构造地质学双语7joints

Chapter 4 Joint KeywordsBedding jointsStrike jointsDip jointsOblique jointsFracturesDiagonal joint层节理走向节理倾向节理斜向节理破裂斜节理Joint setsJoint systemSystematic joints:Non-systematic jointsPlumose structure节理组节理系系统节理非系统节理羽饰构造Shear jointTension jointen-echelonLongitudinal jointsTransverse joints剪节理张节理雁列（脉，节理）纵节理横节理Hackleplume axistension gashesRegional joint Stylolites羽脉羽轴张裂脉区域性节理缝合线Fractures are surfaces along which rocks or minerals have broken. Joints, be one of the most commonly found pervasive mesoscopic structure, are the fractures without apparent displacement in rocks. These discontinuities will be encountered in many engineering tasks involving national and local needs. Cracks in rocks carry ground water far more efficiently than the bulk rock with its relatively low permeability. In contrast the recovery of geothermal energy requires cracks in hot rock so that heat fluids pumped along the cracks can act as the medium with which heated water is transported to the surface. The recovery of petroleum is best accomplished when wells cut a fractured network containing large amounts of petroleum. The property, orientation and distribution of joints are closely related to the folds, faults and regional structures. So , the studies of joints are very helpful to analyze the geological structure.1. Classification of jointsThe classification of joints is based on two main factors: (1) the geometrical relationship with other related structures. (2) Mechanisms of the joints forming.Joints are relatively belonging to small-scale structures. The orientations of joints often have geometrical relationship with other structures.(1) A: The relationship between joints strike and sense of associated strata.l Strike joints. Strike of the joint is parallel to the strike direction of bed.l Dip joints. The strike of the joint is perpendicular to the strike of bed.l Oblique joints. The strike of the joint is oblique to the strike of bed.l Bedding joints. Joint plane is parallel to the bedding planes.B: The relationship between joint and fold axis l Longitudinal joints: Strike of the joint is parallel to the fold axes. l Transverse joints: Strike of the joint is perpendicular to fold axes.l Diagonal joint: Strike of the joint is a oblique to fold axes.(2) According to the mechanism of jointsl Shear joint. In compression, joints develop in the conjugate shear directions (the orientation of symmetric fracture planes) making the lower angle with the major principal stress direction. Shear joints are often grooved, striated, polished or slickensided by even small amounts of shear displacement. Conjugate shear fractures may be used to map maximum principal stress in the vicinity of a fault zone.Shear jointTensile jointl In tension, joints develop by stretching normal to the tensile stress direction which is usually the minor principal stress. Tension joints are rough (unless subsequently weathered). In coarse grained rocks such surfaces may be very rough. Tensional joint include columnar joints in basalts, dykes and sills, tension gashes, calcite and quartz filled veins. Extension joints are not one long discontinuity but rather several joints that form end-to-end in a joint zone.(3) Joint sets and joint systemsl Joint sets: many adjacent joints with similar geometry and orientation. Systematic joints: roughly planar, sub-parallel orientations, regular spacing. Non-systematic joints: curved and irregular in geometry, usually terminate against systematic joints.l Joint system: made up of two or more sets of joints, may be persistent over large regions. The geometry of joint systems are described with the following measurements and observations: orientation, scale and shape, spacing and aperture, intersections and terminations, spatial patterns .3. Surface morphologySome joint surfaces display beautiful surface ornamentation, that is plume structure. Plumose structure consists of a number of morphological components: plume axis, hackles, and fringe zone. Curved hackle marks radiate outward from the point (origin) where the joint originated. These structures indicate the propagation direction of the joint. The fringe of a joint may be marked by closely spaced en-echelon joints oblique to the main joint face. 3. En echelon veins Veins are fracture filled with a precipitate, commonly quartz or calcite, rarely mud. En echelon vein is kind of vein which can be found in many rocks, especially in carbonate rocks. En echelon are parallel or subparallel, closely-spaced, overlapping or step-like minor structural features in rock, such as faults and tension fractures, that are oblique to the overall structural trend Sigmoidal tension gashes, generally S- or Z-shaped, form along zones of ductile shear. This type of extensional fracture is usually mineral-filled and form en echelon arrays along the shear zone. En Echelon tension fractures(gashes) may indicate the direction of displacement by two features: 1) Sigmoidal profiles of fractures, and 2) offset direction of vein-filled fractures The sigmoidal profiles of fractures are cross-sectional views that aid in determining the direction of rotation. The profiles resemble an S-shaped feature with the tips pointing in the direction of rotation, either clockwise or counter-clockwise.The gash fractures form as extensional fractures that are perpendicular to the minimum compressive stress, sigma 3 (see diagram). The fractures may be rotated by ductile deformation either during or after formation. As gash fractures develop at different times of ductile shearing they show different amounts of rotation. Since sigma 3 is normal to the unrotated portion of the gash fracture, either the tips of the sigmoidal fracture or the newly formed fractures tells the stress orientation. The youngest fractures may not show any or very little rotation. (see diagram).4. Stylolites Stylolites are irregular surfaces that commonly appear as dark, jagged lines on exposed surfaces of carbonate rock. Their origin is usually attributed to solution that occurs after the host rock was formed. The dark layers are insoluble residues. Stylolites often cross the whole rock, cutting grains ,matrix and cement stylolites belong to pressure solution structures formed at strain localisation sites within rocks, and can be used to indicate the stress direction.4. Relationships Between Joints and Other Structures Columnar joints: polygonal fractures formed during cooling of lava flows and shallow intrusions. Due to tensile stresses set up as igneous rock contracts during cooling. Sheet (or exfoliation) joints: curved joints that are sub-parallel to the topography. Earth surfaces is eroded and relieves vertical stress but lateral stress is not reduced proportionally. Therefore vertical stress becomes minimum principal stress and joints form perpendicular to land surface. Shrinkage of cooling plutons (large homogenous igneous masses ) may also produce sheeting joints or exfoliation. Joints associated with folds: Several sets of joints may develop in response to folding. Conjugate shear joints oblique to the fold axis are develpoed by compression. Tension joints can develop due to bending, particularly in the vicinity of the fold hinge. Joints parallel to the strike of the fold axis are called strike joints; those parallel to the limb dip are known as dip joints. Such joints are generally tensile fractures. Joints associated with faults: high stress levels in the hangingwall and footwall of a fault can result in the formation of conjugate shear joints, with one set parallel to the fault plane and the other set at an oblique angle of about 65 . Joints in plutonic rocks: several joint sets are commonly observed in plutons. Such joints are thought to be related to stresses set up during cooling of the rock mass within a regional stress field. Regional joint sets: remarkably consistent sets of joints are observed at the regional scale, particularly in weakly deformed sedimentary platform sequences. Such joints are usually tectonic joints: that is extensional joints formed due to horizontal tectonic compression under conditions of elevated pore fluid pressure.5.Working with Joints and Veins in the Field Study way of joints varies with different purposes, but in general, all of the studies on joints are based on the measurement, observation, and statistics. (1) Localities . The observation localities should have the following features:l Well exposed outcropsl Clearly observable structurel Beddings with table orientation l Clear relationship of joint sets and joint systems. (2) Contents of observationl Geological background observation. Including the orientation and rock types of beddings, the features of fault and folding .l Classification of joints, joint sets and joint systems. Describing the intersection and termination of different joint sets.l Timing : Younger joints commonly terminate into older, existing joints. Because fracture stops at free surface of the preexisting joint and cannot propagate across it. Also, joints tend to become parallel or more commonly perpendicular to the free surface because the free surface cannot support shear stress.l Measurement of joint density. The joint density can be measured by joint numbers in unit length or total unit lengths in unit area. l Observation of joint surface. Special attention should be paid to striation, plume structure and other small scale structures.l Whether the joint has been filled with minerals. The distribution and scale of veins.(3) Records Using specially designed tables to records the result of measurement and observation.