245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I

We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D

Age, composition and history of the rifted Southern California mafic forearc basement

Paper number: 245-4
Program: Technical Sessions
Time: 8:55 AM – 9:10 AM

Authors

Abstract

Understanding the origin and nature of oceanic lithosphere or ophiolite underlying forearc regions is key for deciphering the convergence history of continental margins. Jurassic ophiolitic rocks occur as forearc basement at many locations along the western North American convergent margin. This includes the mid-Jurassic Coast Range Ophiolite (CRO), which is part of a triad of lithotectonic belts in northern CA that represents a “type” convergent margin. Similar ophiolitic forearc basement occurs in west-central Baja CA and includes the 173 Ma Cedros Island Ophiolite and the 221 Ma Vizcaíno Ophiolite. The southern CA/northern Baja CA segment of the Mesozoic convergent margin is much more poorly understood because it is largely submerged. This is due to a Neogene micro-plate capture event that slivered off ~400x75km of the forearc from coastal southern CA. Forearc rifting, transtensional faulting and transrotation displaced the forearc to the outer borderland, northern Channel Islands and Santa Monica Mountains. Zircon U-Pb ages measured from three Willows Plutonic Complex (WPC) samples from Santa Cruz Island were 167-166 Ma. A plagiogranite from the saussurite gabbro on Catalina Island was 174 Ma. We also indirectly sampled forearc basement by analyzing mafic clasts from the Miocene syn-rift San Onofre Breccia. Nearly half of results from our mafic plutonic cobble suite yielded 164-158 Ma U-Pb ages similar to the WPC. The remainder yielded a broad spectrum of Jurassic, Mid-Cretaceous, and Late Cretaceous ages. Intragrain spot analyses and depth profiling of natural zircon surfaces confirm that zircon in mafic forearc basement underwent profound recrystallization. The age distributions define U-Pb age maxima coincident with the timing of accretion of the Mid-Cretaceous Catalina Schist and Late-Cretaceous Pelona-Orocopia-Rand Schists. We postulate that forearc basement represented by the San Onofre Breccia cobbles resided close to the subduction zone and was infiltrated by metasomatic subduction fluids. This is supported by results from metagabbro cobbles present within metaconglomerate of the Catalina blueschist unit that exhibit the same zircon U-Pb age systematics. Both autochthonous and allochthonous models exist for the origin of mafic forearc basement along the margin. We argue that in southern CA, forearc basement formed in close proximity to SW North America based upon 1.1, 1.4 and 1.7 Ga zircon inheritance and clearly expressed continental arc zircon trace element signatures.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413272

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