EXHUMATION AND TOPOGRAPHIC DEVELOPMENT OF THE OKANOGAN RANGE, NE NORTH CASCADES

Recent studies in the Coast Mountains, BC have linked rapid valley incision and relief production to 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 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 ~60-70ºC 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. While this speculative interpretation is only one possible explanation, it is supported by the presence of a relict, moderate relief mid-Eocene landscape preserved as erosional remnants on high peaks across the range.

Work is currently underway to quantify the relief preserved within the relict Eocene landscape, to place the Okanogan Range in a regional context based on its exhumation history, and to evaluate the effects of increasing relief on isostatically driven peak uplift across the North Cascades.