JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B12, PAGES 19,495-19,516,NOVEMBER 10, 1990 Heat Flow in the State of Washingtonand Thermal Conditions in the CascadeRange DAVID D. BLACKWELL,JOHN L. STEELE,AND SHARI KELLEY Departmentof GeologicalSciences, Southern Methodist University, Dallas, Texas MICHAEL A. KOROSEC WashingtonDepartment of Natural Resources,Division of Geologyand Earth Resources,Olympia Heat flow data for the state of Washingtonare presentedand discussed.The heat flow in the OkanoganHighland averages 75 mW m -2, andthe gradient averages 25øC km -I . Theheat flow in the ColumbiaBasin averages 62 mW m -2, andthe mean gradient is 41øC km -1 . Bothof theseprovinces areinterpreted to havea mantleheat flow of about55-60 mW m -2, thesame value as in theBasin and Range province to the south and the intermountainregion of Canada to the north. These areas comprisethe high heat flow, back arc regionof the Cordillera. In the coastalprovinces and the western part of the southernWashington Cascade Range the heat flow is below normal and averages40 mW m-2 withan average gradient of 26øCkm -1 . Thislow heat flow is related to theabsorption of heatby the subductingslab, part of the Juan de Fuca plate, that is beneaththe Pacific Northwest. Thus the low heat flow area represents the outer arc part of the subductionzone. Within the volcanic arc, the Cascade Range, the heat flow pattern is complicated.The heat flow is best characterized in the southernWashington Cascade Range. The heatflow there averages 75 mW m-2 andthe gradient averages45øC km- •. Theheat flow peaks at over 80 mW m-2 alongthe axial region that coincides with the Indian Heaven, Mount Adams, and Goat Rocks centersof Quaternary volcanism. As is the case in northernOregon and southernBritish Columbia,the westernedge of the regionof highheat flow has a half width of 10 km, implyinga heat sourceno deeperthan about10 km. In the northernWashington CascadeRange the data are too sparseto determinethe averageheat flow. There are two saddlesin the heat flow patternin Washington,along the ColumbiaRiver andin centralWashington. The origin for the contrastingheat flow may be segmentationof the heat sourceor somemore local effect. The heat flow of the CascadeRange is well characterizedin severallocations, and the pattern is similar at all localities.The most strikingfeature is the 10km half width of the westernside of the high heat flow regionwhere it abutsthe low heatflow outerarc region. The axialheat flow rangesfrom about80 to greaterthan 100 mW m -2. Themidcrustal temperatures are interpreted torange from about 400øC to 800øC.The sourceof thesehigh temperatures is interpretedto be a long-livedmidcrustal zone of magmaresidence that is characteristicof the CascadeRange regardless of crustaltype, rate of volcanism,or compositionof volcanism.For example, in southernWashington the region of highheat flow spansthe widthof the Quaternaryzone of volcanismwith MountSt. Helensand Mount Adams at the west andeast edges, respectively. On the otherhand, most of the Oregonstratovo!canoes are near the center of anomalous region. INTRODUCTION of the conductiveportion of the heat transfer.The compli- cated nature of heat transfer requires detailed heat flow It has become clear in the last 10 years that the coastal studiesand extensivedata coverage. The interest in geother- provincesof the Pacific Northwest of the United States mal energypotential has sparkedmuch study of volcanic representa more typical subductionzone terrain than was arcs. The Cascadiasubduction zone is unique in terms of the initiallythought. The 1980 eruptionsof Mount St. Helens emphasizedthe active volcanism already demonstratedby amountand quality of thermaldata available.One surprising thefact that many of the Cascadestratovolcanoes have been result is that the altered volcanic rocks that comprise the activein the last 200 years. Detailed seismicstudies have bedrock of much of the area are of generally low permeabil- identifiedclear evidenceof a subductingslab under Wash- ity so that convectiveheat transfer by groundwatermotion ington[Weaver and Malone, 1987;Weaver and Baker, 1988; doesnot predominateand a generallyconductive regional Ludwinet al., 1990].In addition,evidence has accumulated picture can be detected.The thermal characteristicsof indicatingthat large subduction zone earthquakes may occur several areas in the Cascade volcanic arc in the United in thePacific Northwest, although perhaps on a time scale States have been described [Blackwell et al., 1982; Mase et more extended than those in areas with faster subduction a!., 1982;Blackwell and Steele, 1983]. Recent studies in the rates[Atwater, 1987;Heaton and Hartzell, 1987]. OregonCascade Range are discussedin a companionpaper The thermal structure of volcanic arcs is of great interest, [Blackwellet al., this issue]. The heat flow data in the but regionalthermal characteristicsof volcanic arcs are CanadianCascade Range have alsobeen recentlydescribed difficultto obtain becauseof the nonconductiveheat transfer [Lewiset al., 1988].The first objectiveof this paperis to processesthat operatein suchareas and the complexnature presentand discuss thermal data in Washington;the second Copyright1990 by the AmericanGeophysical Union. is to discussin detail new heat flow data in the Washington Papernumber 90JB01435. partof the CascadeRange; the third is to summarizeand 0148-0227/90/90JB-01435505.O0 comparethe regionalthermal characteristics of the whole 19,495 19,496 BLACKWELLET AL.: HEATFLOW, WASHINGTON STATE AND CASCADERANGE CascadeRange. The implications of the thermal results for ness,1983a, b]. Theselogs are not incorporated into this magmatismin the crust will be discussedin detail. report. Summaw of tectonics. The tectonicsetting of the Wash- ington CascadeRange is summarizedby several papersin DATA PRESENTATION this volume [Leeman et al., this issue; Wells, this issue; Weaver, 1989;Sherrod and Smith, this issue; Stanley et al., Geothermalgradient, heat flow, and ancillary informatio•n this issue], and more references may be found in those fordrill holes in thestate of Washington,excluding pre-1974 papers. The details of the subductionof the Juan de Fuca publisheddata, are summarized inTable 1. Only holes where plate are becomingclearer, and the locationof the subduct- gradientsor heatflow are considered to be of C quality(see ing slab has been mapped even in areas with no active below)or better are included inTable 1. Results for the poor seismicityusing seismictomography and electrical studies qualitydata are given by Blackwell et al. [ 1985,1989]. More [Rasmussen and Humphreys, 1988; Booker and Chave, completeinformation onthe sites in Table1 aregiven by 1989, Wannamaker et al., 1989]. Blackwell et al. [1989]. The slab is seismically active to a depth of up to 80 km Individualholes are located by latitudeand longitude aM beneath northern California and in the Puget Sound region. by townshipand range.Thermal conductivity values are In Oregon, the slab does not appear to be seismicallyactive, basedon measurementsof core or cuttingsamples, or are although because of the lack of detailed station coverage, estimatedfrom lithology(values in parentheses).Terrain small-sizeslab earthquakesmay not be recorded. All studies correctionswere made for all holeswhere the correctionwas showthat the slab dips at moderate anglesfrom the trench to estimatedto exceed5% of the measuredvalue. The terrain the west edge of the Cascade Range. At that point the dip correctionswere madeby the techniqueof Blackwellet el. rapidly increasesso that the slab is typically at depths of [1980] or Birch [1950]. Other aspects of the heat flow/ 100-150 km beneath the volcanic arc. geothermal gradient measurement and calculation tech- niques are discussedby Roy et al. [1968] and Sasset al. BACKGROUND OF THERMAL MEASUREMENTS [1971].A recent summaryof hardwaretechniques for heat flow studiesis givenby Blackwelland Spafiord[1987]. The earliest geothermal measurement (directed by Van Summarymaps of the heat flow and geothermalgradient Ostrand)have been tabulated by Spicer [1964] and Guffanti datafrom Table 1 areshown as Figures 1 and2, respectively. and Nathenson [ 1981]. Estimated heat flow values have been Only one symbolis shownfor eachsite, andwhere appro- calculated for two of these wells (20N/28W-8 and 11N/26E- priate, the results from holes close together have been 20CC) based on thermal conductivityvalues that we mea- averaged.Both the geothermalgradient and heatflow maps sured on surface samplesfrom litho!ogicunits encountered have been contoured.The contouringwas done by handand in these holes. in areasof sparsecontrol is basedon physiographicsetting. In the efirly 1960s,R. F. Roy madeheat flow measure- This procedureis necessaryin the northern Washington ments in northeasternWashington [Roy, 1963; Roy et al., CascadeRange and coastalprovinces because of the scarcity 1968] and measuredtemperatures in Development Associ- of data there. Another large data gap exists in southeastern ates BasaltExplorer 1, a deep hydrocarbonexploration well Washington.There is only shallow groundwaterdevelop- drilled in the Columbia Basin (DABE-1, 21N/31E-10CB). ment along a broad northwest-southeast trending zone Also in the 1960s Sass et al. [1971] made some heat flow throughthe east central Columbia Basin. With the exception measurementsand investigationswere started by Blackwell of a few points near the center of this zone, an area 60-70km [1969]. Preliminary results of additional statewide studies wide