Palaeostresses inferred from macrofractures, Colorado Plateau, western U.S.A.

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Tectonophysics, 206 (1992) 219-243 Elsevier Science Publishers B.V., Amsterdam

Palaeostresses inferred from macrofractures, Colorado Plateau, western U.S.A. Fransoise Bergerat, Catherine Bouroz-Weil and Jacques Angelier Tectonique Quantitative, U.R.A. 1315 C.N.R.S., Bte 129, Universitk P. and M. Curie, lplace Jussieu, 75252 Paris cedex 05, France

(Received January 11, 1991; revised version accepted November 6, 1991)

ABSTRACT Bergerat, F., Bouroz-Weil, C. and Angelier, J., 1992. Palaeostresses western U.S.A. Tectonophysics, 206: 219-243.

inferred from macrofractures,

Colorado Plateau,

In order to (1) understand the tectonic significance of jointing, and (2) reconstruct the regional palaeostress history, we have studied macrofractures, especially joints, in the Colorado Plateau. Nine major joint set trends have been identified in the Colorado Plateau. Each set includes joints that may differ in significance and in age. The joints are commonly extensional joints but shear joints were also found. The main tectonic events reconstructed from macrofractures analysis include: (1) a pre-Laramide compressional event with 01 N 45”E; (2) three Laramide compressional events with 01 N 65”E, N 95”E and N 115”E successively; and (3) three Neogene extensional events with a3 N 65”E, N 85”E and N 120”E successively. These events are correlated with major tectonic events described in neighbouring areas of the Western American Cordillera.

Introduction The purpose of this paper is to reconstruct the palaeostress evolution of the Colorado Plateau by means of macrofracture analysis. Particular attention is given to joints (as defined later), including comparison with the results of fault slip analysis in the same area (Angelier and Bergerat, 1989). Tectonic studies in the European platform adjacent to the Alpine orogen have demonstrated that the major tectonic events are well documented in the sedimentary succession of the platform, although the amount of total deformation remains very small (Letouzey and TrCmolG?res, 1980; Bergerat, 1985,1987; Letouzey, 1986). Thus, minor tectonic features (e.g., minor faults, stylolites, tension gashes, etc.) observed in the platform domain provide a key to understanding the

Correspondence to: F. Bergerat, Tectonique Quantitative, U.R.A. 1315 C.N.R.S., Bte 129, UniversitC P. and M. Curie, 4 place Jussieu, 75252 Paris cdex 05, France.

relationships between regional stress and deformation in the neighbouring orogens. According to the regional results presented herein, significant levels of consistency emerge from comparisons between independent palaeostress reconstructions based on analyses of joint patterns and of fault slip data sets. The inversion methods used for fault slip data sets (e.g., Angelier, 1984) are not applicable to joints, so that a specific methodology must be adopted. This paper deals with the orientation analysis and interpretation of macro-fracture data excluding faults (i.e. joints and tension gashes). Different types of macrofractures are thus related through investigations on their mechanical significance which highlights the interest of structural analyses taking all kinds of fractures into account. The Colorado Plateau is not a simple foreland platform, but occupies a particular location within the Cordilleran domain, east of the Overthrust Belt and west of the Rocky Mountains front. Thus, it was especially interesting to determine how far this kind of platform, located within an

OO40-1951/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

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orogen, may have recorded the main tectonic events of which one. As a result, our analyses in the Colorado Plateau suggest that intra-erogenic platforms may provide reliable records of major tectonic events, as platforms in front of orogen do. Methods Definition of joints

In this paper, we use the term “macrofractures” for faults, joints and tension gashes. The term “joints” is used exclusively for fractures without displacement detectable for the naked eye. Joints were often studied, especially in fractured reservoirs (e.g., Friedman, 1975; Stearns and Friedman, 1972; Nelson, 19791, but their mechanical interpretation remains difficult because of the absence of perceptible displacement. For this reason, their interpretation has often been disputed. For example, two orthogonal vertical joint sets in the Appalachian Plateau (New York area) were interpreted by Engelder and Geiser (1980) as tension fractures, and by Muehlberger (1961) and Scheidegger (1982) as shear joints. In the present section, our interpretation (Bouroz, 1990) of the mechanical significance of jointing is simply summarized in order to support the following regional analysis of jointing in the Colorado Plateau. Mechanical significance of joints

Some geologists believe that joints are only created under tension. This assumption cannot account for the existence of conjugate sets of synchronous joints which intersect at 30”-60” (Fig. 1A). Furthermore, in many cases, such joints are oblique to maximum (al) and minimum (~3)

palaeostress axes independently determined from the distribution of tension gashes and faults. They consequently develop with a major shear component, the bisector of the acute dihedral angle indicating the direction of al (Muehlberger, 1961; Price, 1966; Hancock, 1985). A third type of joints has been defined by Hancock (1985, 1987) as “hybrid joints”, transitional between pure extension and shear joints; these joints clearly have a shear component along their surface at the time of fracturing. Experimentally induced shear fractures showed very little displacement even at the microscopic scale (Friedman, 1963). Thus, the lack of visible shear alone does not indicate that a fracture is a tensile one. Some characteristic surface features (Figs. lB1D) observed on joint surfaces (Hodgson, 1961) are generally interpreted as related to joint propagation. However, for many geologists, some of them have a possible tectonic significance (Murgatroyd, 1942; Syme-Gash, 1971; Bahat, 1979; Engelder, 1982; Bahat and Engelder, 1984; Ernstson and Schinker, 1986). Among these features, the hackle marks, also called twist-hackles (Kulander et al., 19791, form en-echelon fractures developed in fringe zones. In the Colorado Plateau, many sets of joints bear abundant hackle marks (Fig. lD), the sense of the en-echelon pattern being generally remarkably homogeneous for a given set of joints. Consequently, we think that hackle marks may occur more frequently on surfaces of shear and hybrid joints, and used them as a criterion to determine the sense of the shear component of jointing. Chronological criteria

Establishing the chronology between several joint sets is often difficult and requires a combination of observations.

Fig. 1. Some examples of joint patterns and of the main surface features. (A) System of joints dipping oblique to horizontal bedding (Triassic sandstones, Zion National Park, Utah). (B) Arrest-lines, conchoidal steps (Triassic sandstones, Zion National Park, Utah). (C) Plumose marks (Triassic sandstones, Zion National Park, Utah). (D) En-echelon pattern of hackle marks (Permian sandstones, Monument uplift, Utah). (E) North-south-trending tension joints (shales and sandstones of Upper Cretaceous, Uncompahgre uplift, Colorado). (F) System of orthogonal joints trending N 6O”E and N 1SO”E(shales and sandstones of Upper Eocene, Uinta basin, Utah).

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Fig. 3. Example of extension joints reactivated as strike-slip faults during a later tectonic event (Chime Formation, Triassic, Canyon de Chelly, Arizona). (a) Extension joint planes corresponding to ENE-WSW compression and NNW-SSE extension (large white arrows indicate probable directions of compression and extension). (b) Reactivation of some of these joints as right-lateral strike-slip faults (accompanied by neoformed left-lateral strike-slip faults) in a younger stress field corresponding to ESE-WNW compression and NNE-SSW extension. Faults shown as thin curves, slickenside lineations as dots, strike-slip as double arrows. Computed palaeostress axes crl, 02 and ~3 shown as 5, 4- and 3-branch stars, respectively (direct inversion method, Angelier, 1984, 1990). Large solid arrows indicate computed directions of compression (~1) and extension (~3).

Fig. 2. Example of relative chronology between two joint sets on a “pavement” near Mexican Hat (Utah) (after Bouroz, 1990). (A) Joints of set b abut against older joints of set a. (B) Horizontal profiles of roughness of a and b joint sets, surfaces being smoother for older joints a than for younger joints b. (C) Stereographic projection of a and b joints, with larger dispersion of younger joint set b. Equal-area projection, lower hemisphere. N = geographic north; M = magnetic north.

(a) Geometrical relationsh&s of joint sets. The easiest way to establish the relative chronology between various joint sets consists of studying “X” or “T” relationships between joints (Fig. 2A, Kulander et al., 1979; Hancock, 1985; DeGraff and Aydin, 1987). (b) Roughness and dkpersion of azimuths. Commonly, among two or more sets of joints planes, the older one presents smooth surfaces and the younger show rougher surfaces (Fig. 2B; Barton, 1984). Cc) Geometrical relationships with motwclines. The analysis of angular relationships between

Fig. 4. Examples of comparison between geometry of joint and faults systems. Legend of diagrams as for Figure 3, except for diagrams (b) and cd): normal slip shown as centrifugal arrows, reverse slip as centripetal arrows. (A) Normal shear joint system (a) and conjugate normal fault system (b) in Jurassic limestones of the San Rafael Swell (Utah). (B) Reverse shear joint system (a) in the Jurassic limestones near Bryce Canyon (Utah) and reverse fault system (b) in the Jurassic sandstones of the Uncompahgre Uplift (Colorado).

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joint sets and strata may allow to determine the ages of different joint sets, relative to the different stages of the flexuration process (Friedman and Stearns, 1971; Stearns and Friedman, 1972; Bouroz et al., 1989b). cd) Correlations between joints and faults. Three types of chronology may be established after the comparison with faulting: (1) the reactivation of some joints of a joint set as faults during a later tectonic event (Fig. 31, (2) a similar geometry between shear-hybrid joint sets and fault sets (Fig. 41, and (3) the correlation between trends of extension joints and palaeostress axes computed by fault slip data analysis in the same area. (e) Stratigraphical criteria. A system of joints which affects a stratigraphic section beneath a given level, and is never found above it (in similar rock types), is thus dated. Unfortunately, in the Colorado Plateau, outcrops of Oligocene or younger rocks are scarce while some major tectonic events postdate the Oligocene, so that this criterion could not be extensively used.

The Colorado Plateau in the structural framework of the Western American Cordillera The Colorado Plateau forms a broad platform (Figs. 5-7) where major deformations are located in flexure zones (Kelley, 1955; Reches, 1976,1978; Davis, 1978, 1979; Reches and Johnson, 1979) and along some large faults (Kelley and Clinton, 1960; Thompson and Zoback, 1979; Wong and Humphrey, 1989). The existence of intensively deformed areas around the Plateau such as in the Rocky Mountains, the Rio Grande Rift, and the Basin and Range province (Fig. 51, suggests that it behaved as a stable rigid block while these surrounding areas were strongly tectonised. The knowledge of tectonic history enables one to understand the orientation and evolution of the main tectonic events at its boundaries, and to date by comparison some of the palaeostress field characterized within the Colorado Plateau. Thus, it is indispensable to summarize first the succession of the major tectonic phases already identified around the Plateau.

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The Western American Cordillera

The morphostructural features of the Cordillera are interrupted to the north by the Lewis and Clark Lineament, which marks the southern limit of the Canadian cordilleran segment, and to the south by the Texas Lineament which marks the northern limit of the Mexican Sierra Madre (Fig. 5). These lineaments represent complex strike-slip fauh zones that have been active several times during the Mesozoic and the Cenozoic. However, it is probable that the tectonic importance of these lineaments during the Phanerozoic is limited. The broadening of the Cordillera in the U.S.A. was principally related to the occurrence of Cretaceous-Palaeogene horizontal subduction there (Dickinson and Snyder, 1978; Bird, 1988). From west to east, the Western American Cordillera may be divided in three geological areas: the coastal range, the Pluto-volcanic domain and the cordilleran domain. The Colorado Plateau and neighbouring units belong to the cordilleran domain, in which autochthonous and allochthonous units are distinguished. The autochthonous part includes the Rocky Mountains, the Rio Grande Rift and the Colorado Plateau. The Gverthrust Belt and most of the Basin and Range Province (Great Basin) belong to the allochthonous part (Fig. 5). The horst and graben structure of the Basin and Range Province is due to a widespread extensional tectonism, principally Mio-Pliocene in age. The earlier system of thrusts of Mesozoic age is completeIy cut by high-angle and low-angle normal faults (Wernicke, 1981; Allmendiger et al., 1983; Howard and Jahn, 1987) and accompanied by block tilting (Stewart, 1980; Brun and Choukroune, 1983). Two main extensional events have been characterized based on classical geological observations as well as fault slip data analysis (Anderson and Ekren, 1977; Zoback and Zoback, 1980; Zoback et al., 1981; Faugere, 1985; Angelier et al., 1985; Michel-No&l, 1988). The first event (BRl) is a NE-SW-trending extension, the second one (BR2) is an ESE-WNW-trending extension. The age of the change a3 stress may be Middle Miocene (17 Ma) according to Ekren

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et al. (19681, Late Miocene (between 10 and 6 Ma) according to Zoback et al. (1981), or PlioQuaternary (between 4 and 2 Ma) according to Michel-Noel(1988). The most recent studies show that this clockwise rotation is at least younger than 10 Ma, but its actual age is not accurately known (Michel-No&l et al., 1990). An E-W-trending extension with uncertain chronology has also

been characterized locally (Faugere, 1985; Michel-Noel, 1988). The Overthrust Belt (O.T.B.), also called the Sevier Orogenic Belt (Armstrong, 19681, extends along more than 4800 km of the North American Cordillera, from Alaska to Mexico. It includes several east-vergent thrust units resulting from tectonism during the Late Jurassic, the Middle

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Fig. 5. Simplified structural map of the Western American Cordillera (modified after Aubouin et al., 1986): I = lineament; 2 = normal or undifferentiated fault; 3 = thrust fault; 4 = Quaternaty volcanism; 5 = Tertiary volcanism; 6 = main basins; 7 = large granitic batholiths; 8-11: Cordilleran domain-autochthonous deformed during Laramide orogeny (8); autochthonous, undeformed or moderately deformed (9); allochthonous (10); major metamorphic core complexes (II); 12 = Pluto-volcanic area; 13 = Coast Ranges. D. = Denver; L. A. = Los Angeles; L.V. = Las Vegas; P. = Phoenix, S. L.C. = Salt Lake City; C. L. = Caltam Lineament; L.C.L. = Lewis and Clark Lineament; T.L. = Texas Lineament; S. A. F.S. = San Andreas Fault System.

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and Late Cretaceous and the Paleocene (Nevadan, Sevier and Laramide orogenies; Armstrong, 1968; Villien, 1980; Price, 1981; Allmendiger et al., 1983; Le Vot, 1984; Blanchet et al., 1986; Oriei, 1986). The Rio Grande Rift, was only recognised in the 1970s as a coherent tectonic feature. It is a complex zone of Cenozoic deformation (Callender et al., 1989). Two extensional events have been characterized (Morgan and Golombek, 1984; Morgan et al., 1986): a NE-SW-trending extension, characterized by low-angle normal faulting, occurred from the Late Oligocene to the Early Miocene; and an E-W-trending extension, characterized by high-angle faulting, occurred during the Late Miocene-Early Pliocene. Seismic activity shows that the rift is still active. The major structures of the Rocky Mountains, trending northwest-southeast to north-south, developed during the Laramide orogeny. Classically, they are attributed to a combination of vertical movements and thrusts (Berg, 1962). Recently, two events of the Laramide orogeny have been described in New Mexico and southeastern Arizona (Cabezas, 1989; Rosaz, 1989; Sosson, 1989; Sosson and Bouroz, 1989): the first one (Ll) generated folds and thrusts during the Late Cretaceous-Paleocene, whereas the second one (L2) was characterized by ESE-WNW-trending left-lateral strike-slip faulting during the Middle Eocene. The Colorado Plateau

The main structures of the Colorado Plateau are dominated by a few major faults and by asymmetric uplifts. The Hurricane fault, the Toroweap-Sevier fault, the Bright Angel fault system and the Mesa Butte fault system trend generally N-S to NE-SW (Fig. 6). The uplifts (or swells) and basins are commonly bounded by large monoclines (flexure zones), such as the East-Kaibab, Grand, San Rafael, Nacimiento, Waterpocket, Comb Ridge, Hogback, Defiance and Echo Cliffs monoclines (Fig. 6). The flexuration process has been interpreted as a drape-folding process or as a buckling process (Reches, 1978; Reches and Johnson, 1978) above major

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Fig. 6. Major structural features of the Colorado Plateau (after Kelley, 1955; Hodgson, 1961; Oetking et al., 1967; Davis, 1978). Monoclines (thick lines): CR = Comb Ridge; D= Defiance; EC= Echo Cliffs; EK = East Kaibab; G = Grand Canyon: H = Hogback; N = Nacimiento; SR = San Rafael; W = Waterpocket. Faults (barbed lines): BAF = Bright Angel fault system; HF = Hurricane fault; MBF = Mesa Butte fault system; SF = Sinyala fault: TSF = Toroweap-Sevier fault. Dotted pattern: major basins.

basement faults (Davis, 1978). These monoclines are considered to be Laramide in age, although evidences for this age are not definitive. In some places (Waterpocket, East Kaibab), Eocene formations rest unconformably on Cretaceous formations (Gilbert, 1877; Gregory and Moore, 1931); the Grand monocline, however, includes Eocene strata. According to Baltz (1953, referred to in Kelley, 19551, the Hogback monocline was initiated in latest Cretaceous time, continued to develop during the Paleocene and was completed by the Early Eocene. Many other monoclines (such as Comb Ridge, San Rafael, Echo Cliffs) can not be dated directly. Despite these ambiguities, most of the monoclines probably result from Laramide tectonism, their major deformation ~~es~nding to the major erogenic events of the adjacent Rockies (Kelley, 1955). Correlatively with the study of joint patterns, fault slip data analysis has been carried out in the

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whole Colorado Plateau (Angelier and Bergerat, 1989); detailed results will be given in a forthcoming paper. We summarize herein the main results in order to compare them with the jointing analysis.

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Six main directions of palaeostress have been recognized. Three events correspond to horizontal compressional stresses (al) with average directions N 45”E, N 65”E and N 1lO”E. Three other events correspond to horizontal extensions

Prg. 1. tieological map of the Colorado Plateau. States (noted at four corners): U = Utah; C = Colorado; A = .Arizona; NM = New Mexico. I = Quaternary; 2 = undifferentiated Tertiary; 3 = Mio-Pliocene; 4 = Eocene; 5 = undifferentiated Cenozoic; 6 = Cretaceous; 7 = Jurassic; 8 = Triassic, Permian and Pennsylvanian; 9 = undifferentiated Pennsylvanian to Jurassic; 10 = Cambrian to Devonian; 11= Precambrian; 12 = undifferentiated Precambrian to Devonian. Asterisks: location of sites of joint study (except for Boyce, Zion and Grand Canyon where only few selected sites are shown; for complete study and site location in this area, see Bouroz, 1990).

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((~3) that trend N 65”E, N 85”E and N 120”E. Palaeostress regimes are in fact more numerous. Especially, a single orientation of horizontal compression may correspond to domination by reverse faulting, to domination by strike-slip faulting or to both modes. Likewise, a single orientation of extension may correspond to normal faulting, strike-slip faulting or both. Where strike-slip predominates, the presence of compatible normal or reverse faults enables us to identify the tectonic event as extensional or compressional. The succession of these main six events has been established using relative chronology observations such as cross-cutting faults or cross-cutting slickenside lineations. In addition, no evidence of the N 45”E compressional event was found in strata younger than Jurassic. Likewise, the N 65”E compressional event was not found in the strata younger than Late Cretaceous, and the N 115”E compressional event was not found in the Middle Eocene-Oligocene formations. We concluded that the N 45”E compressional event corresponds to a pre-Laramide event, probably due to the end of the Nevadan orogeny or to the beginning of the Sevier one (PL). Likewise, we concluded that the two other events are Laramide, the N 65”E one (Ll) being Cretaceous and the N 115”E one (L2) being Early Eocene (Angelier and Bergerat, 1989; Sosson and Bouroz, 1989; Bouroz, 1990). The three other events, which are extensional, affect the whole observable stratigraphic section: N 65”E (BRl), N 85”E (BRl’?) and N 12O”E (BR2) extensions successively. They probably correspond to the extensional tectonism of the Basin and Range Province (Angelier and Bergerat, 1989; Bouroz, 1990). Tectonic significance of joints in the Colorado Plateau Distribution of the main joint sets

About 7200 tectonic joints have been measured in 200 sites of the whole Colorado Plateau, in rocks whose ages range from Mississippian to Oligocene (Fig. 7). In the canyons of Brice and Zion and the Grand Canyon, a more detailed study was carried out (Bouroz et al., 1989a),

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because canyon walls allow observations on a vertical section of more than 3000 m. The complete results of this particular study have been presented in a PhD dissertation (Bouroz, 1990). Most joints in the Colorado Plateau are vertical (Figs. 8-12). Nonetheless, some oblique joints were observed, principally in the Jurassic and the Cretaceous, the angle between fracture planes and bedding ranging from 25” to 65” depending on the site (Figs. 1A and 4). They represent reverse or normal shear joints and may be easily correlated with reverse and normal fault systems (see Methods-section). Only vertical joint sets are discussed below. Nine main joint sets can be characterized by their average azimuthal distribution (Fig. 13): N 25-30”E, N 45”E, N 60-65”E, N 70-80”E, N 95-100”E, N llO-115”E, N 125-130”E, N 145155”E and N 20”W-15”E (i.e. N 160-195”E). Some of these joint sets have tightly defined trends (e.g., the N 95”E set), whereas for others, the dispersion of trends is large (e.g., the 20”W-15”E set). Most sets include tension gashes parallel to tension joints. Distribution of the surface features

Among the main surface features, only the twist-hackles (Fig. 1D) may statistically present a regular geometry on joint sets. In numerous sites, it was noticed that these features show a consistent attitude for a given joint set. Although the mechanism is quite different and the amount of shear is generally very small relative to tension, we interpreted these small oblique features in the same way as nascent Riedel shear fractures, because the component of shear that they indicate is in the same sense. For clarity, we shall describe them as right-lateral or left-lateral twist-hackles. Figure 14 shows the remarkably homogeneous distribution of such right-lateral or left-lateral twist-hackles on some joint sets. Three main groups of twist-hackles may be characterized: N 20”W-15”E, that are right-lateral, and N 30-40”E and N 125-135”E, that are left-lateral. The N 65-80”E-trending joint set presents both leftlateral and right-lateral twist-hackles, depending on the sites (Fig. 14).

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If we consider the joints bearing regular twisthackles as shear joints or hybrid joints, the bisecting line of the acute angle between associated left- and right-lateral systems may represent the direction of the al maximal stress. This hypothesis, however, is difficult to check in most cases because (1) it is not common to find in the same site both left- and right-lateral twist-hackles, and (2) the common presence of several successive

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sets of joints of different types makes the identification of contemporaneous sets difficult. As a result, we tried to associate opposite-vergent sets of shear (or hybrid) observed in different sites. For example, we identified a N 20”E compression/N 1lO”E extension, related to N 160-180”E “right-lateral” and N 30-40”E “left-lateral” joint sets, a N 95”E compression/N 5”E extension, related to N 60-70”E “right-lateral” and N 120-

Fig. 8. Macrofracture data (excluding faults) in the San Rafael Swell and Uinta Mountains area (Utah). Diagrams are numbered according to site reference numbers in geological map of Figure 7. For each site: (1) the total number of joints and tension gashes is indicated in parentheses on the right top of the diagram, and (2) where chronology between two or more sets of joints is determined, the sequence numbers of sets are indicated in the diagram (in square brackets, I, 2, 3).

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Fig. 9. Macrofracture

I-G-

data (excluding faults) in the Grand Junction area and in the San Juan basin (New Mexico, Colorado). Explanations as for Figure 8.

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130”E “left-lateral” sets, and a N 45”E compressionfN 135”E extension related to N 0-15”E “right-lateral” and N 75-80”E “left-lateral” sets (Fig. 14). Incidentally, the large azimuthal dispersion of the approximately north-south set of joints might thus be explained by the grouping of tension and shear (or hybrid) joints due to different tectonic events. Chronological rdationships Geometrical relationshipsbetweenjoints

Figures 8-12 show examples of relative chronological relationships between different sets of joints. Figure 15 contains overall information on the relative chronology of joints, based on

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Fig. 10. Macrofractu~

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their “T” geometrical relationships and additional roughness and dispersion data. The resulting relationships indicate that chronology is as follows: the N 15”W-15”E system is probably older than the N 2%30”E and N 40-45”E systerns, and generally older than the N 60-65”E one. The N 25-30”E system is older than the N 9%lOO”E,the N 125-130”E and the N 150-160”E ones; it is also generally older than the N llO115”E system. The N 45”E system is clearly older than the N 60-65”E and the N llO-115”E ones, and generally older than the N 150-160”E systern. The N 60-65”E and the N 95-1OO”Esystems are older than the N 125-130”E one. Concerning other relationships, they were often found inconsistent: apparent contradictions may result from

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data (excluding faults) in the Cedar City and Boyce Canyon area (Utah). Explanations as for Figure 8.

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different phenomena such as permutation of stresses or stress release, as well as from pure coincidence in trend of joint sets formed during separate tectonic events. Our classification in nine main joint sets may seem somewhat arbitrary, especially for joint sets with the largest azimuthal dispersion. Nonetheless, various analyses have been carried out with narrower or broader azimuthal classes (implying

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larger or smaller numbers of joint sets, respectively) and the results did not significantly differ, suggesting that the nine-sets solution is acceptable. Geometrical relationships between joint sets and monoclines

Four Plateau,

major monoclines of the Colorado regarded as Laramide in age as dis-

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Fig. 11. Macrofracture data (excluding faults) in the Grand Canyon area and the Zion National Park (Arizona, Utah). Explanations as for Figure 8.

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data (excluding faults) in the Mexican Hat area and in the Canyon de Chelly Witma, as for Figure 8.

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Utah). EZxplanations

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cussed before, have been analyzed (Bouroz et al., 1989b): East Kaibab (near Pine Valley), Comb Ridge (near Mexican Hat) and Hogback (near Farmington), trending NNE-SSW, and Waterpocket (near Escalante), trending NNW-SSE (Fig. 6). The monocline axes trending NNW-SSE have probably been formed during the first Laramide episode (Ll), those trending NNESSW during the second Laramide episode (L2). A joint set which trends N 80-85”E is clearly the oldest, and it is older than the two successive flexuration processes. Two other joint sets are younger than the N 80”-85”E one; they are older than the L2 flexuration process and their trends are N O-15”E and N 145-150”E. The development of N 90”E and N 110”E-trending joint sets is contemporaneous with the second flexuration

Age

1Cretaceous

AL

process, and some N lo-20”E-trending joints clearly represent extrados tension joints of the flexures. A N 40”E trending joint set clearly is also contemporaneous of the L2 flexuration process. Finally, a N 165-170”E-trending joint system clearly developed later than the L2 flexuration process.

Correlations between joint trends and principal stress axis trends

We have seen in a previous section that the main tectonic events have been characterized by fault slip data analysis (Angelier and Bergerat, 1989; Bouroz, 1990), with N 45”E, N 65”E and N 115”E compressional events, and then N 65”E, N 85”E and N 120”E extensional events successively.

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a

Fig. 13. Distribution of the macrofracture data (excluding faults) in relation to the ages of the stratigraphic series (Y-axis) and to their direction (X-axis). KI = undifferentiated joints; n = mineralized joints and tension gashes. Each square represents one set of joints in one site, at the level of the formation in which the measurements were done and below the corresponding azimuth. Note that names, facies and thicknesses of stratigraphic formations change in different regions of the Colorado Plateau (the stratigraphic column shown is synthetic, with the names of the major formations, the most characteristic lithologies and the average thicknesses).

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The joint sets which are most easily compared with palaeostress axes independently computed from fault slip data sets are extensional (tension gashes and extension joints), because they directly indicate the direction of horizontal stresses, minimum trr3) and m~mum (~1 or ~2). The main trends of such tension gashes are: N 15”W15”E, N 25”E, N 45”E, N 7.5-80”E, N 95”E and N 125-13O”E. In many sites these tension gashes are parallel to joint systems, which suggests that the latter probably represent extension joints. Some of these extension joint sets may be related to impressional tectonic events. For exampIe, the N 45”E-trending set is parallel to the al axis of the pre-Laramide compressional event (PL). Note that N 45”E tension gashes were not found in rocks younger than Triassic.

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The N 75-80”E-trending set is approximately parallel to the compression of the first Laramide compressional event (average al trends N 657O”E). The N 125-130”E-trending set is almost parallel to the impression of the second Laramide compressional event (average al trends N 115125”E). Other extension joint sets may be related to extensional events, such as the N 25”E one which is perpendicular to the extension of the second Basin and Range event (a3, N 115”E). Finally, two tension gashes and joint sets are not directly reiated to known tectonic events: the N 95”E- and the N lS”W-lS”E-trending sets. We consider that these sets probably correspond to tectonic events that were not characterized by

o EoceneR Oligocene 0

Cretaceous

0 N Jurassic 0 * w 5 Triassic

u - Permian 0 N 0 z Pennsylvanian 4 L Miss~ssip~en Fig. 14. Distribution of sense of shear on shear or hybrid joints, based on hackle marks. A = right-lateral shear component; A = left-lateral shear component. Three systems of associated right- and left-lateral shear and/or hybrid joints suggest N 20%, N 45”E and N 95”E trends of compressional axes {see explanations in text).

F BEKGEKAI'L.,

N

60 - 65E +

N

70-&E

N

95-IOOE

I N 110 - 115E 2

N 125 - 130E

N 150- 160E

Fig. 15. Chronological relationships among macrofracture sets (excluding faults). Nine macrofracture sets characterized and listed by azimuthal classes. Relative chronology successions established based on “T” relationships, roughnesses and azimuthal dispersions of joints. For each relationship, arrow goes from older to younger set. The number on each arrow indicates how many times this chronological relationship has been observed.

faulting due to the low level of tectonic stress; the N lS’W-15”E set, nonetheless, may partly correspond to Basin and Range extensional events. Although non-vertical joints are rare in the Colorado Plateau, some sets of dipping joints clearly display the same geometry as fault systems related to known tectonic events. These joints have consequently been interpreted as reverse or normal shear joints (Fig. 4). For instance, NWSE-trending joints that dip about 30-40” (e.g., Fig. 10, site 30) are interpreted as reverse joints related to the N 45”E pre-Laramide compressional event (PL). They affect the Carmel Formation, Late Jurassic in age, but they do not affect younger formations, suggesting an age already indicated by faults and extension joints. Other joint systems that trend NNW-SSE, N-S and NNE-SSW, and dip about 50-60”, affect all exposed formations including the Eocene; they may correspond to N 65”E (BRl), N 85”E (BRl’I and N 1lo-120”E (BR2) Basin and Range extensional events, respectively (e.g., Fig. 8, sites 3 and 14). As discussed before, the distribution of twisthackles on joints surfaces indicates the presence

A,

of a shear component of stress along these joints and its sense. It is possible to characterize three shear-hybrid joint sets, with a N 45”E compression/N 135”E extension, a N 95”E compression/ N 5”E extension and a N 20”E compression/N 110”E extension, respectively. The N 45”E compression may be due to a pre-Laramide tectonic event (PL) already characterized by tension gashes, reverse shear joints and faulting (see above). Likewise, the N 1lO”E extension may be due to the last event of Basin and Range tectonism (BR2). The N 95”E compression does not correspond to a tectonic event identified by fault analysis; however, many tension gashes and joints trending N 95”E are probably due to the same event. Significance of the different joint sets

The nine main vertical joint sets discussed herein were formed, in terms of mechanical significance, as follows (Fig. 16).

Fig. 16. Mechanical significance of the different sets of macrofractures (excluding faults) in the Colorado Plateau in relation to the Mesozoic and Cenozoic tectonic events. Y-axis = azimuths of the different sets of macrofractures; X-axis = successive tectonic events. PL = Nevadan or Sevier Orogeny; Ll, LI’, L2 = successive events of the Laramide phase; BRl, ERl’, BR2 = successive tectonic events of the Neogene extensional period. 2 = undifferentiated macrofractures; 2 = leftlateral shear and/or hybrid joints; 3 = right-lateral shear and/or hybrid joints; 4 = reverse shear and/or hybrid joints; 5 = normal shear and/or hybrid joints; 6 = extension joints and tension gashes.

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The N 15”W-15”E set contains: (1) tension gashes and joints probably related to a tectonic event not characterized by faulting in our sites and older than the recognized Nevadan or Sevier compressional event: (2) tension gashes and joints related to a part of the Basin and Range extension (at least BRl and BRl’): and (3) right-lateral shear or hybrid joints related partly to the N 45”E Nevadan or Sevier compressional event (PL) and partly to the N 120”E Basin and Range extensional episode (BR2). The N 2%30”E set includes (1) tension gashes and joints resulting from the N 120”E extensional event BR2, and (2) probably some left-lateral shear or hybrid joints related to the same event BR2. However, based on geometrical relationships among joints, some joints of this set are older than this last Basin and Range extension (Fig. 15): they may be due to the Laramide flexuration process as extrados extension fractures. The N 45”E set consists of some shear or hybrid joints of the N 120”E extensional episode (BR2) and of numerous tension gashes and joints related to the pre-Laramide event (PL). The N 60-65”E probably includes extension joints due to the first Laramide event (Ll). The adjacent N 70-85”E set of tension gashes and joints may correspond to a small clockwise rotation of the stress field during this event. Some N 65”E right-lateral shear or hybrid joints may belong to a N 95”E compressional event. Some 70-80”E left-lateral shear or hybrid joints are related to the N 45”E compressional event (PL). The N 95-100”E set essentially includes tension gashes corresponding to a tectonic event that has not been characterized by fault slip data analysis: an east-west compressional event or a north-south extensiona one. The reIative chronology reconstructed after the geometrical relationships among joint systems shows that the N 9%100”E tension gashes and joints may correspond to a N 95”E compressional event, Laramide in age, occurring between Ll and L2 (Ll’). The N 125-130”E set consists of (1) numerous tension gashes and joints, probably due to the second Laramide event (L2), and (2) left-lateral shear or hybrid joints corresponding to the N 95”E compressional event (Ll’).

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Finally, the N 145-155”E set is poorly developed and does not clearly belong to any tectonic event described before. Succession of the main tectonic events in the Colorado Plateau and the evolution of the Western American Cordillera

(1) The oldest tectonic episode in the Colorado Plateau recognized herein corresponds to a palaeostress field with Noah-south compression and east-west extension, characterized by extension joints and tension gashes trending approximately north-south. This event predates the N 45”E compression attributed to the Nevadan or Sevier orogeny, which is not accurately dated itself. It is difficult to compare this event with a known tectonic event, because its stress orientations are oblique to the major structural trends since the Jurassic (Blanchet et al., 1986). Two other enigmatic sets of joints, trending N 80-9O”E and N 140-lSO*E, have been recognized as older than Laramide flexure processes; their mechanical significance is unclear and their correlation to any tectonic event hazardous. Bouroz (1990) observed that these pre-Laramide joints trend parallel to major basement faults, and assumed that they propagated upward into the sedimentary cover during reactivations of basement faults. According to this interpretation, the directions of such joints are inherited and thus do not significantly reflect the orientations of regional stress during their propagation. (2) The analysis of both faulting and jointing (AngeIier and Bergerat, 1989; Bouroz, 1990; this paper) allowed identification of a pre-Laramide event (PL), with N 45”E compression, postdating the Jurassic and characterized by reverse and strike-slip faulting, northeast-southwest trending tension gashes and joints, northwest-southeast reverse shear joints and probable conjugate hybrid or shear vertical joints. This event may correspond to one of the tectonic events that occurred at the end of Jurassic times (Nevadan orogeny) or during the early Cretaceous (e.g., Oregonian episode of the Sevier orogeny) and resulted in

238

successive thrust faults in the OTB (Villien, 1980; Le Vot, 1984; Oriel, 1986). In Arizona, before the Senonian, thrusting of basement slices occurred, as well as ductile deformation (Piman phase of Drewes, 1981; Sosson, 1989). To the southwest, in the Sonora, some tectonic units with a northeast vergence were also emplaced after the Albian or the Cenomanian (Rangin, 1982; Pubellier, 1987; Calmus and Radelli, 1987; Sosson and Calmus, 1989; Sosson et al., 1989). To the east (Chihuahua, New Mexico), some folds have been described as sealed by the Campano-Maastrichtian molassic Ringbone Formation (Cabezas, 1989). In fact, the age constraints of the N 45”E compressional event recognized in the Colorado Plateau, are weak and the attribution to late Nevadan or early Sevier tectonism remains questionable. The Laramide tectonism

South of the Colorado Plateau, the Laramide tectonism included two major events: Ll during the Late Cretaceous-Paleocene and L2 before the Late Eocene (Rosaz, 1989; Sosson, 1989; Sosson and Bouroz, 1989). The analysis of faulting in the Colorado Plateau (Angelier and Bergerat, 1989; Sosson and Bouroz, 1989) confirms the existence of these two events; the analysis of joint patterns demonstrates the presence of an additional event. As a result, the succession of Laramide tectonic events is established as follows in the Plateau. The first Laramide event (Ll), with N 65”E trending compression, is characterized by reverse and strike-slip faults (Angelier and Bergerat, 1989; Bouroz, 1990) and N 75-80”E-trending extension joints and tension gashes. This event has also been recognized in the Rocky Mountains (Chapin, 1983), where it corresponds to east-vergent major crustal thrustings (Cabezas, 1989). In southeastern Arizona (Sosson, 1989), this event resulted in the development of folds and thrusts that trend N 150”E, with ductile deformation indicating a northeastward displacement. According to Sosson and Bouroz (19891, this first tectonic deformation probably occurred as soon as the Campanian; in fact, it may be partly attributed to the Sevier orogeny. An important

F. BERGERA.I

EI‘ AL,

event of overthrusting has also been distinguished in Sonora during the early Late Cretaceous (Rangin, 1982; Pubellier, 1987; Sosson and Calmus, 1989). The structural development of the Sierra Madre Orientale and the Chihuahua basin also occurred during the Maastrichtian-Danian (Tardy, 1980). A second Laramide event (Ll’) is characterized by N 95”E trending tension gashes and joints; some conjugate shear or hybrid joints probably belong to this event. The geometrical relationships among joint sets and between joints and monoclines clearly show that this N 95”E compressional event has occurred between Ll and L2. The last Laramide event (L2), with compression trending N 115”E, is characterized by reverse and strike-slip faults (Angelier and Bergerat, 1989; Bouroz, 1990), and N 125-130”E extension joints and tension gashes. This episode has been described in the Rocky Mountains by Cather and Chapin (1990). A transpressive tectonism with N-S right-lateral strike-slip faulting along the eastern boundary of the Colorado Plateau is attributed to this event by Cabezas (1989), but it is geometrically inconsistent with a ESE-WNW compression. In fact, the age constraints are weak enough, so that the two events need not to coincide. This ESE-WNW compression played a major role in the structural development of the Eastern Sierra Madre (Mexico), and in the reactivation of ancient fractures of southeastern Arizona. On the southern boundary of the Colorado Plateau, the Sawmill Canyon faulted zone, which trends N 140”E, underwent reactivation as a leftlateral strike-slip zone (Sosson and Bouroz, 1989). In southeastern Arizona, this event is clearly Palaeogene in age: it postdates the Late Cretaceous (i.e. the Campano-Maastrichtian molassic formation of Fort Crittenden) and predates the Late Eocene (i.e. the lavas sealing the last compressional features), as noticed by Sosson (1989). In southwestern New Mexico, the volcanic Hidalgo Formation, Paleocene in age, is affected by folds with vertical axes as the probable result of N 120”E-trending strike-slip faulting of the second Laramide episode. The lavas of the Playas Peak Formation (Eocene-Early Oligocene in age)

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COLORADO

unconformably overlie older formations and seals the strike-slip structures of the second Laramide episode (Rosaz, 1989). The Neogene extensional tectonics West of the Colorado Plateau, studies in the Basin and Range area (Anderson and Ekren, 1977; Zoback et al., 1981; Faugere, 1985; Angelier et al., 1985, 1987; Faugere et al., 1986; Michel-No&l, 1988) showed a succession of two main extensions. The first extension trends southwest-northeast (N 50-6O”E computed it3), whereas the second one trends west-northwesteast-southeast (N 100-l 15”E computed ~3). Analyses of macrofractures in the Colorado Plateau show the same succession and allow identification of an other extensional event, with approximately N 85”E computed ~3; this event had been locally recognized but not accurately dated in the Basin and Range area (Faugere, 1985; Michel-Noel, 1988). The succession of these extensional tectonic events is reconstructed as follows. The first neogene event (BRl), with extension trending N 65”E, is characterized by normal and strike-slip faulting (Angelier and Bergerat, 1989; Bouroz, 1990), by NNW-SSE-trending normal shear or hybrid joints and probably by N 165170”E-trending extension joints. This event has also been clearly recognized in the Great Basin, where it corresponds to the first major Basin and Range event with development of faulted and tilted blocks (Anderson and Ekren, 1977; Zoback et al., 1981; Faugbre, 1985; Angelier et al., 1985, 1987; Malavieille, 1987; Michel-Noel, 1988). Syndepositional tectonism related to this N 55-6O”E extension began during the Early Miocene in Nevada (Rainbow Canyon, Mesquite basin; Michel-Noel, 1988) and the clockwise rotation of the regional direction of .extension is younger than 10 Ma (Michel-Noel et al., 1990). Along the southeastern boundary of the Colorado Plateau, the Rio Grande Rift is also characterized by a NE-SW-trending extension, Late OligoceneEarly Miocene in age. Comparison with these adjacent areas (the Basin and Range Province and the Rio Grande Rift) indicates that the N

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65”E-trending extension of the Colorado Plateau probably is Early-Middle Miocene in age. A second E-W-trending extensional event (BRl’) is characterized by normal and strike-slip faulting (Angelier and Bergerat, 1989) and N-S normal shear or hybrid joints; some N 15*W15”E-trending tension gashes and joints probably belong to this tectonic event. A similar extension trending N 80-90”E was locally recognized in the Basin and Range area. In the Cordilleran metamorphic core complexes of northeastern Basin and Range Province, east-west-trending extension is Oligocene-Early Miocene in age (Malavieille, 1987). For Michel-Noel (1988), the E-W-trending extension, in the Mesquite basin, may also correspond to an early extensional episode of the Basin and Range tectonics, maybe contemporaneous with the ductile deformation of the large decollements described in the metamorphic core complex by Malavieille (1987); however, there is no chronological evidence to support this hypothesis. For Faugbre (1985), there is a single neogene N 80-9O”E extensional trend in the Eldorado Mountains. In the Rio Grande Rift, the same direction of extension has been characterized as Late Miocene-Early Pliocene in age (Morgan and Golombek, 1984; Morgan et al., 1986). In the Colorado Plateau, the study of faults displays rare evidences of relative chronology, with the N 85”E-trending extension occurring between the N 65”E and N 115”E ones (Angeiier and Bergerat, 1989). The study of joints does not allow confirmation of this hypothesis due to uncertainties in the mechanical significance of critical joint sets. The third Neogene extensional episode (BR2), with u3 trending N 115”E, is characterized by normal and strike-slip faulting (Angelier and Bergerat, 1989; Bouroz, 1990), by NNE-SSWtrending tension gashes and joints, and probably by N 165-17O’E and N 40”E shear or hybrid joints (respectively right- and left-lateral). This ESE-WNW extension has also been recognized for a long time in the Great Basin area (Ekren et al., 1968, Anderson and Ekren, 1977; Zoback and Thompson, 1979; Zoback et al., 1981; Faugere, 1985; Angelier et al., 1985, 1987; Michel-Noel, 1988). This extensional period has affected the

basins when they where created and filled up (Michel-Noel, 1988; Michel-No&l and Angelier, 1989). The clockwise rotation of about 50“ between the first extensional trend (N &YE) and the last one (N 115”E) has been related to block rotations close to large shear zones, such as the left-lateral Lake Mead shear zone and the rightlateral Las Vegas shear zone (Faugere, 198.5). Although the existence of local rotations near large shear zones is undoubted, it is impossible to explain the whole apparent rotation of extensional trends by block rotation processes (Angelier et al., 1987). In the Colorado Plateau, (3) the existence of these two major extensional trends fN 65”E and N 115”E) far away from large shear zones, (2) the clear relative chronology between them, and (3) the probable existence of a minor extensional episode trending E-W between the two major ones, lead us to confirm the significant clockwise change (50”) in regional trend of extension during the Neogene. Conclusion: contribution of jointing studies in the Colorado Plateau to mechanical understanding of joints and regional teetonic knowledge The lack of displacement marks on the joints (with the probable exception of hackle marks) and some “traps” due to stress release or permutation caused the major difficulties in this study. As a result, it was indispensable to carry out carefu1 analyses of: (1) azimuthal distributions of joint sets; (2) joint surface features; (3) geometrical and chronological relationships among joints and between joints and monoclines; and (4) available faulting data (for comparison). In the case of the whole Colorado Plateau, this kind of analysis led us to identify the main Mesozoic and Cenozoic tectonic events including: (1) a N 45”E compressional event of the Nevadan or Sevier orogeny; (2) three compressional events of the Laramide phase, with N 65”E, N 95”E and N ilYE compressions (crl) successively; and (3) three extensional events of the Neogene extensional period, with N 75”E, N 85”E and N 115”E extensions (~3) successively. These tectonic events are easily correlatable with the compressional and extensional major phases recognized in

the neighbouring areas (Rocky Mountains, Basin and Range, Rio Grande Rift). Accurate directions of palaeostresses of the successive major phases are thus recognized. Such paiaeostress orientations are now well characterized in the Basin and Range Province (Anderson and Ekren, 1977; Zoback and Thompson, 1979; Zoback and Zoback, 1980; Zoback et al., 1981; Faugere, 1985; Angelier et al., 1985; Malavieille, 1987; MichelNoel, 1988). In contrast, in the Rocky Mountains, only few local studies have been carried out in terms of stress orientations (Sosson and Bouroz, 1989; Sosson, 1989; Cabezas, 1989). The present study led us to specify the directions of the main tectonic events. In addition, three tectonic events have never been recognized before: (1) the N 45”E compressional event attributed to a preLaramide orogeny; (2) the N 95”E inte~ediate compressional event of the Laramide phase (characterized only by means of tension gashes and joints analysis); and (3) the N 85”E (probably intermediate) extensional event of the Neogene extensional period. The existence of the latter two stress fields bring indirect confirmation of the clockwise rotations of stress fields during the Mesozoic compression (crl: from N 45”E to N 115”E) and during the Cenozoic extension (~3: from N 75”E to N 115”E), respectively. We conclude that the analysis of tectonic joints provides a reliable tool, though difficult to use alone, in the reconstruction of tectonic palaeostresses. The results obtained from joints in a tabular and poorly deformed area, such as the Colorado Plateau, show that they are quite significant with respect to tectonic history of a large region. As a consequence, it is possible to extrapolate the results obtained in the Plateau to areas with larger amounts of deformation around the Plateau. Acknowledgements We acknowledge fruit~l discussions with C.C. Barton and R.L. Wheeler (U.S.G.S., Denver, Colorado). We thank the anonymous referee for helpful comments and M. Gordon for improving the English manuscript. This work was supported by C.N.R.S.-N.S.F. and N.A.T.O. grants.

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