Structural Analyses aiding Identification of Water Conductive Fracture Zones in Crystalline Rock

Abstract

Development of hydraulic conductive zones in crystalline rock can result from a wide range of geological conditions which include primary structures, post crystalline tectonics, fluid solution and movement within a developing and eroding regolith.Crystalline rock areas mostly have low water resource potential due to inherent extremely low storage and water conductive properties. Therefore, fracture zones of high hydraulic conductivity have an important role in developing groundwater resources that may be distant or where it may be difficult to obtain alternative sources in these areas.Mechanisms for development of open tension or pull-apart fractures in brittle rocks are similar to those involving development of mineralised veins. The same structural analytical techniques can be applied for water bearing structures.Crystalline rock fracture zones can be amenable to rapid recharge through rainfall runoff. They are also significant in that they provide a mechanism for underdrainage through ‘delayed yield’ of surrounding or enclosing low conductive rocks such as saprock/saprolite, pelite and phyllite.In addition to brittle rocks, open tension fracture zones of enhanced hydraulic conductivity may also occur in more fissile pelitic rocks such as slate and phyllite. These conductive zones are often associated with crestal zones of folds, strike deviations produced by conjugate shears and fracture zones discordant with layering and foliation. The development of conjugate joint sets in a region frequently provide a significant basis for this type of fracture analyses.This presentation provides examples of water supplies developed from crystalline rock structures in a range of geological and earth environments.