{"id":1203,"date":"2023-12-04T22:06:03","date_gmt":"2023-12-04T22:06:03","guid":{"rendered":"https:\/\/books.gw-project.org\/structural-geology-applied-to-fractured-aquifer-characterization\/?post_type=chapter&p=1203"},"modified":"2023-12-08T15:59:38","modified_gmt":"2023-12-08T15:59:38","slug":"3-2","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/structural-geology-applied-to-fractured-aquifer-characterization\/chapter\/3-2\/","title":{"raw":"3.2 Influence of the Tectonic Regime on the Connected Fracture Network","rendered":"3.2 Influence of the Tectonic Regime on the Connected Fracture Network"},"content":{"raw":"Joints and faults are not generated simultaneously because the stress conditions (as expressed by the Mohr diagram) required for producing them are different. Throughout the duration of a tectonic event in which the stress orientation remains the same, the magnitude of the stresses can change. This may lead to the generation of both joints and faults at different times in the same event and may enhance connectivity and, possibly, the aperture of some fractures.\r\n\r\nThrust and normal conjugate faults produce horizontal intersection lines, whereas strike-slip fault intersections are vertical. Thus, depending on the tectonic regime, horizontal and vertical flow channels can potentially be formed along fault intersections. The conjugate faults generated by the compressive and extensional regimes would favor horizontal flow channels, and the strike-slip regime, vertical flow channels.\r\n\r\nLow dip or horizontal fractures (compressive regime) are more likely to show a decrease in hydraulic conductivity (K<\/span><\/em>) with depth due to the increasing overburden stress of the rock column weight (e.g., Morin & Savage, 2003). The depth from which the decrease of aperture starts to be significant may vary from place to place in response to the variation of the in-situ stress magnitudes. On the other hand, K <\/span>of vertical fractures, either joints or faults, is likely less affected by depth (increasing rock column) (e.g., Morin & Savage, 2003). In general, the ratio of average horizontal stress to vertical stress of the current in-situ stresses decreases with depth as shown in Figure 31 (Hoek & Brown, 1980). This suggests that, in general, the closer to the surface, the larger the aperture of the subhorizontal fractures becomes. This is consistent with the data obtained in several regions as discussed in Section 4.1<\/a>.\r\n\r\n\"\"\r\n

Figure 31 -<\/strong> In-situ stress data from several regions throughout the globe (Australia, United States, Canada, Scandinavia, southern Africa, and others). Close to the surface (above 500 m in depth), the average horizontal stress can be roughly three times larger than the vertical stress (modified from Hoek & Brown, 1980).<\/span><\/p>\r\nThe persistence of sheeting joints and their large apertures favor connectivity and flow. Often, however, a significant number of them are in the unsaturated zone. The largest apertures, exceeding millimeters or centimeters, can be just below or very close to the ground surface and their permeability tends to sharply decrease with depth, as is the case for many other subhorizontal fractures.\r\n\r\nWhere strike-slip faults overlap, transpression and transtension zones can be formed (Figure 29b,c<\/a>) and this induces the generation of thrust and normal faults, respectively. These structures provide connection between otherwise separate strike-slip faults. Depending on the stress magnitude, horizontal and vertical joints may be developed in the transpression and transtension zones, respectively; this process enhances not only fracture connectivity, but also groundwater flow.\r\n\r\n \r\n\r\n ","rendered":"

Joints and faults are not generated simultaneously because the stress conditions (as expressed by the Mohr diagram) required for producing them are different. Throughout the duration of a tectonic event in which the stress orientation remains the same, the magnitude of the stresses can change. This may lead to the generation of both joints and faults at different times in the same event and may enhance connectivity and, possibly, the aperture of some fractures.<\/p>\n

Thrust and normal conjugate faults produce horizontal intersection lines, whereas strike-slip fault intersections are vertical. Thus, depending on the tectonic regime, horizontal and vertical flow channels can potentially be formed along fault intersections. The conjugate faults generated by the compressive and extensional regimes would favor horizontal flow channels, and the strike-slip regime, vertical flow channels.<\/p>\n

Low dip or horizontal fractures (compressive regime) are more likely to show a decrease in hydraulic conductivity (K<\/span><\/em>) with depth due to the increasing overburden stress of the rock column weight (e.g., Morin & Savage, 2003). The depth from which the decrease of aperture starts to be significant may vary from place to place in response to the variation of the in-situ stress magnitudes. On the other hand, K <\/span>of vertical fractures, either joints or faults, is likely less affected by depth (increasing rock column) (e.g., Morin & Savage, 2003). In general, the ratio of average horizontal stress to vertical stress of the current in-situ stresses decreases with depth as shown in Figure 31 (Hoek & Brown, 1980). This suggests that, in general, the closer to the surface, the larger the aperture of the subhorizontal fractures becomes. This is consistent with the data obtained in several regions as discussed in Section 4.1<\/a>.<\/p>\n

\"\"<\/p>\n

Figure 31 –<\/strong> In-situ stress data from several regions throughout the globe (Australia, United States, Canada, Scandinavia, southern Africa, and others). Close to the surface (above 500 m in depth), the average horizontal stress can be roughly three times larger than the vertical stress (modified from Hoek & Brown, 1980).<\/span><\/p>\n

The persistence of sheeting joints and their large apertures favor connectivity and flow. Often, however, a significant number of them are in the unsaturated zone. The largest apertures, exceeding millimeters or centimeters, can be just below or very close to the ground surface and their permeability tends to sharply decrease with depth, as is the case for many other subhorizontal fractures.<\/p>\n

Where strike-slip faults overlap, transpression and transtension zones can be formed (Figure 29b,c<\/a>) and this induces the generation of thrust and normal faults, respectively. These structures provide connection between otherwise separate strike-slip faults. Depending on the stress magnitude, horizontal and vertical joints may be developed in the transpression and transtension zones, respectively; this process enhances not only fracture connectivity, but also groundwater flow.<\/p>\n

 <\/p>\n

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