Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 30

The geological processes that lead to the three main rock types also give rise to a variety of geological structures, such as faults and folds. Faulting and folding can affect any rock type.

Faults are breaks or discontinuities in bedrock where rocks have failed in a brittle fashion. Faults may be small and little offset is seen between the two opposing rocks, or they may be very large features that extend to great depths and lengths. Faults may arise due to compressional, extensional, and transverse forces applied to the Earth’s dynamic crust, as well as combinations of these forces through geological time (Figure 2.14). Larger fault systems are sometimes referred to as shear zones and can comprise anastomosing networks of faults. Fault systems can provide preferential pathways for magmas and hydrothermal fluids to ascend within the crust, and also result in the juxtaposition of geochemically and geologically contrasting rock types.

Folds also represent past tectonic activity acting upon the Earth’s crust; however, they require plastic deformation as opposed to brittle deformation. Folds and resulting foliation are most strikingly observed in sedimentary rocks that started out as layered or stratified and were subjected to predominantly compressional forces in the Earth’s crust (Figures 2.15 and 2.16). The response of the rocks in a compressional regime is to shorten in the direction of compression and the Earth accommodates those forces through folding, just as a stack of loose papers would develop folds if pushed from the sides. Folds can also develop in extensional or tensional stress regimes, such as thick sedimentary basins with normal faulting and deformation or detachment features in extensional shear zones. Similar to faults, folding patterns may result in juxtaposed rock types and pathways through which magmas and hydrothermal fluids might preferentially travel along or through. These settings are important for laying the specific conditions in which certain gemstones deposits may form. Generally speaking, the most dramatic folding happens on a very large scale during continental building and collisional events, and fold patterns can extend over hundreds of kilometers with complex geometries.