- UW Department of Geology and Geophysics
In the Rincon Range, north of Mora, New Mexico, a relatively abrupt regional change in dominant fabric orientation occurs within Paleoproterozoic rocks which are nearly continuously exposed for ∼70 km in adjacent Laramide uplifts of the southern Sangre de Cristo Mountains. Near the village of Guadalupita, these rocks display a smooth but abrupt south-to-north change from subhorizontal to subvertical dominant foliation (S2) over a distance of ∼2 km. This change in dominant fabric orientation coincides with a regional change in metamorphic grade from near-granulite grade (∼650°C, 4–6 kbar) in rocks with a subhorizontal fabric to amphibolite grade (∼500°C, 4–6 kbar) in rocks with a subvertical fabric. The shallowly dipping S2 fabric and highest temperature assemblages are both centered around an ∼1682-Ma granitic orthogneiss, the Guadalupita pluton, which engulfs the overturned lower limb of an ∼15 kmscale, north-facing F1 fold. Porphyroblast-matrix microstructural studies suggest that S1 and S2 formed during a progressive event that was synchronous with pluton emplacement and regional metamorphism at ∼1982 Ma. Granite emplacement and its incorporation into the core of a fold-nappe at ∼1.68 Ga appears to have facilitated subhorizontal S2 fabric development late during the progressive S1/S2 event and heat from the granite enhanced regional metamorphic conditions to create the ∼150° C temperature gradient. However, metamorphic monazites aligned in S2 yield U-Pb dates of ∼1421 Ma, suggesting that monazite grew during renewed tectonism that reactivated the older subhorizontal fabric during ∼1.42-Ga regional metamorphism. Present geometries therefore reflect a superposition of major tectonometamorphic events at 1.68 and 1.42 Ga. This study suggests that: (1) large temperature gradients around plutons can cause regionally heterogeneous middle-crustal pressure-temperature–time–deformation (P–T–t–D) paths; (2) plutons may both localize and be localized by subhorizontal shear zones; and (3) middle-crustal rheologies are strongly influenced by thermal weakening near plutons.