Surface Uplift and Exhumation Trends of the Washington Cascade Range From Warping of the Grande Ronde Basalt and Apatite (U-Th)/He Ages

The warping of Columbia River Basalt (CRB) on the eastern flank of the Washington Cascade Range has long been used to suggest the extent of post-Miocene surface uplift of the range, and has raised the question of the extent of paleorelief of the Cascades prior to and during the eruption of CRB. We superimpose the outcrop pattern of the 17-15.5 Ma Grande Ronde Basalt of the CRB onto a 10-m DEM of the Cascades to constrain post-CRB surface uplift, and project the tilted CRB surface westward over the range to estimate the magnitude and rate of post-CRB exhumation. We compare these exhumation rates to rates estimated from 20 apatite (U-Th)/He ages and a corresponding 1-D thermo-kinematic model coupled with a cooling rate-dependent kinetic model. In the southern Washington Cascades, the CRB is found in the core of the range and ramp up from elevations of about 500 m just east of the range to elevations of 1900-2100 m near the core. Excluding Quaternary volcanoes, the maximum elevation in that area is <2500 m, suggesting low pre-CRB elevations with much of the range's present height due to post-CRB uplift. In the central Cascades, (U-Th)/He ages indicate increasing exhumation rates from about 0.02 to 0.2 mm/yr from east to west, which are compatible with rates estimated by projecting the now-tilted CRB surfaces over the range. To the north, the western limit of CRB is deflected progressively east, and north of Snoqualmie Pass CRB does not reach elevations higher than 1000 m, compared to maximum non-volcanic elevations of over 2200 m within the range. Two hypotheses accounting for the change in south-to-north distribution of the CRB in and adjacent to the Cascades are: (1) CRB ponded against a pre-existing high surface north of Snoqualmie Pass, or, (2) CRB flowed across low topography throughout the range but was uplifted and eroded more extensively in the north, perhaps due to greater erosion in this area by extensive Pleistocene glaciation. Analysis of additional helium samples collected from the northern Cascades should allow the distinction between these two possibilities; exhumation rates comparable to the rest of the range may indicate a pre-existing topographic high, while significantly higher exhumation rates may be compatible with extensive post-uplift erosion.