Presentation on theme: "The Cordilleran Orogen of western North America"— Presentation transcript:
1 The Cordilleran Orogen of western North America Mercator map projection about pole at 25° N 15 ° EUSAIsland arc assemblageOceanicsubductioncomplexRiftSnowwUS_Intro.ppt1%10%/m.y.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 place.The Cordilleran Orogen of western North Americais part of the circum-Pacific orogenic belt.
2 } Present I’m assuming a flat slab Here are some things it did… The western U.S. Cordilleran Orogen is defined by broad zones of uplift, deformation and magmatismThe area of uplift (which is unusually wide in the western U.S.) is where the Farallon slab is thought to have been flat againstgPE and resulting stress1234 kmNorth America lithosphere during the Laramide orogeny.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 amountsof gPE (gravitational potential energy), which acts to drive extension.kmS-wave velocity± 5.0%Cordilleran uplifttensionMexicoCanadaPacific PlateNorth American PlateSurface Velocity1/211248mm/yrcompressionThe 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 deformationis mostly extension, whereas near the transform plate margin deformation is dominated by right-lateral shearing.Cordilleran interiordeformationI’m assuming a flat slabHere are some things it did…But how did it do it?Stress field created by combination of gPE and plate interaction. Light gray areas are weak.Velocity of points relative to stable North America. Projection s such that the Pacific plate moves toward the top of the page.Figure to left shows stresses caused by plate interaction alone; figure at top right shows stresses created by gPE alone.Plate interactionsSummary> Plate interaction> Grav. PE> Plate weaknessInheritance}Yellow arrowsshow plate interactionforces
3 PrecambrianNorth 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 thePrecambrian (pink and orange in top map) forms stable continental cores .western continent. The continent margin (west of the“miogeoclonal’ margin, with a normal sedimentaryend of the Precambrian that rifting split away thegray line at above) formed an Atlantic-typeshelf (see cross section to right).Simplified Geologic Maps.Lund (2008)
4 Main Geologic Events of the Southwest U.S. Dickinson (2006)PaleozoicPassive Marginand the MiogeoclineDickinson (2006)Main Geologic Events of the Southwest U.S.(for the area shown in the Blakey figure at bottom left)core complexesRiftNevadanLaramideSonomaAntlerSevierAncestralRockiesTectonicEventLatest 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).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.This state of affairs continued through much of the Paleozoic.hingelinesedimentPrecambrianlithospherepost-rift lithospheremiogeoclineocean crustcrustBlakey (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 paleo-Pacific. Deposits were thin on the continental platform east of the hingeline, and thicken rapidly to the west of the hingeline.
5 and magmatic arc accretion and growth Dickinson (2002)and magmatic arc accretion and growthMesozoicPaleozoic
6 Siletzia accretion and the Ignimbrite flareup 212834273743Cascade ArcmagmatismChallis-trendMaSiletzia accretion and the Ignimbrite flareupCenozoicLaramidePost-Laramideignimbrite flareup. (SMO, PaleogeneSouthern corecomplexesMaNortherncore complexes55-30 MaDickinson(2002)