Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Abstract

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c. 1.65 Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42 Ga and probably during collisional assembly of island arc crust into new (1.7–1.6 Ga) continental lithosphere. Supracrustal rocks record 650–750 °C, 6–8 kbar metamorphism (M2); these high temperatures may have been reached during sandwiching between c. 1.65 Ga granitoids. However, the early history has been obscured by renewed tectonism at c. 1.4 Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4 kbar (M3). Metamorphic temperature variations from uppermost‐amphibolite to amphibolite facies rocks may be associated with c. 1.65 and/or 1.4 Ga plutonism, but not to a 1.4 Ga extensional shear zone as previously proposed. Syn‐ and post‐1.4 Ga contraction is suggested by high‐ and low‐temperature microstructures showing top‐to‐the‐south‐east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c. 1.65 Ga, but that the present fabric also records 1.4 Ga tectonism involving high‐T metamorphism and fabric reactivation.