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Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America

J.K. Madsen*1, D.J. Thorkelson*1, R.M. Friedman*2 and D.D. Marshall*3

¹ Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
² Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Science, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
³ Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada

View larger version (59K): [in this window] [in a new window]   Figure 1. Paleocene to Oligocene forearc, arc, and backarc magmatism of the Pacific Northwest, USA. Also shown are the Coast Range Basalt Province, and the Chugach and Yakutat terranes, which accreted to the forearc area during Tertiary time. Arc magmatism is depicted in the pink fields and the Challis-Kamloops Belt and other Eocene extended-arc to backarc magmatism in the light-green fields. Within these fields, the intrusions are pink and volcanic rocks are green. Shown in light gray are selected metamorphic core complexes exhumed during Eocene time. MCC—metamorphic core complex. Where plutons and volcanic exposures are small, they are depicted as diamonds or squares. Major strike-slip faults are shown as thin black lines.

View larger version (32K): [in this window] [in a new window]   Figure 2. Tertiary forearc magmatism of coastal western North American from 61 Ma to 20 Ma. The vertical axis corresponds to geographic location and extent of magmatism along the coastline at right. The horizontal axis corresponds to time in Ma. Note synchronicity of forearc magmatism at different times in widely separated positions along the coast. Geochronology methods used to constrain timing of magmatism are shown in italics. Color of boxes matches specific locations of plutons depicted on the coastline. Alaska oroclinal bending occurred between 66 and 44 Ma (Hillhouse and Coe, 1994).

View larger version (52K): [in this window] [in a new window]   Figure 3. Location map of Eocene forearc magmatism, structures, and terranes that were accreted to Vancouver Island during Tertiary time. Eocene intrusions of the Mount Washington and Clayoquot suites and the Flores volcanics are shown in areal groupings that demonstrate similar intrusive styles, petrography, and geochemistry. U-Pb ages and pluton names are displayed for select intrusions visited in this study. New U-Pb dates obtained for this study are shown in boxes. Dates shown in italics are previously published ages. Structures: BRF—Beaufort range fault, FF—Fulford fault, YCF—Yellow Creek fault, CLF—Cowichan Lake fault, CR—Chemainus River fault, SMF—Survey Mountain fault, LRF—Leech River fault, WCF—West Coast fault. Also shown is the approximate location of the pole of oroclinal bending on southern Vancouver Island possibly related to crescent accretion. Structures shown in gray are related to the Cowichan fold-and-thrust system. Cities/towns: Z—Zeballos, TS—Tahsis, TF—Tofino, U—Ucluelet, PA—Port Alberni, V—Victoria, N—Nanaimo. Other abbreviations: KL—Kennedy Lake, MI—Meares Island, FI—Flores Island, NI—Nootka Island.

View larger version (25K): [in this window] [in a new window]   Figure 4. Concordia plots of zircon fractions used for U-Pb age determinations from the Pacific Centre for Isotopic and Geochemical Research (PCIGR). Insets show close-ups of ellipses and fractions used to make the age determination. Age of inheritance is provided on the divergent line pointing toward the upper intercept. A: Kennedy Lake stock. B: Labour Day Lake intrusion. C: Zeballos stock. D: Moriarty Lake sill complex. E: Tahsis Mountain. F: Tofino pluton. B and D show evidence of Jurassic inheritance, possibly from the island intrusions. Sample F intrudes the Pacific Rim Complex and demonstrates Precambrian inheritance. A–D belong to the Mount Washington Intrusive Suite. E and F are Clayoquot intrusions.

View larger version (12K): [in this window] [in a new window]   Figure 5. Concordia diagram for U-Pb zircon results from the Royal Ontario Museum. Mount Patlicant is located in the Nanaimo Lakes area and has an interpreted U-Pb age of 38.6 ± 0.1 Ma. Mount Washington stock is located in the Mount Washington area and is dated at 35.2 ± 0.3 Ma. Both are concordant.

View larger version (50K): [in this window] [in a new window]   Figure 6. The first frame of the tectonic model at 53 Ma, which represents a best-fit plate configuration for the Pacific Basin developed to account for the forearc magmatic record from Alaska to Oregon. The oceanic plates include the Kula plate, Resurrection plate, Pacific plate, and Farallon plate. The oceanic portions of these plates are labeled to the west of the trench (represented by the toothed line) and the subducted components are to the east and north of the trench. Subducted oceanic crust was geometrically modeled to move at the same vectors as the attached plates in the ocean basin. Arrows on the plates represent vectors for 3 m.y. of plate motion. The vectors were calculated from published stage poles of Norton (1995) and Lonsdale (1988), and Resurrection plate vectors were estimated for this study as discussed in the text. Subducted slabs are diagrammatically shown to terminate at the 300 km isobath on this and all subsequent figures. White areas to the east and north of the trench represent mantle. Mantle-filled gaps between subducted plates are slab windows. The 53 Ma model frame displays the generalized slab windows beneath Alaska and British Columbia and the Pacific Northwest that were inherited by this model.

View larger version (48K): [in this window] [in a new window]   Figure 7. Time slice from the tectonic model showing the best-fit plate configurations at 49 Ma, after 4 m.y. of forward plate movement. The vectors on the oceanic plates represent 3 m.y. of plate motion. The black Y-shaped feature represents the abandoned Kula-Farallon-Pacific triple junction, which is now preserved as the Great Magnetic Bight near Alaska.

View larger version (50K): [in this window] [in a new window]   Figure 8. Time slice from the tectonic model showing the best-fit plate configurations at 45 Ma. A plate reorganization event at 47 Ma slightly altered the ridge-transform geometries. Vectors on the oceanic plates represent 3 m.y. of plate movement. A large promontory on the Kula plate intersected the trench at 47 Ma and divided the Resurrection plate. This created the Eshamy plate to the north of the promontory, and a Resurrection plate remnant to the south. The paleotrench near Washington and Oregon made an outboard jump at ca. 48 Ma due to accretion of the Coast Range Basalt Province (shown as instantaneous for the purposes of the model).

View larger version (46K): [in this window] [in a new window]   Figure 9. Time slice of the tectonic model at 39 Ma. This is the second phase of the tectonic model and was performed with respect to hotspots using the stage poles of Engebretson et al. (1985). At 40 Ma, the Kula and Eshamy plates became fused to the Pacific plate. The Pacific plate began to move in approximately transform motion with respect to the Queen Charlotte transform and initiated extension in the Queen Charlotte Basin (discussed in text). Note the small North America vector. The subducted Resurrection remnant is assumed to move at the same vector as before it was fully subducted. The newly extinct ridge in the Gulf of Alaska is still able to impart a thermal pulse into the forearc of Alaska. Vectors represent 3 m.y. of plate motion.

View larger version (48K): [in this window] [in a new window]   Figure 10. Time slices of the tectonic model at (A) 35 Ma and (B) 30 Ma. In frame A, slab windows are opening beneath Washington, Vancouver Island, and Queen Charlotte Islands, and asthenosphere underlies most of British Columbia. The subducted portion of the Resurrection plate has foundered into the mantle since 39 Ma. The extinct ridge in the Gulf of Alaska remains in a stationary position during subduction. In Frame B, the slab window, which was underlying Washington, has migrated north to underlie southern British Columbia. The Resurrection plate remnants have equilibrated with the mantle, and a vast slab window is present beneath British Columbia. Late Oligocene to Recent magmatism is beginning inboard of the trench. Arrows shown represent 3 m.y. of plate motion.

View larger version (40K): [in this window] [in a new window]   Figure 11. Late Oligocene to present forearc, arc, and within-plate magmatism in British Columbia, Yukon, and southeast Alaska. Volcanic fields are outlined. Individual volcanic centers of the northern Cordilleran volcanic province, Anahim belt, Wells Gray–Clearwater, and Wrangell fields are depicted by shapes within outlined fields. The Chilcotin Plateau is represented by the gray field in interior British Columbia. Arc magmatism is represented by squares and triangles of the Pemberton and Garibaldi belts, respectively. Oligocene to present forearc magmatism is found on Queen Charlotte Islands (Kano intrusions and Masset volcanics) and northern Vancouver Island (Alert Bay volcanic belt). The volcanic fields are described in the text. This figure is provided for comparison with the late Oligocene to present slab window reconstructions in Figures 10 and 12–14.

View larger version (33K): [in this window] [in a new window]   Figure 12. Time slice of the tectonic model at 20 Ma. After 28 Ma, the Farallon plate is referred to as the Juan de Fuca plate. The black Y-shaped feature represents the Great Magnetic Bight now preserved offshore of Alaska. During this interval, a large slab window was present under northern British Columbia and parts of Yukon Territory, and was responsible for the anomalous inboard magmatism shown in Figure 11. Vectors represent 3 m.y. of plate motion

View larger version (37K): [in this window] [in a new window]   Figure 13. Time slices from the tectonic model at (A) 10 Ma and (B) 5 Ma. A large, stable slab window present underneath northern British Columbia has persisted until present. Queen Charlotte Islands are possibly underlain by a small protrusion of Pacific plate, but the plate boundary may instead be transpressional. Regardless, Queen Charlotte Islands are dominantly underlain by asthenosphere and not subducted Pacific slab. Vectors displayed on plates represent 3 m.y. of plate motion.

View larger version (53K): [in this window] [in a new window]   Figure 14. The final frame of the tectonic model, which depicts the present-day tectonic setting of Alaska, Yukon Territory, British Columbia, Washington, and Oregon. At 4 Ma, the Explorer plate broke free from the Juan de Fuca plate, possibly along the small circle defined by the Juan de Fuca Euler pole, as depicted at top. Superimposed on this diagram are forearc and inboard magmatic features. These magmatic features are late Oligocene to Recent in age. LM—Level Mountain, ME—Mount Edzizza

View larger version (46K): [in this window] [in a new window]   Movie 1. Tectonic model for the Pacific Basin and northwestern North America from 53 Ma to 39 Ma. Also visit http://dx.doi.org/10.1130/GES00020.s1 to view the animation.

View larger version (49K): [in this window] [in a new window]   Movie 2. Tectonic model for the Pacific Basin and northwestern North America from 39 Ma to present day. Also visit http://dx.doi.org/10.1130/GES00020.s2 to view the animation.