THE ALABAMA RECESS: A COMPOSITE RECESS IN THE APPALACHIAN THRUST BELT

In thrust belts, salients are sweeping curves convex toward the foreland; recesses are more angular bends concave toward the foreland. Many recesses are narrow cross-strike bands of intersecting structures; others are more complex, as the Alabama recess in the southern Appalachian thrust belt between the Tennessee (east) and Ouachita (west) salients.

The Alabama recess was defined from a nearly orthogonal intersection between northeast-striking frontal Appalachian structures and northwest-striking frontal Ouachita structures in the subsurface in eastern Mississippi. The leading Appalachian structure, the Sequatchie anticline, trends 040 tangent to the apex of the Tennessee salient, but ends southwestward in western Alabama, transferring the Appalachian thrust front southeast across strike. Farther west, the Appalachian thrust front curves to ~090 and truncates the leading Ouachita frontal ramps, which strike 340 at the intersection and curve westward around the Ouachita salient.

In the Tennessee salient, the trailing structures curve gradually southward to trend ~000. In contrast, the Alabama thrust-belt structures trend ~040, and the trailing structures project toward an angular intersection with the trailing structures in the southern arm of the Tennessee salient. Near the projected intersection, the 040 trend curves to ~065. At the trailing edge of the sedimentary thrust belt, the leading metamorphic thrust sheets (000-striking Great Smoky and 065-striking Cartersville faults) define an angular intersection in northwestern Georgia.

The Alabama recess encompasses cratonward-concave angular intersections of frontal structures in Mississippi and trailing structures in Georgia, ~400 km apart along strike. Between the two intersections, the Appalachian thrust belt is relatively straight. Fault truncation shows that Appalachian thrusting was later than Ouachita thrusting. Progradation of synorogenic clastic wedges indicates that tectonic loading began earlier in the Tennessee and Ouachita salients than in Alabama. In a diachronous sequence of thrusting, the 040-trending structures in Alabama are younger than those in the salients on either side. The Alabama recess is a composite structure, in which the latest stage of thrusting linked the arms of two otherwise unconnected salients.