Contrasting accreted terranes in the southern Appalachian Orogen, basement beneath the Atlantic and Gulf Coastal Plains, and West African orogens

Abstract

A variety of lithostratigraphic units may be outlined within the pre-Mesozoic crystalline basement of the Atlantic and Gulf Coastal Plains. These include: (1) a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern Alabama and southeastern Mississippi (Wiggins Uplift). Hornblende and biotite within higher-grade units record 40Ar/ 39Ar post-metamorphic cooling ages of ∼310−300 Ma; (2) a suite of contrasting igneous rocks (granite, basalt, and agglomerate) and serpentinite which occurs along the Brunswick-Altamaha Magnetic Anomaly in southwestern Alabama; and (3) an extensive, apparently coherent tectonic element (the Suwannee Terrane) comprised of undeformed granite (in which biotite records 40Ar/ 39Ar post-magmatic cooling ages of ∼530−525 Ma), low-grade felsic metavolcanic rocks, a suite of high-grade metamorphic lithologies (St. Lucie Metamorphic Complex in which hornblende yields 40Ar/ 39Ar post-metamorphic cooling ages of ∼515−510 Ma), and a succession of Lower Ordovician-Middle Devonian sedimentary rocks (characterized by Gondwanan fauna). The West African Orogens (Mauritanides, Bassarides, and Rokelides) record a locally complex, polyphase tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African Craton at ∼700 Ma. This led to development of a rift-facies lithotectonic succession which included sedimentary units and intracontinental igneous sequences. Rifting was limited, and a western ensialic arc began to develop by ∼680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ∼650 Ma (Pan-African I orogenesis). Following widespread deposition of late Paleozoic glacial and flyschoid sediments, a second tectonothermal event occurred between ∼550 and 500 Ma (Pan-African II orogenesis). The West African Orogens were marked by general tectonic quiescence throughout most of the early and middle Paleozoic. Late Paleozoic (∼300 Ma) collision of Gondwana and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes which include sequences with uncertain palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record an associated penetrative tectonothermal overprint. Characteristics of pre-Mesozoic basement units which comprise the Suwannee Terrane in the Florida subsurface suggest they represent extensions of the Bassaride-Rokelide Orogen. There is no apparent record of Paleozoic tectonothermal activity in the Suwannee Terrane. This contrasts markedly with penetrative late Paleozoic reworking of basement units in the Wiggins Uplift. This suggests that a major dextral transcurrent fault was active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various basement units of the Coastal Plain are not correlative with any of the non-Laurentian terranes exposed in the southern Appalachian Orogen (e.g., Carolina terrane of the eastern Piedmont) which accreted in the Ordovician-Devonian to exterior positions along the eastern margin of the North American craton. These exotic Appalachian terranes were transported into their present structural positions during late Paleozoic collision of Gondwana and Laurentia. They are separated from Gondwanan elements of the Suwannee Terrane by a suture approximately marked by the Brunswick-Altamaha Magnetic Anomaly.