A TECTONIC MODEL FOR EVOLUTION OF THE CASCADE RANGE P. E. Hammond* Department of Earth Sciences Portland State University Portland, Oregon 97207 ABSTRACT King (1959, 1977), the belt is composed of a series of smaller metamorphic belts and batholiths but is Four volcanic 'and plutonic episodes in the largely covered by broad expanses of volcanic rocks. evolution of the Cascade Range are recognized: (1) The belt was formed in the late Mesozoic as part of dominantly andesitic volcanism of the lower Western the Nevadan orogeny. Its northern, or Columbia, Cascade Group (WCG) with comagmatic plutonism in the embayment was filled with volcanic rocks and accreted North Cascades, 50-30 Ma; (2) mixed andesitic and to the continent during the Cenozoic. Wise (1963), silicic volcanism with continuing plutonism of middle following the global tectonics of Carey (1958), WCG, 30-15 Ma; (3) slightly greater silicic than interpreted the spreading pattern of the belt to have andesitic volcanism and plutonism culminating WCG, continuously formed since the Paleozoic in response to 15-5 Ma; and (4) dominantly basaltic volcanism of a regional right-lateral distortion, or megashear, High Cascade Group (HCG), 5-0 Ma. Epizonal plutons which is causing east-west extension in the Basin and form two north-trending belts underlying the northern Ranges and north-south compression in the Columbia part of the range. Folding in northern part of the embayment. Hamilton (1966, 1969) pictured the belt as range was along northwest to north trends while broad an orocline that was caused principally by westward subsidence prevailed in the southern part 50-12 Ma. extension during the Cenozoic when the Klamath East-trending Yakima folds were superimposed on the Mountains and Sierra Nevada moved northwestward and northern part of the range while the southern part the Basin and Ranges spread apart behind them. continued subsiding 12-5 Ma. Uplift began in the Hamilton further believed that during the Cenozoic the North Cascades possibly as early as 50 Ma, in Mesozoic terrain of northeastern Oregon pivoted away southern Washington 20 Ma, attaining as much as km from the Idaho batholith, and that the Cenozoic vol- elevation. In the south uplift has been negligible canic rocks of southwestern Washington and northwes- and the HCG accumulation accounts for the topoqraDhy. tern Oregon covered former oceanic crust of the With a change in the subduction regime, the style of Columbia embayment. WCG volcanism shifted to HCG basaltic outpourings along north-south structures and extension pervaded Geophysical studies sustain Hamilton's picture. the range, diverging plutonic belts and bowing the According to Warren and Healy (1973), the crust range westward in southern Oregon. thickens from about 15 km beneath the western margin This evolution can be traced in a tectonic model of the Coast Range to more than 30 km along the which starts with the growth of the Cascade volcanic eastern margin of the Cascade Range (Fig. 1). The arc on the back side of the Coast Range-Klamath Willamette Valley is not a major structure and the Mountains block. They rifted from the continent along ranges lack roots (Thiruvathukal and others, 1970). the Olympic-WalIowa lineament (OWL) and together The crustal thickness beneath the Klamath Mountains rotated about a pivot in the Olympic Mountains, with ranges from 20 in the west to about 35 km in the east, OWL becoming a transition zone between the southern continuing at that thickness beneath southeastern Cascade arc and the stationary North Cascades. Active Oregon. Below the central Columbia Plateau the crust subduction pressed the northern pivotal end of the thins to 23 km (Hill, 1972). Deep resistivity studies block against the continent. Rotation occurred during across the Olympic-WalIowa lineament (OWL in Fig. 1) episodes 1 and 2, with the ranges reaching north-south indicate low resistivity value in north-cental Oregon positions before outpourings of Columbia River Basalt. and high resistivity values in southern Oregon and During rotation the Blue and Ochoco Mountains were easternmost Washington (Cantwell and Orange, 1965). dragged northward by the Cascade arc along probable In addition, seismic-refraction studies of the upper northwest- to west-trending faults and uplifted during mantle beneath the central Columbia Plateau (Hill, Yakima folding. Thinned Mesozoic rocks underlie the 1972) and west of the Cascade Range in Washington and central Columbia Plateau; rifted and rafted pre-middle northern Oregon (Berg and others, 1966) reveal Vp Miocene volcanic rocks underlie southeastern Oregon. velocities of more than 7 km/sec. These determina- Active subduction culminated in episode 3, with tions suggest that granitic crust underlies the Blue wanning subduction and extension characterizing Mountains (Thiruvathukal and others, 1970). Basaltic episode 4. oceanic crust, although thicker than average, is in- terpreted to underlie the Coast and Cascade Ranges and INTRODUCTION possibly the southwestern Columbia Plateau and Klamath Mountains. The sprawling tectonic pattern of the Pacific Northwest has puzzled geologists for years. How was Refinements of Hamilton's concept were forth- a narrow belt of the Cordillera deformed to a Z-shaped coming. Dickinson (1976) postulated that basaltic orogenic belt (Fig. 1)? When was the belt formed? seamounts collided with the North American continent And what is the tectonic relationship of the oblique during middle Eocene to form the Coast Range and a Coast and Cascade Ranges to the belt? According to magmatic arc (the Challis-Absaroka arc of Snyder and others, 1976) stepped rapidly westward from north- eastern Washington and central Idaho to form the Cas- •Present address: Department of Geology, City of cade Range during middle and late Eocene. London Polytechnic, London El 2NG, England. 219 Pacific Coast Paleogeography Symposium 3: Cenozoic Paleogeography of the Western United States, 1979 Copyright © 2012 Pacific Section, SEPM (Society for Sedimentary Geology) ho 7 K5 CANADA _H °_\ - CANADA, __4<f CD US. A N _ WASH. 46° OREGON A / I I IDAHO / / / I^ I ( \D;y 1 bas'nT L/ A AND RANGE | 42°- [:::vX::::| Eocene basalts | | Cen. fold axes HIGH CASCADE GROUP [ | Cascade vole. rx. |p * [ Feeder dikes WESTERN CASCADE GROUP: Columbia R. basalt | ..-.-.-•-'-"'"'| Intrusive belt INTRUSIONS Tertiary pluton I jigy I Strike-slip fault UPPER |aq Volcano/caldera | l|| | High Cascade fissure MIDDLE ] LOWER 1% | Meso. struc. I Normal fault Figure 1. Generalized geologic map of the Pacific Northwest, showing the Figure 2. Generalized geologic map of the volcanic Cascade Range, major Cenozoic tectonic elements, based on King and Beikman (1974), with showing distribution of the parts of the Western Cascade Group, additions from King (1959), Lawrence (1976), Wright (1976), Walker (1977), High Cascade Group, and Tertiary granitic intrusions; taken from Swanson and Wright (1978), and Hammond (1976, 1979a, b), vfTTh modifica- Figure 1 with modifications. Patterns as in Figure 1. tions. Cities: Po=Portland, Se=Seattle. Fault zones: B=Brothers, D=Denio, E=Eugene, HL=Honey Lake, JD=John Day, K=Klickitat, L=Likely. Landmarks: MD=Monument dike swarm, PSL=Puget Sound lowland, SN=northern end of the Sierra Nevada, WV=Willamette Valley. Volcanoes: A=Mount Adams, B=Mt. Baker, CL=Crater Lake, G=Glacier Peak, H=Mt. Hood, J=Mt. Jefferson, L=Lassen Peak, ML=Medicine Lake caldera, N=Newberry caldera, R=Mt. Rainier, S=Mt. Shasta, SH=Mt. St. Helens, TS=Three Sisters. 23 7 EVOLUTION OF THE CASCADE RANGE From paleomagnetic directions, Simpson and Cox or rifted, from the North American continent along a (1977) determined that part of the Oregon Coast Range zone now part of the Olympic-Wallowa lineament. To- rotated about 70 clockwise into position during mid- gether coastal block and Cascade arc rotated westward dle Eocene to middle Miocene. They proposed two against an active subduction zone, arriving at ap- paleogeographic models to explain the position of the proximate north-south positions in middle Miocene. The Coast Range. In model 1, the Coast Range block, as a volcanic arc was partly uplifted and capped by vol- slab of oceanic plate undergoing subduction, rotated canic rocks of the High Cascade Group to form the Cas- during early Eocene from a southern point near the cade Range, under conditions of wanning offshore sub- Klamath Mountains. The block rafted against the con- duction during PIio-Pleistocene. tinent by middle Eocene but continued to rotate until middle Miocene. In model 2, after being rafted FRAMEWORK OF THE CASCADE RANGE against the continent in middle Eocene as an oceanic aseismic ridge, the Coast Range, together with the The Cascade Range extends about 1100 km from Klamath Mountains, thereafter rotated from a point at Mount Garibaldi in British Columbia southward to Las- its northern end. During this time the Blue Mountains sen Peak in northern California (Fig. 1). The 250-km rafted away from the Idaho batholith, creating gaps long North Cascades, bounded on the south by the Olym- which were filled with old crust or volcanic rocks, pic-Wallowa lineament (OWL) at Snoqualmie Pass, is most of which were eventually covered by younger vol- composed predominantly of pre-Cenozoic sedimentary, canic rocks. Model 2 follows Hamilton's concept. metamorphic, and plutonic rocks. Remnants of Tertiary Although both models have major unresolved problems volcanic rocks occur locally; granitic plutons are (Simpson and Cox, 1977), evidence that the Coast Range abundant. The North Cascades, like the entire range, has rotated is convincing. Choiniere and others are crowned by snow-clad stratovolcanoes, Glacier (1976), Cox and Magill (1977), and Plumley and Beck Peak, Mount Baker, and Mount Garibaldi. In contrast, (1977) show that a large segment of the Coast Range the southern 850-km extent is underlain almost exclu- has rotated as a unit, as suspected by Simpson and sively by Cenozoic volcanic and intrusive rocks, for- Cox. If the Coast Range has rotated, how has the ming the volcanic Cascade Range.