Related Research Articles
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).
The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. 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 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. Although it is known as an orogeny, some of the classic features of orogeny as commonly defined such as metamorphism, and granitic intrusives 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.
Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (14,500–58,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness causes the tendency to split, or have schistosity. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.
The Yilgarn Craton is a large craton that constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts. Zircon grains in the Jack Hills, Narryer Terrane have been dated at ~4.27 Ga, with one detrital zircon dated as old as 4.4 Ga.
The Gascoyne Complex is a terrane of Proterozoic granite and metamorphic rock in the central-western part of Western Australia. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two Archean cratons; the Pilbara craton to the north and the Yilgarn craton to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma.
The 10 kilometer wide Ross Lake fault zone (RLFZ) is part of a 500 kilometer long zone of high-angle faults in the North American Cordillera of Washington and Canada. The RLFZ consists of two major sets of faults. The eastern set of the Hozameen and Slate Creek faults and more southerly North Creek fault form the western boundary of the Jurassic-Cretaceous Methow River basin and in part separate it from metamorphic equivalents of Methow strata. Minor structures along the North Creek fault record dextral strike-slip events that occurred between approximately 88 and 50 Ma. The same formations lie on both sides of the faults, implying modest slip. The northernmost strand of the western fault set, the Ross Lake fault itself, is a vertical zone of horizontally-lineated mylonite that separates upper-amphibolite-facies rocks of the Cascades crystalline core from sub-greenschist-facies rocks to the east. Some dextral shear and 6–12 km of NE-side down normal slip occurred from 50(?) to post-45 Ma. At Elijah Ridge, the Ross Lake fault steps westward across a gently dipping extensional zone to the Gabriel Peak tectonic belt. This approximately 100 kilometer long, northeast-dipping mylonite zone is dominated by flattening, but kinematic indicators record dextral shear in the north and reverse shear farther south. This transpressional deformation occurred from 65 Ma to 58 Ma when at least 7–24 km of dextral slip was probably transferred to the eastern faults by ENE-striking shear zones. Younger ENE-striking sinistral faults at least locally accommodated 5–10 km of dextral strike slip by vertical axis rotation. The fault sets merge southward to form the Foggy Dew fault zone where mylonites record oblique dextral-normal slip (down-to-E). Slip is bracketed between 65 and 48 Ma; some occurred after 60 Ma and the zone records the regional transition from approximately 65–58 Ma transpression to approximately 57–45 Ma transtension. The fault zone is truncated to the SE by the 48 Ma Cooper Mountain batholith, which also obliterates its intersection with the southern continuation of the Pasayten fault. South of this batholith, only a narrow, discontinuous shear zone is on strike with the Foggy Dew fault and similar units lie on both sides of this projection of the RLFZ.
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 Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 billion years (Ga). They form the basement on which the Torridonian and Moine Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.
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 Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into close contact with eclogite facies metamorphic rocks of the Western Gneiss Region. It formed towards the end of the Caledonian Orogeny and was mainly active during the Devonian. It has an estimated displacement of at least 70 km and possibly as much as 110 km. It was reactivated during the Mesozoic and may have influenced the development of fault structures in the North Sea rift basin.
The Cordillera Darwin Metamorphic Complex is a geologic complex composed chiefly of metamorphic rocks located in southern Tierra del Fuego. It has been suggested that the Cordillera Darwin Metamorphic Complex is analogous to the Eastern Andes Metamorphic Complex. The Cordillera Darwin Metamorphic Complex is the only metamorphic complex in the southern Andes known to have amphibolite facies rocks containing kyanite and sillimanite which evidences high-grade metamorphism. High-grade metamorphism took place during the Cretaceous purportedly in association with the closure of the Rocas Verdes Basin.
The Norfolk Basin is synclinal basin, partially bounded by faults, running east-northeast between the Dedham Block and the Foxborough Block. It contains the folded and cleaved, but unmetamorphosed Wamsutta Formation and Pondville Conglomerate, which both formed in the Pennsylvanian, also known as the Late Carboniferous 323 to 298 million years ago.
The Bloody Bluff fault is the boundary between the Milford-Dedham zone and the Nashoba zone in eastern Massachusetts. It runs between Westborough, Massachusetts and Lynnfield, Massachusetts. In the south, into Connecticut, the component Lake Char fault is poorly exposed. The Bloody Bluff fault zone is 3.2 kilometers wide in Framingham, Massachusetts and forms the five kilometer wide Burlington mylonite zone to the north. Rocks along the fault have experienced both ductile and brittle deformation.
The Nantucket Basin is a northeast trending basin, formed in the Triassic and Jurassic, buried beneath Atlantic Coastal Plain sediments under Nantucket. The basin is believed to be 25 kilometers wide and 100 kilometers long, containing sandstone and basalt. The rock units within it appear to dip north, based on seismic-reflection profiles and taken together the rocks are up to one kilometer thick. It is one of several failed Mesozoic rift basins in Massachusetts.
The Middleton Basin is a small sedimentary basin, in northeastern Massachusetts, containing Late Triassic and Early Jurassic red beds. Estimated to be 5.7 kilometers long and at most half a kilometer wide, it is one of the smallest basins in the state and is closely associated with the Newbury basins.
The geology of Sudan formed primarily in the Precambrian, as igneous and metamorphic crystalline basement rock. Ancient terranes and inliers were intruded with granites, granitoids as well as volcanic rocks. Units of all types were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny. Dramatic sheet flow erosion prevented almost any sedimentary rocks from forming during the Paleozoic and Mesozoic. From the Mesozoic into the Cenozoic the formation of the Red Sea depression and complex faulting led to massive sediment deposition in some locations and regional volcanism. Sudan has petroleum, chromite, salt, gold, limestone and other natural resources.
The geology of South Korea includes rocks dating to the Archean and two large massifs of metamorphic rock as the crystalline basement, overlain by thick sedimentary sequences, younger metamorphic rocks and volcanic deposits.
References
- ↑ Goldsmith, Richard (1991). Structural and Metamorphic History of Eastern Massachusetts. USGS. p. H36-H37.
 | This Massachusetts state location article is a stub. You can help Wikipedia by expanding it. |