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39-537: Rifting leading to the breakup of supercontinent Rodinia , which had formed in the mid-Stenian, occurred during this period, starting from 900 to 850 Mya. The first putative metazoan ( animal ) fossils are dated to the middle to late Tonian ( c. 890-800 Mya). The fossils of Otavia antiqua , which has been described as a primitive sponge by its discoverers and numerous other scholars, date back to about 800 mya. Even earlier sponge-like fossils have been reported in reefs dating back to 890 million years before

78-449: A lacustrine environment or in a restricted marine environment, although not all rifts contain such sequences. Reservoir rocks may be developed in pre-rift, syn-rift and post-rift sequences. Effective regional seals may be present within the post-rift sequence if mudstones or evaporites are deposited. Just over half of estimated oil reserves are found associated with rifts containing marine syn-rift and post-rift sequences, just under

117-433: A certain depth range - the so-called alternating zone , where brittle fracturing and plastic flow coexist. The main reason for this is found in the usually heteromineral composition of rocks, with different minerals showing different responses to applied stresses (for instance, under stress quartz reacts plastically long before feldspars do). Thus differences in lithology , grain size , and preexisting fabrics determine

156-414: A different rheological response. Yet other, purely physical factors, influence the changeover depth as well, including: In Scholz's model for a quartzo-feldspathic crust (with a geotherm taken from Southern California), the brittle–semibrittle transition starts at about 11 km depth with an ambient temperature of 300 °C. The underlying alternating zone then extends to roughly 16 km depth with

195-403: A kind of orogeneses in extensional settings, which is referred as to rifting orogeny. Once rifting ceases, the mantle beneath the rift cools and this is accompanied by a broad area of post-rift subsidence. The amount of subsidence is directly related to the amount of thinning during the rifting phase calculated as the beta factor (initial crustal thickness divided by final crustal thickness), but

234-438: A mid-oceanic ridge and a set of conjugate margins separated by an oceanic basin. Rifting may be active, and controlled by mantle convection . It may also be passive, and driven by far-field tectonic forces that stretch the lithosphere. Margin architecture develops due to spatial and temporal relationships between extensional deformation phases. Margin segmentation eventually leads to the formation of rift domains with variations of

273-455: A quarter in rifts with a non-marine syn-rift and post-rift, and an eighth in non-marine syn-rift with a marine post-rift. Shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes

312-468: A single basin-bounding fault. Segment lengths vary between rifts, depending on the elastic thickness of the lithosphere. Areas of thick colder lithosphere, such as the Baikal Rift have segment lengths in excess of 80 km, while in areas of warmer thin lithosphere, segment lengths may be less than 30 km. Along the axis of the rift the position, and in some cases the polarity (the dip direction), of

351-526: A temperature of about 360 °C. Below approximately 16 km depth, only ductile shear zones are found. The seismogenic zone , in which earthquakes nucleate, is tied to the brittle domain, the schizosphere . Below an intervening alternating zone, there is the plastosphere . In the seismogenic layer , which occurs below an upper stability transition related to an upper seismicity cutoff (situated usually at about 4–5 km depth), true cataclasites start to appear. The seismogenic layer then yields to

390-569: A wide depth-range, a great variety of different rock types with their characteristic structures are associated with shear zones. A shear zone is a zone of strong deformation (with a high strain rate ) surrounded by rocks with a lower state of finite strain . It is characterised by a length to width ratio of more than 5:1. Shear zones form a continuum of geological structures, ranging from brittle shear zones (or faults ) via brittle–ductile shear zones (or semibrittle shear zones ), ductile–brittle to ductile shear zones . In brittle shear zones,

429-402: Is also affected by the degree to which the rift basin is filled at each stage, due to the greater density of sediments in contrast to water. The simple 'McKenzie model' of rifting, which considers the rifting stage to be instantaneous, provides a good first order estimate of the amount of crustal thinning from observations of the amount of post-rift subsidence. This has generally been replaced by

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468-469: The Moho topography, including proximal domain with fault-rotated crustal blocks, necking zone with thinning of crustal basement , distal domain with deep sag basins, ocean-continent transition and oceanic domain. Deformation and magmatism interact during rift evolution. Magma-rich and magma-poor rifted margins may be formed. Magma-rich margins include major volcanic features. Globally, volcanic margins represent

507-400: The lithosphere is being pulled apart and is an example of extensional tectonics . Typical rift features are a central linear downfaulted depression, called a graben , or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley , which may be filled by water forming a rift lake . The axis of

546-645: The 'flexural cantilever model', which takes into account the geometry of the rift faults and the flexural isostasy of the upper part of the crust. Some rifts show a complex and prolonged history of rifting, with several distinct phases. The North Sea rift shows evidence of several separate rift phases from the Permian through to the Earliest Cretaceous , a period of over 100 million years. Rifting may lead to continental breakup and formation of oceanic basins. Successful rifting leads to seafloor spreading along

585-478: The alternating zone at 11 km depth. Yet big earthquakes can rupture both up to the surface and well into the alternating zone, sometimes even into the plastosphere. The deformations in shear zones are responsible for the development of characteristic fabrics and mineral assemblages reflecting the reigning pressure – temperature (pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling

624-450: The deformation is concentrated in a narrow fracture surface separating the wall rocks, whereas in a ductile shear zone the deformation is spread out through a wider zone, the deformation state varying continuously from wall to wall. Between these end-members, there are intermediate types of brittle–ductile (semibrittle) and ductile–brittle shear zones that can combine these geometric features in different proportions. This continuum found in

663-469: The development of isolated basins. In subaerial rifts, for example, drainage at the onset of rifting is generally internal, with no element of through drainage. As the rift evolves, some of the individual fault segments grow, eventually becoming linked together to form the larger bounding faults. Subsequent extension becomes concentrated on these faults. The longer faults and wider fault spacing leads to more continuous areas of fault-related subsidence along

702-437: The direction of movement. With the aid of offset markers such as displaced layering and dykes , or the deflection (bending) of layering/foliation into a shear zone, one can additionally determine the sense of shear. En echelon tension gash arrays (or extensional veins), characteristic of ductile-brittle shear zones, and sheath folds can also be valuable macroscopic shear-sense indicators. Microscopic indicators consist of

741-406: The following structures: The width of individual shear zones stretches from the grain scale to the kilometer scale. Crustal-scale shear zones (megashears) can become 10 km wide and consequently show very large displacements from tens to hundreds of kilometers. Brittle shear zones (faults) usually widen with depth and with an increase in displacements. Because shear zones are characterised by

780-448: The form of a fracture called a fault . In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile . That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side. Because shear zones are found across

819-514: The green algae from the Dolores Creek Formation. The first large evolutionary radiation of acritarchs occurred during the Tonian. Vase-shaped microfossils abound in late Tonian sediments and represent the earliest testate amoebozoans. This geochronology article is a stub . You can help Misplaced Pages by expanding it . Rifting In geology , a rift is a linear zone where

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858-502: The history of a specific terrane . Starting at the Earth's surface, the following rock types are usually encountered in a shear zone: Both fault gouge and cataclasites are due to abrasive wear on brittle, seismogenic faults. Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by adhesive wear . Pseudotachylites can still be encountered here. By passing into greenschist facies conditions,

897-557: The localisation of strain, some form of strain softening must occur, in order for the affected host material to deform more plastically. The softening can be brought about by the following phenomena: Furthermore, for a material to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without fracturing, the following deformation mechanisms (on a grain scale) have to be taken into account: Due to their deep penetration, shear zones are found in all metamorphic facies . Brittle shear zones are more or less ubiquitous in

936-425: The main rift bounding fault changes from segment to segment. Segment boundaries often have a more complex structure and generally cross the rift axis at a high angle. These segment boundary zones accommodate the differences in fault displacement between the segments and are therefore known as accommodation zones. Accommodation zones take various forms, from a simple relay ramp at the overlap between two major faults of

975-567: The majority of passive continental margins. Magma-starved rifted margins are affected by large-scale faulting and crustal hyperextension. As a consequence, upper mantle peridotites and gabbros are commonly exposed and serpentinized along extensional detachments at the seafloor. Many rifts are the sites of at least minor magmatic activity , particularly in the early stages of rifting. Alkali basalts and bimodal volcanism are common products of rift-related magmatism. Recent studies indicate that post-collisional granites in collisional orogens are

1014-535: The present, but their identity is highly debated. This dating is consistent with molecular data recovered through genetic studies on modern metazoan species; more recent studies have concluded that the base of the animal phylogenetic tree is in the Tonian. Tonian rocks preserve some of the earliest fossils of macroalgae, such as the benthic macroalgae from the Longfengshan biota of the Luotuoling Formation or

1053-404: The product of rifting magmatism at converged plate margins. The sedimentary rocks associated with continental rifts host important deposits of both minerals and hydrocarbons . SedEx mineral deposits are found mainly in continental rift settings. They form within post-rift sequences when hydrothermal fluids associated with magmatic activity are expelled at the seabed. Continental rifts are

1092-481: The pseudotachylites disappear and only different types of mylonites persist. Striped gneisses are high-grade mylonites and occur at the very bottom of ductile shear zones. The sense of shear in a shear zone ( dextral , sinistral , reverse or normal) can be deduced by macroscopic structures and by a plethora of microtectonic indicators. The main macroscopic indicators are striations ( slickensides ), slickenfibers , and stretching– or mineral lineations. They indicate

1131-419: The rift area may contain volcanic rocks , and active volcanism is a part of many, but not all, active rift systems. Major rifts occur along the central axis of most mid-ocean ridges , where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates . Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically

1170-432: The rift axis. Significant uplift of the rift shoulders develops at this stage, strongly influencing drainage and sedimentation in the rift basins. During the climax of lithospheric rifting, as the crust is thinned, the Earth's surface subsides and the Moho becomes correspondingly raised. At the same time, the mantle lithosphere becomes thinned, causing a rise of the top of the asthenosphere. This brings high heat flow from

1209-636: The same polarity, to zones of high structural complexity, particularly where the segments have opposite polarity. Accommodation zones may be located where older crustal structures intersect the rift axis. In the Gulf of Suez rift, the Zaafarana accommodation zone is located where a shear zone in the Arabian-Nubian Shield meets the rift. Rift flanks or shoulders are elevated areas around rifts. Rift shoulders are typically about 70 km wide. Contrary to what

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1248-510: The sites of significant oil and gas accumulations, such as the Viking Graben and the Gulf of Suez Rift . Thirty percent of giant oil and gas fields are found within such a setting. In 1999 it was estimated that there were 200 billion barrels of recoverable oil reserves hosted in rifts. Source rocks are often developed within the sediments filling the active rift ( syn-rift ), forming either in

1287-418: The structural geometries of shear zones reflects the different deformation mechanisms reigning in the crust, i.e. the changeover from brittle (fracturing) at or near the surface to ductile (flow) deformation with increasing depth. By passing through the brittle–semibrittle transition the ductile response to deformation is starting to set in. This transition is not tied to a specific depth, but rather occurs over

1326-754: The thrust type is the Moine Thrust in northwestern Scotland . An example for the subduction zone setting is the Japan Median Tectonic Line . Detachment fault related shear zones can be found in southeastern California, e.g. the Whipple Mountain Detachment Fault . An example of a huge anastomosing shear-zone is the Borborema Shear Zone in Brazil . The importance of shear zones lies in the fact that they are major zones of weakness in

1365-437: The transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot . Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen . Most rifts consist of a series of separate segments that together form the linear zone characteristic of rifts. The individual rift segments have a dominantly half-graben geometry, controlled by

1404-1063: The underlying dominant sense of movement of the terrane at that time. Some good examples of shear zones of the strike-slip type are the South Armorican Shear Zone and the North Armorican Shear Zone in Brittany , the North Anatolian Fault Zone in Turkey , and the Dead Sea Fault in Israel . Shear zones of the transform type are the San Andreas Fault in California , and the Alpine Fault in New Zealand . A shear zone of

1443-426: The upper crust. Ductile shear zones start at greenschist facies conditions and are therefore restricted to metamorphic terranes. Shear zones can occur in the following geotectonic settings: Shear zones are dependent neither on rock type nor on geological age. Most often they are not isolated in their occurrence, but commonly form fractal -scaled, linked up, anastomosing networks which reflect in their arrangement

1482-491: The upwelling asthenosphere into the thinning lithosphere, heating the orogenic lithosphere for dehydration melting, typically causing extreme metamorphism at high thermal gradients of greater than 30 °C. The metamorphic products are high to ultrahigh temperature granulites and their associated migmatite and granites in collisional orogens, with possible emplacement of metamorphic core complexes in continental rift zones but oceanic core complexes in spreading ridges. This leads to

1521-530: Was previously thought, elevated passive continental margins (EPCM) such as the Brazilian Highlands , the Scandinavian Mountains and India's Western Ghats , are not rift shoulders. The formation of rift basins and strain localization reflects rift maturity. At the onset of rifting, the upper part of the lithosphere starts to extend on a series of initially unconnected normal faults , leading to

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