Thermal footprint of an eroded thrust sheet in the southern Appalachian thrust belt, Alabama, USA

Abstract

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.

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