The New York-Alabama Magnetic Lineament: its reflection character and relationship to the Grenville front

• Main Author: Hopkins, Debbie L.
• Other Authors: Geophysics
• Format: Others
• Language: en
• Published: Virginia Tech 2014
• Subjects: earth formations; LD5655.V856 1995.H665
• Online Access: http://hdl.handle.net/10919/38141; http://scholar.lib.vt.edu/theses/available/etd-06062008-160240/

The source of the New York - Alabama Magnetic Lineament (NY AML) is revealed on newly reprocessed seismic reflection data, and the Grenville Front Tectonic Zone (GFTZ) is imaged beneath it in eastern Tennessee. Industry data, correlated to lower crustal depths, image a wedge-shaped block beneath the shelf strata of the Cumberland Plateau and Valley and Ridge provinces of eastern Tennessee. Two dimensional gravity and magnetic modeling corroborate the interpretation that the contrast in density and magnetic susceptibility between the wedge and the adjacent crust produces the Lineament. The boundary across which the contrast is generated dips approximately 30° northwest. East-dipping reflections imaged below 7 seconds can be extended northwest to the surface where they align with the position of the Grenville Front. The reflections are interpreted as evidence of deformation related to the GFTZ in Canada. The mid-crustal band of reflectivity visible on most of the reflection profiles lies above the east-dipping reflections and is interpreted to delineate the eastern margin of the GFTZ. The crust southeast of the wedge-shaped block exhibits high reflectivity with well-developed west-dipping events. The west-dipping events might correlate to those reported in Ohio on COCORP data, suggesting that they are pervasive in the basement throughout the eastern United States. The fabric is interpreted to have formed during the continent-continent collision of the Grenville Orogeny. The absence of west-dipping reflections within the wedge suggests that the wedge is younger than the development of the fabrics recorded by the reflections. Vertical dike swarms are interpreted to intrude the west-dipping fabric. The swarms model as felsic and appear on migrated data to be older than uplift, erosion, and deposition of the shelf strata. Crustal thickness estimates by previous authors of over 45 km are corroborated with interpreted images of the Moho on two deeper reflection profiles. The thick crust might be the locus of anatectic melting following Grenville collision. The emplacement of granitic or granodioritic magmas provides an explanation for the density, magnetic susceptibility, and difference in reflectivity of the wedge-shaped block. The New York - Alabama Magnetic Lineament diverges from the location of the Grenville Front north of the study area. The position of the NYAML can be interpreted to represent the axis of anatectic melting following collision, and indicates that the thickest part of the crust formed farther east of the Front in Canada than in Tennessee. Pseudomagnetic field investigations permit the distinction between the source of the New York - Alabama Magnetic Lineament and adjacent high susceptibility sources to the northwest. The sources to the northwest appear from the modeling to be mafic intrusions that might be related to the Norris Lake peridotite. Earthquake locations in the Eastern Tennessee Seismic Zone (ETSZ) are aligned along the southwest edge of the gradient of the NY AML, and fall within the crust characterized by strong west-dipping reflections. Because the contact between the wedge and the region of west-dipping reflections is dipping to the northwest, the relationship between the NYAML and the ETSZ is not clear. More accurate hypocenter locations are necessary to clarify whether the earthquakes are restricted to the region of the crust typified by west dip. If not, the relationship between the earthquakes and the NYAML might be coincidental. A velocity model that considers the dipping boundaries in these reflection data should result in hypocenter locations that can constrain the relationship. === Ph. D.