Chapter 27: Reflection seismic studies of crustal structure in the eastern United States
Robert A. Phinney, Kabir Roy-Chowdhury, 1989. "Chapter 27: Reflection seismic studies of crustal structure in the eastern United States", Geophysical Framework of the Continental United States, L. C. Pakiser, Walter D. Mooney
Download citation file:
Since 1978, several seismic reflection transects have been run across the Appalachian and Ouachita orogens; several special topical reflection lines in the craton have shown the complexity and variability of the Precambrian basement. New methods of signal extraction and display make it possible to reprocess these stacked sections to provide standardized crustal images. The limitations to image fidelity induced by the presence of noise and unresolvable structure variations, as well as the limitations inherent in the physics of wave propagation, are discussed in a tutorial section to provide tools for the understanding of reflection profiles. Of particular importance is the need for the interpreter to work interactively with the final processing step in which processed digital data are transformed into a graphical image.
Reprocessed sections for a set of transects across the Appalachians provide a framework for a rediscussion of the structure and evolution of the orogen. Features that appear on several lines are (1) an east-dipping suture that separates the exotic terranes of the central and eastern regions from the post-Taconian North American continent; (2) west of the suture, thin-skinned, west-directed thrusting of elements of the post-Taconian margin onto the edge of the craton; (3) east of the suture, a compressed central package extending to near Moho; and (4) the east-dipping boundary produced by Alleghanian collision of an Avalonian terrane. Of particular interest is the indication of a partially rifted fragment of Grenville basement, which forms an antiformal buttress just west of the principal suture. On the lines best showing the eastern Avalonian terrane is an indication that the collision resulted in midcrustal delamination, leading to west-directed overthrusting and underplating of the crust by the colliding terrane. The relationship between Moho reflection strength and Mesozoic extensional faults leads to the inference that the extension extends regionally throughout the interior of the orogen and out to the present continental margin, and that the extension was responsible for the enhanced reflectivity of Moho and lower crust.