We characterize the composition, timing, geometry, and deformation style of the syntectonic Miocene Chemehuevi dike swarm exposed in the footwall of the regionally developed low-angle Chemehuevi detachment fault system (southeastern California, USA). Our data support mafic to felsic dike emplacement from ∼1.5 ± 1 to 3.8 ± 1 m.y. after initiation of regional extension (ca. 23 Ma), followed by rapid slip and denudation with minor magmatism. Pb/U zircon ages indicate intermediate to felsic dike emplacement adjacent to the Mohave Wash fault, part of the regional fault system, as it was active across the upper limit of the brittle-plastic transition, from 21.45 ± 0.19 to 19.21 ± 0.15 Ma. Intermediate to felsic dikes are undeformed at structurally shallow levels (<9 km minimum paleodepth), but are rotated and locally folded, and host a well-developed mylonitic foliation and lineation at deeper structural levels (≥9 km paleodepth), even where the country rock is nonmylonitic. Syntectonic mafic to intermediate dikes were emplaced into the footwall, hanging wall, and fractures and cataclasites hosted in the Mojave Wash fault zone. Dike emplacement therefore occurred into and adjacent to a low-angle normal fault zone during its early history across the upper brittle-plastic transition, with dikes locally composing as much as 25% of the footwall adjacent to fault zone, and <2% of the total extension regionally. The predominant east-west and northeast-southwest orientations of dikes within the swarm are unique to this core complex, and differ from the predicted emplacement orientation for northeast-directed extension and other complexes in the region. Dikes have moderate to subvertical dips at the highest crustal levels (domains 1–3), and are subhorizontal in the deepest exposures of the fault system (domains 4 and 5), where they host mylonitic fabrics. The elemental geochemistry of the Chemehuevi dike swarm is similar to that of local volcanism exposed in tilted hanging-wall blocks to the regional fault system and regional intrusive magmatism, and the swarm is proposed to have fed the now rootless volcanic systems as part of the regional magmatic system.

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