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New England Geology

Giant Dikes: Patterns and Plate Tectonics
J. Gregory McHone, Don L. Anderson & Yuri A. Fialko


Giant dikes typically exceed 30 m in width and 100 km in length, with some examples over 100 m wide and 1,000 km long. Dikes are self-induced magma-filled fractures, and they are the dominant mechanism by which basaltic melts are transported through the lithosphere and the crust. These spectacular intrusions are likely to have fed flood basalts in large igneous provinces (LIPs), including provinces where the surface basalts have been diminished or removed by erosion.

Although giant dikes can intermingle with denser swarms of smaller dikes of similar composition (and probably similar origin), others occur in sets of several to a few dozen extremely large quasi-linear or co-linear intrusions, which may gently bend and converge/diverge at low angles across many degrees of latitude. Tectonic controls on the formation of giant dikes appear to be independent and different from structures related to smaller dike swarms. Theoretical modeling and field observations help us to understand the essential physics of magma migration from its source to its final destination in the upper lithosphere.

…in northeastern North America, huge but widespread dikes in Canada and New England diverge to the NE and ENE from a focus point east of New Jersey, but that is also not a plume center. The dikes change their trends across the “New England Salient,” which is a bend in terrane suture zones and primary structures of this section of the Appalachian Orogen. In addition, the giant dikes did not form together in a radial generation, but instead decrease systematically in age from the SE toward the NW (Figure 6).



Geological History of Jamestown, Rhode Island

Building the Northern Appalachian Mountains  and New England

The Appalachians are a complex mix of mountains formed by a series of continental collisions that took place over a period of more than 1 billion years.

This page focuses on important mountain-building and landscape altering milestones in the history of the Northern Appalachians and New England. It utilizes a set of illustrative cross sections to depict the cumulative effects of these milestone events. The example cross-section runs parallel to, but slightly north of the present southern coasts of Connecticut, Rhode Island and Massachusetts, and extends westward into New York state and eastward beyond Cape Cod. Each geological event mentioned on this page is described more fully on other pages on this site; and those pages, in turn, have links to external resources.
Each of these mountain building events (orogenies) included incorporation of new land masses seaward of the former coast line, distortion of existing land forms and metamorphosis of existing rocks. Erosion of the each newly-formed mountain chain resulted in deposition of massive amounts of sediment – both inland and along the coast.

Over the more recent past (tens of thousands of years) the region has been further shaped by two major glacial advance and retreat cycles….

See http://www.jamestown-ri.info/northern_appalachians.htm




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