Lesson 20 - Module E. Planetary Engineering: Mesozoic...

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Module E. Planetary Engineering: Mesozoic Tectonics Lesson 20: Cordilleran Terranes and Mesozoic Paleogeographic Evolution Introduction Different parts of the western margins of North and South America are presently interacting with the oceanic plates to the west in a variety of different ways, as shown in the figures below. In some parts, oceanic crust is being subducted, and a magmatic arc is being constructed, whereas in other portions of the margin (especially in southern California and the northern BC coast) the oceanic plate is simply sliding alongside the continental plate (a transform margin). The geological evidence from western North America tells us that this margin has been very active in the past as well. Recognition that the Cordillera actually consisted largely of separate terranes , some of which have traveled far from their place of origin, was a major breakthrough in understanding how the Cordilleran belt in western North America formed. This is the topic for this Lesson. These are two key definitions that are widely used in tectonic studies and are significant to this Lesson: terrane - a crustal block or fragment that preserves a distinctive geologic history that is different from the surrounding areas and that is usually bounded by faults accreted terranes - terranes that become attached (or accreted) to a continent (e.g., western North America) as a result of tectonic processes
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Figure E-58. Terranes of Western North America A map of western North America showing some important plate tectonic features and the mosaic of far-traveled exotic terranes plastered against the long-lived, stable interior of the continent. Figure courtesy of USGS as modified from an illustration provided by Oceanus Magazine; original figure by Jack Cook, Woods Hole Oceanographic Institution. Figure E-59. Present Day Plate Interactions Along the North and South American Cordilleran Belts Note the unusual perspective of this map! The Cordilleran Belt The figure below is a simplified (believe it or not!) map showing the distribution of various terranes that make up the Cordilleran belt in western Canada. The two shades of blue show portions that have a close affinity to the interior of North America. These are the areas that are underlain by rocks that are presently located pretty close to where they were originally formed. All of the rest of the terranes farther to the west are believed to have formed at least some distance away, and were moved into their current positions within the Canadian Cordillera by tectonic processes.
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Figure E-60. A Map of Sedimentary Rocks of Western North American Continent Note the rocks in two shades of blue (encircled legends). These portions have a close affinity to the interior of North America. Rocks in light blue areas are directly linked to the continent. Rocks in darker blue areas are the same package of rocks that slid to the north a few hundred km along faults ~55 Ma. Everything else is made up of terranes that have come in and become attached to the western edge of N. America
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This note was uploaded on 11/18/2011 for the course EOSC 116 taught by Professor Randell during the Winter '09 term at The University of British Columbia.

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Lesson 20 - Module E. Planetary Engineering: Mesozoic...

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