Thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. A thrust fault is a type of reverse fault. If the angle of the fault plane is lower and the displacement of the overlying block is large the fault is called an overthrust or overthrust fault. Erosion can remove part of the overlying block, creating a fenster – when the underlying block is exposed only in a small area; when erosion removes most of the overlying block, leaving island-like remnants resting on the lower block, the remnants are called klippen. If the fault plane terminates before it reaches the Earth's surface, it is referred to as a blind thrust fault; because of the lack of surface evidence, blind thrust faults are difficult to detect until they rupture. The destructive 1994 quake in Northridge, was caused by a undiscovered blind thrust fault; because of their low dip, thrusts are difficult to appreciate in mapping, where lithological offsets are subtle and stratigraphic repetition is difficult to detect in peneplain areas.
Thrust faults those involved in thin-skinned style of deformation, have a so-called ramp-flat geometry. Thrusts propagate along zones of weakness within a sedimentary sequence, such as mudstones or salt layers, these parts of the thrust are called decollements. If the effectiveness of the decollement becomes reduced, the thrust will tend to cut up the section to a higher stratigraphic level until it reaches another effective decollement where it can continue as bedding parallel flat; the part of the thrust linking the two flats is known as a ramp and forms at an angle of about 15°-30° to the bedding. Continued displacement on a thrust over a ramp produces a characteristic fold geometry known as a ramp anticline or, more as a fault-bend fold. Fault-propagation folds form at the tip of a thrust fault where propagation along the decollement has ceased but displacement on the thrust behind the fault tip is continuing; the continuing displacement is accommodated by formation of an asymmetric anticline-syncline fold pair.
As displacement continues the thrust tip starts to propagate along the axis of the syncline. Such structures are known as tip-line folds; the propagating thrust tip may reach another effective decollement layer and a composite fold structure will develop with characteristics of both fault-bend and fault-propagation folds. Duplexes occur where there are two decollement levels close to each other within a sedimentary sequence, such as the top and base of a strong sandstone layer bounded by two weak mudstone layers; when a thrust that has propagated along the lower detachment, known as the floor thrust, cuts up to the upper detachment, known as the roof thrust, it forms a ramp within the stronger layer. With continued displacement on the thrust, higher stresses are developed in the footwall of the ramp due to the bend on the fault; this may cause renewed propagation along the floor thrust until it again cuts up to join the roof thrust. Further displacement takes place via the newly created ramp.
This process may repeat many times, forming a series of fault bounded thrust slices known as imbricates or horses, each with the geometry of a fault-bend fold of small displacement. The final result is a lozenge shaped duplex. Most duplexes have only small displacements on the bounding faults between the horses and these dip away from the foreland; the displacement on the individual horses is greater, such that each horse lies more or less vertically above the other, this is known as an antiformal stack or imbricate stack. If the individual displacements are greater still the horses have a foreland dip. Duplexing is a efficient mechanism of accommodating shortening of the crust by thickening the section rather than by folding and deformation. Large overthrust faults occur in areas; these conditions exist in the orogenic belts that result from either two continental tectonic collisions or from subduction zone accretion. The resultant compressional forces produce mountain ranges; the Himalayas, the Alps, the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults.
Thrust faults occur in the foreland basin. Here, compression does not result in appreciable mountain building, accommodated by folding and stacking of thrusts. Instead thrust faults cause a thickening of the stratigraphic section. Foreland basin thrusts usually observe the ramp-flat geometry, with thrusts propagating within units at a low angle "flats" and moving up-section in steeper ramps where they offset stratigraphic units. Identifying ramps where they occur within units is problematic. Thrusts and duplexes are found in accretionary wedges in the ocean trench margin of subduction zones, where oceanic sediments are scraped off the subducted plate and accumulate. Here, the accretionary wedge must thicken by up to 200% and this is achieved by stacking thrust fault upon thrust fault in a melange of disrupted rock with chaotic folding. Here, ramp flat geometries are not observed because the compressional force is at a steep angle to the sedimentary layering. Thrust faults were unrecognised until the work of Arnold Escher von der Linth, Albert Heim and Marcel Alexandre Bertrand in the Alps working on the Glarus Thrust.
Economic geology
Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals, construction-grade stone, petroleum minerals and water. Economic geology is a subdiscipline of the geosciences. Today, it may be called the scientific study of the Earth’s sources of mineral raw materials and the practical application of the acquired knowledge; the term refers to metallic mineral deposits and mineral resources. The techniques employed by other earth science disciplines might all be used to understand and exploit an ore deposit. Economic geology is practiced by geologists. Economic geology may be of interest to other professions such as engineers, environmental scientists, conservationists because of the far-reaching impact that extractive industries have on society, the economy, the environment; the purpose of the study of economic geology is to gain understanding of the genesis and localization of ore deposits plus the minerals associated with ore deposits.
Though metals and other geologic commodities are non-renewable in human time frames, the impression of a fixed or limited stock paradigm of scarcity has always led to human innovation resulting in a replacement commodity substituted for those commodities which become too expensive. Additionally the fixed stock of most mineral commodities is huge (e.g. copper within the earth's crust given current rates of consumption would last for more than 100 million years. Nonetheless, economic geologists continue to expand and define known mineral resources. Mineral resources are concentrations of minerals significant for future societal needs. Ore is classified as mineralization economically and technically feasible for extraction. Not all mineralization meets these criteria for various reasons; the specific categories of mineralization in an economic sense are: Mineral occurrences or prospects of geological interest but not economic interest Mineral resources include those economically and technically feasible and those that are not Ore reserves, which must be economically and technically feasible to extract Geologists are involved in the study of ore deposits, which includes the study of ore genesis and the processes within the Earth's crust that form and concentrate ore minerals into economically viable quantities.
Study of metallic ore deposits involves the use of structural geology, the study of metamorphism and its processes, as well as understanding metasomatism and other processes related to ore genesis. Ore deposits are delineated by mineral exploration, which uses geochemical prospecting and resource estimation via geostatistics to quantify economic ore bodies; the ultimate aim of this process is mining. See main articles Coal and Petroleum geologyThe study of sedimentology is of prime importance to the delineation of economic reserves of petroleum and coal energy resources. Important publications in economic geology Mineral economics Mineral resource classification Ore Ore genesis Coal Lindgren, W. 1933. Mineral Deposits. 930 pp. McGraw-Hill, New York. U. S. Geological Survey Circular 831, Principles of a Resource/Reserve Classification for Minerals Dill, H. G. 2010. The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium. Earth-Science Reviews Volume 100, pp. 1–420, 2010
Drainage system (geomorphology)
In geomorphology, drainage systems known as river systems, are the patterns formed by the streams and lakes in a particular drainage basin. They are governed by the topography of the land, whether a particular region is dominated by hard or soft rocks, the gradient of the land. Geomorphologists and hydrologists view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff and groundwater flow; the number and shape of the drainage basins found in an area vary and the larger the topographic map, the more information on the drainage basin is available. According to the configuration of the channels, drainage systems can fall into one of several categories known as drainage patterns. Drainage patterns depend on the geology of the land. A drainage system is described as accordant if its pattern correlates to the structure and relief of the landscape over which it flows. Dendritic drainage systems are the most common form of drainage system.
In a dendritic system, there are many contributing streams, which are joined together into the tributaries of the main river. They develop. Dendritic systems form in V-shaped valleys. A parallel drainage system is a pattern of rivers caused by steep slopes with some relief; because of the steep slopes, the streams are swift and straight, with few tributaries, all flow in the same direction. This system forms on uniformly sloping surfaces, for example, rivers flowing southeast from the Aberdare Mountains in Kenya. Parallel drainage patterns form. A parallel pattern develops in regions of parallel, elongate landforms like outcropping resistant rock bands. Tributary streams tend to stretch out in a parallel-like fashion following the slope of the surface. A parallel pattern sometimes indicates the presence of a major fault that cuts across an area of steeply folded bedrock. All forms of transitions can occur between parallel and trellis patterns; the geometry of a trellis drainage system is similar to that of a common garden trellis along a strike valley, smaller tributaries feed into from the steep slopes on the sides of mountains.
These tributaries enter the main river at 90 degree angle, causing a trellis-like appearance of the drainage system. Trellis drainage is characteristic of folded mountains, such as the Appalachian Mountains in North America and in the north part of Trinidad. Rectangular drainage develops on rocks that are of uniform resistance to erosion, but which have two directions of joining at right angles or 90 degrees; the joints are less resistant to erosion than the bulk rock so erosion tends to preferentially open the joints and streams develop along the joints. The result is a stream system in which streams consist of straight line segments with right angle bends and tributaries join larger streams at right angles; this pattern can be found with the Arun River in Nepal. In a radial drainage system, the streams radiate outwards from a central high point. Volcanos display excellent radial drainage, they can sometimes be found on tops of mountains. Other geological features on which radial drainage develops are domes and laccoliths.
On these features the drainage may exhibit a combination of radial patterns. The radical pattern develops when streams flow in different directions from a central peak or dome like structure. In India the Amarkantak range shows the best example of radial drainage pattern; the centripetal drainage system is similar to the radial drainage system, with the only exception that radial drainage flows out versus centripetal drainage flows in. A deranged drainage system is a drainage system in drainage basins where there is no coherent pattern to the rivers and lakes, it happens in areas. The classic example is the Canadian Shield. During the last ice age, the topsoil was scraped off, leaving bare rock; the melting of the glaciers left land with many irregularities of elevation and a great deal of water to collect in the low points, explaining the large number of lakes which are found in Canada. The drainage basins are still sorting themselves out; the system will stabilize. In an annular drainage pattern streams follow a circular or concentric path along a belt of weak rock, resembling in plan a ringlike pattern.
It is best displayed by streams draining a maturely dissected structural dome or basin where erosion has exposed rimming sedimentary strata of varying degrees of hardness, as in the Red Valley, which nearly encircles the domal structure of the Black Hills of South Dakota. Angular drainage patterns form where bedrock joints and faults intersect at more acute angles than rectangular drainage patterns. Angles are both less than 90 degrees. A drainage pattern is described as discordant if it does not correlate to the topography and geology of the area. Discordant drainage patterns are classified into two main types: antecedent and superimposed, while anteposition drainage patterns combine the two. In antecedent drainage, a river's vertical incision ability matches that of land uplift due to tectonic forces. Superimposed drainage develops differently: a drainage system develops on a surface composed of'younger' rocks, but due to denudative activities this surface of younger rocks is removed and the river continues to flo
Passive margin
A passive margin is the transition between oceanic and continental lithosphere, not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins; the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere, created by rifting is known as a passive margin. Passive margins are found at every ocean and continent boundary, not marked by a strike-slip fault or a subduction zone. Passive margins define the region around the Atlantic Ocean, Arctic Ocean, western Indian Ocean, define the entire coasts of Africa, Greenland and Australia, they are found on the east coast of North America and South America, in western Europe and most of Antarctica. East Asia contains some passive margins; this refers to whether a crustal boundary between oceanic lithosphere and continental lithosphere is a plate boundary or not.
Active margins are found on the edge of a continent. These are marked by uplift and volcanic mountain belts on the continental plate. Less there is a strike-slip fault, as defines the southern coastline of W. Africa. Most of the eastern Indian Ocean and nearly all of the Pacific Ocean margin are examples of active margins. While a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. Active subsidence, growth faulting, pore fluid formation and migration are all active processes on passive margins. Passive margins are only passive in plate boundaries. Passive margins consist of both onshore coastal plain and offshore continental shelf-slope-rise triads. Coastal plains are dominated by fluvial processes, while the continental shelf is dominated by deltaic and longshore current processes; the great rivers drain across passive margins. Extensive estuaries are common on mature passive margins. Although there are many kinds of passive margins, the morphologies of most passive margins are remarkably similar.
They consist of a continental shelf, continental slope, continental rise, abyssal plain. The morphological expression of these features are defined by the underlying transitional crust and the sedimentation above it. Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes have a similar morphology. In addition, the shelf break seems to mark the maximum Neogene lowstand, defined by the glacial maxima; the outer continental shelf and slope may be cut by great submarine canyons, which mark the offshore continuation of rivers. At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as the fjords of Norway and Greenland; the main features of passive margins lie underneath the external characters. Beneath passive margins the transition between the continental and oceanic crust is a broad transition known as transitional crust; the subsided continental crust is marked by normal faults.
The faulted crust transitions into oceanic crust and may be buried due to thermal subsidence and the mass of sediment that collects above it. The lithosphere beneath passive margins is known as transitional lithosphere; the lithosphere thins seaward. Different kinds of transitional crust form, depending on how fast rifting occurs and how hot the underlying mantle was at the time of rifting. Volcanic passive margins represent one endmember transitional crust type, the other endmember type is the rifted passive margin. Volcanic passive margins are marked by numerous dykes and igneous intrusions within the subsided continental crust. There are a lot of dykes formed perpendicular to the seaward-dipping lava flows and sills. Igneous intrusions within the crust cause lava flows along the top of the subsided continental crust and form seaward-dipping reflectors. Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation and is due to subsidence of the transitional crust.
Subsidence is caused by gravitational equilibrium, established between the crustal tracts, known as isostasy. Isostasy controls the uplift of the rift flank and the subsequent subsidence of the evolving passive margin and is reflected by changes in heat flow. Heat flow at passive margins changes over its lifespan, high at the beginning and decreasing with age. In the initial stage, the continental crust and lithosphere is stretched and thinned due to plate movement and associated igneous activity; the thin lithosphere beneath the rift allows the upwelling mantle to melt by decompression. Lithospheric thinning allows the asthenosphere to rise closer to the surface, heating the overlying lithosphere by conduction and advection of heat by intrusive dykes. Heating elevates the lower crust and lithosphere. In addition, mantle plumes may cause prodigious igneous activity. Once a mid-oceanic ridge forms and seafoor spreading begins, the original site of rifting is separated into conjugate passive margins and migrates away from the zone of mantle upwelling and heating and cooling begins.
The mantle lithosphere below the thinned and faulted continental oceanic transition cools
Strike-slip tectonics
Strike-slip tectonics is concerned with the structures formed by, the tectonic processes associated with, zones of lateral displacement within the Earth's crust or lithosphere. In the early stages of strike-slip fault formation, displacement within basement rocks produces characteristic fault structures within the overlying cover; this will be the case where an active strike-slip zone lies within an area of continuing sedimentation. At low levels of strain the overall simple shear causes a set of small faults to form; the dominant set, known as R shears, form at about 15° to the underlying fault with the same shear sense. The R shears are linked by a second set, the R' shear that form at about 75° to the main fault trace; these two fault orientations can be understood as conjugate fault sets at 30° to the short axis of the instantaneous strain ellipse associated with the simple shear strain field caused by the displacements applied at the base of the cover sequence. With further displacement the Riedel fault segments will tend to become linked with the development of a further set of shears known as'P shears', which are symmetrical to the R shears with respect to the overall shear direction, until a throughgoing fault is formed.
The somewhat oblique segments will link downwards into the fault at the base of the cover sequence with a helicoidal geometry. In detail many strike-slip faults at surface consist of en echelon and/or braided segments in many cases inherited from formed Riedel shears. In cross-section the displacements are dominantly reverse or normal in type depending on whether the overall fault geometry is transpressional or transtensional; as the faults tend to join downwards onto a single strand in basement, the geometry has led to these being termed flower structure. Fault zones with dominantly reverse faulting are known as positive flowers, those with dominantly normal offsets are known as negative flowers; the identification of such structures where positive and negative flowers are developed on different segments of the same fault, are regarded as reliable indicators of strike-slip. Strike slip duplexes occur at the step over regions of faults, forming a lens shaped near parallel arrays of horses; these occur between two or more large bounding faults which have large displacement.
An idealized strike-slip fault runs in a straight line with a vertical dip and has only horizontal motion, thus there is no change in topography due to motion of the fault. In reality, as strike slip faults become large and developed, their behavior changes and becomes more complex. A long strike slip fault follows a staircase-like trajectory consisting of interspaced fault planes that follow the main fault direction; these sub parallel stretches are isolated by offsets at first, but over long periods of time they can become connected by step overs in order to accommodate the strike slip displacement. In long stretches of strike-slip the fault plane can start to curve, giving rise to structures similar to step overs. Right lateral motion of a strike slip fault at a right step over gives rise to extensional bends characterised by zones of subsidence, local normal faults, pull apart basins. On extensional duplexes, normal faults will accommodate the vertical motion, creating negative relief. Left stepping at a dextral fault generates contractional bends.
On contractional duplex structures, thrust faults will accommodate vertical displacement rather than being folded, as the uplifting process is more energy efficient. Strike slip dulexes are passive structures; each horse has a length that varies from half to twice the spacing between the bounding fault planes. Depending on the properties of the rocks and the fault, the duplexes will have different length ratios and will develop on either major or subtle offsets, although it is possible to observe duplex structures that develop on nearly straight fault segments; because the motion of the duplexes may be heterogeneous, the individual horses can experience a rotation with a horizontal axis, which results in the formation of scissor faults. Scissor faults exhibit normal motion at a thrust motion ant the other end; because strike slip duplexes structures have more horizontal motion than vertical motion, they are best observed on a map rather than a vertical projection, are a good indication that the main fault has a strike slip motion.
An example of strike slip duplexes were observed in New Jersey. Flemington and the Hopewell faults, the two main faults in the region, experienced 3 km of dip slip and over 20 km of strike slip motions to accommodate regional extension, it is possible to trace the lensoidal structures. The lens structures observed in the 3M quarry are 10 meters wide; the main duplex is 30 m in length and other smaller duplexes are present. Areas of strike-slip tectonics are associated with: Mid-ocean ridges are broken into segments offset from each other by transform faults; the active part of the transform links the two ridge segments. Some of these transforms can be large, such as the Romanche fracture zone, whose active portion extends for about 300 km. Transform faults within continental plates include some of the best known examples of strike-slip structures, such as the San Andreas Fault, the Dead Sea Transform, the North Anatolian Fault and the Alpine Fault. Major lateral offsets between large extensional or thrust
Ancient Greek
The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is roughly divided into the Archaic period, Classical period, Hellenistic period, it is succeeded by medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earliest form it resembled Attic Greek and in its latest form it approaches Medieval Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects. Ancient Greek was the language of Homer and of fifth-century Athenian historians and philosophers, it has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article contains information about the Epic and Classical periods of the language. Ancient Greek was a pluricentric language, divided into many dialects; the main dialect groups are Attic and Ionic, Aeolic and Doric, many of them with several subdivisions.
Some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are several historical forms. Homeric Greek is a literary form of Archaic Greek used in the epic poems, the "Iliad" and "Odyssey", in poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects; the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period, they differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not than 1120 BCE, at the time of the Dorian invasion—and that their first appearances as precise alphabetic writing began in the 8th century BCE.
The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians; the Greeks of this period believed there were three major divisions of all Greek people—Dorians and Ionians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-west Greek is the strongest marked and earliest division, with non-west in subsets of Ionic-Attic and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Non-west is called East Greek. Arcadocypriot descended more from the Mycenaean Greek of the Bronze Age.
Boeotian had come under a strong Northwest Greek influence, can in some respects be considered a transitional dialect. Thessalian had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek, spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Most of the dialect sub-groups listed above had further subdivisions equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, Northern Peloponnesus Doric; the Lesbian dialect was Aeolic Greek. All the groups were represented by colonies beyond Greece proper as well, these colonies developed local characteristics under the influence of settlers or neighbors speaking different Greek dialects; the dialects outside the Ionic group are known from inscriptions, notable exceptions being: fragments of the works of the poet Sappho from the island of Lesbos, in Aeolian, the poems of the Boeotian poet Pindar and other lyric poets in Doric.
After the conquests of Alexander the Great in the late 4th century BCE, a new international dialect known as Koine or Common Greek developed based on Attic Greek, but with influence from other dialects. This dialect replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, spoken in the region of modern Sparta. Doric has passed down its aorist terminations into most verbs of Demotic Greek. By about the 6th century CE, the Koine had metamorphosized into Medieval Greek. Ancient Macedonian was an Indo-European language at least related to Greek, but its exact relationship is unclear because of insufficient data: a dialect of Greek; the Macedonian dialect (or l
