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Analysis of Minor Fractures Associated with Joints and Faulted Joints

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Analysis of Minor Fractures Associated with Joints and Faulted Joints Journal o/Structural Geology, Vol. 13. No. 8, pp. 865 to 886, 1991 0191-8141/91 $03.00+0.00 Printed in Great Britain © 1991 Pergamon Press plc Analysis of minor fractures associated with joints and faulted joints KENNETH M. CRUIKSHANK, GUOZHU ZHAO and ARVID M. JOHNSON M. King Hubbert Structural Geology Laboratory, Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, U.S.A. (Received 6 April 1990; accepted in revised form 7 April 1991) Abstract--In this paper, we use fracture mechanics to interpret conditions responsible for secondary cracks that adorn joints and faulted joints in the Entrada Sandstone in Arches National Park, U.S.A. Because the joints in most places accommodated shearing offsets of a few mm to perhaps 1 dm, and thus became faulted joints, some of the minor cracks are due to faulting. However, in a few places where the shearing was zero, one can examine minor cracks due solely to interaction of joint segments at the time they formed. We recognize several types of minor cracks associated with subsequent faulting of the joints. One is the kink, a crack that occurs at the termination of a straight joint and whose trend is abruptly different from that of the joint. Kinks are common and should be studied because they contain a great deal of information about conditions during fracturing. The sense of kinking indicates the sense of shear during faulting: a kink that turns clockwise with respect to the direction of the main joint is a result of right-lateral shear, and a kink that turns counter- clockwise is a result of left-lateral shear. Furthermore, the kink angle is related to the ratio of the shear stress responsible for the kinking to the normal stress responsible for the opening of the joint. The amount of opening of a joint at the time it faulted or even at the time the joint itself formed can be estimated by measuring the kink angle and the amount of strike-slip at some point along the faulted joint. Other fractures that form near terminations of pre-existing joints in response to shearing along the joint are horsetail fractures. Similar short fractures can occur anywhere along the length of the joints. The primary value in recognizing these fractures is that they indicate the sense of faulting accommodated by the host fracture and the direction of maximun tension. Even where there has been insignificant regional shearing in the Garden Area, the joints can have ornate terminations. Perhaps the simplest is a veer, where the end of one joint segment turns gradually toward a nearby joint segment. The veer is a result of a nearby, shear-stress-free face such as a joint surface. Our greatest difficulty has been explaining long overlap of parallel joint segments, that is, the lack of veer. The only plausible explanation we know is suggested by the research of Cottrell and Rice, that high compression parallel to the joint segments will tend to prevent the joints from turning toward one another. The most interesting and puzzling fractures are stepped joints and associated echelon cracks, in which the slight misalignment of the stepped joints suggests mild left-lateral shear, while the strong misalignment of echelon cracks that continue the traces of the stepped joints suggests strong right-lateral shear. The stepped joints are thought to reflect local left-lateral shearing that acted over an area of several thousand square metres, whereas the stepped echelon cracks reflect local interaction between the tips of nearby joints propagating in different directions. INTRODUCTION faulting--all were extremely mild deformations---so one can compare secondary cracks that formed where joints THE understanding of rock fracture has progressed far were subjected to negligible shearing with secondary enough for us to infer conditions of faulting and jointing cracks that formed where there was significant faulting from the forms of faults and joints, provided their following the jointing. Some of the secondary cracks geologic history has been simple. Studies combining appear to have formed at the same time as the joints theoretical and field work have provided basic rules with themselves, a result of interaction between adjacent which we can interpret conditions responsible for eche- joint segments. Others appear to be individual kinks or lon cracks, tail cracks and bridge fractures associated groups of horsetail cracks that formed near terminations with joints or faulted joints. of pre-existing joints as a result of later faulting along the The purpose of this paper is to use the understanding joints. of fracturing to develop some rules for interpreting In most areas it is no simple matter to differentiate traces of secondary cracks associated with joints and fractures that developed at the time the joints opened faulted joints. Our examples are along fractures in from fractures that developed when the joints sub- Entrada Sandstone in the Garden Area (Fig. 1) of sequently slipped as faults. We are able to make such a Arches National Park, near Moab, Utah, although differentiation in the Garden Area, however, because in Segall & Pollard (1983a) and Davies & Pollard (1986) part of the area there was only jointing (zero slip) and in have described analogous secondary cracks on joints and other parts there was faulting following the jointing. The faulted joints in Sierran granite. secondary fractures in the area of zero slip resulted from The Garden Area provides an excellent opportunity interaction of the joints at the time they formed, essen- to study joints and faulted joints. The area was subjected tially in response to mode I loading, resulting from to two periods of jointing and two periods of strike-slip regional extension. In most parts of the Garden Area, SG 13:8-A 865 866 K.M. CRU~KSHANK, G. ZHAO and A. M. JOHNSON -6 ~i i iii~:.. N 0 0.5 1.0 km Fig. 1. The Garden Area of the Arches National Park, U.S.A., showing fracture traces visible on air photographs. Inset shows that the Garden Area is on the SW limb of the Salt Valley anticline. the joints have subsequently been subjected to shearing from shearing superimposed on the joints, in addition to (mode II and III deformations; Lawn & Wilshaw 1975): secondary fractures due to shear imposed through inter- a left-lateral slip in the northern third and a right-lateral action. slip in the central half of the area (Fig. 2).Comparison of The theoretical basis of our analyses is broad, but we joint terminations between areas of shear and no shear rely most heavily on investigations of crack stability and allows us to examine secondary fractures that result effect of mode II shearing on mode I fracturing by Minor fractures associated with joints and faulted joints 867 Cottrell & Rice (1980), Sumi et al. (1985) and Olson & the lower part of the Morrison Formation (Cater & Pollard (1989), and on the investigation of effects of Craig 1970). mode III shearing on fracturing by Pollard et al. (1982). There are three sets of joints in the Garden Area (Fig. To study crack interactions, we use an alternating 2) which Dyer (1983) termed J-l, J-2 and J-3, with J-3 method similar to that explained by Pollard & Holzhau- being the youngest. Traces of J-3 joints trend about sen (1979). Results of these analyses are applied to some N10°W throughout the Garden Area and dip vertically field examples in order to infer conditions at the time the or steeply eastward. Joints J-1 trend about N60°E and secondary cracks formed. occur only in the southern part of the area. Joints J-2 trend about N30°E and occur in both the northern and southern parts of the area (Fig. 2). Reinvestigation of the fractures (Zhao & Johnson in review) indicates that SETTING joints J-1 and J-2 are jointed faults; that is, they are band faults or fault zones that formed in a shearing (mode II The Garden Area is on the SW flank of the Salt Valley or III) deformation and subsequently opened in mode I anticline, the northwesternmost of the salt-cored anti- deformation, and are in this respect similar to stepped clines of the Paradox fold and fault belt in western fault segments described in a large landslide in central Colorado and eastern Utah (Dane 1935, Elston et al. Utah by Fleming & Johnson (1989). In the Garden 1962, Cater & Craig 1970, Doelling 1985). The area is Area, the trends of the J-1 and J-2 joints were inherited largely within Arches National Park, and about mid- from the trends of strike-slip faults that formed in distance between the Salt Valley anticline in the NE and response to NE-SW compression. In the vicinity of the Moab fault zone in the SW (Fig. 1). The rocks in the joints J-2, joints J-3 degenerate into numerous short Garden Area have been only mildly deformed and dip segments, bridge cracks or horsetail cracks, in some about 7 ° toward the SW. places oblique to the regional trend, particularly in the The joints in the Garden area occur within the white northern part of the area (Fig. 2), indicating that joints sandstone of the Moab Member which, here, is about J-3 are younger than joints J-2, as indicated by Dyer 10 m thick. According to Dyer (1988), underlying the (1983). Although Dyer decided that joints J-1 are the Moab Member are 70-95 m of red, cross-bedded sand- oldest, their age is unknown. Here we are going to stone of the Slickrock Member, and overlying the Moab discuss structures associated with the J-2 and J-3 joints of Member are about 12 m of thinly bedded, alternating Dyer.
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