GEOMETRIC LIMITS OF SOUTHERN APPALACHIAN ACCRETED TERRANES

The southern Appalachian décollement defines the lower boundary of accreted terranes emplaced onto Laurentian crust of the Alabama promontory during Alleghanian thrusting. The southern Appalachian décollement dips at a low angle southeastward beneath the foreland thrust belt and beneath the Piedmont metamorphic thrust sheets, including the Pine Mountain internal basement massif. The elevation of the top of Precambrian crystalline basement rocks beneath the décollement ranges from -2 km on the northwest to approximately -8 km on the southeast. Southeast of the Pine Mountain basement massif, a more steeply southeast-dipping structural fabric (imaged by COCORP seismic reflection profiles) extends to the base of the crust, truncating the shallow top of basement, defining the southeastern limit of Laurentian crust, and characterizing the Suwannee-Wiggins suture zone between Laurentian crust and Gondwanan crust of the Suwannee terrane.

Palinspastic reconstruction of the Alleghanian foreland thrust belt shows that pre-Alleghanian Laurentian passive-margin and overlying cover sediments originally extended approximately to the present location of the Pine Mountain basement massif, covering nearly all of the shallow basement. The Talladega slate belt at the leading (northwestern) edge of the Piedmont metamorphic terranes is tied stratigraphically to the Laurentian passive-margin succession, indicating a palinspastic location along the margin of shallow Laurentian crust. Therefore, all of the metamorphic thrust sheets southeast of the Talladega slate belt have palinspastic sites outboard of the Laurentian margin, and the accreted metamorphic terranes between the Talladega slate belt and the Suwannee-Wiggins suture overlie shallow Laurentian basement. Some of these terranes may have been accreted along or adjacent to the Laurentian margin during the Taconic and/or Acadian orogenies; however, they were emplaced onto the shallow Laurentian crust during Alleghanian thrusting, coincident with tectonic stripping and imbricating of the original sedimentary cover. The Suwannee terrane evidently includes a full thickness of Gondwanan (African) continental crust in contrast to the thin terranes above the shallow basement rocks of the Laurentian continental crust northwest of the Suwannee-Wiggins suture.