HELIUM AND FISSION-TRACK THERMOCHRONOLOGY OF THE WASHINGTON CASCADES

Although it has been the locus of ongoing arc magmatism since the Eocene, the Washington Cascades differ from the southern part of the arc in Oregon and California in that deep-seated rock uplift has produced high surface elevations and topographic relief, and abundant exposure of crystalline rock, rather than volcanic burial or edifice construction. We have measured apatite (U-Th)/He and fission-track (AFT) ages from crystalline bedrock throughout the Washington (and southern B.C.) Cascades to constrain the timing and rates of exhumation of the upper ~2-5 km of crust. With very few exceptions, apatite He ages of samples from the east and west sides of the range are distinct: west flank samples are 6-12 Ma, while east flank samples (Mt. Stuart to Glacier Pk. Region) vary from ~15-60 Ma. Vertical transects require Late Miocene exhumation rates of at least 0.5-1.0 mm/yr (possibly faster) on the west flank, but pre-Miocene exhumation rates of ~0.1 mm/yr (and slower) on the east flank. Typically, AFT ages (~40-80 Ma; 16-30 in the North Cascades) are signficantly older than apatite He ages throughout the range. One exception is a sample with indistiguishable 60-62 Ma AFT and He ages on the east flank near Mt. Stuart. The combination of high local relief (2-3 km) and old cooling ages on the east side suggest a recent increase in uplift/exhumation rates in this region. Taken together, these data indicate an increase in exhumation rates in the Late Miocene (9 +/- 3 Ma), especially on the west side of the range, where rocks were exhumed from 2-5 km depths at rates of at least 0.5 mm/yr. Prior to this, exhumation was relatively slow (< ~ 0.1 km/myr since the Early Eocene). This overall exhumation history is very similar to that of the Coast Mountains of B.C. and southeast Alaska, some 1500 km to the north. Onset of relatively rapid exhumation in the Late Miocene over this distance of the coastal northwestern Cordillera requires a large-scale mechanism at this time such as change in relative plate motions, lower-crustal delamination, or possibly increased erosional exhumation caused by climatic changes.