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

Borderland Basins Record Pacific-North America Plate Boundary Evolution

Paper number: 30-1
Program: Technical Sessions
Time: 8:05 AM – 8:25 AM

Authors

Abstract

Oblique-rifting along the southern California and northern Baja California continental margin followed the southward migration of the Rivera Ridge-Trench-Transform triple junction and produced a long, complex, system of basins. Two distinct styles of basins are recognized in the inner Borderland. Along the coast, initial extension formed two elongate parallel basins between tilted blocks above an east dipping detachment. A change in relative plate motion changed the basin style to rhombic nascent seafloor spreading basins offset by right-stepping transforms. The northwest-trending dextral San Clemente and related Inner Borderland faults exhibit right-stepping echelon character of transform faulting along the nascent Pacific-North America plate boundary. Transtensional basins exist along the faults with the larger San Clemente basin resembling a rhombic nascent seafloor spreading center opening between echelon fault sections. This process progressed southward as the Rivera Triple migrated south. Changes in the relative plate motion are recorded in the orientation of the transform faults. The sediments in the Miocene basins record the history of these basins. Uplift of basin sediments along a major restraining segment of the San Clemente fault that aligns with Miocene relative plate motion enable estimates of timing for significant changes in the relative plate motion and evolution of the Pacific-North America transform plate boundary. Subsequent opening of the Gulf of California and evolution of the San Andreas fault system transferred the locus of relative plate motion inland. Changing relative plate motions initiated tectonic inversion of the numerous Miocene basins lying within the Inner Borderland Rift. Widespread and abundant seismic reflection profiles from exploration energy companies, government, and academic geophysical cruises provide excellent images of the basin structure and stratigraphy. Offshore basin sampling is provided by a few deep cores, seafloor samples, and outcrops on numerous islands where transpression inverted former basin sequences including basement rocks (e.g. Catalina and other Borderland islands). Examples from several prominent Borderland basins provide important constraints on basin formation and evolution of the offshore transform plate boundary before opening of the Gulf of California and growth of the San Andreas fault system, and subsequent transpressional deformation resulting from changes in the plate boundary and relative motion vectors.

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

doi: 10.1130/abs/2026CD-411074

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