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Mid-crustal Late Cretaceous metamorphism in the Nason Terrane, Cascades crystalline core, Washington, USA; implications for tectonic models


Mid-crustal Late Cretaceous metamorphism in the Nason Terrane, Cascades crystalline core, Washington, USA; implications for tectonic models



Memoir - Geological Society of America 200: 211-232



The Cordilleran Coast Plutonic Complex comprises the roots of a middle Cretaceous to Paleogene magmatic arc and orogenic belt that extends from the Yukon Territory to Washington State. Exceptional rock exposure and mineral preservation have made the Cascades crystalline core, the southernmost exposure of the Coast Plutonic Complex, a laboratory for understanding mid-crustal processes in contractional magmatic arcs. Perhaps surprisingly, after decades of study, fundamental tectonic models for the Late Cretaceous evolution of the core remain in question. This study evaluates tectonic models using phase equilibrium modeling, thermobarometry, and high-precision geochronology to constrain present crustal structure and the magnitudes, rates, and lateral preservation of Late Cretaceous regional metamorphism across the Nason terrane, Wenatchee block, Cascades crystalline core. Garnet Sm-Nd ages of 88-86 Ma restrict the last regional metamorphism to no less than 3 m.y. after emplacement of the 96-91 Ma Mount Stuart batholith. These ages and petrologic data reflecting only negligible to moderate pressure increases (0-3.6 kbar) during garnet growth indicate a time lag between a fairly rapid pressure increase (up to 0.5 kbar/m.y.) subsequent to low-pressure contact metamorphism associated with emplacement of the Mount Stuart batholith and temperature increases during initial garnet growth. This thermal relaxation signature supports a thrust-loading model for post-Mount Stuart regional metamorphism. The lateral extent and magnitude of regional metamorphism across the Nason terrane and Mount Stuart domain offer additional support for a tectonic loading model for regional metamorphism. Peak recorded pressures decrease from >8 kbar in the northeast of the terrane to 5 kbar southwest of the Mount Stuart batholith, compatible with loading by a tapered thrust sheet, followed by exhumation and shortening after peak metamorphism. The lack of structural evidence for thrusting and steepening of the paleobarometric gradient across the Nason terrane north of the batholith suggest that strain-partitioned folding was dominant at the exposed crustal level during and after the last regional metamorphism. Thus, a tectonic model compatible with metamorphic P-T data may include a decoupling of upper-crustal and mid-crustal shortening accommodation mechanisms during Late Cretaceous regional metamorphism.

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