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1% 10%/m.y. Island arc assemblage S USA At any time, geology is made over only a small fraction of the Earth’s surface. The colored area in the map above show current-day strain rate, i.e., where geology currently is happening. This occurs at spreading centers, subduction zones, and in areas of continental orogeny. The western U.S. is such a The Cordilleran Orogen of western North America is part of the circum-Pacific orogenic belt. USA Rift now wUS_Intro.ppt USA Mercator map projection about pole at 25° N 15 ° Oceanic subduction complex Present The western U.S. Cordilleran Orogen is defined by broad zones of uplift, deformation and magmatism Cordilleran uplift The area of uplift (which is unusually wide in the western U.S.) 4 km 3 2 gPE and resulting stress is where the Farallon slab is thought to have been flat against North America lithosphere during the 100-175 km S-wave velocity Laramide orogeny. 1 0 Currently, the upper mantle is very slow to seismic waves beneath the uplifted area (the mantle is hot or partially molten). Also, the uplifted area has high amounts of gPE (gravitational potential energy), which acts to drive extension. tension compression ± 5.0% Cordilleran interior deformation Surface Velocity mm/yr 48 The area of active deformation (which is unusually wide in the western U.S.) occurs both at the plate margin and where intense magmatism occurred recently (in the Basin and Range). Only the western portion of the high gPE area is extending, which argues that the deforming area is relatively weak. This is consistent with its history of recent, intense magmatism. Interior deformation is mostly extension, whereas near the transform plate margin deformation is dominated by rightlateral shearing. 12 1 1/2 0 Velocity of points relative to stable North America. Projection s such that the Pacific plate moves toward the top of the page. Plate interactions Yellow arrows show plate interaction forces Stress field created by combination of gPE and plate interaction. Light gray areas are weak. Figure to left shows stresses caused by plate interaction alone; figure at top right shows stresses created by gPE alone. Summary > Plate interaction > Grav. PE > Plate weakness } Inheritance Lund (2008) North America (above). Archean craton blocks were assembled at mobile belts; a series of Proterozoic arcs accreted to the south side of the Archean continent. Proterozoic rifts emplaced intrusions (mid-continent rift, 1.1 Ga) and made basins (diagonal dashes below), but it wasn’t until the end of the Precambrian that rifting split away the western continent. The continent margin (west of the gray line at above) formed an Atlantic-type “miogeoclonal’ margin, with a normal sedimentary shelf (see cross section to right). Precambrian (pink and orange in top map) forms stable continental cores . Simplified Geologic Maps. Precambrian Paleozoic Passive Margin Dickinson (2006) and the Miogeocline Main Geologic Events of the Southwest U.S. (for the area shown in the Blakey figure at bottom left) Latest Precambrian rifting stretched the margin of North America (the area between the hingeline and the western edge of purple crust in the tilted figure below). Tectonic Event Post-rift lithospheric cooling caused subsidence and creation of a passive “Atlantic-type” margin that received a thick accumulation of sediment in the “miogeocline” west of the hingeline. Laramide Sevier This state of affairs continued through much of the Paleozoic. Nevadan Sonoma Ancestral Rockies Antler QuickTime™ and a decompressor are needed to see this picture. Rift Rift Blakey (web site) Cambrian (~510 Ma) paeogeography. The passive margin setting is established. Carbonate rocks, fluvial and shallow marine sandstone and mudstone grade westward into the paleoPacific. Deposits were thin on the continental platform east of the hingeline, and thicken rapidly to the west of the hingeline. Dickinson (2006) Paleozoic and Mesozoic Dickinson (2002) magmatic arc accretion and growth Dickinson (2002) 55-30 Ma Northern core complexes Southern core complexes 30 -17.5 Ma Siletzia accretion and the Ignimbrite flareup ignimbrite flareup. (SMO, Paleogene Post-Laramide Laramide Cenozoic