THE SUWANNEE-SABINE CONNECTION: UNITING THE ALLEGHANIAN AND OUACHITA OROGENS IN SOUTHERN LAURENTIA

Accurately characterizing the tectonics associated with the simultaneous accretion of the undeformed Gondwanan Suwannee and Sabine terranes to adjacent deformed terranes in southern Laurentia is critical to understanding the accretionary history of Laurentia, the character of the Alleghanian and Ouachita orogenies, and the geodynamics of the formation of Pangea. The unexposed Suwannee terrane is separated from the southern Appalachian orogen (SAO) by the Suwannee suture. The suture is typically considered an Alleghanian feature spatially coincident with the Brunswick-Altamaha magnetic anomaly (BMA), which cuts across the tectonic grain of the SAO. The Sabine terrane is an isolated block of thick crust in the subsurface of Texas and Louisiana; it is not separated from Laurentia by any obvious structure or anomaly. In the SAO U-Pb ages of zircon xenocrysts and Sm-Nd depleted mantle model ages from Neoproterozoic and Alleghanian granites north and south of the BMA suggest Mesoproterozoic (Grenville) sources. In contrast, Suwannee terrane sedimentary rocks contain no Mesoproterozoic zircons. This suggests that the Suwannee terrane is composite, likely containing both African (upper) and Amazonian (lower) crust. Samples from the Sabine terrane have been interpreted to represent ~250 Ma arc magmatism. In contrast, Alleghanian arc magmatism is not present in the SAO or Suwannee terrane. This is a substantial tectonic problem because the Alleghanian orogeny is typically associated with closure of the Iapetan and Rheic oceans. This “missing link” of arc magmatism can be accommodated, however, by subduction along the northern-western margin of a combined Suwannee-Sabine terrane. In this interpretation, a combined Suwannee-Sabine terrane was emplaced along a primarily transpressional margin to the east (Florida) and a more compressional margin to the west (Ouachita orogen of Texas-Oklahoma). The transpressional nature of Suwannee accretion suggests the Suwannee may contain internal thrust displacements that extended to transpressional over-thrusting that produced Alleghanian magmatism and metamorphism in the southernmost SAO (e.g., Uchee terrane). Rifting associated with the Gulf of Mexico has likely disrupted original continuity of the Suwannee-Sabine terrane in the late Paleozoic.