New Mexico middle crustal cross sections: 1.65 Ga macroscopic geometry, 1.4 Ga thermal structure and continued problems in understanding crustal evolution

Williams, M.L., Karlstrom, K.E., Lanzirotti, A., Read, A.S., Bishop, J.L., Lombardi, C.E., Wingstead, M.B., 1999, New Mexico middle crustal cross sections: 1.65 Ga macroscopic geometry, 1.4 Ga thermal structure and continued problems in understanding crustal evolution, in: Rocky Mountain Geology: Lithospheric structure and evolution of the Rocky Mountains Part II, (special volume), University of Wyoming, K.E. Karlstrom (ed), v.34, no. 1, p. 53-56.

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 cryptic bedding-parallel foliation and fold nappes. D ₂ involves north-verging, km- scale inclined folds, the main shortening foliation, and D ₂ structures that further attenuate or reactivate F ₂ folds. D ₃ involves east-west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S ₂ 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 D ₃ occurred at ca. 1.42. The age of D ₂ is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M ₁ -M ₃ , 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 degrees 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 (D ₁ -D₂ ) were established by 1.65 Ga, and that regional amphibolite-grade metamorphism and associated D ₃ 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.