1 Tectonic Controls on Cenozoic Volcanism in the Gulf of California-Salton Trough Region and the Adjacent Southwestern North American Plate Boundary

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 - 10:00 AM
LMH, Deli

Geochronology of Bimodal Volcanism and Fault Kinematics Suggest Plate Boundary Evolution Drove Miocene Extension at the SE Colorado Plateau Margin, New Mexico

Paper number: 1-1
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Abstract

Eocene to recent extension and volcanism in western North America occurred during Farallon slab rollback and subsequent growth of the Pacific-North America transform boundary; in general, core complex-style extension gave way to distributed high-angle normal faulting and volcanism changed from intermediate and voluminous (ignimbrite flare-up) to bimodal. Although initial Pacific-North America contact occurred ~29 Ma, the boundary lengthened irregularly in the Miocene and Pacific-North America motion rotated to ~northwest-southeast. It has been hypothesized that extensional changes were caused by evolving stresses along the Pacific-North America margin. If so, a simple spatio-temporal correlation should exist between plate-boundary evolution and extensional history. Conversely, a complex correlation may suggest importance of local/regional factors. Extension in regions far from the plate boundary, such as the Rio Grande Rift (RGR), may elucidate these controls.

We studied extensional evolution in the Basin and Range (BR)-RGR transition zone, using 40Ar/39Ar analyses of syntectonic volcanic rocks and fault kinematic measurements from the northeast-trending Reserve and Luna half grabens, New Mexico, adjacent to the southeast Colorado Plateau (CP). Bimodal volcanism and basin fill were coeval with high-angle normal faulting and post-date Eocene-Oligocene ignimbrites and other intermediate volcanics. Basalts in lowermost Reserve Graben basin fill record earliest extension at ~16.5 Ma, and detrital sanidine from uppermost basin fill and a post-tectonic basalt bracket final faulting between ~7.5 and 1.9 Ma. Kinematic indicators from the Reserve Graben’s northeast-striking master fault record oblique dextral-normal slip followed by normal dip slip; sanidine ages from a rhyolite unit that experienced normal-sense offset along this fault suggest primarily dip slip after ~15.2 Ma. Other variously-oriented faults consistently show northwest-southeast extension, constrained to mostly <15 Ma by additional basalts. Thus, extension direction appears to have changed from northeast-southwest to northwest-southeast about ~16-15 Ma.

We conclude that far-field stress transmitted from the Pacific-North America boundary controlled mid-Miocene changes in extension direction in the BR-RGR transition zone, and likely in the main RGR where similar changes are inferred. In contrast, initial Oligocene RGR extension and slightly older ignimbrite volcanism probably resulted from Laramide flat-slab rollback, when the Pacific-North America margin was relatively small. Extension-controlling stress levels in the RGR may have increased due to lengthening or motion changes along the evolving but far-removed Pacific-North America margin.

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

doi: 10.1130/abs/2026CD-411042

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