James F. Tull

Structural evolution of a major Appalachian salient-recess junction: Consequences of oblique collisional convergence across a continental margin transform fault

Most orogenic belts exhibit broadly arcuate regional map-view trends of structural patterns. Commonly, the inflection between opposing curves (recesses and salients) is a transverse zone marking a linear array of along-strike contrasts in regional structure and stratigraphy. Analysis of such transverse zones can provide insight into the processes affecting orogenic curvature. Curvature is especially distinctive along the west flank of the Appalachians, where the tightest salient/recess juncture is a thinned-skinned transverse zone in Georgia, separating the Alabama recess and Tennessee salient, and extending entirely across the width of deformed Laurentian cover rocks. This zones most notable feature is a large oblique hanging-wall ramp within the frontal metamorphic allochthon formed during Alleghanian collisional events. The basal decollement steps several kilometers stratigraphically upward across this ramp from Proterozoic basement and its overlying Late Proterozoic rift sequence (Tennessee salient), southwestward into basal Cambrian and younger rocks (Alabama recess). This ramp resulted from oblique sinistral displacement of the allochthon against an earlier, east-facing, dextral continental margin transform fault that initially separated adjacent rifted margins of opposite polarity. Differences in the distribution, thickness, and facies of units indicate that important variations in basin geometry existed across the curvature inflection before Paleozoic deformation, suggesting that the orogenic curvature in part results from an inherited basin architecture. Translation of the allochthon over the ramp caused deflection of the tectonic transport trajectory, major cross folding, and rotation of earlier structures. Associated out-of-plane deformation propagated outward across the foreland, producing the array of thinned-skinned transverse structures aligned into the transverse zone.

New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians

The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate lithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.

Polyphase late Palaeozoic deformation in the southeastern foreland and northwestern Piedmont of the Alabama Appalachians

Abstract Late Palaeozoic deformation in the southern Appalachians is believed to be related to the collisional events that formed Pangaea. The Appalachian foreland fold and thrust belt in Alabama is a region of thin-skinned deformed Palaeozoic sedimentary rocks ranging in age from Early Cambrian to Late Carboniferous, bounded to the northwest by relatively undeformed rocks of the Appalachian Plateau and to the southeast by crystalline thrust sheets containing metasedimentary and metaigneous rocks ranging in age from late Precambrian to Early Devonian. A late Palaeozoic kinematic sequence derived for a part of this region indicates complex spatial and temporal relationships between folding, thrusting, and tectonic level of decollement. Earliest recognized (Carboniferous(?) or younger) compressional deformation in the foreland, observable within the southernmost thrust sheets in the foreland, is a set of large-scale, tight to isoclinal upright folds which preceded thrafing, and may represent the initial wave of compression in the foreland. Stage 2 involved emplacement of low-angle far-traveled thrust sheets which cut Lower Carboniferous rocks and cut progressively to lower tectonic levels to the southwest, terminating with arrival onto the foreland rocks of a low-grade crystalline nappe. Stage 3 involved redeformation of the stage 2 nappe pile by large-scale upright folds oriented approximately parallel to the former thrusts and believed to be related to ramping or imbrication from a deeper decollement in the foreland rocks below. Stage 4 involved renewed low-angle thrusting within the Piedmont rocks, emplacement of a high-grade metamorphic thrust sheet, and decapitation of stage 3 folds. Stage 5 is represented by large-scale cross-folding at a high angle to previous thrust boundaries and fold phases, and may be related to ramping or imbrication on deep decollements within the now mostly buried Ouachita orogen thrust belt to the southwest. Superposed upon these folds are stage 6 high-angle thrust faults with Appalachian trends representing the youngest (Late Carboniferous or younger, structures in the kinematic sequence.

Southeastern margin of the middle Paleozoic shelf, southwesternmost Appalachians: Regional stability bracketed by Acadian and Alleghanian tectonism

Lower Devonian–Lower Mississippian(?) metasedimentary rocks in the southernmost Appalachian orogen near the southeast corner of Laurentia record a period of continental-margin stabilization bracketed by orogenic events best assigned to the early Acadian and earliest Alleghanian orogenies. These strata are in the Talladega belt, a far-traveled allochthon forming the southwestward extent of the western Blue Ridge. In the allochthon, lower Paleozoic trailing-margin carbonate-shelf rocks are unconformably overlain by a >2.5-km-thick early Acadian (Silurian(?)–Lower Devonian) clastic wedge, derived from basement-cored uplifts to the south and southeast. Deposition of this clastic wedge was followed by continental-margin stabilization and deposition of a starved-basin sequence containing widespread Lower Devonian–Lower Mississippian(?) chert near the outer shelf margin. The chert sequence grades northeastward through a shelf–slope–basin-plain transition into a thick black-shale basinal facies. These units were deposited during the classic Acadian orogeny in New England and the Catskill deltas progradation across the central Appalachian foreland—the major clastic response to Acadian uplift. The Talladega sequence subsequently was subjected to low-grade regional metamorphism and pervasive deformation, possibly as late as middle Mississippian, during what were probably the initial stages of the Alleghanian orogeny. Deposition of the Lower Devonian–Lower Mississippian(?) sequence of the Talladega belt thus represents a significant hiatus in deformation along southeast Laurentia, bracketed first by orogenic activity related to early Acadian (Silurian(?)–Early Devonian) foundering of the early Paleozoic shelf and deposition of a clastic wedge and later by earliest Alleghanian regional dynamothermal events.

Analysis of a regional middle Paleozoic unconformity along the distal southeastern Laurentian margin, southernmost Appalachians: Implications for tectonic evolution

The highly allochthonous Talladega belt in the southernmost Appalachian orogen of Alabama and Georgia represents the most completely preserved outboard fragment of the early Paleozoic Laurentian continental margin in this part of the orogen. Palinspastic restoration indicates that this Alleghanian thrust sheet was originally located at or near this continental margin. The Talladega belt is within a structural recess (Alabama recess) in the orogen, which is interpreted to have originated as a continental promontory during the opening and initial closing of the Iapetus ocean. Because continental promontories protrude oceanward along a continental margin, any significant early deformation of this margin should be recorded within this thrust sheet. Regional geologic mapping has revealed a middle Paleozoic regional unconformity within this belt, and distortion of this unconformity by late Paleozoic regional folding provides for examination of subcrop geometries below the unconformity over a large area (∼640 km 2 ). Retrodeformation of the late Paleozoic folds indicates that only very mild deformation predated the unconformity. The origin of this early deformation is unclear. Possibilities include very mild contractional deformation during the Taconic orogeny, or folding associated with wrench or extensional faulting during Silurian(?)-Devonian Acadian successor-basin formation. If the latter is the case, then no vestiges of Taconic deformation can be found in this outboard fragment of the margin, implying that a major continental promontory at the southwesternmost Laurentian margin may have escaped any significant effects of this orogeny.

Volcanic arc emplacement onto the southernmost Appalachian Laurentian shelf: Characteristics and constraints

In the southernmost Appalachians, the Hillabee Greenstone, an Ordovician volcanic arc fragment, lies directly atop the outermost Laurentian Devonian-earliest Mississip- pian(?) shelf sequence at the structural top of the greenschist facies Talladega belt, the frontal metamorphic allochthon along this orogenic segment. The Hillabee Greenstone was emplaced between latest Devonian and middle Mississippian time. It and the upper- most Laurentian section were later repeated together within a series of map-scale imbri- cate slices of a postmetamorphic, dextral, transpressional, Alleghanian thrust duplex system that placed the high-grade eastern Blue Ridge allochthon atop the Talladega belt. Geochemical and geochronologic (U-Pb zircon) studies indicate that the Hillabee Greenstones interstratifi ed tholeiitic meta- basalt and calc-alkaline metadacite/rhyolite formed within an extensional setting on con- tinental crust ca. 460-470 Ma. Palinspastic reconstructions of the southern Appalachian Ordovician margin place the Hillabee Green- stone outboard of the present position of the Pine Mountain terrane and suggest links to Ordovician plutonism in the overlying east- ern Blue Ridge, and possibly to widespread K-bentonite deposits within Ordovician plat- form units. The tectonic evolution of the Hil- labee Greenstone exhibits many unusual and intriguing features, including: (1) premeta- morphic emplacement along a basal cryptic thrust, which is remarkably concordant to both hanging wall and footwall sequences across its entire extent (>230 km), (2) forma- tion, transport, and emplacement of the arc fragment accompanied by minimal deforma- tion of the Hillabee Greenstone and under- lying outer-margin shelf rocks, (3) emplace- ment temporally coincident with the adjacent collision of the younger, tectonically indepen- dent Ouachita volcanic arc with southeastern Laurentia. These features highlight strong contrasts in the Ordovician-Taconian evolu- tion of the southern and northern parts of the Appalachian orogen.

Nested Paleozoic successor basins in the southern Appalachian Blue Ridge

Field studies in the southern Appalachian Blue Ridge and its southwest extension, the Talladega belt, indicate that in at least three regions, polydeformed and metamorphosed turbidite-dominated sequences unconformably overlie rifted-margin continental-terrace wedge clastic rocks and overlying carbonate-platform deposits. These sequences are (1) the Talladega Group (in the Talladega belt), (2) the Walden Creek Group (along the west flank of the Blue Ridge), and (3) the Mineral Bluff Formation (within the core of the Blue Ridge). Paleontologic evidence indicates that the Talladega and Walden Creek Groups are in part as young as Silurian-Devonian. The presence of these anomalously young sequences unconformably above the trailing-margin stratigraphy in the Blue Ridge brings into question conventional ideas of the timing and nature of the tectonic evolution of the ancient continental margin.

Laurentian magmatism of the southern Appalachian Blue Ridge: Post-Iapetan rifting

Tectonically significant postrift metaigneous rocks occur within three genetically distinct cover-rock sequences along a 400 km strike segment of the southeastern margin of Laurentia, in the southern Appalachian western Blue Ridge. These include (1) Neoproterozoic rift facies rocks, (2) lower Paleozoic drift facies rocks, and (3) unconformably overlying postdrift Paleozoic successor basin sequences. This region is divisible into two along-strike, ensialic, continental margin volcanic belts, separated by a probable Neoproterozoic transfer fault. To the northeast, igneous rocks are intrusive into sequences 1 and 2 above, whereas in the overlying successor basins on both sides of the transfer boundary, they occur predominantly as eruptive rocks. These igneous rocks can be separated into several suites that are distinct from the Neoproterozoic rift-related igneous rocks, based on stratigraphic position, geographic location, and composition. Mafic dikes and sills intrusive into the rift facies and mafic metavolcanic rocks of the Hillabee Greenstone represent low-K tholeiitic magmatism associated with derivation from a “depleted” mantle source, whereas most igneous rocks in the northeastern belt exhibit an alkaline basalt affinity. The successor basin sequences associated with the volcanic activity formed above extended and thinned continental crust, near and parallel to the southeastern Laurentian margin. This igneous activity can be constrained broadly between stratigraphic position and age of metamorphism (Middle Ordovician to earliest Mississippian time). Most likely this magmatic activity was associated with a destructive plate boundary during Paleozoic A-type subduction, but was largely decoupled from slab-derived magmatism, being instead more likely associated with backarc or pull-apart basin evolution.

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: characteristics, extent, and tectonic significance

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically significant Ordovician (ca. 480–460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deep-water (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin’s predominantly metasedimentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal metavolcanic rocks indicate formation in a suprasubduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia’s Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the back-arc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

Appalachian Blue Ridge cover sequence ranges at least into the Ordovician

The first direct evidence that stratified rocks of the central core of the southern Appalachian Blue Ridge range in age into the Paleozoic comes from a pelmatozoan echinoderm column discovered within a unit directly above the Murphy Marble in North Carolina. Before this discovery most geologists had considered all stratified rocks of the Blue Ridge east of the frontal imbricate thrust blocks to be Late Proterozoic or Early Cambrian(?). The echinoderm fragment is in a lower amphibolite facies interbedded mica schist-impure marble zone that lies directly above the Murphy Marble. Rocks above the Murphy Marble are dominantly turbiditic metaclastic rocks with minor carbonate and metavolcanic rocks, interpreted as having formed within a successor basin unconformably above upper Precambrian rift facies and lower Paleozoic drift facies rocks of the Laurentian passive margin. An upper bound for the age of the successor basin in the Murphy belt has not been established; similar sequences in the Talladega belt to the southwest, and possibly the Foothills belt to the west, range at least into the Devonian. Most Appalachian tectonic models assert that during the Taconic orogeny a Middle Ordovician synorogenic clastic wedge, now located in the easternmost Tennessee foreland salient, was derived by erosion from the metamorphosed pre-Ordovician Blue Ridge basement and cover sequence to the east, which was uplifted as part of an advancing Taconic crystalline thrust wedge. The presence of Ordovician or younger rocks described here, which were deposited east of the proposed Taconic orogenic front, suggests the need to modify models for Taconic clastic wedge formation in the southern Appalachians. The results presented here also suggest that peak metamorphism in the region was post-Ordovician, and thus was probably not contemporaneous with the Taconic orogeny, as previously thought.