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The choice of modeling material—dry sand or wet clay—affects the style and distribution of deformation in scaled experimental (analog) models of extension. For example, fault-zone widths are greater in sand than in clay, possibly reflecting the marked difference in maximum grain size of the modeling materials (<0.5 mm for dry sand vs. < 0.005 mm for wet clay).Most differences in the deformation patterns, however, reflect differences in the ductility of the modeling materials. Normal faults are long, planar and hard-linked (i.e., directly connected) in the dry sand with its low ductility, whereas normal faults are short, curved and soft-linked (i.e., not directly connected) in the wet clay with its higher ductility. A few large normal faults accommodate most deformation in the sand models, whereas a few large faults and numerous small faults accommodate most deformation the clay models. Little folding occurs in the sand models, but folds are common in the clay models.
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April 2007 Houston Geological Society Bulletin 1
Volume 49, Number 8 April 2007
Houston Geological Society
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31 Scaled Experimental Models of Extension:
Dry Sand vs. Wet Clay
by Martha Oliver Withjack, Roy W. Schlische and
Alissa A. Henza
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April 2007 Houston Geological Society Bulletin 31
Scaled Experimental Models of Extension:
Dry Sand vs. Wet Clay
by Martha Oliver Withjack, Roy W. Schlische, and Alissa A. Henza
Department of Geological Sciences, Rutgers University, Piscataway, NJ
Abstract
The choice of modeling material—dry sand or wet clay—affects
the style and distribution of deformation in scaled experimental
(analog) models of extension. For example, fault-zone widths are
greater in sand than in clay, possibly reflecting the marked differ-
ence in maximum grain size of the modeling materials (<0.5 mm
for dry sand vs. < 0.005 mm for wet clay). Most differences in the
deformation patterns, however, reflect differences in the ductility
of the modeling materials.
Normal faults are long, planar and hard-linked (i.e., directly con-
nected) in the dry sand with its low ductility, whereas normal
faults are short, curved and soft-linked (i.e., not directly connect-
ed) in the wet clay with its higher ductility. A few large normal
faults accommodate most deformation in the sand models,
whereas a few large faults and numerous small faults accommo-
date most deformation the clay models. Little folding occurs in
the sand models, but folds are common in the clay models.
Introduction
For more than seventy-five years, geologists have used scaled
experimental (analog) models to simulate extensional deforma-
tionin the upper crust (e.g., H. Cloos, 1928, 1930; E. Cloos, 1968;
McClay and Ellis, 1987a, 1987b; Withjack et al., 1990, 1995;
Withjack and Callaway, 2000; Schlische et al., 2002; Withjack and
Schlische, 2006; Schreurs et al., 2006). These models provide
valuable information about deformational processes. For exam-
ple, they suggest how normal faults nucleate, propagate and link.
Geologists can use this information to better understand a basin’s
petroleum system (e.g., its depositional patterns, migration path-
ways) and to minimize the uncertainties and risks associated with
hydrocarbon exploration and production.
How well do these scaled experimental models simulate nature?
The goal of this article is to address this fundamental question by
looking at the influence of modeling materials on modeling
results. First, we describe the key properties of the most common
modeling materials—dry sand and wet clay—and discuss the
basics of scaling. Second, we compare the results of sand and clay
models for three common experimental setups of extensional
deformation. Finally, we compare the results of the sand and clay
models with natural examples of extensional deformation from
rift basins and passive margins.
Modeling materials and scaling
The strength of most upper crustal rocks increases with depth,
obeying a Mohr-Coulomb criterion of failure (e.g., Byerlee,
1978). According to this criterion,
t= C0+msn(1)
where tand snare, respectively, the shear and normal stresses
on a potential fault surface, C0is the cohesive strength and mis
the coefficient of internal friction. This empirical criterion of fail-
ure describes the initiation of new faults, but not the reactivation
of existing faults. For most sedimentary rocks, the coefficient of
internal friction ranges from about 0.55 to 0.85 (e.g., Handin,
1966; Byerlee, 1978). For many intact sedimentary rocks, the
cohesive strength is about 20 MPa (Handin, 1966), whereas for
highly fractured sedimentary rocks, the cohesive strength is sig-
nificantly less (e.g., Byerlee, 1978; Brace and Kohlstedt, 1980).
The two most common modeling materials used to represent
upper crustal rocks are dry sand and wet clay. The dry sand is com-
posed of fine quartz grains with diameters of less than 0.5 mm. The
wet clay is composed predominantly of kaolinite particles (less
than 0.005 mm in diameter) and water (~40% by weight). The
properties ofdrysand and wet clay are well documented (e.g.,
Richard and Krantz, 1991; Vendeville et al., 1995; Withjack and
Callaway, 2000; Eisenstadt and Sims, 2005; Withjack and Schlische,
2006; Schreurset al., 2006). Both modeling materials have similar
densities (r~1600 kg m-3). Like upper crustal rocks, both model-
ing materials have strengths that obey a Mohr-Coulomb criterion
of failure. Their coefficients of internal friction are similar (0.5 for
drysand; 0.6 for wet clay), but their cohesive strengths are different
(negligible for dry sand; ~50 Pa for wet clay).
Twoconditions must be satisfied to create a scaled experimental
model (e.g., Hubbert, 1937; Weijermars et al., 1993; Vendeville et
al., 1995; Withjack and Callaway, 2000). First, the coefficient of
friction ofthe modeling materials must be similar to that of
upper crustal rocks. Second,
C0* = r* • g* • L*, (2)
where C0*, r*, g* and L* are ratios of model to natural prototype
for cohesive strength, density, gravity, and length, respectively. In
our models, the values of r*and g* areabout 0.7 and 1, respec-
tively, and L* is 10–4 to10–5 (i.e., 1cm in the models equals 100
to 1000 m in nature). Thus, the second condition requires that
the cohesivestrength of the modeling materials must be approxi-
mately 10–4 to 10–5 of the cohesive strength of upper crustal
rocks. These two conditions ensure that: 1) all forces, stresses and
strengths in the models are scaled down by the same amount as
the corresponding forces, stresses and strengths in nature, and
2) the strikes Scaled Experimental Models of Extension continued on page 33
April 2007 Houston Geological Society Bulletin 33
Figure 1. Experimental setups before and after deformation
showing the map view (top) and a cross sectional view
(bottom). In all models, a homogeneous layer of either dry
sand or wet clay(blue), 4-cmthick, covers the flat base.
Growth layers (yellow) fill in any depressions that form
during the model runs. a) Setup 1: diverging, overlapping
metal plates simulate a detached normal fault. Movement
of the lower plate causes a normal fault to develop in the
sand or claylayer. The fault emanates from the edge of the
xed upper plate. b) Setup 2: rubber sheet straddling
diverging metal plates simulates distributed extension.
Movement of one plate stretches the rubber sheet and the
overlying sand or clay layer. In response, normal faults
develop in the sand or clay layer. c) Setup 3: 45° dipping
precut simulates a dipping normal fault. Movement along
the precut creates a normal fault in the overlying sand or
clay layer. The fault emanates from the edge of the fixed
upper plate.
Scaled Experimental Models of Extension
Scaled Experimental Models of Extension continued on page 35
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April 2007 Houston Geological Society Bulletin 35
and dips of the faults (relative to the principal stress axes) that
develop in the models are similar to those in nature. Both condi-
tions are satisfied with either dry sand or wet clay as the modeling
material.
Although both dry sand and wet clay are suitable modeling mate-
rials to represent upper crustal rocks, they have different
deformational styles related primarily to their different ductili-
ties. Ductility reflects the capacity for distributed deformation at
the scale of observation (Rutter, 1986). A great variety of defor-
mation mechanisms ranging from fracturing/faulting to
intracrystalline plasticity to diffusive mass transfer can produce
ductile behavior (e.g., Rutter, 1986). In both dry sand and wet
clay, the primary deformation mechanism is fracturing/faulting
(e.g., Maltman, 1987; Richard and Krantz, 1991; Withjack and
Callaway, 2000). Dry sand has a low ductility because most defor-
mation is localized on a few major faults, even when strains are
small. Wet clay has a high ductility because deformation is dis-
tributed on numerous minor to major faults. With increasing
strain, however, most deformation becomes localized on a few
major faults, even in the clay. To better understand the role of
ductility on the results of scaled experimental models, we have
compared the results from three identical models of extension
using drysand and wet clayas the modeling materials.
Experimental design
Wehave modeled extensional deformation using three common
experimental setups (Figure 1). In all setups, a 4-cm thick, homo-
geneous layer of dry sand or wet clay overlies the flat base of the
apparatus. In setup 1, two overlapping plates form the base of the
apparatus. As the lower plate moves outward (at a constant rate),
a normal fault propagates upward from the fixed edge of the
upper plate through the overlying sand/clay layer. In setup 2, two
plates form the base of the apparatus. An 8-cm wide sheet of rub-
ber, attached to both plates, straddles the plate boundaries. As
one plate moves outward (at a constant rate), the rubber sheet
stretches and normal faults develop in the overlying sand/clay
layer. In setup 3, two blocks separated by a 45°dipping precut sur-
face form the base of the apparatus. As the hanging-wall block
moves downward (at a constant rate), a normal fault propagates
upward from the fixed edge of the footwall block through the
overlying sand/clay layer. Photographs of the top surface of the
models, taken at regular intervals, record the surface deformation
throughtime. In several experiments, we fill in subsiding areas
with either dry sand or wet clay at regular intervals to simulate
deposition during deformation. These growth layers initially have
aflat upper surface. After these experiments, we vertically slice
the models, creating serial cross sections.
Comparison of sand and clay models
Similarities
Overall, fault patterns are similar in the sand and clay for all three
experimental setups. High-angle (dipping 60°–65°) normal faults
develop in both the dry sand and the wet clay (Figure 2). The
faults strike approximately perpendicular to the extension direc-
tion in all models. For example, Figures 3a and 3b show the top
surface of the sand and clay models for setup 2 (distributed
extension) after 4 cm of displacement.
Differences
Deformation patterns differ in the sand and clay in several funda-
mental ways. Fault-zone widths are greater in the sand models
than in the clay models (e.g., Figure 2a, right side). Faults in the
sand are several millimeters wide, whereas faults in the clay are
less than 0.1 mm wide. This difference in fault-zone width
reflects the significant difference in the grain size of the modeling
materials. Normal faults are long, relatively planar and hard-
linked (i.e., directly connected to each other) in the sand models
in both cross-sectional view (Figure 2) and map view (Figure 3).
In the clay models, the normal faults are shorter, curved and soft-
linked (i.e., not directly connected) in cross-sectional view
(Figure 2) and map view (Figure 3). Previous work (Maltman,
1987) and recent studies (Granger et al., 2006) show that the sur-
faces of normal faults in the clay have numerous small-scale
undulations that parallelthe displacement direction (Figure 3e).
Fault distributions vary in the sand and clay models. Major faults
accommodate most of the deformation in the sand models,
whereas minor faults accommodate most deformation in the clay
models. For example, 85% of the imposed displacement is
accommodated by major faults, either the main normal fault or
majorsecondary faults, in the sand model of setup 1 (Figure 2a).
Only15% of the imposed displacement is accommodated by
minor secondary faults or cataclastic flow. In the corresponding
claymodel of setup 1, only 44% of the imposed displacement is
accommodated by major faults, either the main normal fault or
major secondary faults (Figure 2a). More than 55% of the
imposed displacement is accommodated by minor secondary
faults or by cataclastic flow.
Another major difference between the sand models and clay
models is the lack of folds in the sand compared to the clay.
Numerous relay ramps and fault-displacement folds develop in
the clay where they provide displacement transfer between the
normal faults that die out along strike. (e.g., Figure 3b, 3d, and
4a). Fault-bend folds also develop in the clay. For example, a
faultedfault-bend fold (rollover fold) develops in the hanging
wall of the main normal fault in the clay model of setup 1 (Figure
2a, bottom; Figures 4b, 4c). In contrast a series of relatively rigid
fault blocks forms in the hanging wall of the main normal fault in
the sand model of setup 1 (Figure 2a, top). As displacement on
Scaled Experimental Models of Extension
Scaled Experimental Models of Extension con tinu ed f rom pag e 33 ___________________________________________________________
Scaled Experimental Models of Extension continued on page 36
36 Houston Geological Society Bulletin April 2007
Figure 2. Cross sections through the sand and clay models for the three experimental setups. All cross sections are shown at the same scale. Black lines
are interpreted faults. a) Cross sections through setup 1 for sand (top) and clay (bottom) after 4 cm of displacement on moving plate. The two photo-
graphs on the right show enlargements of fault zones from the models. The red outline boxes show the photographs’ locations. Diagrams on the left
showthe distribution of deformation in the sand and clay models. b) Cross sections through setup 2 for sand (top) and clay (bottom) after 4 cm of
stretching of the rubber sheet. c) Cross sections through setup 3 for sand (top) and clay (bottom) after 1.4 cm of displacement. The photograph on the
right shows the faulting and folding in the clay model.
Scaled Experimental Models of Extension
Scaled Experimental Models of Extension continued on page 41
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April 2007 Houston Geological Society Bulletin 41
Figure 3. Map viewsof sand and clay models for setup 2 after 4 cm of stretching of the rubber sheet. a) Photograph of top surface of sand model. b)
Photograph of top surface of clay model. c) and d) Close-up photographs of sand and clay models. The dashed lines in a) and b) show photographs’
locations. e) Crenulations on a fault surface from clay model in setup 2 (after Granger et al., 2006).
Scaled Experimental Models of Extension continued on page 43
Scaled Experimental Models of Extension
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April 2007 Houston Geological Society Bulletin 43
Figure 4. Cross sections of folds in the clay models. a) Relay ramps and fault displacement folds from setup 2. b) Development of fault-bend folds
(rollover folds) from setup 1 as displacement of the moving plate increases from 0 to 8 cm. c) Photograph of clay model from setup 1 after 8 cm
of displacement of the moving plate.
Scaled Experimental Models of Extension
Scaled Experimental Models of Extension continued on page 45
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April 2007 Houston Geological Society Bulletin 45
the moving plate increases, the rollover fold in the clay grows
wider, bedding dips increase and bedding thickness decreases
(Figure 4b). Numerous minor to major normal faults accommo-
date the rollover folding. Fault-propagation folds also develop in
the clay but not the sand. For setup 3, a major normal fault and a
minor antithetic fault develop in the sand model (Figure 2c, top),
whereas a fault-propagation fold develops in the identical clay
model (Figure 2c, bottom). Numerous small-scale faults cut the
folded beds (Figure 2c, photograph).
Summary and discussion
Overall, fault patterns are similar in the extensional sand and clay
models in that high-angle normal faults develop that strike
roughly perpendicular to the extension direction. Deformation
patterns, however, have several fundamental differences.
Individual fault-zone widths are much greater in the sand than in
the clay, reflecting the significant difference in grain size of the
modeling materials (< 0.5 mm for dry sand vs. < 0.005 mm for
wet clay). Most differences in the deformation patterns, however,
reflect the different ductilities of the modeling materials. Normal
faults are long, planar and hard-linked in the sand, whereas they
are shorter, curved and soft-linked in the clay. A few large normal
faults accommodate most deformation in the sand models,
whereas a fewlargefaults and numerous minorfaults accommo-
datemost deformation in the clay models. Little folding occurs in
the sand models, but folds (relay ramps, fault-displacement,
fault-propagation and rollover) are numerous in the clay models.
Which modeling material best replicates nature? The answer to this
question depends on the ductility of the natural example at the scale
of observation. The dry sand, with its low ductility, best represents
rock that deforms primarily by localized faulting. Figure 5a shows an
outcrop from Greece where most deformation is localized on two
fault zones. The localized deformation on the normal faults resem-
bles that in the sand model of setup 3 (Figure 2c). The wet clay, with
its greater ductility, best represents rock that deforms by distributed
minorand majorfaulting.The distributed deformation on numer-
ous normal faults of varying size and the presence of relay ramps and
fault-displacement folds in the North Sea (Figure 5b) resembles the
deformation patterns in the clay model of setup 2 (Figure 4a). The
undulations on the fault surfaces are similar to those on the fault
surfaces in the clay models (Figure 3e). The Blackberry normal fault
in the Gulf of Mexico with its rollover fold cut by numerous small-
scale normal faults (Figure 5c) resembles the deformation in the clay
model of setup 1 (Figures 4b, 4c). The fault-propagation folds from
the Gulf of Suez (Figures 5d, 5e) resemble the fault-propagation
folds from the clay models of setup 3 (Figure 3c).
Do these conclusions have implications for cross-section restora-
tion? Restorations of cross sections from sand and clay models
suggest that the assumed angle of simple shear used in many
restoration programs depends on ductility (Withjack and
Schlische, 2006). Specifically, the effective shear angle is similar to
the dip of observed normal faults if the ductility is low. The effec-
tive shear angle, however, can differ significantly from the dip of
the observed faults if the ductility is high. For example, in the
sand model of setup 1, restorations show that the effective shear
angle is 60°–65°, the same as the dip of the major antithetic faults
(Figure 2a, top). In the clay model of setup 1, numerous minor to
major normal faults (antithetic and synthetic) accommodate the
hanging-wall deformation (Figure 2a, bottom). The effective
shear angle (35°–50°) is considerably less than the dip of the anti-
thetic normal faults, reflecting the combined effect of the
antithetic and synthetic normal faults. n
Acknowledgments
This research benefited from many thought-provoking discus-
sions with Mark Baum, Jennifer Elder Brady, Amber Granger and
William Rizer. NSF grant EAR-0408878 supported some of this
research.
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Scaled Experimental Models of Extension cont inue d fr om p age 43 __________________________________________________________
April 2007 Houston Geological Society Bulletin 47
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Scaled Experimental Models of Extension
Scaled Experimental Models of Extension cont inue d fr om p age 47 _____________________________________________________________
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... Handin 1966;Byerlee 1978); both wet clay and dry sand are thus considered suitable modelling materials because they have a coefficient of internal friction of approximately 0.6 (e.g. Sims 1993;Eisenstadt & Sims 2005;Withjack et al. 2008); and (b) the model-to-nature ratios for cohesive strength (C * 0 ), density (r * ), gravity (g * ) and length (L * ) must follow the scaling equation: ...
... Handin 1966), and up to 100 MPa for intact crystalline and metamorphic rocks (Handin 1966;Schellart 2000). The cohesive strength of wet clay is approximately 50 Pa (Sims 1993;Eisenstadt & Sims 2005;Withjack et al. 2008): therefore, C * 0 in the models ranges between 10 24 and 10 25 . Thus, 1 cm in the models represents approximately 100-1000 m in nature. ...
... Wet clay deforms by both cataclasis and ductile deformation, producing both faults and folds (e.g. Eisenstadt & Sims 2005;Withjack et al. 2008) Wet clay is ideal for studying fault-segment boundaries because it generates more numerous, smaller faults and, therefore, a larger number of fault interactions compared with dry sand models (e.g. Eisenstadt & Sims 2005;Withjack et al. 2008). ...
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Fault-segment boundaries initiate, evolve and die as a result of the propagation, interaction and linkage of normal faults during crustal extension. However, little is known about the distribution, evolution and controls on the development of relay ramps, which are the key structures developed at synthetic segment boundaries. In this study, we use a series of scaled physical models (wet clay) to investigate the distribution and evolution of fault-segment boundaries within an evolving normal-fault population during orthogonal extension. From the models, we can establish a simple geometrical classification for segment boundaries, analyse their spatial and temporal evolution, and identify key factors that influence their variability. Development of overlapping fault tips is a prerequisite for fault growth via segment linkage. Synthetic segment boundaries are the most common segment boundary type developed in the models. The proportion of synthetic segment boundaries in the total fault population increases with increasing strain, whereas conjugate (antithetic) segment boundaries are very rare. Hanging-wall-breached relay ramps are the most common type (>70%) of breached-segment boundary, followed by footwall-breached relay ramps (<25%). Transfer faults are uncommon in our models. The type of breached segment boundary that develops cannot be predicted based on fault overlap to fault spacing aspect ratio alone. Instead, we show that fault linkage occurs in a range of styles across a wide range of fault overlap to fault spacing ratios (1:1–7:1). Furthermore, we show that fault spacing is constrained by stress-reduction shadows at the time of fault nucleation, whereas fault overlap changes during fault growth and interaction. Our study thus shows that scaled physical models are a powerful tool to assess the style, distribution and controls on the evolution of synthetic segment boundaries developing in rifts. Predictions from these models must now be assessed with data from natural examples exposed in the field or imaged in the subsurface.
... Similar results are reported in analogue studies (Eisenstadt and Sims, 2005;Withjack et al., 2007). For example, dry sand with low ductility has been shown to cause wider deformation zone outside the major fault (Withjack et al., 2007). ...
... Similar results are reported in analogue studies (Eisenstadt and Sims, 2005;Withjack et al., 2007). For example, dry sand with low ductility has been shown to cause wider deformation zone outside the major fault (Withjack et al., 2007). ...
Article
Stepovers and bends along strike-slip faults are where push-up ranges and pull-apart basins are formed. They are also commonly where fault ruptures terminate. Field study and analogue models suggest that the configuration of faults plays a key role in crustal deformation around bends and stepovers, but the related mechanics of stress perturbation, strain partitioning, and fault evolution remains poorly understood. Here we present results of systematical mechanical models of stress changes and strain partitioning around simple stepovers and bends, using three-dimensional viscoelasto-plastic finite element code. Our model predicts elevated deviatoric stress around all stepovers and bends, with higher stresses around the restraining ones. Narrow stepovers localize strain between the fault gaps to form connecting faults, whereas wide stepovers localize strain on the tips of fault segments so the stepovers may evolve into subparallel faults. We explored how various configurations of stepovers and bends change the stress field and strain distribution, and show that these results can help explain some key differences between the pull-apart basins in the Dead Sea Trough and Death Valley, and the push-up ranges along the San Andreas Fault.
... How rheological differences between these two classes of materials affect modeling observations is a long-debated issue. Several investigators compared results obtained using wet and dry materials, concluding that both can be used (e.g., Eisenstadt and Sims, 2005;Withjack and Schilsche, 2006;Withjack et al., 2007;Bonini et al., 2014) taking into account their specificities, e.g. dry sand does not behave as an ideal frictional-plastic material and clay has high cohesion preventing gravitational collapses (Mandl, 2000). ...
... In these studies thin mechanical discontinuities are simulated by introducing thin weak layers of materials with less friction or strength than the dry granular materials representing fault-hosting rocks (e.g., Sassi et al., 1993;Bonini et al., 2011Toscani et al., 2014;Di Domenica et al., 2014;Faccenna et al., 1995;Bonini, 1998;McClay et al., 2000;Dubois et al., 2002;Gartrell et al., 2005;Del Ventisette et al., 2005, 2006Konstantinovskaya et al., 2007;Sani et al., 2007;Cerca et al., 2010;Pinto et al., 2010). Alternatively, another technique consists in extending the model before compressing it, in case of positive inversion, or the opposite, in case of negative inversion (e.g., McClay, 1989;Withjack et al., 2007;Marques and Nogueira, 2008). So far, wet-clay models have not been much used because the only known method for introducing pre-existing discontinuities was to pre-form the material before the experiment (e.g., Henza et al., 2010). ...
Article
We use wet-clay analogue models to investigate how pre-existing discontinuities (i.e. structures inherited from previous tectonic phases) affect the evolution of a normal fault at the Earth’s surface. To this end we first perform a series of three reference experiments driven by a 45° dipping master fault unaffected by pre-existing discontinuities to generate a mechanically isotropic learning set of models. We then replicate the experiment six times introducing a 60°-dipping precut in the clay cake, each time with a different attitude and orientation with respect to an initially-blind, 45°-dipping, master normal fault. In all experiments the precut intersects the vertical projection of the master fault halfway between the center and the right-hand lateral tip. All other conditions are identical for all seven models. By comparing the results obtained from the mechanically isotropic experiments with results from experiments with precuts we find that the surface evolution of the normal fault varies depending on the precut orientation. In most cases the parameters of newly-forming faults are strongly influenced. The largest influence is exerted by synthetic and antithetic discontinuities trending respectively at 30° and 45° from the strike of the master fault, whereas a synthetic discontinuity at 60° and an antithetic discontinuity at 30° show moderate influence. Little influence is exerted by a synthetic discontinuity at 45° and an antithetic discontinuity at 60° from the strike of the master fault. We provide a ranking chart to assess fault-to-discontinuity interactions with respect to essential surface fault descriptors, such as segmentation, vertical-displacement profile, maximum displacement, and length, often used as proxies to infer fault properties at depth. Considering a single descriptor, the amount of deviation induced by different precuts varies from case to case in a rather unpredictable fashion. Multiple observables should be taken into consideration when analyzing normal faults evolving next to pre-existing discontinuities.
... Because the properties of the wet kaolin depend on the water content , we follow procedures described in to prepare the wet kaolin with a target strength that equates 1 cm of clay in the experiment to 1-2 km of crust. The benefits of wet kaolin over other crustal analog materials are that (a) wet kaolin creates very clear faults that can be tracked (e.g., Oertel, 1965;Tchalenko, 1970), (b) the low but non-zero cohesion of wet kaolin ensures that faults are long-lived (Cooke et al., 2013;Withjack et al., 2007), and (c) the viscoelastic behavior of wet kaolin can simulate off-fault relaxation of stresses within the crust . Many studies have used kaolin to simulate evolution of strike-slip fault systems (e.g., Cooke et al., 2013;Hatem et al., 2015Tchalenko, 1970). ...
Article
Full-text available
Crustal deformation occurs both as localized slip along faults and distributed deformation off of faults. While there are few robust estimates of off‐fault deformation in nature, scaled physical experiments simulating crustal strike‐slip faulting allow direct measurement of the ratio of fault slip to regional deformation, quantified as kinematic efficiency (KE). We offer an approach to predict KE using a 2D convolutional neural network (CNN) trained directly on fault maps produced by physical experiments. Experiments with different loading rates and basal boundary conditions generate the fault maps throughout the evolution of strike‐slip faults. Strain maps allow us to directly calculate KE and its uncertainty, utilized in the loss function and performance metric. The trained CNN achieves 91% custom accuracy in the KE prediction of an unseen data set. Although the CNN model is trained on scaled experiments, it can predict off‐fault deformation of crustal faults that matches available geologic estimates.
... The Riedel shear experiment documented that smaller enechelon patterns formed at low amplitude and approximately perpendicular to principal stress (σ1) (Dooley and Schreurs, 2012). The behavior of high density (low water content) clay and sandstone in the Riedel experiment provides a suitable analog for the clay-rich beds and sandstone of the Northern Oklahoma Permian Red Reds (Eisenstadt and Sims, 2005;Withjack et al., 2007;and Dooley and Schreurs, 2012). The density of the clay increases the brittleness of fracturing in the upper crust, resulting in discrete and planar patterns similar to the en-echelon patterns in Figure 2B ( Arch et al., 1988, Dooley andSchreurs, 2012). ...
Article
Unmanned aerial systems (UAS) provide a framework for recording perishable surficial data or information. Open fractures exhibiting regular en-echelon patterns were captured by a 12-megapixel, FL-9 mm camera attached to a Phantom IV UAS over the epicenter of the magnitude (Mw) 5.8 earthquake of September 3, 2016, 15 months later. The Digital Surface Models (DSMs) and orthoimagery offered a spatial resolution (∼1 cm) sufficient to identify small-scale plastic deformations that appear to be controlled by en-echelon joint sets developed in the underlying formation. The fissure boundaries and intersections are remarkably linear and sharp. They appeared to have been recently formed, presumably by seismic swarms believed to have been associated with wastewater injection. The DSMs revealed a series of conjugate patterns suggestive of regional systematic joints with apparent subsidence of infilling up to 50 cm. The earthquakes emanated from the Precambrian metamorphic basement, with epicentral clusters at ∼5- and 8-km depths. Low energy release from depths >1.5 km appears to be locally attenuated by an unconsolidated “soil cap,” which likely formed an impedance contrast. The maximum deformation direction from the cumulative energy of earthquakes correlates with a wrench fault tectonics model that could conceivably produce the observed en-echelon joint sets observed in the orthoimagery and DSMs. These features were observed within 275 m of the reported Mw 5.8 epicenter. The remarkably linear repeating pattern of deformation appears to express fissures that preserve the wrench fault fractures generated by the Mw 5.8 earthquake emanating from discontinuity suites within marine sandstone, shale, and limestone of Pennsylvanian to Permian age.
... Despite the numerous and different analogue materials employed in the experimental laboratories worldwide (Schreurs et al., 2006), experimentalists are always looking for the best material with optimal characteristics in terms of frictional properties, number and high-quality structural detail of the developed structures, and ease of supply. The most frequently used analogue materials are quartz sand (e.g., Richard and Krantz, 1991;Vendeville et al., 1995, Schreurs et al., 2006Montanari et al., 2007;Rosas et al., 2017), silica powder (e.g., Galland et al., 2006;Graveleau and Dominguez, 2008), wet clay (usually kaolin with variable water content; e.g., Withjack and Jamison, 1986;Clifton et al., 2000;Withjack et al., 2007;Henza et al., 2010;Cooke and van der Elst, 2012;Cooke et al., 2013;Sims et al., 2013) and gypsum, both wet (plaster; e.g., Gabrielsen and Clausen, 2001) and dry (van Gent et al., 2010). However, each of these analogue materials presents major or minor issues, for instance the difficulty of wetting pure gypsum powder at the end of the experiments, the high strength of clay, and the high dilatancy of quartz sand (Naylor et al., 1986;Mandl, 1988). ...
Article
We have performed a series of sandbox models addressing the influence of sand mixtures (quartz and feldspar sand in different proportions) with different grain-size on the development of normal faults. The overall model evolution suggests that fault orientation, width of the deformed area, and average subsidence are not significantly influenced by the different sand mixtures. Conversely, the number of secondary structures, mean fault length, and average fault dip are strongly dependent on the material and grain-size. Modelling results suggest that a Log-normal distribution of fault length and segment length distribution betst describes the experimental fault populations, particularly when the amount of fine-grained (K-feldspar) sand is ~30%. The length distribution and the number of faults are related, specifically with increasing the amount of the K-feldspar sand (i.e., higher the amount of fine-grained material) the shorter the minimum detectable fault length. This behavior is described by a Log-normal distribution, with a few long faults and many intermediate to short faults. We thus suggest that (for length ratios of 10−5 to 2 10−6) a sand-mixture composed of 70% quartz sand and 30% K-feldspar sand is the one (among those we have tested) that provides the best compromise in terms of frictional properties, density, structural detail, number and length distribution of the faults, easiness of use, and general behavior.
... We chose wet clay and dry sand as our preferred analogCE7 materials; this allowed us to reproduce different aspects of the faulting process and to make the most out of the characteristics of both these materials. Recent studies compared results obtained using wet clay and dry sand (e.g., Eisenstadt and Sims, 2005;Withjack and Schlische, 2006;Withjack et al., 2007), highlighting differences and similarities. On the one hand, wet clay has been used extensively to analyze brittle deformation related to folding (e.g., Cloos, 1968;Withjack and Jamison, 1986;Withjack and Schlische, 2006;Henza et al., 2010;Miller and Mitra, 2011), and its effectiveness as analog material has been stressed recently by new rheological tests (Cooke and van der Elst, 2012). ...
Article
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Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on field investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogs with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously. We used observations and models of the 6 April 2009, Mw 6.3, L’Aquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean basin, and was chosen as a paradigm of the nature of the associated geological evidence, along with observational difficulties and ambiguities. Thanks to the available high-resolution geologic, geodetic and seismological data aided by analog modeling, we reconstructed the full geometry of the seismogenic source in relation to surface and sub-surface faults. We maintain that the earthquake was caused by seismogenic slip in the range 3–10 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. Based on our results we propose a scheme of normal fault hierarchization through which all surface occurrences related to faulting at various depths can be interpreted in the framework of a single, mechanically coherent model. We stress that appreciating such complexity is crucial to avoiding severe over- or under-estimation of the local seismogenic potential.
... Clay, sand box, sand-rubber sheet and sand-silicone models are usually used for fault activity modeling (Withjack et al., 2007), but under different situations. Clay is usually used for compressive or shearing experiments due to its combined property of both sticky and capable of break. ...
Article
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Glass microbeads are frequently used in analog physical modeling to simulate weak detachment zones but have been neglected in models of thrust wedges. Microbeads differ from quartz sand in grain shape and in low angle of internal friction. In this study, we compared the structural characteristics of microbeads and sand wedges. To obtain a better picture of their mechanical behavior, we determined the physical and frictional properties of microbeads using polarizing and scanning electron microscopy and ring-shear tests, respectively. We built shortening experiments with different basal frictions and measured the thickness, slope and length of the wedges and also the fault spacings. All the microbeads experiments revealed wedge geometries that were consistent with previous studies that have been performed with sand. However, the deformation features in the microbeads shortened over low to intermediate basal frictions were slightly different. Microbeads produced different fault geometries than sand as well as a different grain flow. In addition, they produced slip on minor faults, which was associated with distributed deformation and gave the microbeads wedges the appearance of disharmonic folds. We concluded that the glass microbeads may be used to simulate relatively competent rocks, like carbonates, which may be characterized by small-scale deformation features.
Article
To broaden the availability of granular materials that are suitable for the analog modeling of upper crustal deformation, we investigated the mechanical behaviors of pure quartz sand and two sand mixtures (quartz sand-powdered barite and quartz sand mica crystals) using ring-shear tests and simple convergent sandbox experiments. The ring-shear test results indicate that the three materials have similar peak friction angles (between 39.25° and 36.02°), but the magnitude of the shear strain and the shear strength required to cause their failure are different. The differences between the analog models are identified by distinct fault kinematics and different grain flows, which are primarily related to differences in the plastic elasto-frictional rheology. We conclude that the use of the quartz-mica mixture, which showed the strongest distributed (plastic) deformation, can improve analog models where different materials are required to simulate crystalline basement (sand) and supracrustal rocks (sand mica mixture). This is a common situation in extension and inversion tectonics, such as, for example, in inversion tectonics, when a granitic basement block acts as a buttress.
Article
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Laboratory measurements of rock strength provide limiting values of lithospheric stress, provided that one effective principal stress is known. Fracture strengths are too variable to be useful; however, rocks at shallow depth are probably fractured so that frictional strength may apply. A single linear friction law, termed Byerlee's law, holds for all materials except clays, to pressures of more than 1 GPa, to temperatures of 500°C, and over a wide range of strain rates. Byerlee's law, converted to maximum or minimum stress, is a good upper or lower bound to observed in situ stresses to 5 km, for pore pressure hydrostatic or subhydrostatic. Byerlee's law combined with the quartz or olivine flow law provides a maximum stress profile to about 25 or 50 km, respectively. For temperature gradient of 15°K/km, stress will be close to zero at the surface and at 25 km (quartz) or 50 km (olivine) and reach a maximum of 600 MPa (quartz) or 1100 MPa (olivine) for hydrostatic pore pressure. Some new permeabiltiy studies of crystalline rocks suggest that pore pressure will be low in the absence of a thick argillaceous cover.
Article
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Dry sand and wet clay are the most frequently used materials for physical modeling of brittle deformation. We present a series of experiments that shows when the two materials can be used interchangeably, document the differences in deformation patterns and discuss how best to evaluate and apply results of physical models.Extension and shortening produce similar large-scale deformation patterns in dry sand and wet clay models, indicating that the two materials can be used interchangeably for analysis of gross deformation geometries. There are subtle deformation features that are significantly different: (1) fault propagation and fault linkage; (2) fault width, spacing and displacement; (3) extent of deformation zone; and (4) amount of folding vs. faulting. These differences are primarily due to the lower cohesion of sand and its larger grain size. If these features are of interest, the best practice would be to repeat the experiments with more than one material to ensure that rheological differences are not biasing results.Dry sand and wet clay produce very different results in inversion models; almost all faults are reactivated in wet clay, and few, if any, are significantly reactivated in sand models. Fault reactivation is attributed to high fluid pressure along the fault zone in the wet clay, a situation that may be analogous to many rocks. Sand inversion models may be best applied to areas where most faults experience little to no reactivation, while clay models best fit areas where most pre-existing normal faults are reactivated.
Article
Clay models have been used to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault. The experimental results show that fault shape influences the style of secondary faulting and folding. Most antithetic normal faults form above concave-upward fault bends, whereas mostly synthetic normal faults form above low angle fault segments and convex-upward fault bends. Generally, secondary faulting and folding are youngest at fault bends and become progressively older past fault bends. The observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous, inclined simple shear as the hanging-wall deformation mechanism. -from Authors
Article
Synthetic seismic sections created with a two-dimensional, ray-tracing modeling program reveal the seismic characteristics and interpretational pitfalls for several structural styles. Salt structures appear as zones lacking stratal reflections. Surrounding reflection packages change thickness and have synclinal and bow-tie patterns associated with folding produced by salt piercement and withdrawal. Interpretational pitfalls are (1) salt bodies appear too large, (2) adjacent structural and stratigraphic features are concealed, and (3) underlying flat-lying strata appear deformed because of velocity pull-up. Seismic evidence of thrust faulting includes vertical repetition of reflection packages. Fault-surface reflections and aligned terminations of stratal reflections characterize thrust-fault ramps. Folds have anticlinal or synclinal reflection patterns, although deep and/or tight synclines have bow-tie patterns on unmigrated sections. Interpretational pitfalls are (1) bedding-parallel thrusts are difficult to recognize, (2) reflections from steep fold limbs are lacking, and (3) velocity pull-ups and push-downs distort structural geometries beneath thrust-fault ramps. Divergent wrench faults appear as upward-widening zones of reflection terminations and overlapping stratal reflections. Offsets and thickness changes of reflection packages are inconsistent across these zones. Aligned terminations of stratal reflections and anticlinal, synclinal, and bow-tie reflection patterns characterize secondary normal faults and folds, respectively. Interpretational pitfalls are (1) wrench fault locations and geometries are difficult to define and (2) velocity pull-ups and push-downs distort structural geometries near wrench faults.
Article
The geometries of extensional fault systems have been studied by use of experimental sand analogues and are compared with examples in the literature. Extensional faulting above a uniformly extending basement produces a domino-style fault array involving both planar rotational faults and listric faults. The faults evolve with time and may change from listric geometries (concave upward) through planar fault segments to convex-upward geometries. At high extensional strains early faults are cut by later high-angle planar extensional faults. Hanging-wall deformation above a simple listric extensional detachment is characterized by faults that nucleate and propagate into the hanging wall and produce crestal collapse grabens. Listric extensional faults with a ramp/flat geometry also produce hanging-wall crestal collapse grabens and local reverse faults. The experiments show that hanging-wall blocks in listric extensional fault systems must undergo significant internal strains in order to accommodate progressive deformation over nonplanar fault surfaces.
Article
We use geometric and experimental models to study the development of extensional fault-bend folds. The geometric models show that fault shape, fault displace- ment, and patterns of aggradation/erosion profoundly affect the distribution of growth beds, the magnitude and direction of dip of pregrowth and growth beds, and the location and dip of the outer limit of folding in pregrowth and growth beds. Complex structural and stratigraphic patterns develop if the rate of aggradation/erosion relative to the rate of fault displacement changes through time. The experimental models (with dry sand and wet clay) show that several deformational styles can accommodate extensional fault-bend folding. In sand models, a few, relatively major, secondary antithetic normal faults accommodate most hanging wall deformation. Pregrowth layers, although faulted, remain flat. The effective shear direction parallels the antithetic normal faults, and the shear angle is about 608-658. In clay models, numerous, relatively minor, secondary normal faults (antithetic and synthetic) and cataclastic flow accommodate most hanging wall deformation. The deformed pregrowth and growth layers dip gently toward the main fault. The effective shear angle (358-508) is considerably less than the dip of the antithetic normal faults. In the sand models and geometric models with a large shear angle (608), more displacement occurs on the main normal fault and the hanging wall collapses in a relatively narrow zone. In the clay models and geometric models with a small shear angle (358), less displacement occurs on the main normal fault. Instead, the hanging wall stretches substantially and collapses in a relatively wide zone.
Article
Experimental deformation of water-rich argillaceous sediments has shown that they deform not by pervasive homogeneous flow, as has sometimes been surmised in the past, but by intense slippage within very narrow, discrete zones of shear. These shear zones enable large total strains to be accomplished whilst leaving the bulk of the material undisturbed. Macroscopically the zones are shiny, finely-grooved planes which may be stepped where sub-structures intersect them. Under the microscope the zones are seen to result from pronounced particle reorientation into sub-parallelism with the zone margins, presumably by slippage at the grain scale. The zones commonly curve and anastomose, and various sub-fabrics may be discernible. Although the details vary, the shear zones are strikingly consistent throughout the range of experimental conditions. In specimens with 15% water content, the shear zones lack cohesion and are analogous to shear fractures in brittle rocks. Between approximately 15-45% water content the same overall geometry persists, but the shear zones maintain cohesion. At greater water contents, up to at least 60%, the clay sediments are extremely weak and although they may appear to be undergoing pervasive flow, microscopic examination reveals that even here arrays of short, narrow zones of concentrated displacement are being generated.
Article
The progressive development of extensional fault geometries in a sedimentary cover sequence above a rigid or deforming basement has been experimentally investigated using analogue models. Quartz sand (700 μm) was used as a modelling material. The experiments were recorded using time-delay 16 mm cine photography and 35 mm photography. Final models were impregnated with resin and serially sectioned to investigate the three-dimensional fault geometry. Four series of experiments were carried out: (i) extension above a linear basement dislocation; (ii) extension above a uniformly stretching basement; (iii) extension controlled by a planar fault; and (iv) extension controlled by a listric fault. Extension above a linear basement dislocation produced a single graben structure in which initial high-angle bounding faults were cut by later listric faults. Fault nucleation occurred into the hanging wall of the graben and only minor rotation occurred in the fault blocks within the graben. Extension above a uniformly extending basement produced a variety of fault structures. Heterogeneous nucleation of faults occurred with initial planar geometries giving way to more listric faults. Significant fault-block rotation was observed. In some instances rotation of pre-existing fault planes produced a negative (convex upward) listric-fault geometry. Extension controlled by a planar fault produced a single graben structure in which new faults developed into the graben. Little rotation of fault blocks occurred. Extension controlled by a predetermined listric fault showed the progressive development of a rollover anticline. The crest of the anticline collapsed producing a second-order crestal collapse graben. The nucleation of new faults was consistently in the hanging wall above the major detachment faults. In all of the experiments carried out to date, once a major fault had developed the second-order fault nucleation was consistently in the hanging wall fault block. Footwall collapse and hence fault migration into the footwall was not significant. Listric faults produced considerable rotation of hanging wall blocks. In some instances heterogeneous rotation of preexisting faults generated negative listric-fault shapes. In rollover structures collapse of the crestal region of the fold produced a second-order graben structure.