Earth and Planetary Sciences ETDs

Publication Date

Summer 7-15-2024

Abstract

The well-exposed basement transect in the Lower Granite Gorge (LGG) of western Grand Canyon provides a middle-crustal record of suturing crustal blocks during the Proterozoic accretionary history of southwestern Laurentia. Orogenic evolution included at least three stages: (1) subduction-related ~1.75-1.72 Ga volcanic arc magmatism, turbidite deposition, and suturing of arc terranes with different crustal thicknesses; (2) intense northwest-southeast crustal shortening at ca. 1.71-1.68 Ga to form the Vishnu Mountains during the Yavapai orogeny; (3) dextral strike-slip shear zones development at ~1.4 Ga during long-term crustal residence and slow cooling above ~250 °C (~10 km) until ~1.3 Ga. Final cooling from ~150-200 °C (~6-8 km) to the surface took place by 0.5 Ga. The Gneiss Canyon shear zone (GCsz) is a ~15-km-wide set of ductile and brittle-ductile shear zones that forms part of a transitional Paleoproterozoic tectonic boundary between the Mojave crustal province to the northwest and Yavapai province to the southeast. High-temperature, northeast-striking (S2) subvertical high-strain zones of the GCsz extend from River Miles 234 to 242 and record oblique west-side-up thrusting with a dextral strike slip component. This took place at migmatite conditions at ~1.70-1.68 Ga based on field relationships and in-situ monazite geochronology. 40Ar/39Ar thermochronology of hornblende suggest cooling through ~500 °C on both sides of the GCsz between 1.70 and ~1.62 Ga. Muscovite 40Ar/39Ar cooling ages of ~1.55 Ga record post-orogenic slow cooling through 350 °C during a time of little or no tectonism in the region. Biotite 40Ar/39Ar cooling ages indicate rocks cooled through 300 °C by ~1.45 Ga, at depths of ~10 km. A family of newly mapped mylonite–ultramylonite–pseudotachylyte zones reactivated the GCsz as lower temperature dextral strike-slip shear zones. Individual shear zones are up to 5-m-wide and are characterized by shallow to moderately plunging stretching lineations, mylonitic foliation, and low-temperature (~300 °C) deformational features and hence, are inferred to be approximately dated by the 40Ar/39Ar biotite cooling ages. The main contributions of this paper include a new 1:24,000 geologic map of the Lower Granite Gorge available in GeMS digital format, a synthesis of western Grand Canyon Proterozoic evolution, and evidence for ~1.45 Ga dextral strike-slip reactivation of the orogen.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Dr. Karl E. Karlstrom

Second Committee Member

Dr. Matthew T. Heizler

Third Committee Member

Dr. Ryan S. Crow

Fourth Committee Member

Dr. Adrian J. Brearley

Language

English

Keywords

Lower Granite Gorge, Grand Canyon, Gneiss Canyon shear zone, Proterozoic basement geology

Document Type

Thesis

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Geology Commons

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