The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. [1] [2] [3] Most of the evidence for this event is in Nevada but the limits of its reach are unknown. A great volume of conglomeratic deposits of mainly Mississippian age in Nevada and adjacent areas [4] testifies to the existence of an important tectonic event, and implies nearby areas of uplift and erosion, but the nature and cause of that event are uncertain and in dispute. [5] Although it is known as an orogeny (mountain building event), some of the classic features of orogeny as commonly defined such as metamorphism, and granitic intrusives [6] have not been linked to it. In spite of this, the event is universally designated as an orogeny and that practice is continued here. This article outlines what is known and unknown about the Antler orogeny and describes three current theories regarding its nature and origin.
There are two principal facies of lower Paleozoic rocks in Nevada. [1] In the eastern part of the state, a north-trending fossil-rich carbonate shelf of Ordovician to Devonian age, termed the carbonate or eastern assemblage, gives way westward to a contemporaneous expanse of siliceous sedimentary deposits and minor mafic volcanic rocks termed the siliceous or western assemblage. [2] Crafford assigned these two facies respectively to the shelf domain and the basin domain. [3] The dark color of the western assemblage, the scarcity of carbonate rocks, and a near absence of shelly fossils, are generally interpreted to indicate a relatively deep-water depositional environment. [2] [3] The western assemblage also differs from the eastern assemblage in its components of bedded chert, basalt bodies, barite deposits, and sulfide deposits. [3] The nature of the two assemblages and their relation to one another are critical for an understanding of the Antler orogeny. The western facies assemblage is generally thought to be displaced from the west and to constitute the upper plate of an extensive thrust fault—the Roberts Mountains thrust. The eastern facies assemblage is thought to extend westward under the thrust plate. [1] The basis for this belief is that the western facies domain is dotted with anomalous blocky exposures of contemporaneous eastern facies shelf strata, some of mountain size, surrounded by exposures of western facies rocks. [2] These have been interpreted almost universally as exposures of the carbonate shelf facies in windows of the Roberts Mountains thrust sheet, and to prove the existence of that thrust sheet. [2] [3]
From an early date, [7] geologists have struggled to explain the presence in Nevada and adjacent areas of the Antler orogenic deposits without achieving a consensus. The advent of plate tectonic theory provided a variety of possible mechanisms by which the Roberts Mountains thrust and the orogenic deposits could be explained, but none of them has been universally accepted. As outlined in the following paragraphs, plate motion along the western margin of the North American continent in the Late Devonian has been offered as the cause of the orogeny and three varieties of it have been tried—east dipping subduction, west-dipping subduction, and strike-slip motion. None of them is without serious problems [5] and the nature and driving force of the orogeny remain uncertain.
This much is known concerning the Antler orogeny:
Based on stratigraphic relations near Antler Peak, of the Battle Mountains, Roberts introduced the term Antler orogeny in an abstract as follows: The earliest orogeny, here named the Antler orogeny ... took place during Mississippian (?) and early Pennsylvanian time. [10] That abstract was followed in 1951 by his geologic map of the Antler Peak quadrangle in the text of which he described the Antler orogeny in detail and somewhat refined its age span: During the Antler orogeny, formations in Battle Mountain ranging in age from Ordovician to Mississippian (?) were complexly folded and faulted. As these rocks are unconformably overlain by the Battle Formation of Early Pennsylvanian (Des Moines) age, the orogeny probably took place during the Late Mississippian. The orogeny may have continued into Early Pennsylvanian, however, for the coarse conglomerates of the Battle Formation indicate derivation from a rugged highland area. [11] In a subsequent influential paper, Roberts and others adjusted the age of the Antler orogeny as follows: This belt is now known to have been the locus of intense folding and faulting during the Antler orogeny in latest Devonian or Early Mississippian time ... [1] In the same paper the authors established a connection between the Antler orogeny and a major thrust fault as follows: A belt along the 116°-118° meridians—the Antler orogenic belt—was the locus of intense folding and faulting that culminated in the Roberts Mountains thrust fault... That age range and connection with the Roberts Mountains thrust were confirmed in a widely quoted paper by Silberling and Roberts: During the Late Devonian or Early Mississippian ... the Antler orogenic belt was intensely folded and faulted, and during Mississippian time the Roberts Mountains thrust sheet was emplaced. [12] The effect of this revision in the age of the orogeny was to exclude the evidence in the Antler Peak quadrangle cited above for a Late Mississippian to mid-Pennsylvanian age, on which the concept of the Antler orogeny originally had been based, and to establish the conventional age of that orogeny as Late Devonian to Early Mississippian.
The original date of the Roberts Mountains thrust fault was post-Paleozoic. [13] However, with publication of the 1958 and 1962 papers cited above, the authors revised the age of the Roberts Mountains thrust to coincide with the Late Devonian to Mississippian Antler orogeny and to extend the name far beyond the Roberts Mountains.
Over a period of 22 years numerous reports relating the Antler orogeny and Roberts Mountains thrust to plate convergence were published in various journals, and because their basic tenets have been widely accepted, they are here termed the conventional theories. The earliest effort to relate plate tectonics specifically to the Antler orogeny was briefly outlined by E.M. Moores: A collision of this continental margin with a subduction zone dipping away from it in late Devonian-early Mississippian time ... resulted in deformation of the pre-existing continental marginal rocks in the Antler Orogeny. [14]
Two principal contrasting tectonic theories were published in greater detail between 1972 and 1992 as related below. One theory involved closure of a back-arc basin between the western continental margin and a volcanic arc over an east-dipping subduction zone. A second theory involved collision of the continent with an island arc above a west-dipping subduction zone. Both were based on the basic understanding that the western facies assemblage is composed of oceanic deposits and that it is underlain by an extensive thrust fault.
Burchfiel and Davis presented the first detailed paper that explained the Antler orogeny and the Roberts Mountains thrust in terms of the subduction aspect of plate tectonics, stating: ... the paleogeography of this part of the Cordilleran geosyncline probably consisted of an offshore island complex separated from the continental slope and shelf by a small ocean basin of behind-the-arc type. Initial regional deformation within the Cordilleran geosyncline—the Mid-Paleozoic Antler orogeny—was characterized by the eastward displacement (Roberts Mountains thrust) of eugeosynclinal units from within the small ocean basin over miogeosynclinal strata deposited on the continental shelf. [15] Their now-outdated terms eugeosynclinal and miogeosynclinal refer respectively to the western facies and eastern facies domains. In that paper, Burchfiel and Davis set the parameters for future discussions of the nature and origins of the Antler orogeny and associated thrusts. Their basic concept of east-dipping subduction was reflected in modified form by others, including Miller and others. [16] [17]
Dickinson and others argued for an opposing theory, that west-dipping subduction and volcanic arc-continent collision were the fundamental processes. [18] [19] They stated in the abstract of their 1983 report that The Roberts Mountains allochthon was probably the subduction complex or accretionary prism of an intra-oceanic Antler arc-trench system that faced east (west-dipping), with subduction downward to the west. Its emplacement by thrusting over the Cordilleran miogeoclinal terrane of lower Paleozoic strata occurred in earliest Mississippian time during an inferred arc-continent collision that began in latest Devonian time and is termed the Antler orogeny. [19] Their term "miogeoclinal terrane" referred to the eastern facies assemblage. This was followed by papers offering modified versions of the same theory. [20] [21] [22] Other papers supplied definitive reviews and confirmed the Antler orogeny as a result of plate convergence. [17] [23] [24]
As an alternative to the two conventional theories described above, Ketner proposed that (1) left-lateral strike-slip faulting along the western margin of the North American continent, rather than plate convergence, was the engine of Paleozoic tectonics in the region; (2) the Roberts Mountains allochthon, as such, does not exist, and the Ordovician to Devonian western facies assemblage was deposited essentially in situ; and (3) blocks of shelf carbonate rocks earlier thought to be exposures of the shelf in windows of the Roberts Mountains allochthon are slide blocks from the carbonate shelf. The slide blocks probably were dislodged by the Alamo impact event of Late Devonian age. [5] In this scheme, the deep-water aspects of the western facies assemblage are due to sea-level rise in the Cambrian [25] [26] rather than displacement from an ocean basin. [8]
The sedimentary effects of the Antler orogeny are well known and well described in many published reports, [2] [3] [4] [27] but the exact nature of that event and the driving force remain unsettled. Among the unanswered questions are these: what aspect of plate tectonics was involved; what effect did the Alamo impact event have; why did marine basins appear in the area of general uplift; why did the western facies assemblage, and not the eastern assemblage, include bedded chert, basaltic bodies, barite deposits, and sulfide deposits.
Orogeny is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involves a series of geological processes collectively called orogenesis. These include both structural deformation of existing continental crust and the creation of new continental crust through volcanism. Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere. A synorogenic process or event is one that occurs during an orogeny.
The geology of the Appalachians dates back more than 1.1 billion years to the Mesoproterozoic era when two continental cratons collided to form the supercontinent Rodinia, 500 million years prior to the later development of the range during the formation of the supercontinent Pangea. The rocks exposed in today's Appalachian Mountains reveal elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor – strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangea with the Appalachians and neighboring Anti-Atlas mountains near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.
The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 80 to 70 million years ago, and ended 55 to 35 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.
The Acadian orogeny is a long-lasting mountain building event which began in the Middle Devonian, reaching a climax in the early Late Devonian. It was active for approximately 50 million years, beginning roughly around 375 million years ago, with deformational, plutonic, and metamorphic events extending into the Early Mississippian. The Acadian orogeny is the third of the four orogenies that formed the Appalachian orogen and subsequent basin. The preceding orogenies consisted of the Potomac and Taconic orogeny, which followed a rift/drift stage in the Late Neoproterozoic. The Acadian orogeny involved the collision of a series of Avalonian continental fragments with the Laurasian continent. Geographically, the Acadian orogeny extended from the Canadian Maritime provinces migrating in a southwesterly direction toward Alabama. However, the Northern Appalachian region, from New England northeastward into Gaspé region of Canada, was the most greatly affected region by the collision.
The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kinds of orogenic processes occurring. During the early stages of orogenesis an "Andean type" continental magmatic arc developed due to subduction of the Farallon oceanic plate beneath the North American Plate. The latter stages of orogenesis, in contrast, saw multiple oceanic arc terranes accreted onto the western margin of North America in a "Cordilleran type" accretionary orogen. Deformation related to the accretion of these volcanic arc terranes is mostly limited to the western regions of the resulting mountain ranges and is absent from the eastern regions. In addition, the deformation experienced in these mountain ranges is mostly due to the Nevadan orogeny and not other external events such as the more recent Sevier and Laramide Orogenies. It is noted that the Klamath Mountains and the Sierra Nevada share similar stratigraphy indicating that they were both formed by the Nevadan orogeny. In comparison with other orogenic events, it appears that the Nevadan Orogeny occurred rather quickly taking only about 10 million years as compared to hundreds of millions of years for other orogenies around the world.
The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the continents and terranes of Laurentia, Baltica and Avalonia collided.
The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador to Mexico, as well as to Scotland.
The Sevier orogeny was a mountain-building event that affected western North America from northern Canada to the north to Mexico to the south.
The Lachlan Fold Belt (LFB) or Lachlan Orogen is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age. It dominates New South Wales and Victoria, also extending into Tasmania, the Australian Capital Territory and Queensland. It was formed in the Middle Paleozoic from 450 to 340 Mya. It was earlier known as Lachlan Geosyncline. It covers an area of 200,000 km2.
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland, known as the Hebridean Terrane. During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.
The Rhenohercynian Zone or Rheno-Hercynian zone in structural geology describes a fold belt of west and central Europe, formed during the Hercynian orogeny. The zone consists of folded and thrust Devonian and early Carboniferous sedimentary rocks that were deposited in a back-arc basin along the southern margin of the then existing paleocontinent Laurussia.
The Sonoma orogeny was a period of mountain building in western North America. The exact age and structure of the Sonoma orogeny is controversial. The orogeny is generally thought to have occurred during the Permian / Triassic transition, around 250 million years ago, following the Late Devonian Antler orogeny. The Sonoma orogeny was one of a sequence of accretionary events along the Cordilleran margin, possibly caused by the closure of the basin between the island arc of Sonomia and the North American continent. Evidence of this event has been reported throughout western North America, but most distinctly in northwest Nevada.
The Massif Central is one of the two large basement massifs in France, the other being the Armorican Massif. The Massif Central's geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has also left its imprints, probably causing the important Cenozoic volcanism. The Massif Central has a very long geological history, underlined by zircon ages dating back into the Archaean 3 billion years ago. Structurally it consists mainly of stacked metamorphic basement nappes.
The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.
The geology of North America is a subject of regional geology and covers the North American continent, the third-largest in the world. Geologic units and processes are investigated on a large scale to reach a synthesized picture of the geological development of the continent.
The Vinini Formation is a marine, deep-water, sedimentary deposit of Ordovician to Early Silurian age in Nevada, U.S.A. It is notable for its highly varied, mainly siliceous composition, its mineral deposits, and controversies surrounding both its depositional environment and structural history. The formation was named by Merriam and Anderson for an occurrence along Vinini Creek in the Roberts Mountains of central Nevada and that name is now used extensively in the State.
The main points that are discussed in the geology of Iran include the study of the geological and structural units or zones; stratigraphy; magmatism and igneous rocks; ophiolite series and ultramafic rocks; and orogenic events in Iran.
The geology of Nevada began to form in the Proterozoic at the western margin of North America. Terranes accreted to the continent as a marine environment dominated the area through the Paleozoic and Mesozoic periods. Intense volcanism, the horst and graben landscape of the Basin and Range Province originating from the Farallon Plate, and both glaciers and valley lakes have played important roles in the region throughout the past 66 million years.
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