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Landscape photoalbum Fault Historic and Ongoing Subduction and Terrane Accretion along the Western Margin of the North American Plate Terrane accretion is one of the fundamental methods of continental growth. It was presumably the most important tectonic or geologic process affecting western North America during Mesozoic to early Cenozoic time. The collision and subsequent accretion of island arcs, oceanic crust, and continental fragments onto the craton built almost all of Alaska and much of western North America from the Yukon to Mexico. The western margin of the North American Plate consists of three distinguishable zones of tectonic activity (cartoon below). To the south lies a subduction zone, where the oceanic lithosphere of the Jaun de Fuca Plate (and its recent off-shoot, the Explorer Plate) sinks northeastward beneath the North American Plate near southern British Columbia in Canada and Washington and Oregon in the United States of America. The ongoing convergence has built and continued to feed the Cascade Volcanic Arc, which runs from Mount Garibaldi in southwestern British Columbia southward to Mount Shasta in northern California (Price, 1994). The active arc is highlighted most notably by Mount St. Helens in southwestern Washington, which erupted in 1980 and has drawn the eyes of seismologists and civilians alike ever since with ongoing seismic activity. The central segment is a transform boundary of right-hand strike slip faulting. The Pacific Plate is moving northwestward with respect to the North American Plate, along the Fairweather-Queen Charlotte Fault (Bruhn et al., 2004). The northern segment of the boundary is marked by the Aleutian Trench (pictured on the Home page of this site) and the Chugach-Saint Elias Faults. Here the Pacific Plate is being subducted beneath the North American Plate, marked by the magmatic arc that forms the Aleutian Islands. Riding along the Pacific Plate is the Yakutat terrane, which is being partially subducted and partially accreted along the transition from transform boundary to subduction zone. The interactions between this terrane and the over-riding plate have led to noticeable deformation and accompanying seismicity in the region of the Saint Elias orogen. By studying this modern-day terrane accretion and deformation, scientists hope to better understand the processes that shaped the margin of western North America beginning in the Mesozoic farther to the south. Geology Glossary Structural Geology Tectonic Structures Tectonics is the study of crustal deformation and structural behavior. Plate Tectonics is the deformation and structural behavior of crustal plates. Stress Stress is any force which acts to deform rocks. Compression - a stress that acts to press or squeeze rocks together. Tension - a stress that acts to stretch a rock, or pull a rock apart. Shear - a stress which acts tangential to a plane through a body, causing two contiguous parts to slide past each other. Structural Behavior As a general rule: 1) Rocks tend to have a relatively high compressive strength 2) Rocks tend to have a relatively low tensile and shear strength Strain When a stress is applied, deformation may occur Depending on the rate of stress Depending on the amount of stress Strain is the change in shape or volume of a body as a result of stress; deformation. Brittle and Ductile deformation. Ductile Deformation During ductile deformation rocks bend or flow. Folding or bending of material without breaking Specifically defined as a rock that is able to sustain, under a given set of conditions, 5-10% deformation before fracturing. Folds can be microscopic in size or kilometers in extent. Brittle Deformation During brittle deformation rocks break or fracture. Two main styles of fracture: Joints and Faults. Both are the result of relatively rapid stress. For example: modeling clay will break if stress is applied rapidly, but will bend if stress is applied slowly. Joints Joints are fracture surfaces along which there has been no displacement. Joints can form from compressional, tensional and shear stress, and can range in size from microscopic to kilometers in length. Joint sets and jointing has a major influence on landform development. Erosion is able to occur at a faster rate along joints Faults Faults are fractures along which there has been displacement of the material on either side of the fault. Faults are classified based on: 1) the sense of movement (the direction in which the blocks on either side of the fault move) - this is controlled by the type of stress that is applied. 2) the orientation of the fault surface (the angle of the plane of fracture) Fault Terminology Fault Plane - the plane along which the rock or crustal material has fractured. Hanging Wall Block - the rock material which lies above the fault plane. Footwall Block - the rock material which lies below the fault plane. Photo journal Faults Origin of Hydrocarbons Earth Structures In order to understand the structure we find near the surface of the earth, we must understand the interior of the earth and its gross structure. Much of the what we know, or think we know, about the structure of the earth is related to indirect observations we have made that help us make hypotheses about it. The structure of the earth appears to unique, at least within our solar system and may be related to the way we believe the earth formed Terranes Terrane accretion is the process of continent building in which smaller units of exotic crust (both oceanic and continental) collide and become welded to a larger continental craton. Terranes differ sufficiently from the craton petrologically and stratigraphically and are often bounded by known or suspected faults. Terrane is a general term for an exotic geologic unit, and they need not possess specific characteristics, except that they differ from the craton. Often, terranes are separated by clear physical boundaries between rock groups and breaks in stratigraphy that cannot be explained by conventional facies changes or unconformity (Coney et al., 1980). Many terranes display sedimentary or volcanic rock characteristics that are of oceanic origin rather than continental, but some simply are of a different continental origin than the one to which they are accreted. Paleomagnetic data are sometimes used to differentiate between terranes and craton. When available, faunal variations can be clues to terrane classification. Terrane accretion during primarily Mesozoic to early Cenozoic time into the Cordillera of western North America extended the continental margin to the west by over 500 km in some areas and caused deformation as far east as the Great Plains. The terranes were accreted as a result of the closing of the Paleozoic Pacific Ocean and consist of pieces of oceanic and possibly continental crust, as well as some oceanic arcs (Coney et al., 1980). Terranes collide and accrete onto continents at convergent plate boundaries. The terranes “ride” along the subducting slab until they are scraped off by the over-riding plate. This causes folding and faulting in both the terrane and the continent. While the terrane is more buoyant than the subducting lithospheric slab, and most of it remains near the surface, some of the terrane may, in fact, be partially subducted with the slab beneath the over-riding plate. This partial subduction of oceanic or continental crust exposes those rocks to increased temperature and pressure. If the leading edge of the lithospheric slab were to neck and release from the system, removing the main driving force of the subduction, these metamorphosed crustal rocks could be re-exposed as the positively buoyant material rises back towards the surface. This bouyant re-surfacing also leads to uplift and seismic activity in the over-riding continental plate. The silicone putty-flubber-corn syrup analog model in this study attempts to simulate this partial subduction and subsequent re-surfacing of the terrane in an oceanic-continental subduction zone. A second model created an idealized subduction zone with a manually plunged Plexiglas slab that allowed for the mathematical and graphical displays of flow and strain components in the mantle, which lead to observable deformation in the back-arc region of the over-riding continental plate. Aleutian Islands San Andreas fault Introdusing to features TERRANE MAP OF EUROPE Tectonic Terrane Map of San Francisco Paleotectonic Evolution of Tibet Moon terrane Terrane deformation and accretion in an oceanic-continental subduction zone Berceley Glossary Soil liquefaction Soil liquefaction liquefaction !!! About geology Rocky Mountain Mineral Law Foundation seismo flovage - salt geology Rivers and meander loops Aerial Photography of Southern California Mass wasting Rivers photo Sole and turbedites Kentucky Geology |
