The bulk of Alaska is material that was added to the North American continent in fairly recent geologic time. The parks are within the North American Cordillera, a broad region that has been the site of terrane accretion and other tectonic activity over the past 200 million years. Elias and Glacier Bay) and another in Washington State (North Cascades). Information on four parks is the focus of this chapter, three in Alaska (Denali, Wrangell–St. Olympic National Park and Oregon Caves National Monument could therefore also be part of this “accreted terrane” discussion, as could numerous other NPS sites. Regions in the western part of the country, such as the Olympic Mountains in Washington and the Klamath Mountains in Oregon, were also affixed to North America in fairly recent times. Thus sites such as Shenandoah National Park and the Blue Ridge Parkway, discussed elsewhere in the “Collisional Mountain Range” part of these web pages, could also included here. The Appalachian Mountains contain oceanic islands, continental fragments, and sedimentary, volcanic, and metamorphic rocks that were added to North America as an ancient ocean closed. There are many NPS sites across the country that have rocks that are out of place-manufactured elsewhere then carried long distances before being affixed to the North American continent. Continents are thus older in their interior regions, known as cratons, and get progressively younger outward, as seen in the above map of North America. Over time, the small continents grew outward as material was added to their edges through sedimentation, subduction, and the collision of oceanic islands and other continental fragments. Those blocks of coalesced material formed the nuclei of the continents. When island arcs collided with other island arcs, rock and sediment were scraped off the top of subducting plates. ![]() Where plates converged, thicker crust formed at island arcs as magma poured out on the surface as lava flows, and was added to the base of the crust as igneous intrusions. When the Earth cooled enough for a shell of lithosphere to form, the crust that formed on top of the plates was thin like today’s oceanic crust. ![]() Modified from “Earth: Portrait of a Planet, by S. Numbers are the ages (in billions of years) of the igneous and metamorphic rocks that are exposed at the surface or lie beneath younger sedimentary layers. North America has been growing outward over time as terranes are added along the edges. Suture zones mark the boundaries between different terranes. Extinct volcanic arcs on still-older accreted terranes reflect the positions of earlier subduction zones. An active volcanic arc develops on crust of an older accreted terrane. In the digram below, an oceanic island or continental fragment (incoming terrane) approaches a subduction zone, where it will eventually attach (accrete) to the edge of the continent. Continents Grow Outward by Terrane Accretion This addition, or “accretion,” is one of the ways that continents tend to grow in size over geologic time. ![]() Instead, they crash into the edge of the continent and become permanently attached. But when continental fragments or oceanic islands approach a subduction zone, their crusts may be too thick to subduct. IntroductionĪ plate capped by thin oceanic crust will subduct beneath one with much thicker continental crust. It consists of rocks that formed elsewhere, and then were deformed, metamorphosed, and accreted to North America. Freeman and Co., 768p.Ĭontinental Collision Zones: Seismotectonics and Crustal Structure Deep Earthquakes Earthquake Mechanisms Earthquake Mechanisms and Plate Tectonics Earthquakes: Hazards and Prediction Earthquakes: Location Techniques Earthquake Seismology Earthquakes and Seismicity Earth Structure: Global Lithosphere, Oceanic: Formation and Evolution Mantle, Upper: Structure Mantle Convection and Plumes Mantle Dynamics Seismicity: Intraplate Seismicity: Midocean Ridge Seismicity: Subduction Zone Seismic Source: Observations Seismic Source: Theory Seismology: History Subduction Zones.North Cascades National Park, Washington. Sykes, 1979, Seismic gaps and plate tectonics: seismic potential for major plate boundaries, Pageoph. ![]() Kanamori, H., 1977, The energy release in great earthquakes, Jour. Molnar, 1971, Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes, Rev. Sykes, 1968, Seismology and the new global tectonics, Jour. Princeton, N.J.: Princeton University Press, 310p. Richter, 1954, The Seismicity of the Earth, 2nd ed.
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