Fault formation in porous sedimentary rocks at high strain rates:
First results from the Upheaval Dome impact structure, Utah, USA
Wendy R.O. Key1,2† and Richard A. Schultz1
1
Geomechanics–Rock Fracture Group, Department of Geological Sciences and Engineering, MS 172, University of Nevada, Reno,
Nevada 89557, USA
2
Now at AMEC Geomatrix, Rancho Cordova, California 95670, USA
ABSTRACT
Previous theoretical work has suggested
that porous sedimentary rocks subjected to
sufficiently high strain rates may not form
deformation band damage zones (DBDZs).
This hypothesis is evaluated by an examination of faults within the porous Navajo Sandstone at the Upheaval Dome impact structure
in Canyonlands National Park, southeastern
Utah, where high strain-rate conditions are
known to have occurred. We found no evidence for DBDZ formation along the accessible fault planes at Upheaval Dome. Instead,
a layer of pulverized quartz grains within
the Navajo Sandstone occurs adjacent to the
fault planes. Similar material has been observed in association with dynamic rupture
in crystalline and sedimentary rocks along
the San Andreas fault in southern California
and in metamorphic rocks along the Bosman
fault in South Africa. Measured grain sizes
obtained from the pulverized material collected at Upheaval Dome are consistent with
strain rates of ~1–3 × 103 s–1. Pulverized sedimentary rocks along the San Andreas fault
imply strain rates of ~10–2 to 101 s–1. Strain
rates along the faults at Upheaval Dome are
well above the average values associated with
intraplate tectonics but are consistent with, or
faster than, seismic slip rates along faults
such as the San Andreas fault. Our results
support the hypothesized rate dependence of
deformation in porous rocks.
INTRODUCTION
Several mechanisms for faulting that depend
on rock type are well known in the literature. In
low-porosity crystalline (or compact) rock such
as granite, faulting occurs as a result of sliding
on preexisting cracks (Segall and Pollard, 1983;
†
E-mail: [email protected]
Martel and Pollard, 1989; Lockner et al., 1991;
Lockner, 1995) or by failure of strain-softening
damage zones of cracks resulting from shear
deformation (Wawersik and Fairhurst, 1970;
Wawersik and Brace, 1971; Lockner et al.,
1991). In contrast, rocks of substantially higher
porosity (>5%; e.g., Paterson and Wong, 2005),
such as sandstones, first generate deformation
band damage zones (DBDZs), identified as a
network of strain-hardening deformation bands
with reduced porosity and grain reorganization, that subsequently become faulted (Aydin
and Johnson, 1978; Shipton and Cowie, 2001,
2003). Shear deformation of compact rocks may
thus be characterized as crack-dominated deformation, whereas shearing of high-porosity rocks
(e.g., Paterson and Wong, 2005) is initially compaction dominated. The compaction-dominated
sequence is well documented in Navajo Sandstone (Aydin, 1978; Aydin and Johnson, 1978,
1983; Davis, 1999; Davis et al., 1999; Shipton
and Cowie, 2001), a porous, quartz-rich, crossbedded sandstone unit, and Entrada Sandstone
(Aydin, 1978; Aydin and Johnson, 1978, 1983;
Fossen and Hesthammer, 1997) on the Colorado
Plateau, as well as in other units throughout the
world (Aydin et al., 2006; Fossen et al., 2007).
Recent experimental and theoretical work suggests that deformation mechanisms in a porous
rock may depend on strain rate (Fossum and
Brannon, 2006). Using a combination of viscoplastic numerical models and experimental validation (Kolsky bar tests), Fossum and Brannon
(2006) found for Salem Limestone, under the
conditions they evaluated, that the characteristic response time for compaction-dominated
deformation to fully develop is approximately
three times longer than for dilation-dominated
deformation. According to Fossum and Brannon
(2006), at low, quasistatic strain rates (i.e.,
~10–5 s–1), either deformation mechanism can
potentially occur in a porous rock. At higher,
dynamic, strain rates, however (i.e., 102 s–1 for
Salem Limestone), compaction-dominated defor-
mation may not have sufficient time to develop,
so that deformation of a porous rock would occur
with cracking-dominated mechanisms only, as
in a compact rock. This intriguing and novel result suggests that a transition from compactiondominated deformation to cracking-dominated
deformation in other porous rock types, such as
the Navajo and Entrada Sandstones, may also
occur as a function of strain rate.
To evaluate the hypothesis of a rate dependence of deformation in porous sedimentary
rocks subjected to high strain rates, we examined macroscopic and microscopic data collected in the field along faults inferred to have
formed under these conditions. The Upheaval
Dome impact structure was chosen for this investigation because high strain-rate conditions
are known to have been applied at this location
during its formation, and faults associated with
the impact event are well documented (Kriens
et al., 1997, 1999; Kenkmann et al., 2005).
Rock units affected by the impact and preserved
within the perimeter of Upheaval Dome include
Navajo Sandstone, which provides an ideal
lithology for comparing faults formed at low
strain rates in continental tectonic settings, such
as the Colorado Plateau, to those generated at
high strain rates.
FAULTING IN POROUS
SEDIMENTARY ROCKS
The processes leading to fault formation in
porous sedimentary rocks have been extensively studied, particularly within the Colorado Plateau of the western United States
(Aydin, 1978; Aydin and Johnson, 1978, 1983;
Davis, 1999; Davis et al., 1999; Shipton and
Cowie, 2001, 2003; Schultz and Balasko,
2003; Schultz and Siddharthan, 2005; Okubo
and Schultz, 2005; Fossen et al., 2007). In
porous granular rocks such as sandstones,
deformation band damage zones are the precursors to faulting in porous rocks in tectonic
GSA Bulletin; May/June 2011; v. 123; no. 5/6; p. 1161–1170; doi: 10.1130/B30087.1; 7 figures; 1 table.
For permission to copy, contact [email protected]
© 2011 Geological Society of America
1161
Key and Schultz
settings such as the Colorado Plateau (Aydin
and Johnson, 1978; Johnson, 1995; Okubo and
Schultz, 2005). Deformation bands are identified as tabular discontinuities within a porous
granular rock that accommodate strain through
compaction, dilation, shear, or a combination
of these (Antonellini et al., 1994; Aydin et al.,
2006; Fossen et al., 2007; Schultz and Fossen,
2008). Among the several kinematic varieties
of deformation bands, the most common involve both compaction (associated with porosity reduction and grain reorganization within
the band) and cataclasis (cracking, rotation,
and interlocking of grains within the band)
(e.g., Aydin et al., 2006; Fossen et al., 2007).
Individual cataclastic deformation bands
(Fig. 1A) are commonly millimeters in width
and accommodate shear displacement on the
order of several millimeters (Aydin, 1978; Aydin
and Johnson, 1978). Further deformation within
the host rock is accommodated by the formation of additional parallel or subparallel deformation bands (Aydin and Johnson, 1978). The
presence of two or more deformation bands
defines a DBDZ (Aydin and Johnson, 1978).
Deformation band damage zones characteristically display a lenticular or wavy pattern when
observed in the field in map view (Aydin, 1978;
Aydin and Johnson, 1978; Schultz and Balasko,
2003) and a backward-breaking linking geometry in cross section (Davis, 1999; Schultz
and Balasko, 2003; Okubo and Schultz, 2005;
Fig. 1B). Deformation band damage zones are
typically on the order of decimeters to meters in
total thickness (Aydin and Johnson, 1978; Shipton and Cowie, 2001). The total amount of offset accommodated by a DBDZ can be estimated
by summing the displacements of each individual deformation band within the damage zone
resulting in offsets up to several centimeters
(Aydin and Johnson, 1978). To accommodate
larger displacements on the order of decimeters
to meters, widening of the DBDZ ceases and a
slip surface forms; this surface eventually grows
into a throughgoing fault that slices through
the DBDZ (Aydin and Johnson, 1978, 1983;
Shipton and Cowie, 2001, 2003; Schultz and
Siddharthan, 2005; Fig. 1C).
PREVIOUS WORK AT
UPHEAVAL DOME
Upheaval Dome is a circular structure ~5.5 km
in diameter located on the Colorado Plateau in
southeastern Utah (Fig. 2). It has been estimated
to be Jurassic (Alvarez et al., 1998) or Cretaceous (Kenkmann et al., 2005) in age. Upheaval
Dome is characterized by a complex central
uplift, a ring syncline, and an outer circumferential monocline dipping away from the center
1162
A
B
C
Figure 1. Deformation and faulting in porous Entrada Sandstone near the San Rafael Swell
in southeastern Utah. (A) Several individual deformation bands; foot shown for scale.
(B) A deformation band damage zone (DBDZ) with linking geometries visible in cross section; width ~1 m. Deformation bands in both (A) and (B) display positive relief in comparison to the surrounding, undeformed Entrada Sandstone. (C) A normal fault with several
meters of offset cutting the Entrada Sandstone above and the Carmel Formation below. The
fault cuts through a DBDZ that is ~4 m in width (shown by arrow).
of the structure (Shoemaker and Herkenhoff,
1983; Kriens et al., 1997, 1999; Jackson et al.,
1998). A sedimentary sequence composed of
the Permian White Rim Sandstone of the Cutler
Group, Triassic Moenkopi and Chinle Formations, and Jurassic Wingate Sandstone, Kayenta
Formation, and Navajo Sandstone is exposed
within the structure (Fig. 3). Radially oriented
thrust faults with offsets ranging from 5 to
500 m are associated with structural thickening within the central uplift (Kriens et al., 1999;
Kenkmann et al., 2005). Concentric listric normal faults are documented in the ring syncline
(Kriens et al., 1999).
For decades, the origin of Upheaval Dome
had been a source of debate; however, in recent
years the theories for its formation have focused
on salt intrusion or hypervelocity meteorite impact. According to the salt intrusion hypothesis,
Upheaval Dome was formed by upward migration of salt from the underlying Paradox Formation, which has been regionally identified as
the source of numerous salt structures throughout the area (Harrison, 1927; McKnight, 1940;
Mattox, 1975). Jackson et al. (1998) suggested
that a salt diapir had passed through Upheaval
Dome’s center and had subsequently been
pinched off and eroded away. Using structural
and stratigraphic relationships, Jackson et al.
(1998) suggested that the structure underwent
prolonged growth beginning in the Jurassic and
which continued for ~20 million years. They
cited a concentric fold system composed of an
outer rim monocline, a rim syncline, and an inner
dome as geomorphic evidence for salt diapirism (Jackson et al., 1998). Additional evidence
cited in favor of salt diapirism was synsedimentary deformation (i.e., truncations, channels,
and growth folds) within the Chinle Formation,
Wingate Sandstone, and Kayenta Formation, and
emplacement of clastic dikes along faults inferred to have formed in the neck of the pinchedoff salt structure (Jackson et al., 1998).
An origin by impact cratering was advocated by Shoemaker and Herkenhoff (1983)
and Kriens et al. (1997, 1999) on the basis of
detailed mapping of the structure and comparisons with known terrestrial impact sites.
Impact cratering is characterized by very high
strain rates that generate extreme shock pressure and temperature conditions (Osinski and
Spray, 2005; Pati and Reimold, 2007). Crater
Geological Society of America Bulletin, May/June 2011
High strain-rate faulting in porous sedimentary rocks: First results from the Upheaval Dome impact structure
UPHEAVAL
DOME
UTAH
JW
PWR , TM, TC
JK
JN
Fault 1
Fault 2
N
1 km
Figure 2. Oblique aerial view of the Upheaval Dome impact structure located in southeast Utah (see inset location map in the upper-left corner; photograph courtesy of Tom Till
Photography, Moab, Utah). The central uplift is characterized by the light-colored mounds
in the center of the crater structure and is composed of the White Rim Sandstone (PWR)
and the Moenkopi (TM) and Chinle (TC) Formations. The ring syncline is composed of the
cliff-forming Wingate Sandstone (JW) located outside the central uplift, the topographic low
formed by the Kayenta Formation (JK), and another cliff-forming unit, the Navajo Sandstone (JN), located along the perimeter of the crater. The ring syncline extends outward from
the Navajo Sandstone to the flat-lying strata surrounding the impact structure. The locations of the faults evaluated in this study, Fault 1 and Fault 2, are shown.
formation can be divided into three sequential
stages including contact and compression, excavation, and modification (Melosh, 1989). During the earliest contact and compression stage,
kinetic energy is transferred from the impactor
to the target rocks. Structures formed in the target rocks during this high-pressure stage include
shock metamorphism, cataclastic dikes, and
shatter cones (French, 1998; Sagy et al., 2004).
A bowl-shaped transient crater is formed during
the excavation stage that is faulted during the
modification stage as a result of gravity-driven
radial collapse (Croft, 1981a; Melosh, 1989).
Structures forming during the modification
stage include concentric normal faulting, folding, and uplift of the floor in the crater center
(Croft, 1981b). Impact structures are known to
form as a result of strain rates well in excess of
10–2 s–1 (Melosh, 1989; Pati and Reimold, 2007).
Huntoon (2000) presented a compilation of
identified strain indicators documented at Up-
heaval Dome to show that the structure’s morphology is consistent with proven impact craters
rather than salt intrusion. He correlated the occurrence of cataclastic dikes, shatter cones, thrust
faults, and folds within the Wingate Sandstone
with the excavation phase of crater formation.
Evidence cited for the modification phase includes the central uplift, imbricated thrust sheets,
outwardly plunging anticlines, ring syncline, and
listric normal faults (Huntoon, 2000). Shocked
quartz grains have also been identified in interbedded sandstone units within the Kayenta
Formation of Upheaval Dome’s ring syncline
(Buchner and Kenkmann, 2008). Shocked quartz
is considered to be an undisputed indicator of an
impact origin (Ashworth and Schneider, 1985;
Gratz et al., 1988; French, 1998).
Seismic-reflection studies conducted across
the structure have revealed that deformation
decreases with depth and that layers above the
Paradox Formation are relatively flat lying,
favoring an impact origin for Upheaval Dome
and contradicting the salt intrusion model
(Kanbur et al., 2000). Additional evidence supporting an impact origin for Upheaval Dome
includes morphology and structural geology
consistent with an impact structure (Shoemaker
and Herkenhoff, 1983; Kriens et al., 1997,
1999), fault kinematics (Kenkmann et al., 2005),
clastic dikes (Kenkmann, 2003), and spatially
distributed cataclastic deformation bands in the
Wingate Sandstone (Okubo and Schultz, 2007).
Based on the evidence contained in the literature
and summarized above, Upheaval Dome is best
interpreted as an eroded complex impact crater,
and faults mapped within the structure were
formed in response to high strain rates generated
during a hypervelocity impact event.
Much emphasis has been placed in terrestrial impact research on crater morphology
(Shoemaker, 1960; Melosh and Gaffney, 1983;
Melosh, 1989; Grieve, 1991; Morgan and
Warner, 1999; Grieve and Therriault, 2004),
mineralogical characteristics such as the presence of high-temperature and high-pressure
minerals and shock (or planar) deformation features in quartz and feldspars (Grieve and Therriault, 2004; Osinski and Spray, 2005; Buchner
and Kenkmann, 2008), and presence of shatter
cones (Grieve and Therriault, 2004; Sagy et al.,
2004; Pati and Reimold, 2007). In contrast,
studies of faulting at impact structures are less
common. For example, Spray (1997) referred to
some faults at impact craters as “superfaults,”
or faults that accommodate large displacements
(≥100 m) during single slip events in which
velocities greater than 0.1 m/s occur. Detailed
structural mapping studies have been conducted
at several impact structures in sedimentary
target rocks including Kentland (Laney and
Van Schmus, 1978), Upheaval Dome (Kriens
et al., 1997, 1999; Kenkmann et al., 2005), and
Haughton crater in Canada (Osinski et al., 2005;
Osinski and Spray, 2005). Structural and stratigraphic mapping at Upheaval Dome was conducted by Kriens et al. (1997, 1999), and in the
central uplift by Kenkmann (2003), Kenkmann
et al. (2005), and Scherler et al. (2006). Our
paper builds on the geometry and kinematics of
faults at Upheaval Dome by focusing on their
potential strain-rate dependence.
OBSERVATIONS RELATED TO FAULTS
AT UPHEAVAL DOME
Faults investigated in this study were identified at Upheaval Dome using the geologic
map published by Kriens et al. (1999) with accessibility evaluated initially by using Google
Earth software and then verified in the field.
Two faults, Fault 1 and Fault 2, were chosen for
Geological Society of America Bulletin, May/June 2011
1163
Age
Key and Schultz
Thickness
(in meters)
Unit description
J
Entrada Sandstone: Light to red,
usually massive sandstone. (Not
drawn to scale.)
J
Carmel Formation: Red sandstone, siltstone, and shale with
basal limestone. (Not drawn to
scale.)
J
Navajo Sandstone: Porous,
highly cross-bedded buff to red
sandstone.
J
Kayenta Formation: Red, gray,
and purple shale and
sandstone.
J
Wingate Sandstone: Massive,
red, cliff-forming sandstone.
T
Chinle Formation: Partly silty
and sandy variegated shales,
with thin limestone and conglomerate lenses.
T
Moenkopi Formation: Primarily
red to brown shale, siltstone,
and sandstone with gray limestone interfingering.
Jurassic
E
C
900
N
Deformation bands (Jurassic)
documented in the JE at
Arches National Park.
Deformation bands (Late
Cretaceous to late Paleogene)
documented in the JN in
western Utah.
K
700
W
600
C
500
M
400
P
WR
Units exposed within Upheaval Dome
800
Triassic
Figure 3. Stratigraphic column
of units exposed at Upheaval
Dome and younger eroded units
after Jackson et al. (1998) (data
from the Buck Mesa #1 well log
[Fiero, 1958; Mattox, 1968; and
Woodward-Clyde Consultants,
1983]). Measured thicknesses are
reported beginning at the unconformity underlying the Permian
rock sequence to the currently
exposed surface. Unit descriptions are from Molenaar (1975).
Ages of deformation are from
Antonellini et al. (1994) (Entrada
Sandstone), Davis et al. (1999)
(Navajo Sandstone), and Okubo
and Schultz (2007) (Wingate
Sandstone).
Additional comments
Deformation bands (post–early
Jurassic) documented in the
J W at Upheaval Dome.
White Rim Sandstone observable
at the top of the Cutler Group.
Permian
300
200
P
Cutler Group- Red arkosic
sandstone and conglomerate,
sandy shale, and limestone.
C
100
0
detailed study from a larger set of candidates
that all cut Navajo Sandstone, given their accessibility (i.e., not located along a cliff face)
and state of preservation of fault-zone materials (Fig. 2). Fault 2 has been mapped as a
thrust fault (Kriens et al., 1999); however, the
sense and magnitude of offset at Fault 1 are
difficult to determine given the absence of pas-
1164
sive markers. Faults observed in association
with the impact at Upheaval Dome are known
to have formed during a single event with no
significant offset along the faults generated
subsequent to crater formation; these faults are
thus distinct from those outside the structure
that are not related to the impact (Kriens et al.,
1999). Such conditions are ideal for the present
study, because the sequence of fault formation
in porous Navajo Sandstone will be preserved
with no additional deformation or overprinting.
Cataclastic deformation bands of post–Early
Jurassic age have been documented at Upheaval
Dome in the fine-grained, low-porosity Wingate
Sandstone (Okubo and Schultz, 2007). Based
on calculated stress magnitudes in excess of 0.7
Geological Society of America Bulletin, May/June 2011
High strain-rate faulting in porous sedimentary rocks: First results from the Upheaval Dome impact structure
GPa, these deformation bands have been attributed to the formation of Upheaval Dome during
the early excavation stage of the impact event
(Okubo and Schultz, 2007). During our investigation, zones of deformation bands were also
observed in the Navajo Sandstone in the ring
syncline. The deformation bands were observed
to be millimeters in width and occasionally accumulated into small DBDZs on the order of
several centimeters in width. Observed deformation bands and DBDZs within the Navajo
Sandstone exhibit no evidence (i.e., slip surfaces) for faulting.
The architecture of the faults we examined
in Navajo Sandstone at Upheaval Dome is distinctly different from that commonly observed
in this unit elsewhere on the Colorado Plateau.
Instead of DBDZs that were subsequently
faulted, we observed a friable rock with a powdery texture exposed adjacent to distinct fault
planes. At Fault 1 (Figs. 4A and 4B), this rock
is white in color and ~2–6 cm thick. At Fault 2
(Fig. 4C), the rock is tan in color and ~2–3 cm
thick. Varying degrees of friability within the
Navajo Sandstone along observed faults at Upheaval Dome were noted in thin section, with
the most friable rock observed at Fault 1 when
compared to rock at Fault 2 and the undeformed
Navajo Sandstone. At both faults, the Navajo
Sandstone is observed above the fault with
sheared Kayenta Formation below; no DBDZs
are observed adjacent to either fault in the Navajo Sandstone as is commonly the case elsewhere on the Colorado Plateau in these units.
The friable rock was identified as deformed
Navajo Sandstone based on field relationships
shown in Figure 4, in which it was consistently
observed between the fault plane and relatively
undeformed Navajo Sandstone. Thin-section
observations were also used to identify this
material as Navajo Sandstone (i.e., quartz rich,
high porosity). Iron oxidation coating the quartz
grains was noted in thin sections of the friable
rock collected along Fault 2, and likely is responsible for the tan color observed.
In thin section the friable rock differs from
fault gouge (Engelder, 1974) by the lack of
significant rotation or shear of the grains after
fragmentation and by the absence of fabrics that
would be indicative of shear. As shown in Figure 5, the grains of the friable rock have clearly
been shattered yet maintain their original grain
boundaries. Shattered clasts within these grains
have been visibly reduced to the microscale.
Gouge zones are typically characterized by the
progressive development of shear fabrics including en echelon R1-shears, conjugate R2-shears,
antithetic P- and X-shears, and Y-shears parallel
to country rock interfaces (Gu and Wong, 1994).
Fault material observed along Fault 1 and Fault 2
displayed no evidence for shear localization or
cataclastic flow after fragmentation, and is therefore not consistent with fault gouge.
We interpret the friable rock observed along
fault planes at Fault 1 and Fault 2 as “pulverized” Navajo Sandstone, a material consistent
with cracking-dominated deformation. Pulverized rock is defined by Dor et al. (2006) and
Dor (2007) as rock that typically yields a white,
powdery texture in the field and is observed
in thin section to be shattered in-place to the
microscale, while maintaining its original grain
fabric with little to no evidence for rotation or
shear after fragmentation. Five damage classes
based on the intensities of fractured and pulverized rock have been identified (Dor et al., 2006;
Dor, 2007) and are summarized in Table 1.
Weak and/or selective pulverization, in which
some grains remain intact while others shatter,
is identified in thin section along the faults at
Upheaval Dome, which corresponds approximately to damage class IV as defined by Dor
et al. (2006) and Dor (2007).
Pulverized rock along a fault is now recognized as distinct from gouge (Brune, 2001; Dor
et al., 2006, 2009; Dor, 2007) and is considered
to be diagnostic of high-velocity slip events along
a fault (Brune, 2001; Wilson et al., 2005; Reches
and Dewers, 2005; Dor et al., 2006, 2009; Dor,
2007). Damage along a fault plane has been
shown to be asymmetric, with increased damage
on the stiffer side of the fault (Dor et al., 2006,
2009). This is consistent with our observations
at Upheaval Dome in which we find rock pulverization in the stiffer Navajo Sandstone above
the fault plane and shear deformation in the
softer clay and siltstone units of the Kayenta
Formation below the fault plane.
Current research on high strain-rate faulting
has linked dynamic slip events to pulverization
in a variety of rock types ranging from granite to
porous sedimentary rocks such as those found in
the Juniper Hills and the Hungry Valley Formations along the San Andreas fault (Brune, 2001;
Dor et al., 2006, 2009; Dor, 2007), to quartzites
along the Bosman fault in South Africa (Reches
and Dewers, 2005; Wilson et al., 2005). The
origin of pulverized rock has been closely associated with seismogenic faulting (i.e., dynamic
slip events) (Prakash et al., 2008). Two mechanisms have been proposed for the formation of
pulverized rock including a localized reduction
in fault-normal stress during rupture passage
(Brune, 2001; Wilson et al., 2005; Dor et al.,
2006) or pulverization due to changing stress
conditions localized at the rupture tip during
earthquake propagation (Reches and Dewers,
2005; Wilson et al., 2005). Field studies by several research groups worldwide are demonstrating that material along some major faults may
be pulverized rock, instead of gouge as previously mapped or inferred (Brune, 2001; Dor
et al., 2006), with important revised implications for the mechanical behavior of faults.
ANALYSIS AND DISCUSSION
Grain-Size Analysis
Several different methods have been employed to evaluate the reduced grain sizes of
fault rocks including water disaggregation (An
and Sammis, 1994; Reches and Dewers, 2005),
sieve analysis (An and Sammis, 1994), Coultercounter analysis (An and Sammis, 1994), and
digital analysis of grain perimeter ratios from
thin-section images (Dor, 2007; Dor et al.,
2009). For this study, we chose to measure grain
sizes directly from images taken from thin sections. This method has been used in the evaluation of grain-size reduction within deformation
and compaction bands (Sternlof et al., 2005;
Torabi et al., 2007; see also Holcomb et al.,
2007) and displays the grain sizes and packing
conditions as they exist in the field.
For this study, grain sizes were measured by
summing the measured length and width of each
individual grain and dividing the resulting value
by two (Key, 2009). Pulverized grain sizes were
measured petrographically from each of the
individual rock fragments observed within
the original grain boundaries (see Fig. 5B). Grain
sizes ranging from 3.75 × 10–6 m to 1.40 × 10–4 m
were obtained for pulverized rock at Upheaval
Dome. In order to compare the pulverized grain
sizes to the original grain sizes prior to deformation, measurements of the original quartz grains
were also collected. These measurements were
possible because the rock’s original fabric was
maintained after pulverization, thus preserving
original grain boundaries. Original grain sizes
of the pulverized material were measured using
the method described above.
Figure 6 presents a log-log plot of measured
grain sizes of pulverized rock at Fault 1 (see
Fig. 5). We obtain a two-dimensional (2D)
power-law slope of 0.77 for the original grain
sizes and 1.55 for the pulverized grain sizes collected from the Upheaval Dome samples. Although the regression lines shown in Figure 6
have a negative slope, the power law is reported
as a positive value by convention to allow for
more direct comparisons to previous work evaluating the grain-size distribution of fault gouge
(An and Sammis, 1994; Sammis and Steacy,
1995; Sammis and King, 2007). The 1.55 powerlaw slope obtained from the pulverized grain
sizes is remarkably close to the 1.6 2D slope
obtained from fault gouge reported by Sammis
et al. (1987) and Sammis and Steacy (1995).
Geological Society of America Bulletin, May/June 2011
1165
Key and Schultz
A
Undeformed
Navajo
Sandstone
Fault Plane
B
Undeformed
Navajo Sandstone
Pulverized
Navajo Sandstone
Fault Plane
Sheared
Kayenta Formation
Pulverized
Navajo Sandstone
Sheared
Kayenta Formation
C
Navajo
Kayenta
Figure 4. (A) Schematic diagram of Fault 1 with pulverized Navajo Sandstone shown in black just above the
fault plane; field notebook for scale. (B) Photograph of Fault 1 oriented facing south and away from the center
of the crater (coordinates: 38°25′36.684″, 109°56′13.258″, North American Datum of 1983). Pulverized Navajo
Sandstone is shown as the thin white unit (2–6 cm in width) to the right of the field notebook and above the
fault. In both (A) and (B), sheared siltstone of the Kayenta Formation is observed beneath the fault plane and
to the left of the notebook. The Kayenta Formation–Navajo Sandstone contact left of the main fault plane is
shown by the dashed line on the left of the cave. (C) Outcrop exposure of Fault 2; geologist, circled, is sitting on
the fault plane separating Navajo above from Kayenta below.
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Geological Society of America Bulletin, May/June 2011
High strain-rate faulting in porous sedimentary rocks: First results from the Upheaval Dome impact structure
environment, although the power-law slope
associated with our pulverized grain sizes is
comparable to the 1.6 2D power-law slope for
“low-strain gouge and breccia” reported by
Sammis et al. (1987) and Sammis and Steacy
(1995). Our data suggest a degree of nonuniqueness in grain-size distributions of pulverized
rock and fault gouge.
Strain Rates
0 μm 10
20
30
A
0 μm 10
20
30
B
Figure 5. Photomicrographs of quartz grains
in undeformed Navajo Sandstone (A) and
pulverized Navajo Sandstone (B). (A) Grains
shown in the undeformed rock taken from
outside the fault zone are subangular to
subrounded and display little to no intragranular fracturing. (B) Grains shown in the
pulverized rock display significant fracturing. Images taken in plane-polarized light.
It has been suggested that the distribution of
grain sizes within fault gouge can be correlated
to strain-rate conditions (Sammis et al., 1987;
An and Sammis, 1994; Sammis and Steacy,
1995; Sammis and King, 2007). For example,
a 2D power-law slope of 2.0 has been reported
in association with higher strain rates, with a 2D
power-law slope of 2.6 associated with “lowstrain gouge” (Sammis and King, 2007). The
pulverized rock collected at Upheaval Dome
is inferred to have formed in a high strain-rate
Results from laboratory and controlled blasting experiments used to investigate dynamic
fragmentation in rock have defined a relationship between grain size and the strain rate responsible for grain-size reduction (Grady and
Kipp, 1987; Zhou et al., 2005). Blasting experiments were conducted within oil shales in
Rifle, Colorado, in order to prepare the rock for
retorting (heating), allowing access to liquid oil
(Grady and Kipp, 1987). Strain rates applied to
the rock during these experiments ranged from
100 to 104 s–1 (Grady and Kipp, 1987). The relationship derived by Grady and Kipp (1987) was
most recently applied by Reches and Dewers
(2005) in their study of dynamic rupture along
faults. This relationship is given by:
⎡
d=⎢
⎢
⎣
(
)
2
20K IC ⎤ 3
⎥,
ρCd ε ⎥
⎦
(1)
where d is the diameter of the fragmented
grains, KIC is the mode-I fracture toughness of
the unfractured grain, ρ is the rock density, Cd
is the compressive (P-wave) velocity of Navajo
Sandstone, and ε is the strain rate applied to
the rock. The reduction of grain size during
dynamic fragmentation has been found to be
dependent on the kinetic energy applied to
the rock affected. Therefore, the role of initial
grain size is relatively insignificant in comparison to the rock’s properties and the strain rate
applied to the rock (Grady and Kipp, 1987;
Reches and Dewers, 2005). Additionally, overburden pressure is negligible in comparison to
the extreme loading conditions applied to the
rock during dynamic fragmentation (Grady
and Kipp, 1987).
Zhou et al. (2005) reevaluated Grady and
Kipp’s (1987) relationship between grain size
and strain rate by comparing data generated
from one-dimensional (1D) armor ceramic system models to the data presented by Grady and
Kipp. The comprehensive model presented by
Zhou et al. (2005) took a numerical approach
to the evaluation of dynamic fragmentation,
considering a wide range of strain rates (101 to
5 × 106 s–1) and incorporating various spatial
distributions of intrinsic defects and material
strengths. Zhou et al. (2005) also considered
elastic wave interaction with cohesive crack
opening, resulting in a prediction of the number of fragments a rock will break into under
a given strain rate. Grain sizes calculated from
these simulations, in which original grains with
randomly oriented flaws were subjected to high
strain-rate conditions (defined as ≥103 s–1), revealed that grain sizes are reduced by ~40%
compared to those predicted by Grady and Kipp
(1987) for the same set of conditions (see Zhou
et al., 2005). At low (quasistatic) strain rates
(≤101 s–1) where only a portion of the strain
energy goes toward the development of new
cracks, fragment sizes are not dependent on
strain rate, and a constant grain size is predicted
(Zhou et al., 2005). We define a parameter, f,
which accounts for the shift in values obtained
by Grady and Kipp’s relation (1987) as suggested by Zhou et al. (2005) to account for their
more realistic (i.e., nonuniform) initial flaw distribution. Rearranged for strain rate, Grady and
Kipp’s (1987) adjusted equation is given by:
ε=
20K IC
3
,
(2)
⎛ d⎞2
ρCd ⎜ ⎟
⎝ f⎠
where f = 1 in Grady and Kipp’s (1987) Equation 1, and f = 0.4 following Zhou et al. (2005).
We have applied the adjusted Grady and
Kipp (1987) relation (Equation 2) to the samples of pulverized rock obtained from the field
TABLE 1. SUMMARY OF DAMAGE CLASSES RANGING FROM DEGREES OF FRAGMENTATION AND/OR FRACTURING
(CLASSES I–III) TO DEGREES OF PULVERIZATION (CLASSES IV AND V) AS DEFINED BY DOR ET AL. (2006) FROM
OBSERVATIONS ALONG THE MOJAVE SECTION OF THE SAN ANDREAS FAULT IN SOUTHERN CALIFORNIA
Damage class
Description
I
Weak fracturing
Rock with large macroscopic fractures exceeding background fracture density.
II
Fragmentation
Rock that is fragmented with cm-scale fractures. Rock yields a rugged appearance with fragments that can be crushed
to smaller pieces by hand.
III
Intense fracturing
Rock fractured at the grain scale. Rock yields a texture similar to grus and is characterized by rough, rounded surfaces.
IV
Weak and/or selective
Rock with grains fractured to the microscale. Some crystals or grains remain intact, some break along subcrystal or
pulverization
grain fractures, and some yield a powdery texture. Rock is easily eroded with smooth and rounded outcrops.
V
Pervasive pulverization
Rock with grains fractured to the microscale. All crystals or grains yield a powdery texture when crushed by hand.
Rock is easily eroded with smooth and rounded outcrops.
Note: Damage classes are determined by the intensity of observed deformation in proximity to faulting and range from weak fracturing at the macroscopic scale to pervasive
pulverization at the microscopic scale (Dor et al., 2006; Dor, 2007).
Geological Society of America Bulletin, May/June 2011
1167
Key and Schultz
1000
locations at Upheaval Dome in order to estimate
strain rates along the faults at the time of impact. Our measured grain sizes are plotted in
Figure 7 along with Equation 2. We chose values of KIC of 10–100 MPa*m1/2, which brackets
the value of 30 MPa*m1/2 used by Reches and
Dewers (2005), ρ of 2250 kg/m3 appropriate to
undeformed Navajo Sandstone (Sternlof et al.,
2005), and Cd of 2483 m/s obtained using:
Cd =
μ+λ
,
ρ
Pulverized grains
Number (n)
Original grains
10
y = 1.67x –0.77
(4)
E
2 (1 + ν )
(5)
0.01
0.1
1
Figure 6. Log-log plot comparing grain sizes in the undeformed
and pulverized Navajo Sandstone as measured in thin section.
10 –1
10
KIc = 30 MPa*m1/2
100
10 –2
10 –3
Grain size (d, m)
(Timoshenko and Goodier, 1970), with Young’s
modulus (E) of 20 GPa and Poisson’s ratio
(ν) of 0.2 (Sternlof et al., 2005) appropriate to
Navajo Sandstone.
Using a representative grain size of 4 × 10–5 m
for pulverized rock along faults at Upheaval
Dome with f = 1 in Equation 2 (Grady and
Kipp, 1987), we obtain an approximate strain
rate of 88 s–1. Using maximum and minimum
measured grain sizes of 3.75 × 10–6 to 1.40 ×
10–4 m, strain rates ranging from 5 to 1 × 104 s–1
are obtained. Using f = 0.4, we obtain a strain
rate of 2 × 101 s–1, with values ranging from 1
to 3 × 103 s–1 using maximum and minimum
measured grain sizes.
To compare strain rates calculated along
faults within Upheaval Dome to rates generated elsewhere, strain rates associated with the
pulverization of sedimentary rocks along the
Mojave section of the San Andreas fault were
also calculated. Doan and Gary (2008) estimated strain rates associated with pulverized
rock along the San Andreas fault of at least 1.5 ×
10–2 s–1 using high strain-rate laboratory testing
methods. Using grain sizes ranging from 2.56 ×
10–3 to 1.51 × 10–4 m obtained by Dor et al.
(2009) for pulverized sedimentary rocks along
the San Andreas fault and Grady and Kipp’s
(1987) relationship (Equation 2, with f = 1),
we obtain maximum and minimum strain rates
ranging from 6 × 10–2 to 4 × 101 s–1. Using
f = 0.4, we obtain strain rates ranging from 2 ×
10–2 to 2 × 101 s–1. These values are consistent
with those obtained by Doan and Gary (2008).
Our results suggest that strain rates calculated for faults at Upheaval Dome (1 – 3 ×
103 s–1) are consistent with, or faster than, strain
1168
1
0.001
Grain size (d, mm)
Eν
,
(1+ ν) (1− 2ν)
μ=
y = 0.36x –1.55
(3)
(Reches and Dewers, 2005). Values were obtained for Lamé’s coefficient (λ) and the shear
modulus (μ) using:
λ=
100
San Andreas fault (Dor, 2007)
10 –4
Upheaval Dome fault
10 –5
10 –6
10 –7
10 –15 10 –13 10 –11 10 –9 10 –7 10 –5 10 –3 10 –1 10 1
10 3
10 5
Strain rate (s –1)
Figure 7. Plot of grain size versus predicted strain rate (Equation 2) compared to data collected along faults at Upheaval Dome (this work) and sedimentary rocks along the San
Andreas fault (Dor et al., 2009). The dashed line on the right side of the plot represents
KIC = 100 MPa*m1/2 for f = 1; the solid and dashed lines offset to the left represent KIC ranging from 10 MPa*m1/2 to 100 MPa*m1/2 for f = 0.4.
Geological Society of America Bulletin, May/June 2011
High strain-rate faulting in porous sedimentary rocks: First results from the Upheaval Dome impact structure
rates calculated along the Mojave section of the
San Andreas fault (2 × 10–2 to 2 × 101 s–1). Strain
rates calculated for Upheaval Dome are up to
20 orders of magnitude larger than the highest strain rates generated in intraplate tectonic
settings (i.e., 10–20 to 10–17 s–1, Gordon, 1998)
as well as those used in standard uniaxial and
triaxial laboratory experiments (e.g., American
Society for Testing and Materials [ASTM] standard D7012, 2004). Our findings are consistent
with a general rate-dependence for deformation
of porous sedimentary rocks with DBDZs developing in response to strain rates <10–2 s–1 and
pulverized rock forming in response to strain
rates in excess of 10–2 s–1.
According to Wilson et al. (2005), rock
pulverization is a function of strain rate and
rupture velocity resulting from a single earthquake event, but not a function of cumulative offset along a fault. Zones of pulverized
rock are observed to approach 100–200 m in
thickness along the Mojave section of the San
Andreas fault (Brune, 2001; Dor et al., 2006,
2009), where cumulative fault offset is on the
order of tens to hundreds of kilometers (Wilson
et al., 2005). By contrast, zones of pulverized
rock observed along the faults located within
the ring syncline of Upheaval Dome are several
centimeters in thickness. Maximum fault offsets within the ring syncline at Upheaval Dome,
where the faults evaluated in this study are located, are estimated to be on the order of meters
to tens of meters (Kenkmann et al., 2005). Although preliminary, it appears that the width of
pulverized zones might be related to the amount
of fault offset.
CONCLUSIONS
Our findings suggest that faulting in porous
Navajo Sandstone within the ring syncline of
Upheaval Dome was associated with the formation of pulverized rock along fault planes and not
by the formation of deformation band damage
zones (DBDZs). This sequence of faulting contrasts with that on the Colorado Plateau outside
the impact structure, in which DBDZs form during the early stage of deformation. Using the relationships obtained by Grady and Kipp (1987)
and refined by Zhou et al. (2005), we infer that
the two faults studied at Upheaval Dome likely
formed in response to strain rates ranging from 1
to 3 × 103 s–1. Using the same method, we obtain
strain rates ranging from 6 × 10–2 to 2 × 101 s–1
for the Mojave section of the San Andreas fault
that are consistent with dynamic earthquake slip
rates. Our findings support the inference of Fossum and Brannon (2006) that deformation mechanisms in porous rock such as Navajo Sandstone
are strain-rate dependent.
Pulverized rock has been documented in all
rock types (i.e., sedimentary, crystalline, and
metamorphic) where faulting is observed. Understanding strain-rate conditions at an impact site
provides a new source of information regarding
the mechanical responses of rock types affected
by deformation at high strain rates. Complex
impact craters, such as Upheaval Dome, where
faults display greater offsets at the center of the
crater than near the crater rim, may provide ideal
locations for testing the relationship of fault offsets to widths of pulverized zones.
ACKNOWLEDGMENTS
We thank Thomas Kenkmann, Chris Okubo, and an
anonymous referee for their helpful reviews, and Ken
Herkenhoff, and especially Associate Editor W.U.
Reimold, for comments that improved the clarity
of the final paper. Discussions with Ze’ev Reches,
Yehuda Ben-Zion, John Louie, and Steve Martel were
helpful during various stages of the work. This work
was supported by a grant from National Aeronautics
and Space Administration’s Planetary Geology and
Geophysics Program.
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Geological Society of America Bulletin, May/June 2011