LATE TRIASSIC TO MID-CRETACEOUS RECONSTRUCTION OF MEXICO

I update a model proposing that Mesozoic arc-related rocks of Mexico formed in the upper plate of a single long-lived, east-dipping subduction zone.^{1, 2} Recent work has linked the geometry of the subducted Cocos slab (the longest on Earth) to the upper plate geology of Mexico, over 220 myr of eastward subduction.³ I expand on this model by suggesting alternatives to models that propose multiple subduction zones.

Following the termination of Permian-Triassic (280-240 Ma) subduction, the paleo-Pacific Mexico margin was dominated by a huge siliciclastic wedge (Potosí fan), sourced by Grenville, Pan-African and Permian rocks. Its zircons were dispersed far onto the ocean floor, forming a unifying signature in Late Triassic to mid-Cretaceous oceanic arc-related rocks that compose much of the western third of Mexico (Mesozoic Pacific System of Mexico⁴), tying them to Mexico (i.e. they are not exotic). Subduction initiated at 221 Ma on the most outboard part of the margin, in the oceanic realm (Vizcaino Peninsula), producing a supra-subduction zone ophiolite with Potosí fan zircons as xenocrysts in its chromatites. Previous workers have proposed that this requires west-dipping subduction, but I suggest that the zircons were deposited on the seafloor before the trench formed, and then subducted eastward. The magmatically robust Jurassic to mid-Cretaceous arc of western Mexico and Baja California formed above this east-dipping slab, and shifted positions with time.

The problem of the ”double arc” (Late Triassic “Cordilleran arc” and Jurassic “Nazas arc), which would require a separate slab under eastern Mexico, is solved by regarding continental basins of eastern Mexico as rifts related to the breakup of Pangea (Mesozoic Atlantic System of Mexico⁴). These are localized sedimentary basins with much lesser volcanic rocks, and they lack the major volcanic centers and plutons of arcs. The arc-like signature of the volcanic rocks is due to proximity of these continental rift basins to the magmatically-robust arc of western Mexico.

In summary, only one single east-dipping subduction zone is needed to reconstruct the Mesozoic history of Mexico.

¹Busby et al. 1998, Geology 26/3, 227-230.

²Busby 2004, Tectonophysics 392, 241-277.

³Boschman et al. 2018, G³ 19, 4649-4672.

⁴Martíni and Ortega-Gutiérrez, 2018, Earth Science Reviews 183, 38-55.