New Mexico Geological Society Annual Spring Meeting — Abstracts

The regional significance of ca. 1400 Ma deformation of Proterozoic continental crust in the southwestern United States is controversial. Some workers have proposed that metamorphism and deformation were restricted to the margins of ca. 1400 Ma plutons. Others suggest that ca. 1400 Ma deformation and metamorphism were regional in extent. Our work supports the latter interpretation, but indicates that strain was distributed heterogeneously in New Mexico during middle Proterozoic deformation.

The Pecos complex of northern New Mexico provides an ideal site to study Proterozoic defonnation and metamorphism, as it includes dated granitoid plutons of three distinct ages: ca. 1718±5 Ma (Yavapai orogeny), 1650 Ma (Mazatzal orogeny), and ca. 1480 Ma. All three plutons exhibit a generally easterly striking, south-dipping, solid-state foliation; where present, a stretching lineation plunges shallowly. Locally developed kinematic indicators in both the oldest and youngest intrusions record dextral strike-slip shear. The foliation is variably developed in all three plutons; much· of the youngest pluton is not foliated. Definitive evidence of older structures has not been found; however, intrafolial folds in one locality and variations in orientation of the foliation in the older plutons suggest that an older foliation may have been overprinted. Deformation was accompanied by amphibolite facies metamorphism. Local retrogression of hornblende to actinolite appears to have largely post-dated deformation.

⁴⁰Ar/³⁹Ar dates on hornblende fall within the range of 1372± 10 Ma. Cooling through the closure temperature of hornblende (~500°C) therefore postdates intrusion of the ca. 1480 Ma granitoid. The timing of dextral strike-slip shear, which occurred under amphibolite facies conditions, therefore is bracketed by dates on intrusion and the cooling of hornblende. We thus have evidence of a regionally extensive, ca. 1400 Ma fabric in the Pecos complex. It is significant that this fabric formed at least in part through strike-slip shear, since previous models of this deformation event emphasized either regional shortening or extension. This work suggests that deformation was regional in extent, that strain was distributed heterogeneously, and that current models are inadequate to explain the kinematics of the structures we observe in the field.