Thermal evolution of the Shames River mylonite zone, Coast Mountains, British Columbia

Beverly Marie House, University of Texas at El Paso


The Shames River mylonite (SRM) zone is a major extensional shear zone that represents the tectonic boundary between the Coast Plutonic Complex and the Stikine accreted terrain in western British Columbia. Based on U-Pb and 40Ar/39Ar geochronology, the SRM zone was active between 54 and 48 Ma. This study applied the hornblende-plagioclase thermometer to orthogneiss and amphibolite samples collected from within the Shames River mylonite zone. Conventional thermobarometers were also applied to rocks from the hanging wall and footwall of the shear zone. Rocks of the Central Gneiss Complex in the footwall followed an isothermal decompression pressure-temperature (P-T) path from ∼8 kbar to ∼3 kbar at temperatures above ∼800°C. Rocks of the Stikine Terrane in the hanging wall followed isobaric heating and cooling P-T paths through the andalusite stability field with peak metamorphic P-T conditions restricted to the margins of plutons. The Shames River mylonite deformed plastically at temperatures of ∼750°C and then cooled during shearing, deforming by brittle failure at the end of deformation. Recrystallized pseudotachylite from plutons immediately above the mylonite records greenschist facies metamorphic conditions of ∼400°C and 2 kbar indicating the onset of brittle deformation between the two terranes at ∼5 to 6 km depth between 51 and 48 Ma. The combined metamorphic data set indicates approximately 16 km of uplift of the footwall during extension. These results show that contrasting metamorphic and tectonic histories between the Stikine Terrane and central gneiss complex are the result of large scale extensional deformation that juxtaposed upper crustal rocks with deep crustal rocks during arc magmatism. ^

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Recommended Citation

House, Beverly Marie, "Thermal evolution of the Shames River mylonite zone, Coast Mountains, British Columbia" (2004). ETD Collection for University of Texas, El Paso. AAIEP10562.