39 The Mesozoic Convergent Margin of Alta and Baja California

This session will cover aspects including the structure, petrology, geochronology, and stratigraphy of the Mesozoic convergent margin of North America from Alta California to Baja California Sur.

Friday, 24 April 2026
1:30 PM - 5:30 PM
LMH, Deli

Unexpected Very Late Jurassic–Early Cretaceous Granitic Magmatism on Isla Tiburón, Sonora, México: Implications for Late Paleozoic and Mesozoic Subduction Models

Paper number: 39-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

Mesozoic granitic magmatism in northwestern México and the southwestern US has commonly been interpreted as the product of Jurassic–Early Cretaceous continental arc activity associated with east-dipping subduction along the western margin of North America, as envisioned in the fringing arc model. However, this interpretation does not fully accommodate growing evidence that Cordilleran arc magmatism in the region began in the Early Permian (~284 Ma), persisted through the Triassic, and continued into the Late Jurassic, implying a more protracted and complex magmatic history than traditionally assumed.

In this context, our new discovery of very Late Jurassic to very Early Cretaceous granitic intrusions on Isla Tiburón (~150–145 Ma; three new U–Pb zircon ages) provides further evidence for a longer-lived and more continuous record of arc magmatism that has been previously recognized in Sonora; additional evidence is the presence of Early Cretaceous inherited zircons in Late Cretaceous granitoids. Interpretation of these ages extends the duration of magmatic activity into a period that is difficult to reconcile with single-phase subduction scenarios and highlight the need to consider complex temporal and spatial evolution of arc processes in northwestern México.

It is challenging to integrate these new age results into the classic east-dipping fringing arc model, as it would require initiating the arc in Early Permian time and sustaining a wide continental arc for >150 Myr, which is increasingly inconsistent with the geological record of Sonora. Additionally, the model invoking a single west-dipping subduction zone cannot adequately explain the long-lived magmatism present in Sonora. In contrast, tectonic models proposing two coeval subduction systems better accommodate current geological observations for this part of the cordillera. Among these, a model with two east-dipping subduction zones seems more consistent with the available evidence than one invoking opposing east- and west-dipping subduction zones and the existence of a Mezcalera oceanic plate. 

Regardless of the configuration of the Late Paleozoic-Mesozoic convergent margin of North America, both compressional and extensional episodes of arc magmatism are required, beginning in the Early Permian and continuing until the collision of the Alisitos arc in the late Early Cretaceous. Following this collisional event, a single east-dipping subduction zone likely persisted until the final cessation of arc activity in the Early Miocene at this latitude of the Mexican cordillera.

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

doi: 10.1130/abs/2026CD-411011

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