PALEOMAGNETISM OF THE SILURIAN RED MOUNTAIN FORMATION: IMPLICATIONS FOR THRUST TRANSLATION WITHIN THE BIRMINGHAM ANTICLINORIUM IN THE APPALACHIAN THRUST BELT, ALABAMA

The northeast-striking, northwest-verging, large-displacement Jones Valley thrust fault in the Alleghanian Appalachian thrust belt in Alabama ends northeastward along strike within the Birmingham anticlinorium, the crest of which includes several thrust faults with smaller displacement. The footwall and hanging wall of the Jones Valley fault are continuous along strike with the forelimb and trailing limb, respectively, of the Birmingham anticlinorium. Along-strike termination of the Jones Valley fault implies vertical-axis rotation, and to test for rotation, analysis of magnetic remanence directions in the Silurian Red Mountain Formation was conducted on samples from three sites in the footwall/forelimb and seven sites in the hanging wall/trailing limb. The Red Mountain Formation includes hematite ore beds and reddish hematitic sandstones exposed on both sides of the Jones Valley fault. The beds dip steeply northwest to overturned on the footwall and gently southeast on the hanging wall, indicating a detachment fold. Paleomagnetic analysis reveals a stable pre-folding magnetization that exhibits a Pennsylvanian direction. The magnetization is likely a chemical remagnetization that occurred in the Pennsylvanian prior to any deformation. Sites from both limbs of the Birmingham anticlinorium (both sides of the Jones Valley fault) reveal no difference in declination, thus showing no rotation. To accommodate motion on the Jones Valley fault within the Birmingham anticlinorium, we propose that, northeast of the end of the Jones Valley fault, translation was partitioned among several small faults in the crest area of the Birmingham anticlinorium, whereas to the southwest, translation was accommodated entirely on the Jones Valley fault, allowing for no relative rotation on the trailing limb of the Birmingham anticlinorium.