Eocene Extension in the Northen Rockies

Eocene metamorphic core complexes comprise a belt of hyper-extended terrains exposed from western Montana to northeastern Washington and British Columbia.  Exposure of the core complexes resulted from widespread extension that began ca. 55 to 53 Ma during the final stages of or immediately afterwards shortening ended in the Cordilleran thrust belt (Foster et al., 2007). Several hypotheses have been proposed to explain the onset of rapid Eocene extension and associated magmatism in the northern Cordillera including:  orogen collapse, asthenospheric upwelling within a slab window, rapid rollback of Farallon slab, regional transtension associated with northward motion of the Kula plate, or accretion of the Siletzia terrane (e.g., Armstrong et al., 1977; Severinghaus and Atwater, 1990; Morris et al., 2000 Breitsprecher et al., 2003; Haeussler et al., 2003; Foster et al., 2007; Humphreys, 2009a).

We are currently focusing research on the Anaconda metamorphic core complex, which is the eastern-most major core complex in the Cordilleran. We are applying detailed geochronology, thermochronology, and structural analysis to unravel the exhumation history and kinematics in this complex. Our results have implications for the timing, duration, and rate of extension in the Anaconda complex, as well as the Eocene tectonic and magmatic environment in the northern Cordillera.