THE DEVELOPMENT OF ECHELON VEIN ARRAYS IN THE McKIM LIMESTONE: EASTERN MONUMENT UPWARP, UTAH Solomon Seyum and David D. Pollard Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 email: [email protected]
Abstract Keywords: The McKim Limestone unit is a well-exposed, 3m- Echelon veins, shear zones, pressure solution seams, thick stratum that stretches over much of Raplee McKim limestone, Raplee, Comb, anticline, monocline, anticline and across the antiformal hinge of Comb fold, fractures monocline as nearly continuous, kilometer-scale outcrops in some areas. Two sets of systematic, Introduction carbonate-filled, echelon veins and complementary Shear deformation of sedimentary strata is often echelon pressure solution seams are common and localized within relatively narrow zones. The varieties apparently present only within this unit. Each set of shear zones are placed into three general categories, maintains sub-parallel orientations across those parts brittle, brittle-ductile, and ductile, based on the inferred of the folds with modest limb dips. Left-stepping and gradient in displacement across the shear zone. The end right-stepping echelon vein arrays have mean trends of member of idealized brittle shear zones would be 097°and 140°, respectively. These two sets of marked by a sharp discontinuity where the approximately planar shear zones often cross-cut in a displacement gradient is zero elsewhere in the material conjugate geometry. They also are observed to abut one but infinite at the discontinuity. A shear zone where another or appear as only one set. The initial formation sharp discontinuities in displacement develop in of thin echelon veins suggests a brittle elastic conjunction with significant displacement gradients is deformation. However, some veins have greater considered a brittle-ductile shear zone. A ductile shear apertures to trace length ratios, and some have zone has a continuous gradient in displacement between sigmoidal shapes, suggesting ductile inelastic the zone boundaries. deformation. Two kinds of brittle-ductile shear zones are Current conceptual models of brittle-ductile shear observed in the Earth’s crust. One example is that of zones of this type infer principal strain and stress dragged and folded rocks adjacent to a fault. The orientations based on geometries and displacement second example is the development of echelon fractures indicators, such that the regional greatest compressive in a narrow zone that otherwise has deformed by stress bisects the acute angle between a pair of continuous and significant displacement gradients conjugate shear zones at the time of formation. (Ramsay, 1980). Limestone strata exposed across Previous studies have suggested that the shear strain Raplee anticline and Comb monocline of the eastern across a localized zone produces echelon extension Monument Upwarp, in southeastern Utah, display fractures, whose initial orientation within the zone is systematic arrays of echelon veins and pressure solution defined by the local principal strain direction. seams (Fig. 1) that are classified as the second example Other limestone and grainstone strata in this field of brittle-ductile shear zones. We suggest that area do not exhibit conjugate shear zones with echelon understanding their origin and development will vein arrays. A suggestion is that the specific contribute to the currently established tectonic history compositional structure of the McKim Limestone of the area that has, to date, been inferred from regional favored vein formation over jointing under the same jointing events and fold development. stress state. These observations open the door to Echelon veins found on the eastern Monument investigate the constitutive properties of the different Upwarp have been found exclusively within the thin, lithologies and to compare models of brittle-ductile 3m-thick, McKim Limestone unit. The McKim unit and shear zones based on a complete mechanics to those several other similar limestone units in the area act as based on kinematic relationships alone. The final cap rocks for trapping oil in source rocks below. objective of such an investigation would be to relate the Understanding the nature of deformation of this unit causative stress field to the regional tectonic history. may shed light on the structural integrity and permeability potential of limestone cap rocks in oil reservoirs.
Stanford Rock Fracture Project Vol. 22, 2011 L-1 displacement, and strain fields can not be related to a causative, stress field using a purely kinematic approach. A few studies (e.g. Olson and Pollard, 1991) have used a complete mechanics approach to investigate echelon veins, but these are limited to elastic deformation. A comparison of shear zone formation with jointing in adjacent rock layers suggests that lithology plays a key role in governing the difference in failure mechanisms that is apparently manifest in strength and ductility contrasts.
Geographic and Geologic Settings The study area is located in the far, southeastern corner of the state of Utah, between the towns of Bluff Figure 1. Echelon veins within conjugate shear zone and Mexican Hat. Comb monocline forms the 130 km arrays. Image of the top surface of the McKim boundary of the eastern Monument Upwarp, a Limestone on the antiformal axis of Comb prominent topographic high of the Colorado Plateau monocline. Left-stepping echelon veins (left of rock tectonic province (Fig. 2). The San Juan River, a major hammer) often trend east-west. Right-stepping tributary to the Colorado River, bisects the two main veins (right of hammer) often trend northwest- southeast. folds; having incised deep valleys that showcase the red hues of the jointed stratigraphy. The river marks the northern border of the Navajo Nation. Identification of these previously undocumented Comb monocline and Raplee anticline are adjacent, echelon vein and pressure solution seam arrays adds north-south trending, arcuate, reservoir-scale folds new information about the tectonic history of the dipping away from each other as much as 40° west eastern Monument Upwarp. They are investigated at (Raplee) and 60° east (Comb). Raplee anticline is a Raplee anticline and Comb monocline to make double-plunging fold that dwarfs in size next to Comb interpretations of the evolving tectonic stress state. monocline with a respectable 14 km fold axis length. Analyses of their geometries, mechanical interactions, The spatial relationship of the two folds and their vein fill textures, spatial and structural distributions, vertical geometries are illustrated in the geologic map and the textural features of the host rock composition and cross-section in Fig. 3. The age and lithology of are used to provide parameters to model deformation. strata exposed across Comb monocline range from Most previous models of echelon vein shear zones are Pennsylvanian marine sedimentary rocks to Jurassic limited to analyzing deformation as homogeneous terrestrial sedimentary rocks. Raplee anticline reveals simple shearing or variable simple shearing ina pre- Pennsylvanian- to Permian-age marine strata (Fig. 4). defined zone. Because the equations of motion are not invoked and no constitutive laws are prescribed, the
Figure 2. Geologic map of southeastern Utah displaying the geographic locations of Monument Upwarp, Raplee Ridge and Comb Ridge. Modified from map by Bump and Davis (2003); (Mynatt et al., 2009). A white dashed line traces the length of Comb Ridge. Raplee Ridge is outlined with a rectangular box.
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Figure 3. a) Geologic map (modified from USGS map, Bull. 11, Plate 1, O’Sullivan et al. (1964)). The stratigraphic column to the right highlights the PPr unit (the portion of the Rico Formation exposed across the folds whose the uppermost stratum is the McKim Limestone) and the Pch unit (Halgaito Tongue that rests on top of the McKim in the study area). Green outlined regions indicate the extent of the McKim Limestone, so far visited, that exhibit echelon vein and pressure solution seam arrays. Red outlined regions are the extent of the McKim visited that do not display these structures. To the far left of the map is an incomplete red-outlined region that is partially outside of the map limits. The red arrow at the top right corner points in the direction of a McKim roadside outcrop visited 2 km outside of the map limits that does not have echelon vein or solution seam arrays. b) Geologic cross section from the linear transect displayed on the map.
Both folds are interpreted as forming in response to Moore, 1931). Since there are no exposures of thrust slip along deep-seated thrust faults during an east-west faults associated with these folds, this interpretation is tectonic compressional event, likely due to the based on their similarity to other major folds found Cretaceous to Eocene Laramide orogeny (Gregory and elsewhere on the Colorado Plateau (Kelley, 1955;
Stanford Rock Fracture Project Vol. 22, 2011 L-3 and/or thrust faulting processes (Mynatt et al., 2009). Fig. 5 is a regional geologic structure map of systematic joint distribution that illustrates this method of determining fracture-fold age relationships. Abutment relationships of joint sets at the outcrop scale was used to determine the relative ages of jointing (Fig. 6). Abutting joints are suggested to be younger than the joints they terminate against. Mynatt et al. (2009) identified three main, systematic joint sets across Raplee anticline, and they form the basis of the current conceptual tectonic model of the geologic area (Fig. 7). The first joint set to develop, Set I, initiated prior to folding and has a mean orientation of 092°/90°. Set II joints are approximately orthogonal to Set I with a mean orientation of 197°/88°, and also initiated prior to folding. The third dominant joint set, Set III, initiated and developed during folding with a 320°/89° mean orientation. Some Set I joints have been reactivated in shear during folding, and infilling of the joint sets likely occurred at various times over the tectonic history.
Observations The McKim Limestone has the largest bedding top exposure area across Raplee anticline, and is only 1m to Figure 4. The stratigraphic column of units exposed 3m thick. It is the topmost stratum of the Rico at Raplee anticline. Fractures were measured and Formation, overlying a shaley siltstone unit (Shale 7), mapped in the Goodrich, Shafer, Mendenhall, and and is the unit just below the Halgaito Tongue of the Unnamed members of the Rico Formation. Modified from Ziony (1966); (Mynatt et al., 2009). The McKim Cutler Formation (Fig. 4). The compositional texture of Limestone and Halgaito Tongue are highlighted. the McKim Limestone can vary substantially over tens of meters. Common properties found in varying Tindall and Davis, 1999). Using inverse theory, the abundances are fossil clasts, quartz silt grains, micritic geometry of the thrust fault beneath Raplee anticline carbonate cement, and sparry calcite partially, or was estimated by Hilley et al. (2010) using an elastic totally, filling pore spaces (Fig. 8). boundary element model to displace originally Systematic joints are pervasive throughout some of horizontal layers to best match the fold geometry. The the sedimentary layers exposed in the area. In addition best-fit thrust fault strikes 003°, dips 47° to the east, to joints, the McKim Limestone contains two sets of and has an upper tip line that is eight-hundred meters systematic, conjugate shear zones marked by echelon below the current surface near the center of Raplee veins and complementary echelon pressure solution anticline. seams. These features have been observed on the Deep incision due to surface drainage processes in northern edge of Raplee anticline and on the antiformal the last several Ma (Wolkowinsky and Granger, 2004), hinge of Comb monocline (Fig. 3), not previously and an arid climate that has prohibited the growth of recorded by Ziony (1966) or Mynatt et al. (2009). The pervasive vegetation, provides exceptional outcrops to top and bottom surfaces of the McKim Limestone are map deformation structures across varying structural hummocky and jagged. Numerous attitude positions of the folds. measurements in a given area reveal that bedding dips in the locations where these structures have been found Structural History do not exceed 10°. The forelimbs of either fold have not yet been explored for these structures, so there is a Previous research on fractured strata of the eastern possibility that they are present there as well. Similar Monument Upwarp has suggested a multi-stage process structures are shown in figure 11c by Mynatt et al. associated with folding (Mynatt et al., 2009; Ziony, (2009); though their interpretation of the structure is not 1966). Systematic joint sets found both on and off the similar to what is being proposed here. anticlinal and monoclinal folds are thought to have The arrays of echelon veins and pressure solution developed prior to folding, whereas joints restricted to seams maintain sub-parallel orientations across the the folds are attributed to stresses induced by folding
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Figure 5. A regional map of fracture Sets I (red dashes), II (blue dashes), and III (green dashes) in the area on and around Raplee Ridge, UT. Set I and II are more or less widely distributed. Set III joints are concentrated at the fold forelimbs. Structure contours are estimated for the top of the McKim limestone. Black lines with arrows are fold hinges, where the arrows are pointing toward the forelimbs. Map modified from Ziony (1966); (Mynatt et al., 2009).
Figure 6. Image on left. Fracture map of the Goodrich sandstone on the southern plunge of Raplee anticline showing the abutting relationships of Set I (red lines) and Set II (blue lines) joints (Mynatt et al., 2009).
Figure 7. Block diagrams on the right. Conceptual models of four deformation events and fracture formation at Raplee anticline, with inferred orientations and relative magnitudes of the local principal stresses at each relative time step. Set I and Set II joints (red and blue lines) formed within the layers when they were flat- lying. Set III joints (green lines), reactivation of Set I joints in shear, and the development of less dominant joint sets (orange and purple lines) developed in strata that was tilted, with an associated remote principal stress configuration that was parallel and perpendicular to the tilted strata.
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Figure 8. Thin section images of the McKim Limestone from four different locations. Dark areas are micritic carbonate matrix, and the white “specks” in a), b), and d) are quartz grains. All four limestone samples are considered wackestones according to the Dunham textural classification scheme. a) Quartz grains and pore spaces filled with sparry calcite. Fossils are a minor component. Of the four images shown, this is the only limestone sample taken from a relatively undeformed portion of an outcrop containing echelon veins. b) Large fossil fragments are entrained within a dispersion of quartz-grains in a carbonate matrix. Vacant or filled pore spaces are not significant. c) Abundant fossil fragments with sparry calcite that has filled pore spaces within fossils and the matrix. There is a low concentration of grains and, therefore, might be classified as a mudstone/wackestone. d) Porosity and fossil fragments are insignificant as quartz grains are found clustered and dispersed throughout the carbonate matrix. folds (Fig. 9). Left-stepping and right-stepping echelon their width can vary from 2cm to 10cm. Shear zones vein arrays have mean trend values of 097°and 140°, often cross-cut in a conjugate geometry, but do not respectively. Note that most field data thus far has been necessarily imply contemporaneous formation. They collected on the northern edge of Raplee anticline. Fig. may also abut one another, or appear as only one set. 1 is an example of two array sets observed on Comb Fractures sometimes develop through the centers of monocline that agree closely with the mean orientation shear zones, sub-parallel to the zone boundaries, and data in Fig. 9. Satisfactory field measurements of array provide a vertical cross sectional view of echelon veins dip angles are lacking due to uncertainty in interpreting (Fig. 11). the planar geometry of shear zones, so a vertical dip is A relative age relationship between the two sets of assumed. The length of a single echelon vein array shear zones is not apparent. The observations made thus ranges from tens of centimeters to several meters, and far do not show consistent abutting or offsetting
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Figure 9. Stereonet plots of all measured array strike orientations using SpheriStat™ v2.2. Satisfactory dip values were not attainable in the field and are not used in this analysis. No correction was made for bedding orientation. Bedding dip did not exceed 10° where measurements were made. a) Gaussian density distribution contours (K=100) are used to indicate areas of data concentration. The scale bar indicates the number of points that have significant influence on a given position centered within a 1% counting circle. E is the expected count within the circle if the data points would be evenly distributed across the net. In this case, E = 1.67; E = N/K; where N is the number of data points and K is the counting kurtosis (from 100 points across the net, in this case). Using a cluster analysis tool, two distinct sets of arrays are identified (red and green dots), and their mean value is given by the plotted lines that represent the mean planes of the two sets; the red line for arrays with left-stepping veins, and the green line for arrays with right-stepping veins. b) Illustrates the range of array orientations as measured in the field without using the cluster analysis tool. The mean orientations are shown as lines, and vary only by a degree from those shown in a).
a)
Figure 10. a) An outcrop map of conjugate arrays of b) c) echelon veins and pressure solution seams. b-c) Accompanying stereonets show the mean trends of veins and pressure solution seams measured from the map. Stereonet b) is associated with the 140° trending array and stereonet c) with the 095° trending array.
Stanford Rock Fracture Project Vol. 22, 2011 L-7 relationships between shear zones. Those oriented sigmoidal geometries, and their three-dimensional approximately east-west with left-stepping veins and features suggest that the echelon vein arrays may owe right-stepping pressure solution seams appear as if to their development to a localized shear zone, whose have undergone a greater intensity of deformation boundaries can be approximated near the points where compared to the northwest-southeast shear zone set in veins terminate. Since the tips of multiple, individual most places, but this is not always true. veins within an array are present (Fig. 10a), the echelon Most veins are oriented between 25° and 75° to the geometry is not a result of material removal of a single trace of an array, and they tend to parallel the array of vein by multiple pressure solution seams (Hancock and the adjacent set, which can be compared in Figs. 10b-c. Atiya, 1975). There is typically a large overlap between neighboring Echelon pressure solution seams are abundant and veins with little, if any, noticeable mechanical complementary to the echelon veins within many shear interaction (Fig. 10a). Sigmoidal vein shapes are zone arrays. Pressure solution seams are present where present (Fig. 12), but they are not abundant. The veins are not, but it is not true for the opposite. When spacing between veins varies from millimeters to found with complementary veins, they are typically centimeters, and their apertures are wider than joints perpendicular to the veins and exhibit a wavy, positive- observed in the McKim and adjacent units. Pull-aparts, relief trace on bedding tops (Fig. 10a). At the outcrop as described by Peacock and Sanderson (1995), are not scale, their range in length is a few millimeters to a few common, but there does appear to be linkage of centimeters with similar spacing distances to veins. neighboring veins to form wider aperture veins (Fig. A relative age relationship between veins and 12). In cross-section (Fig. 11), echelon veins are pressure solution seams is not yet evident. Material observed to be isolated from one another with finite removal due to pressure dissolution is seen in most lengths in the horizontal and vertical directions, places (Figs. 10, 11), and is portrayed by discontinuous revealing that they are not a result of growth from a vein geometries. A neighboring pressure solution seam single dilatant parent fracture (Pollard et al., 1982). within the same shear zone may terminate against a The orientation of veins with respect to the trend of vein as well. Termination of a seam against a vein the associated array, the amount of vein overlap, might be a coincidence, in the sense that it has not yet
Figure 11. Echelon vein arrays in cross section. The arrows point in the trend direction of the conjugate pair of shear zones. Left-stepping echelon veins that trend 097° do not extend perpendicular through the bedding. Right-stepping echelon veins that trend 155° do appear to extend through bedding.
Figure 12. Sigmoidal veins on the top surface of the McKim. This photo was taken from the location indicated on the image of Fig. 11.
Stanford Rock Fracture Project Vol. 22, 2011 L-8 had the opportunity to propagate or dissolve any further interpretations, but also infer the orientation of the before ceasing in its present state. It is also possible that regional principal stress state at the time of formation of pressure solution seams and veins developed at the echelon veins in localized shear zones. Based on the same time, since their orientations suggest a similar orientation of vein arrays (Fig. 8) and the stepping local stress state. If so, precipitates that now fill veins arrangement of veins therein, it is inferred that the mean may have derived from the dissolution of carbonates acute bisector (~119°) might represent the regional from local pressure solution seams (Beach, 1974; greatest compressive stress orientation at the time of Durney, 1972). deformation (Ramsay, 1980b); resolving right-lateral shear stress on 097°-oriented arrays and left-lateral Kinematics shear stress on 140°-oriented arrays. In order to test this interpretation, we suggest utilizing a realistic range of An objective of this study is to reconcile the constitutive properties and the equations of motion to occurrence of joints and shear zones in adjacent strata represent the deformation. that apparently experienced the same variations in No evidence has been found of original host rock stress state before and during folding, but deformed textures in the McKim that traverse the shear zones to quite differently. If the deformation mechanisms are give a quantitative measure of localized shear understood for the two structures, an inference can be displacement. This would provide a direct evaluation of made about the local and regional stress states at the the theories that infer strain direction based on vein and time of fracture development. array orientations alone (Peacock and Sanderson, Shainin (1950) is one of the earliest geologists to 1995). In thin section, we are able to determine relative record field observations of systematic echelon tension displacement of individual veins from fossils and older fractures that resemble the features observed in the veins that have been offset. Fig. 13d shows at least two McKim Limestone (Fig. 4). His objective was also to veins offsetting an elongate fossil and suggests that compare the inferred stresses that formed these there was predominantly opening displacement structures with the inferred jointing and folding associated with those veins. Future analysis of vein stresses. In the West Virginian Alleghenies of the infill texture might provide data that can be used to Appalachian orogeny, where Shainin (1950) conducted infer the magnitude and direction of relative his field work, conjugate sets of echelon tension displacement within a vein at the time of precipitation fractures occur exclusively within the Athens (Ramsay and Huber, 1983; Segall and Pollard, 1983). Limestone unit. The inferences from kinematic The ratio of vein aperture to trace length is a indicators, supported by analogue experiments analyses measure of the average strain in the surrounding rock (Cloos, 1932), is that echelon tension fractures formed due to vein opening. For brittle elastic deformation the by shear across a zone of a given width. Assuming upper bound on strain would be a few percent so one homogeneous simple shear strain within the shear zone, would expect aperture to length ratios less than about the local maximum extension would be at 45° to the 1/100. Some observed vein aperture to length ratios are shear zone boundaries (Shainin, 1950). For soft clay much greater than one would infer a deformation that shear deformation experiments by Cloos (1955), exceeded the elastic limit. Some echelon veins exhibit opening fractures formed at 45° from the shear sigmoidal shapes (Figs. 11a, 13a). Ramsay (1967) direction when the clay was wet, and at 60° when dry. suggests that their development is due to continued The width of the zone depends on the thickness of the shearing of the zone after initially linear veins have clay cake. Shainin (1950) hypothesized that either the already formed (Fig. 14). For a localized zone of greatest compressive stress bisected the acute angle homogeneous simple shear strain, the direction of between conjugate incipient shear planes, or that two greatest extension is at 45° to the zone boundary, so this distinct events of deformation took place. The interpretation of the formation mechanism of is the direction proposed for initial vein formation. echelon veins has stimulated a lively debate since the Ramsay suggests that continued shearing may rotate the ° 1950s (Olson and Pollard, 1991). Two interpretations centers of the veins to an angle greater than 45 with the deduced from early field studies of echelon veins are: shear zone boundary, while propagation at the vein tips 1) the initiation of fractures in extension within a shear will proceed at an angle of 45°. If there is a longitudinal zone (Cloos, 1955; Shainin, 1950); and 2) pre-existing strain normal to the shear zone boundary, referred to as echelon fractures served as a zone of weakness where a tranpressive or transtension, veins may initiate at an subsequent shearing can be resolved (Beach, 1975; angle greater or less than 45°, respectively. While the Roering, 1968). More recent studies (Mazzoli and Di relationship between initial vein orientation and the Bucci, 2003; Ramsay, 1980; Rickard and Rixon, 1983; local greatest extension direction may be qualitatively Smith, 1996; Srivastava, 2000; Wiltschko et al., 2009) useful for interpreting field observations, fracture continue to employ geometrical data and kinematic mechanics offers no basis for such a relationship and it has not been tested in rock mechanics experiments.
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b) d)
Figure 13. Outcrop and thin section images of very different vein geometries with similar array orientations. Field locations are meters apart. Arrows and circles indicate the approximate location from where the thin sections came from. The orientation of the thin section images does not match that of the outcrop images. Veins are filled with euhedral and fibrous calcite crystals. a) Sigmoidal vein with a wide aperture. b) Smaller veins merge with, or propagate away from, the main vein. c) Thin, closely spaced, left-stepping echelon veins. d) A tight cluster of sub-parallel veins. Several locations indicate vein opening displacements.
Figure 14. Conceptual model of simple shear resolved across a localized zone, with the principal extension at a 45° angle within the zone relative to the zone boundary (modified from Ramsay (1967)). As shearing continues, propagation of the joint advances at 45° and the centers of the initial veins rotate to an angle greater than 45°. The shear zone width gets larger with continued shearing.
Stanford Rock Fracture Project Vol. 22, 2011 L-10 Field data shows a wide variety of echelon vein geometries for shear zone arrays that trend in similar orientations (Figs. 10, 13a, 13c). Ramsay and Huber (1987) suggest that the orientation of an array with respect to the remote stress state will produce vein geometries whose orientations may be more or less than 45° to the array trace; a volumetric component of displacement as well as shear displacement. What is the sensitivity of echelon vein geometry to the remote stress state and array orientation? Fig. 15 is a conceptual model of the initial geometries and boundary conditions of the cracks and the zone. The cracks may open or shear, and their propagation is driven by these modes of deformation. An elastoplastic yield criterion for crack propagation determines if the extensional and/or shear strain produces crack tip stress that exceed the yield strength of the material. The orientation of the potential shear zone to the inferred greatest compressive stress, based
on kinematic analysis described previously, varies from Figure 15. A proposed 2-dimensional model design 10° to 32° with a mean orientation of 19°. The that may capture shear zone formation that best orientation of initial cracks with respect to the array resembles the geometries observed in the field. trace varies from 25° to 75°. Varying one, or both of Initial echelon cracks, with array-crack measured these orientations under a fixed remote loading geometries (thick dashed lines), are embedded in a condition, may produce resulting echelon crack homogeneous material with prescribed elastic geometries that can be compared with kinematic property values for limestone similar in compositional properties as the McKim limestone. interpretations. Shear displacement boundary conditions are The effects of internal fluid pressure, pressure applied parallel to the trace (thin dashed lines) of dissolution, and interaction with a conjugate shear zone echelon cracks to ultimately calculate the stress array are ignored for this preliminary setup of echelon and displacement fields in the material. vein development within a brittle-ductile shear zone. It is conceivable that a three-dimensional approach to the Although sigmoidal vein geometries described above shear zone problem would be appropriate since echelon do exist, the sigmoidal shape in Fig. 12 does not reflect veins have finite horizontal and vertical lengths. that. In this example, the center of the vein has a Advancement in numerical modeling of the two- smaller angle with respect to the shear zone boundary dimensional, single shear zone of echelon cracks, than the vein tips. This is also noticed in Fig. 13a. shown in Fig. 15, may lead to analyses of more complex systems that employ these additional Model Setup parameters. Ratios of vein aperture to trace length greater than 1/100, sigmoidal geometries of veins, and pressure Objectives of Future Work solution seams are evidence that the development of Field and petrographic observations and shear zones on the McKim Limestone did not occur in a measurements, and detailed mapping of systematic purely elastic material. The initiation of veins in these shear zones, serve to identify mechanisms of shear zones might have occurred when the local deformation and relative age relationships, as well as deformation was dominantly elastic, but continued their spatial variations with respect to structural shearing and vein opening was at least partly influenced position on the folds. Mechanical interpretations of by inelastic deformation. Therefore, any model that deformation will contribute to the tectonic history, and intends to represent the formation of echelon veins will will employ appropriate material properties extracted need to consider a material with constitutive properties from the literature, and boundary conditions inferred that are not limited to linear elasticity. Finite element from the geologic and tectonic setting. This information programs such as ANSYS or Abaqus FE are will motivate and constrain mechanical models of the considered. structures using finite element methods to compute stress and displacement fields throughout the material.
Stanford Rock Fracture Project Vol. 22, 2011 L-11 We suggest that this investigation will lead to a better Peacock, D.C.P., and Sanderson, D.J., 1995, Pull-aparts, shear understanding of the constitutive properties and fractures and pressure solution: Tectonophysics, v. 241, strengths of sedimentary rocks prior to, and/or during, p. 1-13. folding. Pollard, D.D., Segall, P., and Delaney, P.T., 1982, Formation and interpretation of dilatant echelon cracks: Geological Society of America Bulletin, v. 93, p. 1291-1303. Acknowledgments Ramsay, J.G., 1967, Folding and Fracturing of Rocks: New Nate Levine and Chris Zahasky for their field York, McGraw-Hill Book Company, 568 p. assistance in the summer of 2009 and for being the first Ramsay, J.G., 1980, Shear zone geometry: A review: Journal of Structural Geology, v. 2, p. 83-99. to recognize echelon veins. NSF-CMG program, grant Ramsay, J.G., and Huber, M.I., 1983, The Techniques of number EAR 0417521, and the Rock Fracture Project Modern Structural Geology, Volume 1: Strain Analysis: for their support. London, England, Academic Press, 307 p. Rickard, M.J., and Rixon, L.K., 1983, Stress configurations in References conjugate quartz-vein arrays: Journal of Structural Geology, v. 5, p. 573-578. Beach, A., 1974, A geochemical investigation of pressure Roering, C., 1968, The geometrical significance of natural en- solution and the formation of veins in a deformed echelon crack-arrays: Tectonophysics, v. 5, p. 107-123. greywacke: Contributions to Mineralogy and Petrology, Segall, P., and Pollard, D.D., 1983, Joint formation in granitic v. 46, p. 61-68. rock of the Sierra Nevada: Geological Society of Beach, A., 1975, The geometry of en-echelon vein arrays: America Bulletin, v. 94, p. 563-575. Tectonophysics, v. 28, p. 245-263. Shainin, V.E., 1950, Conjugate sets of en echelon tension Cloos, E., 1932, "Feather Joints" as Indicators of the fractures in the Athens Limestone at Riverton, Virginia: Direction of Movements on Faults, Thrusts, Joints and Bulletin of the Geological Society of America, v. 61, p. Magmatic Contacts: Proceedings of the National 509-517. Academy of Sciences of the United States of America, v. Smith, J.V., 1996, Geometry and kinematics of convergent 18, p. 387-395. conjugate vein array systems: Journal of Structural Cloos, E., 1955, Experimental analysis of fracture patterns: Geology, v. 18, p. 1291-1295. Geological Society of America Bulletin, v. 66, p. 241- Srivastava, D.C., 2000, Geometrical classification of 256. conjugate vein arrays: Journal of Structural Geology, v. Durney, D.W., 1972, Solution-transfer, an Important 22, p. 713-722. Geological Deformation Mechanism: Nature, v. 235, p. Tindall, S.E., and Davis, G.H., 1999, Monocline development 315-317. by oblique-slip fault-propagation folding: the East Gregory, H.E., and Moore, R.C., 1931, The Kaiparowits Kaibab monocline, Colorado Plateau, Utah: Journal of Region; a geographic and geologic reconnaissance of Structural Geology, v. 21, p. 1303-1320. parts of Utah and Arizona, U.S. Geological Survey Wiltschko, D.V., Lambert, G.R., and Lamb, W., 2009, Professional Paper, p. 161. Conditions during syntectonic vein formation in the Hancock, P.L., and Atiya, M.S., 1975, The development of footwall of the Absaroka Thrust Fault, Idaho-Wyoming- en-échelon vein segments by the pressure solution of Utah fold and thrust belt: Journal of Structural Geology, formerly continuous veins: Proceedings of the v. 31, p. 1039-1057. Geologists' Association, v. 86, p. 281-286. Wolkowinsky, A.J., and Granger, D.E., 2004, Early Hilley, G.E., Mynatt, I., and Pollard, D.D., 2010, Structural Pleistocene incision of the San Juan River, Utah, dated geometry of Raplee Ridge monocline and thrust fault with 26Al and 10Be: Geology, v. 32, p. 749-752. imaged using inverse Boundary Element Modeling and Ziony, J.I., 1966, Analysis of systematic jointing in part of the ALSM data: Journal of Structural Geology, v. 32, p. 45- Monument Upwarp, southeastern Utah [Ph.D. thesis], 58. University of California, Los Angeles. Kelley, V.C., 1955, Monoclines of the Colorado Plateau:
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804. Mazzoli, S., and Di Bucci, D., 2003, Critical displacement for normal fault nucleation from en-échelon vein arrays in limestones: a case study from the southern Apennines (Italy): Journal of Structural Geology, v. 25, p. 1011- 1020. Mynatt, I., Seyum, S., and Pollard, D.D., 2009, Fracture initiation, development, and reactivation in folded sedimentary rocks at Raplee Ridge, UT: Journal of Structural Geology, v. 31, p. 1100-1113. Olson, J.E., and Pollard, D.D., 1991, The initiation and growth of en échelon veins: Journal of Structural Geology, v. 13, p. 595-608.
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