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High-Resolution Temporal and Stratigraphic Record of Siletzia's Paleogene nonmarine basin evolution in central and western Washington High-resolution temporal and stratigraphic record of Siletzia’s accretion and triple junction migration from nonmarine sedimentary basins in central and western Washington Michael P. Eddy1,†, Samuel A. Bowring1, Paul J. Umhoefer2, Robert B. Miller3, Noah M. McLean4, and Erin E. Donaghy2 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA 3Department of Geology, San Jose State University, San Jose, California 95192, USA 4Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA ABSTRACT strike-slip faulting is consistent with ridge- tion of the effects of ridge-trench interaction, trench interaction and supports the presence as they are preserved in the North American The presence of early Eocene near- of an oceanic spreading ridge at this latitude geologic record. trench magmatism in western Washing- along the North American margin during Previous studies have identified two areas in ton and southern British Columbia has led the early Eocene. the Pacific Northwest that may record Paleo- to speculation that this area experienced gene ridge-trench interaction: the terranes that ridge-trench interaction during that time. INTRODUCTION compose the southern Alaska margin (Fig. 1A) However, the effects of this process as they and the Paleogene forearc of southern British are preserved in other parts of the geologic Ridge-trench interaction is a fundamental Columbia and western Washington. The recog- record are poorly known. We present high- tectonic process that can dramatically alter the nition of these areas has led to several potential precision U-Pb zircon geochronology from geology of convergent margins. Such inter- plate reconstructions that place oceanic spreading Paleogene nonmarine sedimentary and actions have occurred several times over the past centers intersecting North America along the volcanic sequences in central and western 60 m.y. in western North America as oceanic southern Alaska margin, at the latitude of Wash- Washington that preserve a record of tec- plates were consumed in E-dipping subduction ington, or simultaneously in both areas between tonic events between ca. 60 and 45 Ma. The zones. Most recently (ca. 30 Ma–present), the ca. 60 Ma and 50 Ma (Fig. 1A; Wells et al., data reveal that the Swauk, Chuckanut, and intersection of the Pacific- Farallon Ridge with 1984; Engebretson et al., 1985; Cowan, 2003; Manastash Formations formed a nonmarine the continent has led to the fragmentation of the Haeussler et al., 2003; Madsen et al., 2006). sedimentary basin along the North Ameri- Farallon plate into multiple microplates (Stock Yet, while the effects of ridge-trench inter- can margin between ≤59.9 and 51.3 Ma. This and Lee, 1994), the development of the San action are well documented in southern Alaska basin experienced significant disruption that Andreas fault ( Atwater, 1970), and the gen- (Bradley et al., 2003, and references therein), culminated in basinwide deformation, uplift, eration of geochemically anomalous magmas evidence for ridge-trench interaction at the lati- and partial erosion during accretion of the above slab windows and areas of slab breakoff tude of Washington relies almost exclusively Siletzia terrane between 51.3 and 49.9 Ma. (e.g., Johnson and O’Neil, 1984; Benoit et al., on the existence, and age, of geochemically Immediately following accretion, dextral 2002). Plate reconstructions for the northern anomalous near-trench and backarc magmatism strike-slip faulting began, or accelerated, on Pacific Basin also require subduction of the (Breit sprecher et al., 2003; Groome et al., 2003; the Darrington–Devil’s Mountain, Entiat, Kula-Farallon Ridge or Kula-Resurrection Haeussler et al., 2003; Madsen et al., 2006; Ick- Leavenworth, Eagle Creek, and Straight and Resurrection- Farallon Ridges along North ert et al., 2009). Ridge-trench interaction has Creek–Fraser fault zones between 50 and America’s western margin during the early also been invoked, at least in part, to explain 46 Ma. During this time, the Chumstick For- Cenozoic (Fig. 1A; Atwater, 1970; Grow and the origin and accretion of the oceanic Siletzia mation was deposited in a strike-slip basin Atwater, 1970; Wells et al., 1984; Engebret- terrane to this part of North America during coeval with near-trench magmatism. Fault- son et al., 1985; Haeussler et al., 2003; Mad- the early Eocene (McCrory and Wilson, 2013; ing continued on the Entiat, Eagle Creek, sen et al., 2006). However, the precise timing Wells et al., 2014). Nevertheless, few studies and Leavenworth faults until a regional sedi- and location(s) of this ridge-trench interaction have considered other manifestation of Paleo- mentary basin was reestablished ≤45.9 Ma, remain uncertain, as the oceanic crust needed gene ridge-trench interaction at this latitude. and may have continued on the Straight to constrain the position of these ridges is In central and western Washington, a series Creek–Fraser fault until 35–30 Ma. This completely subducted. Instead, the location of well-studied Paleogene nonmarine sedimen- record of basin disruption, volcanism, and of past triple junctions, and consequently the tary and volcanic sequences preserve a record geometry of past plate configurations, must of sedimentation, deformation, and volcanism †mpeddy@ mit .edu be constrained through the careful identifica- that may be related to ridge-trench interaction GSA Bulletin; March/April 2016; v. 128; no. 3/4; p. 425–441; doi: 10.1130/B31335.1; 9 figures; 2 tables; Data Repository item 2015323; published online 28 September 2015. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 128, no. 3/4 425 © 2015 Geological Society of America Eddy et al. triple junction migration. However, the tec- Figure 1 (on following page). (A) Map of the Pacific Northwest that shows possible locations tonic significance of these rocks remains con- for the Kula-Farallon spreading ridge during the early Paleogene, modified from Haeussler troversial due to disagreements over whether et al. (2003). If the terranes that compose southern Alaska are far-traveled from a more they record sediment accumulation within southerly location in the early Paleogene (e.g., Cowan, 2003), the range of possible posi- regional or local basins. Precise geochronology tions would narrow along the Washington and British Columbia coasts. Alternatively, if the could resolve this issue by temporally correlat- Chugach terrane was not far-traveled, the Resurrection oceanic plate may have existed and ing these sequences and constraining the age, would have resulted in two or more ridges intersecting North America (Haeussler et al., 2003; duration, and spatial extent of tectonic events, Madsen et al., 2006). (B) Generalized geologic map of central and northwest Washington, and we present 22 high-precision U-Pb zircon including southern Vancouver Island, modified from Walsh et al. (1987), Stoffel et al. (1991), dates with this goal in mind. The data reveal an Schuster et al. (1997), Dragovich et al. (2002), and Massey et al. (2005). Paleogene-aged sedi- abrupt tectonic change at ca. 50 Ma marked by mentary formations and regional strike-slip faults discussed in this paper are labeled, and deformation, bimodal volcanism, and initiation, abbreviations are: MH—Mount Higgins area, BP—Barlow Pass area, ECF—Eagle Creek or significant acceleration, of dextral strike-slip fault, LFZ—Leavenworth fault zone, and DDMFZ—Darrington–Devil’s Mountain fault faulting. This change coincides with near-trench zone. Stars denote the locations of Paleogene-aged adakites, abbreviated: BH—Bremer- magmatism along the Washington and British ton Hills, MPV—Mount Persis volcanics, PT—Port Townsend, and peraluminous granites, Columbian coasts (e.g., Madsen et al., 2006), as abbreviated: WCI—Walker Creek intrusions, MPS—Mount Pilchuck suite (Tepper et al., well as the accretion of the Siletzia terrane to 2004; Madsen et al., 2006; MacDonald et al., 2013). The heavy dashed line marks the in- North America (Wells et al., 2014), and we sug- ferred boundary between Siletzia and North America (Wells et al., 1998). gest that all of these events are related to triple junction migration. detailed stratigraphic, paleocurrent, and petro- are presented in Table DR1 and shown as con- PALEOGENE NONMARINE logic data are available for each sequence, exist- cordia plots in Figure DR1.1 Our preferred date SEDIMENTARY AND VOLCANIC ing geochronology is too imprecise to make for each sample is shown in Table 1. SEQUENCES accurate temporal correlations. Despite this dif- All reported dates represent a weighted ficulty, previous correlation efforts have used a mean of 238U-206Pb zircon dates, or a single- Nonmarine sedimentary and volcanic rocks variety of stratigraphic arguments, in addition grain 238U-206Pb date for samples where taking of Paleogene age are exposed as isolated to existing geochronology, to create regional a weighted mean is inappropriate. A correction sequences along and between the Straight histories for sedimentation (e.g., Tabor et al., for preferential exclusion of 230Th during zir- Creek–Fraser, Darrington–Devil’s Mountain, 1984; Cheney, 1994) and fault motion (Gresens con crystallization was made using a calculated Entiat, and Leavenworth fault zones in central
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