8 Cenozoic Cordilleran Tectonics, Magmatism, and Crustal Evolution: Perspectives from Mexico and Western North America (Posters)

Tectonic reconstructions of the southwestern North American plate boundary provide an excellent framework for understanding Cenozoic volcanism (arc, rift, slab window, seafloor) in this largely extensional and transtensional setting. Geologic, geochemical and geophysical submissions encouraged.

Wednesday, 22 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Initiation of the Northwest Cordillera Large Igneous Province—Volcanism, Geochemistry, and Climate

Paper number: 8-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Early Eocene was a tumultuous time in Earth’s history with major tectonic and magmatic events including the opening of the North Atlantic, collision of the Indian and Eurasian plates, and kimberlite eruptions of the Lac De Gras kimberlite field. This was also a time of major climatic excursions of the Late Paleocene to Early Eocene Thermal Maximum (PETM) and Early Eocene Climactic Optimum (EECO). Coincident with these events was widespread volcanism of the Northwest Cordillera extending minimally from south Yukon to Wyoming. Previously referred to as the Chalis-Kamloops belt, our geologic compilations along with geochemistry and high precision geochronology indicate that the Early Eocene magmatic rocks of the Northwest Cordillera represent a Large Igneous Province (LIP). Volcanism had a rapid onset at about 56 Ma and offset at about 45 Ma, coeval with the PETM and EECO, and becomes younger to the south.

Rocks are primarily subalkaline basaltic andesites and dacites with lesser exposures of rhyolitic rocks. Although most andesitic LIPs are associated with subduction, this volcanism is associated with extension along pull-apart basins related to major dextral faulting, and crustal delamination. The Cordilleran lithosphere in the region had a long history of subduction modification which is reflected in the trace element ratios of the rocks. In addition, these rocks are about eight times higher in phosphorous than average continental crust.

Although generally characterized by thick sequences of primarily basaltic-andesite flows, the initiation of volcanism in south-central BC was characterized by highly silicic pyroclastic flows interbedded with carbonaceous siltstones, shales, and sandstones, coal lenses and chert. Large fragments of plant material are incorporated at the base of the flows. Although coal lenses and carbonaceous sediments are abundant near the base of the sequence, the consistent and voluminous volcanism during a 10 Ma timespan changed the surface of the planet in the region from highly vegetated to more baren with only isolated lakes and forests. This volcanism potentially impacted climate by contributing to a change to surface albedo over a large area, burial of coal swamps and forests, increasing phosphorous to aquatic and marine ecosystems with weathering of the rocks, and contributing volcanic gases to the atmosphere.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410991

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