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Deformation history and basin-controlling faults in the Mesozoic sedimentary rocks of the Somerset coast
No full textStructures in the Triassic and Liassic sedimentary rocks of the Somerset coast indicate spatial and temporal variations in the deformation history. From Kilve to Watchet, there was Mesozoic north-south extension and Tertiary north-south contraction. At Lilstock, about 3 km to the east, there was a more complex history, including: (1) the development of joints, normal faults and veins striking about 060°; (2) approximately north-south extension on 095° striking normal faults, with sinistral transtension; (3) east-west contraction, with sinistral shear on some 095° striking normal faults; (4) dextral reactivation of some 095° striking normal faults; (5) north-south contraction, and thrusts and strike-slip faults, with the reverse-reactivation of the largest 095° striking normal faults. The joints in the district mostly postdate the faults.Two large approximately east-west striking faults are postulated to form the northern edges of the Quantock and Exmoor hills, here called the North Quantocks Fault (NQF) and the North Exmoor Fault (NEF). These were basin-bounding normal faults during the Mesozoic, with maybe more than 1000 m of throw, suggesting that the Bristol Channel Basin is not a half-graben developed above a south-dipping Variscan thrust which underwent Mesozoic extensional reactivation. The NQF and NEF may have been reverse-reactivated during the Alpine contraction. The NQF probably caused variations in the stress history of the Mesozoic sedimentary rocks of the Somerset coast over only a few kilometres along strike. The possible relationships of the NQF and NEF with the Cothelstone Fault are discussed.Several broad orders of relay ramp scale occur, within which are developed smaller antithetic normal faults. Between the north-dipping NQF and NEF, the relay ramp contains south-dipping (antithetic) normal faults with tens to hundreds of metres displacement. In relay ramps between the south-dipping faults are north-dipping normal faults with metre-scale displacements, which in turn have relay ramps with south-dipping normal faults with millimetre-scale displacements.<br/
Strike-slip relay ramps
No full textAreas of reorientated bedding at contractional oversteps between strike-slip faults are here called strike-slip relay ramps. Metre-scale examples are described from the Jurassic sediments at East Quantoxhead, Somerset U.K. Larger strike-slip relay ramps occur in the Rio de Peixe Basin, NE Brazil, along the Newport-Inglewood Trend, California, and in the Bovey Basin, SW England. Although the geometry and development of strike-slip relay ramps are similar to those of relay ramps in normal fault systems, there are differences in the structures which accommodate the transfer of displacement between the overstepping faults. Whereas strike-slip relay ramps are typically transpressional, with pressure solution often occurring, relay ramps in normal fault systems are dominated by extension or transtension. Care needs to be taken when interpreting areas of reorientated bedding between overstepping faults, particularly when displacement directions are unknown, for example when using seismic data. This is because relay ramps can occur in both strike-slip and normal fault zones.<br/
Effects of propagation rate on displacement variations along faults
No full textField observations indicate that much of the variability in the displacement-distance (d-x) profiles and length-displacement relationships of faults is caused by factors which can affect the propagation of faults. These factors include the interaction and linkage of segments, fault bends, conjugate relationships and lithological variations. Existing models for the d-x profiles of faults do not take these effects into account. Fault development can be modelled assuming faults accumulate displacement by a series of slip events, and using a function (p) to describe the rate of fault propagation. When p is constant during fault development, an approximately linear d-x profile eventually develops. When p decreases, such as when interaction occurs, the d-x profile rises above the linear profile. When p increases, the d3x profile initially falls below the linear profile. Such variations in finite d-x profiles mean that the analysis of finite fault displacement gives little information about the d-x profiles of individual slip events.Variations in p cause variations in r/dMAX ratios (where r is the distance between the maximum displacement point and the fault tip, and dMAX is maximum displacement). Interaction tends to hinder propagation, but displacement continues to increase, causing relatively low r/dMAX ratios. Inelastic deformation can occur at fault tips, especially where strain is concentrated at oversteps, causing steep d-x profiles and low r/dMAX ratios to develop.<br/
Strain and scaling of faults in the chalk at Flamborough Head, U.K.
No full textAnalysis has been made of the orientations, displacements and spacings of 1340 extensional faults, with displacements of up to 6 m, along an almost completely exposed 6 km length of cliff. This data set has been used to study how fault populations account for strain in a region and to study relationships between different scales of fault. Strains have been estimated; the maximum and intermediate extensions are sub-horizontal, with approximately equal extension (e ? 0.01) in all horizontal directions. It can be inferred that the minimum extension (maximum compression) was sub-vertical, but that the wide variety of fault orientations and cross-cutting relationships resulted from variable horizontal extensions. Some faults have oblique-slip slickenside lineations, which imply a period of later, dominantly NNW-SSE extension, which possibly developed as the exposure-scale faults linked up E-W-striking larger-scale normal faults, effectively forming a single wide fault zone.Graphs of displacement per unit distance are used to illustrate variations in displacement. The scaling of fault displacement appears to follow a power-law relationship. The differences in orientation between the small-scale and large-scale faults precludes a simple estimation of the total strain over all scales.<br/
Estimating strain from fault-slip using a line sample
No full textA method is presented that enables data for faults with different orientations and displacements, measured along a single straight line, to be used to estimate the magnitudes and orientations of the principal strain axes. The method combines two well-established techniques. When sampling along a line, the probability of intersecting a fault is affected by its orientation. This sampling bias may be minimized by the use of a weighting, w = 1/cos ?, where ? = angle between the perpendicular to the fault and the sample line. The displacement gradient and Lagrangian strain tensors may then be used to describe the deformation with respect to the undeformed state. The method can also be applied to such structures as veins and stylolites. As an example of the use of the method, 1340 normal faults have been measured along a 6 km length of Cretaceous chalk cliffs at Flamborough Head, Humberside, U.K. Consistent strain estimates have been obtained for different portions of the cliff.<br/
Effects of layering and anisotropy on fault geometry
No full textThe geometry and orientation of faults are examined using several field examples of small-scale (<1m displacement) fault systems. For isotropic rocks under triaxial compression, faults normally develop in conjugate sets about the maximum compressive stress (1), with dihedral angles usually of about 50°, as predicted by the Coulomb theory of failure. In layered rocks, the geometry of faults varies with the orientation of layering with respect to the stress field. Where 1, is approximately normal to layering or anisotropy, conjugate faults also develop symmetrically about 1. Where rocks have interbedded layers with different mechanical behaviours, however, faults tend to initiate orthogonal to the more brittle layers (i.e. originate as extension fractures sub-parallel to 1), but oblique to the less brittle layers. As the fault steps through the layering, pull-aparts are developed which may reduce the dihedral angle. Where 1 is oblique (c. 25–75°) to anisotropy, one set of faults is developed at a high angle to layering, with another at a low angle, usually showing a ramp-flat geometry. Large dihedral angles (up to 90°) may result and 1, does not bisect this angle. Where 1 is approximately parallel to layering, two cases can be recognized. Where 3 is approximately normal to layering, faults with layer-parallel flats and contractional ramps develop. Where 2 is approximately normal to layering, conjugate faults develop which are symmetrical about 1, the geometry resembling that in isotropic rocks. These observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation, but the observations incorporate the effects of layering of materials with different deformation characteristics. <br/
Pull-aparts, shear fractures and pressure solution
No full textVein arrays are often composed of pull-aparts which are linked by shear fractures, good examples of which occur in the Lower Jurassic limestones of Somerset, southwestern England. Such pull-apart arrays have displacement-distance characteristics which are similar to fault zones, with maximum displacement (indicated by the largest pull-apart widths) near the centre of the array, and with displacement decreasing towards the tips. Pull-apart arrays usually die out into enéchelon or pinnate veins. Evidence for pressure solution along the shear fractures which connect pull-aparts include their dark and braided nature, their obliquity to the displacement direction, the high dihedral angles (often > 90°) between conjugate shear fractures, and the dissolution of earlier structures. A range of geometries occurs, with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression. There is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases. There is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays. The concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults.<br/
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