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New Mexico middle-crustal cross sections

1.65-Ga macroscopic geometry, 1.4-Ga thermal structure, and continued problems in understanding crustal evolution

¹ Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, U.S.A.
² Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

Recent detailed work in key regions along two north–south transects in northern New Mexico highlights continued controversy about Proterozoic tectonic evolution. Ductile deformation features (folds, ductile thrusts, and associated foliations and lineations) are grouped into three deformation generations. D₁ includes crytic bedding-parallel foliation and fold nappes. D₂ involves north-verging, km-scale inclined folds, the main shortening foliation, and D₂, structures that further attenuate or reactivate F₂ folds. D₃ involves east–west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S₂ is the dominant regional foliation that could possibly be imaged seismically. Map relationships around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast-matrix studies of dated minerals show that D₃ occurred at ca. 1.42. The age of D₂ is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M₁–M₃, that may relate to the deformational events. New geochronology indicates that most metamorphic minerals grew (or were reset) at ca. 1.47–1.35 Ga. U-Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet, and tourmaline suggest regional metamorphism to 550–700° C at 1.47–1.42 Ga. Metamorphic aureoles are present around plutons, but the highest grades of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism is interpreted to record a regional mantle-driven thermal event, the latter parts of which correspond to a time of pluton emplacement. ⁴⁰Ar/³⁹Ar dates record post–1.42-Ga cooling: the highest grade rocks yield the youngest cooling ages, indicating slow cooling and gradual unroofing of the 1.42-Ga thermal profile following 1.42-Ga metamorphism. Our preferred model is that macroscopic geometries (D₁–D₂) were established by 1.65 Ga, and that regional amphibolite-grade metamorphism and associated D₃ deformation at 1.47–1.42 Ga produced localized high-strain domains and fabric reactivation at exposed levels. At deeper levels, structures and assemblages may increasingly record 1.42-Ga reactivation.

Key Words: Proterozoic geology • New Mexico • Mazatzal • Yavapai • tectonic evolution

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