Plutons shown in red note Mount Stuart stitching pluton at lower right

Transcription

1 Tape here Tape here Page 4 Subduction zone backsteps Seattle Spokane Quesnellia Terrane Part of the Intermontane Superterrane Farallon Plate Cut along dotted line Portland Sedimentary Basins of the Eocene, Oligocene, and Miocene Tape here Tape here Wrangellia Insular Superterrane Recap: How did we 5. Docking of Okanogan Highlands assemble pre-eocene (~ m.y.) Washington? Aka: Insular Superterrane Page 5 C-7 & C5 Major Terrane Groupings of North Cascades Skagit Crystalline Core High Grade Met. Seattle Spokane NW Cascades and San Juan Thrust System Medium Grade Met. Portland 6. Docking of the North Cascades Ma Melanges Low Grade Met. Mount Stuart batholith - Stitching Pluton Nanaimo Formation: Overlap Sequence 54 42? Ma 90 Ma Overlap sequences sit atop both terranes and the rocks they dock against therefore they can provide a minimum age for both rock bodies So called Stitching plutons also provide minimum ages for terrane docking since they are in contact with both the terrane and the body of rock it docked with. 1

2 Plutons shown in red note Mount Stuart stitching pluton at lower right Pickett Range Page 6 Seattle Spokane Portland 7. Eocene to Miocene Sandstones and Volcanics (55 to 20 m.y.) 2

3 Photo donated by Larry McMIllan Eocene Palm fossils in the Chuckanut Sandstone near Bellingham show a warmer climate prevailed ~50 Ma Eocene McIntosh Formation: Tenino Sandstone quartz sandstones, but no evidence of Cascades volcanism yet! Northcraft Formation volcanic rocks, early evidence of Cascades Volcanism ~37Ma Pe Ell volcanic rocks near Rock Creek Skookumchuck Formation: view to south in Centralia Coal Mine Metasequoia fossil

4 Coal seam in the Centralia Coal Mine: Carina Henderson points to a tonstein, decomposed volcanic ash Eocene bivalve Bivalve from outcrop on Gold St. in Centralia <= Portunites triangulum Crab, Eocene, Wahiakum Co., Washington Outcrop of Lincoln Creek Formation at Galvin E-1 8. Docking of Olympic Peninsula (25 to 30 m.y.) Seattle Olympic Mountains, Willapa Hills & Oregon Coast Range Spokane Olympic Subduction Zone Siletzia Terrane Portland Tape here Cut alon g dotte d line 4

5 Geology Map /parks/olym/geolmap.html AZARDS/CASCADIA/turbidite_record.html /32/turbidite.gif 20 my old Hoh Formation turbidite deposits (overturned) at Beach #4 Hoh Formation turbidite deposits (looking down on overturned beds) Miocene rocks at Ruby Beach

6 Major Rock Units of the Olympics E-4 1. Peripheral Rocks 2. Core Rocks Metasediments from the core of the Olympics at Ruby Beach Separated by reverse faults Peripheral Rocks: Mostly the Crescent Fm E-4 Crescent Formation <= from big1.jpg Age: Mostly Eocene (58 45 Myrs) Rocks Pillow Basalts Some Vesicular Basalts km thick Limestone(Red) w/deep water forams Basalt of the Crescent Formation on Klahhane Ridge above Port Angeles; from Core Rocks E-4 Core Rocks Age: Eocene-Miocene (40-20 Myrs) Metamorphosed Myrs Rocks Sandstones w/graded beds Shale Youngest Oldest Metamorphism East - Low Grade (11 km burial) West - Not Met. Fig. 4. Thick beds of sandstone in the Valhallas and geologists at work. (from: lym2.html) Fig. 7. Sandstone beds with thin shale interbeds on Windfall Peak.

7 Olympic Structures Core Rocks Peripheral Rocks E-3 Interpretation: Peripheral Rocks E-4 Terrane (fault bounded) Hot Spot Island (Yellowstone?) Note faults place older rock over younger rock Basalt Chemistry = Hot Spot & Divergent Zone Modern Analogy for Crescent basalts - Iceland Possible Eocene Reconstruction 35 Myrs 55 Myrs E-1 ogy/hotspots.htm graphy-of-iceland Plate Reconstruction: 55 Myrs D-2 Supporting Evidence: Rock Age E-3

8 E-4 Interpretation: Core Rocks Olympic Accretionary Wedge Accretionary Wedge Trench Sediments Olympic History E-4 But Why is the North Wedge (Olympics) so much larger than the South Wedge (Willapa Hills/Oregon Coast Range)? Western Core rocks accreted first & were buried deepest (explains why they are oldest and most metamorphosed) 1. Shoved in the Corner Insular ST Olympics Coast Range Washington Oregon But Why is the North Wedge (Olympics) so much larger than the South Wedge (Willapa Hills/Oregon Coast Range)? Steep Subduction Evidence for the Bend 1. NW Volcanoes Shallow Subduction 100 Depth 2. There is a bend in the Juan de Fuca Plate under the Olympics Olympics Cross Section North South N. America Juan de Fuca QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture.

9 Volcanoes, faults and EQ outline big blocks Why is there a Bend at all? Basin & Range Extention to the South OC SN From Wells et al., 1998, after Magill et al., 1981, 1982; Guffanti and Weaver, 1988; Sherrod and Smith, 1990 Big blocks are rotating - north-seeking magnetic directions captured when Coast Range lavas cooled now point eastward. W. Oregon has been rotating clockwise for millions of years. Rotation rate of 1.2 per Million years for at least 50 million years, based on paleomagnetic study. Rotations are less to N, S, and E.. v N (pie-shaped wedges are uncertainties about mean rotation from expected north seeking direction; modified from Gromme et al., 1986) v Coast Range basalt - similar to Hawaiian Islands - was 52 millionmillion-yearyear-old submarine accreted to the continent about 50 million years ago; pillow basalt, Trask River then rotated clockwise. Northwest Block model for long-term motion Sierra Nevada dragged northward by Pacific Plate Oregon (pink) pivots clockwise to get out of the way Washington (green) gets squeezed against Canadahence all the earthquakes and faults. SE Oregon and Nevada Stretch to fill in the gap behind rotating block PBO GPS receivers installed in PNW (green dots) what are they going to measure? Plate and block motions Deformation of crust (its change in shape) along plate boundaries especially above the Cascadia megathrust 9

10 GPS velocities Velocities rotate clockwise (block rotation) decrease away from coast (compression above SZ) Blue are campaign sites, red are continuous sites (PBO) Many institutions: RPI, UW, GSC, USGS, OSU, NGS, PANGA, PBO McCaffrey, Qamar, King, Wells, et al, s GJI, 2007