RAPID EARLY EOCENE EXTENSION FOLLOWING LARAMIDE SHALLOW SUBDUCTION IN THE NORTHERN ROCKY MOUNTAINS REGION

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in early Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40Ar/39Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. U-Pb zircon and 40Ar/39Ar data from the Anaconda complex, Priest River complex indicates a similar timing for the onset of major extension and exhumation. 40Ar/39Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. Rapid Eocene extension and exhumation of middle-crustal rocks in the northern Rockies was concentrated in areas that experienced voluminous early Eocene mid-crustal magmatism. Dextral transtension leading to the exhumation of metamorphic core complexes was also synchronous with the extensive Absaroka-Challis-Colville-Kamloops magmatism. On a larger scale, transtension along the northern edge of the area affected by the Laramide flat slab, occurred along the southern extent of early Eocene transtension in the Canadian Cordillera. Extension was probably initiated by a change in subduction parameters such as the demise of the Laramide flat slab or interaction with the Farallon and Kula (or Resurrection) triple junction. This caused a rapid influx of heat from the asthenosphere as a slab gap opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.