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Topographic Form of the Coast Ranges of the Cascadia Margin in Relation to Coastal Uplift Rates and Plate Subduction

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Topographic Form of the Coast Ranges of the Cascadia Margin in Relation to Coastal Uplift Rates and Plate Subduction JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B6, PAGES 12,245-12,255, JUNE 10, 1994 Topographic form of the Coast Ranges of the Cascadia Margin in relation to coastal uplift rates and plate subduction Harvey M. Kelsey,• David C. Engebretson,'•Clifton E. Mitchell,3 and Robert L.Ticknor'• Abstract. The CoastRanges of theCascadia margin are overriding the subductedJuan de Fuca/Gordaplate. We investigatethe extent to whichthe latitudinal trend in averagetopography of the CoastRanges is a functionof the latitudinalchange in attributesrelated to the subduction process.These attributes include the variableage of the subductedslab that underlies the Coast Rangesand average vertical crustal velocities of thewestern margin of theCoast Ranges for two markedlydifferent time periods, the last 45 yearsand the last 100kyr. Thesevertical crustal velocitiesare computedfrom the resurveying of highwaybench marks and from thepresent elevationof shoreplatforms that have been uplifted in thelate Quaternary, respectively. Topographyof theCoast Ranges is in parta functionof theage and bouyancy of theunderlying subductedplate. This is evidentin the fact that the two highesttopographic elements of the CoastRanges, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subductedoceanic crust. The subductedBlanco Fracture Zone in southernmostOregon is responsiblefor an age discontinuityof subductedcrust under the KlamathMountains. The northernterminus of the topographicallyhigher Klamaths is offsetto the northrelative to the positionof the underlyingBlanco Fracture Zone, the offsetbeing in thedirection of migrationof the fracturezone, as dictated by relativeplate motions. Vertical crustal velocities at thecoast, derivedfrom benchmark surveys, are as much as an orderof magnitudegreater than vertical crustalvelocities derived from uplifted shore platforms. This uplift rate discrepancy indicates thatstrain is accumulatingon theplate margin, to be releasedduring the nextinterplate earthquake.In a latitudinalsense, average Coast Range topography is relativelyhigh where benchmark-derived, short-term vertical crustal velocites are highest. Because the shore platform verticalcrustal velocites reflect longer-term, permanent uplift, we inferthat a smallpercentage of theinterseismic strain that accumulates as rapid short-term uplift is notrecovered by subduction earthquakesbut rathercontributes to rockuplift of theCoast Ranges. The conjecturethat permanentrock uplift is relatedto interseismicuplift is consistentwith the observationthat thosesegments of the subductionzone subject to greaterinterseismic uplift ratesare at approximatelythe samelatitudes as thosesegments of the the CoastRanges that have higher magnitudesof rock uplift over the long term. Introduction The topographyof the Coast Rangesis the net result of uplift of rocks due to plate convergenceand erosion by surface The coastal ranges of northern California, Oregon and processes. We hypothesize that latitudinal variations in Washingtonform a belt of elevatedtopography along the Coast Range topographyare a functionof changingphysical northwest coast of the United States. This area includes the attributes of the Cascadia subduction zone from north to south. northern California Coast Ranges, the Klamath Mountains, To test this hypothesisempirically, we compare latitudinal the OregonCoast Ranges, the Willapa Hills, and the Olympic variation in averagetopography to the latitudinalvariation in Mountains(Figure 1). For our purposes,we collectivelycall vertical crustal velocity at the coast, as measuredby uplift these features the Coast Ranges. The Coast Ranges are rates relative to sea level, and to the latitudinal variation in situatedon the leadingedge of the North Americanplate, and the age of the plate being subductedbeneath the CoastRanges. crest elevations are 120-200 km east of the deformation front For our analysis, we present data on latitudinal and of the Cascadia subduction zone. The ranges are thus longitudinal trends in the topographyof the Coast Ranges, overridingthe subductedJuan de Fuca/Gordaplate (Figure1). variation in uplift rate along the westernmargin of the Coast Ranges, and latitudinal variation in the relative elevation of the subductingplate beneath the Coast Ranges, as derived 1Departmentof Geology,Humboldt State University, Arcata, from the relationshipbetween lithospheric subsidence and age California. of oceanic crust. 2Departmentof Geology,Western Washington University, For the coastal uplift rates, the data include surfaceuplift Bellingham. 3Departmentof Geological Sciences, University of Oregon,Eugene. rate for two differenttime periodsusing datums appropriate to thesetime periods:the last 45 yearsof uplift usingresurveyed Copyright1994 by the AmericanGeophysical Union. highway benchmarksand the last 80,000-125,000 years of uplift using uplifted shore platforms. Surface uplift rates Papernumber 93JB03236. derived from both geodeticand shoreplatform surveysare 0148-0227/94/93JB-03236505.00 referencedto eustatic high standsof sea level, where eustatic 12,245 12,246 KELSEY ET AL.: TOPOGRAPHIC FORM OF COAST RANGES A 50 ø .:::::::.•:::.......... ..:.:.:.:.::.:.:....:.:.:.:.:. \ JUAN DE FUCA 45 ø PLATE 7 PACIFIC PLATE 40 ø 130 ø CASCADIA CFW DEFORMATION YB c FRONT CA WINDOW I I I I I I I I I I I I I 126 ø I I Figure 1. (a) Locationmap of the Pacific Northwestin the contextof plate tectonics;triangles are Cascade volcanoes.(b) Locationmap of the coastalranges situated on the leadingegde of the North Americanplate betweenthe Mendocinoescarpment and the Canadianborder. Trianglesdenote selected Cascade volcanoes. CR, mouthof the ColumbiaRiver; CFW, CapeFoulweather; YB, YaquinaBay; CA, CapeArago; CB, Cape Blanco;CF, Cape Ferrelo;HB, HumboldtBay; CM, Cape Mendocino. (c) Shadingshows the area of the CoastRanges, 65 arc min of longitudewide, for which the digital elevationmodel ETOPO-5 was employed to computeaverage topography. The box depictsthe size of the computationalwindow (65 arc minx 15 arc min) that was employed for each individual calculation of average topography. See text for further explanation. KELSEY ET AL' TOPOGRAPHIC FORM OF COAST RANGES 12,247 sea level is an expression of Earth's geoidal surface. The three adjacentlines of latitude (computationalwindow (Figure geodetically derived uplift is in referenceto present sea level lc) is 65 arc min of longitude x 15 arc min of latitude on a 5 and the shore platform uplift is in reference to the arc min grid), with 13 out of 39 of the grid pointschanging for apppropriatelate Plesitoceneeustatic sea level high stand(see each calculation. Thus each calculationof averagetopography below). Becauserock uplift relative to the geoid is equal to representsan areaof about2400 km2 (27.8 km x 86.6 km). surface uplift minus exhumation [England and Molnar, 1990] Because we will subsequentlycompare uplift rates at the and because there has been negligible erosion of either the coast to trends in averagetopography for the Coast Rangesas shoreplatforms or the highway benchmarks,the coastaluplift a whole, we determined whether north-southtrends in average rates that we discussare both uplift rates of rock relative to the topographyfor the coastal (western) side of the Coast Ranges geoid as well as surfaceuplift rates. Subsequentuse of the term are sinfilar in form to trends of average topography for the "uplift rate" should thus be unambiguous. Using uplift rate Coast Ranges as a whole. Using the same computational data integratedover the last 45 years and that integratedover technique but applied to narrower widths, we determined the last 80,000-125,000 years, we can compare interseismic average topographyfor a 25-arc-min-wide swath on the west uplift with long-term uplift at the coast along the Cascadia side of the Coast Ranges, and we also computed average margin. topography for a nonoverlapping 25-arc-min-wide swath on the eastern side of the Coast Ranges. Comparing the western Average Topography Along the Crest profile to the profile for the Coast Ranges as a whole (Figure of the Coast Ranges 2), the profiles show that first, the highest average topography is between 41 ø and 45 ø, correspondingto the We calculated average topography for the Coast Ranges Klamaths Mountains; second, average topographyfluctuates (Figure 2) using the public domain 5 arc min digital elevation around 200 m between 43.5 and 46ø; and third, a spike in model ETOPO-5 [National GeophysicalData Center, 1988]. average topographyoccurs at 47.5o-48ø , correspondingto the Using ETOPO-5, we calculated a running average for the Olympic Mountains. The two profiles differ considerably topography of the Coast Ranges in a north-south trending along two segmentswhere coastal plains or embaymentsare swath extending from the coast inland for 65 arc min of unusually wide, extending as much as 20 km inland. These longitude(Figure l c). The swathextends between latitudes 41 ø segments include the southern Oregon coastal plain (Cape and 47øN along the trend of the Coast Ranges, for a total Blanco to Coos Bay; latitudes 43ø-43.3ø) and the embayments length of 778 kin. The averagewidth of the swathis 86.6 km of southernWashington (Columbia River, Willapa Bay, Grays (90.9 km at latitude 41 ø and 82.2 km at latitude 47ø). Each Harbor; latitudes 46ø-47ø). With the exception of these individual calculation of average topography at 5-arc min coastallowlands, the westernportion of the Coast Rangeshas latitudinal intervals is the running average of the average latitudinal
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