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

CONTRASTING ACCRETED TERRANES IN THE SOUTHERN APPALACHIAN OROGEN, BASEMENT BENEATH THE ATLANTIC AND GULF COASTAL PLAINS, AND WEST

AFRICAN OROGENS

R.D. DALLMEYER

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

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

Abstract

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 4°Ar/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 4°Ar/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 4°Ar/'~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 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

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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.

Introduction

Most late Paleozoic continental reconstructions place western Africa adjacent to southeastern 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 potential 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 successions which could have formed:

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

(2) in early-middle Paleozoic settings removed 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 associated with opening of the present Atlantic Ocean.

Results of recent collaborative field and geochronological work in West Africa and the southern Appalachian Orogen have helped to resolve the origin of many of these exotic terranes and establish their accretionary chronology. 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 characterized by a distinctive group of lithologies, metamorphic grade, and/or structural style (e.g., King, 1955; Hatcher, 1972, 1978; Rankin, 1975; Glover et al., 1983). Stratigraphic and/or paleontological characteristics suggest that sequences within the Valley and Ridge and allochthonous 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 gravity 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 southern 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 terrane (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 4°Arff9Ar whole-rock plateau ages of ~ 460 Ma for penetratively 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

C.

~ 34 o- I

• ~ ; ' _Belair 1. B e l t

c o °

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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 emplacement 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 4°Ar/Z9Ar mineral cooling ages in the allochthonous eastern Piedmont of Georgia and South Carolina are a result of the exposure of variable crustal levels which developed by regional flexure during late Paleozoic translation over thrust ramps. Their model suggests that the Charlotte and Carolina Slate belts were initially contiguous and underwent a regional metamorphism outboard of Laurentia some- time prior to ~360-340 Ma. Pre- to syn-tectonic granitic plutons were emplaced into the Carolina Slate Belt between 320 and 310 Ma. This heat influx resulted in widespread metamorphism and establishment of an amphibolite-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 temperatures. 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 result of stepping over frontal ramps in the basal thrust. This was accompanied by dextral ductile shearing which resulted in differential offset of the high-grade core and lower-grade flanking zones along the polygenetic Modoc zone. Dynamically recrystallized biotite within the zone records 267 Ma 4°ArffgAr plateau ages (Dallmeyer et al., 1986) which probably date final stages of this ductile activity. Deeper crustal levels of the Charlotte Belt which had been maintained above hornblende Ar closure tern- peratures since initial Devonian metamorphism 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 suggests 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-metamorphic 4°ArffgAr cooling ages recorded by hornblende in westernmost portions of the In- ner Piedmont near Atlanta (Dallmeyer, 1978). Dallmeyer (1988a) suggested that emplacement 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 southern Appalachian lithotectonic belts. These include: (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 .

Introduction

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 various 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 intruded by numerous diabase dikes. Three contrasting 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 southeastern Mississippi (Wiggins Uplift);

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

(3) an extensive, apparently coherent tectonic element comprised of undeformed granite, 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

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SUWANNEE TERRANE ~ rdovician-Devonian

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

metaiqneous suite

[ ] high-grade metamorphic rocks [ ]

Q

MESOZOIC continental sedimentary rocks and diabose intrusions volcanic sequences

WIGGINS UPLIFT ;~-'~ g n eiss, am phi bolit e,

granite ] p h y l l i t e

SW ALABAMA basalt, granite, ogg Iomerate,

[ ] serpentinite

COST GE-I OFFSHORE low grade metasedimentary

] and metavolcanie rocks

BRUNSWICK-ALTAMAHA MAGNETIC ANOMALY

[ ~ magnetic low (<400 gammas)

~ ] magnetic high (>800 gammas)

Fig. 2. Lithotectonic units within pre-Mesozoic crystalline basement of the Atlantic and Gulf Coastal Plains, southeastern United States (adapted from Chowns and Williams, 1983; Thomas et al., 1988). All contacts with Mesozoic sequences shown as high-angle faults. Trace of Brunswick-Altamaha Magnetic Anomaly from Zeitz (1982). Suwannee suture from Thomas et al. ( 1987 ). The wells discussed in the text are located ( 1-13 ).

tary rocks. The last association has been termed the Suwannee Terrane by Thomas et al. (1988).

Suwannee Terrane

Osceola Granite An area of undeformed granite consti tutes a

large portion of the pre-Mesozoic crystalline basement of central peninsular Florida (Fig. 2 ). This has been termed the Osceola Granite by Chowns and Williams ( 1983 ). Dallmeyer et al. { 1987) described the pluton as heterogeneous, and represented 'by biotite granodiorite, leuco- cratic biot i te-quartz monzonite, and biotite granite. Most of the samples examined by Dall- meyer et al. were composed dominantly of oli-

goclase, quartz, perthitic alkali feldspar, and biotite. The feldspars typically display a magmatic character (plagioclase occurs as zoned crystals and alkali feldspar appears to have re- tained most of its original albite component in the form ofperthi t ic lamellae). There is no tex- tural evidence for significant subsolidus migra- tion, exsolution, and /or recrystallization of feldspar components. Because of these petrographic characteristics, Dallmeyer et al. suggested the pluton experienced relatively rapid post-magmatic cooling and was probably emplaced at relatively shallow crustal levels.

Bass (1969) reported R b - S r analytical results from several density fractions of feldspar from two portions of a core from a well in

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Osceola County {well 1, Fig. 2). The data were extremely scattered and tentatively interpreted by Bass to reflect a crystallization age of ~ 530 Ma. Dallmeyer et al. (1987) reported 4°Ar/39Ar incremental-release ages for five biotite concentrates from cuttings recovered from four wells penetrating the Osceola Granite (wells 2- 5, Fig. 2 ). All the samples are unaltered, and the biotite concentrates display well-defined plateau ages ranging between ~ 535 and 527 Ma. Dallmeyer et al. suggested that the biotite plateau ages probably closely date emplacement of the pluton in view of its high-level character and apparently rapid post-magmatic cooling.

St. Lucie metamorphic complex A suite of high-grade metamorphic and as-

sociated, variably deformed igneous rocks occurs southeast of the Osceola Granite. This has been termed the St. Lucie metamorphic complex by Thomas et al. (1988). Predominant lithologies include amphibolite, biotite-muscovite schist and gneiss, and quartz diorite. The complex has a distinctive aeromagnetic signature (Taylor et al., 1968; Klitgord et al., 1983), with marked NW-SE trending magnetic line- ations. Thomas et al. (1988) suggested these may reflect structural strike.

Bass (1969) reported isotopic ages for the high-grade complex, including a 503 Ma K-Ar date for a hornblende concentrate from amphibolite and a 530 Ma Rb-Sr model age for a biotite concentrate from interlayered gneiss within a well in St. Lucie County (well 6, Fig. 2). Hornblende concentrates prepared from amphibolite cuttings recovered from wells in St. Lucie and Martin Counties (wells 7 and 8, Fig. 2 ) record well-defined 4°Ar/39Ar plateau ages of ~513 and 511 Ma (Dallmeyer, 1988b). These have been interpreted to date post-metamorphic cooling through appropriate Ar retention temperatures.

Felsic volcanic-plutonic complex A felsic volcanic-plutonic complex has been

penetrated in separated areas of the Coastal

Plain pre-Mesozoic basement. Lithologic var- iants include felsic vitric tuff, felsic ash flow tuff, and tuffaceous arkose with subordinate ande- site and basalt. Epizonal felsic plutons occur within some wells and are probably subvolcanic equivalents of the volcanic sequences. Mueller and Porch ( 1983 ) presented geochemical anal- yses for representatives of the felsic complex which suggested calc-alkaline affinities. The complex is generally undeformed; however, it nearly everywhere displays a low-grade metamorphic overprint.

The felsic igneous complex appears to be unconformably overlain by Lower Ordovician sandstone in one well in central peninsular Florida, and on this basis Chowns and Williams (1983) suggest a late Proterozoic-early Paleo- zoic age. This is consistent with stratigraphic relationships inferred from seismic characteristics in northwestern Florida (Arden, 1974). Whole-rock, K-Ar ages ranging between ~ 480 and 165 Ma have been reported for many mem- bers of the felsic complex (summarized by Chowns and Williams, 1983 ). A representative suite of seven volcanic samples has been ana- lyzed with whole-rock, 4°Ar/39Ar incremental- release techniques (Dallmeyer, unpublished data). All samples display markedly discordant age spectra indicating widespread disturbance of initial intracrystalline Ar systems. These results suggest that the published K-Ar whole- rock ages may not be used to constrain the time of either magmatic or metamorphic events.

The COST GEl well was drilled ~ 100 km east of the northernmost Florida coast (Fig. 2 ) and penetrated ~ 600 m of low-grade metasedimentary rocks (argillite) overlying variably metamorphosed trachyte and sandstone (Scholle, 1979). The relationship of this sequence to the mainland felsic igneous complex is uncertain. Whole-rock K-Ar ages of 374 and 346 Ma were reported for metasedimentary rocks recovered from the well (Simonis, in Scholle, 1979). A slate sample from 11,600 ft displays an internally discordant 4°Ar/39Ar age spectrum defining a total-gas age of ~ 341 Ma

(Dallmeyer, unpublished data). A felsic metavolcanic rock from 12,350 ft also displays an internally discordant age spectrum; however, in- termediate- and high-temperature increments correspond to an ~ 375 Ma plateau date. This is generally similar to a 363+7 Ma Rb-Sr whole-rock isochron reported for seven samples from the COST well by Simonis (in Scholle, 1979). These Devonian ages are more probably related to metamorphic overprinting than to initial magmatic events.

Paleozoic sedimentary rocks A succession of generally undeformed sedi-

mentary rocks occurs in several separate areas of the Coastal Plain crystalline basement. The base of the section is marked by Lower Ordov- ician littoral quartz sandstones (Carroll, 1963). These are overlain with presumed conformity by Ordovician to Middle Devonian shales with locally significant horizons of siltstone and sandstone. A nearly continuous succession appears to be present; however, Cramer (1973) noted that the absence of Lower Silurian faunas may indicate a disconformity. Cold-water, Gondwanan paleontological affinities are dis- played by all fauna throughout the entire Pa- leozoic sequence (Whittington, 1953; Andress et al., 1969; Goldstein et al., 1969; Cramer, 1971, 1973; Whitt ington and Hughes, 1972; Pojeta et al., 1976). Opdyke et al. (1987) reported ~ 1800-1650 Ma U-Pb zircon ages for detrital zircons within a core of Ordovician-Silurian sandstone from a well penetrating the north Florida basin in Alachua County, Florida (well 9, Fig. 2). Opdyke et al. presented paleomagnetic results from the sandstone core which suggest a paleolatitude of ~49 °. Dallmeyer ( 1987 ) reported a 4°Ar/39Ar plateau age of ~ 504 Ma for detrital muscovite from a well penetrating Lower Ordovician sandstone in Marion County, Florida (well 10, Fig. 2).

Structure

The spacing of basement penetrations pre- cludes reliable determination of the nature of

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the contacts between the various lithotectonic units which constitute the Suwannee Terrane. Applin (1951), Barnett (1975), and Chowns and Williams (1983) suggested that the Paleo- zoic sedimentary sequence occupies a regional synclinal structure (termed the North Florida Basin by Thomas et al., 1988). Later development of horsts and grabens during Mesozoic faulting significantly affected the subcrop distribution of basement units, particularly in south Georgia, southeastern Alabama, and northwestern Florida (e.g., Smith, 1983).

Relationship to Appalachian elements

Parts of the pre-Cretaceous crystalline basement beneath the Atlantic and Gulf Coastal Plains were initially correlated with Appala- chian sequences exposed in the Valley and Ridge Province (Campbell, 1939) and eastern Pied- mont (Milton and Hurst, 1965). However, the undeformed character and Gondwanan paleontological affinities of the Suwannee succession contrast markedly with sequences of similar age in the Valley and Ridge Province. In addition, the age and character of the Osceola Granite and bordering high-grade metamorphic sequences are unlike that of any Appalachian elements in either the Blue Ridge or Piedmont. Because of these inconsistencies, most recent workers (e.g., Chowns and Williams, 1983 ) have suggested that the pre-Cretaceous basement units are unrelated to exposed Appalachian tectonic elements.

Wiggins Uplift

The Wiggins Uplift is an elevated block of pre-Mesozoic crystalline rocks bordered by Mesozoic faults which has been penetrated by several wells drilled in southwestern Alabama and southeastern Mississippi (e.g., Cagle and Khan, 1983). Predominant rock types include phyllite, chlorite schist, metasandstone, amphibolite, gneiss, and variably metamorphosed

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and deformed granite. K-Ar whole-rock ages ranging between ~ 300 and 275 Ma have been reported for various lithologies within the complex (Cagle and Khan, 1983). 4°ArffgAr incremental-release ages have been determined for several units within the Wiggins Uplift (Dall- meyer, 1988c). These include:

( 1 ) a sample of deformed granite from a well in Jackson County, Mississippi (well 11, Fig. 2 ) which records a markedly discordant whole- rock age spectrum corresponding to a total-gas date of ~ 138 Ma;

(2) phyllite from a well in Mobile County, Alabama (well 12, Fig. 2) which records a whole-rock plateau age of ~ 318 Ma; and

(3) plateau ages of 310 and 305 Ma for hornblende and biotite concentrates from interlayered amphibolite and felsic gneiss within core recovered from a well in Jackson County, Mis- sissippi (well 13, Fig. 2).

Southwestern Alabama igneous complex

Several wells have penetrated various igneous rocks (including granite, basalt, and volcanic agglomerate) along the trace of the Brunswick-Altamaha Magnetic Anomaly in southwestern Alabama (Neathery and Thomas, 1975; Thomas et al., 1988). K-Ar whole-rock ages of ~335 and 267 Ma were reported by Neathery and Thomas (1975) for granite and basalt within the complex. Thomas et al. (1988) reported that massive serpentinite was penetrated in a well along the southern gradient of the anomaly.

Regional tectonic relations

Available basement penetrations allow de- marcation of the boundary between Appala- chian sequences and the Suwannee Terrane shown in Fig. 2 (Chowns and Williams, 1983; Thomas et al., 1988). This approximately co- incides with the trace of the Altamaha-Bruns- wick Magnetic Anomaly in Alabama, and Nel- son et al. (1985a,b) suggested that the anomaly

everywhere marks a suture between Appala- chian elements and the Suwannee Terrane. However, traced eastward across Georgia the anomaly and subsurface terrane boundary di- verge (Fig. 2). On the basis of these relationships, Chowns and Williams (1983) suggested that although the anomaly may mark the deep crustal expression of the suture, it is likely that shallower crustal levels have been thrust north- ward carrying subcrop expression of the boundary over the deeper crustal interface.

The southern boundary of the Suwannee Terrane in peninsular Florida is defined by a major fault (the Jay Fault; Smith, 1983 ) which is probably a projection of the Bahamas fracture zone (Klitgord et al., 1983 ). This may con- nect northwestward with the Pickens-Gilber- ton Fault System (Smith, 1983) which can be traced into the midcontinent. A Mesozoic volcanic sequence occurs south and west of the Jay Fault. This succession probably developed in response to opening of the present Atlantic Ocean (Mueller and Porch, 1983), but developed on older continental crust (Ross et al., 1986). Several fault-bounded blocks of crystalline basement with characteristics similar to that of the Suwannee Terrane appear to occur in southern Florida (Thomas et al., 1988). On the basis of geophysical characteristics, Klit- gord et al. (1984) have suggested that several tracts of fault-bounded continental crust also occur in the Gulf of Mexico west of Florida. In addition, continental crust with Pan-African age affinities was penetrated in two DSDP holes drilled in the Gulf of Mexico northeast of Yu- catan (Dallmeyer, 1984).

An extensive series of NE-SW trending Me- sozoic grabens is developed along the boundary between the Suwannee Terrane and the various Appalachian elements. Higgins and Zietz (1983) suggested that these developed in response to initial phases of rifting of the present Atlantic Ocean. In northwestern Florida and southeastern Alabama a NW-SE trending series of Mesozoic faults intersects the grabens, producing a complex series of smaller horst and

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graben structures (Smith, 1983). The Wiggins Uplift appears to be localized within one of these horsts. The relationship of the Wiggins Uplift crystalline basement to that of the Suwannee Terrane is uncertain; however, it has clearly been extensively overprinted by late Paleozoic ductile strain and metamorphism. The relationship of the southwestern Alabama igneous suite to either the Suwannee Terrane or the basement of the Wiggins Uplift is uncertain.

Pre-Mesozoic basement in the southeastern Gulf o f Mexico

Drilling at Holes 537 and 538A of the Deep Sea Drilling Project Leg 77 (Fig. 3 ) penetrated metamorphic rocks beneath abbreviated Me- sozoic-Cenozoic sedimentary sequences cap- ping relatively high-standing fault blocks in the southeastern Gulf of Mexico (Dallmeyer, 1984; Buffler and Schlager, 1984; Schlager et al., 1984). At Hole 538A, located on Catoche Knoll,

a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite has been intruded by post- tectonic diabase dikes. Hornblende from the amphibolite displays internally discordant 4°Ar/39Ar age spectra (Dallmeyer, 1984) sug- gesting initial post-metamorphic cooling at ~ 500 Ma followed by a mild thermal disturbance at ~ 200 Ma. A lower-grade phyllitic metasedimentary sequence was penetrated at Hole 537. Whole-rock phyllite samples display internally discordant 4°Ar/39Ar age spectra with plateau segments clearly documenting an early Paleozoic metamorphism at ~ 500 Ma.

4°Ar/39Ar results from Holes 537 and 538A are mutually consistent and indicate that Me- sozoic-Cenozoic sedimentary sections in the southeasternmost Gulf unconformably overlie a polymetamorphic terrane of variable grade. Initial metamorphism appears to have occurred at ~ 500 Ma, with markedly higher-grade conditions maintained in that portion of the base-

Fig. 3. Location of sites 537 and 538A drilled during Leg 77 of the Deep Sea Drilling Project (from Dallmeyer, 1984): bathymetic contours in meters.

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ment terrane penetrated in Hole 538A. The variations in metamorphic grade between Holes 537 and 538A may indicate that different crustal levels of the same fragmented basement complex were penetrated in the two fault blocks drilled on Leg 77. Such juxtaposition could reflect differential offset along normal faults that were active during initial continental extension preceding late Mesozoic formation of the Gulf. The basement terrane at both Holes 537 and 538A records the effects of a mild geologic re- heating at ~ 200 Ma. This most probably resulted from local elevation of temperatures caused by intrusion of diabase dikes during initial late Mesozoic crustal extension.

Mauritanide, Bassaride, and Rokelide Orogens

Introduction

The Mauritanide, Bassaride, and Rokelide Orogens occur along the western edge of the West African Craton (Fig. 4). Geological re- views of these areas have been provided by Al- len (1967, 1969), Williams (1978), L~corch~ (1980), L6corch~ et al. (1983, 1988), Roussel et al. (1984), and Villeneuve and Dallmeyer (1987).

Geologic setting

Crystalline basement for the various West African orogens is exposed within the Reguibat and L~o Shields and in the Kayes and Kdnidba (Eastern Senegal) inliers (Bessoles, 1977; Wil- liams, 1978). Basement sequences include Lib- erian ( ~ 2700 Ma) high-grade gneisses and associated catazonal granitic plutons together with less metamorphosed volcanic and sedimentary rocks of Eburnian age ( ~ 2000-1800 Ma). Widespread granitic plutonism occurred throughout the basement at ~ 1800 Ma.

An extensive late Proterozoic sedimentary sequence (Supergroup 1 of Ldcorchd et al., 1983) unconformably overlies the basement.

This appears to range in age between ~ 1100 and 700 Ma (Bassot et al., 1963; Clauer, 1976; Clauer et al., 1982), and is dominated by shallow marine rocks in the north and by continental clastic units in the south. North of the Ro- kelide Orogen, a western allochthonous sequence of variably deformed and metamorphosed volcanic and metasedimentary rocks represents a late Proterozoic rift sequence which could, in part, be mutually time correlative (Villeneuve, 1984) with upper portions of Su- pergroup 1. The atlochthonous units include volcanic-volcaniclastic rocks (with tholeiitic and alkaline basalt: Dupont et al., 1984; Remy, 1987), serpentinite and intercalated marine sequences (chert, jasper, and graywacke). This may be divided into two distinct, internally imbricated structural units: a slightly metamorphosed, tholeiitic to alkaline eastern volcanic sequence, and a higher-grade (amphibolite facies) metavolcanic and metasedimentary western sequence. Within the central Mauritanides, the western sequence includes continental supracrustal components and intraplate, rift-related metabasalts (Remy et al., 1988). Rocks within the western sequence are interpreted to have formed, in part, within a miogeoclinal setting along the margin of a continental block which rifted westward from the West African Craton in the late Proterozoic (L~corch~ et al., 1988). The eastern sequence probably developed along the western margin of the West Af- rican Craton during this rifting. Blanc (1986) reported U-Pb zircon crystallization ages of ~ 680 Ma for alkaline and peralkaline felsic intrusions believed to have developed during this rifting event.

Westernmost segments of the West African orogens are dominated by variably retrogressed, mylonitic gneissic rocks. In the Maur- itanides and Bassarides these are host to a generally calc-alkaline, variably deformed and metamorphosed igneous sequence. This includes felsic volcaniclastic units together with associated comagmatic granitic plutons. Rb-Sr whole-rock isochron and a°ArffgAr mineral ages

3 9 7

Mesozoic - Cenozoic Basin

1 ~ AIIochthonous Basement

S i l u r i a n - Devonian Sedimentary IT'EM'MI Rocks Marine / EpJcontinental

Cambrian - Ordovician Sedimen- tary Rocks Continental / Epi- cont inental

M i d d l e Cambrian Lower ~ Ordovician Mo l a s se

Late Proterozoic Cambrian Flysch (basal tHite) Calc - Alkaline Igneous Rocks (c. 650-700 Ma) and Host C o n t i n e n t a l Terrane

M i d d l e - L a t e P r o t e r o z o i c Ri f t r ~ Sequence (c. 700 Ma)

M i d d l e - La te P r o t e r o z o i c FT--"-I C r a t o n i c C o v e r (c. 700-1100 Ma) A u t o c h t h o n o u s Basement West

I + I African Craton (1800-2500 Ma)

Late Paleozoic

..=.-.+- Late Proterozoic - Cambrian -

( ~ L o c a t i o n s of the schematic cross sec t ion

w A~AR

AkiouJt ~2 IJIBITEN ~2 tp~ E ~ . . . . . . . - > -" _ ' : ~

1 (L~orch~. 1980) lO__.Jm

w Cangarafa TAGANT E

I

(Ola. t979 ) +o--Jim

w E M'eout y ~p, y, ASSABA

, :~.__~.~-- ~,~-~- ; - - - -

~ + + + + + 1 rLe Page. 1983) lo.~J=_

wsw Tiagniaf f Bakel Gabou ~ ENE [ ~'"l /I ~" T.,

® +

(Oia e ta / . rgao / to-do._

NW *'' £P .~/0 e " IPt KI~Ou+ OS~

® ,ojo ( Vdleneuve, 1982 !

sw Gaoual ~ Mali NE

I ( Vtlleneuve 1980 ) l°'~Jm

sw ~°l NE

® + + +

[ VJl/eneuve r98f ] lO4m

WSW (pl ENE -, , 'S- ,J

( Wlll/$m$ and Cu l+ r , r982)

sw NE , Gibi Mountains

(9 . . . . . 4 -f 5/- ,;ho2o , ; ,+ , m

Fig. 4. Tectonostratigraphic sett ing of the West African orogens (adapted from Villeneuve and Dallmeyer, 1987}: M = Mauritanides, B = Bassarides, R = Rokelides) .

suggest that the igneous sequence developed at ~685-675 Ma (Bassot and Caen-Vachette, 1983; Dallmeyer and Villeneuve, 1987).

An epicontinental to marine clastic sequence (Supergroup 2 of L~corch~ et al., 1983 ) unconformably overlies Supergroup 1 and various, pre-deformed and metamorphosed western structural units of the West African orogens (Fig. 4). Basal portions of this sequence are marked by a distinctive upper Proterozoic tillite which is often associated with baritic car- bonate, chert, and stromatolitic dolostone. Cul- ver et al. (1988) described Early Cambrian

microfossils (Aldanella) from conformably overlying flyschoid shales in northern Guinea. Structural units in external portions of the West African orogens contain deformed and slightly metamorphosed sequences which appear to be correlative with lower portions of Supergroup 2 successions in the foreland. These are locally thrust eastward over deformed but nonmeta- morphosed foreland rocks. Traced westward the external sequences become progressively imbricated with more internal tectonic units.

Upper portions of Supergroup 2 are most extensively developed in the Mauritanide

398

foreland where they are represented by cross- bedded, feldspathic red sandstones which locally unconformably overlie lower portions of Supergroup 2. The red sandstones are conformably overlain by a sequence of cleanly washed, scolithic white sandstones. Basal units contain inarticulate brachiopod faunas which span the Cambrian-Ordovician boundary (Legrand, 1969). In the Bassaride and Rokelide Orogens, coarse, molassic red sandstones unconformably overlie various deformed and metamorphosed structural units. Here the scolithic white sandstones are absent.

Sequences related to a widespread Upper Or- dovician glaciation (Deynoux, 1978) mark the base of a third foreland succession in the Maur- itanide foreland (Supergroup 3 of L~corch~ et al., 1983 ). The basal erosional disconformity is locally complicated by an angular unconformity of uncertain regional significance (Dia et al., 1969). In the Bassaride and northern Rokelide Orogens, possibly related, cross-bedded quartz- itic sandstones occur in basal units of the Bov~ Basin (Fig. 1). These unconformably overlie molassic red sandstones, pre-defbrmed late Proterozoic-Early Cambrian flyschoid shales, or pre-deformed and metamorphosed structural units within central parts of the orogens. In Mauritania, basal sequences of Supergroup 3 are conformably overlain by Lower Devonian sandstone and shale, Middle Devonian lime- stone and Late Devonian siltstone and shale which range up into the Frasnian. In the Bov~ Basin, the basal formations are conformably overlain by Silurian shales which may be traced continuously upward into Devonian sandstones and shales which range into the Fam- menian. No Carboniferous sequences have been reported from the West African orogens.

Metasedimentary rocks within several allochthonous units exposed along internal portions of the West African orogens have uncertain foreland correlations. These include a sequence of very low-grade, deformed sandstones which contain Early Devonian faunas and structurally overlie pre-deformed, high-

grade units in the area west of Moudjeria (Fig. 4). The low-grade metasedimentary sequence may be correlated with either upper portions of Supergroup 2 or with Supergroup 3. Similar uncertain correlations apply to an extensive, mylonitic quartzite nappe exposed near Akjoujt in northwestern Mauritania (Fig. 4).

In northwestern Mauritania, the mylonitic quartzite nappe and previously imbricated external and axial formations are structurally overlain by an internally imbricated allochthonous complex (the internal nappes of L~corch~ (1980) and L~corch6 et al. (1983)). Lower structural levels of the nappe complex are represented by alternating structural units comprised of low-grade metapelitic rocks, mafic volcanic sequences, and Fe quartzites. The oc- currence of K-poor, undifferentiated tholeiitic basalts within the metavolcanic intervals suggests an island arc or back arc origin (Ba Gatta, 1982; Kessler, 1986). Upper structural levels of the internal nappe complex are represented by variably mylonitic and thoroughly retrogressed gneissic rocks with structurally interlayered horizons of amphibolite (Giraudon and Sougy, 1963; Marcelin, 1975). An internally imbricated nappe complex occurs as an extensive klippe northwest of the Reguibat Shield (Sougy, 1962; Bronner et al., 1983). A variety of variably metamorphosed and deformed rocks are included within the nappe complex and it may, in part, be structurally correlative with the Akjoujt supracrustal nappes.

Tectonothermal history

Field relationships and limited geochronological controls suggest that the effects of three distinct tectonothermal events are locally recorded in the various West African orogens. In the Bassarides, volcanic and associated sedimentary units of the late Proterozoic rift sequence were deformed and variably metamorphosed (greenschist to amphibolite facies) prior to deposition of the unconformably overlying late Proterozoic tillite (Villeneuve, 1984).

399

Dallmeyer and Villeneuve (1987) reported 660- 650 Ma 4°Ar/39Ar plateau ages for muscovite within penetratively cleaved metasedimentary rocks of the rift sequence exposed in southern Senegal, and interpreted these to date a Pan- African I phase of tectonothermal activity. Similar 650-620 Ma mineral ages (K-Ar and Rb-Sr) were presented by Lille (1967) for variably mylonitic, calc-alkaline metagranites within internal allochthonous units in the southern Mauritanides north of Bakel (Fig. 4 ). The available geochronological controls and the widespread sedimentological expression of tectonic instability in the West African Craton at ~650 Ma (Bronner et al., 1980) suggest that Pan-African I orogenesis was of regional significance.

In the Bassarides of southern Senegal the late Proterozoic-Early Cambrian Mali Group was locally folded prior to deposition of unconformably overlying molassic sequences (Villeneuve, 1984). This tectonothermal event appears to have increased in intensity westward where calc-alkaline igneous rocks within internal allochthonous formations record metamorphic assemblages up to greenschist facies (Ville- neuve, 1984). Dallmeyer and Villeneuve (1987) reported ~ 550 Ma 4°Ar/39Ar plateau ages for muscovite within penetratively cleaved felsic metavolcanic rocks from this area, and suggested that these date a Pan-African II phase of tectonothermal activity. The effects of Pan- African II tectonothermal activity are penetratively recorded throughout the Rokelide Oro- gen. In Sierra Leone components of a western gneissic succession are imbricated with both cover sequences (Rokel River Group which is, in part, equivalent to the Mali Group; Ville- neuve, 1984) and penetratively mylonitic and thoroughly retrogressed structural units of West African Shield basement (Allen, 1967, 1969; Williams, 1978). Cover sequences are absent in Liberia where the Rokelides are characterized by imbrication of the western gneissic sequence and West African Shield basement (Thorman, 1976). In northern portions of the Rokelide Or-

ogen (southern Guinea and northern Sierra Leone), hornblende records K-Ar and 4°Ar/ 39Ar plateau ages ranging between ~ 580 and 550 Ma (Allen et al., 1967; Beckinsale et al., 1981; Dallmeyer, 1988b). The hornblende ages are interpreted to date post-metamorphic cooling through the temperatures required for intracrystalline retention of Ar. Post-metamorphic cooling following Pan-African II tectonothermal activity appears to have been younger in the southern Rokelides where hornblende yields K-Ar dates between ~ 530 and 485 Ma (Hurley et al., 1971; Hedge et al., 1975). This probably reflects an increased diachron- ism between at tainment of maximum Pan-Af- rican II thermal conditions and subsequent cooling through hornblende argon retention temperatures. This is interpreted to reflect exposure of increasingly deeper structural levels southward in the Rokelide Orogen (consistent with preservation of cover sequences only in northern areas). Pan-African I orogenesis has not been documented within the Rokelide Or- ogen. It is uncertain whether this reflects an initial absence of a Pan-African I record, or its complete removal during the penetrative Pan- African II tectonothermal activity.

Late Paleozoic folding and thrusting affect foreland sequences (which range into the Late Devonian) along the length of the Mauritanide Orogen. Limited geochronological data suggest that tectonothermal activity of similar age is recorded within more internal portions of the orogen. Ldcorchd and Clauer (1983) reported ~ 300 Ma K-Ar ages for < 2/zm illite size fractions within samples collected adjacent to a major thrust near Akjoujt (Fig. 4). In the same area, Dallmeyer and Ldcorchd (1988b) reported 310-300 Ma 4°Ar/39Ar plateau ages for dynamically recrystallized muscovite within mylonitic metagranite from the structurally highest allochthonous units. In the central Mauritanides, Dallmeyer and Ldcorchd (1988a) reported 4°Ar/39Ar age spectra for muscovite separated from samples from the GaSua quartzite nappe. These become increasingly

400

more discordant structurally downward in the nappe. Dallmeyer and L~corch~ (1988a) interpreted these variations to reflect the increasing effects of an ~ 310-300 Ma thermal overprint (associated with nappe emplacement) on intracrystalline muscovite Ar systems which had earlier cooled below closure temperatures at ~ 590 Ma. Complete rejuvenation of muscovite Ar systems at ~ 300 Ma was effected in westernmost structural units of cleaved and variably metamorphosed calc-alkaline metavolcanic sequences west of Moudjeria (Fig. 4: Dallmeyer and L~corch~, 1988a). Together, the K-Ar and 4°Ar/39Ar results clearly indicate that at least components of late Paleozoic thrusting and metamorphism affected internal portions of the Mauritanide orogen. Deformation of similar age is probably observed throughout the northern Bassarides and probably extends south at least to the Bov~ Basin where Late Ordovician-De- vonian sedimentary successions are folded with local development of a fracture cleavage (Vil- leneuve, 1984). Dallmeyer and Villeneuve (1987) reported 4°Ar/39Ar plateau ages of ~ 280-270 Ma for dynamically recrystallized muscovite within fluxion fabrics developed in penetratively mylonitized calc-alkaline granites in northeastern Senegal west of Kedougou (Fig. 4 ). These suggest that significant late Pa- leozoic tectonothermal activity is at least locally recorded in northwesternmost portions of the Bassarides. These effects do not extend into central Guinea (Villeneuve and Dallmeyer, 1987), and there is no record of late Paleozoic tectonothermal activity in the Rokelide Orogen.

Geophysical control on tectonic models

The Mauritanide Orogen is bordered on the east by the West African Craton and on the west by a thick section of Mesozoic and younger sedimentary sequences within the Senegal-Maur- itania coastal basin (Fig. 4 ). The regional Bou- guer gravity anomaly pattern suggests that the

orogen extends westward beneath the basin (L~corch6 et al., 1983; Roussel et al., 1984). Three distinct anomaly domains have been outlined:

(1) a prominent, nearly continuous, NNW- SSE trending belt of positive anomalies (Mauritanian anomaly) which, although par- allel to the axis of exposed portions of the orogen, are displaced slightly westward;

(2) a broad regional negative anomaly east of the Mauritanian anomaly which is characterized by NE-SW gravity trends; and

(3) a generally positive anomaly west of the Mauritanian anomaly. L~corchd et al. (1983) and Roussel et al. (1984) interpreted the regional gravity pattern east of the Mauritanian anomaly to reflect largely the signature of Pre- cambrian basement beneath the West African Craton. This distinctive signature may be traced westward beneath exposed portions of the orogen, clearly reflecting its allochthonous nature. The strong positive Mauritanian anomaly has been interpreted as an asymmetric mantle ridge (oversteepened eastward) which has a crest at a depth of -~ 15 km. East of 15°30'W the gravity pattern over the Mesozoic basin is very similar to that defined by Precambrian crystalline rocks beneath the West African Craton. This led Ponsard et al. (1982), L~corch6 et al. (1983), Roussel et al. (1984), and Ponsard (1984) to propose that a coastal crustal block similar in character to the West African Craton exists in the subsurface west of the Mauritan- ian anomaly. The two crustal blocks are there- fore separated by the nearly continuous positive Mauritanian anomaly which probably marks a remnant, west-dipping Pan-African suture zone. Intracontinental rift sequences occur east of the suture throughout the Mauritan- ide and Bassaride Orogens. Calc-alkaline igneous suites which occur outboard (west) of the rift sequences, remain east of the suture in the Mauritanides. They cross it north of the Bas- sarides and extend westward along the Roke- lide Orogen.

Geodynamic models

Mauritanide and Bassaride Orogens Field relationships, regional variations in ra-

diometric ages, igneous geochemical affinities, and geophysical characteristics suggest a polyphase tectonothermal evolution for the Mauri- tanide and Bassaride orogens. The following tectonic development is proposed (Fig. 5):

(1) Middle-late Proterozoic ( ~ 1100-700 Ma: Fig. 5A): Deposition of continental sedimentary sequences on Liberian-Eburnean crystalline basement of the West African Shield.

(2) Late Proterozoic (~700-680 Ma: Fig. 5B): Development of an intracontinental rift

700-680(~) Ma ~ ~

680-660(~ Ma - ~

660-650 Ma *""~'+ + + +~1~'+ + + * + @ ~ + - - ' - 650-575 Ma

®

575-550 Ma ®

550-300 Ma + + + (~) ÷ + + + + •

+ + . . . . . ... ,. 300-275 Ma ~ ~'-

(~) + + ~ +

Fig. 5. Proposed late Paleozoic-early Paleozoic tectonothermal evolution of the southern Mauritanide, Bassaride, and northern Rokelide orogens (adapted from I_~corch$ et al., 1988). Refer to text for discussion.

401

system with emplacement of alkaline through tholeiitic basaltic suites (e.g., Remy, 1987). Blanc (1986) reported a 680 + 10 Ma, U-Pb zircon crystallization age for a syenitic phase of the igneous rift sequence.

(3) Late Proterozoic (~680-660 Ma: Fig. 5C): Compression and imbrication of continental crust. This led to partial melting at deeper crustal levels and formation of a western, ensialic volcanic arc. Bassot and Caen- Vachette (1983) reported a Rb-Sr whole-rock isochron crystallization age of 683 _+ 17 Ma for a calc-alkaline granite of this suite.

(4) Late Proterozoic (~660-650 Ma: Fig. 5D): Continental collision resulted in metamorphism and deformation (Pan-African I orogenesis) in the Bassarides and central Mauritanides.

(5) Late Proterozoic-Early Cambrian (~ 650-575 Ma: Fig. 5E): Deposition of flysch sequences with basal tillite following post-collisional extension.

(6) Cambrian ( ~575-550 Ma: Fig. 5F): De- formation and metamorphism in the Bassar- ides and central Mauritanides (Pan-African II orogenesis) with an apparent southward in- crease in intensity. This may record distal effects of a continent-continent collision within the Rokelide Orogen. Associated uplift and ero- sion leads to deposition of molasse (Youkoun- koun-Taban Groups).

(7) Cambrian-Late Devonian ( ~ 550-360 Ma: Fig. 5G): Deposition of sedimentary successions in the Taoudeni and Bov~ Basins.

(8) Late Carboniferous-Early Permian (~300-275 Ma: Fig. 5H): Collision of Gond- wana and Laurentia results in an eastward relative movement of the western continental block. This produces: (1) variable metamorphism and internal imbrication of pre-deformed western sequences (internal nappes of L~corch~ et al., 1983 ); (2) ductile imbrication of late Proterozoic rift sequences; (3) folding and thrusting of intracontinental foreland sequences (foreland fold and thrust belt and, in part, external nappes of L~corch~ et al., 1983).

402

This activity was concentrated along the eastern margin of the western continental block, extending from northeastern Senegal to the Reguibat promontory.

Rokelide Orogen A penetrative record of Pan-African II oro-

genesis occurs throughout the Rokelide Oro- gen. There are no surviving traces of potential Pan-African I metamorphism and/or deformation, and there is no evidence of late Paleo- zoic orogenesis. The intense Pan-African II activity is interpreted to reflect continent- continent collision during assembly of northwestern Gondwana, and involving relative mo- tion between the West African and Guiana Cra- tons. Very deep erosional levels are now exposed in the Rokelide Orogen, and, as a result, most supracrustal manifestations of oceanic closure have been removed. A suture zone is probably represented by the ductile shear zones which now separate the Kasila Group (variably retrogressed and penetratively mylonitic western gneiss terrane) and the Kenema Assemblage- Marampa Group (variably retrogressed and mylonitic structural units of West African Cra- ton basement; e.g., Williams, 1978).

Appalachian-West African correlations

The crystalline basement rocks penetrated beneath the Atlantic and Gulf Coastal Plains were initially correlated with successions in either the Valley and Ridge or Piedmont provinces of the Appalachians (e.g., Campbell, 1939; Milton and Hurst, 1965). However, on the basis of the Gondwanan paleontological affinities of the Paleozoic sedimentary succession, correlations with West African sequences have been suggested by most recent workers (e.g., Wilson, 1966; Rodgers, 1970). Recent collaborative field and geochronologic studies in the Mauritanide, Bassaride, and Rokelide Orogens of West Africa have helped resolve the tectonothermal evolution of these areas, thereby per- mitting direct correlation with counterparts

comprising the Suwannee Terrane beneath the Coastal Plain of the southeastern United States. These include (Fig. 6):

(1) Correlation of the subsurface Osceola Granite and the post-tectonic Coya Granite exposed in the northern Rokelide Orogen (Guinea). Both record ~530 Ma crystallization ages (Dallmeyer et al., 1987) and display similar petrographic characteristics. Dallmeyer et al. proposed that the two plutons were initially part of a sequence of post-kinematic plutons emplaced along the northwestern margin of Gondwana following an ~550 Ma Pan- African II tectonothermal event.

(2) Correlation of the subsurface Paleozoic sequence in the North Florida Basin with sequences of similar age in the Bov~ Basin (Sen- egal and Guinea; Chowns and Williams, 1983; Villeneuve, 1984 ). This is suggested by similar- ities in fauna and stratigraphic successions. In addition, the ~ 505 Ma 4°Ar/39Ar plateau age recorded by detrital muscovite within Ordovi-

e O ~ / ~ " / / / / C ~ ~

O0~" ~x 1 /

t ~ ~

- Meso C e n o Z o i c + q *, ~ '

Fig. 6. Schematic reconstruction of the northwestern margin of Gondwana illustrating the relationship of pre-Cre- taceous lithotectonic units in the subsurface of the southeastern United States to correlative sequences in West Africa and northeastern South America (from Dallmeyer et al., 1987 ).

cian sandstone in the Florida subsurface suggests a metamorphic source similar in age to that presently exposed within the Bassaride and Rokelide Orogens. The ~ 1800-1650 Ma U-Pb ages reported by Opdyke et al. (1987) for detrital zircons within Ordovician-Silurian sandstone in the Florida subsurface suggest deriva- tion from a source similar in age to the basement of the West African Craton (e.g., Bessoles, 1977). Paleomagnetic results from the sandstone core suggest a paleolatitude of ~49 ° which is in marked contrast to the ~ 28 ° paleolatitude suggested for Laurentia in the Ordov- ician-Silurian, and clearly supports a Gond- wana linkage.

(3) Correlation of the subsurface St. Lucie metamorphic complex and portions of the Ro- kelide Orogen (Chowns and Williams, 1983). The effects of a penetrative ~ 550 Ma Pan-Af- rican II tectonothermal event are recorded throughout the Rokelide Orogen. Here components of a western exotic gneissic succession are imbricated with cover sequences and penetratively mylonitic and retrogressed structural units of West African Shield basement (Allen, 1967, 1969; Thorman, 1976; Williams, 1978). In northern parts of the orogen hornblende records K-Ar and 4°Ar/39Ar post-metamorphic cooling ages ranging between ~ 580 and 550 Ma (Allen et al., 1967; Beckinsale et al., 1981; Dall- meyer, 1988b). Post-metamorphic cooling appears to have been younger in the southern Ro- kelides where hornblende records K-Ar dates between ~ 530 and 485 Ma (Hurley et al., 1971; Hedge et al., 1975). The ~515-510 Ma 4°Ar/ 39Ar plateau ages recorded by hornblende within the St. Lucie metamorphic complex clearly support a linkage with central portions of the Rokelide Orogen.

(4) Correlation of the subsurface felsic igneous complex with a calc-alkaline, variably deformed and metamorphosed igneous sequence that occurs along western portions of the Mauritanide, Bassaride, and northernmost Ro- kelide Orogens (Dallmeyer and Villeneuve, 1987; Dallmeyer et al., 1987). This sequence in-

403

cludes felsic volcaniclastic units together with associated, hypabyssal subvolcanic plutons. Radiometric ages suggest that the calc-alkaline igneous sequence developed between ~ 700 and 650 Ma (Lillie, 1967; Bassot and Caen-Vach- ette, 1983; Dallmeyer and Villeneuve, 1987).

Terrane accretion in the southern Appalachian Orogen

Geophysical, lithologic and/or faunal characteristics suggest that all southern Appala- chian lithotectonic units east of the Hayesville thrust fault (Fig. 1) are allochthonous, non- Laurentian terranes structurally overlying autochthonous or parautochthonous North American successions (e.g., Cook et al., 1979;

Fig. 7. Schematic reconstruction illustrating the proposed tectonic role of the West African coastal structural block during late Paleozoic amalgamation of Pangea: EAT = exotic Appalachian terranes which had earlier (Ordovician-De- vonian) accreted to Laurentia and were thrust into their present structural positions on the North American margin during Late Carboniferous collision of Gondwana and Lau- rentia; SB=Suwannee Basin (Florida subsurface); BB =Bov~ Basin; BFZ=Bahamas Fracture Zone (Meso- zoic feature).

404

Harris et al., 1981; Secor et al., 1983, 1986b; Horton and Drake, 1986 ). Paleomagnetic characteristics of plutons intruding the Inner Pied- mont and the Kings Mountain, Charlotte and Carolina Slate belts are not consistent with significant post-Devonian latitudinal movement. This requires that the various contrasting terranes were at least proximal to North America before the Carboniferous (e.g., Ellwood, 1982; Dooley, 1983; Barton and Brown, 1983). Sedi- mentological expression of outboard (eastern) tectonic instability is clearly documented in Silurian and Devonian successions deposited within the Laurentian miogeocline (e.g., Tull, 1982), and it is likely that at least some of the metamorphism and ductile thrusting recorded within the crystalline southern Appalachians is a result of initial terrane accretion to North America {e.g., metamorphism and transport of the Alto allochthon; pre-Carboniferous metamorphism and folding of the Carolina Slate and Charlotte belts). Geochronological results within these southern Appalachian belts sug-

gest that they are comprised of contrasting tectonic elements which accreted to Laurentia at different times between the Ordovician and the Devonian.

Westward transport of previously accreted terranes into their present structural positions on the North American margin occurred during the Alleghanian orogeny (Fig. 7) which resulted from the collision of Laurentia and Gondwana (e.g., Dallmeyer, 1986a,b, 1988a; Secor et al., 1986b). Initial phases of Allegh- anian tectonothermal activity occurred between ~ 315 and 295 Ma, and involved folding, metamorphism, and emplacement of felsic plutons at middle crustal levels. The second episode of Alleghanian activity was associated with crustal uplift and resultant rapid post-metamorphic cooling between ~ 295 and 285 Ma. This was accompanied by westward-vergent folding as crystalline nappes moved over ramps during thrust transport. Regional post-metamorphic cooling appears to have occurred slightly earlier (between ~ 335 and 305 Ma) in

Fig. 8. Continental reconsti~ction prior to Mesozoic rifting and opening of the Gulf of Mexico and the Atlantic Ocean (from Ross and Scotese, 1988). Major tectonic elements include: FSB = Florida Straits Block; CB =Cuba Block; YB = Yucatan Block; CB = Chortis Block; MSM = Mojave-Sonora Megashear; TMVB = Trans-Mexican Volcanic Belt. Continental crust attenuated during Mesozoic opening of the Gulf of Mexico has been approximately restored to original dimensions (RC).

the e a s t e r n Blue Ridge a n d wes t e rn P i e d m o n t .

T h e s e sequences were p r o b a b l y m a i n t a i n e d a t

e leva ted t e m p e r a t u r e s fol lowing a L a t e Devon - ian or ear l ier m e t a m o r p h i s m which p r o b a b l y a c c o m p a n i e d the i r ini t ia l acc re t ion to L a u r e n -

tia. F ina l cool ing is i n t e r p r e t e d to have occur red dur ing t r a n s p o r t to h igher c rus ta l levels as t hey were t h r u s t on to the N o r t h A m e r i c a n marg in . T h e f inal p h a s e of A l l eghan ian defor- m a t i o n resul ted in d e v e l o p m e n t of dext ra l shear zones in the ea s t e rn P i e d m o n t b e t w e e n ~ 290

a n d 268 Ma. T h i s s t r a in has been i n t e r p r e t e d to have deve loped as a resu l t of re la t ive r o t a t i o n be tween G o n d w a n a a n d L a u r e n t i a dur ing f inal s tages of P a n g e a a m a l g a m a t i o n (Secor et al., 1986b). E a s t w a r d t r a n s p o r t of the Wes t Afri-

can coas ta l b lock also occur red a t th i s t ime, a n d was a c c o m p a n i e d by d e v e l o p m e n t of duct i le s t r a in zones a long its bo rde r s ( D a l l m e y e r a n d

Vil leneuve, 1987). R e c e n t c o n t i n e n t a l recon- s t ruc t ions (e.g., Ross e t al., 1986; Rowley et al., 1986; Ross and Scotese, 1988) suggest t h a t f inal a m a l g a m a t i o n of L a u r e n t i a a n d G o n d w a n a resu l ted in a P a n g e a conf igu ra t ion s imi la r to t h a t p o r t r a y e d in Fig. 8. F r a g m e n t s of G o n d w a n a c o n t i n e n t a l c rus t were s t r a n d e d dur ing Meso- zoic open ing of the Gu l f o f Mexico a n d the At- lant ic Ocean ( b a s e m e n t in the s o u t h e a s t e r n G u l f a n d b e n e a t h the At lan t i c a n d Gu l f Coas ta l P l a in s ) .

Acknowledgments

Various p h a s e s of the work s u m m a r i z e d here were s u p p o r t e d by g r an t s f r o m the U.S. Na- t iona l Science F o u n d a t i o n (EAR-8020469; EAR-8514013) a n d the P e t r o l e u m R e s e a r c h F o u n d a t i o n of the A m e r i c a n Chemica l Socie ty ( P R F 13920-AC2).

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