POLYGENETIC TOPOGRAPHY OF THE WASHINGTON CASCADES AND TOPOGRAPHIC SIGNATURE OF A GLACIAL BUZZSAW?

We evaluate three existing models for Washington Cascade range topographic development using new techniques to analyze topography, geology, and exhumation. Models include: 1) post-Miocene uplift superimposed on a long-existing “Ancestral” Cascades, 2) post-Miocene uplift of an initially low-relief western Washington, and 3) a polygenetic model of high northern but low southern Cascades prior to the Miocene and additional syn- and post-Miocene uplift. Based on our analyses, we favor the polygenetic model, proposed by J.Hoover Mackin. South of Snoqualmie Pass, warping of the east-derived 15.5 Ma Columbia River Basalt (CRB) indicates that much of the modern elevation may be a result of post-Miocene uplift, the basalt is not limited to the Cascade flank, reaching nearly as far west as the modern drainage divide, and exhumation rates from apatite (U-Th)/He (AHe) data are consistent with the projection of the CRB over the range. These observations require relatively subdued relief in the southern Cascades during the late Miocene. In the northern Cascades, the CRB is limited to the far eastern edge of the range, CRB deformation only reflects 30-60% of the total relief, and AHe exhumation rates are not consistent with a greater westward CRB extent. Despite the polygenetic nature of the topography, the north and south Cascades have similar topographic form, with maximum peak elevations forming a west-dipping “surface,” with few non-volcanic peaks rising < 200 m above their neighbors. The trend in peak elevations is strikingly parallel to the trend in the glacial equilibrium line altitude (ELA), but is independent of strong variations in precipitation, exhumation rate, or erosion index. Our analyses show that the peak concordance may be due to the effects of a “glacial buzzsaw,” in which glacial erosion has acted to limit peak elevations to no higher than 600 m above the Quaternary average ELA, independently of prior topography, exhumation or uplift rate.