A DISCUSSION ON THE ALLOCHTHONOUS HISTORY OF THE PINE MOUNTAIN TERRANE GIVEN NEW EVIDENCE FROM ITS NORTHEASTERN EXTENT

The Pine Mountain terrane (PMt), southern Appalachians (SA), consists of massive Grenville basement nonconformably overlain by Cambrian(?) metasedimentary strata exposed in a complex fault-bounded window. We present a possible allochthonous history of the PMt given structural and temporal constraints based on recent detailed geologic mapping and U-Pb zircon geochronology at the northeastern end of the PMt, combined with other data from the SA Inner Piedmont (IP).

Bounding faults of the window consist of the Alleghanian Towaliga fault (Tf) to the NW, the mid-Paleozoic Box Ankle fault to the E, and the Alleghanian Bartletts Ferry-Goat Rock and Dean Creek faults to the S and SW. Crosscutting relationships and contrasting deformation temperatures in fault rocks reveal the sequence in which these faults formed. The Box Ankle thrust fault juxtaposes IP rocks above the PMt and is the oldest bounding fault. It is truncated to the NW and SE by younger faults, contains upper amphibolite facies mylonite, and is isoclinally folded axial planar to regional D₂ (Acadian/Neoacadian) fabric that is crosscut by Alleghanian plutons. The Tf formed as a dextral strike-slip fault at garnet grade, and evidence from the NE end of the PMt suggests only small-displacement, sinistral reactivation/inversion occurred ~200 Ma. The Tf truncates the Lloyd Shoals plutonic complex (LSpc), a batholith-scale amalgamation of mostly Devonian megacrystic granite cut by stocks of Pennsylvanian-Permian granodiorite. Our observations dictate that: 1) the IP overrode the PMt prior to the Alleghanian orogeny; and 2) truncation of the LSpc requires the Tf be related to PMt emplacement, as the kinematics of the Tf and truncation of the LSpc do not permit emplacement of the PMt prior to Tf displacement.

Palinspastic restoration of the bounding faults, and major SA faults, indicates the PMt may have originated near the Virginia promontory. The PMt was then translated SW with the IP along the Brevard fault during the Acadian/Neoacadian orogeny, and along the Tf during the Alleghanian orogeny. The current configuration of the PMt far outboard in the orogen, combined with its near-shore Cambrian(?) cover strata, suggest it may represent a peri-Laurentian microcontinent that rifted from the margin during the Cambrian.