INSIGHTS INTO THE ALLEGHANIAN THRUST BELT IN CENTRAL ALABAMA FROM A BALANCED CROSS SECTION IN THE BESSEMER TRANSVERSE ZONE

In the central Alabama Alleghanian thrust belt, multi-channel seismic and well log data are used to analyze the structural elements of the thrust belt. Six horizons were interpreted on the seismic data in two-way travel time 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 the thrust belt is a forward propagating thrust sequence with four main thrust faults branching from a basal detachment. Basement along the profile is at a minimum depth of 4,700 m in the Black Warrior basin and a maximum depth of 7,200 m in the Birmingham Graben system, yielding 2,500 m of relief in this Precambrian rift system. The coherent reflectors in this cross section show no evidence for a ductile duplex; thus, the section was balanced using line length balancing techniques. Similar to other balanced cross sections to the north and south in Alabama, this balanced cross section yields a minimum shortening of 30 km or 24%. Although this cross section is located within the Bessemer transverse zone, it has a forward propagating thrust sequence and amount of shortening similar to other cross sections in Alabama not within transverse zones. Thus, transverse zones in Alabama do not change the overall architecture or shortening within the Alleghanian thrust belt. Instead, the transverse zones appear to be accommodation regions where structural elements from the NE and SW combine producing a complex area of folding and faulting.