Thomas, William A. and Kanda, Ravi V. S. and O'Hara, Kieran D. and Surles, D. Matthew (2008) Thermal footprint of an eroded thrust sheet in the southern Appalachian thrust belt, Alabama, USA. Geosphere, 4 (5). pp. 814-828. ISSN 1553-040X http://resolver.caltech.edu/CaltechAUTHORS:THOgeos08
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Erosion of the leading hanging-wall cutoffs of thrust sheets commonly obscures the magnitude of thrusting. The Jones Valley thrust fault in the southern Appalachian thrust belt in Alabama, USA, is exposed along a northwest-directed, large-scale frontal ramp, and the leading part of the thrust sheet has been eroded. Previously published and newly collected vitrinite reflectance data from Pennsylvanian coal beds document a distinct, northeast-trending, elongate, oval-shaped thermal anomaly northwest of the trace of the Jones Valley fault. The northwest edge of the thermal anomaly is ~18 km northwest of the fault trace, suggesting the original extent of the eroded thrust sheet. The anomaly ends both northeastward and southwestward along strike at lateral ramps. The southeast edge of the anomaly corresponds to the location of a footwall frontal ramp. A three-dimensional heat conduction model for simultaneous horizontal (two-dimensional) and vertical heat flow in a rectangular thrust sheet is designed to test whether the documented thermal anomaly (%Ro = 1.0–1.6) may reflect the former extent of thrust-sheet cover. The model uses a 3-km-thick thrust sheet with horizontal dimensions of 10 x 30 km, as well as a three-dimensional analytical solution to the heat conduction equation, whereby the thrust sheet cools both laterally and vertically. The model reproduces the magnitude and oval shape of the vitrinite reflectance anomaly at 100–500 k.y. after thrust emplacement. The geothermal gradient reaches a steady state at ~2 m.y., and is never fully reestablished even for long times because of lateral cooling in the hanging wall. Thickness and extent of the thrust sheet from the thermal model are consistent with balanced and restored cross sections of the Jones Valley thrust sheet based on geologic data; a thrust sheet ~3 km thick was emplaced ~18 km onto the foreland over the site of the thermal anomaly. The three-dimensional thermal evolution of both the hanging wall and the footwall is distinct from that predicted from one-dimensional models; a three-dimensional model predicts less heating of the footwall because of horizontal heat loss across bounding ramps.
|Additional Information:||Copyright © 2008 by Geological Society of America. MANUSCRIPT RECEIVED BY THE SOCIETY December 7, 2007. REVISED MANUSCRIPT RECEIVED May 8, 2008. MANUSCRIPT ACCEPTED May 14, 2008. Acknowledgment is made to the donors of the Petroleum Research Fund (38965), administered by the American Chemical Society, for support of this research. Jack Pashin provided the graphics files for Figure 3 and the volatile-matter contour map in Figure 2, as well as helpful advice on sample selection and a review of a draft of the manuscript. Richard Carroll provided numerous core samples from boreholes. Brian Cook, Liz Dodson, and Carrie Kidd assisted in the compilation and computation for the tables and Figure 4. We thank John Costain, Rick Groshong, Bob Hatcher, and Sid Jones for helpful reviews of the manuscript.|
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|Deposited On:||29 Oct 2008 20:50|
|Last Modified:||26 Dec 2012 10:28|
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