Earth Science Student Research Grants Recipients 2006

Exhumation and topographic development of the Okanogan Range, northeast North Cascades

Owen Callahan

Recent studies have linked relief production in the Coast Mountains, BC to rapid valley incision associated with glaciation ~2 Ma. The effect of relief production on isostatically driven peak uplift is poorly constrained, because there is little information about pre-incision topography. The goal of this study is to better constrain the boundary conditions of the North Cascades and BC Coast Mountains, by identifying pre-incision topography and exhumation trends within the Okanogan Range. Apatite (U-Th)/He cooling ages were obtained from 13 samples from the Okanogan Range, northeast North Cascades. Five samples were collected in a 1.7 km vertical transect on the eastern border of the range, and eight samples are from a broad, N-S transect sub-parallel to the range front. Sample ages from the vertical transect vary directly with elevation, ranging from a low of 32.9 Ma (+12.6 -9.11 m.y.) at 0.5 km to 62.1 Ma (+1.50 -1.52 m.y.) at 2.2 km. The vertical transect was collected over a horizontal distance of ~3.6 km. Over this short horizontal distance the effect of topography on the shape of the ~60-70°C closure isotherm is assumed to be negligible, therefore the slope of the line in an age-elevation plot is presumed to approximate the rate of exhumation. The slope calculated from these samples suggests an exhumation rate of 0.05-0.06 km/m.y. from ~62 Ma until ~33 Ma. Sample ages from the horizontal transect range from 25.0 Ma (+1.63 -1.90 m.y.) to 63.0 Ma (+7.11 -6.83 m.y.). The ages of samples older than ~40 Ma vary inversely with elevation, while samples younger than ~40 Ma show a positive correlation between age and elevation. This apparent trend could be attributed to a period of decreasing relief from ~63 Ma to 40(?) Ma, followed by increasing relief at some time either during or after the late Eocene.

GIS reconstruction of the Eocene landscape by interpolation of preserved unconformable deposits suggests paleorelief of ~1500 m, and allows for refined thermochronology interpretations by identifying sample paleodepth. When combined, these two methods reveal an age distribution that is possibly the result of a long-lived high geotherm or late Eocene top-to-the-west tilting and normal faulting on the eastern border of the range following the development of the Okanogan Dome, a metamorphic core complex ~30 km to the east. These results illustrate the importance of comparing thermochronology data with surficial deposits as a means of evaluating the paleogeothermal gradient.

Ongoing thermochronology will help refine the exhumation and paleorelief reconstructions of the Okanogan Range, especially by evaluating the high geotherm or top-to-the-west tilting hypotheses presented above. Mapping of the unconformable Eocene deposits will also work in conjunction with the thermochronology data. The refined topographic model will be used to evaluate the effects of increasing relief on isostatically driven peak uplift across the North Cascades.