Insights into the Alleghanian thrust belt from a balanced cross section in the Bessemer transverse zone, southeastern United States

ABSTRACT In the central Alabama Alleghanian thrust belt, multichannel seismic and well-log data were used to analyze the structural elements associated with the Bessemer transverse zone. Six horizons were interpreted on the seismic data in two-way traveltime and were depth-converted using interval velocities derived from a synthetic seismogram as well as from two sonic logs. The structural interpretation from the depth-converted data shows that the thrust belt is a mainly forward-propagating thrust sequence with six principal thrust faults branching from a basal detachment, along with a few smaller thrust sheets, three normal faults, and some out-of-sequence faulting in the thrust sheet closest to the foreland. Basement along the profile is at a minimum depth of 4.7 km in the Black Warrior basin and a maximum depth of 7.2 km in the Birmingham graben system, yielding 2.5 km of relief in this Precambrian rift system. In this cross section, the coherent reflectors show no evidence for a ductile duplex; thus, the section was balanced using line length balancing techniques. Restored sections reveal minimum shortening estimates of 37 km (43%), which is 5%–10% greater than previous estimates from outside of the Bessemer transverse zone. Perhaps the reason why shortening is greater in the Bessemer transverse zone is that faults are combining, producing higher shortening estimates.