Geological Society of America Field Guide 4 2003 Tectonic geomorphology and the record of Quaternary plate boundary deformation in the Olympic Mountains Frank J. Pazzaglia Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Glenn D. Thackray Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA Mark T. Brandon Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520-8109, USA Karl W. Wegmann Washington Department of Natural Resources, Division of Geology & Earth Resources, Olympia, Washington 98504-7007, USA John Gosse Department of Earth Sciences, Room 3006, Life Sciences Centre, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Eric McDonald Desert Research Institute, Division of Earth and Ecosystem Sciences, 2215 Raggio Parkway, Reno, Nevada 89512, USA Antonio F. Garcia Department of Physics, California Polytechnic State University, San Luis Obispo, California 93407, USA Don Prothero Department of Geology, Occidental College, 1600 Campus Road, Los Angeles, California 90041-3314, USA ABSTRACT We use Quaternary stratigraphy to reconstruct landscape evolution and measure tectonic deformation of the Olympic Mountains section of the Pacifi c Northwest Coast Range. An important motivation for understanding orogenesis here, and throughout the Coast Range, is the concern about the relationship of active deformation to seismic hazards associated with the Cascadia subduction zone. There is also much interest in apportioning the nature of the deformation, whether cyclic or permanent, whether it involves mainly shortening parallel or normal to the margin, and how the deformation on the pro- versus retrowedge sides of the orogen compare. Pre-Holocene stratigraphy and structure provide the only records of suffi cient duration to separate long-term permanent deformation from earthquake-cycle elastic deformation. For this reason, active-tectonic studies have focused on deformation of Quaternary deposits and land- forms, which are best preserved along the Pacifi c Coast and offshore on the continen- tal shelf. At least four major glacial advances are recorded in the valley and coastal deposits along the western margin of the Olympic Peninsula. Both numeric and relative dating, including soils of these deposits, establish a stratigraphic anchor that is used to document the relationship between margin parallel and margin normal deformation in Pazzaglia, F.J., Thackray, G.D., Brandon, M.T., Wegmann, K.W., Gosse, J., McDonald, E., Garcia, A.F., and Prothero, D., 2003, Tectonic geomorphology and the record of Quaternary plate boundary deformation in the Olympic Mountains, in Swanson, T.W., ed., Western Cordillera and adjacent areas: Boulder, Colorado, Geological Society of America Field Guide 4, p. 37–67. For permission to copy, contact [email protected]. © 2003 Geological Society of America. 37 38 F.J. Pazzaglia et al. the Olympic Mountains, which, on a geologic time scale (>103 yr), seems to be the fast- est deforming part of the Cascadia forearc high. The glacial stratigraphic framework is extended to fl uvial terraces of the Clearwater drainage, which remained unglaciated during the late Pleistocene and Holocene, preserving a record of river incision, with each terrace recording the shape and height of past long profi les. We assess how fl uvial terraces are formed in this tectonically active setting and then use features of the ter- races to estimate incision rates along the Clearwater long profi le. The long fl uvial his- tory preserved in the Clearwater ensures that the unsteady deformation associated with the earthquake cycle is averaged out, leaving us with a record of long-term rock uplift as well as horizontal shortening. We show, however, that the earthquake cycle may play an important role in terrace genesis at the millennial time scale. Keywords: geomorphology, active tectonics, subduction wedges, glacial geology, soils, terraces. INTRODUCTION stratigraphic order, from oldest to youngest. Throughout the trip, we will show the data and reasoning for the spatial correlation of We designed a 4-day fi eld trip to exhibit the geology, geo- deposits, their numeric age, and the resulting tectonic implica- morphology, and active tectonics of the Olympic Peninsula. The tions. An important consideration in understanding deformation trip is the culmination of over a decade of collaborative research in this setting is how rocks move horizontally through the sub- in the Olympic Mountains by the authors and their colleagues. duction wedge. We present geomorphic and stratigraphic data to The following text and fi gures are culled from numerous pub- help resolve the horizontal translation of rocks and thus provide lished papers and theses that report the fi ndings of this research, some constraints for shortening over geologic time scale. namely, Brandon and Vance (1992), Thackray (1996, 1998, 2001), As noted, the glacial-interglacial stratigraphy is critical to Wegmann (1999), Brandon et al. (1998), Pazzaglia and Brandon the analysis of tectonism, and additionally has implications for (2001), Wegmann and Pazzaglia (2002), and Tomkin et al. (2003). paleoclimatic processes. Glaciers have descended repeatedly into It is organized around four major topics that should generate lively the coastal lowlands, constructing extensive glacial and glacial- discourse on how to use and interpret basic fi eld relationships in fl uvial landforms and depositing a variety of sediments. Abundant tectonic geomorphology research: (1) How are complexly juxta- organic matter facilitates detailed radiocarbon dating of glacial posed glacial, fl uvial, and eolian surfi cial deposits and their cor- events, and magnetostratigraphy provides additional age control responding soils assembled into a dated stratigraphic framework on older deposits. In the Queets and Hoh River valleys, the gla- in a tectonically active setting?; (2) What is a river terrace, how is ciogenic sediments include lacustrine-outwash-till sequences. In it made, and what do river terraces tell us about active tectonics?; sea-cliff exposures, a last interglacial wave-cut platform separates (3) What is driving orogenesis for the Olympic Mountain segment early and middle Pleistocene sediments from late Pleistocene sedi- of the Cascadia subduction zone? Is it shortening parallel to the ments. Two older glacial units—the Wolf Creek and Whale Creek direction of plate convergence, shortening normal to the direction drifts—predate the last interglaciation. The Whale Creek drift is of plate convergence, or some combination of both? Are there of middle Pleistocene age and the Wolf Creek drift is associated any geomorphic or stratigraphic fi eld relationships that can actu- with magnetically reversed sediments and is therefore of probable ally be used to track the horizontal movement of rocks and thus early Pleistocene age. Three additional stratigraphic units—the interpret the shortening history over geologic time scales? How Lyman Rapids, Hoh Oxbow, and Twin Creeks drifts—docu- do the tectonics on the prowedge versus the retrowedge sides of ment six late Pleistocene glacial advances. The maximum late the Olympic Mountains compare?; (4) We know that uplift along Pleistocene advance occurred between 54,000 yr B.P. and the last Cascadia includes the effects of cyclic earthquake-related defor- interglacial sea-level highstand (ca. 125,000 yr B.P.), most likely mation and long-term steady deformation. How do these different between 55,000 and 75,000 yr B.P. The second glacial maximum types of uplift infl uence incision and aggradation in the rivers of occurred ca. 33,000–29,000 cal. yr B.P., while the advance cor- the Olympic Mountains? relative with the ca. 21,000 cal. yr B.P. ice-sheet maximum was The trip begins by laying out the big-picture tectonic setting. far less extensive. Correlation of glacial fl uctuations with pollen The concept of a pro- and retrowedge is introduced and exhibited fl uctuations determined from Kalaloch sediments by Heusser on the eastern fl ank and core of the range. Geologic and thermo- (1972) indicates that the glacial advances were driven dominantly chronologic evidence for margin parallel versus margin normal by sustained moisture delivery and may refl ect insolation-modu- shortening is introduced. We then begin building a Quaternary lated variations in westerly atmospheric fl ow. stratigraphic foundation anchored along the western coast of the Fluvial terraces are the main source of geologic and Quater- Olympic Peninsula that will be extended landward into the Clear- nary stratigraphic data used in our tectonic interpretations. Terraces water drainage. As far as possible, we will present the deposits in are landforms that are underlain by an alluvial deposit, which in Tectonic geomorphology and the record of Quaternary plate boundary deformation 39 turn sits on top of a strath, which is an unconformity of variable deposits are located and there are good exposures of the accretion- lateral extent and local relief. Typically, the strath is carved into ary wedge rocks. The fi eld relationships for permanent shortening bedrock, but it can also be cut into older alluvial deposits. At the of the Olympic wedge will also be explored. The third day will coast, we recognize straths and their accompanying overlying allu- be devoted to the Clearwater drainage and an investigation of ter- vial deposits, and then show how those features continue upstream races of various size, genesis, and tectonic