Long-term Orogenic Growth, Decay, and Rebirth: A Cretaceous-Recent Exhumational Lag Time Perspective from Arctic Alaska

The Colville foreland basin of Arctic Alaska contains >3 million km³ of Cretaceous-Recent sediment that spans multiple phases in the development of the >2000-km-long Brooks Range and Chukotka orogens. Detrital zircon fission-track (ZFT) and U-Pb double-dates from the foreland stratigraphic succession quantify the lag time between exhumational cooling <250°C in the orogen and deposition in the basin, and provide a consistent, spatially-averaged dataset for evaluating long-term orogenic evolution. In the Early Cretaceous, U-Pb ages in the western Colville basin are consistent with Chukotkan provenance and ZFT lag times as short as 4 Myr suggest erosion rates >1 km/Myr, akin to the most rapidly exhuming modern orogens. The large percentage of young detrital ZFT ages is consistent with previous interpretations of widespread shortening during arc-continent collision and bedrock cooling ages throughout the orogen. In contrast, in the central Colville basin during the Early Cretaceous, much longer lag times that decrease upsection suggest prolonged unroofing of pre-orogenic material of possibly mixed Brookian and Chukotkan provenance. In the Late Cretaceous, U-Pb ages suggest that Chukotka-derived sediment was no longer a major source to the basin. Moreover, lag times in the central Colville basin increased relative to a static ZFT age of ~140 Ma and suggest minimal exhumation during orogenic decay. Late Cretaceous quiescence is consistent with decreased sediment flux during filling of relict accommodation and a general lack of coeval bedrock cooling ages in the orogen. In the Cenozoic, ZFT lag times in the central Colville basin decreased and were consistently ~20 Myr, suggesting erosion rates of ~0.4 km/Myr during rejuvenation of the Brooks Range. However, the small percentage of young ZFT ages suggests a restricted region of deep, rapid exhumation, consistent with previous interpretations of thick-skinned contraction involving modest crustal shortening. In general, we show how lag time trends characterize long-term orogenic evolution by quantifying exhumation rates and extents for regions that have long since eroded away. Our Alaska example corroborates decades of geologic study and highlights the persistence of the Brooks-Chukotka orogen with implications for Arctic tectonics, climate, and resources.