38 Plate Boundary Evolution and Lithospheric Dynamics of Western North America (Posters)

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 - 5:30 PM
LMH, 5th Floor Chapel

Surface Wave Tomography of Subduction Fragments and Geochemistry of Miocene Volcanism from Northern Baja to Southern California

Paper number: 38-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The tectonic aftermath of the collision of a spreading center with an active subduction zone is a rare occurrence which has been suggested to cause the termination of spreading and subduction processes, formation of triple junctions, fragmentation of subducting and overriding plates, and anomalous volcanism in a wide ~500 km zone surrounding the plate boundary. Differential plate spreading and convergence rates caused the East Pacific Rise (EPR) mid-ocean ridge spreading center axis to approach the Farallon-North American trench more than 50 Ma. During collision, the subduction zone consumed the EPR at the location of the modern day southern California coastline ~30 Ma. We investigate hypotheses for slab windows, slab fragments, and observations of Miocene volcanism formed after subduction termination and northward migration of the remaining EPR and triple junction along the California coastline and southward migration along the Baja California coastline. We present surface wave inversion results from a marine seismic offshore deployment with sensitivity that crosses the border between northern Baja to southern California from 31o to 37o latitude. Marine seismic data is inverted together with permanent land stations near the continental margin for Rayleigh wave phase velocities. A second stage shear wave inversion using phase velocities as data obtains 1D and 2D shear wave velocity structure. Preliminary results indicate that lithospheric velocities in the oceanic mantle are at least 2% higher than the continental region. The lithosphere-asthenosphere boundary is observed at 50 ± 20 km beneath 15-35 Ma oceanic seafloor. Preliminary seismic tomography indicates the presence of a high velocity anomaly beneath the inner and outer Borderlands that is ~9% higher than the surrounding mantle extending from 80 km to at least 160 km depth. The absence of deep seismicity suggests that if this high velocity anomaly is a slab fragment, it has heated up beyond the brittle threshold for elastic deformation. Seismic tomography results are compared to geochemical studies of anomalous Miocene volcanism beneath the Borderlands and coastline to test for arc volcanism, slab windows, or melting of a young slab. We investigate the possibility that a fossil slab fragment was formed in northern Baja California and translated northward to the southern California Borderlands during Pacific plate capture.

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

doi: 10.1130/abs/2026CD-410965

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