Farallon and Kula Plates David Reed Off of the Pacific Coast of North American lie the remnants of old tectonic plates. They current go by the name of the Cocos Plate (currently located in Central America) and the Juan De Fuca Plate (located near the Oregon and Washington coast). Connecting these old subsiding plates is the San Andreas Fault. The past history of these plates can be reconstructed by looking at the seafloor of the northern Pacific as well as the continental features along the edge of the coast. From that reconstruction, a large amount of the geology of the western United States and Canada can be uncovered. Looking at the seafloor magnetic anomalies and making an estimate of the seafloor spreading rates, it is possible to move backwards in time and find the location of the tectonic plates. Starting from the shore line, oceanic crust increase in age. This implies that the spreading center is located in the open sea and that there must also be a subduction zone at or near the coast line. Seafloor spreading rates of the past can be assumed based on the magnetic anomalies as well as change in direction of the plate s movement. Looking at past triple junctions and magnetic anomalies, the California coast shows east-west spreading and the coast off of Alaska show north-south spreading. The Pacific plate is sliding to the northwest in a roughly parallel direction to North America, this show the San Andreas Fault. With the above assumptions, the past plate dynamics can be recreated. With a reconstructed past, it s possible to begin to draw conclusions about the geological features around these plates. With the subduction zone shown as a blue line in the figures, an area of volcanism can be draw out that is linked to this plate subduction. The current volcanos are due to this melting plate. This subduction process has lead to the creation of most of the mountains in the United States and Canadian west. The rocky mountains in Canada are due to steep thrust faults tied to the steep angle of the decent of the Kula plate and the American Rockies were created from continental uplift from the more shallow angle of decent of the Farallon plate. As the old Farallon plate was moving towards the North American plate, strong compression forces were acting across the area. As the plate began to break up and sink down into the mantle, those forces reversed and the area was forced apart. This created the basin and range area in the American west.
By recreating the plate motions off the western coast of North America, and then replaying the plate motion, most of the geological features in the entire area can be linked to the tectonic dynamics. The Kula and Farallon Plates, while long since past into the mantle, have shaped the current geology as we know today. References Architecture of the Georgia Basin, southwestern British Columbia England, T D J; Bustin, R M University of British Columbia Bulletin of Canadian Petroleum Geology, vol.46, no.2, pp.288-320, Jun 1998 Eocene volcanism in the Buck Creek basin, central British Columbia (Canada): transition from arc to extensional volcanism J. Dostal, B. N. Church, P. H. Reynolds and L. Hopkinson Journal of Volcanology and Geothermal Research Volume 107, Issues 1-3, June 2001 Figures Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay http://www.uwgb.edu/dutchs/platetec/kula.htm
The Basin and Range (John Whitlock) The Basin and Range province (Fig. 1) was first defined and named by N.M. Fenneman (1928, 1931), based solely on physiography; the area defined as such has an area of ~800,000km 2 (Eaton, 1982). Others (Pardee, 1950; Lawrence, 1976; Reynolds, 1979; Eaton 1979) have found evidence that the fundamental structures of the Basin and Range exist far outside the area sensu Fenneman; the tectonophysical extent of the Basin and Range exceeds one million square kilometers (Eaton, 1982). The structure of the province is of middle Miocene age (17mya+), but extension deformation of the region began nearly 30mya regionally, and up to 37mya locally (Eaton, 1982). The Basin and Range is divided into five sections (Fenneman 1931); most notable among these is the Great Basin. Eaton (1982) is quick to point out that it is not, in fact, a single large depression, nor is it technically a basin. Along its northern and southern extents, it is almost a kilometer higher than the next section (Eaton, 1982). The province itself is characterized most clearly by the feature for which it is named, the basin and range structure (Eaton, 1982). Basin and range topography is similar to horst and graben structures, wherein a series of normal faults appear as a result of lateral crustal expansion. Grabens are formed when blocks of this crust depress between these faults. This leaves behind a horst, or a block of crust that has not descended and remains at or slightly above the original crust height (Bates and Jackson, 1984). Mountains, such as the Sierra Nevada and Wasatch mountains, are formed along the upthrown sides of fault scarps, which then erode, filling the
grabens with sediment. This pattern of sediment-filled basins bordered by fault scarps is the classic basin and range structure (Bates and Jackson, 1984). References Bates, R.L. and J.A. Jackson. 1984. Dictionary of Geological Terms, third edition. Anchor Books and the American Geological Insitute, New York. Eaton, G.P. 1979. A plate-tectonic model for late Cenozoic crustal spreading in the western United States. In Rio Grande Rift: Tectonics and Magmatism, R.E. Rielacker, ed. pp 7-32. American Geophysical Union, Washington D.C. Eaton, G.P. 1982. The Basin and Range province: Origin and tectonic significance. Annual Review of Earth and Planetary Science. 10:409-440. Fenneman, N.M. 1928. Physiographic divisions of the United States. Annals of the Association of American Geographers. 18:261-353. Fenneman, N.M. 1931. Physiography of Western United States. McGraw-Hill, New York. Lawrence, R.D. 1979. Strike-slip faulting terminates the Basin and Range province in Oregon. Geological Society of America, Bulletin. 87:846-850. Pardee, J.T. 1950. Late Cenozoic block faulting in western Montana. Geological Society of America, Bulletin. 61:359-406. Reynolds, M.W. 1979. Character and extent of basin-range faulting, western Montana and east-central Nevada. In 1979 Basin and Range Symposium, G.W. Newman, H.D. Goode eds. pp. 185-193. Rocky Mountain Association of Geologists and Utah Geological Association.
From Eaton, 1982.
Avani Naik GS 420 Info on: Colorado Plateau The Colorado Plateau region near the four corners, consists of plateaus, mesas, and deep canyons, the oldest rocks of which are billions of years old. It is believed that many small land masses collided on a large scale over a billion years ago to form the nucleus of the North American continent. Most of these metamorphic rocks, formed during Precambrian, make up the basement of the Colorado Plateau. Injections of igneous rocks make up the darker parts of the metamorphic basement. Numerous lava flows are evident on the plateaus and mesas in the region. Volcanic activity dominated from the Tertiary to much of the Quaternary. Through uplift, erosion, transport and deposition, sedimentary layers were formed in the plateau, which are exposed in canyon walls. During the Miocene, about 20 million years ago, the Colorado Plateau region was uplifted about 3 kilometers, owing to the tectonic activity in the west. The topography west of the plateau was altered as a result of this crustal stretch. However, the Colorado Plateau topography was only thrust upward as a single block. The mechanics of this process is still not fully understood. Bibliography Colorado Plateau Mosaic. (http://disc.gsfc.nasa.gov/geomorphology/geo_1/geo_plate_i-1.html)
2001. Colorado Plateau Region. (http://vulcan.wr.usgs.gov/livingwith/volcanicpast/notes/colorado_plateau_region.html).
Tom Hawkins Mendocino T.J. GeoScience 420 Peter Van Keken The Mendocino Triple Junction is an area off the coast of the western United States where 29 Million Years ago the large oceanic Farallon plate began subducting beneath the North American continental plate. When the Farrallon plate s oceanic spreading center reached the subduction zone, it refused to go beneath the North American plate. The growing Pacific plate and the opposing North American plate forced the junction of these three plates to begin migrating northward along the North American plate boundary, replacing the Cascadia subduction zone with the strike-slip San Andreas fault. While the Pacific plate south of the Mendocino fracture zone has been adjacent to the strike-slip fault system since before 25 Ma, new San Andreas fault is being created in the North American crust. The remains of the Farrallon plate became known as the Juan de Fuca plate. The corner of the Pacific plate includes a wedge of felsic rocks, the Vizcaino block, that formed as an accretionary prism and was sliced off the continental margin some time after initiation of the strike-slip plate boundary. Seismic refraction data indicated the presence of a layer beneath the Vizcaino block that has a high velocity like that of mafic rocks, which were taken to be oceanic crust.. The western margin of North America adjacent to the northernmost San Andreas fault consists of Tertiary accretionary prism rocks of the Franciscan terrane. A high-velocity mafic layer, similar to that offshore, is observed in the lower crust beneath the Franciscan terrane. North of the triple junction, the active Cascadia accretionary prism overlies oceanic crust of the subducting Juan de Fuca plate. As the subducted Juan de Fuca slab migrates northward, a slabless window is created beneath North America. This window must be filled either by asthenospheric upwelling caused by the northward-migrating Pacific plate that extends beneath the continent, or portions of the Juan de Fuca slab that do not subduct. It is assumed that the mafic lower crust beneath North America was accreted to the base of the continent by magmatism derived from pressure-release melting of the upwelling asthenosphere. Othe possibilities are the mafic rocks are oceanic crust attached to the Pacific plate or that the mafic rocks are oceanic crust of the Juan de Fuca plate attached to North America.
Picture is courtesy of www.oceansjsu.com. The bubbles aren t very informative, but you can clearly see the plate boundaries. References: Gsajournal.com The above link is where I obtained a large amount of my information and provides an interesting read.
Karen Kimm Geosci 420 10-9-05 Yellowstone Snake River Plain The Snake River Plain (SRP) geologic province is a depression that stretches approximately 650 kilometers from southeastern Oregon to Yellowstone National Park in northwestern Wyoming. Compared to other geological features in North America, the SRP is very young. It began forming only 16 million years ago (Ma), and is still tectonically active today. The SRP is a result of the movement of the North American Plate over the Yellowstone hot spot. A hot spot, also known as a mantle plume, is a column of hot rock that rises through the mantle to the base of the lithosphere, producing basaltic magma. The magma then penetrates the lithosphere and erupts at the surface, forming a volcano. As the plate continues to move over the hot spot, new volcanoes are formed and the older ones become inactive, eventually collapsing and forming basin-like depressions known as caldera. The SRP is essentially a series of old calderas that have resulted as the North American Plate moves to the southwest over the hot spot at a rate of approximately 25 mm/year. The oldest caldera in the chain formed approximately 16 Ma in southeastern Oregon, and the most recent eruption occurred approximately 600,000 years ago in Yellowstone. Since the North America plate continues to move to the southwest, the SRP will gradually lengthen to the northeast as more volcanism occurs in the future.
Figure from Press et al 2004. References: Johnson, Gary, Donna Cosgrove, and Mark Lovell. Origin of the Snake River Plain. Idaho Water Resource Research Institute, University of Idaho, December 1998. 8 October 2005. <http://imnh.isu.edu/digitalatlas/hydr/snakervr/osrp.htm>. Press et al. Understanding Earth, 4 th ed. W. H. Freeman, New York: 2004.
Sean Lenhard 10/10/05 Geosci 420.001 The Cascade Mountains The Cascades are located in the western United States, about 100-150 miles inland from the Pacific Ocean. It stretches for about 700 miles and its highest peak is Mount Rainier at 14,410 feet. The Cascades are generally divided into two areas: the Western Cascades and the High Cascades. The Western Cascades are believed to be older and more eroded than the High Cascades, and the High Cascades are characterized by volcanism. The Cascades have been an active area of volcanism for approximately the past 36 million years. The reason for the volcanism is due to the collision of the Juan de Fuca tectonic plate and the North American plate. These plates are converging into each other at nearly 4 cm per year. The Juan de Fuca plate is being forced under the North American plate because its oceanic crust is more dense than the continental crust. After it is forced down into the mantle, the increase of pressure and temperature melts down the oceanic crust of the Juan de Fuca plate and it travels upwards through the North American plate, breaking through the surface in the form of volcanoes. Many of the volcanoes in the Cascades have had eruptions that are more explosive than other volcanoes. This is due to the amount of gases that is being released from the melting oceanic crust. Mount Saint Helens is a familiar example of how explosive the
eruptions can be. When it erupted, much of the mountain exploded as if a bomb went off and its ash and dust traveled thousands of miles in the atmosphere. References http://www.peakware.com/areas.html?a=293 http://vulcan.wr.usgs.gov/volcanoes/cascades/description_cascade_range.html
The Laramide Orogeny Jesse Ortega In The United States, The Laramide Orogeny is represented by a group of block uplifts which form by the exhumation of deep crustal (basement) rocks (English - Thermal). For those unfamiliar with geological terminology: an orogenic event, also called an orogeny, is defined as literally, the process of formation of mountains (Bates). The mountains formed by Laramide style deformation arise on deep faults, and involve dense basement rocks. Maxson dates Laramide activity as occurring between 80 and 45 Million years ago (Ma). According to Maxson, the Laramide-style basement-involved uplifts are often seen in environments experiencing crustal collision: think India colliding with Asia. However, this raises two issues when investigating the Laramide orogeny in the U.S. First, the uplift is thought to be about 1000 km from the ancient continental margin - and there is no Laramide style deformation in between (English-Laramide). Secondly, the colliding continents responsible for the basement-involved uplift in the Himalayas are well defined. However, The U.S. contains no foreign terrane which shows evidence of a collision with the ancient coast during the time of the Laramide orogeny. This write-up presents two hypotheses investigating how this geometry can exist. The literature contains many hypotheses, but these two are popular and interesting. The first hypothesis cites flat-slab subduction as the mechanism for creating a compressional state in the interior of the continent (Figure 1, English-Laramide). In this model, the subducting oceanic lithosphere does not descend into the mantle Figure 1: right away (English-Thermal). Instead, in rides just below the lithosphere, for a greater-than -typical distance, bringing Figure stresses 2: farther into the interior of the continent (English-Thermal). The second hypothesis is known as the hit and run collision model (Figure 2). Maxson describes an alien terrane that collided with the ancient coast, causing the basement-involved deformation: the hit. The foreign crust was then pushed northward into Canada: the run (Maxson). This is an attempt to explain why no such terrane is found at the latitude of the basement-involved uplift. The moral of the Laramide story is that no one knows what happened. It is anomalous to find deep, basementinvolved, thrusts so far from the continental margin - - especially when there is no evidence, at the scene of the crime, suggesting collision (Maxson). More research, such as seismic or gravity surveys might reveal subsurface conditions, adding information to the problem, and allowing the different camps to agree.
WORKS CITED (The Laramide Orogeny) Bates, Robert L, Jackson, Julia A. Dictionary of Geological Terms Third edition. American Geological Institute. 1984. English, Joseph M; Johnston, Stephen T. The Laramide Orogeny; what were the driving forces? International Geology Review, vol.46, no.9, pp.833-838, Sep 2004. English, JM; Johnston, ST; Wang, K. Thermal modelling of the Laramide orogeny: testing the flat-slab subduction hypothesis. Earth and Planetary Science Letters [Earth Planet. Sci. Lett.]. Vol. 214, no. 3-4, pp. 619-632. Sep 2003. Maxson, Julie A; Tikoff, Basil. Hit-and-run collision model for the Laramide Orogeny, Western United States. Geology (Boulder), vol.24, no.11, pp.968-972, Nov 1996.
Mary Carnagie GeoSci 420 Sevier Fold and Thrust Belts At a very elementary level, thrust faults are simply reverse dip-slip faults with a dip angle of less than 45 where the overlying block of earth is overthrusted on top of the other dip block. Folds are rock features that have been deformed by compressional forces. Basically, the Sevier fold-and-thrust belt is a region of almost exclusively horizontal, convergent forces. The Sevier orogeny is the deformation that occurred along the eastern edge of the Great Basin in Utah, also referred to as the North American Cordillera, during the time between the Nevadan orogeny to the west and the Laramide orogeny to the east, ending early in the Late Cretaceous period. The North American Cordillera has three main geologic components: 1. The dominantly thin-skinned Sevier fold-and-thrust belt. 2. A belt of silicic intrusive and extrusive igneous rocks 3. Laramide-style, basement-cored uplifts The Sevier thrust-and-fold fault is an anomalous fault in that it is very thin skinned and that magma is allowed to seep to the surface at a convergent margin (one would think that horizontal compression would leave less space for magma). There is often extensive mantle influence underneath thrust-and-fold faults, and Sevier is no exception. The only difference is that plutons and batholiths feed magma to a depth of merely 1-10 km under the surface. This would also account for the silicic rocks. Silicic rocks form at much lower temperatures, as opposed to mafic ones, so it would make sense that the mantle activity was closer to the surface of the earth.
Figure 1
Figure 2
Figure 3 Figure 4 Works Cited Kalakay, Thomas J., John, Barbara E., and Lageson, David R. Fault-controlled pluton emplacement in the Sevier fold-and-thrust belt of southwest Montana, USA. Journal of Structural Geology 23 (2001): 1151-1165. Miller, Marli. Earth Science World ImageBank. Photographs hf8mbw and hf8me9. th Press, et. al. Understanding Earth, 4 ed. New York: W. H. Freeman and Company, 2004.