TECTONICS, VOL. 23, TC1007, doi:10.1029/2001TC001329, 2004
Kinematic implications of joint zones and isolated joints
in the Navajo Sandstone at Zion National Park, Utah:
Evidence for Cordilleran relaxation
Christie M. Rogers,1 Douglas A. Myers,2 and Terry Engelder
Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA
Received 24 September 2001; revised 15 February 2003; accepted 1 August 2003; published 24 January 2004.
[1] At Zion National Park (ZNP) the landscape is a
consequence of differential weathering of the Navajo
Sandstone where closely spaced vertical joints
constitute joint zones that erode to form regularly
spaced (half kilometer) slot canyons striking 351.
Between these joint zones is a set of isolated joints
striking 339. Fracture interaction and horsetail/wing
crack development indicate that the 339 striking joint
set is younger than the 351 striking joint zones,
despite the lateral extent of stress reduction shadows
in the vicinity of the large-scale joint zones. In addition
to an older, less pervasive, 020 joint set, this
sequence of jointing records a counterclockwise
rotation of the regional extension directed from
WNW to WSW in the Navajo Sandstone at ZNP.
ZNP is located at the western margin of the Colorado
Plateau, 100 km east of the major normal faults of the
northeastern central Basin and Range subprovince.
Extension within the eastern central Basin and Range
initiated during the Miocene and exhibited a WSW
extension direction [Anderson, 1971; Wernicke et al.,
1988; Snow and Wernicke, 2000]. The correlation
between nearby Basin and Range extension and the
extension direction for the 351 tending joint zones of
ZNP is so close that the jointing at ZNP is interpreted
as evidence for modest, yet pervasive Basin and Range
extension in the western margin of the Colorado
INDEX TERMS: 8010 Structural Geology: Fractures
Plateau.
and faults; 8020 Structural Geology: Mechanics; 8164
Tectonophysics: Stresses—crust and lithosphere; 9350
Information Related to Geographic Region: North America; 9604
Information Related to Geologic Time: Cenozoic; KEYWORDS:
joints, joint zones, stress shadow, Zion National Park, Navajo
Sandstone, Cordilleran relaxation. Citation: Rogers, C. M., D. A.
Myers, and T. Engelder (2004), Kinematic implications of joint
zones and isolated joints in the Navajo Sandstone at Zion
National Park, Utah: Evidence for Cordilleran relaxation,
Tectonics, 23, TC1007, doi:10.1029/2001TC001329.
1
Now at ExxonMobil Exploration Company, Houston, Texas, USA.
Now at Anadarko Petroleum Corporation, The Woodlands, Texas,
USA.
2
Copyright 2004 by the American Geophysical Union.
0278-7407/04/2001TC001329
1. Introduction
[2] In foreland regions, joint patterns are helpful for the
dynamic analyses of tectonic events because the strike of
one vertical joint set marks the trajectory of the maximum
horizontal stress (SH) at a specific time during tectonic
events [e.g., Engelder and Geiser, 1980; Hancock and
Engelder, 1989]. The complexity of tectonic events is
reflected by multiple jointing episodes that record the
reorientation of the regional stress state as the tectonic event
progresses [e.g., Gray and Mitra, 1993; Zhao and Jacobi,
1997; Younes and Engelder, 1999]. Joint patterns are also
helpful for the kinematic analysis of tectonic events mainly
because joints propagate normal to the direction of instantaneous extension [Segall, 1984]. Several studies of regional
kinematics have used joints to understand tectonic regimes
dominated by extension [i.e., Hancock et al., 1984; Hancock
and Bevan, 1987]. The objective of this paper is to present
a kinematic analysis of jointing indicative of penecontemporaneous extension in the western Colorado Plateau during
tectonic relaxation of the Cordillera.
[3] Our interest in the edge of the Colorado Plateau
is focused on the region about Zion National Park
(ZNP) (Figures 1 and 2), Utah, where pervasive large-scale
jointing indicates a modest extension slightly south of west
(Figure 3). The relatively undeformed Colorado Plateau has
been a distinct tectonic province since the development of
block uplifts during Laramide deformation at the end of the
Cretaceous [Gries, 1983; Humphreys, 1999]. Extension in
the Cordillera of the western United States has occurred
from the Paleocene to the present [e.g., Axen et al., 1993],
while that of the Basin and Range extension event, in
particular, has been occurring since Oligocene [Zoback et
al., 1981; Graf et al., 1987; Wernicke, 1992]. Several largescale Basin and Range fault systems affect the Colorado
Plateau in the vicinity of ZNP (Figure 4) [Davis, 1999]. The
Hurricane Fault is west of ZNP and represents the western
margin of the Colorado Plateau tectonic province [Davis,
1999]. Although there may have been a left-lateral component to the fault displacement on the Hurricane Fault
prior to the Quaternary [Anderson and Barnhard, 1993],
this N10– 20E striking, high-angle, west dipping normal
fault has exhibited purely dip-slip displacement from the
Quaternary to the present with as much as 2520 m of
total stratigraphic separation in SW Utah [Stewart and
Taylor, 1996]. The Sevier Fault is east of ZNP, strikes
N30E, and is also a high-angle, west dipping, normal
fault zone that has exhibited dip-slip displacement ranging
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Figure 1. Schematic showing the position of the Cordilleran Thrust Front and the Colorado Plateau in the western
United States. Arrows signify the three extension directions
responsible for regional jointing at Zion National Park in
southwest Utah.
from 600– 750 m. The area of interest, ZNP, is located
within the relatively undeformed fault block bounded by the
Hurricane and Sevier Faults.
[4] In the study area, the Jurassic Navajo Sandstone, an
eolian sandstone, constitutes the bulk of the exposures
within ZNP (Figure 5). The flat-lying Navajo exhibits
large-scale to very large-scale trough, tabular-planar, and
wedge-planar cross-stratification [Marzolf, 1983]. Maximum thickness of the Navajo in the Zion region is about
670 m, and on average the formation is about 610 m thick at
the Park [Biek et al., 2000]. The Navajo Sandstone hosts a
set of widely spaced, long, continuous joint zones [Gregory,
1950; Davis, 1999; Biek et al., 2000], and along these
widely spaced joint zones, differential erosion produces
the spectacular rounded cliffs and slot canyons of the Park
(Figures 3 and 6).
2. Brittle Fracturing in the Navajo Sandstone
at Zion National Park
[5] Three styles of joint development have been identified
at ZNP. Within the Navajo Sandstone, two styles of jointing
appear to have distinct scales and orientations [Eardley,
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1965; Hamilton, 1984]. The larger-scale lineaments are
weathered joint zones [Gregory, 1950]. West of Zion
Canyon, these large-scale lineaments trend 020, whereas,
throughout the park, east and west of Zion Canyon, largescale lineaments strike 350. These large-scale lineaments
represent the two sets of slot canyons cutting the Navajo
Sandstone. West of Zion Canyon the 350 slot canyons
terminate against the 020 slot canyons (Figure 3). A
second style of jointing, striking 340, appears between
the joint zones and consists of large-scale, isolated joints that
do not erode into slot canyons [Eardley,1965] (Figure 7).
Throughout the park, the 340 joints neither abut against
nor cross cut the slot canyons. The third style of jointing at
ZNP involves exfoliation along canyon walls [Robinson,
1970; Bahat et al., 1995].
[6] Zion Canyon and its lateral canyons cut the entire
Navajo Sandstone section (Figure 3). These cliffs offer
views of the NNW trending joint zones in cross section.
The NNW trending joint zones cut the entire thickness
of the Navajo Sandstone and consist of closely spaced
(sub)vertical joints with an average strike of 351. Differential erosion of one or more joint zones produces each of
the regularly spaced NNW trending slot canyons that
collectively are the major topographic feature of ZNP. These
351 joints are very rarely located outside of the joint zones
within the Navajo Sandstone. Rather, in between the slot
canyons, the 339 isolated joint set predominates. The largescale, isolated joints are more closely spaced than the slot
canyons that have eroded from the joint zones. These
isolated joints do not penetrate the entire thickness of the
Navajo Sandstone, do not commonly promote differential
weathering, and thus do not factor significantly in the
shaping of the topography of the park.
[7] The slot canyons, which erode from the joint zones
within the Navajo Sandstone at ZNP, have a spacing that is
approximately equal to the thickness of the sandstone unit
[Rogers and Engelder, 2004]. Furthermore, the two sets of
joint zones at ZNP are confined to the Navajo Sandstone
and traceable within neither the underlying Kayenta and
Moenave Formations nor the overlying Temple Cap and
Carmel Formations. This joint zone height (equal to bed
thickness) to spacing relationship is consistent with other
joint sets confined within individual beds [Narr and Suppe,
1991; Gross et al., 1995]. The uniform spacing of joints
confined to one bed may be a consequence of stress
reduction shadows that prevent infilling between pairs of
joints spaced at about the thickness of the bed. The spacing
of the ZNP joint zones is consistent with this theory in so far
as infilling of additional 350 joints between the joint zones
appear to have been inhibited by the presence of stress
shadows. This mechanical scenario is also consistent with
the hypothesis by Eardley [1965] that the isolated joints
should have propagated before the larger-scale 351 joints.
[8] Eardley’s [1965] field notes suggested that the largescale, isolated joints are older than the joint zones (W. L.
Hamilton, personal communication, 2001). The reason for
his interpretation is unclear. A secondary objective of this
study is to examine the validity of the 1965 Eardley
hypothesis that the isolated joints are older than the joint
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Figure 2. Central Basin and Range subprovince and the relatively undeformed Colorado Plateau and
Sierra Nevada to the east and west, respectively. Within the central Basin and Range, two highly extended
regions, the Lake Mead and Death Valley extensional belts, differ in extension direction and are separated
by the Sheep Range and Spring Mountains Blocks. Zion National Park is located at the western margin of
the Colorado Plateau just east of the northern Lake Mead extensional belt (modified from Snow and
Wernicke [2000]; reprinted by permission of American Journal of Science). Location given in Figure 1.
zones. To accomplish this and the overall objective of
understanding how Cordilleran relaxation overprints the
Colorado Plateau, we will (1) characterize the joint spacing
distribution of the two NNW trending joint sets within
outcrops at ZNP and within aerial photographs of the Park,
(2) characterize the geometry of the 339 trending isolated
joints in relation to the 351 trending joint zones, and
(3) infer the age relationship between these two joint sets
at ZNP and the resulting stress history they represent, and
(4) place these jointing events into a context of Cordilleran
relaxation by extension.
2.1. Orientation and Spacing Data
[9] Spacing and orientation data were collected from
joint-perpendicular scan lines, joint-parallel scan lines, and
aerial photograph scan lines mainly along highway 9 within
ZNP (Figure 8). The joint zones and the isolated joints are
distinguished, first, by orientation with the strike and dip of
the plane to the vector mean pole for fractures in several
joint zones at 171/87 (right hand rule for the 351 trending
joint zones) and isolated joints at 159/90 (the 339 joints)
(Figure 9). These two sets of brittle structures are also
distinguished by their spacing. Two methods of determining
the spacing from aerial photographs were utilized: the
standard joint-perpendicular scan line method (D) and the
area method (S) of Wu and Pollard [1995]. For the joint
zones, the linear scan line spacing, D, is approximately
435 m (line a – a0 in Figure 8), whereas the spacing, S,
determined by the area method is approximately 454 m
Figure 10a). For the isolated joints, the scan line spacing, D,
is approximately 21 m (line b – b0 in Figure 8) and the
spacing, S, is approximately 23 m (Figure 10b).
2.2. Descriptive Morphology of Joint Zones
[10] In aerial photographs, the slot canyons resemble a
large-scale, regularly spaced joint set in the Navajo Sandstone (Figure 6). On the ground, however, each canyon
erodes from one or more joint zones composed of closely
spaced (sub)vertical joints. Horizontal exposures of joint
zones were observed in the eastern half of the Park along
Highway 9. Thirty-eight joint perpendicular scan lines were
taken in the Navajo Sandstone across these joint zones, and
at many stations, the vector mean pole defines a joint strike
(i.e., 351) that is consistent with the trend of the associated
slot canyons observed in aerial photographs and topographic
maps.
[11] Exposures of joint zones in cross section occur in
cliffs of Navajo Sandstone in the western half of the Park.
Here, where the entire Navajo section is exposed, joint
zones cut to the base of the Navajo and are associated with
deeply eroded slot canyons. No stratigraphic offset was
observed on fractures within the zones, and although joint
zones cut the entire thickness of the Navajo Sandstone,
individual fractures within the zones do not. Individual
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Figure 3. Field area at Zion National Park. The NNW trending slot canyons are labeled A-S, of which R
slot joint zone, Court of the Patriarchs, and M slot joint zone, Refrigerator Canyon (asterisks), are described
in detail in the text. The orientation of these NNW trending joint zones reflects an average 261 regional
extension direction. West of Zion Canyon, note that the NNW joint zones (canyons) terminate against the
presumably older NNE joint zones (canyons). Locations of subsequent figures indicated by boxes.
fractures are (sub)vertical, and local wing crack growth
indicates vertical propagation of joints within these
zones.
[12] Slot canyon morphology is a function of the number
of associated joint zone(s). A single joint zone controlled
the erosion of R slot in the Court of the Patriarchs where the
canyon tip is observed in the joint zone (Figures 3 and 11).
This slot canyon exhibits a tilted ‘‘V’’ shape in cross section
and cuts 90% of the thickness of the Navajo Sandstone. In
contrast, two joint zones define M slot, Refrigerator Canyon, where it meets Zion Canyon (Figures 3 and 12). The
associated slot canyon exhibits a ‘‘box’’ morphology and
cuts 480 m of Navajo Sandstone. Fractures within this
pair of zones exist symmetrically below and define the box
canyon edges at the base of the canyon.
2.3. Descriptive Morphology of
339°° Striking Isolated Joints
[13] Although the 339 striking isolated joints are indeed
small relative to the height, length, and spacing of the joint
zones at ZNP, they are large-scale features by most measures. While these joints may seem tall (>20 m) when
viewed in outcrop, none cut through the entire thickness
of the Navajo Sandstone.
[14] Isolated joints have three important morphological
characteristics. First, the plumose structure apparent on the
joint face indicates that the joint propagated in the subhorizontal direction (Figure 13), not the vertical direction as in
the case for joints in the joint zones. Second, the joint face
illustrates that the isolated joints are longer in the horizontal
dimension than in the vertical dimension. The ellipticity of
the joint face in Figure 13 was calculated at greater than 5:1.
Thirdly, larger members of this joint set carry well-developed fringe cracks. Fringe cracks are out of plane en
enchelon joints that occur at the margins of the main joint
face [e.g., Younes and Engelder, 1999]. Isolated joints in
this category have a central surface characteristic of a parent
joint and right stepping en echelon fringe cracks (Figure 14).
The isolated joints of Figure 14 are exposed on an inclined
surface; therefore, the oblique cross section of the joints
shows right stepping en echelon fringe cracks both above
and below the parent joint.
[15] The isolated joints are regularly spaced; however,
their spacing is much closer than is expected for typical
joints in bedded sedimentary rocks [see Olson, 1993,
Figure 14], such that spacing is less than the vertical
dimension of the joints. Joints, confined by bed boundaries,
often show a one to one relationship between bed thickness
and joint spacing. The height to spacing of the isolated joints
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Figure 4. Fault systems of the Colorado Plateau in southwestern Utah. These Basin and Range-style
faults, the Hurricane, Sevier, and Paunsaugunt, are large-scale down-to-the west normal faults (modified
from Davis [1999]).
at ZNP is characterized by fracture spacing ratios (FSRs),
i.e., bed thickness/average joint spacing [Gross et al., 1995],
much higher than unity (Figure 15). The value of the FSR
ranges from 4 to 20, but peaks in the range of 9 to 12.
[16] At some places within the Navajo Sandstone isolated
joints become so closely spaced that they appear as a joint
zone, but at ZNP they do not erode as slot canyons. Not
only does this type of zone differ from the large-scale 351
trending joint zones in orientation, but it also differs in zone
geometry. The isolated joints are more consistently vertical,
paralleling one another across the zone (e.g., Figure 16). In
contrast, the steeply dipping subvertical joints of the largescale joint zones have a tendency to dip toward the
centerline of the associated slot canyon [Rogers and
Engelder, 2004] resulting is a different geometry/distribution of joints within the zone (e.g., Figure 12).
2.4. Age Relationship Between Joint
Zones and Isolated Joints
[17] In identifying the relative age of joint sets, evidence
that older, preexisting, joints have influenced the propagation of younger joints is key to making the determination.
Younger joints, generally, do not cross older joints. Additionally, the orientation of tail and horsetail fractures represents a younger joint orientation along requisite older joints.
Furthermore, younger joints can curve in the vicinity of
older joints.
[18] These particular characteristics are used to determine
the relative age relationship between the three joint sets,
and, in particular, the 351 trending joint zones and the 339
trending isolated joints, at ZNP. Aerial photograph interpretation reveals that the 339 trending isolated joints do not
cross the 351 trending slot canyons (Figure 7), and the
351 trending slot canyons do not cross the 020 trending
slot canyons (Figure 3) indicating that the isolated joint are
the youngest joints of the three.
[19] Isolated joints, in the form of tail and horsetail
fractures, have been identified along the 351 trending joint
zones, also indicating that the isolated joints are the youngest. Both tail and horsetail fractures form when a parent
joint is later sheared due to a change in the regional/local
stress field. Tail cracks form at the parent crack tip and
horsetail fractures initiate at discontinuities along the parent
joint [Cruikshank and Aydin, 1995]. For example, a tail
fracture has propagated from the termination of I slot joint
zone (Figure 17), and horsetail fractures are present along
the L slot joint zone (Figure 18). These tail and horsetail
fractures have propagated along the tips of the joint zones in
the orientation of isolated joints. Since the parent joint must
be present before a tail or horsetail fracture can propagate,
the isolated joints are inferred to be younger than the joint
zones.
[20] Finally, isolated joints are also observed to curve
parallel to 351 trending joint zones (Figure 17). Isolated
joints between slots (joint zones) H and I follow a curved
trajectory to parallel the slot canyons. The existence of this
curving parallel relationship indicates that 351 trending
joints were present before the propagation of the isolated
joints [Dyer, 1988].
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K
I-J pair
H
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G
N
hwy 9
L
0
0.5
0
0.5
1 mi
Trend of isolated joints
1 km
A
Figure 5. Stratigraphic column of the units exposed
within the Zion National Park area, Utah [Peterson and
Pipiringos, 1979; Marzolf, 1983; Hamilton, 1984; Hintze,
1988].
[21] Some isolated joints propagate as a smooth curve
when the local stress field of the joint tip begins to be
influenced by a local stress field generated in the vicinity
of a joint zone. Many isolated joints curving parallel to
joint zones have right stepping en echelon fringe cracks
(Figures 14 and 19a). Manifestation of this curving parallel
Figure 6. Aerial view of the NNW trending slot canyons,
eroded joint zones, in the Navajo Sandstone. This view
encompasses the field area at Zion National Park (Figure 3)
(note that photo distortion exists due to camera tilt). The
different slot canyons are labeled with capital letters and
cross-referenced to Figure 3. The canyons generally trend
350, and they are spaced approximately a half-kilometer
apart.
Joint zones/slot canyons
Figure 7. Aerial photograph over eastern Zion National
Park along Highway 9 (see location, Figure 3). Joint zones
of the Navajo Sandstone, labeled with capital letters, have
preferentially eroded into slot canyons. In between the joint
zones, a set of isolated joints is evident.
relationship differs depending on the level at which the
isolated joint is observed, i.e., whether the main joint face or
its fringe zone is exposed at the surface.
[22] The trace of the isolated joints in plan view is
dependent on the relative vertical position of the bedrock
surface cutting the joint face. For example, if the top fringe
of the isolated joint was eroded leaving only the curving
portion of the main joint face (Figure 19b, surface B) the
trace of the isolated joint would be 339 in the center of
the joint with each joint tip curving smoothly parallel to a
joint zone. On the other hand, if the fringe of the isolated
joint is exposed at the surface (Figure 19b, surface A) the
trace would also have a 339 striking middle joint
segment, but have similarly striking, right stepping en
echelon fringe cracks on either end of the middle joint
segment. The series of the right stepping en echelon fringe
joints traces a curving path that will eventually parallel the
joint zone, while each individual en echelon crack maintains
the orientation of the parent joint.
[23] Data on right and left stepping en echelon cracks
were collected from nine joint-parallel scan lines. The sense
of en echelon stepping is used to confirm the nature of
curving of the isolated joints as seen in aerial photographs
(Figure 17). Seventy-three percent of all measured en
echelon joints step to the right with the remainder stepping
to the left (Table 1). Right stepping joint segments are
consistent with a joint curving in a clockwise direction.
Because these en echelon cracks are propagating from a
parent joint as visualized in Figure 19, then the distance
between adjacent en echelon cracks (i.e., step distance) and
overlap distance of these same cracks should correlate in a
systematic manner. The overlap distance is about 1 order of
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I
J
E
G
D
D2
M
K10
K9
K11
M11
N1
I3
I5 b2 H9
G3
H2H3
J4
G2
I4
H4 G1
H5
E3
E4
E1
Hwy 9
A1
C1
C
A
D3
B
7
Figure 1
L
N
(F)
H
H8
K
O?
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a'
N5
N6
O4 N4
M10
e 18
Figur
a
L2
b
Temple cap and Carmel Fm
SCALE
1
0
0.5
1
0.5
0
b'
1 mile
N
NW trending slot canyon/joint
zone, dashed where inferred
1kilometer
Navajo Fm.
Kayenta Fm, Moenave Fm,
and/or alluvium
Figure 8. Sites of joint spacing and orientation scan line data collection (i.e., circled field locations and
lines a – a0 and b – b0) (this location given in Figure 3). Locations of subsequent figures indicated by boxes.
magnitude larger than the distance between adjacent en
echelon pairs (Figure 20). This is typical for en echelon
joints [i.e., Younes and Engelder, 1999].
[24] The expected angle, q, between the en echelon
stepping zone and the main joint trend can be calculated
by taking the inverse tangent of the slope of the best fit line.
This angle is approximately 3.4. Presumably, this is representative of the misalignment of the zone of en echelon
fringe cracks and the parent joint.
[Dyer, 1988] and in the Jurassic limestones of the Bristol
Channel [Engelder and Peacock, 2001]. Like the rocks at
Arches and the Bristol Channel, the regional stress field
reaches between the preexisting joint set to control the
direction of later propagation.
[26] The isolated joint set in the Navajo Sandstone at
Zion has in filled to reach a state of closely spaced
3. Mechanical Explanation for Propagation
of En Echelon Fringe Cracks From a
Curving-Parallel Parent Joint
[25] Joint spacing can give clues about boundary conditions leading to joint propagation in bedded sediments.
For example, those joint sets displaying a fracture spacing
ratio of about unity are considered saturated and appear to
have growth restricted within stress shadow zones [Narr
and Suppe, 1991; Gross et al., 1995]. The NNW trending
slot canyons, which are localized along joint zones, at ZNP
have an FSR near unity. However, the isolated joints
postdate the joint zones and most have propagated within
the stress shadow zone of the joint zones. The exact
boundary conditions driving propagation within the stress
shadow regions of existing joint zones are not entirely clear
at present. It is, however, a common phenomenon as seen in
the Jurassic Entrada Sandstone of Arches National Park
Figure 9. Lower hemisphere stereonet plots composed of
joint-perpendicular scan line data from the joint zones and
the set of isolated joints. The strike and dip of the plane to
the vector mean pole is given (right-hand rule applies).
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A)
1200
Spacing (m)
1000
800
600
S = 454m
400
D = 435m
200
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Position (m)
B)
60
Spacing (m)
50
40
30
S = 23m
20
D = 21m
10
0
0
100
200
300
400
500
600
700
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explanation for this behavior is based on the fact that the
size and lateral extent of a crack-tip stress field scales with
the length of the initial crack. A crack that is 10% as long as
another crack will have a crack-tip stress field that reaches
10% as far from the crack tip (Figure 21).
[28] The propagation path of a joint is governed by the
location of the maximum circumferential stress, which, in
turn, is defined by the shape of the crack-tip stress field
[Erdogan and Sih, 1963; Lawn, 1993]. As long as the cracktip stress field is symmetrical, the maximum circumferential
stress remains in line with a planar crack and in-plane
propagation takes place. If the crack-tip stress field is
distorted for any of a number of reasons, the maximum
circumferential stress is no longer in line with the crack. In
this latter case, the crack will propagate out of its plane and
toward the maximum circumferential stress. This leads to
a curved joint. Isolated joints in the Navajo Sandstone
propagate in their plane until the crack-tip stress field
contacts a joint zone, becomes asymmetrical, and out-ofplane propagation takes place (Figure 21).
[29] Fringe cracks on the isolated joints propagate in the
direction of the regional maximum horizontal stress, SH.
This indicates that when the fringe cracks propagated, there
was a traction across the joint zones and the regional stress
was transmitted without being deflected by the joint zones.
This situation may differ from the situation during propagation of the parent crack with a curving propagation
trajectory. Then traction across the joint zones was small
enough to permit the modification of the crack-tip stress
field of the parent joint as its tip propagated toward the joint
zone. The en echelon fringe cracks continued in-plane
propagation, isolated from and lagging behind that of the
800
Position (m)
Figure 10. Plot of joint spacing versus position for (a) the
350 trending joint zones and (b) the set of 339 trending
isolated joints as determined from the scan line, D, analysis.
The dashed line indicates average spacing determined in the
scan line analysis. For comparison, the value derived from
the area method, S, is plotted as the solid line.
fracturing [Bai and Pollard, 2000]. Joints that are more
closely spaced than expected for standard saturation are
believed to have propagated from flaws near the interface of
beds [Bai and Pollard, 2000], but the closely spaced
isolated joints within the Navajo Sandstone appear to have
propagated from inside the bed. At this point, evidence
suggests that the isolated joints are late, but their driving
mechanism remains unknown.
[27] The behavior of the en echelon fringe cracks of the
isolated joints presents a paradox. Despite being near a joint
zone, these fringe cracks propagate in the direction of the
parent crack before it curved to propagate subparallel to a
joint zone. The question is why the fringe cracks continue to
propagate toward the joint zones long after the parent crack
has been forced to curve parallel to the joint zones. An
Figure 11. View of the Navajo Sandstone at R slot as
viewed from the Court of the Patriarchs (Figure 3). A single
fracture zone and a tilted ‘‘V’’ shaped slot canyon
characterize R slot. See color version of this figure in the
HTML.
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a counterclockwise rotation of extension in the region. Two
sets of joint zones include an early, less developed, NNE
(020) trending set and the later NNW (351) set that
dominates the landscape (Figures 3 and 6). A set of isolated
joints striking NNW (339) occurred later than the NNW set
of joint zones as indicated by the geometric characteristics
described above. Kinematically, these three fracture sets
reflect a regional extension of the Cordilleran foreland that
was first north of west and then rotated in a counterclockwise direction to south of west. While the absolute timing of
regional extension responsible for the propagation of these
fracture sets is uncertain, we can make some inferences
focusing on the kinematics of extension in the Cordilleran
foreland. Previously, the jointing at ZNP was loosely
attributed to Basin and Range extension to the west
[Eardley, 1965; Biek et al., 2000]. Here we have presented
a detailed account of the jointing at ZNP and attempt to
identify a specific regional context.
[31] The joints at ZNP are kinematic indicators of past
events that have occurred locally in the vicinity of the
Figure 12. Oblique view of the Navajo Sandstone at M
slot, Refrigerator Canyon, as viewed from Zion Canyon
(Figure 3). Two fracture zones, separated by an unfractured
volume of rock, and a ‘‘box canyon’’ morphology characterize M slot. See color version of this figure in the HTML.
parent joint, and due to their small size, their crack-tip stress
field does not come in contact with the joint zone.
4. Discussion: Regional Kinematics
[30] At Zion National Park the evolution of brittle fracturing is related to at least a portion of the post-Mesozoic
Cordilleran relaxation; the question is which part? Three
episodes of brittle fracturing, with clear relative ages, define
Figure 13. Face of an isolated joint with plumose structure
(note dashed lines) indicating subhorizontal joint propagation illustrates that the isolated joint is longer in the
horizontal dimension than in the vertical dimension. See
color version of this figure in the HTML.
Figure 14. Photograph of isolated joints with en echelon
fringe cracks. The location is marked in Figure 8 as b2,
between H and I slots. Note the right stepping en echelon
joints above and below the parent joint (B2) and that the
spacing of the isolated joints (B2 – B5) is much less than the
joint height. A geologist is barely visible in the lower left of
the picture for scale (arrow). See color version of this figure
in the HTML.
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ROGERS ET AL.: JOINTS AT ZNP—EVIDENCE FOR CORDILLERAN RELAXATION
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6
of the regional set of isolated joints and, most notably, that
of the NNW trending joint zones. The question is whether
the extension at ZNP is penecontemporaneous with the
regional Cordilleran extension and, therefore, a manifestation of the same deep-seated process. If so, then the western
Colorado Plateau exhibits additional evidence for Basin
and Range extension beyond the classic Basin and Range
normal faults (i.e., the Hurricane and Sevier Faults) recognized there. In addition, we speculate on the events that
5
Frequency
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4
3
2
1
0
0-3
3-6
6-9
9-12
12-15
15-18
18-21
21-24
FSR
Figure 15. Histogram of the Fracture Spacing Ratio (FSR)
range measured from the isolated joints in Figure 16. The
peak FSR range is 9 – 12.
western edge of the Colorado Plateau. Joints serve as
kinematic indicators since they propagate normal to the
direction of instantaneous extension [Segall, 1984]. Consequently, the events that led to their development may be
recognized in the tectonic events that have affected the
region, targeting, specifically, those that share common
directions of extension with the joint sets at ZNP. In
particular, this discussion narrows to focus on early extension in the central Basin and Range subprovince (CBR),
directly west of ZNP (Figure 2), where evidence for WSW
directed extension [Anderson, 1971; Wernicke et al., 1988;
Snow and Wernicke, 2000] parallels the extension direction
Figure 16. Photograph of the closely spaced, isolated
joints resembling a joint zone as viewed south across
highway 9 between J and K slots (note car in lower left for
scale) (Figure 8). This type of joint zone is different in
character from the widely spaced, 351 trending, slot
canyon forming joint zones. See color version of this figure
in the HTML.
Figure 17. Aerial photograph tracing of the curvingparallel relationship of the isolated joints relative to the joint
zones between H and I slots (location given in Figure 8).
The propagation of the younger isolated joints is influenced
by the presence of the older 351 trending joint zones. In
addition, a tail fracture has been interpreted at the south end
of I slot.
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ROGERS ET AL.: JOINTS AT ZNP—EVIDENCE FOR CORDILLERAN RELAXATION
Horsetail Fractures
Hwy. 9
L
N
Scale
300 m
Horsetail Fractures
Figure 18. Aerial photograph tracing of horsetail fractures
propagating in the isolated joint set orientation from L slot
joint zones (location given in Figure 8).
resulted in the counterclockwise rotation of extension in the
western margin of the Colorado Plateau at ZNP.
4.1. Postcompressional Relaxation of the Cordillera
[32] The Cordilleran foreland of the western United
States exhibits a multistage postcompressional relaxation.
Tectonically driven, late Mesozoic compression caused
Sevier-style and Laramide-style shortening, each imprinting
distinctive structures on the region. Postcompressional
relaxation of thickened crust of the Cordilleran thrust front
has been proposed as a mechanism for several processes
including (1) thickening the crust of the Colorado Plateau
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85– 50 Ma [McQuarrie and Chase, 2000], (2) early collapse
of the northern Cordilleran foreland 49– 20 Ma [Constenius,
1996], as well as (3) a component of central Basin and
Range extension since 20 Ma [Wernicke et al., 1987; Axen
et al., 1993; Zandt et al., 1995].
[33] Two end-member models of Cordilleran gravitational
collapse/relaxation are considered: (1) east versus (2) west
directed collapse. Eastward flow of crustal material, in
conjunction with gravitational collapse of the Cordillera, is
proposed in models by Zandt et al. [1995] and McQuarrie
and Chase [2000]. These models suggest that eastward flow
of thickened Cordilleran crust resulted in compression in the
Colorado Plateau [see also Kilty et al., 2000], expressed as
Laramide deformation there. Compression within the Colorado Plateau does not favor conditions for generating the
regional joint network at ZNP, particularly as it is developed
within the flat-lying Navajo Sandstone and unrelated to
folding or faulting.
[34] Westward directed collapse of the Cordilleran thrust
front offers a more favorable mechanism for extension in
the western margin of the Colorado Plateau. For example,
decreasing convergence rates between the Pacific and North
American plates, coupled with slab roll back, 40– 20 Ma
is cited as a ‘‘trigger’’ for gravitational collapse of the
unsupported Cordilleran thrust belt from SE British Columbia to NE Utah [Constenius, 1996]. Cordilleran-normal
deviatoric tension due to reduction in horizontal compression was enough to allow the belt to extend horizontally
westward along preexisting thrust faults, an episode distinct
from the Basin and Range extensional event [Constenius,
1996]. This is consistent with two distinct extensional
episodes proposed for the northern and southern Basin
and Range subprovinces and attributed to (1) early Tertiary
gravity collapse of thickened crust following slab roll back
and (2) late Tertiary extension resulting from plate boundary forces arising from the development of the San Andreas
transform boundary [e.g., Coney, 1987; Zandt et al., 1995],
of which boundary forces associated with the transform
boundary appear to have contributed as a mechanism of
extension for the western CBR, demonstrated by the
Sierran-Great Valley block extension history [Wernicke
and Snow, 1998; Atwater and Stock, 1998]. Although
considered distinct events to the north and south, these
events converge in time and space at the CBR [Zandt et
al., 1995] where early WSW directed extension occurred
in the eastern CBR [Wernicke et al., 1988; Snow and
Wernicke, 2000].
[35] The CBR exhibits an extensional history distinct
from the northern and southern Basin and Range subprovinces in both timing [Wernicke, 1992; Axen et al., 1993]
and magnitude [Wernicke et al., 1988]. Two strongly
extended belts, the Lake Mead (Las Vegas system of
Wernicke et al. [1988]) and the Death Valley belts, separate
relatively undeformed crustal blocks of the CBR subprovince [Wernicke et al., 1988; Snow and Wernicke, 2000].
CBR extension closest to ZNP is exhibited in the northern
component of the Lake Mead extensional belt between the
Colorado Plateau and the Sheep Range block (Figure 2).
Predominantly down to the west normal faults in the
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Figure 19. (a) A 3-D visualization of the formation of the isolated joint set with respect to the joint
zones. The joint zones are parallel with the page. Along the main joint face, the isolated joint propagates
with a curving-parallel relationship to the joint zone. However, along the fringe areas of the isolated joint,
the joint propagates as a series of right stepping en echelon fringe cracks that maintain the isolated joint
orientation. (b) Plan view of two distinct erosional surfaces through the joint in Figure 19a illustrating the
fringe cracks along Surface A and the curving-parallel relationship along Surface B. See color version of
this figure in the HTML.
Mormon Mountains and Tule Springs Hills region, 80 km
west of ZNP, partially reactivated as well as obliquely cut
preexisting thrust faults [Axen et al., 1990].
[36] It is the WSW directed extension of the CBR that
deserves considerable attention. Relative motion vectors in
the CBR near the latitude of Las Vegas show opening
directions between 6 and 20 south of west [Wernicke et
al., 1988], representative of extension directions between
264 and 250 respectively. There is a remarkable correspondence in extension directions for the 351 trending
joint zones and the 339 trending isolated joints, which
represent extension directions of 261 and 249, respectively.
A total finite displacement grid for the CBR shows an
extension of 9 south of west for the area immediately west
of the Colorado Plateau at the latitude of ZNP [Snow and
Wernicke, 2000] (Figure 2). Correlation between an average
260 extension direction documented in the northern Lake
Mead belt and the 261 extension direction of joint zones is
so strong that coincidence seems unlikely.
4.2. Extension in the Colorado Plateau
[37] The western margin of the Colorado Plateau has been
cited as a transition zone to the Basin and Range extensional
province [Schramm and Taylor, 1994; Stewart and Taylor,
1996] on the basis of the presence of high-angle normal
faults (e.g., the faults of Figure 4) there. In addition to these
high-angle normal faults, jointing at ZNP may represent
earlier modest, yet pervasive, Basin and Range extension
overprinting the western margin of the Colorado Plateau.
[38] WSW extension of the Lake Mead extensional belt
(Las Vegas system) occurred mainly from 15 to 11 Ma and
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ROGERS ET AL.: JOINTS AT ZNP—EVIDENCE FOR CORDILLERAN RELAXATION
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Table 1. A Listing of Joint Parallel Scan Lines Taken of Isolated
Joints Including the Total Measured Length of Each Isolated Joint,
and the Right Stepping and Left Stepping Statistics of the Isolated
Jointsa
Nearest Joint Zone
Station Name
Joint Length, m
R/L Steps
I/J
E
G
H
I
I
I
K
b2
E4a
G1
Hra
I3a
I3b
I5
K6
166
45
50
16
55b
42
22b
23b
18/4
13/1
16/5
5/1
9/3
6/4
18/4
17/15
a
Joint zone and station designations cross reference to Figure 8. R, right
stepping; L, left stepping.
b
One end of the joint was covered and total length could not be
measured.
was nearly complete by 10 Ma [Wernicke et al., 1988]. We
infer that the joint zones and isolated joints at ZNP had their
origin during this period from 15 – 11 Ma. The NNW
regional jointing at ZNP may, in fact, be restricted to the
western margin of the Colorado Plateau. Continuing west
from ZNP 30 km, in the vicinity of Hurricane, Utah each
of the three joint trends observed within ZNP appear to
persist in varying degrees [Lefebvre, 1961]. However, the
350 joint trend associated here with the CBR extension is
notably absent in the Navajo Sandstone of south central
Utah’s Lake Powell region (R. A. Nelson, personal communication, 2001), 150 km east of ZNP and conceivably
out of the range of CBR influence.
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[39] In the western CBR, the Death Valley belt began
extending at approximately the same time as the Lake Mead
belt; however, it exhibited a NW extension direction and
increased in rate as the Lake Mead belt slowed [Snow and
Wernicke, 2000]. Because the NNE trending joint zones are
older than the NNW trending joint zones, they are not
attributed to concurrent and later NW extension associated
with the Death Valley Belt, which appears to be too remote
to appear as a fabric of jointing at ZNP.
[40] Throughout the Basin and Range, the extension
direction rotated clockwise during the last 10 Ma [Zoback
et al., 1981]. Clockwise rotation is coincident with the
northward change in direction of the Pacific plate relative
to the North American plate 8 Ma [Atwater and Stock,
1998] and consistent with an increase in dextral shear along
the developing San Andreas transform boundary [Zoback et
al., 1981; Zoback, 1989]. This NW extension remains active
today within the Colorado Plateau’s ZNP region, demonstrated by recent motion along the Hurricane Fault [Stewart
and Taylor, 1996]. However, this clockwise rotation is not
consistent with the kinematics at ZNP.
4.3. Context for CCW Rotation of Extension
[41] Placing the CCW rotation of extension, as inferred
from the sequence of jointing at ZNP, into a proper context
of post-Mesozoic Cordilleran relaxation is uncertain.
Although the relationship between the NNE and NNW
trending joint zones consistently demonstrates that the
NNE trend is representative of an older joint set, the origin
of these NNE joints is the most uncertain of the three sets at
Zion.
100
B
80
A
joint
Step Distance (cm)
joint
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
Overlap (cm)
Figure 20. Step distance (inset: length A) as a function of segment overlap (inset: length B) for the set
of isolated joints. The inverse tangent to the best fit line defines the angle, 3.4, between the zone of en
echelon fringe cracks and the main joint face.
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ROGERS ET AL.: JOINTS AT ZNP—EVIDENCE FOR CORDILLERAN RELAXATION
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for the northern and southern Basin and range subprovinces,
the same cannot be said of the CBR where proposed
gravitational collapse and boundary force mechanisms for
extension may coincide in time [Zandt et al.,1995]. Furthermore, early extension in the CBR appears to represent
the NNW jointing at ZNP, excluding the older NNE joints.
ZNP, although directly west of the CBR, is also situated just
southeast of the northern Basin and Range subprovince
(Figure 2). This would suggest that the NNE joints could
be related to extension in the northern Basin and Range,
prior to the onset of extension in the CBR. Timing there
may accommodate an older joint set that developed from
WNW extension possibly due to gravity collapse normal
to the overall NE trending Cordilleran thrust front across
Utah (Figure 1). Consequently, as the area to the north
extended and potential energy was relieved, the potential
energy contours associated with the thickened Cordilleran
crust could have rotated CCW directing extension along
gradients, a process that could have continued south as
extension progressed (C. H. Jones, personal communication,
2002).
5. Conclusions
Figure 21. Scaling of crack-tip stress fields with crack
length related to fringe cracks. As the larger main joint
face approaches the joint zone, the local crack-tip stress
field overlaps the joint zone and the joint curves
subparallel to the joint zone. The fringe cracks, however,
are much smaller, and their local crack-tip stress fields are
not large enough to overlap the joint zone. Therefore the
fringe cracks continue to propagate in the original joint
orientation.
[42] The NNE joints are representative of WNW directed
extension. Recent NW extension of the Basin and Range is
an obvious candidate for inspection. It is, however,
unlikely that the NNE trending joints originated in connection with recent events since (1) NW extension has
dominated CBR since the late Miocene [Wernicke and
Snow, 1998] and (2) recent earthquake motion along the
Hurricane Fault [Stewart and Taylor, 1996] is consistent
with NW trending least compressive stress trajectories
documented by Zoback and Zoback [1980] in SW Utah.
With this evidence for NW extension occurring since the
late Miocene, neglecting any CCW rotation of extension, it
becomes a challenge to place the NNW jointing into any
context of regional extension. Therefore the connection
between NNW jointing and WSW extension of the CBR is
not discounted by any late Miocene and younger events,
and the NNE joints are inferred to have originated prior
to WSW extension in the Lake Mead extensional belt of
the CBR.
[43] Although two periods of extension, associated with
post-Mesozoic Cordilleran relaxation, have been proposed
[44] This investigation proposes that the large-scale
NNW trending joint zones and isolated joints of the Colorado Plateau at Zion National Park have their origins in
regional Miocene extension directed WSW, average 260,
exhibited in the northern Lake Mead belt of the central
Basin and Range subprovince [Anderson, 1971; Wernicke et
al., 1988; Snow and Wernicke, 2000]. This direction is
remarkably consistent with the 261 extension direction
reflected by joint zones at ZNP indicating that they may
be a manifestation of the same deep-seated crustal process
characterized, overall, by westward directed gravity collapse of the thickened Cordilleran thrust front.
[45] The sequence of jointing events indicates that a
counterclockwise rotation of extension has affected the
western margin of the Colorado Plateau in the vicinity of
Zion prior to the cessation of extension in the Northern Lake
Mead belt 10 Ma [Wernicke et al., 1988]. CCW rotation of
extension is inferred from abutting relationships between
the older NNE and younger NNW (351) trending joint
zones. In turn, as relatively older features, the large-scale
NNW (351) trending joint zones influenced the development of the NNW (339) isolated joints indicated by the
curving parallel relationship of the isolated joint set into the
joint zones. Furthermore, the relationship between the NNW
trending joint zones and isolated joints is characterized by
an en echelon stepping analysis and reinforced by the
presence of both tail and horsetail fractures propagating in
the isolated joint orientation from the NNW joint zone
trend. CCW rotation of extension may be rooted in changing potential energy gradients associated with north to south
collapse along the Cordilleran thrust belt.
[46] The particular sequence of jointing in the Navajo
Sandstone of ZNP is, most likely, restricted to the western
margin of the Colorado Plateau adjacent to the central Basin
and Range. In addition, it is the spectacular joint zone
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ROGERS ET AL.: JOINTS AT ZNP—EVIDENCE FOR CORDILLERAN RELAXATION
development in this portion of the Colorado Plateau that
makes Zion National Park unique in its dramatic topography. Although the initial jointing event that established the
joint zones is inferred to be >10 Ma in age, the preferential
erosion of the joint zones into slot canyons has occurred
over the past 2 million years [Biek et al., 2000]. A post 6 Ma
phase of uplift of the Colorado Plateau [Morgan and
Swanberg, 1985; Parsons and McCarthy, 1995] and concomitant high angle normal faulting (i.e., the Hurricane
Fault) [Biek et al., 2000] are responsible for subsequent
removal of overburden and headward erosion of rivers
required to excavate the slot canyons at ZNP. Much of the
secondary fracturing in the joint zones occurred during this
stage and is a function of local mechanical conditions
[Rogers and Engelder, 2004].
[47] Positioned on the western edge of the Colorado
Plateau, the jointing at Zion provides further evidence for
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the narrow transition between the highly extended central
Basin and Range province and the stable Colorado Plateau.
In respect to the NE trending, classic Basin and Range
normal faults (i.e. the Hurricane and Sevier Faults) of this
transition zone, the jointing at ZNP is inferred to be older.
This is based on orientation and timing of central Basin and
Range WSW extension, 15– 11 Ma, and an estimated onset
of Hurricane fault motion 11– 5 Ma [Stewart and Taylor,
1996], consistent with age of block faulting and NE
extension in the northern Basin and Range since the late
Miocene [Zoback et al., 1981].
[48] Acknowledgments. We thank the staff and administrators at
Zion National Park for their enthusiastic help and support during field
seasons at the Park. This work was funded by Penn State University’s Seal
Evaluation Consortium (SEC), AAPG Grants-In-Aid, and a Krynine Grant
from Penn State University. Early versions of this paper were reviewed by
D. Elsworth, D. Fisher, C. Jones, W. Taylor, B. Voight, and B. Wernicke.
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Engelder, Department of Geosciences, Pennsylvania State University, University Park, PA 16801,
USA. (engelder@geosc.psu.edu)
D. A. Myers, Anadarko Petroleum Corporation,
1201 Lake Robbins Drive, The Woodlands, TX 772511330, USA. (douglas_myers@anadarko.com)
C. M. Rogers, ExxonMobil Exploration Company,
233 Benmar, Houston, TX 77060, USA. (christie.m.
rogers@exxonmobil.com)