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40Ar/39Ar dating of slate and phyllite in the Nambucca Block, southern New England Orogen, indicates that deformation took place between 264 and 260 Ma. These data further show that indentation of the Early Permian turbidite successions by the southward-moving Coffs Harbour Orocline ceased at this time. Folding of the Sakmarian to Sarginian sequences in the Silver Spur Basin (294.6 - 275.6 Ma) during the megafolding event constrains the formation of the Texas Orocline to be post-ca 276 Ma. The presence of brachiopod faunas indicates that orocline formation commenced in the early Kungurian (ca 273 Ma) during the early stages of the Hunter - Bowen Orogeny. Thus, development of the Texas and Coffs Harbour Oroclines took place within a period of 13 million years, from 273 to 260 Ma. Oblique convergence is believed to be responsible for the formation of the Texas and Coffs Harbour Oroclines and associated constrictional structures during the Hunter - Bowen Orogeny. Slip on a dextral, strike-slip fault formed during convergence, buckled and rotated the meridional-trending arc - forearc - subduction complex to form north-northwest-trending, Z-shaped megafolds. [ABSTRACT FROM AUTHOR]

Three late Paleozoic, angular unconformities, each tightly constrained in age by biostratigraphy, are exposed in Carlin Canyon, Nevada. These record deformation as well as erosion. Folding associated with these deformation events is roughly coaxial; all three sets of fold axes trend northeast. Each unconformity represents tectonic disruption of the middle part of the western North American margin between the times of the initiation of the Antler orogeny (Late Devonian—Early Mississippian) and the Permian—Triassic Sonoma orogeny. This paper focuses on one of these unconformities in the Middle Pennsylvanian—the C6 unconformity—and the deformation and age constraints associated with it. Our data from Carlin Canyon yield detailed glimpses of how the Antler foreland evolved tectonically in Mississippian and Pennsylvanian time. Middle Pennsylvanian (Desmoinesian) northwest-southeast contraction resulted in thin-skinned folding and faulting, uplift, and erosion. These data require reinterpretation of the tectonic setting at the time of the Ancestral Rocky Mountains orogeny and suggest that plate convergence on the west side of the continent played a significant role in late Paleozoic tectonics of the North American continent. [ABSTRACT FROM AUTHOR]

The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this "greater Green River Basin" become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming-Idaho-Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre-Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious. Within context of a measured... [ABSTRACT FROM AUTHOR]

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, were involved in a ∼1.15–1.12 Ga Grenville-age collisional orogenic event along the southern margin of Laurentia. This event is characterized by polyphase ductile deformation synchronous with transitional amphibolite-, granulite- to eclogite-facies metamorphism. Results of detailed structural mapping in the southeastern part of the Llano Uplift show that these rocks record a deformational history involving northeast-directed ductile thrusting and regional-scale polyphase folding, all related to northward collision of a southern continent and an arc with the southern margin of Laurentia. We use the structural, metamorphic, and kinematic history of the southeastern Llano uplift to critically evaluate collisional models for the Grenville orogen in Laurentia and discuss the implications for reconstructions of Rodinia. [ABSTRACT FROM AUTHOR]

As in many continental margins, Mesozoic rifting in the Gulf of Mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin. Unlike the situation in most other margins, however, extension in the Gulf of Mexico did not involve widespread reactivation of these older tectonic features. This difference may be related to the unusual facts that (1) the antecedent passive continental margin was a transform rather than a rift margin and (2) the prior orogeny did not substantially thicken the crust. In order to characterize the interaction between Mesozoic rifting and prior tectonic events, two geophysical transects were constructed across the central Gulf of Mexico coastal plain and continental shelf. The transects show that the early Paleozoic passive continental margin buried beneath the late Paleozoic Ouachita orogen was little affected by compressional tectonism or Mesozoic extension during opening of the Gulf of Mexico. The thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km, consistent with previous interpretations that this was a transform margin. The Ouachita fold-and-thrust belt is shown to be a thin-skinned doubly vergent orogen that formed above a well-preserved southward-dipping subduction system. Relict Cambrian oceanic crust may exist beneath the southern part of the orogen, but its presence is not required by the data. The thin crust and shallow mantle in the Ouachita suture created a zone of relatively high strength that acted as a barrier to Mesozoic extension, which is restricted to regions south of the orogen. Extension was initially distributed over a broad area that included the Mississippi Interior Salt Basin and regions south of the Wiggins Arch. Extension became progressively more focused south of the Wiggins Arch with time, culminating in the onset of seafloor spreading beneath the modern continental rise. Mesozoic extension factors are β = 1.4-1.9 beneath the Mississippi Interior Salt Basin, β ≈ 2.9 immediately south of the Wiggins Arch, and β ≈ 4 adjacent to the oldest oceanic crust in the central Gulf of Mexico. Keywords: Gulf of Mexico; North America; rifting; Ouachita orogeny; gravity profiles. [ABSTRACT FROM AUTHOR]

Late Paleozoic synorogenic clastic wedges in the Appalachian foreland basin contain erosional detritus from the Alleghanian orogenic belt; however, the relative importance of detrital contributions from the frontal orogen, the orogenic hinterland, and the distal craton has not been resolved. To evaluate these components, we analyzed detrital zircons from two sandstones in the Pennington-Lee clastic wedge in eastern Tennessee: the Lower Pennsylvanian Sewanee Conglomerate and the Middle Pennsylvanian Cross Mountain Formation. Ages of detrital zircons are similar in the two sandstones, ranging from 365 to 2860 Ma. The age range spans the Acadian and Taconic orogenies, the Grenville province, and pre-Grenville rocks (Granite-Rhyolite province, midcontinent orogens, and Superior province) of the Laurentian craton. The Acadian-, Taconic-, and Grenville-age detrital zircons have sources in Alleghanian frontal crystalline thrust sheets. The pre-Grenville zircons indicate an original source in the Laurentian craton (Ca-nadian shield), and a longitudinal river system headed in the shield might have supplied detritus directly to the Appalachian foreland basin in Tennessee. Alternatively, all of the Proterozoic and Archean ages of zircons represented in the Sewanee and Cross Mountain sandstones have been identified in the detrital zircon populations of preorogenic Iapetan synrift and passive-margin strata along the eastern Laurentian margin, suggesting a source of recycled zircons from preorogenic rocks exposed in Alleghanian thrust sheets. A lack of detrital zircons contemporaneous with the Alleghanian orogeny shows that Alleghanian igneous rocks had not been integrated into the drainage network and/ or had not been exhumed. The age distribution of the detrital zircons suggests deformation and erosion of rocks of the Laurentian margin and emphasizes the importance of preorogenic architecture of the continental margin in the construction and erosion of the orog [ABSTRACT FROM AUTHOR]

Abstract A major problem with the current use of porphyroblast–matrix microstructural relationships to infer orogenic histories, such as multiple orthogonal orogenic events, is that other evidence for these events is typically lacking. For example, a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the Foothills Terrane, Sierra Nevada, California, shows that: (1) except in shear zones, contact aureoles and local zones along lithological contacts, the Foothills Terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations, and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts; (4) complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional complexity. Because of these conclusions, multiple datasets, rather than data acquired only from porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified. [ABSTRACT FROM AUTHOR]

Tests the orogenic fluid hypothesis in the central Appalachians in West Virginia. Comparison of the paleomagnetic and rock magnetic results with geochemical data; Paleomagnetic studies of the Devonian Helderberg Group; Remagnetization by orogenic fluids.

Studies the mesoproterozoic chronostratigraphy of the southeastern Llano uplift found in central Texas. Crystalline rocks affected by a Grenvillian orogeny; Protolith ages of quartfeldspathic rocks from the Packsaddle Schist and Valley Spring Gneiss; Metamorphic age of the gneiss.

Summarizes the Upper Cretaceous to Eocene stratigraphy and age data used to interpret the paleography and tectonic development of the southern Cordillera foreland basin between the Sevier and Laramide orogenies. Reference of `Laramide' to structural style of basement-cored uplifts and intervening sedimentary basins.

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 °C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the Turonian-Coniacian, 20 m.y. after the HP-LT metamorphism, and 25 m.y. before the terminal Paleocene continental collision. The Cretaceous sub-duction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia. [ABSTRACT FROM AUTHOR]