30 Cenozoic dynamics of the western North American plate boundary: Insights from offshore marine geology and geophysics into Cordilleran tectonics

We invite presentations on Cenozoic tectonics, volcanism, sedimentation, and geohazards offshore and along the margin of western North America. Example topics include crustal structure, mantle processes, triple-junction migration, paleomagnetism, active faulting, marine stratigraphy, and seismicity.

Friday, 24 April 2026
8:00 AM - 12:00 PM
LMH, Deli

Is Guadalupe Island a Symptom of Triple-Junction Migration or Mantle-Plume Magmatism?

Paper number: 30-5
Program: Technical Sessions
Time: 9:15 AM – 9:35 AM

Authors

Abstract

Elongated alkaline seamounts that develop on fossil oceanic spreading centers (FSC seamounts) record plate-divergence termination and provide insight into triple-junction migration, mantle heterogeneity, and/or mantle-plume magmatism. In particular, a system of FSC seamounts offshore of the California borderlands discontinuously spans ~1800 km NW–SE and tracks the late Cenozoic, southward-propagating interaction between the East Pacific Rise oceanic spreading center and the active western margin of North America. Guadalupe Island is the most volumetrically significant component of this system: the seamount developed on a ~12.5 Ma FSC and rises ~5 km from the seafloor, subaerially exposing ~30 km (N–S) x ~10 km (E–W) of its uppermost ~1 km. Although its FSC context suggests a connection to the East Pacific Rise, Guadalupe Island also defines the eastern terminus of the ~1000 km-long, WNW–ESE-trending Fieberling–Guadalupe seamount trail that was previously interpreted as a mantle-plume hotspot.

 

We present recent findings from fieldwork on Guadalupe Island and ongoing petrography, petrology, and laboratory analytical work for volcanic-rock samples including bulk-rock major–trace-element geochemistry, mineral chemistry, and K/Ar geochronology. The subaerially exposed seamount includes older, shield-building strata that account for most of its volume and younger, fissure- and caldera-collapse-related strata and landforms that cover most of its surface. The composite volcanic edifice includes collapsed calderas in the south (where more denudation obscures its geometry) and in the north (where less denudation outlines a ~10 km diameter). The volcanic rocks are alkaline, mostly sodic, and highly fractionated (LaPM/YbPM = 10–20). Shield- and fissure-lava SiO2 (47–62%) records a range of melt differentiation that partially overlaps with those for scoria cones (45–51%), lava domes (55–65%), and caldera-collapse-related tuffs (63–66%). Less-differentiated samples are enriched in high-field-strength elements and record OIB-like melts. Mineral-chemical measurements for olivine, clinopyroxene, and plagioclase further elucidate melt evolution. Volcanic-groundmass K/Ar ages for four samples record older volcanism in the south (1.8–1.7 Ma) and younger volcanism in the north (1.0–0.7 Ma), corroborating the qualitative pattern apparent from surface denudation. Available geochemical and geochronological data for Guadalupe Island are integrated with those for its host FSC seamount system to evaluate potential geodynamic relationships among mantle flow, mantle melting, and triple-junction migration as they pertain to microplate formation and capture of the Farallon plate by the Pacific plate during the late Cenozoic.

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

doi: 10.1130/abs/2026CD-410975

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