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

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  • Precambrian Research, 42 (1989) 387-409 387 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands




    Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.)

    (Received March 31, 1987; revision accepted August 15, 1987)


    Dallmeyer, R.D., 1989. Contrasting accreted terranes in the southern Appalachian Orogen, basement beneath the Atlantic and Gulf Coastal Plains, and West African orogens. Precambrian Res., 42: 387-409.

    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 4Ar/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 4Ar/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 4Ar/'~gAr 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 tecton- othermal 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 sedi- mentary 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 se- quences 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 tectono- thermal 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

    0301-9268/89/$03.50 1989 Elsevier Science Publishers B.V.

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    Laurentia. They are separated from Gondwanan elements of the Suwannee Terrane by a suture approximately marked by the Brunswick-Altamaha Magnetic Anomaly.


    Most late Paleozoic continental reconstruc- tions place western Africa adjacent to south- eastern North America (e.g., Van der Voo et al., 1976; Scotese et al., 1979; Lefort, 1980; Pilger, 1981; Klitgord and Schouten, 1981; Van der Voo, 1983; Ross et al., 1986; Rowley et al., 1986; Ross and Scotese, 1988). These clearly suggest po- tential tectonothermal linkages between the central-southern Appalachian Orogen and the Mauritanide, Bassaride and Rokelide orogens of West Africa. All of these areas are dominated by variably allochthonous sequences which traced tectonostratigraphically upward become increasingly more exotic relative to structurally underlying miogeoclinal sequences. The exotic lithostratigraphic terranes represent succes- sions which could have formed:

    (1) outboard of the Laurentian or Gond- wanan miogeoclines during late Paleozoic clo- sure of'the Iapetus oceanic tract;

    (2) in early-middle Paleozoic settings re- moved from either Laurentia or Gondwana and accreted prior to or during amalgamation of Pangea; or

    (3) Laurentian or Gondwanan continental fragments stranded during Mesozoic rifting as- sociated with opening of the present Atlantic Ocean.

    Results of recent collaborative field and geo- chronological work in West Africa and the southern Appalachian Orogen have helped to resolve the origin of many of these exotic ter- ranes and establish their accretionary chronol- ogy. These results are briefly summarized in this report.

    Southern Appalachian Orogen

    The southern Appalachian Orogen may be broadly subdivided into several NE-SW trend-

    ing lithotectonic belts (Fig. 1 ), each character- ized by a distinctive group of lithologies, meta- morphic grade, and/or structural style (e.g., King, 1955; Hatcher, 1972, 1978; Rankin, 1975; Glover et al., 1983). Stratigraphic and/or pa- leontological characteristics suggest that se- quences within the Valley and Ridge and allo- chthonous western Blue Ridge provinces formed along the Paleozoic margin of Laurentia. Rocks within the eastern allochthonous lithotectonic belts have uncertain palinspastic origins. Re- sults of seismic reflection studies (e.g., Cook et al, 1979; Harris et al., 1981 ) and regional grav- ity and magnetic characteristics {e.g., Hatcher and Zietz, 1978, 1980; Cook and Oliver, 1981) indicate that autochthonous North American basement probably underlies most of the south- ern Appalachian allochthonous sequences.

    Lithostratigraphic sequences in the eastern Blue Ridge and throughout the Piedmont are not comparable with successions of similar age in either the Valley and Ridge or the western Blue Ridge which appear to have originated along the eastern margin of Laurentia. The presence of Atlantic Province trilobite fauna (Secor et al., 1983) within the Carolina Slate Belt requires that at least these sequences (and the petrogenetically linked Charlotte Belt) were faunally isolated from Laurentia during the Middle Cambrian (Secor et al., 1983). To- gether these characteristics suggest that at least the eastern Piedmont represents an exotic ter- rane (the Carolina Terrane of Secor et al., 1983 ) that accreted to North America subsequent to the Middle Cambrian. The timing of accretion is uncertain. In the Albemarle area of North Carolina, Noel et al. (1988) reported 4Arff9Ar whole-rock plateau ages of ~ 460 Ma for pene- tratively cleaved slate/phyllite within the Car- olina Slate Belt. They interpreted these to date cleavage formation which they suggested devel-

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    186 . :;: ::: ~ I ..... ~ Kings Mr. 3 6 -

    o loo . . . . . Belt | i I : : F " .

    / ' ..:.:: km P ~ ' / : : i i i i i i i i ~/ChougOsequence


    ~ 34 o- I

    ~ ; ' _Belair 1. B e l t

    c o

    8 2 8 0 " I 1

    Fig. 1. Index map of the southern Appalachian orogen locating geographic and geologic features discussed in the text (adapted from Secor et al., 1986a). Grenville-age basement rocks within the Blue Ridge are shown (black): AA = Alto allochthon.

    oped during accretion of the exotic Carolina Slate Belt to Laurentia. No clear record of this Middle Ordovician activity is obvious in any of the geochronological results presented by Dall- meyer et al. ( 1986 ) and Dallmeyer (1988a) for the Piedmont in Georgia or South Carolina. In these areas the oldest record of tectonothermal activity is ~ 360-340 Ma. If this was associated with accretion of Piedmont terranes to Lauren- tia, distinctly different tectonic elements must comprise the Carolina Slate Belt.

    Three distinct late Paleozoic deformational events affected the eastern Piedmont in Geor- gia and South Carolina between ~ 315 and 270 Ma. Together these comprise the Alleghanian orogeny (Dallmeyer et al., 1986; Secor et al., 1986a). The first was associated with regional metamorphism of variable grade and the em- placement of felsic plutons at mid-crustal depths between ~315 and 295 Ma. A second Alleghanian event resulted in folding of iso- thermal surfaces between ~ 295 and 285 Ma. The final phase of Alleghanian deformation led to development of dextral ductile shear zones between ~ 290 and 265 Ma. Secor et al. (1986b) and Dallmeyer et al. (1986) suggested that re-

    gional variations in metamorphic grade and 4Ar/Z9Ar mineral cooling ages in the allo- chthonous eastern Piedmont of Georgia and South Carolina are a result of the exposure of variable crustal levels which developed by re- gional flexure during late Paleozoic translation over thrust ramps. Their model suggests that the Charlotte and Carolina Slate belts were ini- tially contiguous and underwent a regional metamorphism outboard of Laurentia some- time prior to ~360-340 Ma. Pre- to syn-tec- tonic granitic plutons were emplaced into the Carolina Slate Belt between 320 and 310 Ma. This heat influx resulted in widespread meta- morphism and establishment of an amphibol- ite-grade infrastructure (Kiokee Belt) and a greenschist-grade superstructure. In higher crustal levels the late Paleozoic thermal event variably reset intracrystalline Ar systems which had previously cooled through closure temper- atures. However, at deeper crustal levels the ambient country rock temperatures had been continuously maintained above hornblende Ar closure temperatures ( ~ 500 C) since initial Devonian metamorphism. This crustal section underwent westward transport onto the North

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    American margin following emplacement of 280-260 Ma late- to post-kinematic plutons

    within the infrastructure. During translation, the high-grade infrastructure thickened as a re- sult of stepping over frontal ramps in the basal thrust. This was accompanied by dextral duc- tile shearing which resulted in differential off- set of the high-grade core and lower-grade flanking zones along the polygenetic Modoc zone. Dynamically recrystallized biotite within the zone records 267 Ma 4ArffgAr plateau ages (Dallmeyer et al., 1986) which probably date final stages of this ductile activity. Deeper crus- tal levels of the Charlotte Belt which had been maintained above hornblende Ar closure tern- peratures since initial Devonian metamor- phism were brought to higher crustal levels as a result of regional flexure over a western, higher-level thrust ramp.

    The Alto allochthon within the western Piedmont appears to have been emplaced by west-directed thrusting from a Piedmont root zone following Late Devonian or earlier high- grade metamorphism (Hopson, 1984; Dall- meyer, 1988a; Hopson and Hatcher, 1988). This thermal chronology is consistent with work elsewhere in the western Piedmont which sug- gests at least local homogenization of Sr iso- topes between ~400 and 380 Ma (Higgins et al., 1980; van Breemen and Dallmeyer, 1984). It is also consistent with ~ 355 Ma post-meta- morphic 4ArffgAr cooling ages recorded by hornblende in westernmost portions of the In- ner Piedmont near Atlanta (Dallmeyer, 1978). Dallmeyer (1988a) suggested that emplace- ment of the Alto allochthon occurred after Late Devonian or earlier high-grade metamorphism and prior to regional cooling through muscovite Ar closure temperatures at ~ 315-300 Ma. Up- ward transport to higher crustal levels during nappe transport is probably recorded by the diachronous cooling of the allochthon through hornblende Ar closure temperatures between

    360 and 335 Ma. Generally similar times for ductile faulting are suggested by Rb-Sr whole- rock isochron ages reported for mylonitic rocks

    within thrusts bordering several of the south- ern Appalachian lithotectonic belts. These in- clude: (1) the Brevard fault zone (356 _+ 28 Ma; Odom and Fullagar, 1973); and (2) the Great Smoky fault (368+ 9 Ma; Hatcher and Odom, 1980).

    Pre-Cretaceous crys ta l l ine basement beneath the At lant ic and Gul f Coastal P la ins o f the southeas tern U .S .A .


    The nature of the pre-Mesozoic crystalline basement beneath the Atlantic and Gulf Coastal Plains of the southeastern United States has been revealed by penetrations associated with deep oil test drilling. Buried extensions of Ap- palachian elements (including the Valley and Ridge Province, Talladega Slate Belt, and var- ious Piedmont terranes) extend ~ 50-60 km southeast of the Coastal Plain unconformity (Fig. 2 ). These are bordered to the south by a series of fault-bounded basins containing Me- sozoic, continental clastic rocks which are in- truded by numerous diabase dikes. Three con- trasting lithotectonic elements constitute the pre-Mesozoic crystalline basement which has been penetrated south of the Mesozoic basins. These include (Fig. 2):

    ( 1 ) a group of metamorphic rocks of variable grade together with deformed and retrogressed granite in southwestern Alabama and south- eastern Mississippi (Wiggins Uplift);

    (2) a suite of contrasting igneous rocks (granite, basalt, and agglomerate ) and serpen- tinite which occurs along the Brunswick-Alta- maha Magnetic Anomaly in southwestern Al- abama; and

    (3) an extensive, apparently coherent tec- tonic element comprised of undeformed gran- ite, low-grade felsic metavolcanic rocks, a suite of high-grade metamorphic rocks (gneiss and amphibolite ), and a succession of undeformed, Lower Ordovician-Middle Devonian sedimen-

  • Foreland Of The ' Appalachian Orogen

    ::(:~. ff ., Talladega " ~ i : ~ S l a t e Belt

    Terranes In " . The Piedmont


    SUWANNEE TERRANE ~ rdovician-Devonian

    sedimentary rocks [ ] Osceola Granite [ ] ealc-alkaline, felsic, low-grade [ ]

    metaiqneous suite

    [ ] high-grade metamorphic rocks [ ]


    MESOZOIC continental sedimentary rocks and diabose intrusions volcanic sequences

    WIGGINS UPLIFT ;~-'~ g n eiss, am phi b...


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