LATE-OROGENIC EXTENSION IN THE SOUTHERNMOST APPALACHIANS COMPARED WITH THE NORTHERN CALEDONIDES: A KINEMATIC PATTERN OF COLLAPSE?

Recent geological investigations in the southernmost Appalachians indicate a structural and temporal framework for late-orogenic extensional movements that is reminiscent of that now recognized for the northern parts of the Caledonian belt of Norway.  In the southern Appalachians of Alabama and Georgia, the Augusta fault (east flank of the Kiokee belt) is one of the most internal (i.e., eastern) Appalachian faults exposed.  The Augusta fault is a tops-toward-the-hinterland (east) extensional shear zone interpreted to reflect unroofing of the subducted slab underneath Gondwana (?) at and after about 274 Ma.  Kinematic and ⁴⁰Ar/³⁹Ar isotopic investigations, and COCORP seismic data document extensional uplift of the internal Pine Mountain window (PMW) and Uchee belt (UB) at approximately the same time.  Uplift of the PMW/UB was accomplished partly by tops-toward-the-foreland (west) normal movement along the Towaliga fault.  Further westward, toward the foreland, biostratigraphically constrained Laurentian units of the Talladega slate belt (TSB) are excised by a tops-to-the hinterland (east) fault, the Goodwater-Enitachopco fault.  The PMW/UB and TSB, therefore, are uplifted blocks separated by the Piedmont allochthons that occur in a regional synform (i.e., Tallassee synform), which we interpret as a down-dropped, broad, graben-like structure. 

We recognize a similar kinematic pattern for late-orogenic extension in the northern Norwegian Caledonides where excellent, continuous outcroppings of mid-crustal level basement are as abundant as those of the overlying cover.  We explore the idea that tops-to-the-hinterland normal faulting in the most internal parts of the orogen, tops-to-the-foreland in the internal parts, and tops-to-the-hinterland in the external parts may be a pattern common to Earth's orogenic belts.  Strike-slip movements associated with these late-stage normal movements also occur in both orogenic belts and we additionally explore their possible significance.