Suwannee Terrane Research Articles

Crustal and Upper Mantle Structure Beneath the Southeastern United States From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion

AbstractUsing data from 186 stations belonging to the USArray Transportable Array, a three‐dimensional shear wave velocity model for the southeastern United States is constructed for the top 180 km by a joint inversion of receiver functions and Rayleigh wave phase velocity dispersion computed from ambient noise and teleseismic earthquake data. The resulting shear wave velocity model and the crustal thickness and Vp/Vs () measurements show a clear spatial correspondence with major surficial geological features. The distinct low velocities observed in the depth range of 0–25 km beneath the eastern Gulf Coastal Plain reflect the thick layer of unconsolidated or poorly consolidated sediments atop the crystalline crust. The low (1.70–1.74) and slow lowermost crustal velocities observed beneath the eastern Southern Appalachian Mountains (including the Carolina Terrane and Inner Piedmont) relative to the adjacent Blue Ridge Mountains and Valley and Ridge can be interpreted by lower crustal delamination followed by relamination. The Osceola intrusive complex in the central Suwannee Terrane has similar crustal characteristics as the eastern Southern Appalachian Mountains and thus can similarly be attributed to crustal delamination/relamination processes. The Grenville Province and adjacent areas possess relatively high values which can be attributed to mafic intrusion associated with crustal extension in a recently recognized segments of the eastern arm of the Proterozoic Midcontinent Rift.

Evidence of Carboniferous arc magmatism preserved in the Chicxulub impact structure

Abstract Determining the nature and age of the 200-km-wide Chicxulub impact target rock is an essential step in advancing our understanding of the Maya Block basement. Few age constraints exist for the northern Maya Block crust, specifically the basement underlying the 66 Ma, 200 km-wide Chicxulub impact structure. The International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 core recovered a continuous section of basement rocks from the Chicxulub target rocks, which provides a unique opportunity to illuminate the pre-impact tectonic evolution of a terrane key to the development of the Gulf of Mexico. Sparse published ages for the Maya Block point to Mesoproterozoic, Ediacaran, Ordovician to Devonian crust are consistent with plate reconstruction models. In contrast, granitic basement recovered from the Chicxulub peak ring during Expedition 364 yielded new zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) concordant dates clustering around 334 ± 2.3 Ma. Zircon rare earth element (REE) chemistry is consistent with the granitoids having formed in a continental arc setting. Inherited zircon grains fall into three groups: 400–435 Ma, 500–635 Ma, and 940–1400 Ma, which are consistent with the incorporation of Peri-Gondwanan, Pan-African, and Grenvillian crust, respectively. Carboniferous U-Pb ages, trace element compositions, and inherited zircon grains indicate a pre-collisional continental volcanic arc located along the Maya Block's northern margin before NW Gondwana collided with Laurentia. The existence of a continental arc along NW Gondwana suggests southward-directed subduction of Rheic oceanic crust beneath the Maya Block and is similar to evidence for a continental arc along the northern margin of Gondwana that is documented in the Suwannee terrane, Florida, USA, and Coahuila Block of NE México.

Early Mesozoic synrift Eagle Mills Formation and coeval siliciclastic sources, sinks, and sediment routing, northern Gulf of Mexico basin

AbstractThe sedimentary architecture and provenance of the early Mesozoic incipient northern Gulf of Mexico basin remains controversial due to both lack of outcrop exposure and sample scarcity across the southern United States with subcrop depths approaching 6 km. The Eagle Mills Formation and coeval deposition across the northern Gulf of Mexico provides both a stratigraphic foundation for some ∼15-km-thick overlying Mesozoic and Cenozoic deposits, and a coeval pre-salt equivalent for southern synrift deposits, in one of the most economically significant hydrocarbon basins in the world. This study presents more than 3200 new detrital zircon U-Pb analyses from sixteen Late Triassic pre-salt, siliciclastic, subcrop well samples, and combines over 14,000 linear kilometers of 2-D multi-channel seismic reflection data, 1511 geophysical well logs, and biostratigraphic data from 2478 wells to construct basin-scale pre-salt isochore and structure maps spanning the northern Gulf of Mexico margin from Florida to the USA-Mexican border.The data show that incipient Gulf of Mexico paleodrainage pathways held individual distinctions between basement sources and tectonic controls in three primary regions across the northern Gulf of Mexico: (1) The western Gulf of Mexico paleodrainage extended from the Central Texas uplift highlands to the submarine Potosi Fan on the western margin of Laurentia with local tributary sources from the East Mexico Arc, Yucatán/Maya, and Marathon-Ouachita provinces as evidenced by inverse Monte Carlo unmixing of peri-Gondwanan (ca. 700–500 Ma), Appalachian/Ouachita (500–280 Ma), Grenville (1250–950 Ma), and Mid-Continent/Granite-Rhyolite Province (1500–1300 Ma) detrital zircon ages. Isochore and associated geophysical well and seismic data suggest that by Early Jurassic time this depocenter had shifted into the present-day western Gulf of Mexico as East Mexico Arc development continued. (2) Southerly drainage in the north-central Gulf of Mexico region bifurcated around the Sabine and Monroe uplifted terranes with southwestern flow characterized by peri-Gondwanan detrital zircon ages from late Paleozoic accreted basement or discrete flexural successor basins, and southeastern fluvial networks distinguished by traditional North American basement province sources including Grenville, Mid-Continent, and Yavapai-Mazatzal. (3) Eastern Gulf of Mexico regional paleodrainage, with regional southern flow dictated by the brittle extensional tectonics of the South Georgia Rift as well as the regional southern flexure of the South Florida Basin, resulted in almost all pre-salt detrital zircon siliciclastic ages from this region to be dominated by local Gondwanan/peri-Gondwanan aged sources including the proximal Suwannee terrane and Osceola Granite complex. These regional, synrift sediment provenance models provide the first critical allochthonous evidence of Late Triassic–Early Jurassic paleodrainage stemming from the Appalachian-Ouachita hinterlands into the incipient northern Gulf of Mexico basin with critical implications for pre-salt hydrocarbon exploration and carbon sequestration reservoir potential.

Preserved Neoproterozoic Continental Collision in Southeastern North America: The Brunswick Suture Zone and Osceola Continental Margin Arc

AbstractA series of exotic terranes were accreted to the eastern margin of southeastern Laurentia beginning in the Siluro‐Devonian (Acadian event), and to the northeastern Laurentian margin beginning earlier (late Taconian). Many of these terranes have unclear tectonostratigraphic relationships to each other and to their parental cratons, but their accretionary history is critical to understanding the evolution of the Appalachian orogen. Two of these, the Gondwanan Suwannee and Charleston terranes, accreted during the Alleghanian orogeny and now lie beneath the Atlantic Coastal Plain in southeastern North America. Reanalysis of deep seismic reflection and well data reveals a preserved Neoproterozoic continental collision zone and associated continental margin arc, the Osceola arc, related to their juxtaposition. The subduction zone and associated strain are recorded in the newly termed Brunswick suture zone (BSZ). The BSZ is readily identified on a series of eight deep seismic reflection transects across the Brunswick Magnetic Anomaly (BMA), which we interpret as the boundary between the Charleston and Suwannee terranes. While originally interpreted to be the Late Paleozoic Alleghanian suture, new age constraints provided by the overlapping Gondwanan Paleozoic Suwannee Basin strata require the BSZ to predate the Early to Middle Paleozoic passive margin sequence of the Suwannee Basin. These results provide new insights into the tectonostratigraphic evolution of the Charleston and Suwannee terranes, the controversy surrounding the age and origin of the dipping seismic reflectors, previously attributed to the suturing of the Suwannee terrane to Laurentia, and the relationship of this suture zone to the origin of the BMA.

Imaging crustal structure beneath the southern Appalachians with wavefield migration

AbstractTo constrain crustal structures in the southern Appalachians and the suture zone with the Gondwanan‐affinity Suwannee terrane, we applied the 2‐D generalized Radon transform wavefield migration method to the scattered incident P wavefield recorded by the EarthScope Southeastern Suture of the Appalachian Margin Experiment and adjacent Transportable Array stations. We resolve the root of thickened crust beneath the high topography of the Blue Ridge Mountains and estimate its density contrast with the mantle to be only 104 ± 20 kg/m3. A weak velocity contrast across the crustal root Moho is observed and may be related to an ongoing crustal delamination event, possibly contributing to local tectonic rejuvenation. Beneath the Suwannee terrane, we confirm prior observations of a gently south‐southeastward dipping crustal suture, indicating the terminal collision of Laurentia and Gondwana involved several hundred kilometers of overthrusting.

Offshore extent of Gondwanan Paleozoic strata in the southeastern United States: The Suwannee suture zone revisited

The recognition of the Suwannee terrane in southeastern North America was a critical step in advancing our current understanding of the tectonic evolution of ocean basins. Unfortunately, further clarification of the boundaries of the Suwannee terrane has proven difficult due to the thick coastal plain cover, limited well log analyses, and a paucity of seismic reflection data onshore. We present the results from a new compilation and analysis of legacy marine seismic reflection, refraction and well data from offshore the southeastern United States, which reveals in the subsurface a previously unmapped lower Paleozoic sedimentary section spanning the continental shelf from Florida to North Carolina. Previously only identified in two deep offshore wells (COST GE-1 and Transco 1005-1), this Paleozoic platform sequence is identified in seismic reflection profiles as a package of low-frequency, sub-horizontal, laterally-continuous reflectors which are clearly discordant with and distinct from the overlying Mesozoic and Cenozoic stratigraphy above the post-rift unconformity (PRU). The inferred base of the Paleozoic sequence is marked by the downward diminution of parallel seismic reflectors, as well as an increase in seismic velocities to >6km/s observed on numerous offshore seismic refraction surveys. While clearly deformed in some areas, these Paleozoic strata are generally sub-horizontal, range in thickness from 4 to 6km, and can be mapped continuously over an area in excess of 130,000km2. Similar sedimentary rocks have been recognized from onshore exploration wells in Florida since the 1930s, and were subsequently identified to be part of the Suwannee basin within the larger exotic Suwannee terrane of Gondwanan affinity. Recognition of the presence and extent of these Gondwanan strata offshore implies: (1) the inferred position of the Suwannee suture zone offshore lies >200km further north, approximately along the boundary between the Carolina terrane and the Charleston terrane; (2) previously identified terranes (Brunswick, Charleston, Suwannee, Northern Florida) are likely distinct crustal blocks, but their amalgamation predates deposition of the overlying Paleozoic section of the Suwannee basin; (3) collectively, this crust represents the Gondwanan continent based on the size and presence of a stable platform stratigraphy, nominally doubling the size of the last sutured terrane, the Suwannee terrane.

The Suwannee suture: Significance for Gondwana-Laurentia terrane transfer and formation of Pangaea

In southeastern Laurentia the Suwannee suture separates the undeformed part of the Gondwanan Suwannee terrane (SWT) from crust of Laurentia and previously accreted Gondwanan terranes that were deformed during the Alleghanian orogeny. Undeformed early Paleozoic sedimentary rocks of the SWT are considered of Gondwanan-African derivation on the basis of various considerations, including a lack of Mesoproterozoic (Grenville) detrital zircons. The cratonic heritage of Suwannee lithosphere is complicated, however, because Sm–Nd depleted mantle model ages from both Neoproterozoic and Alleghanian (Carboniferous–Permian) granites north and south of the suture zone are Mesoproterozoic. Mesoproterozoic lithosphere has not been reported in conjugate West Africa, but is present in Amazonia, suggesting that the SWT: 1) may be a composite terrane, containing both African upper crust and Amazonian lower crust/lithosphere, 2) was separated from Gondwana during Atlantic Ocean rifting along a Mesoproterozoic–Neoproterozoic suture, and/or 3) contains Laurentian Grenville-age lower crust and lithosphere that must also underlie other accreted proximal Gondwanan terranes (e.g., Uchee terrane). In addition, the ~300Ma, syn- to post-tectonic granitoids emplaced across the Suwannee suture zone are the product of crustal anatexis, not supra-subduction magmatism, on the basis of Mesoproterozoic Sm–Nd Tdm and zircon xenocrysts. The lack of documented ~300Ma, supra-subduction magmatism in both Laurentian and Gondwanan components in southern Laurentia (e.g., southernmost Appalachian and Ouachita orogens) strongly suggests that accretion of the SWT and related Gondwanan terranes occurred during dextral transpressional convergence between Laurentia and a combined African-Amazonian craton, a characteristic shared with conjugate crust in southwestern Europe. In this interpretation, intervening oceanic lithosphere and contemporary volcanic arcs (if any) were largely removed by disaggregation and dispersion along the transpressional boundary possibly combined with highly oblique overthrusting and/or partly subducted at the leading edge(s) of other blocks of Gondwanan lithosphere that were accreted to southern Laurentia in the late Paleozoic (e.g., Wiggins, Sabine, Coahuila, Maya, etc.). Mesozoic opening of the Gulf of Mexico has subsequently and variably altered the original configuration of these terranes.

Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa

Aeromagnetic and gravity data sets obtained for Alabama (United States) have been digitally merged and filtered to enhance upper-crustal anomalies. Beneath the Appalachian Basin in northwestern Alabama, broad deep-crustal anomalies of the continental interior include the Grenville front and New York–Alabama lineament (dextral fault). Toward the east and south, high-angle discordance between the northeast-trending Appalachians and the east-west–trending wedge of overlapping Mesozoic and Cenozoic Gulf Coastal Plain sediments reveals how bedrock geophysical signatures progressively change with deeper burial. High-frequency magnetic anomalies in the Appalachian deformed domain (ADD) correspond to amphibolites and mylonites outlining terranes, while broader, lower-amplitude domains include Paleozoic intrusive bodies and Grenville basement gneiss. Fundamental ADD structures (e.g., the Alexander City, Towaliga, and Goat Rock–Bartletts Ferry faults) can be traced southward beneath the Gulf Coastal Plain to the suture with Gondwanan crust of the Suwannee terrane. Within the ADD, there is clear magnetic distinction between Laurentian crust and the strongly linear, high-frequency magnetic highs of peri-Gondwanan (Carolina-Uchee) arc terranes. The contact (Central Piedmont suture) corresponds to surface exposures of the Bartletts Ferry fault. ADD magnetic and gravity signatures are truncated by the east-west–trending Altamaha magnetic low associated with the Suwannee suture. Arcuate northeast-trending magnetic linears of the Suwannee terrane reflect internal structure and Mesozoic failed-rift trends. Geophysical data can be used to make inferences on surface and subsurface geology and vice versa, which has applicability anywhere that bedrock is exposed or concealed beneath essentially non-magnetic sedimentary cover.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe–reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40 Ar/ 39 Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate “Appalachian” dates (e.g., Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may be related to Carolina zone terranes that accreted during the Alleghanian orogeny.

Mesozoic Igneous Activity in the Suwannee Terrane, Southeastern USA: Petrogenesis and Gondwanan Affinities

Drillhole sampling of the rocks beneath the post-upper JUrdSSiC sedimentary cover of the Southeastern Coastal Plain of the United States has revealed that tholeiitic and alkaline basalts and diabases, in addition to high-K rhyolites and granites, are present in the exotic Suwannee terrane. The most reliable published ages for basaltic rocks range from 183r5 to 199r8 Ma, overlapping the accepted age range for magmatism of the Central Atlantic Magmatic Province (CAMP). Ages for rhyolites range from 16526 to 189's Ma, while the only dated granite has a well-constrained U-Pb zircon age of 159?3 Ma. The geochemistry of all lithologies is consistent with their eruption or emplacement in an extensional tectonic environment. The tholeiitic basalts are derived from continental lithospheric mantle and crust that is approximately 1 billion years old, suggesting an affinity with the South American portions of Gondwana. Alkali basalts found in southwestern Florida may contain a component of sub-lithospheric mantle and have compositions similar to some basalt s from Liberia and Brazil. Associated silicic magmas may have been generated by intracrustal melting induced by the heat of underplated or injected mafic magmas during the waning stages of igneous activity.

Lithospheric sources of North Florida, USA tholeiites and implications for the origin of the Suwannee terrane

Sr, Nd, and Pb isotopic data for Mesozoic tholeiites from six drillholes in northern Florida (Suwannee terrane) indicate derivation of the magmas from a source that was slightly incompatible-element enriched with respect to bulk earth, probably continental lithospheric mantle variably contaminated by more enriched crust. Initial isotopic compositions of the Florida tholeiites show a considerable range: 87 Sr/ 86 Sr =0.7059 to 0.7097, ε Nd=−4.6 to +0.1, 206 Pb/ 204 Pb =18.30 to 18.65, 207 Pb/ 204 Pb =15.57 to 15.66, and 208 Pb/ 204 Pb =38.22 to 38.35. These compositions are distinctly different from those of Mesozoic alkali basalts from the subsurface of southwestern Florida [Heatherington, A.L., Mueller, P.A., 1991. Geochemical evidence for Triassic rifting in southwestern Florida. Tectonophysics 188, 291–302]. It is suggested that the North Florida tholeiites were derived from within the continental lithosphere, in contrast with Mesozoic volcanic rocks from southwestern Florida, which have been previously suggested to be related to a mantle plume. Sm–Nd isotope systematics suggest derivation of the tholeiites from a ∼1-Ga source, a similar age to that proposed by Pegram [Pegram, W.J., 1990. Development of continental lithospheric mantle as reflected in the chemistry of the Mesozoic Appalachian tholeiites, USA Earth and Planetary Science Letters 97, 316–331] for the source of tholeiitic basalts within the Carolina terrane. This supports Proterozoic tectonic models which suggest that both the Suwannee and Carolina terranes had affinities with South American portions of Gondwana containing ∼1 Ga crust, specifically, the Sunsas-Rondonian province [Nance, Murphy, 1996; Mueller, P.A., Kozuch, M., Heatherington, A.L., Wooden, J.L., Offield, T.W., Koeppen, R.P., Klein, T.L., Nutman, A.P., 1996. Evidence for Mesoproterozoic basement in the Carolina terrane and speculations on its origin. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic, GSA Special Paper 304, pp. 207–218].

Blocky calcite cement from the Suwannee Terrane: Evidence for mixing of middle to high latitude meteoric and sea water

Blocky calcite cement from the Suwannee Terrane, an exotic terrane juxtaposed against Gondwanaland during the Ordovician and now located in Georgia, Florida, and Alabama, has δ13C = −7.8 to −16.5‰ PDB and δ18O = −6.8 to −11.6‰ PDB. This cement is interpreted to have formed during sulfate reduction, which is supported by the coeval presence of pyrite and non-ferroan calcite cement. Isotopic values define part of a mixing line between calcite cements precipitated from marine, organic carbon-dominated and meteoric, inorganic-carbon dominated fluids.

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

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.

A Tectonic Linkage between the Rodelide Orogen (Sierra Leone) and the St. Lucie Metamorphic Complex in the Florida Subsurface

The St. Lucie Metamorphic Complex comprises part of the non-Laurentian, Paleozoic Suwannee terrane beneath the Gulf and Atlantic Coastal Plains of the southeastern United States. Hornblende concentrates prepared from cuttings from two deep test wells penetrating the complex display internally concordant $$^{40}Ar/^{39}Ar$$ incremental-release spectra defining plateau ages of $$510.8 \pm 1.1 Ma$$ and $$513.1 \pm 1.8 Ma$$, which are interpreted to date post-metamorphic cooling through temperatures required for intracrystalline argon retention. The Kasila Group constitutes the western segment of the Rokelide orogen in Sierra Leone. Four hornblende concentrates prepared from amphibolite within the Kasila Group yield $$^{40}Ar/^{39}Ar$$Ar plateau ages of $$505.0 \pm 5.2$$, $$508.2 \pm 2.1$$, $$510.5 \pm 2.6$$, and $$546.1 \pm 6.8 Ma$$. They are interpreted to date post-metamorphic cooling through appropriate argon closure temperatures following a ca. 550-560 Ma, Pan-African II phase of tectonothermal activity....

New COCORP profiling in the southeastern United States. Part II: Brunswick and east coast magnetic anomalies, opening of the north-central Atlantic Ocean

New COCORP profiles on the coastal plain of Georgia and northern Florida support the hypothesis that the Brunswick anomaly marks a late Paleozoic suture. They do not support the alternate view that this anomaly is caused by a Mesozoic rift basin. The trend of the Brunswick anomaly relative to the Appalachian gravity gradient indicates that in westernmost Georgia and adjacent Alabama, African basement (Suwannee terrane) is in proximity to autochthonous North American basement (Grenville). Farther east one or more Paleozoic accreted terranes intervene between the North American and African sides of the orogen. Offshore, the Brunswick anomaly closely parallels the east coast magnetic anomaly. This relationship implies that the east coast magnetic anomaly marks not only the present continental/oceanic crustal transition but also the northward continuation of the late Paleozoic suture between North America and Africa. Transitional crust beneath the Carolina Trough, Baltimore Canyon trough, and correlative parts of the African Atlantic margin is thus likely to have formed within a preexisting suture zone. The dramatic change in character of the U.S. Atlantic margin southward from the Carolina Trough to the Blake Plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region.