Joints and Veins - Global Change

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2917-CH07.pdf 11/20/03 5:11 PM Page 155σ 1 σ 3Jointσ 3σ 1Once the crack propagates, the volume of openspace between the walls of the crack increases, sothe fluid pressure in the crack decreases. As a consequence,the crack stops growing until an increasein fluid pressure once again allows the stress intensityat the crack tip to drive the tip into unfracturedrock. Thus, the surfaces of joints formed by naturalhydraulic fracturing tend to have many arrest lines.High P H2 O(a)σ cpσ cgσ cgσ oσ cpσ cging stress exceeds the closing stress so that the crackpropagates; effectively, the outward push of the fluidin the crack creates a tensile stress at the crack tip.Why does the closing stress increase at a slower ratethan the fluid pressure and the opening stress? Thereason is that grains in the rock are cemented to oneanother, so that the grains cannot move freely inresponse to the increase in fluid pressure in the pores.The elasticity of the grains themselves, therefore,takes up some of the push caused by the fluid pressure.Thus, the closing stress acting on the fluid in thecrack where it is in contact with a grain is less thanwhere the fluid in the crack is in contact with a pore,but the outward push of the fluid in the crack is thesame everywhere. 5 As a result, the net outward pushexceeds the net inward push, and tensile stress locallydevelops.σ tσ o > σ cp + σ cg(b)FIGURE 7.16 (a) Block diagram showing the stresses inthe vicinity of a crack in which there is fluid pressure thatexceeds the magnitude of σ 3 . As a result, there is a tensilestress, σ t , along the crack. (b) Enlargement of the crack tip,illustrating the poroelastic effect. The opening stress (σ o ) dueto fluid pressure in the crack exceeds the closing stress (σ c ),which is the sum of σ cp , the closing stress where a pore is incontact with the crack, and σ cg , the closing stress where a grainis in contact with the crack.5 This is known as the poroelastic effect.7.5.4 Joints Related to RegionalDeformationDuring a convergent or collisional orogenic event,compressive tectonic stress affects rocks over a broadregion, including the continental interior. Joints formwithin the foreland of orogens during tectonism for anumber of reasons.Joints from natural hydrofracturing often form onthe foreland margins of orogens during orogeny. Theconclusion that the joints are syntectonic is based ontwo observations. First, the joints parallel the σ 1direction associated with the development of tectonicstructures like folds. Second, the joints locally containmineral fill which formed at temperatures andfluid pressures found at a depth of several kilometers;thus, they are not a consequence of the recent crackingof rocks in the near surface. The origin of suchjoints may reflect increases in fluid pressure withinconfined rock layers due to the increase in overburdenresulting from thrust-sheet emplacement, or fromthe deposition of sediment eroded from the interior ofthe orogen.During an orogenic event, the maximum horizontalstress is approximately perpendicular to the trend ofthe orogen. As a consequence, the joints that form bysyntectonic natural hydraulic fracturing are roughlyperpendicular to the trend of the orogen. Because thestress state may change with time in an orogen, laterformedjoints may have a different strike than earlierformedjoints, and the joints formed during a givenevent might not be exactly perpendicular to the foldtrends where they form. Such joint patterns are typicalof orogenic foreland regions, but may also occur incontinental interiors.Joints are commonly related to faulting, and thesefall into three basic classes. The first class is composedof regional joints that develop in the country rock dueto the stress field that is also responsible for generationand/or movement on the fault itself. Since faults areusually inclined to the remote σ 1 direction, the jointsthat form in the stress field that cause a fault to move7.5 ORIGIN AND INTERPRETATION OF JOINTS155
2917-CH07.pdf 11/20/03 5:11 PM Page 156will not be parallel to the fault (Figure 7.17a). The secondclass includes joints that develop due to the distortionof a moving fault block. For example, thehanging wall of a normal fault bock may undergo someextension, resulting in the development of joints, or thehanging block of a thrust fault may be warped as itmoves over the fault, if the underlying fault surface isnot planar, and thus may locally develop tensilestresses sufficient to crack the rock (Figure 7.17b). Thethird class includes joints that form immediately adjacentto a fault in response to tensile stresses created inthe wall rock while the fault moves. Specifically, duringthe development of a shear rupture (i.e., a fault), an enechelon array of short joints forms in the rock adjacentto the rupture. These joints merge with the fault and areinclined at an angle of around 30° to 45° to the faultsurface; they are called pinnate joints (Figure 7.17c). t 1The acute angle between a pinnate joint and the faultindicates the sense of shear on the fault, as we will seein Chapter 8.When the stress acting on a region of crust isreleased, the crust elastically relaxes to attain a differentshape. This change in shape may create tensilestresses within the region that are sufficient to createrelease joints, such as occurs in relation to folding.Folded rocks may be cut by syntectonic naturalhydrofractures, a process manifested by joints orientedat a high angle to the fold-hinge, as we described previously(Figure 7.18). In addition, during the developmentof folds in nonmetamorphic conditions, jointsoften develop because of local tensile stresses associatedwith bending of the layers (Figure 7.18). Jointsresulting from this process of outer-arc extension havea strike that is parallel to the trend of the fold-hinge,and may converge toward the core of the fold. Ifdevelopment of folds results in stretching of therock layer parallel to the hinge of the fold, thencross-strike joints may develop.Finally, joints may develop in a region ofcrust that has been subjected to broad regionalwarping, perhaps due to flexural loading of thecrust. Like folding, joint formation reflects tensilestresses that develop when the radius of curvatureof a rock layer changes.(a)(b)(c)FIGURE 7.17 (a) Formation of joints in the hanging-wall block of aregion in which normal faulting is taking place. (b) Formation of jointsabove an irregularity in a (reverse) fault surface. (c) Pinnate joints alonga fault.7.5.5 Orthogonal Joint SystemsIn orogenic forelands and in continental interiors,you will commonly find two systematicjoint sets that are mutually perpendicular. Insome cases, the joints define a ladder pattern(Figure 7.19a), in which the joints of one setare relatively long, whereas the joints of theother are relatively short cross joints which terminateat the long joints. In other cases, thejoints define a grid pattern (Figure 7.19b), inwhich the two sets appear to be mutually crosscutting. The existence of such orthogonal systemshas perplexed geologists for decades,because at first glance it seems impossible fortwo sets of tensile fractures to form at 90° toone another in the same regional stress field.Recent field and laboratory studies suggest anumber of possible ways in which orthogonalsystems develop, though their application tospecific regions remains controversial.In orogenic forelands, an orthogonal jointsystem typically consists of a strike-paralleland a cross-strike set, defining a ladder pattern.The two sets may have quite different origins.156 JOINTS AND VEINS
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