35 The Mesozoic Convergent Margin of Alta and Baja California (Posters)

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
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Cataclastic Brecciation of Late Cretaceous (~96 Ma) Granites at Monument Hills, Northwestern Sonora: Timing and Tectonic Implications

Paper number: 35-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Monument Hills locality, ~30 km east of San Luis Río Colorado in northwestern Sonora, consists of small quarry-scale outcrops of Late Cretaceous granitic rocks. These granites were originally described and dated by L.T. Silver using U–Pb zircon geochronology at 95 ± 3 Ma (reported in Anderson and Roldán-Quintana, 1979) and were interpreted as post-deformational relative to Jurassic-deformed rocks elsewhere in NW Sonora, commonly linked to deformation along the proposed Mojave–Sonora megashear. Subsequent studies by Ruiz-Segoviano et al. (2013) confirmed this age, reporting a U–Pb zircon emplacement age of 96.5 ± 1.2 Ma.

In this study, we present four additional U–Pb zircon ages yielding a combined crystallization age of 96.2 ± 0.5 Ma. Zircon U-Pb systematics show no evidence of isotopic disturbance associated with brittle deformation; cataclasis occurred after complete crystallization and solidification of the pluton and this combined U–Pb age provides a maximum constraint on the timing of deformation in the region.

Consistent with previous observations by Ruiz-Segoviano et al. (2013), but in contrast to early interpretations, the granitic unit exhibits cohesive cataclastic brecciation along meter-scale, NNW-striking shear bands. Deformation is dominantly brittle and characterized by cohesive cataclasites, although localized microstructures indicative of limited ductile deformation, including incipient mylonites, are present. 

We interpret the cataclastic rocks at Monument Hills as the product of dynamothermal metamorphism associated with brittle–ductile shear zones formed under low-grade greenschist-facies conditions. Ongoing work aims to determine whether this deformation is related to Laramide compression during the Late Cretaceous–early Tertiary or to a younger Early Miocene extensional event associated with Basin & Range or metamorphic core complex (MCC) formation.

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

doi: 10.1130/abs/2026CD-410809

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