Late Quaternary Stream Incision and Uplift in the Forearc of the Cascadia Subduction Zone, Western Oregon

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B10, PAGES 20,193-20,210, OCTOBER 10, 1995

Late Quaternary stream incision and uplift in the forearc of the Cascadia subduction zone, western Oregon

StephenF. Personius u.s. GeologicalSurvey, Denver, Colorado

Abstract. Documentationof a latestPleistocene/earliest Holocene episode of strath formationand fluvial aggradationin the OregonCoast Range provides a datumfrom which long- termbedrock stream incision rates are determined.Variations in long-termincision rates probably reflectcumulative differential uplift in the forearcof the Cascadiasubduction zone, although factorssuch as bedrock and climaticcontrols and isostaticadjustments to erosionobscure the preciserelationship between surface uplift andstream incision. Patterns of differentialincision are moststriking near the latitudeof Newport,where a steepgradient divides a regionof higherrates (~0.6-0.9 mm/yr) in the northernCoast Range from a regionof lower rates(~0.1-0.3 mm/yr) in the centralCoast Range. The steepincision gradient is nearlycoincident with abruptchanges in marineterrace (~80-125 kyr) uplift rates,the locationsof Quaternaryfaults, and the southernflank of a saddleof low historic(~40-70 years)uplift. The exactcauses of thesevariable patterns of incision/upliftare unknown. Analogieswith uplift patternsin othersubduction zones and comparisonswith otherneotectonic data in theregion indicate that patterns of differentialincision probablyare caused by variationsin permanentstrain accumulation along the Cascadia subduction zone. Suchvariations may be relatedto differencesin seismicmoment release during individual earthquakes,to changesin plategeometry or ratesof wedgeaccretion, to segmentationof earthquakeruptures, and/or to deformationon activestructures in theNorth American plate and accretionarywedge.

Introduction deformationin the region are focusedon coastalareas and are primarily concernedwith late Holocene salt marsh sequences The purposeof this work is to characterizerates and styles and late Quaternarymarine terraces[cf. Atwater et al., 1995, of late Quaternary deformation in the Oregon Coast Range and referencestherein]. As a consequence,existing geologic (hereafterreferred to as the Coast Range), a high-relief, deeply studiesleave large land areas (essentiallythe entire inland dissectedmountain range that is part of a belt of elevated forearc)and time periods(>5 ka and <80 ka) largelyunstudied. coastal mountains reaching from northern California to The goal of the presentstudy is to use the widespreaddistribu- British Columbia. These ranges form a high forearc that lies tion of fluvial terracesin the Coast Range to bridge these tem- 100-200 km east of the deformation front of the Cascadia sub- poral and spatial gaps and to help resolve the earthquake ductionzone (Figure 1). Marine magneticanomaly studiesin- potentialof the Cascadiasubduction zone. dicate that the Juan de Fuca plate is being subductedbeneath Fluvial terraces are used as datums for tectonic studies in the North American plate in a northeasterlydirection at a rate many settingsaround the world, perhapsmost commonlyto of ~4 cm/yr [DeMets et al., 1990]. Deformation in forearc demonstrateQuaternary faulting and folding [e.g., Lensen, regionsin similar tectonic settingscommonly is characterized 1964; Rockwell et al., 1984, 1988]. Fluvial terraces also are by active folding, faulting, and large-scaleuplift, subsidence, used to calculate stream incision rates as proxies for regional and tilting of the land surface. Examples of these phenomena uplift rates[e.g., Pillans, 1986; Gardner et al., 1992;Merritts have been observedhistorically during large subductionzone et al., 1994]. Previous research on fluvial terraces in the earthquakes, such as those in Alaska, Chile, and Japan Coast Range mostly consistsof studiesof streamcapture and [Plafker, 1972; Ando, 1975] and have been inferred from the probableantecedence of ancientwestward flowing streamsin prehistoricrecord, primarily throughstudy of uplifted marine westernOregon [Niem, 1976;Baldwin, 1981] and reconnais- terraces[e.g., Berryman et al., 1989; Ota, 1991]. sancemapping of Quaternarydeposits [e.g., Schlicker and The dearth of interplate seismicity [e.g., Ludwin et al., Deacon, 1974; Snavely et al., 1976a, b]. Only a few studies 1991] has forced earth scientiststo rely on other geophysical have looked at fluvial terraces and stream patterns in the and geologicstudies to describethe modemtectonic setting of context of recent tectonics [Adams, 1984; Rhea, 1993]. the Cascadiasubduction zone. Recent geodeticstudies [Savage The resultspresented here supplementrecently published et al., 1991; Mitchell et al., 1994] have identified both latitu- data on fluvial terraces in the region [Personius, 1993; dinal and longitudinal variations in historic uplift acrossthe Personius et al., 1993] and are focused on broad patterns of forearc. In the prehistoricrecord, most studiesof Quaternary vertical incision determinedfrom heights and ages of bedrock strathsthat underlie most fluvial terraces in the Coast Range. The term "stream incision" as discussed hereafter refers to This paperis not subjectto U.S. copyright.Published in 1995by the vertical incision of stream channels into bedrock and should AmericanGeophysical Union. not be confusedwith processesof lateral incisionthat accom- Papernumber 95JB01684. pany streammeandering. The rivers examinedin this study

20,193 20,194 PERSONIUS:STREAM INCISION IN WESTERNOREGON

126 ø 124 ø 122 ø lain by a more complex sequenceof basaltic submarine(Siletz River Volcanics) and subaerial (Tillamook Volcanics) volcanic rocks and tuffaceous siltstones and sandstones. Stream channelsin massivesandstones and densevolcanic rocks typ- Nehalem(North FRiveror k )•ii•ii iii ii :•::':":'":•!':/::'•ii!•ii•11i!11•:::•i:/"•//•i!:i• ii (North Fork ically are incised into bedrock in narrow inner channels that 46 o vary from meanderingto straight segments(Figure 3). In Nehalem River some reaches, channels are armored with a meter or two of Nestucca River gravelly sediment, but such reaches are short and discontin- Salmon River uous,and they make up only a small percentage(<10-20%) of

Siletz River the nontidalreaches of many streamsin the central and north- em CoastRange. Streamchannels commonly are flanked by Yaquina Riv narrow platforms of beveled bedrock that lie within a few tens Big Elk Cre of centimetersof low-water(late summer)stream levels (Figure Drift Creek 3). Most channelsare flanked by nearly continuoussteep-

Alsea River 44 o walled, 6- to 15-m-highterrace risers that tightly confine the floodplain(Figure 4). The treadsof the flanking terracesare Siuslaw River reoccupiedbriefly duringlarge flood events,such as duringthe Smith River winterfloods of 1964, but meanannual floods commonly peak Umpqua River at 4-6 m above low water levels and ordinarily do not reach these surfaces in nontidal reaches. The annual flood levels locally are marked by discontinuousfill-cut terracesinset into the higher continuousterraces. In their tidal reaches, Coast Range rivers have very low

42 o gradientsand relatively poorly preservedterraces (Figure 5). The lower reachesof the studiedrivers were deeply entrenched during sea level low stands but are now filled with thick sectionsof fine-grained fluvial and estuarine sediment. For example, in the Reedsportarea on the lower Umpqua River, 40ø •_ • water wells have penetratedmore than 40 m of fluvial and estuarine sediment without encounteringbedrock [Curtiss et al., 130 ø 125 ø 120 ø 1984]. A single low fill terrace is present along most tidal Figure 1. Map showing locations of major rivers in the reaches;these terrace treads are physicallycontinuous with the Oregon Coast Range. Heavier lines mark reaches of rivers lowest continuousterraces on upstream reaches, but they are examinedin this study, and trianglesmark locationsof major nowhere underlain by exposed bedrock straths. The lack of Quaternaryvolcanoes in the CascadeRange. exposedstraths and presenceof thick sedimentfills in these lower valleys indicate that eustatic sea level fluctuations induceddeep incision and subsequentaggradation along lower drain the centraland northernCoast Range (Figure 1) and have drainagebasins that range in sizefrom 80 km2 (NorthFork stream reaches. Because of eustatic base level changes and KlaskanineRiver) to 11,800km 2 (UmpquaRiver; Table 1). lack of observable straths in these lower reaches, the rest of this paper will focus on the upstream, nontidal reaches of Streamswere selectedbecause of their relativelygood terrace Coast Range streams. preservation, their distribution in a broad area of western Oregon,and the similar bedrockthat underliesmost drainage Fluvial terracesprovide the datumsfrom which I calculated basins. The first part of this paper is focusedon the methods incision rates in the Coast Range. Fortunately,terraces found and assumptionsused to determine incision rates. The tec- along nontidal stream reaches in this study are remarkably tonic implications of differential incision in the region are similar, regardlessof height above presentriver level. Nearly discussedin the secondpart of the paper. all terraces are strath terraces, cut into bedrock, and have cover sedimentsconsisting of a 1- to 2-m-thick channel facies of sandypebble and cobblegravel, which in turn is overlainby a Geologic Setting of Coast Range Streams 2- to 10-m-thick overbank facies of fine sand and silt (Figure Upstream from the head of tide, which defines the limit of 4). Such terraces are preserved as much as 200 m above tidal influenceon streamflows,Coast Range streamsprimarily modern Coast Range streams, but most higher terraces are flow on bedrock in deeply entrenched, V-shaped valleys. restricted to small eroded remnants that are difficult to corre- Valley morphology and lack of characteristicglacial land- late (Figure 5). With the exception of a few thermo- forms indicate that the Coast Range was not glaciated in the luminescence- (TL) and radiocarbon-dated sites [Personius, late Pleistocene[e.g., Porter et al., 1983]. In most casesthese 1993], most higherterraces are undatedand cannotbe usedin streams presently transport only a thin, discontinuousveneer the incision analysis. of gravelly sediment. Bedrock channels are incised into a Despite the numerous terrace remnants preserved along simple sequenceof Eocene sedimentary and volcanic rocks Coast Range streams, only a single example of a nearly (Figure 2; see also Walker and MacLeod [1991]). Much of the continuousterrace is preservedwell enough to correlate in central Coast Range is underlain by the Eocene Tyee most drainagebasins. Upstreamfrom the tidal limit the tread Formation, a thick sequence of arkosic and lithic turbidite of this terrace is 6-15 m above river level and is underlain by sandstoneand siltstone. The northern Coast Range is under- 2-11 m of fluvial sediment and a beveled bedrock strath. The PERSONIUS: STREAM INCISION IN WESTERN OREGON 20,195

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124 ø 123 ø terrace is commonly several hundred meters wide, is phys- ically continuous along most reaches, may be paired or unpaired, and is present in most drainage basins in the Coast

N. F. Klaskanine Range. Grabau [1990], Personius [1993], and Personiuset al. 46 ø -- River [1993] used numerousradiocarbon ages and the widespread occurrence of this terrace to infer a brief climate-driven episode of increased input of sediment into Coast Range streams during the Pleistocene-Holocene transition. The N. F. Nehalem River combinationof the uniform age (9-12 ka) and near-ubiquitous

Nehalem River presence of the lowest continuous strath terrace makes it an excellent datum for comparing stream incision across the region and thus is a major focus of this work.

Stream Incision in the Coast Range Modern Incision Processes

The processesthat control rates of bedrock stream incision Nestucca River are incompletelyunderstood and difficult to quantify,because processrates are slow and commonly affected by hydrologic and geologic variables [e.g., Shepherd and Schumm, 1974; Salmon River 45 ø Gardner, 1983; Miller, 1991; Wohl et al., 1994], However, a recent study of bedrock incision in coastalstreams of northern Siletz River California and southcentral Oregon [Seidl and Dietrich, 1992] used theoretical and field data to infer that incision on main stem streamsoccurs primarily by knickpointpropagation, by abrasionof the streambedby transportedparticles, and by dis- solution. In this studyI assumedthat suchprocesses are likely Yaquina River to be responsiblefor incision in the modem channelsof most Big Elk Creek Geologic streamsin the central and northernCoast Range.

Drift Units Alsea River .' ;•"] QTs Controls on Paleoincision • Tvs :•':':"::':-..' Ts A detailed analysisof the processesresponsible for creating •"•½-...... Ttv the strathsthat underlie most terracesin the Coast Range is :'""'"":•Tty beyondthe scopeof this paper. However, a generaldescrip- • Tsr tion of the extensivestrath underlyingthe lowest continuous 0 2O I I i terrace is helpful in supportingthe assumptionsused to calcu- km late long-term vertical incision rates. Strathsunderlying the lowest continuous terrace are relatively smooth and slope gently toward the channeland downstream. Cross-valleyvariations in strath height are difficult to measurebecause com- plete exposures of terrace straths rarely are observed. However, examinationsof exhumedstraths, exposures in trib- utary stream cuts, and limited auger, trench, and geophysical surveys [Feiereisen, 1981; Grabau, 1990; Personius, 1993; Personiuset al., 1993] indicate that cross-valleyvariations in strath height are usually less than +1 m. Longitudinalvaria- tions in strath surfaceroughness over distancesof less than a few hundredmeters are rarely morethan about +50 cm andmore Figure 2. Geologic map of western Oregon, generalized commonlyare less than +20 cm. from the digitized version of Walker and MacLeod [1991]. Most streamsin the Coast Range flow acrossbedrock that Units: QTs, Quaternary and minor late Pliocene sediments; has a variety of resistanceto erosion, so rock mass strength Tvs, Pliocene to Oligocene volcanic and volcaniclasticrocks; measurements [Selby, 1980; Moon, 1984] were made at rep- unit includespart of the Miocene ColumbiaRiver BasaltGroup resentative sites to assess the extent of bedrock control on and older volcanic rocks in the Cascade Range; Ts, middle stream incision (Table 1). Most values measured in Eocene Miocene to late Eocene marine siltstone and sandstone; unit sedimentaryrocks are 51-70 and are classifiedas "moderate" locally includes minor basaltic volcanic rocks; Ttv, late and strengthrocks. In a few places,massive sandstone units reach middle Eocene subarial and submarine basaltic volcanic and valuesin the lower part of the "strong"category (71-90), but volcaniclastic rocks; mostly consists of Tillamook ¾ol- most rocks in this categoryare volcanic or intrusive igneous canics; Tty, middle Eocene to Paleocene(?) marine sandstone and siltstone; mostly consistsof the Eocene Tyee Formation; rocks. Strath heightsin the Coast Range are not affectedsig- Tsr, middle Eocene to Paleocene marine basaltic volcanic and nificantly by these variations in bedrock resistance. For volcaniclasticrocks of the Siletz River ¾olcanics;unit locally instance,strath heights are nearly constantalong reachesthat includes mafic intrusive igneous rocks of Eocene and flow acrossinterbedded sedimentary and volcanic or intrusive Oligocene age with similar weathering characteristics. igneousrocks, such as on the Siletz and NestuccaRivers, or PERSONIUS: STREAM INCISION IN WF_3TERN OREGON 20,199

across interbedded massive sandstone and thinner bedded silt- rapids in the modern channel, and bedrock type appears to stone,such as along the Umpqua River. control terrace formation by restricting the ability of streams In contrast, rock type does appear to have a noticeable to migrate laterally. On reacheswhere streamsare flowing on influence on stream channel and valley morphology. Harder hard, dense bedrock, such as massive sandstone and Eocene basalt or massive sandstoneunits commonly are the locations basalt, streamsare restrictedto narrow gorgeswhere gradients of entrenchedinner channelsor form ledges, riffles, and small are higher and extensive terraces have not formed. This responsecannot be simply a reflection of insufficient stream power [cf. Merritts et al., 1994], becauseterraces are commonly present where bedrock changesoccur upstream of narrow gorges [Personius, 1993]. Such limited bedrock control on gradient is the only consistent morphological response of Coast Range streamsto lithology documentedby Rhea [1993] in a recent study of stream morphology in the region. I conclude that lithologic controls are restricted to modifica- tions to channel morphology and inhibition of lateral incision and extensive terrace formation, rather than to significant variations in the rates of vertical stream incision. The morphology of the strathsunderlying the lowest continuous terraces described above clearly indicates that they were formed under hydraulic conditionsdifferent from those of the modern stream channels. Grabau [1990], Personius [1993], and Personiuset al. [1993] used the age and morphology of these terraces to infer a brief episode of increased sediment input into streams that may have clogged the channels and induced lateral incision over a broad area of the valley bottom about 9-12 ka. A decreasein sediment supply in the Holocene allowed the streams to resume primarily vertical, rather than lateral, incision. Although stream channel morphology during the widespread strath-forming episodeis unknown, I found no evidenceof major changesin

"-""-'"'--•treadoflowest SILETZ •.__continuousterrace RIVER •/'"':':'""'"'"':'"'"'"':••i:i

"•""t overbankfacies lorn VE = 10X •i'.'•?:.q channel facies 100 m i•..-"•:..••treadoflowest UMPQUA '"'• ...... ':•:•'•-•:•.:••••,,'"'"'"':".•nuous terrace RIVER

Figure 3. Examplesof streamchannels in the Oregon Coast Range, photographedin August 1992 during near-historiclow water levels: (a) UmpquaRiver at km 128; note wide, irregular :::::::::::::::::::::::i:::.....5::::::::::::-:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::...... ea:...S:....u..r.--...e...... m.....e.,.n....:•:-::..i!•:::?:i::ii• ::.i?::::::½!.i::..:4::..-'..-...... ii:•ii:i:ii...----.•i:-:.•!?:::::!...!•-;i :::::::::::::::::::::::::::::::: i.:iii:?:!::::::i.-:-:.-- -channel bed formed in interbedded sandstone and siltstone (unit Tty); (b) Umpqua River at km 61; note modem abrasion Figure 4. Diagrammaticcross sections of the (a) Siletz and platform, incised channels,and remnantsof a 2- to 3-m-high (b) Umpqua Rivers; both sectionsare located about 80 km exhumed strath (upper left); bedrock is massive sandstone upstreamfrom the coast. Note similar stratigraphicsequences, (unit Tty); (c) Siletz River at km 76; note narrow modem regardlessof terraceheight above the river, and significantly abrasionplatform, flanked by strathriser formed in interbed- greatervertical bedrock incision of Siletz River. Radiocarbon ded sandstoneand siltstone (unit Tty) that confines the chan- dates from near the base of sediments underlying the lowest nel at this location. Strath tread (hidden by vegetation) is continuousterrace on both rivers yield calibratedages of ~9- about 7 m above river level. 12 ka (Table 1). 20,200 PERSONIUS: STREAM INCISION IN WESTERN OREGON

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Figure 5. Profiles of fluvial terracetreads on the (a) Siletz and (b) UmpquaRivers. Line conventions:heavy solid lines are elevationsof the river bottom, lighter solid lines connecttreads that are physicallycontinuous, dashedlines connect treads that are probably correlative, and dotted lines are speculativecorrelations. Note the good preservationof a single low terraceon both rivers and increasinglypoorer preservationof older fluvial terraces. See Table 1 for additional data on radiocarbonand TL ages. paleosinuosityor paleogradientthat could be responsiblefor a Methods and Assumptions significant fraction of the poststrath incision. In addition, paleohydrology studies by Grabau [1990] indicate that the Measuring Incision and Profile Construction coarsestgrains observedin the terrace sedimentsalong Drift Measurementsof vertical incision are restricted mostly to Creek could be transportedunder flow conditionsthat occur nontidalstream reaches that lie >20-50 km upstreamfrom the frequently under the present flow regime, so unusuallyhigh coast. Strathheights were measuredwith tape,level, or digital discharges were not required for transport of the terrace altimet•r, dependingon the height of the strath,from the sur- sediments. Despite the obvious apparent differences in inci- face of the strath to the narrow abrasionplatforms that flank sion behavior, I concludethat the long record of similar strath many CoastRange streams(Figures 3 and 4). Where present, formation and near-ubiquitous confinement of Coast Range theseplatforms are within +20 cm of low water levels and streams in narrow bedrock valleys indicates that extensive commonlyhave little alluviumon their surfaces.The heights straths are caused by processesrelated to brief, climatically of these flanking benchesapparently are maintainedby ero- driven episodesof increasedsediment supply. Such episodes sion during high streamflowsin the winter months,but pro- represent a geomorphically significant but temporally small cessesresponsible for their formationare not well understood part of the incision history of most Coast Range streams. [e.g., Wohl, 1992]. These narrow platformsprovide a more PERSONIUS: STREAM INCISION IN WESTERN OREGON 20,201

uniform datum from which to measurestrath heights than do (Figure 5). With a few exceptions(see sectionon discussion the channelfloors, which can be as much as 5 m deep in inner of results below), older terraces are too poorly preservedand channel reaches. Such wide variations in the depths of inner datedto allow any conclusionsabout the rates or stylesof pre- channels are a common attribute of bedrock streams [e.g., Holocene incision. Bretz, 1924; Shepherdand Schumm,1974]. Small knickpoints are common in the channels of most Heights and distances of terrace straths and treads are CoastRange streams. A few knickpointson the UmpquaRiver plotted as along-channelprofiles (Figures 5 and 6), with dis- are 1-2 m high but typically are less than 50 cm high on most tances measured from 1:24,000 scale topographic maps. streams I studied. Such variations amount to a substantial frac- Correlations of the lower continuous terraces are based on tion of the total strath height along some rivers in the Coast radiocarbondating and physical continuity [Personius, 1993]; Range(i.e., the UmpquaRiver, Figure 6c) and may be respon- correlationsof most higher terrace treads are based on phys- sible for much of the height variability of straths along ical continuity where possible and are otherwise speculative streamsundergoing low rates of incision. Variations of 1-2

124 ø 123o45 ' 1 23ø30' E 100-

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O- 20 40 60 80 1 O0 120 140 Tide Limit VE-400X Distance From Coast (krn) Figure6. Profilesof lowestcontinuous terrace treads and underlying bedrock straths (hatched area) along the (a) Nestucca,(b) Siletz,and (c) UmpquaRivers. Vertical scales are equal, but note differences in verticalexag- geration.Heavier lines on locationmaps mark reaches shown in the profiles. 20,202 PERSONIUS: STREAM INCISION IN WESTERN OREGON

m, however,are relatively insignificantto the measurementof At sites where radiocarboncontrol is completelyabsent, higherstraths (i.e., the Siletz and NestuccaRivers, Figures 6a age estimatesare basedsolely on terraceposition and weather- and 6b). ing characteristics(Table 1). In most casesthese sitesare cor- relatedwith the 9- to 12-ka strathunderlying the lowestcon- Determining Strath Ages tinuousterrace and are locatedalong tributaries of rivers where the ages of correlative strathsare establishedwith radiocarbon Bedrockstraths cannot be dateddirectly, but I assumethat dating. Although the incision rates derived from these sites reasonableminimum ages can be estimatedby determiningthe are not supportedby radiometricdata, the widespreadand char- agesof the overlyingcover sediments [Merritts et al., 1994]. acteristic occurrence of the lowest continuous terraces in the The validity of this assumptionis difficult to test, becausethe Coast Range indicatesthat such correlationsare reasonable. relationshipbetween strath planation and the overlyingsedi- The agesof two older strathsalong the UmpquaRiver are ment is uncertain. However, I interpret the basal channel estimatedby experimentalthermoluminescence (TL) datingof facies (Figure 4) as lag depositsof coarsebedload sediment fine-grainedfluvial sediment[Personius, 1993]. Although thatprobably were the "tools"used by CoastRange streams to these two ages yield incision rates that are consistentwith bevel the strath[e.g., Bull, 1991]; the overlyingoverbank rates from younger,radiocarbon-dated sites, other TL ages depositsstratigraphically represent a return to a period of yielded conflictingresults [Personius, 1993]. None of the TL channeldegradation and periodicflooding that has continued agesobtained in this studyare corroboratedwith other numer- to the present. Scatteredradiocarbon ages obtained from the ical datingmethods, so theseolder agesshould be viewedwith overbank deposits [Personius, 1993] show that the terrace caution. The potential problems with radiocarbonand TL treadshave aggradedthroughout the Holoceneand thuscannot dating discussedabove are the most significantsources of be usedas incisiondatums. I do not know how long the potentialerror in the incisionrate calculations(Table 1). strathswere surfacesof activeplanation, but radiocarbonages from the channel facies or the base of the overbank facies Assumptions should yield reasonableestimates of the times of strath aban- donmentand thuscan be usedto calculateincision rates (Table Interpretationsof the long-term stream incision data ob- 1). tainedin this studyare basedon the followingassumptions: One of the mostserious obstacles to usingthe ageof over- (1) CoastRange streams have undergonea long historyof lying sedimentas a proxy for strathages is the potentialfor incision,as evidencedby numerousstrath terraces of similar subsequentstrath stripping and reoccupation. Localized appearanceat great heights above the modern stream. (2) examplesof strath reoccupationare presenton some Coast Extensivestrath terraces represent short (<1-3 kyr) periodsof Range streams,but most are recognizableas lower inset climate-inducedaggradation, which briefly interrupt long-term terracesor channelfills that yield youngerradiocarbon ages trendsof nearly continuousvertical incision. (3) Radiocarbon [Grabau, 1990;Personius, 1993]. Wholesalestripping and ages from the channel facies or the base of the overbank facies reoccupationof the lowest continuousstraths are unlikely, yield reasonableestimates of the time of strath abandonment giventheir size(Figure 4) andapparent uniform age. and thuscan be usedto calculatelong-term incision rates. (4) Most strath ages (Table 1) are estimatedby radiocarbon Strath-to-strathincision measurements yield the mostaccurate analysisof small (most <5 mm in diameter)detrital charcoal incision rates, but only a single strath is preservedwell fragmentscollected from the baseof the silty overbanksedi- enough to allow regional correlations. As a result, narrow ment,because charcoal rarely occursin the underlyinggrav- benchesthat lie nearlow-water (late summer)stream levels are elly channelfacies. No detailedcomparison of radiocarbon usedas proxiesfor modernstraths. (5) Over the long term ages betweenthese two facies is possible,but the lack of a (tens of thousandsof years), powerful streamsin the Coast prominentunconformity and multiple ages from a few sites Rangeincise their channelsuniformly along any given reach indicateno significantdifference in agebetween the lowerpart at aboutthe rate of regionaluplift. Exceptionsinclude reaches of the overbankfacies and the basalchannel facies [Personius, where major intrinsic hydraulic changeshave occurredand 1993; Personius et al., 1993]. The detrital nature and small timeswhen brief periodsof equilibriumor aggradationprevent size of the charcoalsamples indicate some sample reworking continuousincision. These latter periods force streamsto prior to deposition[e.g., Blong and Gillespie, 1978], but subsequently"catch up" by incisingat fasterrates over shorter Personius [1993] found that inclusionof small pieces of (lessthan a few thousandyears) time intervals[e.g., Bull and burned or partially decomposedroots and/or bioturbationof Knuepfer, 1987; Hamblin, 1994]. youngercharcoal was a more significantproblem than inher- ited age in CoastRange detrital charcoal samples. Thin fluvial Discussion of Results sequencesare especiallysusceptible to samplecontamination by bioturbationand intrusionof anomalouslyyoung organic Spatial and Temporal Incision Patterns matter. For example, several sites in the northern Coast Severalpatterns of differentialincision are apparentin the Rangeyielded anomalously young radiocarbon ages from sed- contouredstream incision data obtainedin this study(Figure iments overlying straths that probably correlate with the 7). The most strikingdifference in regionalincision is evi- widespread lowest continuous strath formed about 9-12 ka dent near the latitude of Newport, where a steepgradient (Table 1). Such ages yield maximum incisionrates. In cases divideswestern Oregon into a regionof higherincision rates where ages were more than 40% younger than correlations (~0.6-0.9mm/yr) in the northernCoast Range and a regionof basedon terraceheights and sedimentweathering character- lower rates (~0.1-0.3 mm/yr) in the central Coast Range. istics,I alsocalculated incision rates with agesestimated from Slight north-south variations in incision in the northern nearby dated sites. These estimated rates are poorly Coast Range appearto define two subregionsor blocks of constrained. higherincision, but incisionsites are so sparsein this region PERSONIUS: STREAM INCISION IN WESTERN OREGON 20,203

126 ø 125 ø 124 ø 123 ø

46 ø '""•'•-. River

!1 River

45 ø

Newport

•o Florence

EXPLANATION O• • Incisionsite ."'%=. Stream incisionrate (mm/yr) Reedsport •'--•..x Geodeticuplift rate (mm/yr) LatePleistocene and T'"'•-•- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Holoceneo faults2o i i i ...... km CoosBay ......

Figure 7. Contouredstream incision rates (heavier lines) and geodeticuplift rates (lighter lines) in the central and northernOregon Coast Range. Symbolsmark locationsof incision sites: solid circles, radiocarbon or TL-dated sites;open circles, radiometricallydated sites that yield anomalouslyyoung ages and maximum incisionrates; and diamonds,undated sites where strathages are estimatedfrom terracecorrelations and sediment weatheringcharacteristics. Dashed incision contours are fitted to rateslisted in Table 1. Dotted incision contoursare speculativeand are basedon variationsin topographyand brief reconnaissanceof streamsdrain- ing the easternflank of the CoastRange. Geodeticcontours are from Mitchell et al. [1994]. Offshore structural data are from a digital version of Goldfinger et al. [1992b]; most north-trendingstructures are thrust faults, and most northwest-trendingstructures are left-lateral strike-slipfaults. North-southline through the CoastRange markslocation of profiles in Figure 8. 20,204 PERSONIUS:STREAM INCISION IN WESTERN OREGON that these small variations could be a reflection of the large to be relatively uniform in the central and northern Coast errors inherent in the incision rate determinations (Table 1). Range. Such consistencyis unusualin comparisonwith areas Incision patterns in northwesternOregon also may reflect a suchas New Zealand that have undergonemore drasticclimatic componentof down-to-the-east tilting, consistentwith geo- changesin the late Quaternary [cf. Pillans, 1986; Bull and detic [Adams, 1984; Mitchell et al., 1994] and geomorphic Knuepfer, 1987]. [Rhea, 1993] studies in the region. Incision sites are too sparseand rates are too low to show any consistentpattern of Climatic Controls on Incision tilt in the central Coast Range. Only limited data are available to enable evaluationof the I interpret the regional nature of the incision patterns extent of incision rate variations through time. Incision rates apparentin Figure 7 as evidencethat local perturbationssuch calculatedfrom radiometricallydated sitesof variousages were as changesin bedrock resistanceor channel morphology are obtained on only two rivers, the Umpqua and Siletz Rivers not major influences on long-term stream incision in the (Figure 5, Table 1). Along the Umpqua River, incision rates CoastRange. However, one factor that could control incision from a ~6.5-kyr terrace (0.2 mm/yr) are similar to rates of 0.2- on a regional scale is climate, particularly precipitation. 0.3 mm/yr from the lowest continuousterrace (~9-12 kyr) and Precipitationin the form of rain falls mostly in the winter older (~125 kyr) terracesat nearby sites. With the exception months, and annual amounts vary significantly in the Coast of one site, incision rates along the Siletz River are consis- Range. Rainfall varies both latitudinally and with elevation, tently ~0.7 mm/yr at sites ranging in age from ~2.2 kyr to with annualamounts ranging from 120-150 cm in the southto ~42 kyr. Such results support my assumptionsthat Coast more than 300 cm in the highestparts of the range in northern Range streamshave undergonea long history of incision and Oregon(Figure 8). Higher ratesof streamincision are coinci- that straths underlying the lowest continuous terraces yield dent with higher annual rainfall amountsin northernOregon, reasonableestimates of late Quaternary incision rates. In con- but this apparentcorrelation does not hold in the centralCoast trast, examplesof varying incision rates suchas thoseon the Range, where a large increase in annual rainfall between Nestucca River probably are related to strath reoccupation, 43.5øN and 44.5øN is not accompaniedby an increasein inci- rather than nonuniform incision. Although the incision data sion rates. However, the coincidence of sharp increasesin are sparse,I concludethat averagedover periods of tens of incision and rainfall rates near latitude 44.5 ø could be inter- thousandsof years, incision rates on any given reach appear pretedas evidenceof a thresholdphenomenon, where incision

_ -3.5 E

annual rainfall _ - 3 ._o geodeticuplifts._ _ O• -2.5

\ _ \ -2 •" marineterrace uplift /' x streamincision ...,,, .• -"----._ /. xx ...... F 1.5 -'•"•"•• ß:-:-:-:.:.:.:.:.:•::::::.:.:.:.:::.:...... :::..._.•._.;!:.:..•:E.... •' ...... •:ii:i:.-•.:.-'-a,i•x •..-..... 1'-•.'--'..-•::(• -• ...... ::l::•::?•.-'•:;;.•? ...... _ •_• ?:__.'•? '...... '"'":'::::-:-:-:---:':':'-FL 1 E

ß 0.5 m

-2 42 43 44 45 46

CA OR Latitude (o N) OR WA

Figure 8. North-southprofiles of'stream incision, annual rainfall, geodeticand marineterrace uplift, and topographyin the Oregon Coast Range. Uplift rates include long-term (~80-125 kyr) rates from marine ter- racesalong the coastand short-term(~40-70 years) ratesfrom relevelingand tide-gaugestudies. Incisionrates p•ofile is shown with +0.2-mm/yr error limits. The marine terracedata from the California-Oregonborder to 45ø are generalizedfrom Kelsey et al. [1994]; marine terracerates north of 45ø are modified from West and McCrumb [1988]. The geodeticdata are from Mitchell et al. [1994], and the rainfall data are from the National Oceanic and AtmosphericAdministration [Gale Research Company,1985]. The incision,geodetic, and rainfall profiles were measuredalong the profile line shownin Figure 7. The generalizedtopographic profile was measuredon 1:250,000 scaletopographic maps along the profile line shownon Figure 7; the averagedtopo- graphicprofile is from a digital elevationmodel measuredin a 80- to 90-km-wide swathalong the lengthof the Coast Range [Kelsey et al., 1994, Figure 2]. Both topographicprofiles yield similar patterns: a region of higher topographyin the Klamath Mountainsin southernOregon (42 ø to 42.75ø latitude), a region of lower topographyin central Oregon (43ø to 44.5ø latitude), and a regionof higher topographyin northernOregon (44.75 ø to 45.75 ø latitude). PERSONIUS: STREAM INCISION IN WESTERN OREGON 20,205

increasesonly when some "threshold"amount of annual rain- response to collapse of the isostatic forebulge that formed fall is reached,in this caseabout 200 cm. Sucha relationship adjacentto the front of the Cordilleran ice complex [Peltier, might explain the sharp increasein incision rates near latitude 1986; Peltier and Tushingham, 1989]. These rates have not 44.5ø , but evidence of thresholdcontrol is lacking at the been substantiated,but nearby studies in the Puget Lowland northernend of the rainfall high near the Oregon-Washington [e.g., Mathews et al., 1970; Thorson, 1989] document that border (Figure 8). Here the area of high incision rates extends rapid isostatic deformation was completed in a few thousand well north of the sharp drop in rainfall, and incision rates year period following deglaciation. Holocene sea level curves decreasegradually toward the Columbia River. Although a constructedfor the Pacific Northwest [Hutchinson, 1992] can- thresholdeffect cannotbe ruled out, the apparentcorrelation not distinguish isostatic effects from eustatic or tectonic con- of rainfall and incision in some basins is more likely the trols on sea level, and regional assessmentsof marine terraces result of orographiceffects on precipitation. along the Oregon coast do not appear to show evidence of A climatically related control on streambehavior is stream rebound-related deformation [Mitchell et al., 1994]. Geodetic power,which can have a stronginfluence on ratesand patterns studiesin the region differ on the importanceof postglacial of incision [Merritts et al., 1994]. Although I made a few reboundin determiningmodem uplift rates [e.g., Savageet al., incisionmeasurements along smaller streams(<200-km 2 1991; Mitchell et al., 1994], so until detailed chronologiesof drainagearea), most of the Coast Range data are from trunk late Quaternary isostatic and tectonic deformation are avail- streamswith drainagebasins larger than 500 km2 (Table1). able, the influence of isostatic adjustmentson regional defor- In regionsof lower and higher incisionrates (Figures7 and 8), mation and patterns of stream behavior in the region cannot all these streamsappear to have more than sufficient stream be adequatelyaccessed. power to actively incise their beds at present and to have A comparison between Coast Range stream incision data createdsequences of multiple strathterraces in the past. Such and better documenteduplift data would be very helpful, but relationsindicate that variationsin stream power cannot be unfortunately,there are no uplift data setsthat are temporally the sole causeof regional differencesin stream incision in the or spatiallycomparable. However, three setsof uplift data at Coast Range. Sufficient streampower also may minimize the different time scales are available: (1) releveling and tide- influence of variations in relief, such as the higher topog- gauge geodetic data, (2) marine terrace uplift data, and (3) raphy in the northern Coast Range, on rates of regional regional topographicdata. The most recent geodeticdata doc- incision (Figure 8). ument land level changesin the last ~40-70 years, both along the Oregon coast and in east-west transectsacross the Coast Relations Between Incision and Uplift Range [Mitchell et al., 1994]. These studiesdocument uplift Given the tectonicsetting of the Coast Range, differential rates of 0-4 mm/yr in the central and northernCoast Range tectonic uplift is a likely cause of regional variations in (Figures 7 and 8). Marine terrace data are restricted to narrow stream incision. Measurementsof tectonic (surface) uplift zones along the coast, so they cannot be contouredat the scale must be corrected for the influence of exhumation or erosional of Figure 7. However, severalrecent studies document uplift in denudationand subsequentisostatic compensation [England the last ~80-125 kyr along the Oregon coast [West and and Molnar, 1990]. However, Kelsey et al. [1994] conclude McCrumb, 1988; Kelsey, 1990; Mclnelly and Kelsey, 1990; that erosionhas a negligibleeffect on the heightsof geodetic Muhs et al., 1990; Ticknor, 1993; Kelsey and Bockheim, benchmarksand marineterraces in westernOregon, and studies 1994; Kelsey et al., 1994]. Generally, marine terrace uplift of hillslope colluvial depositsshow that rates of denudation rates along the Oregon coast are low (<0.4 mm/yr), except may be significantly lower than stream incision and uplift where folding and faulting have deformed the terraces. rates in the region. Hillslope erosion studies in the central Topographicdata are shown as two north-southprofiles on CoastRange [Dietrich and Dunne, 1978; Reneau and Dietrich, Figure 8. Broad regional differencesin topographymay be a 1991] and in the Olympic Mountainsof westernWashington reflection of variationsin uplift over time periods of millions [Reneau et al., 1989] yield similar latest Pleistocene and of years [Kelsey et al., 1994]. Holocene hillslope erosion rates of 0.03-0.07 mm/yr. In Latitudinal variations are evident in the geodetic, marine places these rates are as much as an order of magnitudeless terrace,topographic, and incisionrate profilesshown in than rates of stream incision in the Oregon Coast Range Figure 8. In a region of low topographyin the central Coast (Table 1) and rates of uplift in the Olympic Mountains[West Range (latitude ~43.5ø-44.5ø), rates of stream incision and and McCrumb, 1988; Savageet al., 1991; Brandon and Vance, marine terrace uplift are nearly indistinguishabledespite their 1992; Thackray and Pazzaglia, 1994]. These erosion rates spatial and temporal differences. Both incision and marine imply that the post-latestPleistocene isostatic adjustmentsto terrace uplift rates increasenear the south flank of a region of regional denudationin the Coast Range may be too small to higherCoast Range topography near latitude 44.6 ø and then have a significantimpact on rates of incision along powerful divergenear latitude44.75ø; recent marine: terrace studies in trunk streamsin the region. this area [Ticknor, 1993; Kelsey et al., 1994] indicate that An additionalpotential contribution to surfacedeformation active structuresare in part controllingchanges in uplift rates in the Pacific Northwestis the far-field isostaticresponse to along this part of the Oregon coast. Similar patternsalso are late Pleistocenedeglaciation. Althoughfew detailedstudies of obviousin the marine terracedata in southernOregon, where postglacial sea level history have been conductedin western localized folds and faults are recorded in the marine terrace data Oregon [Hutchinson, 1992], recent geophysicalmodeling of between Coos Bay (43.4ø) and the California border [Kelsey, isostatic adjustments to post-14 ka deglaciation of the 1990; Mclnelly and Kelsey, 1990; Kelsey and Bockheim, Cordilleran ice sheet in Washington and southern British 1994]. The strongestvariations in stream incision occur in a Columbia indicates that western Oregon should be currently broad "saddle"of null geodeticuplift that is roughly coinci- undergoing~0.5-1.0 mm/yr of subsidence(sea level rise) in dent with the region of higher topography in the northern 20,206 PERSONIUS: STREAM INCISION IN WESTERN OREGON

Coast Range. Incision rates remain consistentlyhigher than sion and marine terrace uplift in the central Coast Range may rates of marine terrace uplift in the saddle between 44.8 ø and be indicative of interseismic uplift, but variations in historic 45.8 ø and then mimic the trend of decreasing topography uplift raise questions about the rate and direction of inter- northwardtoward the ColumbiaRiver (Figure 8). seismic deformation in the northern Coast Range. For instance, delineation of a region of null historic uplift be- Tectonic Implications tween Newport and Tillamook (Figures7 and 8) led Mitchell et al. [1994] to conclude that areas where rates of long-term Analogieswith uplift patternsfrom other subductionzones uplift exceedrates of historic deformationmay be subjectto and analysis of other neotectonicdata in the region indicate coseismicuplift rather than subsidenceduring subductionzone that differential incision patterns are related at least in part to events. This idea is intriguing,because it implies that parts of cumulative along-strike variations in permanent strain accu- the northern Coast Range may have accumulatedpermanent mulation along the Cascadia subductionzone. In other sub- strain by "coseismic overshoot," rather than by relatively duction zones, such along-strike variations are attributed to high rates of interseismicrecovery, and calls into questionthe complex interactionsbetween variations in patternsof earth- assertionthat coastal Oregon typically undergoesregional co- quakerupture, wedge accretion, the geometry of thesubducting seismic subsidence during subduction zone earthquakes. plate, rupture segmentation,and deformationon structuresin Although the geodeticrecord probably is too short to describe the upperplate and accretionarywedge. Possibleinfluences of accurately the long-term interseismic behavior of Cascadia, these factors on rates of stream incision and uplift in the the presence of high long-term incision rates in a region of Cascadia forearc are described below. low historic interseismicuplift may mean that parts of north- em Oregon are characterizedby coseismicuplift during sub- Subduction Zone Earthquake Deformation duction zone earthquakes. This possibility is counter to the Late Quaternary uplift records, such as those provided by prevailing view that the northern Oregon coast has undergone marine terraces and stream incision in the Cascadia forearc, repeated coseismicsubsidence [Darienzo and Peterson, 1990] represent cumulative deformation of the earth surface over and lends support to the suggestionsof Goldfinger et al. many seismic cycles [e.g., Fitch and Scholz, 1971]. [1992a], Nelson [1992], and Ticknor [1993] that some sub- Unfortunately, earthquakerecords in most subductionzones sided marsh sequencesalong the Oregon coast are formed by are too short to describe deformation over more than one movement on individual synclines,rather than in responseto seismiccycle, and models of elastic or viscoelasticbehavior regional submergenceduring subductionzone earthquakes. commonly cannot accountfor long-term rates of permanent Plate Geometry (unrecovered) deformation [Thatcher, 1986]. However, Thatcher [1984] used the long earthquakerecord in Japan to Along-strike differencesin uplift patternsmay be attributed describeseveral possible causes of cumulativedeformation at to subductionof seamounts,aseismic ridges, or other topo- subductionzones. Nearest the trench, permanentdeformation graphic features on the downgoing plate [e.g., Gardner et al., results from "coseismicovershoot," where steepeningof the 1992]. In Cascadia, seamountsor other topographic disrup- plate boundaryand splay faulting inducemore coseismicslip tions are restrictedto the southwesternpart of the Juan de Fuca than can be recovered in the interseismic period. These plate [EEZ-SCAN 84 Scientific Staff, 1988, Figure 1] and are regionsmay correspondin part to localizedconcentrations of substantially smaller than the regional patterns of stream slip or momentsuch as thosemodeled by Yabukiand Matsu'ura incisionidentified in the presentstudy. In addition,I found no [1992], which are common attributesof earthquakeruptures at evidenceof topographicdisruptions of the accretionarywedge many plate boundaries[Thatcher, 1990]. If slip or moment that are characteristic of subducted seamounts in other sub- concentrationswere repeatedthrough many earthquakecycles, ductionzones [e.g., Yamazakiand Okamura, 1989]. then suchvariations could have causedsome patterns of differ- Variations in the geometryof the upper or lower platesmay ential uplift and incisionin parts of the Cascadiaforearc that also control patternsof long-term uplift. For example, much lie close to the trench. The oppositesituation is more likely of the anomalousuplift of the Olympic Mountains has been in areas farther from the trench, where permanentdeformation attributedto archingof the Juan de Fuca plate [Brandon and is related to interseismic deformation rates that exceed rates of Calderwood, 1990]. However, determination of the detailed coseismicrecovery [Thatcher, 1984]. The sourcesof such geometry of the Juan de Fuca plate in Oregon is difficult interseismicdeformation are unknown, but geologic studiesof becauseof the scarcityof earthquakedata on the plate interface forearcevolution commonly attribute permanent uplift at con- [e.g., Ludwin et al., 1991], and most studies using vergentmargins to deep-seatedflow, underplating,or aseismic age/subsidencemodels [Kelseyet al., 1994], or geophysical shorteningwithin the accretionarywedge; such processes may techniques[e.g., Rasmussenand Humphreys,1988; Keach et be contributingto uplift of the Olympic Mountainsin north- al., 1989; Wannamakeret al., 1989] are poorly constrainedor ern Washington [Pavlis and Bruhn, 1983; Brandon and too widely spacedto determineslab geometryin detail. An Calderwood, 1990] and otherparts of the Cascadiaforearc. exceptionmay be a recent study by Trehu et al. [1994] in A few areasundergoing high rates of uplift, such as near which seismic reflection profiling is used to describe vari- Cape Blanco in southernOregon, may be attributableto ations in thickness of the Siletz terrane, an accreted oceanic "coseismic overshoot." In contrast, if the northern Oregon terrane that forms the basementof the North American plate and Washingtoncoasts are characterizedby coseismicsub- undermuch of the Cascadiaforearc. In a north-southvelocity sidence[Atwater, 1987, 1992;Darienzo and Peterson,1990], profile that lies 30-50 km east of the Coast Range crest, the thenpermanent uplift of muchof the onshoreCascadia forearc Siletz terrane thins significantly between 45.25ø and 45.75ø should be attributable to unrecoveredinterseismic uplift [e.g., [Trehu et al., 1994, Figure 2], in a region that roughly coin- Kelsey et al., 1994]. Consistentlylow rates of streaminci- cides with an area of higher incision rates in the northern PERSONIUS: STREAM INCISION IN WESTERN OREGON 20,207

Coast Range (Figures 7 and 8). However, no other variations are constrainedtoo poorly in this area to supportspeculations in stream incision appear to correlate with changes in the about possible correlation of incision patterns and active geometryof the Siletz terrane or plate interface, so until more structures. geophysical data are obtained, speculationsabout the rela- Well-documentedevidence of rotation of early Tertiary vol- tionship between patterns of long-term incision/uplift and canic rocks in the Coast Range [e.g., Simpsonand Cox, 1977] plate geometry in western Oregon cannot be substantiated. also supports an upper plate structural origin for some incision patterns. Paleomagnetic data [e.g., Wells and Heller, Rupture Segmentation 1988] are used in a recent model [Wells and Weaver, 1992] to describe the Coast Range as a series of clockwise rotating Another possible cause of differential patterns of uplift in crustal blocks formed as a result of dextral shear and north- the Coast Range is segmentationof the Cascadia subduction south shortening in response to oblique subduction. Some zone. Numerous geological and geophysicalstudies have dis- cussedsegmentation scenarios in Cascadia. Studiesof Cascade recentlydocumented faults offshore of coastalOregon may be Range volcanoes [Hughes et al., 1980; Weaver and secondaryshears within the dextral shearcouple driven by the oblique convergenceof the Juan de Fuca and North American Michaelson, 1985; Guffanti and Weaver, 1988] seismicity [Weaver and Michaelson, 1985; Weaver and Baker, 1988; plates [Goldfinger et al., 1992a]. These faults, which have a Spence, 1989], plate geometry [Weaver and Michaelson, thrust component onshore [Wells et al., 1992], and other related structuresmay form the active boundariesof the rotat- 1985; Spence, 1989], the distribution of prehistoric coseismic subsidenceevents [Nelson and Personius, 1991], vari- ing blocks [Goldfinger et al., 1992a; Wells and Weaver, 1992]. A similar model based on seismic reflection and other ations in historic uplift rates [Kelsey et al., 1994; Mitchell et al., 1994], and the presenceof onshoreand offshore active geophysicaldata in the Puget Lowland [e.g., Johnson et al., 1994; Pratt et al., 1994] envisages the Coast Ranges as a faults [Snavely, 1987; Goldfinger et al., 1991a, 1992a; series of thrust sheets whose master structures sole into a Appelgate et al., 1992; Kelsey et al., 1994] have all identified decollement at the top of the subducting Juan de Fuca plate possible segment boundaries in the upper or lower plates [Unruh et al., 1994; T. L. Pratt, written communication, between 44ø and 46ø latitude in western Oregon. A detailed descriptionof these segmentationmodels is beyond the scope 1993]. Changes in incision rates near Newport and possibly Tillamook (Figure 7) are coincident with these proposedblock of this paper, but one reasonable explanation for regional, along-strikevariations in stream incision is differential cumu- boundary structures. lative deformation on decoupledsegments. The geometry of The relations describedabove may indicate that some inci- possible segments and segment boundaries in northern sion patternsare causedby cumulative late Quaternarydefor- Oregon is open to question, but the coincidenceof sharp mation on active folds and faults in the North American plate gradientsin stream incision, topography,and marine terrace and accretionarywedge. Such deformationon active structures uplift indicates that the Newport area (Figure 8) is a likely may be the most likely causeof uplift variationsdocumented location for a late Quaternary segmentboundary in either the in marine terrace sequencesin Cascadiaand elsewherein the Juande Fuca or North American plates. Pacific rim [Berryman et al., 1989; Muhs et al., 1990; Kelsey et al., 1994]. However, given the coincidenceof active upper Active Structures plate structureswith known segmentboundaries in other subduction zones, the occurrenceof active structuresnear Newport Some details of the structural geology of the North also may supportthe presenceof a segmentboundary in this American plate and accretionarywedge in western Oregon area [Ticknor, 1993; Kelsey et al., 1994]. Thus the incision indicatethat activefolds and faultsmay alsoplay a role in pat- patternspresented here may be consistentwith deformationon ternsof differential incision. For example,recent studieshave active structures and/or segmentation of either the North identifiedseveral active faults in the vicinity of Newport. The American or Juan de Fuca plates. However, the incision data best studiedof thesestructures, the Wecoma fault, splaysinto presentedherein are not preciseenough to determinethe rela- several strands at its eastern end and may project into the tive importanceof thesepossible sources of deformation. Oregon coast 15-20 km north of Newport [Goldfinger et al., 1991a, b, 1992a, b; Appelgate et al., 1992; Kelsey et al., 1994]. Snavelyet al. [1976a, b] also map northwesttrending Conclusion faults in bedrocknorth of Newport that may correlatewith off- Radiometric ages and the heights of fluvial strath terraces shorefaults [Snavely, 1987, p. 319], and Ticknor [1993] and are usedto calculatelong-term bedrock stream incision rates in Kelsey et al. [1994] map the Yaquina Bay fault, which offsets a wide area of the Oregon Coast Range. Several lines of evi- marine terrace platforms just south of Newport. With the dence indicate that broad variations in stream incision are exception of an onshore splay of the Wecoma fault discussed caused by cumulative differential uplift along the Cascadia by Kelsey et al. [1994], nearly all of these faults have down- subduction zone, but bedrock and climatic controls and iso- to-the-south displacementsand lie between the higher inci- static adjustmentsprevent calculation of absoluterates of sur- sion rates documentedalong the Siletz River and lower rates to face uplift. Significantdifferences in rates are useful in deter- the south(Figures 7 and 8). Although there is no evidencethat mining patterns of differential uplift on a regional basis and these faults offset Quaternary depositsinland from the coast, are helpful in describingthe late Quaternarytectonic setting of their displacementdirections are consistentwith changes in the onshorepart of the Cascadiaforearc. late Quaternaryincision rates in this part of the Coast Range. Near the latitude of Newport, changesin regional incision Other large-scale,northwest trending strike-slip faults mapped divide the Coast Range into a region of higher incision rates offshore [Goldfinger et al., 1992a, b] apparentlyproject on- to the north and a region of lower incision rates to the south. shore and offset Quaternary deposits along the coast near Comparisonswith similar uplift patternselsewhere and with Tillamook [Wells et al., 1992], but the stream incision data other neotectonicdata in the region indicate that differential 20,208 PERSONIUS:STREAM INCISION IN WESTERNOREGON patterns of incision probably are causedby along-strike vari- Dietrich, W. E., and T. Dunne, Sedimentbudget for a small catchment ations in permanent strain accumulation along the Cascadia in mountainousterrain, Z. Geomorphol.,Suppl. 29, 191-206,1978. subductionzone. The causesof these along-strike variations EEZ-SCAN 84 Scientific Staff, Physiographyof the westernUnited StatesExclusive Economic Zone, Geology,16, 131-134, 1988. are unknown, but they may be related to variationsin seismic England,P., and P. Molnar, Surfaceuplift, uplift of rocks,and exhum- momentrelease during individual subductionzone earthquakes, ationof rocks,Geology, 18, 1173-1177, 1990. to variationsin plate geometryor rates of wedge accretion,to Feiereisen,J. J., Geomorphology,alluvial stratigraphy,and sediments: segmentationof earthquakeruptures, and/or to deformationon Lower Siuslawand Alsea River valleys,Ph.D. diss.,Univ. of Oreg., Eugene,1981. active structuresin the North American plate and accretionary Fitch, T. J., and C. H. Scholz,Mechanism of underthrustingin southwest wedge. The coincidenceof steep incision gradients, abrupt Japan: A modelof convergentplate interactions,J. Geophys.Res., changesin marine terrace uplift rates, and active faults that 76, 7260-7292, 1971. offset marine terraces indicates that the region near Newport Gale ResearchCompany, Climates of the States,3rd ed., 1175pp., Gale may mark a persistent segment boundary in the Cascadia Res. Co., Detroit, 1985. subduction zone. Gardner,T. W., Experimentalstudy of knickpointand longitudinalpro- ffie evolution in cohesive,homogenous material, Geol. Soc. Am. Bull., 94, 664-672, 1983. Acknowledgments. This work was supportedby the National Gardner, T. W., D. Verdonck, N.M. Pinter, R. Slingerland, K. P. EarthquakeHazards Reduction Program and the Nuclear Regulatory Furlong,T. F. Bullard,and S. G. Wells, Quaternaryuplift astridethe Commission.The authorthanks Steve McDuffy and David Applegate aseismicCocos Ridge, Pacific coast, Costa Rica, Geol. Soc. 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