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Western Limits of the Seattle Fault Zone and Its Interaction with the Olympic Peninsula, Washington

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Western Limits of the Seattle Fault Zone and Its Interaction with the Olympic Peninsula, Washington Western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington A.P. Lamb1, L.M. Liberty1, R.J. Blakely2, T.L. Pratt3, B.L. Sherrod3, and K. van Wijk1 1Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA 2U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA 3U.S. Geological Survey, School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195, USA ABSTRACT INTRODUCTION preted north-dipping backthrusts that are in part beneath the Seattle metropolitan area (Fig. 1). We present evidence that the Seattle fault Oblique subduction of the Juan de Fuca plate The shallow portion of this fault zone is com- zone of Washington State extends to the west beneath the North American continent results in posed of a monocline that bounds the southern edge of the Puget Lowland and is kinemati- northeast migration of coastal regions of Wash- margin of the Seattle Basin, and mapped faults cally linked to active faults that border the ington State relative to stable North America. and folds in the hanging wall just south of the Olympic Massif, including the Saddle Moun- This northeast motion is resisted by Mesozoic monocline. The Seattle fault zone may extend tain deformation zone. Newly acquired high- and older rocks that form the stable craton of to the east beyond the boundaries of the Seattle resolution seismic reflection and marine southwest Canada, resulting in shortening Basin to merge with the active South Whidbey magnetic data suggest that the Seattle fault of the Puget Lowland region of Washington Island fault (Fig. 1; Johnson et al., 1996; Liberty zone extends west beyond the Seattle Basin State (Wells et al., 1998; Mazzotti et al., 2002; and Pratt, 2008; Sherrod et al., 2008; Blakely to form a >100-km-long active fault zone. We McCaffrey et al., 2007). This shortening is et al., 2009). provide evidence for a strain transfer zone, expressed, in part, as a series of northwest- and The Tacoma fault on the south side of expressed as a broad set of faults and folds west-trending active faults that separate basins the Seattle uplift is less well defi ned than the connecting the Seattle and Saddle Mountain and structural uplifts beneath the Puget Low- Seattle fault. The Tacoma fault extends ~20 km deformation zones near Hood Canal. This land, within which are the Seattle and Tacoma along the southern margin of the Seattle uplift connection provides an explanation for the metropolitan areas (Fig. 1; Johnson et al., 1996; between Carr Inlet and the southeastern extent apparent synchroneity of M7 earthquakes Pratt et al., 1997). of Hood Canal (Fig. 1B). The Tacoma fault is on the two fault systems ~1100 yr ago. We The Seattle Basin and Tacoma Basin extend along strike of the White River fault (Fig. 1A), redefi ne the boundary of the Tacoma Basin eastward ~70 km from Hood Canal, beneath which extends through the Cascade Range, but to include the previously termed Dewatto the Seattle-Tacoma urban corridor, to the foot- no direct evidence links these two fault systems basin and show that the Tacoma fault, the hills of the Cascade Range (Fig. 1). The Seattle (Blakely et al., 2007, 2011). Field studies show southern part of which is a backthrust of uplift, separating the basins, is interpreted as a that the Seattle and Tacoma faults are capable the Seattle fault zone, links with a previously pop-up block above the south-dipping Seattle of causing large earthquakes (Atwater and unidentifi ed fault along the western margin thrust fault to the north and the Tacoma back- Moore, 1992; Sherrod et al., 2004), so know- of the Seattle uplift. We model this north- thrust to the south (Pratt et al., 1997; Brocher ing their overall lengths and their interactions south fault, termed the Dewatto fault, along et al., 2001, 2004). Direct geologic evidence for with neighboring faults help us to understand the western margin of the Seattle uplift as the Seattle and Tacoma fault systems is sparse, fault kinematics and earthquake hazards in this a low-angle thrust that initiated with exhu- consisting primarily of uplifted bedrock terraces area. The most recent large rupture occurred on mation of the Olympic Massif and today (Bucknam et al., 1992; Kelsey et al., 2008), topo- the Seattle fault zone in A.D. 900–930, produc- accommodates north-directed motion. The graphic scarps observed in light detection and ing a M7–7.5 earthquake that lifted the hanging Tacoma and Dewatto faults likely control ranging (Lidar) surveys that cover a large area of wall ~6.5 m and generated a local tsunami and both the southern and western boundaries of the Puget Lowland (e.g., Haugerud et al., 2003; landslides (Atwater and Moore, 1992; Bucknam the Seattle uplift. The inferred strain trans- Sherrod et al., 2004, 2008), and faults and folds et al., 1992; Jacoby et al., 1992; Atwater and fer zone linking the Seattle fault zone and found in detailed studies of trench excavations Hemphill-Haley, 1997; Sherrod et al., 2000; ten Saddle Mountain deformation zone defi nes across Lidar scarps (e.g., Nelson et al., 2003; Brink et al., 2006). Trench studies across Lidar the northern margin of the Tacoma Basin, Sherrod et al., 2004). Fault strands and underly- scarps on the Tacoma and Saddle Mountain and the Saddle Mountain deformation zone ing structures are inferred from seismicity, mag- faults (Fig. 1) suggest earthquakes with timing forms the northwestern boundary of the netic, gravity, geologic, and seismic refl ection (within the limits of radiocarbon dating) similar Tacoma Basin. Our observations and model data (e.g., Finn, 1990; Pratt et al., 1997; Brocher to that of the Seattle fault zone event 1100 yr suggest that the western portions of the et al., 2001; Blakely et al., 2002; Johnson et al., ago that may be contemporaneous (e.g., Sherrod Seattle fault zone and Tacoma fault are com- 2004; Stephenson et al., 2006; Liberty and Pratt, et al., 2004, 2008; Blakely et al., 2009). plex, require temporal variations in principal 2008; Sherrod et al., 2008). In this paper, we explore the deformation strain directions, and cannot be modeled as a The ~70-km-long Seattle fault zone is com- caused by convergence across the Seattle fault simple thrust and/or backthrust system. posed of south-dipping thrust faults and inter- zone and eastern portions of the Olympic Massif Geosphere; August 2012; v. 8; no. 4; p. 915–930; doi:10.1130/GES00780.1; 9 fi gures. Received 28 December 2011 ♦ Revision received 6 April 2012 ♦ Accepted 9 April 2012 ♦ Published online 26 June 2012 For permission to copy, contact [email protected] 915 © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/4/915/3342851/915.pdf by guest on 28 September 2021 Lamb et al. 124° W 123° W 122° W 121° W A LRF DMF Figure 1. (A) Map modified from Blakely et al. (2009) show- Canada USA ing the tectonic setting of the EB Puget Lowland and Olympic SWIF Peninsula. The yellow arrow 48° shows the regional direction of Pacific Ocean KA Seattle SCF strain relative to North Amer- Olympic Massif ica (McCaffrey et al., 2007). SB Olympic This strain causes north-south Subduction Complex compression that is buttressed Z SFZ Fall City RMF (OSC) D by the stable Canadian craton M and results in 4.4 ± 0.3 mm/yr S SU SMF of permanent shortening being Region of uplift HRF TF Fig. 1B Tacoma accommodated across the Puget Basin Lowland region. FCF—Frigid CRF TB WRF Thrust fault Creek fault; HRF—Hurricane Region of uplift FCF Fig. 9 Figs 2A and 2B Ridge fault; OF—Olympia NormalBasin fault OF fault; OU—Olympia uplift; 47° Strike-slip fault OU SB—Seattle Basin; SF—Seattle 40 km fault; SMF—Saddle Mountain ′ East and Saddle Mountain 123°30′ W 123° W 122°30′ W West faults; SMDZ—Saddle B Mountain deformation zone; SU—Seattle uplift; TB— OSC LM UM Tacoma Basin; TF—Tacoma 47°45′ ' fault. Other regional faults not referred to in this research but shown in Figure 1: CRF—Can- Lake yon River fault; DMF—Devils l Seattle Fault Zone Mountain fault; EB—Everett Cushman Cana basin; KA—Kingston arch; HRF Peripheral terrane LRF—Leech River fault; HoodFig. 3 RMF—Rattlesnake Mountain ′ ' 47°30 SMDZ Dyes Inlet fault; SCF—Straight Creek Price SMF fault; SWIF—southern Whid- Lake CRF bey Island fault; WRF—White River fault. (B) Geological map Tacoma Fault modifi ed from Schuster (2005) FCF Inlet and Blakely et al. (2009). Carr Quaternary alluvium Tertiary volcanic rocks Division Between Upper Pleistocene glacial deposits Tertiary intrusive rocks and Lower Crescent Formation Tertiary marine sedimentary rocks Eocene Crescent Formation 10 km using new high-resolution seismic profi les and link between several fault systems in the Puget Puget Lowland and the Seattle fault zone (Fig. magnetic data between Hood Canal and Puget Sound region, providing a possible explanation 1A). The complex is cored by severely deformed Sound (Fig. 2). These new data cross the north- for synchronous ruptures of multiple faults dur- and metamorphosed Eocene to Miocene marine west and west fl anks of the Seattle uplift where ing large earthquakes. sedimentary rocks that have been uplifted to structures may defi ne the western limits of the form the Olympic Massif (Fig. 1A). The sedi- Seattle fault zone. We integrate results from GEOLOGICAL AND mentary strata are thrust beneath peripheral these newly acquired data with previously pub- GEOPHYSICAL SETTING rocks of the Siletz terrane, a largely volcanic ter- lished geological and geophysical data to test rane of oceanic affi nity that forms the crystalline whether there is a link between the Seattle fault The Olympic subduction complex, an basement beneath most of the Cascadia forearc zone and structures in the Olympic Massif to the exhumed part of the Cascadia accretionary and reaches thicknesses of as much as 35 km west.
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