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Processes that create and modify continental crust occur at continental margins. We investigate the crustal structure across the U.S. East Coast rifted margin and the convergent margin of Alaska using modem marine seismic techniques. Rifling of the U.S. East Coast margin was characterized by voluminous volcanism. We constrain the landward extent of crustal extension and rift magnetism to within a narrow region beneath the shelf and slope. The crust thinned here by 50-80% and then spreading began. Constraints from seismic imaging show that initial volcanism was accompanied by rapid margin subsidence, suggesting a limited and finite mantle source for the volcanism. We study the evolution of continental crust through terrane accretion and arc magnetism along transects across the Alaska Peninsula, where subduction occurs beneath accreted oceanic arc crust, and across older Bristol Bay crust. The velocity structure of the Peninsular accreted terranes is similar to intraoceanic arc crust and more mafic than continental crust. Accretion has apparently not modified the terranes' composition. The Bristol Bay crust is also composed of accreted oceanic arc terranes but has a continental velocity structure. If this crust was originally more mafic, then both crustal thickening and removal of a mafic component are required to explain its current structure processes consistent with the region's history.
Many of the most important processes that create and modify continental crust occur at continental margins, but recently has the scientific community acquired the necessary intrumentation to image crustal structure across margins in detail. In this thesis we investigate the crustal structure across the U.S. East Coast rifted margin and the convergent margin of southwestern Alaska using modern, deep-penetrating marine seismic reflection/refraction data. We consider U.S. East Coast margin transects along the shelf offshore Georgia and across the mid-Atlantic margin near Chesapeak bay. Results by other workers, based on data from these transects, have shown that voluminous volcanism accompanied formation of the rifted margin during continental breakup. Results presented in this thesis constrain the landward extent of rift-related magmatic emplacement. We find that magmatic intrusion and underplating of pre-existing continental crust occurs primarily in extended crust and that crustal extension is focused in a 75-km-wide region beneath the shelf and slope. The crust thinned by 50 to 80% within this interval and then seafloor spreading began with an unusually large volume of igneous crust production. The initial volcanic extrusives were emplaced subaerially and are now present beneath the sediments in a thick seaward-dipping wedge. We use post-stack depth migration to image this wedge and use the resulting image to consider the early subsidence of the margin. The geometry of the subaerially extruded rift volcanics suggest that the margin subsided rapidly once volncanism began. We infer from the subsidence, the along-margin distribution of magmatic material, and the across-margin localization of magmatic emplacement and deformation that the U.S. East Coast rift volcanics had an anomalously-hot mantle source whose distribution beneath the lithosphere prior to rifting was long (the length of the margin) but not deep. We speculate that the distribution of this material was controlled by topography at the base of the lithosphere inherited from the Paleozoic collision of North America and Africa. Our analysis of the southwestern Alaska convergent margin is based on data from the 1994 Aleutian seismic experiment. The crust of most of Alaska has been built through terrane accretion and arc magmatism, and this experiment was conducted to study the evolution of continental crust through these processes. We consider transects across the westernmost Alaska Peninsula margin, where subduction is occurring beneath protocontinental crust composed of oceanic-arc terranes accreted in the Cretaceous, and across Bristol Bay in the back arc region where the crust has undergone a number of geologic events since accretion. Across the Peninsula, we find that the velocity structure of the accreted terranes differs little from that of the Cenozoic Aleutian oceanic-arc crust west of the Peninsula determined along another transect of this experiment. The accreted oceanicarc terranes are considerably more mafic than continental crust and the process of accretion has apparently not modified the bulk composition of these terranes toward that of average continental crust. It is possible that Cenozoic arc magmatism has been more felsic in composition than that which formed the accreted terranes and the Aleutian oceanic arc to the west, and that these magmas have been emplaced primarily within the crust inboard of the accreted terranes which lie south of the currently active arc. The geology of the Bristol Bay region suggests that the crustal components here had an origin similar to that of the Alaska Peninsula margin- that is, accreted terranes. We find, however, that the crust beneath Bristol Bay has a typically continental velocity structure. If this crust originally had a structure similar to the Alaska Peninsula margin, then at least two processes must have occured to affect the transformation to its current structure: crustal thickening and removal of the mafic lower crust. The geologic events that have affected this region since accretion are consistent with such and evolution.
Approximately 70 percent of the world's population is concentrated in the coastal borderlands, which geologists recognize to be the present continental margins. This new book on these continental margins provides a detailed account of a meeting which brought together specialists in marine and terrestrial geology, geochemistry, and geophysics. The workshop garnered widespread support and enthusiasm for a new direction in margins research focused on interdisciplinary studies of the fundamental processes of continental margin evolution. Scientific problems and solutions were identified for both divergent and convergent margins. Results of the workshop show that many of the fundamental plate interaction processes are common to all margins, whether formed by extension, contraction, or translation. This conclusion suggests a unified approach to margins research. A margins initiative has been proposed to follow up on the workshop results by developing science programs aimed at understanding the processes that control the initiation and evolution of continental margins.
Physical Geography Made Simple focuses on developments in physical geography, including advancements in the study of landforms, weather, climate, water, soils, plants, and animals. The book first offers information on rocks and relief, weathering, slopes, and rivers and drainage basins. Topics include rock structures and landforms, crustal structure and movement, physical and chemical weathering, measurement and description of slopes, and transport, erosion, and deposition. The manuscript then ponders on glacial and periglacial landforms and desert and uropical landforms. The publication takes a look at coastal features, landscape development, and the atmosphere and its energy. The manuscript also elaborates on moisture in the atmosphere, air motion, general circulation, and weather. Discussions focus on fronts, weather prediction, planetary wind belts, pressure variations, upper air motion, adiabatic processes, and evaporation and condensation. The text is a valuable reference for geographers and readers interested in physical geography.
Andean Tectonics addresses the geological evolution of the Andes Mountains, the prime global example of subduction-related mountain building. The Andes forms one of the most extensive mountain belts on Earth, spanning approximately an 8,000 km distance along the western edge of South America, from 10°N to 55°S. The tectonic history of the Andes involves a rich record of diverse geological processes, including crustal deformation, magmatism, sedimentary basin evolution, and climatic interactions. This book addresses the range of Andean tectonic processes and their temporal and spatial variations. This critical resource is ideal for researchers interested in the causes and consequences of Andean-type orogenesis and the long-term evolution of fold-thrust belts, magmatic arcs, and forearc and foreland basins. Evaluates the history of Andean mountain building over the past 250 million years (the Mesozoic and Cenozoic eras) Integrates recent results and provides new perspectives on the complementary records of deformation, magmatism and sedimentary basin evolution, along with their interactions in time and space Provides insights into the development of the northern, central and southern Andes, all of which have typically been considered in isolation