New Mexico Geological Society Annual Spring Meeting — Abstracts

Here we present new mapping, Proterozoic stratigraphy, and structural data from the Ojo Caliente and La Madera quadrangles, New Mexico. These data allow for a revised interpretation of the kinematics and timing of Proterozoic deformational events, and insights into rheology of middle crustal flow. We also examine middle-late Miocene throw along the Vallecitos fault and propose that the location of the fault may be controlled by a Proterozoic shear zone.

Proterozoic units are divided into two groups (in ascending order): 1) the Vadito Group, consisting of aluminous quartzite, schist, metarhyolite, and amphibolite, and 2) the Hondo Group, which includes the Ortega Quartzite. These are intruded by Tres Piedras Granite (~1693 +/- 11 Ma). Metamorphic mineral assemblages indicate Proterozoic units experienced greenschist to amphibolite-grade metamorphism (475-550°C at 12-16 km depth) with metamorphic grade increasing southward. These units experienced three episodes of progressive, ductile deformation D₁, D₂, and D₃. D₁ and D₂ are characterized by folds, fabrics, and shear zones formed as the result of northeast-directed shortening during the Mazatzal Orogeny, ca. 1650 Ma. D₃ is characterized by east-plunging folds formed syntectonically with 1420 Ma granites, and resulted in continued shortening, triple-point metamorphism, and fabric development. Geometry of the resulting regional composite D₂/D₃ structures us controlled by massive Ortega Quartzite that is folded into overturned synclinoria with shallow plunges. D₁/D₂ in the underlying Vadito Group record intense layer-parallel shear before and during the formation of the large synclines. This rheologic contrast produced different deformation styles in different rock types and allowed shear zone detachments to develop near the base of the quartzite.

Detailed mapping of Proterozoic lithologies suggests that the location of the Miocene-age Vallecitos fault may be controlled by this older Proterozoic shear zone. This shear zone is indicated by low-angle truncation of Vadito Group units. The brittle fault reactivation, associated with Rio Grande Rift extension, offsets middle Miocene strata of the Tesuque Formation with west-side-down normal displacement.