Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 6
Presentation Time: 10:00 AM


O'HARA, Kieran, Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506, KANDA, Ravi, Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125 and THOMAS, William A., Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506,

An elongated bull's-eye shaped thermal anomaly is defined by enhanced vitrinite reflectance (R max = 1.2-1.6 %) of Pennsylvanian coal in the Black Warrior basin of Alabama, adjacent to the Appalachian thrust front. The anomaly is bounded on the northeast by a cross strike transverse zone (Bessemer zone) and an associated lateral ramp, and on the southeast by a trailing ramp and a thrust fault (Jones Valley thrust). The southwestern terminus of the anomaly is hidden beneath Cretaceous cover. The origin of the anomaly is inconsistent with igneous activity, excess sedimentary cover, hydrothermal activity, or frictional heating.

Balanced and restored cross sections in the thrust belt indicate that a 3-km-thick thrust sheet with horizontal dimensions of ~10 x 30 km was emplaced northwestward a distance of ~18 km beyond the present eroded thrust front. This thrust was part of a much larger thrust sheet present in the hinterland. A three-dimensional analytical thermal model of a 3-km-thick thrust sheet that cools both laterally and vertically, reproduces the magnitude and shape of the coal rank anomaly between 500,000 and 800,000 years after thrust emplacement. The geothermal gradient reaches a steady state at 2 m.y., and is never fully re-established because of lateral cooling in the hanging wall of the thrust. It is proposed that the thermal anomaly was caused by the excess tectonic cover of the thrust sheet in a manner analogous to that of a hot clothes iron on a shirt. This hypothesis would indicate the southern Appalachian thrust belt was emplaced farther westward onto the Black Warrior foreland basin than previously recognized and that lateral cooling of thrust sheets may produce oval-shaped isograd patterns in orogenic belts.