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

Oligo–Miocene magmatism and transtensional tectonics in the Soledad Rojo Basin of the paleo-eastern California shear zone, western Palo Verde Mountains, lower Colorado River corridor, SE California, USA

Paper number: 1-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Since the mid-Oligocene (ca. 30 Ma), southwestern North America has undergone a complex tectonic transition from subduction to transform motion along the Pacific–North American plate boundary, accompanied by widespread volcanism, extension, and strike-slip faulting. Although much of this deformation was localized along the San Andreas fault system, a significant component (~25%) of plate motion is distributed eastward into a broad transtensional zone encompassing the Walker Lane, eastern California shear zone (ECSZ), and the paleo-ECSZ in the lower Colorado River corridor (LOCO). Compared to adjacent regions, the geologic evolution of the LOCO remains relatively poorly constrained despite its importance as a spatial link between continental transtension in the Walker Lane and the nascent Gulf of California rift system. The Soledad Rojo Basin, located near the southern end of the LOCO, in the low-relief area between the Black Hills and Palo Verde Mountains of southeastern California, preserves a key record of synextensional magmatism, sedimentation, and deformation within the paleo–ECSZ.

 

Our geologic mapping and geochronology indicate that the Soledad Rojo Basin is a late Oligocene (~24–25 Ma) synextensional succession consisting of: (1) a basal sequence of silicic welded ignimbrite outflow sheets and associated bimodal volcanic rocks ; (2) volcaniclastic paleochannel deposits carved into and derived from these underlying volcanic units; and (3) the “Tolbard Fanglomerate,” a regional synextensional alluvial fan–fluvial red-bed unit with metaplutonic and volcanic clasts. In the Soledad Rojo Basin, the Tolbard Fanglomerate is subdivided into a lower subunit of brick-red conglomeratic sandstone and conglomerate with subangular to subrounded (locally rounded) clasts, and an upper subunit of light gray to buff conglomeratic sandstone and conglomerate dominated by subangular clasts. Stratigraphic and structural relationships indicate that the Soledad Rojo Basin formed as a synvolcanic half-graben during regional mid-Cenozoic extension associated with early Pacific–North American plate boundary development. The basin was subsequently dissected and offset by several right-lateral strike-slip and transtensional normal faults, likely during the mid-Miocene (~8-12 Ma), resulting in a cumulative right-lateral displacement of at least 2.5 km. These structures, along with dextral slip documented by recent studies in the paleo-ECSZ, show continued deformation during Miocene reorganization of the plate boundary and suggest that volcanism and basin development were strongly controlled by transtensional tectonics during development of the Gulf of California–Salton Trough rift system.

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

doi: 10.1130/abs/2026CD-410856

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