PERSISTENTLY HIGH GEOTHERMAL GRADIENT ABOVE THE OKANOGAN DETACHMENT: THERMAL WEAKENING IS A CAUSE BUT NOT TRIGGER OF OROGENIC COLLAPSE

Apatite and zircon U-Th/He theromochonometry reveals elevated geothermal gradients in the incipient hanging wall of the Okanogan Valley Fault (OVF) prior to Eocene extension. The Okanogan Valley Fault forms the western boundary of the Okanogan Gneiss Dome, a major structure in the Shuswap Metamorphic Core Complex (MCC), Washington and British Columbia. Deformation and cooling ages from the OVF by other researchers indicate extension during the interval 54-47 Ma. Hanging wall samples were collected from mid-Cretaceous (~110 Ma) plutons in the Okanogan Range west of the west-dipping OVF. For samples in three high-relief vertical transects, the elevation difference between apatite and zircon closure isotherms at 55 Ma indicate earliest Eocene upper-crustal (<2 km) paleogeotherms of 58±17, 48±27, and 28±15 °C/km. High early Eocene geothermal gradients are corroborated by consistency between themochronologically-determined paleotopography and an independently reconstructed, relict basal-Eocene unconformity west of the OVF. We estimate long-interval (time-averaged) paleogeotherms assuming sustained cooling driven by exhumation: paleogeotherms are calculated by dividing the cooling rate (time between zircon and apatite closure in a single sample) by the apparent exhumation rate (derived from vertical transects). For 10 samples at 4 sites, long-interval paleogeotherms are consistently above 50 °C/km from ~95 Ma to ~47 Ma. Other researchers have invoked high early Cenozoic geothermal gradients in the Shuswap MCC to explain reset K-Ar dates, high vitrinite reflectance, and an apparently shallow frictional-viscous transition. These findings suggest that high geothermal gradients preceded extension, perhaps by many 10s of million years. Such gradients, in excess of 50 °C/km, are sufficient to produce the mid-crustal migmatites exposed in the Okanogan dome, and support an interpretation of protracted crustal melting. High paleogeotherms are consistent with geodynamic models that infer thermal weakening prior to orogenic collapse. However, the long persistence of elevated geothermal gradients suggests that “thermally weakened” crust may have been the standard state of the orogenic hinterland, and was not the trigger of temporally focused extension.