TOPOGRAPHIC FORM OF THE COAST RANGES OF THE CASCADIA MARGIN IN RELATION TO COASTAL UPLIFT RATES AND PLATE SUBDUCTION

The Coast Ranges of the Cascadia margin are overriding the subducted J uan de Fuca/Gorda plate. We investigate the extent to which the latitu dinal trend in average topography of the Coast Ranges is a function of the latitudinal change in attributes related to the subduction proces s. These attributes include the variable age of the subducted slab tha t underlies the Coast Ranges and average vertical crustal velocities o f the western margin of the Coast Ranges for two markedly different ti me periods, the last 45 years and the last 100 kyr. These vertical cru stal velocities are computed from the resurveying of highway bench mar ks and from the present elevation of shore platforms that have been up lifted in the late Quaternary, respectively. Topography of the Coast R anges is in part a function of the age and bouyancy of the underlying subducted plate. This is evident in the fact that the two highest topo graphic elements of the Coast Ranges, the Klamath Mountains and the Ol ympic Mountains, are underlain by youngest subducted oceanic crust. Th e subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountai ns. The northern terminus of the topographically higher Klamaths is of fset to the north relative to the position of the underlying Blanco Fr acture Zone, the offset being in the direction of migration of the fra cture zone, as dictated by relative plate motions. Vertical crustal ve locities at the coast, derived from bench mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derive d from uplifted shore platforms. This uplift rate discrepancy indicate s that strain is accumulating on the plate margin, to be released duri ng the next interplate earthquake. In a latitudinal sense, average Coa st Range topography is relatively high where bench mark-derived, short -term vertical crustal velocites are highest. Because the shore platfo rm vertical crustal velocites reflect longer-term, permanent uplift, w e infer that a small percentage of the interseismic strain that accumu lates as rapid short-term uplift is not recovered by subduction earthq uakes but rather contributes to rock uplift of the Coast Ranges. The c onjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduct ion zone subject to greater interseismic uplift rates are at approxima tely the same latitudes as those segments of the the Coast Ranges that have higher magnitudes of rock uplift over the long term.