PROGRESSIVE PALEOCENE-MIDDLE EOCENE EXHUMATION OF THE YUKON-TANANA UPLAND DURING INTRACONTINENTAL TINTINA FAULT STRIKE-SLIP AND ASSEMBLY OF THE CORDILLERA

Assembly of the North America Cordillera occurred through accretion and translation along terrane boundaries that drove intracontinental deformation and exhumation. The >2000-km-long, lithospheric-scale Tintina fault-Northern Rocky Mountain Trench marks the inboard extent of accreted terranes and forms the northern boundary of the Yukon-Tanana Upland (YTU). Published studies indicate >430 km of ~65–42 Ma Tintina dextral offset at a >19 km/Myr implied rate, but the fault’s role in focusing regional exhumation is less well understood. We characterize YTU exhumation patterns with detrital apatite fission-track (dAFT) ages from 13 modern river drainages that cover >45,000 km² of eastern Alaska. We resolve constituent dAFT age populations with mixture modeling. Regionally widespread dAFT age populations record YTU exhumation during Paleogene Tintina strike-slip (50% of grains), mid-Cretaceous extension (30%), and Permo-Triassic collision (10%). Paleogene age populations progressively young from ~65 to 45 Ma northwestward over a 370-km-long, Tintina fault-parallel transect. In addition, bedrock age-elevation data indicate increased exhumation ~60 Ma on the eastern portion and ~42 Ma at the western end of the transect, suggesting that detrital ages are a proxy for Cenozoic exhumation over broad regions where bedrock data are unavailable. The detrital and bedrock data indicate Paleogene exhumation from 110°C crustal temperatures, however, heterogeneity is suggested by the preservation of Late Cretaceous paleo-surfaces in some areas. Paleogene YTU exhumation is coeval with deposition in the 7-km-thick Yukon Flats basin and broad upright folding of Paleozoic-Paleogene rocks in the YTU. Our data suggests Paleogene YTU exhumation propagated westward at a ~22 km/Myr rate similar to the implied Tintina slip rate and is consistent with dextral transpression between the Denali and Tintina faults. Cessation of major Tintina slip and YTU exhumation in the Eocene coincided with a dramatic three-fold drop in northward plate velocity during the demise of spreading between the Kula and Pacific plates. Major slip subsequently migrated south onto the Denali and Fairweather faults, which bound a region where younger exhumation due to Oligocene-Recent Yakutat flat-slab subduction is widespread.