PERSISTENT HIGH TOPOGRAPHY AND CLIMATE INDUCED INCISION IN THE NORTHEAST NORTH CASCADES

The Okanogan Range in northern Washington and southern British Columbia defines the northeast boundary of the North Cascades. The topography of the region is bimodal: the moderate elevation, low-relief upland is a relict of a 55 Ma (Eocene) landscape, while the range has since been incised by several valleys with local relief up to 2 km. Low-temperature apatite and zircon (U-Th)/He thermochronology coupled with GIS reconstructions of the Eocene surface based on unconformable deposits from across the Okanogan Range help to further constrain the timing of incision to within the last 20 m.y. The timing of incision is generally consistent with mid to late Miocene increased exhumation rates in the North Cascades (Reiners et al., 2002) and post-10 Ma exhumation in the British Columbia Coast Mountains (e.g. Farley et al., 2001). However, it is typically difficult to distinguish exhumation related to tectonic surface uplift from climate-change-induced increases in erosion acting on persistent uplands (Molnar and England, 1990). Fortunately, the relative paleoelevation of the Okanogan Range is further constrained by the mid-Miocene Columbia River Basalt group, which onlaps the southern boundary of the range. Preliminary GIS work suggests the contact between Grande Ronde and Priest Rapids members within the Columbia River basalts is essentially flat lying; varying by less than 0.3 km over more than 25 km. The subhorizontal nature of the Columbia River basalts at the southern margin of the Okanogan Range implies limited differential surface uplift since ~15 Ma. The evidence supports the hypothesis that the Okanogan Range and North Cascades are regions of persistent moderate to high elevations that existed as uplands during the eruption of the Columbia River basalts in the middle Miocene. We therefore propose that the post-20 Ma incision of the Okanogan Range represents a response to changes in erosive power driven by climate, and is not a result of tectonic rejuvenation.