Microstructural constraints on the timing of Proterozoic deformation in Central New Mexico

Abstract

The nature and extent of deformation associated with 1.4 Ga tectonism in the south-western USA are poorly understood. Two models have been proposed. Both agree that Proterozoic crustal accretion occurred at 1.65 Ga and that the rocks remained at mid-crustal conditions (c. 12 km depth) until 1.4 Ga. However, one model suggests that 1.4 Ga deformation was regionally extensive, the other that it was localized around 1.4 Ga plutons. Following 1.4 Ga tectonism, the crust cooled below 300 °C. Detailed studies of quartz mylonite microfabrics in samples both adjacent to and removed from 1.4 Ga plutons in the Manzano Mountains, central New Mexico, are used to discriminate between these models of mid-Proterozoic thermotectonic history. In this area, as in much of northern New Mexico, the metamorphic conditions prior to emplacement of 1.4 Ga plutons were 500 °C and 4 kbar. The quartz mylonite microfabrics include ribbon grains, recrystallized grains with serrated boundaries, and strong c-axis crystallographic preferred orientations, which indicate no post-deformational modification. All of these microfabrics are consistent with deformation at upper greenschist/lower amphibolite facies conditions, and could have formed during either 1.65 or 1.4 Ga tectonism. Microfabrics formed during 1.65 Ga tectonism, however, should have been substantially modified by annealing recrystallization during residency in the middle crust and/or thermal/mechanical effects associated with 1.4 Ga tectonism. The observed microstructures are consistent with regional deformation associated with metamorphism at 1.4 Ga. The effects of deformation at 1.4 Ga in New Mexico are therefore more widespread than previously thought.