However, the geometric tests for fault-related folds. were never relationship between folds and faults is studied and applied. to petroleum and. Because of the obvious relationship between faults and earthquake, This iBook covers the basics about faults, folds, shear zones and . A Little Quiz 23 . and geological discussion of the relationship between folding, kinking and faulting R. Hill, J.W. HutchinsonBifurcation phenomena in the plane tension test.
Other studies have been focussed on specific aspects of extension regions, such as the flat Moho Gans or the distributed regional deformation Buck Other than extension-parallel folds, several features of extension regions have not been accounted for by the various models listed earlier.
Also, there are small, but significant, differences between the directions of extension and fold axes in the Aegean Avigad et al. These observations attest to the fundamentally 3-D nature of the deformation. Figure 1 Topographic map of the Basin and Range province. The large area affected by folding may be identified easily from the topographic variations.
Lines in large dashes show boundaries that could be compared with the clamped edges of our laboratory experiments. In this study, we argue that some important deformation characteristics of extension zones are acquired in early phases of extension when the amounts of strain are small, such that deformation proceeds in an elastic regime, before the onset of major normal faulting. Specifically, we propose that extension-parallel folds are due to elastic deformation and discuss geometrical controls on the orientation of fold axes.
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One intrinsic feature of this deformation regime is shortening in a direction perpendicular to extension, which provides a straightforward explanation for the lack of significant crustal thinning that was emphasized by Avigad et al.
The paper is organized as follows. We recapitulate a few observations that demonstrate that folding does occur early during extension and that provide the framework of our study. We then discuss evidence for the important role played by flexural stresses, which are a direct consequence of elastic deformation. We contend that extension-parallel folds are due to a uniaxial extensional stress field and propose that a thin elastic layer controls the initial large-scale deformation pattern.
We carry out laboratory experiments that document precisely the characteristics of folding. We rely on theory originally proposed by Cerda et al.
Simple scaling laws allow easy evaluation of fold characteristics and their dependence on the control variables and physical properties. Because of the 3-D nature of the deformation, we also investigate the influence of the boundary conditions and specifically the influence of the shape and orientation of the rigid domains that bound the extension zone. We show how the intrinsically 3-D nature of elastic deformation can lead to fold axes with different orientations in different parts of the same province.
In a discussion section, we evaluate the conditions that are required for fold generation, which include limitations on the elastic plate thickness and on the dimensions of the deforming region. We also discuss the simple model of an elastic sheet in relation to the more complex rheological properties of continental crust and lithospheric mantle. Finally, we study some aspects of the faulting that occurs in association with folding.
In both regions, as shown by the large-scale deformation pattern and small-scale strain analyses, extension develops in an intrinsically 3-D regime involving extension in one direction and shortening in the other. Extension in the Aegean region started 30 Ma e. In the Aegean Sea, extension was accompanied by EW shortening i. The metamorphic domes have a characteristic spacing in the 20—30 km range, but the Moho discontinuity beneath them shows no associated undulations and is flat Tirel et al.
On a larger scale, the crustal thickness is slightly thicker in the Central Aegean than to the North and South Jolivet et al. According to Avigad et al. Figure 2 View large Download slide Map of the Aegean, and scheme of the domes observed in the central part of the Aegean Sea adapted from Jolivet et al. Domes are restricted to the circled area. Paros, Naxos and Mykonos are domes parallel to the extension direction indicated by large black arrows.
Other domes such as Tinos are perpendicular to the direction of extension. They are not studied here. It is remarkable for its periodic alternation of topographic highs and lows bordered by normal faults Fig. Deformation was achieved in several phases. Many exhumed metamorphic domes are observed in the province and are related to folds with characteristic spacings in the 10—20 km range, similar to those of the Aegean e. Part of the complexity of the observed deformations is due to the change of extension direction and the presence of highly extended corridors Faulds et al.
Reverse or thrust faults occur when rocks are undergoing horizontal shortening. The geology of an area changes through time as rock units are deposited and inserted, and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrusion into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows blanket the surface.
Igneous intrusions such as batholithslaccolithsdikesand sillspush upwards into the overlying rock, and crystallize as they intrude. Deformation typically occurs as a result of horizontal shortening, horizontal extensionor side-to-side strike-slip motion.
These structural regimes broadly relate to convergent boundariesdivergent boundariesand transform boundaries, respectively, between tectonic plates. When rock units are placed under horizontal compressionthey shorten and become thicker. Because rock units, other than muds, do not significantly change in volumethis is accomplished in two primary ways: In the shallow crust, where brittle deformation can occur, thrust faults form, which causes deeper rock to move on top of shallower rock.
Because deeper rock is often older, as noted by the principle of superpositionthis can result in older rocks moving on top of younger ones. Movement along faults can result in folding, either because the faults are not planar or because rock layers are dragged along, forming drag folds as slip occurs along the fault.
Deeper in the Earth, rocks behave plastically and fold instead of faulting. These folds can either be those where the material in the center of the fold buckles upwards, creating " antiforms ", or where it buckles downwards, creating " synforms ". If the tops of the rock units within the folds remain pointing upwards, they are called anticlines and synclinesrespectively. If some of the units in the fold are facing downward, the structure is called an overturned anticline or syncline, and if all of the rock units are overturned or the correct up-direction is unknown, they are simply called by the most general terms, antiforms and synforms.
A diagram of folds, indicating an anticline and a syncline.
Even higher pressures and temperatures during horizontal shortening can cause both folding and metamorphism of the rocks.
This metamorphism causes changes in the mineral composition of the rocks; creates a foliationor planar surface, that is related to mineral growth under stress. This can remove signs of the original textures of the rocks, such as bedding in sedimentary rocks, flow features of lavasand crystal patterns in crystalline rocks.
Extension causes the rock units as a whole to become longer and thinner. This is primarily accomplished through normal faulting and through the ductile stretching and thinning.
Normal faults drop rock units that are higher below those that are lower. This typically results in younger units ending up below older units. Stretching of units can result in their thinning.
Folding in regions of extension | Geophysical Journal International | Oxford Academic
In fact, at one location within the Maria Fold and Thrust Beltthe entire sedimentary sequence of the Grand Canyon appears over a length of less than a meter. Rocks at the depth to be ductilely stretched are often also metamorphosed. These stretched rocks can also pinch into lenses, known as boudinsafter the French word for "sausage" because of their visual similarity. Where rock units slide past one another, strike-slip faults develop in shallow regions, and become shear zones at deeper depths where the rocks deform ductilely.
Geologic cross section of Kittatinny Mountain. This cross section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.
The addition of new rock units, both depositionally and intrusively, often occurs during deformation. Faulting and other deformational processes result in the creation of topographic gradients, causing material on the rock unit that is increasing in elevation to be eroded by hillslopes and channels.
These sediments are deposited on the rock unit that is going down. Continual motion along the fault maintains the topographic gradient in spite of the movement of sediment, and continues to create accommodation space for the material to deposit.
Deformational events are often also associated with volcanism and igneous activity. Volcanic ashes and lavas accumulate on the surface, and igneous intrusions enter from below. Dikeslong, planar igneous intrusions, enter along cracks, and therefore often form in large numbers in areas that are being actively deformed. This can result in the emplacement of dike swarmssuch as those that are observable across the Canadian shield, or rings of dikes around the lava tube of a volcano.
All of these processes do not necessarily occur in a single environment, and do not necessarily occur in a single order. The Hawaiian Islandsfor example, consist almost entirely of layered basaltic lava flows. The sedimentary sequences of the mid-continental United States and the Grand Canyon in the southwestern United States contain almost-undeformed stacks of sedimentary rocks that have remained in place since Cambrian time.
Other areas are much more geologically complex. In the southwestern United States, sedimentary, volcanic, and intrusive rocks have been metamorphosed, faulted, foliated, and folded. Even older rocks, such as the Acasta gneiss of the Slave craton in northwestern Canadathe oldest known rock in the world have been metamorphosed to the point where their origin is undiscernable without laboratory analysis. In addition, these processes can occur in stages. In many places, the Grand Canyon in the southwestern United States being a very visible example, the lower rock units were metamorphosed and deformed, and then deformation ended and the upper, undeformed units were deposited.
Although any amount of rock emplacement and rock deformation can occur, and they can occur any number of times, these concepts provide a guide to understanding the geological history of an area. Methods of geology[ edit ] Geologists use a number of field, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside the Earth.
In typical geological investigations, geologists use primary information related to petrology the study of rocksstratigraphy the study of sedimentary layersand structural geology the study of positions of rock units and their deformation. In many cases, geologists also study modern soils, riverslandscapesand glaciers ; investigate past and current life and biogeochemical pathways, and use geophysical methods to investigate the subsurface.
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