4 Assembling Laurentia: Turning Points in the Geologic Evolution of the North American Continent

Monday, 26 October 2020
1:30 PM - 5:30 PM
Auditorium

LATE TRIASSIC TO MID-CRETACEOUS RECONSTRUCTION OF MEXICO

Paper number: 4-28
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

  • Cathy Busby Presenting Author
    University of California Davis, Davis, CA

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-356641

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