic silicic and alkaline magmatism on the Eastern North American Margin (ENAM) from the Late Triassic to at least the mid-Cretaceous remains an elusive problem, conflicting with aspects of the modern plate tectonic paradigm. Excluding the punctuated (<1 Myr) mafic Central Atlantic Magmatic Province, the Jurassic White Mountain Magma Series (WMMS) represents the only unambiguous representation of rift-related continental magmatism in this region during this period. Although recent zircon U-Pb geochronology and tectonic syntheses delineated the basic sequence of rift-related events on the ENAM (e.g., Kinney et al., 2021; Cooper-Boemmels et al., 2021; Kinney et al., 2022, Withjack et al., 2024), important questions remain about the timing of the cessation of continental magmatism, lithospheric breakup, and the onset of seafloor spreading in the Central Atlantic.

We present new zircon U-Pb ages and Hf isotope compositions for the largest igneous complexes of the WMMS province, providing crucial constraints on the spatiotemporal evolution of regional magmatism. We find no clear spatial or temporal emplacement pattern across the region and interpret emplacement of the youngest magmatic event at ca. 169 Ma. The coincidence of the youngest age within the WMMS and estimated ages for magnetic anomalies on the ENAM, particularly the Blake Spur Magnetic Anomaly and the speculated onset of steady-state seafloor spreading at ca. 168-170 Ma (Bécel et al., 2020), motivate scrutiny on the effects of protracted lithospheric breakup and the timescale of continental rift-related magmatism. Zircon Hf isotopic compositions are highly variable, ranging from -31 to +9.5 εHf, but similar to Hf isotopic signatures observed in CAMP basalts in the region and may reflect a highly heterogeneous mantle source for both.

Kinney et al. (2021); GRL 48(11): e2020GL091390; Cooper-Boemmels et al. (2021), AJS, 321.3 (2021): 365-391.; Kinney et al. (2022), Geology, 50(11): 1301-130; Withjack et al. (2024) Reg. Geo. & Tectonics (2E): 58-83; Bécel et al. (2020), JGR Solid Earth, 125.9: e2020JB020093

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411490

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