Relics of a complex triple junction in the Weddell Sea embayment, Antarctica
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Keywords:
Triple junction
Pangaea
Passive margin
Laurentia
SUMMARY During the Cambrian, two types of continental margins occurred around Gondwana. The eastern margin (Antarctica, Australia and southern South America) was characterized by a narrow continental shelf with a steep slope separating the shallow water environment from a deep‐oceanic one accompanied by mafidultramafic volcanics. The western margin was characterized by a wider continental shelf, probably passing gradually to an unknown outer basin. This comprised three main domains: the Asiatic shelf, composed of distinct cratonic blocks, presumably separated from each other by deeper‐water/ volcanic intracontinental basins; the European shelf, characterized by the development of shallow intracontinental siliciclastic basins; and the Americanc‐African shelf, morphologically and depositionally uniform. The distinction of these two Gondwana continental margins expresses their different geodynamic behaviour during Cambrian extensional tectonics. In fact, the sedimentary/palaeogeographic evolution, suggests the establishment of an active Pacific‐like margin in the eastern domain, and the tentative establishment of a divergent Atlantic‐like margin, in the westem one.
Continental Margin
Siliciclastic
Passive margin
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The Rheic Ocean was one of the most important oceans of the Paleozoic Era. It lay between Laurentia and Gondwana from the Early Ordovician and closed to produce the vast Ouachita-Alleghanian-Variscan orogen during the assembly of Pangea. Rifting began in the Cambrian as a continuation of Neoproterozoic orogenic activity and the ocean opened in the Early Ordovician with the separation of several Neoproterozoic arc terranes from the continental margin of northern Gondwana along the line of a former suture. The rapid rate of ocean opening suggests it was driven by slab pull in the outboard Iapetus Ocean. The ocean reached its greatest width with the closure of Iapetus and the accretion of the peri-Gondwanan arc terranes to Laurentia in the Silurian. Ocean closure began in the Devonian and continued through the Mississippian as Gondwana sutured to Laurussia to form Pangea. The ocean consequently plays a dominant role in the Appalachian-Ouachita orogeny of North America, in the basement geology of southern Europe, and in the Paleozoic sedimentary, structural and tectonothermal record from Middle America to the Middle East. Its closure brought the Paleozoic Era to an end.
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Rift-controlled erosive unconformities are generally absent outside their involved tilted down-faulted blocks, so interpolating the presence of coeval cryptic unconformities in neighbouring passive-margin basins is questionable. In West Gondwana, the supposed correlation of Moroccan (rifting) unconformities to Iberian (passive margin) cryptic unconformities across the Lower–Middle Cambrian boundary interval is neither constrained nor properly argued.
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During the Silurian and Devonian, the sequence of continental collisions that were ultimately to result in the formation of the supercontinent of Pangaea had begun. By the Early to Middle Devonian North America (Laurentia), Acadia, Great Britain, and Northern Europe (Baltica) had collided to form the ‘Old Red Sandstone’ continent (Laurussia). Palaeomagnetic data, however, indicate that the configuration of the continents that made up Laurussia did not resemble the pre-breakup, Mesozoic reassembly. Rather, Britain, Baltica, and Acadia were displaced 10—20° to the south with respect to Laurentia. New palaeomagnetic data for Laurentia and Gondwana, suggests that the ocean separating the northern and southern continents was relatively narrow during the early Devonian, and may have been nearly closed by the late Devonian.
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Laurasia
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The eastern edge of the Appalachian orogen is composed of a collection of Neoproterozoic – early Paleozoic domains, Avalonia, Carolinia, Ganderia, Meguma, and Suwannee, which are exotic to North America. Differences in the geological histories of these peri-Gondwanan domains indicate that each separated independently from Gondwana, opening the Rheic Ocean in their wake. Cambrian departure of Ganderia and Carolina was followed by the Ordovician separation of Avalonia and Silurian separation of Meguma. After separation in the early Paleozoic, these domains constituted the borderline between the expanding Rheic Ocean and contracting Iapetus Ocean. They were transferred to Laurentia by early Silurian closure of Iapetus and Devonian–Carboniferous closure of the Rheic Ocean during the assembly of Gondwana and Laurentia into Pangaea. The first domain to arrive at Laurentia was Carolinia, which accreted in the Middle Ordovician during the Cherokee orogeny. Salinic accretion of Ganderia occurred shortly thereafter and was followed by the Acadian accretion of Avalonia. The Acadian orogeny was immediately followed by Middle Devonian – Early Carboniferous accretion of Meguma and possibly Suwannee which led to the Fammenian orogeny. The episodicity of orogeny suggests that the present location of these domains parallels their order of accretion. However, each of these crustal blocks was translated along strike by large-scale Late Devonian – Carboniferous dextral strike–slip motion. The breakup of Pangaea occurred outboard of the Paleozoic collision zones that accreted Carolinia, Ganderia, Avalonia, Meguma, and Suwannee to Laurentia, leaving these terranes appended to North America during the Mesozoic opening of the Atlantic.
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Devonian
Baltica
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The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identification of synrift rocks and structures, as well as continental-shelf and off-shelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita-Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fill of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upper-plate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530–539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fill of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting reflects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment.
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Continental Margin
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Thrust fault
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