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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.
Rifted margins mark the transition between continents and oceans, which are the two first-order types of land masses on Earth. Rifted margins contribute to our understanding of lithospheric extensional processes and are studied by various disciplines of Earth Science (geology, geophysics, geochemistry). Thanks to better and wider public access to high-quality data, our understanding in these areas has improved significantly over these last two decades. This book summarizes this knowledge evolution and details where we stand today, with a series of case examples included. It is structured in a practical way, with concise text descriptions and comprehensive diagrams. Continental Rifted Margins 2 is a useful resource for students and newcomers to the rifted margin community – a "cookbook” of sorts to facilitate the reading of scientific publications and provide basic definitions and explanations.
Non-continental margins lack thick lavas that are generated as continental crust thins immediately prior to the onset of seafloor spreading. They may form up to 30 per cent of passive margins around the world. This volume contains papers examining an active margin, fossil margins that border present day oceans, and remnants of margins exposed today in the Alps. The papers present evidence across a range of scales, from individual mineral grains, through borelide cores and outcrop, to whole margins at the crustal scale.
Rifted Ocean-Continent Boundaries covers a wide range of topics, from quantitative modelling to current knowledge of the structure and evolution of specific margins around the world. Special emphasis is placed on the structure and evolution of various Atlantic margins. After an introduction to volcanic margin concepts, the first articles report the results of numerical models of the mechanics of rift propagation, melt generation and sources of extensional stresses that may cause break-up. One part of the book is dedicated to current knowledge of the structure and evolution of various Atlantic margins. After a brief incursion into the Mediterranean, succeeding articles report on the transform and active margins of the Ivory Coast-Ghana transform margin and the Sea of Japan.
Developments in Geotectonics, 15: Crustal Properties across Passive Margins covers the papers presented at the symposium ""Crustal Properties across Passive Margins"", held at Dalhousie University in Halifax, Nova Scotia on June 19-23, 1978. The book focuses on theoretical modelling of the rheological properties in the upper mantle beneath oceans and continents and stratigraphic studies of the sedimentary basins. The selection first offers information on seismic refraction study of the continental edge off the Eastern United States and gravity field of the U.S. Atlantic continental margin. Topics include crustal sections, inner and outer quiet zones, alternative models and complications, gravity models, free-air anomaly map, and the ocean-continent transition zone. The text then takes a look at the geologic history of the passive margin off New England and the Canadian Maritime Provinces and geophysical observations bearing upon the origin of the Newfoundland Ridge. The manuscript elaborates on the age and origin of the deepest correlative structures recognized off Canada and Europe and geophysical transects of the Labrador Sea. Discussions focus on magnetics and gravity, stratigraphic control, consequences regarding subduction or rifting, and deep structures of the Grand Banks of Newfoundland. The selection is a dependable reference for readers interested in crustal properties across passive margins.
This reference on the geology and geophysics of continental margins contains a total of 15 papers developed from a session of the Fifth International Congress of the Brazilian Geophysical Society held in Sao Paulo, Brazil in 1997, as well as a number of other contributions. Subjects include the roots of the southeastern continental margin of Brazil, the mosaic of Terranes in central Europe, the evolution of the Angolan passive margin; geological and geophysical interpretation of the San Julian Basin offshore Argentina; and the tectonic evolution of the equatorial South Atlantic. Of likely interest to academic geoscientists working in basin analysis and those engaged in petroleum exploration. Member price, $52.50. Annotation copyrighted by Book News, Inc., Portland, OR.