Holocene Tectonics and Fault Reactivation in the Foothills of the North Cascade Mountains, Washington

Seeing the True Shape of Earth’s Surface themed issue Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington Brian L. Sherrod1, Elizabeth Barnett1, Elizabeth Schermer2, Harvey M. Kelsey3, Jonathan Hughes4, Franklin F. Foit, Jr.5, Craig S. Weaver1, Ralph Haugerud1, and Tim Hyatt6 1U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA 2Geology Department, Western Washington University, Bellingham, Washington 98225, USA 3Department of Geology, Humboldt State University, Arcata, California 95521, USA 4Department of Geography, University of the Fraser Valley, 33844 King Road, Abbotsford, British Columbia V2S 7M8, Canada 5School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164-2812, USA 6Nooksack Tribe, 5016 Deming Road, PO Box 157, Deming, Washington 98244, USA ABSTRACT Creek and Canyon Creek faults formed in McCaffrey et al., 2007). GPS rates are higher in the early to mid-Tertiary as normal faults the southern forearc (6–10 mm/yr) and decrease We use LiDAR imagery to identify two and likely lay dormant until reactivated as to the north (Fig. 1C). With geodetic rates falling fault scarps on latest Pleistocene glacial out- reverse faults in a new stress regime. The to near zero just north of the United States–Brit- wash deposits along the North Fork Nook- most recent earthquakes—each likely Mw > ish Columbia border, questions remain regarding sack River in Whatcom County, Washington 6.3 and dating to ca. 8050–7250 calendar the activity of faults in the northern Puget Low- (United States). Mapping and paleoseismic years B.P. (cal yr B.P.), 3190–2980 cal. yr land (Washington). investigation of these previously unknown B.P., and 910–740 cal. yr B.P.—demonstrate Forearc basins and uplifts in western Wash- scarps provide constraints on the earth- that reverse faulting in the northern Puget ington—from south to north, the Tacoma, quake history and seismic hazard in the Lowland poses a hazard to urban areas Seattle , and Everett Basins—are defi ned mainly northern Puget Lowland. The Kendall scarp between Seattle (Washington) and Vancou- on the basis of high-amplitude geophysical lies along the mapped trace of the Boul- ver, British Columbia (Canada). anomalies, with faults hypothesized where the der Creek fault, a south-dipping Tertiary gradients are strongest (Blakely et al., 2002; normal fault, and the Canyon Creek scarp INTRODUCTION Brocher et al., 2001; Danes et al., 1965). Many lies in close proximity to the south-dipping of the faults found between the basins and the Canyon Creek fault and the south-dipping Geologic and geodetic data show that near- adjacent uplifts are active and present substan- Glacier Extensional fault. Both scarps are surface faults in the Cascadia forearc accom- tial seismic hazard to the region (Fig. 1B). The south-side-up, opposite the sense of displace- modate north-south shortening. Models of the best-known of these basin-bounding faults are ment observed on the nearby bedrock faults. forearc (Fig. 1A) show a series of migrating, the Tacoma, Seattle, Southern Whidbey Island, Trenches excavated across these scarps clockwise-rotating forearc blocks (Wells and and Darrington–Devils Mountain faults (Buck- exposed folded and faulted late Quaternary Simpson, 2001; Wells et al., 1998). This clock- nam et al., 1992; Johnson et al., 2004a, 2004b, glacial outwash, locally dated between ca. wise rotation causes convergence in western 1994; Sherrod, 2001; Sherrod et al., 2008, 12 and 13 ka, and Holocene buried soils and Washington (United States) where the Oregon 2004). The Darrington–Devils Mountain fault scarp colluvium. Reverse and oblique fault- Coast Range block impinges on Tertiary vol- forms a broad boundary between the northern ing of the soils and colluvial deposits indi- canic rocks and sediments, compressing these edge of the Everett Basin and accreted Meso- cates at least two late Holocene earthquakes, Tertiary rocks against the southern edge of the zoic rocks and Tertiary sedimentary rocks in the while folding of the glacial outwash prior to British Columbia (Canada) Coast Mountains. adjacent uplift. North of this uplift lies the Bell- formation of the post-glacial soil suggests This compression results in a series of struc- ingham forearc basin, the northernmost forearc an earlier Holocene earthquake. Abrupt tural basins separated by uplifts in the northern basin. The Bellingham Basin preserves Eocene changes in bed thickness across faults in the Cascadia forearc. Geological models show that to Quaternary sedimentary rocks in its interior Canyon Creek excavation suggest a lateral blocks in the northern Cascadia forearc move and is bounded on the north by pre-Tertiary component of slip. Sediments in a wetland northward relative to the Coast Mountains at sedimentary and metamorphic rocks. The Bell- adjacent to the Kendall scarp record three rates of 7–9 mm/yr (Wells and Simpson, 2001; ingham Basin is anomalous compared to the pond-forming episodes during the Holocene— Wells et al., 1998). other forearc basins in western Washington we infer that surface ruptures on the Boul- Geodetic studies show that the region between because prior to our studies, no known active der Creek fault during past earthquakes 46.5°N and 49.5°N is undergoing north-south faults lined the basin margins, yet geologic and temporarily blocked the stream channel shortening averaging ~3 mm/yr to 4.4 mm/yr geodetic data suggest that the basin should be and created an ephemeral lake. The Boulder (Hyndman et al., 2003; Mazzotti et al., 2002; the locus of active faulting (Kelsey et al., 2012). Geosphere; August 2013; v. 9; no. 4; p. 827–852; doi:10.1130/GES00880.1; 16 fi gures; 5 tables; 1 supplemental fi le. Received 26 October 2012 ♦ Revision received 7 May 2013 ♦ Accepted 17 June 2013 ♦ Published online 16 July 2013 For permission to copy, contact [email protected] 827 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/827/3345917/827.pdf by guest on 27 September 2021 Sherrod et al. Figure 1. (A) Location map of 124°W 121°W B the Pacifi c Northwest, showing A the Cascadia subduction zone, GPS Profile (Fig. 1C) BoundaryBoundary ofof FraserFraser - PPugetuget LLowlandowland BRFB CBCB NORTH R mobile forearc blocks, and British F AMERICAN Columbia stable North America (WA— PLATE VancouverV Isl. V a Washington; OR—Oregon; 50°N n BFGFB Coast Mtns c FG Penticton o CRC F B.C.B.C. buttress. uv R OWL—Olympic-Wallowa lin- 49°N e FFFF C r a Is BBBB U.S.U.S. s l. eament). Thumbtack indicates c AreaArea ofof FigureFigure 1B1B CLFCL a F B d i that the position of crust shown a 36 s WA u mm/yr b d in gray relatively is fi xed based SCFS u OWLOW C c L t F i on very-long-baseline inter- JUAN DE o regon DDMFD 45° O D n OR AreaArea ofof MF FUCA PLATE z ferometry (vlbi) results from o FigureFigure 2 LRFLRF SWIFS EEBB ne Cascade 48°N W Oregon I Penticton, British Columbia, volcanic arc F Coast PacificPacific Range OlympicOlympic which indicate no resolvable PACIFIC OceanOcean MtnsMtns PLATE SBSB NBNB change of position with respect Pacific 53 mm/yr SFSF Ocean Nevada TFTF MBMB to stable North America (Wells ° 0 4 0 50 km et al., 1998). (B) Enlarged map 200 km Californiai OFOF lif TBTB showing geologic and tectonic 130°W 125° 120° features in northwestern Wash- ington and southwestern Brit- SN ish Columbia. (C) Residual C Geologic Units – 1A GPS shortening rates (after 10 Residual GPS removing elastic subduction Sierra Nevada block zone component from GPS Coast Range block 8 signal) from southern Oregon Basin and Range 6 to southern British Columbia Columbia River Basalt Group (redrawn following Mazzotti 4 Washington forearc block et al., 2002). V North equals northward-directed velocities; Basement rocks V north (mm/yr) 2 Extensional magmatism error bars show 95% confi dence 0 level; gray shaded area shows Symbols – 1A and 1B 95% confi dence level for aver- Forearc block motion 44 46 48 50 age velocity. Abbreviations: Lattitude (°N) B—Bellingham; BB—Belling- Relative motion (amount indicated) Geologic Units - 1B ham Basin; BRF—Beaufort North American plate “fixed” Range fault; BFGF—Benson Quaternary sediments Quaternary volcano fault–Ganges fault system; Quaternary volcanics CB—Comax Basin; CRFF— Structural basin (inferred from gravity anomalies) Cameron River–Fulford fault; Tertiary continental deposits Fault, black where Holocene CLF—Cowichan Lake fault; Tertiary marine deposits DDMF—Darrington–Devils movement is known or suspected Mountain fault; EB—Everett Concealed fault, black where Tertiary volcanic rocks (excl. Siletzia) Basin; LRF—Little River Holocene movement is known or Volcanic rocks of Siletzia suspected fault; MB—Muckleshoot Basement rocks (pre-Tertiary and intrusive rocks) Basin; NB—North Bend Basin; OF—Olympia fault; SCF— Straight Creek fault; SB—Seattle Basin; SF—Seattle fault; SWIF—Southern Whidbey Island fault; TB—Tacoma Basin; TF—Tacoma fault; V—Vancouver (Vancouver Island faults from England and Calon, 1991). Digital geologic data from Washington Division of Geology and Earth Resources (2005). This paper demonstrates active faulting on a dence for multiple Holocene earthquakes in an A broad lowland repeatedly glaciated in the set of recently discovered fault scarps along the area of the forearc previously thought to be tec- Quaternary occupies the region (Booth, 1994; eastern edge of the Bellingham Basin in north- tonically quiescent. Easter brook, 1985). Marine waters of Puget western Washington (Fig. 2). These scarps lie Sound partially occupy large areas of this low- adjacent to and represent reactivation of previ- BACKGROUND GEOLOGY land, and Quaternary glacial deposits partially ously mapped bedrock normal faults (Fig.

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