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Primary terms
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aulacogens
The 180-km-long Meers-Willow fault system in the Southern Oklahoma Aulacogen: A potential U.S. mid-continent seismic hazard
Neodymium isotope mapping a polygenetic TTG batholith: failed back-arc rifting in the Central Metasedimentary Belt, southwestern Grenville Province
Cambrian–Lower Ordovician of SW Quebec–NE New York
ABSTRACT The Ottawa aulacogen/graben on the NE US—Canadian (SW Quebec and eastern Ontario) border is a long ENE-trending structure formed with initial late Neo proterozoic rifting of the Rodinia supercontinent. This rifting formed the active spreading arms (New York Promontory and Quebec Reentrant) along the (presently) NE margin of the new Laurentia paleocontinent, with the Ottawa aulacogen commonly regarded as a failed arm of the rifting. However, no sediment accumulation in the aulacogen is recorded until the late early Cambrian subsidence of a SE- trending belt that includes the aulacogen and its extension, the Franklin Basin, in NW Vermont. Late early Cambrian marine onlap (Altona Formation) followed by more rapid late middle Cambrian subsidence and deposition of fluviatile arkoses (Covey Hill Formation of SW Quebec and Ausable Formation/Member of eastern New York) record rapid foundering of this “failed arm.” Subsequent deposition (latest middle Cambrian–Early Ordovician) in the Ottawa aulacogen produced a vertical succession of lithofacies that are fully comparable with those of the shelf of the New York Promontory. One of the greatest challenges in summarizing the geological history of the Ottawa aulacogen is the presence of a duplicate stratigraphic nomenclature with lithostratigraphic names changing as state and provincial borders are crossed. RÉSUMÉ L’aulacogène/graben d’Ottawa, situé sur la frontière entre le NE des États-Unis et le Canada (SW du Québec et est de l’Ontario), est une longue structure d’orientation ENE formée au Néoprotérozoïque tardif durant le rifting initial du supercontinent Rodinia. Ce rifting a aussi mené à la formation de segments à expansion active (promontoire de New York et réentrant de Québec) le long de la marge NE (coordonnées actuelles) du nouveau paléo-continent Laurentia, avec l’aulacogène d’Ottawa qui est généralement considéré comme un segment de rift avorté. Toutefois, aucune accumulation de sediments n’est documentée au sein de l’aulacogène avant la fin du Cambrien précoce, période durant laquelle une ceinture d’orientation SE, representée par l’aulacogène et son prolongement dans le bassin de Franklin vers le NW du Vermont, a subi une subsidence. La sedimentation marine de la fin du Cambrien précoce (Formation d’Altona) a été suivie d’une subsidence rapide à la fin du Cambrien moyen et de la déposition d’arkoses fluviatiles (Formation de Covey Hill dans le SW du Québec et la Formation/Membre d’Ausable dans l’est de l’état de New York) qui ont enregistré un affaissement rapide de ce “bras avorté.” La sédimentation subséquente (Cambrien moyen tardif–Ordovicien inférieur) au sein de l’aulacogène d’Ottawa a produit une succession verticale de lithofaciès qui sont comparables à ceux de la plate-forme du promontoire de New York. Un des principaux défis dans la synthèse de l’histoire géologique de l’aulacogène d’Ottawa demeure la duplication des termes stratigraphiques de part et d’autre des frontières interprovinciales et entre les différents états.
Formation of the Marianna Gap, A Transverse Stream Valley, through Crowley’s Ridge, Northern Mississippi Alluvial Valley, U.S.A.
Testing a back-arc ‘aulacogen’ model for the Central Metasedimentary Belt of the Grenville Province
Epeirogenic transgression near a triple junction: the oldest (latest early–middle Cambrian) marine onlap of cratonic New York and Quebec
The classic failed continental rift or aulacogen is one that intersects a rifted continental margin at a high angle. Based on recent geological and geophysical studies, we have revisited a classic analogy that was drawn between two major intracratonic rifts, the Southern Oklahoma aulacogen in the southern portion of Laurentia and the Dniepr-Donets aulacogen in the southern portion of Baltica. The Southern Oklahoma aulacogen, also known as the Wichita aulacogen, consists of a linear alignment of extensively inverted rift structures that begins at the rifted margin of Laurentia in northeast Texas and extends northwestward at least to southern Colorado. Deep seismic profiles have revealed the upper crustal structure of this feature, and gravity data provide a regional context for interpreting these results. Velocities low enough to indicate the presence of sedimentary rocks extend to a depth of ∼15 km, and the deepest of these sedimentary layers has been interpreted as rift fill. In addition, the main inversion structure of the Southern Oklahoma aulacogen (Wichita uplift) is underlain by very high-velocity and dense mafic material even at upper crustal depths of ∼5 km. The Dniepr-Donets aulacogen has been cited as a type example of an aulacogen and is clearly a “failed rift” in the sense that it did not itself lead to continental breakup and ocean crust formation. The main feature of the aulacogen is a Late Devonian rift basin overlain by a substantial (but variable) postrift sedimentary sequence that records several extensional or transtensional events and at least one moderate compressional reactivation. Recent deep seismic reflection and refraction surveys resolve the geometry of the sedimentary succession in the Donets segment of the basin, indicating an asymmetric form with a steeper basement surface in the south than in the north and a total sedimentary thickness of ∼20 km. A thick (>10 km) high-velocity (>6.9 km/s) lower crustal body lies beneath the rift basin itself and is offset slightly to the north compared to the main basin depocenter. The Moho displays only slight topography around a depth of 40 km although, based on older deep seismic data, it shallows somewhat under the rift axis in the Dniepr segment to the northwest. Thus, major differences between these two major rifts are the nature of the magmatic modification of the crust and degree of inversion. Both the age of initial rifting and subsequent inversion in the Dniepr-Donets aulacogen are redefined compared to what was thought at the time the original analogy was made with the Southern Oklahoma aulacogen.