Descubre la geología de los diamantes. ¿Cómo se formó el diamante? ¿Cuáles son los tipos de yacimientos? ¿Dónde se encuentran los yacimientos de diamantes?
Seismic waves are waves that travel through or over Earth. They are usually generated by movements of the Earth's tectonic plates (earthquakes) but may also be caused by explosions, volcanoes and landslides. They can tell us much about the Earth's structure.
Caves and Karst The Development of Caves In 1799, as legend has it, a hunter by the name of Houchins was tracking a bear through the woods of Kentucky when the bear suddenly disappeared on a hillslope. Baffled, Houchins plunged through the brambles trying to sight his prey. Suddenly he felt a draft of surprisingly cool air flowing down the slope from uphill. Now curious, Houchins climbed up the hill and found a dark portal into the hillslope beneath a ledge of rocks. Bear tracks were all around was the creature inside? He returned later with a lantern and cautiously stepped into the passageway. After walking a short distance, he found himself in a large, underground room. Houchins had discovered Mammoth Cave, an immense network of natural tunnels and subterranean chambers a walk through the entire network would extend for 630 km. Most large cave networks develop in limestone bedrock because limestone dissolves relatively easily in corrosive groundwater. Generally, the corrosive component in groundwater is dilute carbonic acid (H2CO3), which forms when water absorbs carbon dioxide (CO2) from materials, such as soil, that it has passed through. When carbonic acid comes in contact with calcite (CaCO3) in limestone, it reacts to produce HCO31- and Ca2 ions, which then dissolve. In recent years, geologists have discovered that about 5% of limestone caves around the world form due to reactions with sulfuric-acid-bearing water Carlsbad Caverns in New Mexico serves as an example. Such caves form where limestone overlies strata containing oil, because microbes can convert the sulfur in the oil to hydrogen sulfide gas, which rises and reacts with oxygen to produce sulfuric acid, which in turn eats into limestone and reacts to produce gypsum and CO2 gas. Geologists debate about the depth at which limestone cave networks form. Some limestone dissolves above the water table, but it appears that most cave formation takes place in limestone that lies just below the water table, for in this interval the acidity of the groundwater remains high, the mixture of groundwater and newly added rainwater is not yet saturated with dissolved ions, and groundwater flow is fastest. The association between cave formation and the water table helps explain why openings in a cave network align along the same horizontal plane. The Character of Cave Networks Development of karst, dripstone and flowstone. As we have noted, caves in limestone usually occur as part of a network. Cave networks include rooms, or chambers, which are large, open spaces sometimes with cathedral-like ceilings, and tunnel-shaped or slot-shaped passages. Some chambers may host underground lakes, and some passageways may serve as conduits for underground streams. The shape of the cave network reflects variations in permeability and in the composition of the rock from which the caves formed. Larger open spaces developed where the limestone was most soluble and where groundwater flow was fastest. Thus, in a sequence of strata, caves develop preferentially in the more soluble limestone beds. Passages in cave networks typically follow pre-existing joints, for the joints provide secondary porosity along which groundwater can flow faster (figure above a). Because joints commonly occur in orthogonal systems (consisting of two sets of joints oriented at right angles to each other), passages may form a grid. Precipitation and the Formation of Speleothems When the water table drops below the level of a cave, the cave becomes an open space filled with air. In places where downward percolating groundwater containing dissolved calcite emerges from the rock above the cave and drips from the ceiling, the surface of the cave gradually changes. As soon as this water re-enters the air, it evaporates a little and releases some of its dissolved carbon dioxide. As a result, calcium carbonate (limestone) precipitates out of the water and produces a type of travertine. The various intricately shaped formations that grow in caves by the accumulation of dripstone are called speleothems. Cave explorers (spelunkers) and geologists have developed a detailed nomenclature for different kinds of speleothems (figure above b). Where water drips from the ceiling of the cave, the precipitated limestone builds dripstone. Initially, calcite precipitates around the outside of the drip, forming a delicate, hollow tube called a soda straw. But eventually, the soda straw fills up, and water migrates down the margin of the cone to form a more massive, solid icicle-like cone called a stalactite. Where the drips hit the floor, the resulting precipitate builds an upward-pointing cone called a stalagmite. If the process of dripstone formation in a cave continues long enough, stalagmites merge with overlying stalactites to create travertine columns. In some cases, groundwater flows along the surface of a wall and precipitates to produce drape-like sheets of travertine called flowstone (figure above c). The travertine of caves tends to be translucent and, when lit from behind, glows with an eerie amber light. The Formation of Karst Landscapes Features of Karst landscapes. Limestone bedrock underlies most of the Kras Plateau in Slovenia, along the east coast of the Adriatic Sea. The name kras, which means rocky ground, is apt because this region includes abundant rock exposures (figure above a). Geologists refer to regions such as the Kras Plateau, where surface landforms develop when limestone bedrock dissolves both at the surface and in underlying cave networks, as karst landscapes or karst terrains from the Germanized version of kras. Karst landscapes typically display a number of distinct landforms. Perhaps the most widespread are sinkholes, circular depressions that form either when the ground collapses into an underground cave below (as we discussed early in this chapter) or when surface bedrock dissolves in acidic water on the floor of a bog or pond. Not all of the caves or passageways beneath a karst landscape have collapsed, and this situation leads to unusual drainage patterns. Specifically, where surface streams intersect cracks (joints) or holes that link to caverns or passageways below, the water cascades downward into the subsurface and disappears (figure above b). Such disappearing streams may flow through passageways underground and re-emerge from a cave entrance downstream. In cases where the ground collapses over a long, joint-controlled passage, sinkholes may be elongate and canyon-like. Remnants of cave roofs remain as natural bridges. Ridges or walls between adjacent sinkholes tend to be steep-sided. Over time, the walls erode, leaving only jagged, isolated spires a karst landscape dominated by such spires is called tower karst. The surreal collection of pinnacles constituting the tower karst landscape in the Guilin region of China inspired generations of artists who portray them on scroll paintings (figure below). Tower karst forms a spectacular landscape in southern China. Karst landscapes form in a series of stages (figure below a–c). The progressive formation of caves and a karst landscape. The establishment of a water table in limestone: The story of a karst landscape begins after the formation of a thick interval of limestone in which the water table lies underground. The formation of a cave network: Once the water table has been established, dissolution begins and a cave network develops. A drop in the water table: If the water table later becomes lower, either because of a decrease in rainfall or because nearby rivers downcut and drain the region, newly formed caves dry out. Downward-percolating water emerges from the roofs of the caves; dripstone and flowstone precipitate. Roof collapse: If rocks fall off the roof of a cave for a long time, the roof eventually collapses. Such collapse creates sinkholes and troughs, leaving behind hills, ridges, and natural bridges. Life in Caves Despite their lack of light, caves are not sterile, lifeless environments. Caves that are open to the air provide a refuge for bats as well as for various insects and spiders. Similarly, fish and crustaceans enter caves where streams flow in or out. Species living in caves have evolved some unusual characteristics. For example, cave fish lose their pigment and in some cases their eyes. Recently, explorers discovered caves in Mexico in which warm, mineral-rich groundwater currently flows. Colonies of bacteria metabolize sulphur-containing minerals in this water and create thick mats of living ooze in the complete darkness of the cave. Long gobs of this bacteria slowly drip from the ceiling. Because of the mucus-like texture of these drips, they have come to be known as “snotites”. Figures credited to Stephen Marshak.
La energía en un punto en la plataforma continental es una función de la profundidad del agua. Por lo tanto, con cualquier cambio en el nivel del mar, la profundidad del agua en un punto en el esta…
El gas de esquisto podría impulsar la independencia energética de Europa y rebajar el coste de la energía. Pero existen preocupaciones sobre los efectos que podría causar en la salud humana y en el medio ambiente. Los eurodiputados votan este 9 de octubre si la prospección y la explotación de gas de esquisto deberían someterse a evaluaciones obligatorias de impacto medio ambiental. Consulte nuestra infografía para informarse sobre la extracción de este gas.
Take a plunge under the surface of the earth! With this worksheet as your guide, you may explore the geological processes of our planet.
A continuación os dejo los materiales de este segundo tema de Geograía de 2º de Bachiller y que aparecen en vuestro libro. También os dejo ...
Areas of extension are affiliated with horizontal divergent stress and are found in association with constructive or passive plate boundaries and in intra-plate settings. Thus, extensional stress regimes either are associated with subsidence and basin formation (in intra-plate settings) or characterise active break-up of continents (along constructive or passive margins). Although the conditions for development of petroleum systems in areas of active spreading may be meagre, the remnants of the earlier stages of break-up, now situated in passive margins settings, fulfil all the requirements that characterise productive petroleum provinces. This is because such tectonic regimes have undergone crustal thinning and associated subsidence, which involves all the processes essential for petroleum to be generated, trapped and accumulated in sufficient volumes and concentrations for petroleum fields to be commercially interesting. Accordingly,such settings frequently display an attractive combination and distribution of source, reservoir and cap rocks, structural and stratigraphic traps and the conditions for maturation, expulsion, migration and accumulation of hydrocarbons. Extensional Basins The formation of extensional basins may be seen as the first stage of the Wilson Cycle, which begins with thinning, stretching and rifting of the continental crust followed by continental break-up and mid-oceanic spreading. The concept of the Wilson cycle predicts that this process sometimes becomes reversed, causing closure of the ocean, collision between the adjacent continental plates, and hence the construction of a mountain chain along the zone of collision. The junction between the continental plates defines the suture between the two. The major stages in the Wilson Cycle. If we use the present North Atlantic as one example, the highly hydrocarbon-rich northern North Sea basin system is situated in a passive continental margin configuration, where the extensional basin system developed during continental break-up. In contrast, Iceland, where petroleum resources are less abundant, is situated on the top of the mid-oceanic spreading ridge. However, if one looks more closely at the structural configuration at depth, one finds that the northern North Sea basin system, which includes the Viking Graben that developed in Jurassic-Cretaceous times, is underlain by an older (Permo-Triassic) basin system. The Permo-Triassic basin system is in turn superimposed on the even older Caledonian suture, which was subsequently affected by gravitational collapse in Devonian times, representing the last stage in a previous Wilson Cycle. Basic configuration of (a) pure shear and (b) simple shear extensional basins. There are several models for the lithospheric configurations that accompany extensional crustal thinning, the end members of which are the "pure shear" (symmetrical) and "simple shear" (asymmetrical) models. It should be noticed that these models are not necessarily mutually exclusive; we can find basin systems that display elements from more than one model, such as the "delamination model". The "pure-shear model" for extensional crustal thinning has become the most frequently cited in geosciences in modern times. This model assumes thinning of the weak lower crust/lower lithosphere by pure shear, and hence is characterised by the development of a symmetrical configuration. The pure-shear extension of the ductile lower crust is accompanied by thinning of the upper crust by brittle faulting and subsequent development and rotation of fault blocks. In this context it is possible to separate the active stretching stage, which is associated with fault controlled thinning of the upper crust, and later subsidence controlled by thermal processes. As a response to extension, the crust and upper mantle lithosphere becomes thinned and, promoted by extensional faulting, the basin floor will subside quickly. This implies that deeply seated warm rocks are transferred upwards in the lithosphere so that the isotherms in the thinned area become elevated and the thermal gradients become steepened accordingly. These deep processes influence the relief of the basin floor because heating causes rock volumes to expand and elastic, quasi-plastic and isostatic adjustments to occur simultaneously at lithospheric, basin (e.g. by uplift of the basin margins) and fault block scales. In the next stage of development (the post-rift stage), the basin will continue to subside due to a combination of thermal contraction, sediment compaction and sediment loading. This sounds complex, but luckily these processes are well understood and can be modelled with good accuracy on the basis of the algorithms which supplied with additional modelling tools, developed particularly in the late 1990s. For modelling purposes and for the analysis of extensional basins with respect to petroleum exploration, three stages of development can be distinguished. Three major stages in the development of extensional. The pre-rift stage is characterised by gentle flexuring and fracturing of the lithosphere. In some rifts we see the development of a gentle bulge, caused by mantle doming and associated warming and hence expansion of the lithosphere. In other cases, a gentle subsidence, defining a broad, shallow basin is seen, caused by mild extension of the cold (not-yet-heated) lithosphere. In both cases, the lithosphere is prone to develop steep fractures on a crustal or even lithospheric scale. These fractures have the capacity to accommodate magma, generating dikes. Regarding hydrocarbon reservoir potential characterising the pre-rift stage, sand deposits are likely to be sheet-like and relatively thin, with few structural traps developing at this stage. Sediment transport is mainly transverse to the basin axis, but quite homogeneous due to lack of pronounced gradients in the basin. The marginal sediment transport system is prone to act in concert with the axial transport system, feeding the latter with sediments. This may consist of braided or meandering river systems, depending on factors like axial basin gradient and climate. Since most rifts are generated by break-up of continents, a terrestrial depositional environment would be most common for the pre-rift stage, so that source rocks and cap rocks, which are mostly of marine depositional origin, may be scarce. There are, however, numerous examples of both source rocks and cap rocks of terrestrial origin. In the active stretching stage extension, and hence also subsidence, accelerate. Simultaneously, heat input increases due to upheaval of hot layers of the mantle lithosphere. The steep fractures generated in the pre-rift stage will not be able to accommodate the extension and a new set of low-angle planar or listric faults will be activated, separating fault blocks that are detached from the lower crust by a sub-horizontal zone of weakness. Gliding on the system of detachments, the fault blocks and their internal beds will rotate away from the basin axis. From the view of the petroleum explorationist, the active stretching stage deserves particular attention because of the variety of structural and stratigraphic traps that may develop. This stage is also characterised by a complex sediment distribution system that is likely to produce a variety of lithofacies due to the increasing topographic relief associated with high fault activity. The marine transgression that commonly follows the increased subsidence of the basin floor also contributes to this variety in sedimentary facies. Sand that is eroded from the high-standing parts of the basin (basin shoulders and crests of rotated fault blocks) may be trapped in lows in various structural positions and these units are likely later to be covered by transgressive marine sediment accumulations. The sediment transport system in the active stretching stage is likely to be dominated by complex transverse and locally bidirectional fluvial systems that are strongly influenced by the elongated, rotated fault blocks, generating axis parallel transport in segments along the basin margin. The central part of the basin may be less complex and axial-parallel sediment transport would prevail there. In the thermal subsidence stage, thermal contraction of the lithosphere dominates the basin subsidence pattern. Because solids typically contract during cooling, the parts of the basin that have experienced the strongest extension (those that have been thinned the most and hence heated the most) will contract and subside more than other parts. In a pure-shear configuration this is most likely to be the central segment running along the basin axis. This means that the rotation of strata upwards away from the basin axis becomes reversed so that strata begin to rotate downwards towards the basin axis. This rotation is strengthened by sediment loading and compaction (thickest sequence in the central part of the basin). The transverse sediment transport will persist during the thermal subsidence stage, while the basin floor becomes gradually smoothed. An axial transport system may also still be active, but is likely to become less pronounced through this stage of development. Depending on the balance between subsidence and sediment input, the water depth will vary from one basin to another, but the depositional environment is likely to be marine and the central part of the basin may attain great water depth (thousands of metres). The fault systems that dominated the basin floor geometry during the active stretching stage are now quiescent, and stratigraphic hydrocarbon traps rather than structural ones are likely to be the most common. Syn-rift to post-rift transition. The pure-shear model predicts that a simple geometrical change of the outline of extensional basins will accompany the transition from the syn- to the post-rift stage. In this model the margins of the relatively narrow, steep-walled rift, which traps the syn-rift sediments, become overstepped at the syn- to post-rift transition. This implies that the basin becomes wider and the rate of subsidence decreases asymptotically during the following post-rift stage. Thus, one defines the beginning of the post-rift development as the stage by which the syn-rift faults become inactive and subsidence becomes controlled dominantly by thermal contraction and sediment loading. In practical terms, the identification of this stage in the basin development is not trivial, because the transition is frequently not synchronous all over the basin, and the criteria for identifying the transition in reflection seismic data are not always well constrained. To overcome this problem, the syn- to post-rift transition should be defined more precisely as the point in time when net heat out of the system is greater than net heat into the system. It is recognised that a lateral heat flow gradient commonly exists perpendicular to the basin axis. This implies that the area closest to the basin axis, which coincides with the area of greatest thinning, is also the part of the basin displaying the highest heat flux at the end of the syn-rift stage. The lithosphere beneath the central part of the basin will accordingly undergo the greatest vertical contraction during the post-rift stage. The enhanced subsidence at the basin axis is further enhanced in cases where the basin is filled by sediments, creating an extra load and also a greater total compaction. Hence, the syn-topost rift transition coincides with a regional shift in tilt from fault block rotation away from the graben axis during the syn-rift stage to tilting directed towards the basin axis during the post-rift development. This change is due to a shift from bulk thermal expansion to bulk thermal contraction of the lithosphere and is in most cases clearly distinguishable in reflection seismic data. It needs to be emphasised that the syn- to post-rift transition is unlikely to occur simultaneously throughout the entire basin. This is due to differences in structural configurations, e.g. the existence of graben units, and thermal inhomogeneities associated with variable stretching both along and transverse to the basin axis. The entire Cretaceous sequence of the northern North Sea is included in the post-rift development sensu stricto. Furthermore, analysis of the basin topography permits three sub-stages to be identified within the framework of the post-rift development: the incipient, the middle and the mature post-rift stages. The configuration at the syn-rift/post-rift transition is treated separately in the present analysis. In the analysis of basin subsidence it is important to remember that in addition to the effects of fault-related subsidence and thermal expansion and contraction, the basin’s subsidence is affected by elastic deformation and isostasy, and in many cases also by extra-basinal stress. The simple-shear model for extensional basins is in considerable geometrical and mechanical contrast to the pure-shear model for extensional basins in that the simple-shear model assumes that extension is concentrated along one or several inclined fault zone(s) affecting the entire crust. Still, when thermo-tectonic and isostatic responses are concerned the principles are similar to those of the pure-shear model. The simple-shear model is based on observations in the Basin-and-Range of North America. The Basin and Range basin system displays a particular geometry in that the lithosphere is extended to the degree that the lower crust, described as a metamorphic core complex, has become uplifted and exposed in the central part of the basin. The asymmetrical configuration of the basin particularly influences the pattern of isostatic response to extension. An important factor is the relative thickness of the upper mantle/lithosphere. This is because the lower crust commonly is denser than the upper astenosphere, causing large-scale contrasts in differential subsidence and uplift across the basin. Superimposed on this are more local isostatic effects, associated with contrasting thicknesses of layers with different densities and the topography of the basin. Since the same tectono-thermal principles that apply for the pure shear basinal so are valid for simple shear basins, the main basin stages and the conditions for hydrocarbon generation and entrapment are also the same. Even though the simple-shear model was inspired by analysis of the Basin-and-Range basin system it has proved relevant for many other basins too, suggesting that simple shear is a common component in the formation of basins. Model of the Viking Graben, displaying elements of pure and simple shear. The delamination model can be seen as a combination of the simple- and pure-shear models. In this case the upper and middle crust extends by simple shear. At depth, the master fault flattens and merges with the lower crust, which becomes thinned by pure shear. TheViking Graben of the northern North Sea seems to have a configuration that fits the delamination model. Also in this case, the thermo mechanical pure-shear model applies and, with some modifications, can be used to model the basin development. However, the delamination model makes it necessary to take into account an additional variable parameter, namely that the two parts of the lithosphere situated above and beneath the delamination surface have undergone different amounts of extension.
Faults are much more complex and compound features that can accommodate large amounts of strain in the upper crust. The term fault is used in different ways, depending on geologist and context. A simple and traditional definition states: A fault is any surface or narrow zone with visible shear displacement along the zone. This definition is almost identical to that of a shear fracture, and some geologist use the two terms synonymously. Sometimes geologists even refer to shear fractures with millimeter- to centimeter-scale offsets as microfaults. However, most geologists would restrict the term shear fracture to small-scale structures and reserve the term fault for more composite structures with offsets in the order of a meter or more. The thickness of a fault is another issue. Faults are often expressed as planes and surfaces in both oral and written communication and sketches, but close examination of faults reveals that they consist of fault rock material and subsidiary brittle structures and therefore have a definable thickness. However, the thickness is usually much smaller than the offset and several orders of magnitude less than the fault length. Whether a fault should be considered as a surface or a zone largely depends on the scale of observation, objectives and need for precision. Faults tend to be complex zones of deformation, consisting of multiple slip surfaces, subsidiary fractures and perhaps also deformation bands. This is particularly apparent when considering large faults with kilometerscale offsets. Such faults can be considered as single faults on a map or a seismic line, but can be seen to consist of several small faults when examined in the field. In other words, the scale dependency, which haunts the descriptive structural geologist, is important. This has led most geologists to consider a fault as a volume of brittlely deformed rock that is relatively thin in one dimension: A fault is a tabular volume of rock consisting of a central slip surface or core, formed by intense shearing, and a surrounding volume of rock that has been affected by more gentle brittle deformation spatially and genetically related to the fault. The term fault may also be connected to deformation mechanisms (brittle or plastic). In a very informal sense, the term fault covers both brittle discontinuities and ductile shear zones dominated by plastic deformation. This is sometimes implied when discussing large faults on seismic or geologic sections that penetrate much or all of the crust. The term brittle fault (as opposed to ductile shear zone) can be used if it is important to be specific with regard to deformation mechanism. In most cases geologists implicitly restrict the term fault to slip or shear discontinuities dominated by brittle deformation mechanisms, rendering the term brittle fault redundant: A fault is a discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms. By discontinuity we are here primarily referring to layers, i.e. faults cut off rock layers and make them discontinuous. However, faults also represent mechanical and displacement discontinuities. A kinematic definition, particularly useful for experimental work and GPS-monitoring of active faults can therefore be added: Faults appear as discontinuities on velocity or displacement field maps and profiles. The left blocks in the undeformed map a) and profile (b) are fixed during the deformation. The result is abrupt changes in the displacement field (arrows) across faults. A fault is a discontinuity in the velocity or displacement field associated with deformation. Faults differ from shear fractures because a simple shear fracture cannot expand in its own plane into a larger structure. In contrast, faults can grow by the creation of a complex process zone with numerous small fractures, some of which link to form the fault slip surface while the rest are abandoned. Geometry of faults Normal (a), strike-slip (sinistral) (b) and reverse (c) faults. These are end-members of a continuous spectrum of oblique faults. The stereonets show the fault plane (great circle) and the displacement vector (red point). Non-vertical faults separate the hanging wall from the underlying footwall. Where the hanging wall is lowered or down thrown relative to the footwall, the fault is a normal fault. The opposite case, where the hanging wall is up thrown relative to the footwall, is a reverse fault. If the movement is lateral, i.e. in the horizontal plane, then the fault is a strike-slip fault. Strike-slip faults can be sinistral (left-lateral) or dextral (right-lateral) (from the Latin words sinister and dexter, meaning left and right, respectively). Although some fault dip ranges are more common than others, with strike-slip faults typically occurring as steep faults and reverse faults commonly having lower dips than normal faults, the full range from vertical to horizontal faults is found in naturally deformed rocks. If the dip angle is less than 30 the fault is called a low-angle fault, while steep faults dip steeper than 60 . Low-angle reverse faults are called thrust faults, particularly if the movement on the fault is tens or hundreds of kilometres. Listric normal fault showing very irregular curvature in the sections perpendicular to the slip direction. These irregularities can be thought of as large grooves or corrugations along which the hanging wall can slide. A fault that flattens downward is called a listric fault, while downward-steepening faults are sometimes called antilistric. The terms ramps and flats, originally from thrust fault terminology, are used for alternating steep and sub-horizontal portions of any fault surface. For example, a fault that varies from steep to flat and back to steep again has a ramp-flat-ramp geometry. Irregularities are particularly common in the section perpendicular to the fault slip direction. For normal and reverse faults this means curved fault traces in map view, as can be seen from the faults of the extensional oil field. Irregularities in this section cause no conflict during fault slippage as long as the axes of the irregularities coincide with the slip vector. Where irregularities also occur in the slip direction, the hanging wall and/or footwall must deform. For example, a listric normal fault typically creates a hanging-wall rollover. The main faults in the North Sea Gullfaks oil field show high degree of curvature in map view and straight traces in the vertical sections (main slip direction). Red lines represent some of the well paths in this field. A fault can have any shape perpendicular to the slip direction, but non-linearity in the slip direction generates space problems leading to hanging or footwall strain. The term fault zone traditionally means a series of sub-parallel faults or slip surfaces close enough to each other to define a zone. The width of the zone depends on the scale of observation – it ranges from centimetres or meters in the field to the order of a kilometre or more when studying large-scale faults such as the San Andreas Fault. The term fault zone is now also used inconsistently about the central part of the fault where most or all of the original structures of the rock are obliterated, or about the core and the surrounding deformation zone associated with the fault. This somewhat confusing use is widespread in the current petroleum related literature, so any use of the term fault zone requires clarification. A horst (a), symmetric graben (b) and asymmetric graben (c), also known as a half-graben. Antithetic and synthetic faults are shown. Two separate normal faults dipping toward each other create a down thrown block known as a graben. Normal faults dipping away from each other create an up thrown block called a horst. The largest faults in a faulted area, called master faults, are associated with minor faults that may be antithetic or synthetic. An antithetic fault dips toward the master fault, while a synthetic fault dips in the same direction as the master fault. These expressions are relative and only make sense when minor faults are related to specific larger-scale faults. Displacement, slip and separation Illustration of a normal fault affecting a tilted layer. The fault is a normal fault with a dextral strike-slip component (a), but appears as a sinistral fault in map view (b, which is the horizontal section at level A). (c) and (d) show profiles perpendicular to fault strike (c) and in the (true) displacement direction (d). Displacement, slip and separation The vector connecting two points that were connected prior to faulting indicates the local displacement vector or net slip direction. Ideally, a strike-slip fault has a horizontal slip direction while normal and reverse faults have displacement vectors in the dip direction. In general, the total slip that we observe on most faults is the sum of several increments (earthquakes), each with its own individual displacement or slip vector. The individual slip events may have had different slip directions. We are now back to the difference between deformation sensu stricto, which only relates the undeformed and deformed states, and deformation history. In the field we could look for traces of the slip history by searching for such things as multiple striations. Classification of faults based on the dip of the fault plane and the pitch, which is the angle between the slip direction (displacement vector) and the strike. A series of displacement vectors over the slip surface gives us the displacement field or slip field on the surface. Striations, kinematic indicators and offset of layers provide the field geologist with information about direction, sense and amount of slip. Many faults show some deviation from true dip-slip and strike-slip displacement in the sense that the net slip vector is oblique. Such faults are called oblique-slip faults. The degree of obliquity is given by the pitch (also called rake), which is the angle between the strike of the slip surface and the slip vector (striation). Unless we know the true displacement vector we may be fooled by the offset portrayed on an arbitrary section through the faulted volume, be it a seismic section or an outcrop. The apparent displacement that is observed on a section or plane is called the (apparent) separation. Horizontal separation is the separation of layers observed on a horizontal exposure or map, while the dip separation is that observed in a vertical section. In a vertical section the dip separation can be decomposed into the horizontal and vertical separation. Note that this horizontal separation is different from. These two separations recorded in a vertical section are more commonly referred to as heave (horizontal component) and throw (vertical component). Only a section that contains the true displacement vector shows the true displacement or total slip on the fault. The relationship between a single fault, a mapped surface and its two fault cutoff lines. Such structure contour maps are used extensively in the oil industry where they are mainly based on seismic reflection data. A fault that affects a layered sequence will, in three dimensions, separate each surface (stratigraphic interface) so that two fault cut off lines appear. If the fault is non-vertical and the displacement vector is not parallel to the layering, then a map of the faulted surface will show an open space between the two cut-off lines. The width of the open space, which will not have any contours, is related to both the fault dip and the dip separation on the fault. Further, the opening reflects the heave (horizontal separation) seen on vertical sections across the fault. Stratigraphic separation (a) Missing section in vertical wells (well C) always indicates normal faults (assuming constant stratigraphy). (b) Repeated section (normally associated with reverse faults) occurs where the normal fault is steeper than the intersecting well bore (well G). Drilling through a fault results in either a repeated section or a missing section at the fault cut (the point where the wellbore intersects the fault). For vertical wells it is simple: normal faults omit stratigraphy (figure a), while reverse faults cause repeated stratigraphy in the well. For deviated wells where the plunge of the well bore is less than the dip of the fault, such as the well G (figure b), stratigraphic repetition is seen across normal faults. The general term for the stratigraphic section missing or repeated in wells drilled through a fault is stratigraphic separation. Stratigraphic separation, which is a measure of fault displacement obtainable from wells in subsurface oil fields, is equal to the fault throw if the strata are horizontal. Most faulted strata are not horizontal, and the throw must be calculated or constructed. Credits: Haakon Fossen (Structural Geology)
Con el incremento de las temperaturas, la ''Puerta al infierno'' se está abriendo en Siberia. ¿Por qué ha aparecido este agujero?
AS- Birth, development , migration and death of a meander. Click on this link to download this post. Meanders start when friction with the channel bed and banks causes turbulence in the water flow.…
Faults are much more complex and compound features that can accommodate large amounts of strain in the upper crust. The term fault is used in different ways, depending on geologist and context. A simple and traditional definition states: A fault is any surface or narrow zone with visible shear displacement along the zone. This definition is almost identical to that of a shear fracture, and some geologist use the two terms synonymously. Sometimes geologists even refer to shear fractures with millimeter- to centimeter-scale offsets as microfaults. However, most geologists would restrict the term shear fracture to small-scale structures and reserve the term fault for more composite structures with offsets in the order of a meter or more. The thickness of a fault is another issue. Faults are often expressed as planes and surfaces in both oral and written communication and sketches, but close examination of faults reveals that they consist of fault rock material and subsidiary brittle structures and therefore have a definable thickness. However, the thickness is usually much smaller than the offset and several orders of magnitude less than the fault length. Whether a fault should be considered as a surface or a zone largely depends on the scale of observation, objectives and need for precision. Faults tend to be complex zones of deformation, consisting of multiple slip surfaces, subsidiary fractures and perhaps also deformation bands. This is particularly apparent when considering large faults with kilometerscale offsets. Such faults can be considered as single faults on a map or a seismic line, but can be seen to consist of several small faults when examined in the field. In other words, the scale dependency, which haunts the descriptive structural geologist, is important. This has led most geologists to consider a fault as a volume of brittlely deformed rock that is relatively thin in one dimension: A fault is a tabular volume of rock consisting of a central slip surface or core, formed by intense shearing, and a surrounding volume of rock that has been affected by more gentle brittle deformation spatially and genetically related to the fault. The term fault may also be connected to deformation mechanisms (brittle or plastic). In a very informal sense, the term fault covers both brittle discontinuities and ductile shear zones dominated by plastic deformation. This is sometimes implied when discussing large faults on seismic or geologic sections that penetrate much or all of the crust. The term brittle fault (as opposed to ductile shear zone) can be used if it is important to be specific with regard to deformation mechanism. In most cases geologists implicitly restrict the term fault to slip or shear discontinuities dominated by brittle deformation mechanisms, rendering the term brittle fault redundant: A fault is a discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms. By discontinuity we are here primarily referring to layers, i.e. faults cut off rock layers and make them discontinuous. However, faults also represent mechanical and displacement discontinuities. A kinematic definition, particularly useful for experimental work and GPS-monitoring of active faults can therefore be added: Faults appear as discontinuities on velocity or displacement field maps and profiles. The left blocks in the undeformed map a) and profile (b) are fixed during the deformation. The result is abrupt changes in the displacement field (arrows) across faults. A fault is a discontinuity in the velocity or displacement field associated with deformation. Faults differ from shear fractures because a simple shear fracture cannot expand in its own plane into a larger structure. In contrast, faults can grow by the creation of a complex process zone with numerous small fractures, some of which link to form the fault slip surface while the rest are abandoned. Geometry of faults Normal (a), strike-slip (sinistral) (b) and reverse (c) faults. These are end-members of a continuous spectrum of oblique faults. The stereonets show the fault plane (great circle) and the displacement vector (red point). Non-vertical faults separate the hanging wall from the underlying footwall. Where the hanging wall is lowered or down thrown relative to the footwall, the fault is a normal fault. The opposite case, where the hanging wall is up thrown relative to the footwall, is a reverse fault. If the movement is lateral, i.e. in the horizontal plane, then the fault is a strike-slip fault. Strike-slip faults can be sinistral (left-lateral) or dextral (right-lateral) (from the Latin words sinister and dexter, meaning left and right, respectively). Although some fault dip ranges are more common than others, with strike-slip faults typically occurring as steep faults and reverse faults commonly having lower dips than normal faults, the full range from vertical to horizontal faults is found in naturally deformed rocks. If the dip angle is less than 30 the fault is called a low-angle fault, while steep faults dip steeper than 60 . Low-angle reverse faults are called thrust faults, particularly if the movement on the fault is tens or hundreds of kilometres. Listric normal fault showing very irregular curvature in the sections perpendicular to the slip direction. These irregularities can be thought of as large grooves or corrugations along which the hanging wall can slide. A fault that flattens downward is called a listric fault, while downward-steepening faults are sometimes called antilistric. The terms ramps and flats, originally from thrust fault terminology, are used for alternating steep and sub-horizontal portions of any fault surface. For example, a fault that varies from steep to flat and back to steep again has a ramp-flat-ramp geometry. Irregularities are particularly common in the section perpendicular to the fault slip direction. For normal and reverse faults this means curved fault traces in map view, as can be seen from the faults of the extensional oil field. Irregularities in this section cause no conflict during fault slippage as long as the axes of the irregularities coincide with the slip vector. Where irregularities also occur in the slip direction, the hanging wall and/or footwall must deform. For example, a listric normal fault typically creates a hanging-wall rollover. The main faults in the North Sea Gullfaks oil field show high degree of curvature in map view and straight traces in the vertical sections (main slip direction). Red lines represent some of the well paths in this field. A fault can have any shape perpendicular to the slip direction, but non-linearity in the slip direction generates space problems leading to hanging or footwall strain. The term fault zone traditionally means a series of sub-parallel faults or slip surfaces close enough to each other to define a zone. The width of the zone depends on the scale of observation – it ranges from centimetres or meters in the field to the order of a kilometre or more when studying large-scale faults such as the San Andreas Fault. The term fault zone is now also used inconsistently about the central part of the fault where most or all of the original structures of the rock are obliterated, or about the core and the surrounding deformation zone associated with the fault. This somewhat confusing use is widespread in the current petroleum related literature, so any use of the term fault zone requires clarification. A horst (a), symmetric graben (b) and asymmetric graben (c), also known as a half-graben. Antithetic and synthetic faults are shown. Two separate normal faults dipping toward each other create a down thrown block known as a graben. Normal faults dipping away from each other create an up thrown block called a horst. The largest faults in a faulted area, called master faults, are associated with minor faults that may be antithetic or synthetic. An antithetic fault dips toward the master fault, while a synthetic fault dips in the same direction as the master fault. These expressions are relative and only make sense when minor faults are related to specific larger-scale faults. Displacement, slip and separation Illustration of a normal fault affecting a tilted layer. The fault is a normal fault with a dextral strike-slip component (a), but appears as a sinistral fault in map view (b, which is the horizontal section at level A). (c) and (d) show profiles perpendicular to fault strike (c) and in the (true) displacement direction (d). Displacement, slip and separation The vector connecting two points that were connected prior to faulting indicates the local displacement vector or net slip direction. Ideally, a strike-slip fault has a horizontal slip direction while normal and reverse faults have displacement vectors in the dip direction. In general, the total slip that we observe on most faults is the sum of several increments (earthquakes), each with its own individual displacement or slip vector. The individual slip events may have had different slip directions. We are now back to the difference between deformation sensu stricto, which only relates the undeformed and deformed states, and deformation history. In the field we could look for traces of the slip history by searching for such things as multiple striations. Classification of faults based on the dip of the fault plane and the pitch, which is the angle between the slip direction (displacement vector) and the strike. A series of displacement vectors over the slip surface gives us the displacement field or slip field on the surface. Striations, kinematic indicators and offset of layers provide the field geologist with information about direction, sense and amount of slip. Many faults show some deviation from true dip-slip and strike-slip displacement in the sense that the net slip vector is oblique. Such faults are called oblique-slip faults. The degree of obliquity is given by the pitch (also called rake), which is the angle between the strike of the slip surface and the slip vector (striation). Unless we know the true displacement vector we may be fooled by the offset portrayed on an arbitrary section through the faulted volume, be it a seismic section or an outcrop. The apparent displacement that is observed on a section or plane is called the (apparent) separation. Horizontal separation is the separation of layers observed on a horizontal exposure or map, while the dip separation is that observed in a vertical section. In a vertical section the dip separation can be decomposed into the horizontal and vertical separation. Note that this horizontal separation is different from. These two separations recorded in a vertical section are more commonly referred to as heave (horizontal component) and throw (vertical component). Only a section that contains the true displacement vector shows the true displacement or total slip on the fault. The relationship between a single fault, a mapped surface and its two fault cutoff lines. Such structure contour maps are used extensively in the oil industry where they are mainly based on seismic reflection data. A fault that affects a layered sequence will, in three dimensions, separate each surface (stratigraphic interface) so that two fault cut off lines appear. If the fault is non-vertical and the displacement vector is not parallel to the layering, then a map of the faulted surface will show an open space between the two cut-off lines. The width of the open space, which will not have any contours, is related to both the fault dip and the dip separation on the fault. Further, the opening reflects the heave (horizontal separation) seen on vertical sections across the fault. Stratigraphic separation (a) Missing section in vertical wells (well C) always indicates normal faults (assuming constant stratigraphy). (b) Repeated section (normally associated with reverse faults) occurs where the normal fault is steeper than the intersecting well bore (well G). Drilling through a fault results in either a repeated section or a missing section at the fault cut (the point where the wellbore intersects the fault). For vertical wells it is simple: normal faults omit stratigraphy (figure a), while reverse faults cause repeated stratigraphy in the well. For deviated wells where the plunge of the well bore is less than the dip of the fault, such as the well G (figure b), stratigraphic repetition is seen across normal faults. The general term for the stratigraphic section missing or repeated in wells drilled through a fault is stratigraphic separation. Stratigraphic separation, which is a measure of fault displacement obtainable from wells in subsurface oil fields, is equal to the fault throw if the strata are horizontal. Most faulted strata are not horizontal, and the throw must be calculated or constructed. Credits: Haakon Fossen (Structural Geology)
The Southwestern United States is a fantastic place to see geology. The arid climate means that that the rocks are not covered by vegetation. In many places, especially the national parks, the form…
Cours de reconnaissance géotechnique La Géotechnique est l'ensemble des activités liées aux applications de la Mécanique des Sols, d...
In one of our novice activities, we demonstrate to children that Scientists know where Plate Tectonic Boundaries are found. This discovery was made by plotting and viewing a global map of earthqua…
How to Identify a Mineral: Identifying minerals is like playing a sport. You are given a specific set of principles or rules to follow. To be good at a sport like basketball, for instance, one must not only know the rules, but he/she must also practice. The same concepts appl…
Learn the science of volcanic eruption with this cutaway diagram
