CONSTRAINTS ON MID CENOZOIC TOPOGRAPHY, EXHUMATION AND TECTONICS WITHIN THE OKANOGAN RANGE, NORTHEAST NORTH CASCADES

In order to accurately assess changes in topography and surface elevation due to uplift or erosion, it is important to first constrain the initial topographic condition of the region of interest. The goal of our project is to refine the boundary conditions of the North Cascades prior to the onset of late Miocene exhumation by characterizing the topographic, thermal, and exhumation history of the Okanogan Range, northeast North Cascades. We are using GIS to reconstruct the topographic relief preserved within the basal Eocene unconformity, coupled with low-temperature apatite and zircon (U-Th/He) thermochronology to explore the evolution of topography within this region.

In the Okanogan Range a relict Eocene landscape is preserved as a regional unconformity capped by mid-Eocene volcanic and sedimentary deposits. GIS reconstruction of this surface reveals low to moderate relief, typically less than 1000m over 10s of kilometers, while modern relief and structural relief at the margins of the range is closer to 2000m.

Three lines of evidence suggest a high geothermal gradient (~50-70°C/km) existed in the region prior to widespread extension in the middle Eocene. This evidence includes age differences between apatite and zircon closure among samples in a vertical transect, cooling between apatite and zircon within the same sample in a vertical transect, and sample depths below the middle Eocene unconformity.

(U-Th)/He analysis of 42 apatite and 11 zircon samples is currently underway. These samples make up one north-south and four east-west transects across the range. These samples help constrain the spatial and temporal variability of the geothermal gradient, and contribute to our understanding of the tectonics along the borders of the range, including the Okanogan Valley and Pasayten faults. We also compare paleotopographic reconstructions generated from these transects to observed relief within the Eocene unconformity.