Cenozoic Exhumation History Of The Eastern Margin Of The Northern Canadian Cordillera

New low-temperature thermochronology data from clastic sedimentary rocks in the northern Richardson Mountains, Canada, indicate significant exhumational cooling during late Eocene-early Oligocene time. Apatite (U-Th-Sm)/He (AHe) data were collected from 19 Proterozoic-Paleocene rocks across a 115 km transect. Eighty-eight single-grain AHe dates range from 16-300 Ma and are generally younger than stratigraphic ages, indicative of thermal resetting by burial. Additionally, zircon (U-Th)/He (ZHe) dates from two Proterozoic-Cambrian rocks range from 49-123 Ma and suggest burial to >160 degrees C. In contrast, ZHe dates from Jurassic sandstones are older than the stratigraphic age, which limits maximum burial to <160 degrees C. Thermal history modeling reveals three phases of cooling, during the Paleocene-early Eocene (>65-50 Ma), late Eocene-early Oligocene (40-30 Ma), and late Oligocene-early Miocene (30-15 Ma). Most samples cooled during the first and second phases, whereas the third phase is less well constrained. In general, most rocks were below the sensitivity of AHe analysis since the early-middle Miocene. The results suggest a previously unrecognized phase of inferred deformation in the northern Richardson Mountains between 40-30 Ma. Our findings contribute to previous work that recognizes Late Cenozoic deformation along the eastern margin of the Northern Cordillera. We further investigated the potential mechanisms of this widespread deformation and suggest exhumation may relate to kinematic changes of the North American plate relative to structural trends along the margin of the Northern Cordillera.Plain Language Summary We are interested in learning when and why the Richardson Mountains formed. To do this, we use a method called thermochronology, which analyzes isotopes of a mineral to determine its temperature history. During mountain building, rocks that were buried at depth are brought closer to the surface due to deformation, uplift, and erosion. Thermochronology can tell us how far in the past this process occurred. We measured 19 rock samples and used a computer program to model thermal histories based on the thermochronology dates. We found three stages of accelerated cooling in the thermal history. The oldest stage was already known: the rocks were buried until about 65 million years ago (Ma), and then started to cool due to mountain-building processes. The next younger stage of cooling occurred between 40 and 30 Ma. The youngest stage shows that some cooling occurred as late as 15 Ma. We found that the timing of deformation is similar to nearby mountain belts, which supports a widespread driving mechanism for deformation that affected a large area. We suggest that this driving mechanism could be the motion of the North American craton and its changes in pace and direction over time.