Cataclastic deformation

Field and optical microscope studies showed that the fault plane is defined by a 2-3 cm wide cataclastic zone that is bounded laterally by a 1-3 m envelope of plastic and cataclastic deformation. Outside this envelope, the quartz microstructures displayed no evidence of significant deformation. Quartz in the fault envelope contains well-developed deformation bands, deformation lamellae, and intragranular healed fractures, which are visible at all scales of observation. 

The prediction of the sealing effect of brittle deformation in reservoir sandstones involves several steps (Figs. 2 and 6). At present, predictions are limited to clean sandstones. The initial step assesses the potential for and extent of cataclastic deformation. If cataclastic deformation can be shown to be the likely deformation mechanism, then the fault properties can... 

Fig. 8. Matrix porosity versus permeability reduction in faults. The upper trend describes the range of permeabilities observed within actively deforming cataclastic deformation bands. Permeabilities ate enhanced at low porosities, and slightly reduced at high porosities. The lower trend describes how inactive faults after burial show highly reduced permeability relative to the matrix. This permeability reduction gets more significant with increasing matrix porosity.

Cataclastically deformed plagioclase could be formed near the surface of the Moon and asteroids during heavy bombardment by meteoroids. An example of fractured plagioclase is observed in the eucrite Yamato (Y)-792510 (Fig. 7.3) consisting mainly of plagioclase and pyroxene which show undulatory extinction. Compared with pyroxene, plagioclase grains are heavily fractured. 

An example of veins with simple mineralogy is the Schwartz-walder mine, 25 km west of Denver, Colorado. Host rocks for this deposit are metasediments of the late Precambrian Idaho Springs Formation—a complex of sandstone, shale, carbonates and mafic intrusives." Large breccia-reef fault zones formed during late Precambrian cataclastic deformation were reactivated about 50-70 m.y. ago. Structures hosting the uranium are subsidiary to the major structures (Fig. 5). 

Contact metamorphism Localized metamorphism of rocks involving heat, fluids, and minor deformations resulting from the intrusion of a magma (compare with regional and cataclastic metamorphism). 

Figure 9.38. BF image of a healed fracture in the quartz of the deformed envelope of a narrow cataclastic fault zone, which forms part of a thrust belt in the Cantabrian Mountains of northwest Spain. (Courtesy of Martyn Drury.)...

Fig. 6. Brittle fault seal analysis strategy. The strategy aims to quantify the sealing capacity of brittle faults by first predicting the deformation mechanism. Particulate flow faults are treated as non-sealing. Cataclastic faults have variable sealing properties according to fault throw and matrix properties. The chart in the lower left of this figure is reproduced at a larger scale in Fig. 8.

Fig. 9. Permeability versus capillary entry pressure. Entry pressure increases with decreasing permeability. The regression line drawn through the data was derived for a range of lithologies by Ibrahim et al. (1970) (Watts, 1987 Antonellini and Aydin, 1995). The relationship appears to hold for the cataclastic faults as well. Some of the deviations from the regression are due to uncertainties in the thickness of the deformation bands and slip planes, which lead to overestimated fracture permeabilities of up to one order of magnitude.

Faults in sandstones deformed at depths greater than 1 km tend to deform by cataclasis. Permeability and entry pressure of such faults can be predicted from estimates of matrix properties. Static seal capacities of cataclastic faults depend on the minimum sealing properties, which are related to the fault displacements. 

Internally complex, ferroan dolomite-cemented fractures at Ballycastle are closely associated with regional normal faulting and are genetically linked with cataclastic textures typical of brittle deformation in porous sandstones. Cemented fractures described in this paper are distinguished from the principal slip planes ( faults ) on the basis of minimal displacement (centimetre scale at most), and the definition encompasses the tectonodiagenetic products of initial cataclasis, cementation of the... 

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