Solid Earth, 6, 9–31, 2015
www.solid-earth.net/6/9/2015/
doi:10.5194/se-6-9-2015
© Author(s) 2015. CC Attribution 3.0 License.
Analogue experiments of salt flow and pillow growth due
to basement faulting and differential loading
M. Warsitzka1, J. Kley2, and N. Kukowski1
1Friedrich-Schiller-University Jena, Institute for Geosciences, Germany
2Georg-August-University Göttingen, Geoscience Centre, Structural Geology & Geodynamics, Germany
Correspondence to: M. Warsitzka (michael.warsitzka@uni-jena.de)
Received: 23 June 2014 – Published in Solid Earth Discuss.: 17 July 2014
Revised: 14 November 2014 – Accepted: 20 November 2014 – Published: 6 January 2015
Abstract. Salt flow in sedimentary basins is mainly driven
by differential loading and can be described by the concept of
hydraulic head. A hydraulic head in the salt layer can be im-
posed by vertically displacing the salt layer (elevation head)
or the weight of overburden sediments (pressure head). Base-
ment faulting in salt-bearing extensional basins is widely ac-
knowledged as a potential trigger for hydraulic heads and the
growth of salt structures. In this study, scaled analogue ex-
periments were designed to examine the kinematics of salt
flow during the early evolution of a salt structure triggered
by basement extension. In order to distinguish flow patterns
driven by elevation head or by pressure head, we applied a
short pulse of basement extension, which was followed by
a long-lasting phase of sedimentation. During the experi-
ments viscous silicone putty simulated ductile rock salt, and a
PVC-beads/quartz-sand mixture was used to simulate a brit-
tle supra-salt layer. In order to derive 2-D incremental dis-
placement and strain patterns, the analogue experiments were
monitored using an optical image correlation system (parti-
cle imaging velocimetry). By varying layer thicknesses and
extension rates, the influence of these parameters on the kine-
matics of salt flow were tested. Model results reveal that sig-
nificant flow can be triggered in the viscous layer by small-
offset basement faulting. During basement extension down-
ward flow occurs in the viscous layer above the basement
fault tip. In contrast, upward flow takes place during post-
extensional sediment accumulation. Flow patterns in the vis-
cous material are characterized by channelized Poiseuille-
type flow, which is associated with subsidence in regions of
“salt” expulsion and surface uplift in regions of inflation of
the viscous material. Inflation of the viscous material even-
tually leads to the formation of pillow structures adjacent to
the basement faults (primary pillows). The subsidence of pe-
ripheral sinks adjacent to the primary pillow causes the for-
mation of additional pillow structures at large distance from
the basement fault (secondary pillows). The experimentally
obtained structures resemble those of some natural exten-
sional basins, e.g. the North German Basin or the Mid-Polish
Trough, and can aid understanding of the kinematics and
structural evolution of sedimentary basins characterized by
the presence of salt structures.
1 Introduction
Generally, rock salt buried in sedimentary basins behaves as
a viscous fluid (Urai et al., 2008; van Keken et al., 1993)
and flows according to a pressure gradient. Pressure gradi-
ents in a salt layer can be described by the concept of hy-
draulic head, which depends on the sum of elevation head
and pressure head (Kehle, 1988; Hudec and Jackson, 2007).
In the case of basement faulting (Fig. 1a), an elevation head
can be imposed on the salt layer by vertical displacement of
the salt layer itself. An additional pressure head is induced
on the salt layer by differential loading due to lateral changes
in thickness of the sediments accumulating on the irregular
topography of the faulted surface (e.g. Geil, 1991; Hudec and
Jackson, 2007; Jackson et al., 1994; Jackson and Vendeville,
1994; Koyi et al., 1993; Koyi and Petersen, 1993; Krzywiec,
2004b; Remmelts, 1995; Stewart et al., 1996; Vendeville et
al., 1995).
Previous scaled analogue experiments investigating salt
diapirism driven by thick-skinned extension demonstrated
that at the beginning discrete basement faulting is balanced
Published by Copernicus Publications on behalf of the European Geosciences Union.
10 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Legend: Salt
Pre-kinematic cover layer
Syn-kinematic cover layers
A Primary peripheral sink
Secondary peripheral sinkB
C
Figure 1. Conceptual model showing the evolution of a salt di-
apir induced by basement faulting: (a) due to sedimentary loading
a salt pillow develops prior to diapiric piercing, (b) further base-
ment extension leads to faulting of the overburden and (c) reactive
diapirism. Salt flow is governed by vertical displacement of the salt
layer (elevation head) and differential loading due to sediment ac-
cumulation in subsiding areas (modified after Koyi et al., 1993).
by flexural bending of the overburden and decoupled by dif-
fuse faulting (Burliga et al., 2012; Dooley et al., 2003, 2005;
Ge and Vendeville, 1997; Higgins and Harris, 1997; Jackson
and Vendeville, 1994; Koyi et al., 1993; Nalpas and Brun,
1993; Oudmayer and de Jager, 1993; Richard, 1991; Soto
et al., 2007; Stephansson, 1972; Vendeville, 1988; Vendev-
ille et al., 1995; Vendeville and Jackson, 1992; Ventisette et
al., 2005; Withjack and Callaway, 2000). Deformation within
the viscous layer above an active basement normal fault is
characterized by flow towards the hanging wall block under
sediment-starved conditions. Reverse flow towards the foot-
wall block occurs if sufficient sediment accumulates in the
depocentre above the downthrown basement block (Jackson
et al., 1994; Koyi et al., 1993; Nalpas and Brun, 1993; Ge
and Vendeville, 1997). Furthermore, salt flow into a growing
salt structure close to the basement fault can change through
time from the footwall side during an early stage to the hang-
ing wall side in a mature stage (Burliga et al., 2012; Koyi et
al., 1993). However, in most of these experimental studies fi-
nite displacement of the basement faults was large compared
to the thickness of the viscous layer. This obscures incre-
mental flow patterns occurring during the early evolution of
salt structures, when offsets of basement faults are still small
(Fig. 1).
Therefore, our experimental study is designed to examine
incremental strain patterns in a salt layer asserted by both
components of the hydraulic head, elevation and pressure
head during initiation of a salt structures triggered by base-
ment normal faulting. We purposely divided the experimental
procedure into a short pulse of basement faulting and a long
phase of post-extensional sedimentation. Furthermore, a sen-
sitivity study was carried out to test the role of characteristic
parameters, namely salt thickness, cover thickness and ex-
tension rate, in affecting flow patterns and post-extensional
structural evolution. A 2-D optical particle tracking system
(PIV) was applied to observe incremental particle displace-
ments and strain patterns in both the brittle and the ductile
layer during the experiments.
2 Method and procedure
2.1 Scaling analogue experiments
The experiments presented here involve a two-layer ductile-
brittle system covering a rigid basement. The viscous
near-Newtonian behaviour of rock salt (Urai et al., 2008,
van Keken et al., 1993) was simulated by using silicone
putty (PDMS, polydimethylsiloxane; dynamic viscosity of
2.3× 104 Pas at room temperature 20 ◦C and experimen-
tal strain rates of < 10−1 s−1; see Appendix A for details).
The silicone putty shows a power-law viscous rheology
with a low power-law stress exponent (n= 1.3; Rosenau
et al., 2009) so that it can be regarded as near-Newtonian.
Frictional-plastic slip behaviour of overburden sediments is
modelled by a granular mixture of quartz sand and PVC mi-
crospheres (1 : 1) (see Appendix A for details). This gran-
ulate deforms according to the Mohr–Coulomb criterion, a
suitable rheology for the simulation of natural sedimentary
rocks (e.g. Lohrmann et al., 2003).
For representative quantitative and qualitative informa-
tion, analogue models have to be scaled geometrically, kine-
matically and dynamically (Hubbert, 1937; Ramberg, 1967).
This requires dimensionless ratios relating the rheologies and
stresses of nature to be similar to those for experiments. Re-
ferring to previous studies, we employ a geometric scaling
factor 1× of about 10−5, which means 1 cm in the model is
equivalent to about 1 km in nature (Koyi et al., 1993; Nal-
pas and Brun, 1993; Ge and Vendeville, 1997; Withjack and
Callaway 2000; Dooley et al. 2003). Dynamical scaling is
ensured if force ratios and length ratios are similar (gravi-
tational acceleration = 9.81 ms−2). The reliability of mod-
elling results strongly depends on mechanical coupling be-
tween viscous layer and overburden and, therefore, on the
ratio between brittle yield strength Sb and ductile strength
Sd (see Appendix B for a detailed description of the scaling
procedure). Calculation of this strength ratio results in the
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 11
following values (Table 1):
Sb /Sd (nature)=∼ 100− 1000 (1)
and
Sb /Sd (model)= 50− 180. (2)
The model ratio lies within the lower range of the natural
ratio. Hence, the models can be considered as dynamically
scaled. Scaling of time can be achieved by dividing the vis-
cosity ratio η∗ through the normal stress ratio σ ∗N (see Ap-
pendix B). The resulting time scaling factor t∗ =∼ 10−10
means that 1 h in the experiment equals ∼ 1 Ma in nature.
Note that the applied scaling procedure is subject to high
uncertainties, since especially salt viscosity and strain rate
during salt flow are not well constrained and can vary over
2–3 orders of magnitude (Jackson and Talbot, 1986). Nev-
ertheless, similar analogue materials and length ratios have
been used for decades and found suitable for modelling of
salt tectonic processes (see references above).
The mixture of quartz sand and PVC beads possesses
slightly lower frictional properties (ϕ ∼ 27◦), but also a
lower bulk density (ρb = 1300 kg m−3) than pure quartz sand
(ρb = 1580–1710 kg m−3) (see Appendix A). By applying a
low density contrast between cover and viscous layer in our
experiments (300 kg cm−3), buoyancy forces are not overes-
timated (Allen and Beaumont, 2012). Thus, we intend to en-
sure a more realistic stress relation during early stages of salt
pillow development, and closer similarity to natural shallow
(∼ 1000 m), partly compacted sediments, which also possess
densities lower or slightly higher than salt.
2.2 Experimental set-up
In this generic experimental study, we intend to simulate con-
ditions of salt structure evolution during the onset of base-
ment extension. In many rifts and intracontinental basins
multiple extensional phases alternate with phases of tectonic
quiescence as revealed by structural analysis or subsidence
patterns (e.g. Alves et al., 2002; Jackson and Vendeville,
1994; Kockel, 2002; Mohr et al., 2005; van Wees et al.,
2000). Therefore, the experimental procedure applied here
assumes a relatively short extensional pulse followed by a
longer phase of tectonic quiescence accompanied by sedi-
mentation. In order to monitor the characteristic deformation
patterns in both phases, we artificially separated basement
extension and sedimentation in the experimental procedure.
Furthermore, the salt layer in many extensional basins is
overlain by more or less isopachous overburden before base-
ment extension and salt movement begins (e.g. Alves et al.,
2002; Baldschuhn et al., 2001; Duffy et al., 2013; Remmelts
et al., 1995). Thus, we introduced an initial cover layer of
uniform thickness in each experiment, which is here referred
to as “pre-kinematic cover layer”.
Appropriate experimental values for e.g. layer thick-
nesses, displacement of the basement faults or the duration
60°
2
0
 c
m
90 cm
B
C
Engine/ 
frequency converter
D
A
Rigid base
Silicone Pre-extensional sand layer
Post-extensional sand layer
Legend:
10 cm VE = 1
Figure 2. (a) Side view of experimental box comprising analogue
materials silicone and granulate as well as subdivided rigid base-
ment. Basal displacement is driven by a controlled engine. (b) In
general the model procedure starts with a planar layer configura-
tion. (c) Basal displacement is applied. (d) Surface depressions are
filled with sifted granulate. Sieving procedure is repeated daily.
of extension are adapted to the conditions of the Central
European Basin system, which contains prominent exten-
sional segments (e.g. Central Graben, Horn Graben, Mid-
Polish Trough) (Ziegler, 1982) and numerous salt structures
(Maystrenko et al., 2013). Here, the pre-extensional overbur-
den thickness hb varies from a few hundred metres (Mid-
Polish Trough; Krzywiec, 2004a) to ∼ 1000 m (North Ger-
man Basin; Baldschuhn et al., 2001; Maystrenko et al., 2013;
Scheck et al., 2003). The maximum original salt layer thick-
ness hd in the centre of extensional basins mainly lies be-
tween 1000 m (Northeast German Basin) and 1500 m (Mid-
Polish Trough), but can attain 3500 m (Central North German
Basin; Frisch and Kockel, 1999; Maystrenko et al., 2013).
Hence, a ratio of overburden thickness to salt thickness of
Hr < 1 is a reasonable assumption. Throughout this paper, we
define small-offset displacement at the basement fault as be-
ing considerably smaller than the thickness of the salt layer.
Thus, the ratio Dr of basement fault offset d to thickness of
the salt layer hd can also be assumed to be < 1.
Experiments were performed in a 90× 20 cm wide glass-
sided box (Fig. 2a). The rigid base is subdivided into three
blocks by two opposite planes dipping 60◦. The right block
is fixed, the middle one subsides controlled by an engine and
the left one is free to move horizontally. The left block is
wider in order to investigate the post-extensional model evo-
lution at distance from the basal faults. The initially planar
www.solid-earth.net/6/9/2015/ Solid Earth, 6, 9–31, 2015
12 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Table 1. Scaling parameters. Material properties (density, viscosity, friction and cohesion) are stated for analogue materials used in this
experimental study (Appendix A).
Parameter Sign Dimension Model Nature Scaling factor Reference for natural values
(model/nature)
Geometric l∗ [m] 0.01 1000 ∼ 10−5
Thickness (ductile layer) hd [m] 0.01–0.02 1400–2500 10−5 Kockel (2002); Maystrenko
et al. (2013); Nalpas and
Brun (1993); Scheck et al. (2003)
Thickness (brittle layer) hb [m] 0.003–0.015 600–1200 10−5 Nalpas and Brun (1993)
Kinematic
Strain ε – 1 –
Time t [s] 100h= 105 s ∼ 30Ma=∼ 1015 10−10 –
Extension rate u [m s−1] 10−6 1mm a−1 =∼ 10−11 ms−1 ∼ 105 Allen and Allen (2005)
Strain rate in viscous layer ε′ [s−1] ∼ 10−5 10−11–10−15 10−6–10−10 Jackson and Talbot (1986);
Nalpas and Brun (1993)
Dynamic
Gravity acceleration g [m s−2] 9.81 9.81 1 –
Density (ductile layer) ρd [kg m−3] 970 2200 0.44 Jackson and Talbot (1986)
Density (brittle layer) ρb [kg m−3] 1220 1800–2500 0.52–0.72 Jackson and Talbot (1986)
Dynamic viscosity (ductile layer) η [Pa s] 2.3× 104 1017–1019 ∼ 10−14 Nalpas and Brun (1993)
Internal peak friction coefficient µ – 0.51 (±0.006) 0.5–0.6 1 Weijermars et al. (1993)
Internal friction angle φ [◦] 27 ∼ 30 Weijermars et al. (1993)
Cohesion [Pa] C0 [Pa] ∼ 100 5× 106 Byerlee (1978)
Stress and pressure [Pa] ∼ 10−5 –
Brittle strength Sb [Pa] 60–160 ∼ 107 ∼ 10−5 –
Ductile strength Sd [Pa] 0.5–2 ∼ 105 ∼ 10−5 –
Brittle strength/ductile strength Sb/Sd – 50–180 ∼ 100–1000 ∼ 1 –
base is covered with a flat layer of silicone putty as well as a
pre-kinematic granular layer (Fig. 2b).
Each experiment starts with a displacement of the basal
plate (extensional phase), whereby the middle block is pulled
down d =∼ 6–7 mm (Fig. 2c), which causes ∼ 7 mm lateral
stretching of the whole model (∼ 0.8 %). The displacement
rate e on the normal faults of the basal plate ranges from
0.6, 4 to 20 mm h−1, which represents a natural extension
rate of 0.05 to 2 mm a−1 (50 to 2000 m Ma−1). According
to our scaling, the duration of the extensional phase (0.3
to 10 h) is about 0.3 to 11 Ma, which are reasonable val-
ues for a short extensional pulse, as occurred e.g. in the
North German Basin (e.g. Kockel, 2002). After termina-
tion of basal displacement, accumulation of additional sand
was delayed another 15 min in order to record the post-
extensional strain patterns. After this interim period addi-
tional sand (post-extensional layers) was sieved into topo-
graphic depressions to apply a pressure head on the viscous
layer (Fig. 2d) (syn-sedimentary phase). The sieving pro-
cedure was repeated every day. The average sedimentation
rate was ∼ 5 mm d−1 depending on the depth of the depres-
sions. This rate represents a natural sedimentation rate of
∼ 0.002 mm a−1 (∼ 20 m Ma−1), which is 1 order of mag-
nitude lower than the average extension rate. Since we in-
tended to investigate post-extensional structural evolution
with realistic initial density contrasts, the syn-sedimentary
phase lasted much longer than the extensional phase (> 5
days equivalent up to > 100 million years in nature). In na-
ture, sediment compaction would lead to an increase of the
cover density and, therefore, to a faster growth of the salt
structures during (see discussion).
In total, 12 experiments were carried out (Table 2). In or-
der to examine sensitivity on the boundary conditions and re-
producibility of the experiments, a reference experiment with
equal initial conditions was repeated four times (Exp. 1a–c).
The early phase was identical for these three experiments,
although some variations were introduced at a later stage. In
Exp. 1b, an additional phase of basal displacement was ap-
plied after 10 days. Exp. 1c was terminated earlier to observe
intermediate structures. In additional experiments (Exp. 2–8)
initial parameters (layer thicknesses and displacement rates)
were changed systematically to test their influence on struc-
tural evolution and kinematics (Table 2). In Exp. 4b, sand
accumulation was applied simultaneously with displacement
of the basal plate.
We used coloured sand layers to track the structural
evolution. On the basis of digitized photographs of cross-
sections, the final structures were sequentially restored us-
ing a vertical-simple shear restoration algorithm (Rowan and
Ratliff, 2012) in 2-D Move (Midland Valley). Furthermore,
3-D models were constructed in 3-D Move (Midland Valley).
A computer-based displacement data analysis (particle
imaging velocimetry, PIV) tool provided by StrainMaster©
(La Vision GmbH, 2002) was used to monitor incremental
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 13
Table 2. Experiments and key parameters, where hd is the thickness of the ductile silicone and hb the thickness of the brittle sand cover.
Experiment hd [cm] hb [cm] Extension rate Subsidence Total PIV monitoring Comment
e [mm h−1] d [mm] duration [days]
1a 1.5 0.6 4 6 12 side view –
1b 1.5 0.6 4 6+ 6 18 side view two phases of extension
1c 1.5 0.6 4 6 18 side view
1d 1.5 0.6 4 6 4 side view+ top view –
2 1.5 0.6 20 6 4 side view –
3 1.5 0.6 0.6 6 4 side view –
4a 1 0.6 4 6 8 side view –
4b 1 0.6 4 6 8 side view sand accumulation during
basement extension
5 2 0.6 4 6 9 side view –
6 1.5 0.4 4 6 9 side view –
7 1.5 1 4 6 5 side view –
8 1.5 1.5 4 6 6 side view –
B
A
C
+ε
xx
+ε
xx -ε
xx
+ε
xx -ε
xx
Silicone
Sand
Figure 3. Explanation of lateral strain observed with PIV in top
view (see results). (a) Cover extension, which is due to displace-
ment of the basement, is measured as positive lateral strain (+εxx).
(b) Layer bending due to extensional forced folding is accompanied
by extension (+εxx) and compression (−εxx) of the sand surface.
(c) Similarly, surface uplift due to silicone inflation leads to cre-
stal extension (+εxx). Surface subsidence due to silicone deflation
causes compression (−εxx).
strain in the granular and viscous layers, respectively, dur-
ing the experiment. PIV is an optical, non-intrusive method
for particle tracking consisting of digital 12-bit monochrome
CCD (charge-coupled device) cameras (4 Mega Pixel;
∼ 20 pixels cm−1) and analysis software (LaVision 7.0). Im-
ages were taken every minute during the extensional phase
and every 3 minutes during the syn-sedimentary phase. This
is sufficient to follow deformation under low experimen-
tal strain rates. The images were processed with a cross-
correlation algorithm calculating translation and distortion of
textural differences (i.e. grey values) between two sequen-
tial images with a predefined time difference (Adam et al.
2005). Depending on the optical resolution of the cameras
and the precision of the cross-correlation algorithm used, a
spatial resolution < 0.1 pixels can be achieved (Adam et al.,
2005). The resulting accuracy of the displacement vector is
δd =∼ 0.05 mm for an assumed box width of 90 cm. On the
basis of the calculated incremental vector displacement field,
cumulative displacement, x- and y-component of displace-
ment or strain tensor components (e.g. normal strain εxx and
shear strain εxy) as well as long-term average flow velocities
u can be calculated. To achieve a textural pattern in the trans-
parent silicone, it was blended with a small amount of sand
particles (∼ 10 g sand per 1 kg silicone). The effect of this
impurity on density or viscosity of the silicone is negligible
(Boutelier et al., 2008; Warsitzka et al., 2013).
All experiments were monitored in 2-D side view, which
provides strain patterns in the viscous layer close to the glass
wall. This monitored strain can be assumed to be represen-
tative for strain occurring in the centre of the box at least
during early stages of the experiment when deformation of
the cover is similar between the centre and the edges of the
box. During later stages of the experiment, flow patterns in
the centre of the box are no longer parallel to the glass walls.
Thus, strain patterns observed at the glass wall can merely
provide rough estimations of strain occurring in the interior
of the box. The Exp. 1c was additionally recorded in 2-D
top view to observe lateral deformation on the sand surface.
For analysis of flow kinematics, the horizontal displacement
dx in side view or normal strain εxx in top view was used.
The 2-D top view shows lateral extensional (+εxx) or com-
pressional strain (−εxx). This strain can be initiated for in-
stance by thin-skinned extension of the sand cover (Fig. 3a).
Furthermore, flexural bending of the sand produces exten-
sional, as well as compressional domains at the sand sur-
face (Fig. 3b). Similarly, it is possible to identify uplifting
and subsiding areas in the sand layer due to silicone flow
(Fig. 3c). Strain distribution in a folded layer is characterized
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14 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Figure 4. (a) Cross-section of reference Exp. 1a after 12 days. Post-extensional strata reveal the hanging wall peripheral sink (HPS) enclosed
by two pillow-like silicone elevations. Additional sinks as well as an additional secondary pillow occur on the left side of the box. (b) Colour
map displaying the depth of the top of the silicone layer related to the final surface from Exp. 1a. The diameters of the pillows are smaller
than the diameter of the HPS. (c) Cross-section of reference Exp. 1b after 18 days, in which an additional phase of extension was applied
after 10 days. The crests of the pillows are slightly faulted. (d) Depth map of the top of the silicone layer from Exp. 1b showing that the
pillow structures are asymmetric.
by crestal stretching (+εxx) of the sand surface in anticlines,
whereas synclinal bending is associated with compressional
strain (−εxx) (Schultz-Ela and Jackson, 1996).
Strain monitoring with PIV is non-intrusive (Adam et al.,
2005), i.e. only the strain at the model surface can be ob-
served. This restricts monitoring of movement in the silicone,
which is constrained by friction on the glass wall boundaries
and, therefore, not plane-strain. Thus, displacement observed
at the glass wall is merely a fractional amount of displace-
ment, which takes place in the middle of the box (Warsitzka
et al., 2013). We reduced the effect of lateral friction of vis-
cous material on the glass walls by lubricating the glass with
a thin film of glycerine. Due to this the silicone is able to
easily slide along the side walls during the experiment, which
improves strain monitoring by PIV. Without such lubrication,
strain in the silicone layer would only take place at some dis-
tance from the glass walls.
3 Experimental results
3.1 Experimental structures
Cross-sections through the centre of the box of reference ex-
periments 1a and 1b (Fig. 4a, c) show the final structures
obtained after 12 days or 18 days, respectively. A peripheral
sink with thickened sand layers developed above the down-
thrown hanging wall basement block. This sink is here called
the hanging wall peripheral sink (HPS). It is bounded by
two pillows at several centimetres distance from the base-
ment faults. The post-extensional sand layers (red/white;
blue/white) pinch out towards the crests of the two pillows
causing a bowl-shaped structure of the HPS. The width of
the HPS is larger than the width of the underlying basement
graben. The silicone beneath the HPS is depleted, which
eventually brings the base of the sand in contact with the
edges of the basement footwall blocks.
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 15
Figure 5. Restoration of final cross-section of Exp. 1b. (a) At the end of basement extension the hanging wall peripheral sink (HPS) begins
to subside. (b) Additional sand accumulated causes the formation of pillow structures close to downthrown basement block as well as above
the platform at the left side of the box (secondary pillow). (c) The subsidence during the second phase of basement extension is marked by
the dark blue layer. (d) Further post-extensional sand accumulation (blue/white) increases the elevation of the pillow-like structures.
Two types of pillow-like structures can be distinguished in
the experiments. The first (primary pillows) are situated adja-
cent to the basement fault above the footwall block. The sec-
ond type (secondary pillow) is located on the footwall plat-
form near the left-hand end of the experimental box. Depth
maps of the top of the viscous layer (Fig. 4b, d) reveal that
the pillows are elongated and their long axes trend parallel to
the basement offset. An additional peripheral sink (footwall
peripheral sink) containing thickened post-extensional layers
is located between the left-hand primary pillow and the sec-
ondary pillow.
In Exp. 1b the basement was displaced a second time,
which is marked by the blue/white layers. Here, the pre-
kinematic sand layer (yellow) displays minor faulting at the
crest of the pillows. Post-extensional layers of the first phase
(red/white) are bent upward. Different from Exp. 1a, the sil-
icone layer beneath the HPS is not completely depleted.
3.2 Evolution of experimental salt structures
The structural evolution of Exp. 1b is illustrated by means
of restorations of the central cross-section shown in Fig. 4.
During the first syn-extensional stage, a small depression
(HPS) forms due to subsidence of the central basement block
(Fig. 5a). The cover layer above each fault tip is bent into
a monocline roughly 5 cm wide above a ductile layer that
began 1.5 cm thick. Lateral extension in the cover layer is
mainly balanced by peripheral cover grabens near the left
edge of the box. These cover grabens are located close to
the region where the silicone pinches out due to a basement
wedge, and can be regarded as an edge effect. During the
syn-sedimentary stage, surface depressions were filled by
adding sand. The first post-extensional sand layer (orange)
represents the surface subsidence due to basement displace-
ment. Successive growth beds (red–white) reflect downwarp-
ing due to flow of the viscous material. After several phases
of sand accumulation and accompanying expulsion of the
silicone, the HPS widens significantly (Fig. 5b). Areas of
maximum sand accumulation gradually move from the cen-
tre of the HPS towards the rising pillow crests. On the outer
side of the left-hand primary pillow, an additional peripheral
sink (FPS) develops. Eventually, a secondary pillow evolves
above the left footwall block (Fig. 5c).
During the second pulse of rapid extension with additional
6 mm of displacement along the basement faults (displace-
ment rate = 4 mm h−1) (Fig. 5d), the subsidence of HPS in-
creased. This leads to a contact of the base of the HPS with
the edges of the footwall blocks. Consequently, no addi-
tional silicone is expelled from the hanging wall side and
the depocentres move closer to the crests of the pillows
(Fig. 5e). During successive post-extensional sand accumu-
lation (blue–white), the primary pillow structures become
more pronounced and elevated (Fig. 5e). The growth of the
secondary pillow accelerates with increased subsidence of
the footwall peripheral sink.
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16 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Figure 6. Results of PIV monitoring during syn-extensional phase
of the reference experiments 1b and Exp. 1c. (a) Top view of
Exp. 1c: lateral strain εxx observed with PIV displaying zones of
extension (yellow) and zones of compression (blue). Dashed lines
indicate location of basal faults. HPS = hanging wall peripheral
sink. (b) Sketch showing the interpretation of the PIV strain pat-
terns. (c) Side view of Exp. 1b: horizontal displacement dx ob-
served with PIV including a detailed view of the left basement
fault. Coloured areas display rightward (yellow–red) and leftward
(green–blue) movement of the analogue material. (d) Conceptual
sketch (arrows are not to scale) summarizing the interpreted dis-
placement patterns and showing vector profiles representing the ma-
terial movement. The viscous material above the fault tip flows to-
wards the hanging wall side opposed to the shearing along the base-
ment.
4 Displacement and strain patterns
Displacements and strain patterns are visualized using PIV
monitoring for the phases during basement extension, after
termination of basement extension and during accumulation
of post-extensional sand layers.
4.1 Syn-extensional phase
Figure 6 shows the displacement and strain patterns of
Exp. 1b after 6 mm of vertical basal displacement or 1.5 h,
respectively. Patterns of lateral strain (εxx) observed in top
view of the sand surface (Fig. 6a) reveal zones of compres-
sion (−εxx , blue) and zones of extension (+εxx , red). High-
est amounts of εxx are accommodated at a cover graben at
the left side of the box in an extensional zone. Less pro-
nounced zones of extensional strain occur adjacent to the
basement faults. These zones are surrounded by two com-
pressional zones above the edges of the hanging wall base-
ment block. The extensional domains bordering the basement
faults above the footwall blocks are the combined effect of
bending of the cover (see explanation in Fig. 3) and lateral
stretching due to opposite movement in the silicone (Fig. 6b).
Similarly, the less pronounced compressional domains above
the hanging wall block result from layer bending.
In the side view of Exp. 1b, horizontal displacement dx
illustrates the lateral movement of the left basement block
(Fig. 6c). The amount of dx decreases vertically within the
viscous silicone layer and virtually drops to zero in the sand
layer. Hence, shearing at the base is not transmitted into the
cover. As indicated by the vector grid these strain patterns
in the viscous layer can be described as shear flow (Cou-
ette flow). Additionally, significant horizontal movement in
the silicone can be recognized directly above the fault tip in
a 3–4 cm wide, elliptical zone. Although the silicone layer
is sheared in the opposite direction by the leftward moving
basement block, silicone flows towards the subsiding base-
ment block (Fig. 6c). The vector grid in this elliptical zone
shows a vertically parabolic shape, which is characteristic for
channelized Poiseuille-type flow.
4.2 Post-extensional phase
After basement extension had been stopped, deformation was
monitored for approximately 15 min, before the first post-
extensional sand layer was added. During this interim pe-
riod, patterns of the lateral strain εxx observed in top view
of Exp. 1b (Fig. 7a, b) show extensional domains (yellow)
at the edges of the central hanging wall block and at the pe-
ripheral cover graben. Zones of compressional strain (blue)
are identified at the edges of the footwall blocks, in the cen-
tre of the hanging wall block and adjacent to the peripheral
cover graben. As explained by means of Fig. 3, extensional
strains can be attributed to uplift and anticlinal bending of the
thinned cover, whereas areas of compressional strain indicate
subsidence and synclinal bending of the cover. Hence, the
surface subsides at the edges of the footwall blocks, whereas
it is uplifted at the edges of the hanging wall block. Mean-
while the peripheral cover graben is uplifted, whereas adja-
cent areas subside. Complex strain patterns above the cen-
tre of the left footwall block are a result of slightly irregu-
lar thicknesses of the sand layer. The amount of strain (max.
0.025 %) is one order of magnitude lower than strain during
the syn-extensional phase (max. 0.5 %).
The side view of Exp. 1b (Fig. 7c) reveals flow of the
viscous material towards the subsided basement block. The
zones of downward flow are roughly 6 cm wide, as identified
through the coloured area. Therefore, this zone is twice as
wide as in the syn-extensional stage. The vector grid allows
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 17
Figure 7. Results of PIV monitoring during the post-extensional
phase of the reference experiments 1b and Exp. 1c 15 min after
basement extension had ceased. (a) Top view of Exp. 1c: lateral
strain εxx observed with PIV displaying zones of extension (yel-
low) and zones of compression (blue). Dashed lines indicate loca-
tion of basal faults. HPS= hanging wall peripheral sink. (b) Sketch
showing the interpretation of the PIV strain patterns. (c) Side view
of Exp. 1b: horizontal displacement dx observed with PIV includ-
ing a detailed view of the left basement fault. Coloured areas dis-
play rightward (yellow–red) and leftward (green–blue) movement
of the analogue material. (d) Conceptual sketch (arrows are not to
scale) summarizing the interpreted displacement patterns and show-
ing representative vector profiles of the material movement.
identification of a vertically parabolic shape of the vector
profile occurring in the region of active flow. The viscous
material is mainly accumulated directly above the fault step
and above the hanging wall block, which can be inferred by
the horizontally compressed vector grid. Maximum displace-
ment dxmax in the silicone ranges between 0.6 and 0.7 mm in
the 15 min of this interim period. Hence, maximum displace-
ment rates u achieved by viscous flow are 2.4 to 2.8 mm h−1,
which is nearly twice as high as during the syn-extensional
phase (u=∼ 1.5 mm h−1).
The summarizing sketch (Fig. 7d) explains how the zones
of silicone expulsion correspond to areas of lateral cover
compression (−εxx) in the cover layer. The zone where sil-
icone is accumulated is related to surface extension (+εxx)
(Fig. 7d). This indicates that down-slope silicone flux above
Figure 8. Results of PIV monitoring during syn-sedimentary phase
of the reference experiments 1b and Exp. 1c 1h after addition of
the first post-extensional sand layer. (a) Top view of Exp. 1c: lateral
strain εxx observed with PIV displaying zones of extension (yel-
low) and zones of compression (blue). Dashed lines indicate loca-
tion of basal faults. HPS= hanging wall peripheral sink. (b) Sketch
showing the interpretation of the PIV strain patterns. (c) Side view
of Exp. 1b: horizontal displacement dx observed with PIV includ-
ing a detailed view of the left basement fault. Coloured areas dis-
play rightward (yellow–red) and leftward (green–blue) movement
of the analogue material. (d) Conceptual sketch (arrows are not to
scale) summarizing the interpreted displacement patterns and show-
ing representative vector profiles of the material movement.
the fault tip entails uplift above the hanging wall block and
subsidence above footwall blocks. In the same way, exten-
sional strain observed at the peripheral cover graben denotes
uplift. This suggests flow of viscous material towards the
thinned cover.
4.3 Syn-sedimentary stage
One hour after filling the surface depressions, zones of ex-
tension (+εxx) and compression (−εxx) have developed
(Fig. 8a). These zones are wider and the lateral strain (εxx) is
more diffuse than in the post-extensional phase. In contrast
to the post-extensional phase, slight compression occurs di-
rectly above the downthrown basement block. Meanwhile,
areas adjacent to the basement fault on the footwall block are
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18 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
characterized by extension, surrounded by additional zones
of compression further outboard. The region around the pe-
ripheral cover graben at the left edge of the box displays
slight extensional strain. As in the post-extensional stage,
these zones of extensional strain and compressional strain
can be related to surface uplift or subsidence, respectively,
as illustrated in the interpretive sketch (Fig. 8b).
The displacement pattern revealed in side view (Fig. 8c)
indicates that the viscous material flows from the down-
thrown block towards the footwall blocks. Hence, silicone
is expelled from the lowered basement compartment to the
higher basement compartments on both sides. The regions
in the viscous layer affected by horizontal displacement are
wider than in the previous phases. Furthermore, these regions
are roughly twice as wide (∼ 11 to 13 cm) as the monocline
in the cover layer (∼ 6 cm). A parabolic-shaped vector grid
denotes Poiseuille-type flow within this zone. A horizontally
compressed vector grid indicates that viscous material accu-
mulates close to the edge of the basement fault and above the
footwall block. The maximal horizontal displacement dxmax
in the viscous layer was approximately 0.6 to 0.7 mm during
one hour of recording. Thus, displacement rates (u= 0.6 to
0.7 mm h−1) were significantly lower than during the post-
extensional phase (u= 2.4 to 2.8 mm h−1).
Combining the observations from top view and side view,
it can again be shown that the zones of silicone expulsion
and silicone inflation fit with areas of surface compression or
surface extension (Fig. 8d). Hence, horizontal flow in the vis-
cous layer is predominantly compensated by vertical move-
ment in the cover layer. Subsidence of the cover layer on the
hanging wall block expels silicone up the basement fault.
Consequently, silicone inflation onto the footwall block in-
duces surface uplift adjacent to the basement step. Anticli-
nal uplift above the footwall blocks (yellow areas) eventu-
ally results in the formation of the primary pillow structures
(Fig. 4). Furthermore, uplift of the peripheral cover graben
continues. Nevertheless, the amount of uplift is less than dur-
ing the post-extensional phase, which is a result of burial un-
der sand in the centre of the cover graben.
4.4 Subsequent sedimentation phases
During ongoing accumulation of sand, the horizontal flow
pattern in the silicone layer changes gradually (Fig. 9). Ini-
tially, horizontal flow is restricted to the region above the
fault tip (Fig. 9a), while only minor displacement occurs
away from the basement fault above the footwall block. Due
to subsequent sand accumulation in the HPS, the zone af-
fected by viscous flow above the fault tip widens and mi-
grates away from the basement step along the footwall block
(Fig. 9b). After 10 days significant viscous deformation is
induced away from the initial basement fault (Fig. 9c). Al-
though the primary pillows are mainly supplied from the area
above the hanging wall block, a zone of rightward (yellow)
directed flow can be observed. This indicates material in-
flux from the footwall side into the primary pillows, which
is driven by the subsidence of an additional peripheral sink
(footwall peripheral sink, Fig. 5c). Furthermore, leftward di-
rected displacement (blue) at the left edge of the box in-
creases. This reflects material flow during increased growth
of the secondary pillow. At this stage intense flow is active
within the entire box. Note that at late stages, silicone flow
is no longer parallel to the glass walls. The pillow-like struc-
tures developed only in the centre of the experimental box
resulting in circular inflow from all sides. Only a part of this
flow can be observed at the glass wall boundary.
4.5 Second phase of extension
After 11 days, a second phase of rapid extension with 6 mm
of displacement was applied in reference experiment 1b (dis-
placement rate = 4 mm h−1). In general, displacement pat-
terns during this second extensional phase are similar to
those observed in the first extensional phase (Fig. 10). Never-
theless, absolute displacement in the silicone layer above the
fault tip is larger in the second phase than in the first phase.
During the post-extensional and the syn-sedimentary phase,
the lateral extent of viscous flow is smaller after the second
extensional phase. Furthermore, accumulation of the viscous
material predominantly takes place above the footwall side.
This indicates that the drainage channel in the silicone layer
above the fault tip is reduced due to subsidence of the HPS.
5 Sensitivity study
The structural development and flow kinematics are basically
similar in all experiments presented above. However, sys-
tematic variations depending on the thickness of the viscous
layer hd, the thickness of the cover layer hb and the base-
ment displacement rate e can be perceived. A comparison
of all experimental cross-sections and displacement patterns
occurring in the specific experiments can be found in the
Supplement. A summary of these flow patterns for the syn-
sedimentary stage, and the maximum flow velocities in the
centre of the silicone layer is presented in Fig. 11a–c. Flow
velocities u were calculated for the end of the extensional
stage (after 1.5 h), the end of the post-extensional stage (after
15 min), and for each syn-sedimentary phase (1 h after accu-
mulation of sand) by dividing the maximum displacement in
the centre of the viscous layer by the total time (Fig. 11a–
c). In general, displacement rates u are highest during the
post-extensional phase (u= 1 to 5 mm h−1). Displacement
rates during the syn-sedimentary phases are notably lower
(u=< 1 mm h−1). In most experiments, displacement rates
increase gradually with each syn-sedimentary phase.
5.1 Effect of basement extension rate e
Slower basement extension causes a wider and smoother
bending of the monoclines above the fault tips (Exp. 3;
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 19
Figure 9. Time series of PIV displacement patterns of the reference experiment Exp. 1b during syn-sedimentary phase. (a) After 2 days,
pronounced leftward (blue) directed movement occurs close to the basement fault tip. (b) After 6 days, the zone of leftward movement
increases in width and additional flow appears away from the basement fault. (c) After 10 days, significant displacement can still be observed
above the basement fault and away from it.
Figure 10. Comparison of horizontal displacement dx in side view above the tip of the basement fault during (a) first and (b) second phase
of basement extension of the reference experiment 1b after 11 days of experimental duration. Coloured areas denote rightward (yellow–red)
and leftward (green–blue) movement of the analogue material.
Fig. 11a). Furthermore, the position of the cover graben is
located in a greater distance from the basement fault. If e is
large (Exp. 2), a comparably narrow monocline and a small
cover graben develops adjacent to the basement fault.
During the syn-extensional phase, the zone above the fault
tip, which is affected by downward flow, increases in width,
if e is small (Fig. 11a). Maximum displacement dxmax in the
silicone layer is significantly higher at slow basement ex-
tension (Exp. 3: dxmax =∼ 5 mm in 495 min) than for rapid
basement extension (Exp. 2: dxmax =∼ 0.5 mm in 16 min).
However, the average flow velocity u in the viscous layer
is lower in Exp. 3 (u=∼ 0.6 mm h−1) than in Exp. 2 (u=
∼ 1.8 mm h−1; Fig. 11a).
During the post-extensional phase and during the syn-
sedimentary phase, viscous flow above the fault tip spreads
over a wider zone, if basement extension was slow (Fig. 11a).
By contrast, maximum displacement dxmax and flow velocity
u in the viscous layer are higher, when basement extension
was rapid (Exp. 2; Fig. 11a) compared to slow basement ex-
tension (Exp. 3).
In summary, decreasing the displacement rate of the base-
ment fault e leads to an increased width of the monocline in
the cover, a wider zone affected by flow in the viscous layer,
but a lower flow velocity in the viscous layer (Fig. 11a).
5.2 Effect of the thickness of the viscous layer hd
A thicker viscous layer hd causes a wider and smoother bend-
ing of the monoclines above the fault tips (Exp. 5, Fig. 11b).
At small hd (Exp. 4) a cover graben develops adjacent to the
basement fault. In Exp. 4, the mature pillow structures are
comparatively narrow (∼ 6 cm) and are located close to the
basement faults. In experiments with larger hd (Exp. 1a, 5),
pillows are wider, their elevation is higher, and the distance
to the basement graben is larger (see the Supplement).
The zone affected by viscous flow increases in width in
all phases of the experiment, if the thickness of the silicone
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20 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Figure 11. Summarized results of the structures, the kinematics, and the displacement rates u depending on (a/a’) extension rate at the
basement fault e, (b/b’) thickness of the viscous layer hd, and (c/c’) thickness of the cover layer hb. Black arrows indicate extent of flow
in the silicone layer, whereas the thickness of the arrows represents the displacement rates. Triangles and bars in the centre column reflect
increasing or decreasing effect on structures and kinematics. The displacement rates are generally highest during the post-extensional phase.
During the syn-sedimentary phases, the displacement rates are notably lower. Note that only the values of the first four syn-sedimentary
phases are displayed while most experiments last longer.
layer is larger (Exp. 5; Fig. 11b). During basement extension,
flow velocities are higher for a thick viscous layer (Exp. 5:
u=∼ 2.5 mm h−1) than for a thin viscous layer (Exp. 4: u=
∼ 0.7 mm h−1) applying an equal amount of basement ex-
tension (Fig. 11b). A similar dependence can be observed for
the post-extensional and the syn-sedimentary phase (Exp. 4:
u=∼ 0.4 mm h−1; Exp. 1b: u=∼ 0.5 mm h−1; Exp. 5: u=
∼ 0.8 mm h−1). Exp. 5 (hb = 20 mm; Fig. 11b) shows the
highest post-extensional flow velocities of all experiments
(u=∼ 5 mm h−1), which even exceeds the actual basement
extension rate (e = 4 mm h−1). Eventually, fast silicone flux
leads to an initial active piercing of the silicone through the
crest of the pillow structures in this experiment.
5.3 Effect of the thickness of the cover layer hd
A thicker cover layer hb (Exp. 1b, 8; Fig. 11c) causes a
wider and smoother bending of the monoclines above the
fault tips. Compared to this, a narrow monocline occurs if
hb is small (Exp. 6; Fig. 11c). With increasing hb the pil-
lows become wider and located in a larger distance from the
basement graben (see the Supplement). Only if the thickness
of the cover is very large (hb = 15 mm, Exp. 8), do no dis-
tinct pillow structures evolve. In this case, the silicone layer
above the footwall block is thickened over a broad area and
the sinking of the HPS merely lasts a few syn-sedimentary
phases.
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 21
Varying hb has relatively minor effects on flow patterns. In
each phase of the experiments, the lateral extent of silicone
flow is only slightly wider for larger cover thicknesses (e.g.
Exp. 7) compared to experiments with smaller cover thick-
nesses. Flow velocities u in the viscous layer are roughly
similar during the syn-extensional phase, independent of hb
(u= 1 to 1.5 mm h−1; Fig. 11c). During the post-extensional
and syn-sedimentary phases, respectively, flow velocities are
higher in the experiment with a thinner pre-extensional cover
layer (Exp. 6) compared to experiments using a thicker cover
layer (Exp. 7, 1a; Fig. 11c).
6 Discussion
6.1 Structural evolution
Our experimental procedure involved a short pulse of base-
ment extension followed by a long-lasting phase of post-
extensional sedimentation, which is different from most pre-
vious experimental studies (Nalpas and Brun, 1993; Koyi et
al., 1993; Dooley et al., 2005). As in experiments by Nalpas
and Brun (1993) and Dooley et al. (2005), we can distinguish
(1) basement fault-related salt structures and (2) platform salt
structures.
1. The basement fault-related salt structures, here referred
to as primary pillows, are located close to the basement
faults, but entirely on the footwall block. In Fig. 5 we
showed that the main depocentres in the HPS gradually
migrate from the centre of the sink towards the crests
of the rising pillows. This indicates that the viscous ma-
terial is squeezed beyond the basement fault and that
the primary pillows develop further away from the base-
ment fault. This observation is in agreement with most
of the previous experimental studies mentioned above,
which showed that early stage salt structures or cover
grabens are laterally offset from the basement fault tip
if a viscous detachment is present (e.g. Withjack and
Callaway, 2000).
2. The pillows of the second type (secondary pillows) are
located further on the footwall block. In our experi-
ments, decoupled cover faults developed close to the
edge of the experimental box, which is likely an effect
of the side wall (see below). Secondary pillows mainly
grew during later syn-sedimentary phases, when the
footwall peripheral sink (FPS) formed. This suggests
that the formation of secondary pillows does not depend
on layer thinning above the footwall platform. This type
of pillow is rather forced by differential loading pro-
duced by the peripheral sink of the neighbouring pri-
mary pillow. This process is similar to the development
of “secondary diapirs” (Warsitzka et al., 2013), which
are generated by the subsidence of minibasins adjacent
to diapirs (Cobbold et al., 1989; Goteti et al., 2012; Peel,
2014) and which have been exemplarily shown to occur
in nature, e.g. in the North German Basin (Strunck et
al., 1998).
6.2 Kinematics
Two end member types of flow patterns were observed in our
experiments: (1) downward flow above the basement fault
tip occurs during the initial phase of extension and the post-
extensional phase; (2) subsequently, upward flow occurs dur-
ing (post-extensional) sediment accumulation in the HPS.
We interpret this reversal of flow direction in our experi-
ment as a result of the prevailing gradient of the hydraulic
head (e.g. Kehle, 1988; Koyi et al., 1993). Downward flow
is driven by elevation head as soon as the viscous layer is
vertically displaced. Sand accumulation in the hanging wall
peripheral sink creates differential loading. Due to this, a
pressure head in the viscous layer induces a reversal of the
flow direction. Flow velocities during syn-sedimentary up-
ward flow are small compared to those during downward
flow (Fig. 11). Hence, we infer that the pressure head is only
slightly higher than the elevation head. During subsequent
phases of sedimentation, flow velocities in the viscous layer
above the basement fault increase (Fig. 11) denoting a grad-
ually increasing pressure head. During later stages of the ex-
periments, lateral spreading of regions affected by viscous
flow across the footwall platform (Fig. 9) suggests that an ad-
ditional pressure head is imposed above the footwall block.
This is due to the subsidence of the FPS. However, the more
deeply subsided HPS and the strain patterns (Fig. 9) suggest
that the supply of the primary pillow from the footwall side
remains minor throughout the experimental evolution. The
main portion of viscous material within the primary pillow
flows in from the footwall side.
Deformation visualized through vector grids (Figs. 7, 8)
suggests that lateral flow of the viscous material equilibrates
the vertical movement of the overburden and viscous ma-
terial is expelled from subsiding towards uplifting regions.
Therefore, the flow regime within the viscous layer can be
described as squeezed channel flow (e.g. Fuchs et al., 2014),
i.e. the idealized parabolic vector profiles (Poiseuille flow)
are vertically compressed beneath subsiding areas and ver-
tically extended in regions of material accumulation. Such
channel flow occurs in opposite direction to shearing at the
basement during the syn-extensional phase (Fig. 6). This
indicates that stresses applied by the hydraulic head can
exceed shear stress caused by lateral strain. During post-
extensional and syn-sedimentary phases, broad regions af-
fected by channel flow were observed above the basement
step. This demonstrates that small displacements of the base-
ment (Dr < 1) can cause widespread stresses in the viscous
layer.
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22 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
6.3 Sensitivity study
Varying the layer thicknesses and extension rates provides
insights into the reliability of the observed material flow pat-
terns and conditions for pillow formation. By increasing the
thickness of the viscous layer hd, a wider region is affected
by faster viscous flow. This indicates that frictional forces at
the top and the bottom of the viscous layer are less significant
in a thick viscous layer than in a thin viscous layer assuming
the same hydraulic head. Therefore, the growth of pillows is
facilitated if the salt layer is thicker.
In experiments with a thicker cover layer, flow velocities
were generally lower and the formation of pronounced pil-
low structures was suppressed. This results from enhanced
normal stresses at the top of the viscous layer, if the over-
burden is thick. Thus, the resistance against uplift above the
footwall block increases and inflow of viscous material from
the hanging wall block during the syn-sedimentary phase
is impeded. Nevertheless, significant parts of the viscous
layer are affected by horizontal material flow during the syn-
sedimentary phase, even if the thickness of the cover layer is
large (Exp. 8; see the Supplement). This indicates that vis-
cous material accumulated over a larger region leading to
subtle, but broad uplifts.
The basement extension rate e affects flow patterns and
displacement rates u in the viscous layer during the post-
extensional and syn-sedimentary phases. Post-extensional
and syn-sedimentary flow velocities were lower after slow
basement extension (Exp. 3; Fig. 11), because viscous flow
was able to keep pace with the subsidence of the downthrown
basement block. Hence, the space created by basement sub-
sidence was partly equilibrated by increasing the thickness
of the viscous layer (Jackson et al., 1994; Higgins and Har-
ris, 1997). Fewer syn-kinematic sediment could be added to
the hanging wall peripheral sink, which reduced the effect
of differential loading. In experiments applying fast base-
ment extension (Exp. 2) high syn-sedimentary flow velocities
were observed above the basement fault. This is because the
subsidence of the HPS was similar to basement subsidence.
Hence, syn-kinematic layers in the HPS are thicker lead-
ing to a higher pressure head. Therefore, it can be assumed
that fast basement extension promotes post-extensional pil-
low growth, at least when the displacement of the basement
fault was small.
6.4 Limitations of the experimental procedure
All experiments have been set up with a pre-kinematic over-
burden layer before the onset of basement extension. How-
ever, the sensitivity study revealed that flow velocities are
higher and pillow structures are larger in experiments with
a thinner cover layer. Thus, it can be inferred that without
a pre-kinematic cover layer, sand accumulation in the HPS
would trigger the formation of a minibasin and nearby down-
built diapirs (e.g. Burliga et al., 2012; Goteti et al., 2012).
Figure 12. Comparison of two experiments with equal initial con-
ditions, but (a) without syn-extensional sand accumulation and
(b) with syn-extensional sand accumulation in the hanging wall
peripheral sink. Downward flow is prevented in (b). However, no
upward flow can be recognized in (b), although differential load-
ing between the hanging wall side and the footwall side is present.
Coloured areas display rightward (yellow–red) and leftward (green–
blue) movement of the analogue material.
Decoupled cover grabens and secondary pillows devel-
oped close to the edge of the experimental box, where the
silicone layer pinches out. In nature there is no comparable
lateral confinement of the salt layer. However, irregularities
of the basement, above which the thickness of the viscous
layer changes, can also determine the location of decoupled
thin-skinned extension (e.g. Gaullier et al., 1993). Similarly,
secondary pillows can also be caused by a subsiding mini-
basin in a laterally extended salt layer (e.g. Peel et al., 2014).
Basement displacement and syn-kinematic sediment accu-
mulation were artificially separated in our experiments, al-
though these processes are contemporaneous in nature. In
some preliminary experiments, e.g. Exp. 4b, additional sand
was sieved into the subsiding HPS during basement exten-
sion. In these experiments downward flow above the base-
ment fault was non-existent, but no upward flow was induced
during the syn-extensional phase (Fig. 12). This might be due
to shearing in the viscous layer above the basement fault, and
due to the low density contrast between the viscous material
and the cover layer. In other experimental studies simulat-
ing salt diapirism due to basement extension (e.g. Burliga et
al., 2012; Dooley et al., 2005) syn-extensional sedimentation
was applied and no phase of downward flow was described.
However, the density of the sand cover in those experiments
was1ρ = 500–700 kg m−3 higher than the density of the vis-
cous layer. This exaggerates the effect of buoyancy and dif-
ferential loading (Allen and Beaumont, 2012) compared to
natural systems – at least for the initial stage of salt structure
development.
In nature, overburden density increases due to compaction
and exceeds that of salt at depths of 600–1500 m (Jack-
son and Talbot, 1986). This compaction process is difficult
to simulate in analogue models. Assuming a relatively thin
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 23
Figure 13. Conceptual model of the formation of a pillow and corresponding flow patterns in nature and experiment. (a) During initiation
of basement faulting average density of the overburden in the HPS is still lower than that of salt causing only a low amount of differential
loading. (a’) This is similar to the lack of sedimentation in the experimental peripheral sinks. In both cases downward flow occurs above
the fault tip since the pressure head is smaller than the elevation head. (b) Due to further basement extension, thickness and density of
the overburden above hanging wall side increase. (b’) Additional cover material is filled into the HPS. In both cases, upward flow above the
basement fault is initiated by an increasing pressure head. (c/c’) During decreased or stopped extension, the viscous material can continuously
flow upward causing the formation of a pillow structure.
pre-kinematic overburden layer (< 1000 m), the average den-
sity of the natural overburden is lower than that of salt. How-
ever, density increases with burial especially in the hanging
wall peripheral sink, where thick sediments are deposited.
Therefore, differential loading between a peripheral sink and
a nearby salt structure becomes more effective with increas-
ing subsidence of the HPS. For comparison of our exper-
imental results with natural salt tectonics, filling the HPS
with additional sand slightly denser than the viscous material
is similar to the gradual compaction process in nature dur-
ing continuous syn-extensional sedimentation (see below). In
both cases, the pressure head exceeds the elevation head at a
certain amount of subsidence of the HPS (Fig. 13).
6.5 Inferences concerning pillow growth and salt flow
in extensional basins
Both components of the hydraulic head (elevation head and
pressure head), modelled separately in our experiments, tem-
porally and spatially overlap in natural salt-bearing exten-
sional basins. However, we suggest that as in our models,
the elevation head can dominate during early growth stages
of natural salt pillows according to the following conceptual
model (Fig. 13): at the beginning a viscous layer and its over-
burden are gradually displaced by basement faulting. As long
as the average density of the overburden is smaller than that
of salt, differential loading is also small, even if sediment
supply is high enough to keep pace with basement subsi-
dence (Fig. 13a). Hence, early downward flow is driven by
elevation head. If basement displacement continues, density
of the overburden in the HPS increases due to compaction
and, therefore, differential loading becomes more effective.
At a certain depth of the HPS, pressure head increases suf-
ficiently to exceed the elevation head and upward flow is
induced (Fig. 13b). When this stage is reached, progressive
subsidence of the HPS drives further expulsion of the viscous
material even if basement extension terminates (Fig. 13c, c’).
Consequently, the viscous material accumulates above the
higher basement compartment, where the overburden is up-
lifted. Overall, this is a feasible mechanism explaining salt
pillow formation and corresponding peripheral sinks in ex-
tensional settings. Other processes may modify the evolu-
tion described here, e.g. the accumulation of dense sediment
(such as carbonates) in the HPS or erosion of the graben
flanks.
Based on our experimental results, we propose the follow-
ing hypotheses for salt structure evolution in extensional set-
tings:
1. In contrast with previous analogue models (e.g. Dooley
et al., 2005; Ge and Vendeville, 1997; Koyi et al., 1993;
Nalpas and Brun, 1993), no diapirs evolved in our ex-
periments. Therefore, we suggest that small-offset base-
ment faults (Dr < 1) are generally insufficient to trigger
reactive piercing of the salt.
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24 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
2. Furthermore, we suggest that the successful formation
of a salt pillow due to basement displacement requires
a phase of tectonic quiescence, in which the salt is able
to flow upwards and accumulate on the footwall side.
As demonstrated by other analogue model studies (e.g.
Burliga et al., 2012; Dooley et al., 2005) uninterrupted
basement extension leads to enhanced cover faulting
and shearing of the viscous material above the base-
ment step. This reduces material supply from the hang-
ing wall side (Burliga et al., 2012), but promotes reac-
tive diapirism above the basement fault tip without the
occurrence of a pillow stage.
3. In experiments involving large initial overburden thick-
nesses (> 1 cm), no distinct pillow structure evolved.
Scaled to nature, this implies that pillows are sup-
pressed when the overburden is thicker than ∼ 1000–
1500 m. This value is only a rough quantitative estima-
tion, since rock properties in nature (e.g. salt viscos-
ity, strength of the overburden) are more variable than
in analogue models. Salt pillows showing a particularly
thick pre-kinematic overburden layer, e.g. in some parts
of the North German Basin (Baldschuhn et al., 2001),
were possibly generated by another mechanism than the
one simulated in our experiments. Other viable mech-
anisms include regional tectonic contraction (Kossow
and Krawczyk, 2002), the progradation of sedimentary
wedges (Ge and Vendeville, 1997) or the formation of
a residual elevation between two growing salt structures
(Vendeville, 2002).
6.6 Comparison with natural examples
The structural development and kinematics derived from our
experiments can be compared to natural examples of salt
structures in extensional basins such as the North Sea Basin
(Duffy et al., 2013; Korstgård et al., 1993), the Dniepr-
Donets Basin (Stovba and Stephenson, 2003), the Lusitanian
Basin (Alves et al., 2002), the Mid-Polish Trough (Krzy-
wiec, 2004a; Wagner et al., 2002) or the North German Basin
(Baldschuhn et al., 2001; Jaritz, 1987; Kockel, 1998). Cross-
sections displayed in Fig. 14 show some examples of these
basins. The overburden layers of the peripheral sinks have
been reconstructed using vertical simple shear (Rowan and
Ratliff, 2012) and line length unfolding in 2-DMove (Mid-
land Valley). Different from our simplified experimental set-
up, the present-day basement of the salt layer exhibits a com-
plex fault pattern. Nevertheless, differentially subsided base-
ment compartments can be distinguished, which we attribute
to differential loading on the salt layer.
The salt pillow exhibited in Fig. 14a developed above a
slightly deformed basement. The pillow bears a close resem-
blance to the experimental pillow structures observed in our
analogue models (Fig. 4). Hence, we suggest that such salt
pillows were initiated by differential basement subsidence
and continued to grow due to differential loading applied by
nearby peripheral sinks.
In cross-section C of the Weser Trough (North German
Basin) (Fig. 14c), local thickness variations in the post-
salt Middle Buntsandstein (Early Triassic) layer provide ev-
idence for the onset of basement faulting. During a phase
of intensified extension in Middle Keuper time (Late Trias-
sic) in the North German Basin (Kockel, 2002; Mohr et al.,
2005; Scheck-Wenderoth et al., 2008), the HPS in the Weser
Trough had increased in depth and salt pillows evolved.
Based on our modelling results, we suggest that the main
driving forces for salt flow and pillow formation were dif-
ferences in loading between the HPS and the locally faulted
overburden above the footwall compartments. Small offset
basement faults induced widespread expulsion of the salt
into relatively broad pillow structures. Due to renewed exten-
sion during Late Keuper (Late Triassic) and Early to Middle
Jurassic times reactive diapirism took place.
Extension in the Mid-Polish Trough (Fig. 14d) had al-
ready begun during Early Triassic or might have continued
since Late Permian (Krzywiec, 2004b). For this reason, the
pre-kinematic (pre-extensional) overburden above the Up-
per Permian (Zechstein) salt layer was probably very thin or
completely absent. Nevertheless, the reconstruction of cross-
section C suggests that no significant pillow formed before
the Middle Triassic. According to our conceptual model, the
density of the Lower Triassic layers was insufficient to gen-
erate significant differential loading and to support an up-
ward directed salt flow at the beginning (Early Triassic). Dur-
ing the Middle Triassic, a second phase of basement exten-
sion and compaction of the sediments in the HPS created a
pressure head, which was high enough to drive upward flow
and pillow uplift. Syn-kinematic sediments in the HPS re-
veal that the main depocentre migrated towards the left-hand
salt structure during the Middle Late Triassic. According to
our experimental results this indicates that the salt was pro-
gressively expelled towards the footwall block, where it ac-
cumulated in the rising Klodawa salt structure. During the
Late Triassic, the Klodawa salt structure began to pierce as a
diapir due to further basement extension.
Early flow patterns within evolving salt structures cannot
be retrieved from seismic interpretations, but can be pro-
posed on the basis of our experimental results and from
rare outcrop studies in salt mines. Burliga (1996) interpreted
folded salt layers in a salt mine in the Klodawa salt diapir
(Mid-Polish Trough) to be a result of different phases of salt
flow. During an early stage (Early Triassic) salt flowed down-
wards towards the basin axis. During the mature stage of the
Klodawa diapir, salt moved from the downthrown basement
block towards the rising salt structure due to differential load-
ing between the basin centre and the crest of the salt struc-
ture (Burliga, 1996). This interpretation is supported by our
experimental results and shows how experiments can help
to understand the evolution of salt structures and their re-
constructions. Furthermore, examples from salt mines in the
Solid Earth, 6, 9–31, 2015 www.solid-earth.net/6/9/2015/
M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 25
Figure 14. Interpreted seismic profiles of salt structures including reconstruction of the hanging wall peripheral sink (HPS) and the footwall
peripheral sink (FPS). The sub-salt basement has not been restored, but its assumed geometry is schematically denoted. (a) The cross-section
of the Mid-Polish Basin (modified after Dadlez, 2003) shows a typical salt pillow that was probably initiated by differentially subsided
basement. In cross-sections b–c, the main depocentres overlie the downthrown basement part and salt structures are located above basement
highs. (b) The Weser Trough (Central North German Basin) experienced several extensional phases during the Mesozoic, which led to the
formation of salt pillows during Early Triassic and to diapiric piercing during the Late Triassic (modified from Baldschuhn et al., 2001).
(c) The salt structures Klodawa and Wojszyce in the Mid-Polish Trough were initiated during the Middle to Late Triassic above a half graben
(modified from Krzywiec, 2004b). Late Cretaceous inversion caused strong uplift of the sub-salt basement. UP – Upper Permian; LT – Lower
Triassic; MT – Middle Triassic; UT – Upper Triassic, LJ – Lower Jurassic; MJ – Middle Jurassic; UJ – Upper Jurassic, Cr – Cretaceous; C
– Cenozoic. For locations of the profiles see references.
North German Basin disclose complex internal folding of the
evaporite succession even in regions of less external tectonic
imprinting in the basement or the overburden (e.g. Borne-
mann, 1979; Richter-Bernburg, 1980; Strozyk et al., 2013;
van Gent et al., 2010). Flow kinematics in our experiments
demonstrate that significant strain in the viscous layer can
occur despite only slight deformation of the basement and
overburden.
7 Conclusions
Our experimental study simulated flow patterns in a salt layer
during a short phase of basement normal faulting followed
by a long period of sedimentation and the growth of post-
extensional salt pillows.
1. Cross-sections of the final stages of our experiments
demonstrate that two types of pillow structure can be
distinguished. Primary pillows are located adjacent to
basement faults and are mainly driven by differential
loading induced by thicker sedimentary layers above the
downthrown basement block (hanging wall peripheral
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26 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
sink). Secondary pillows develop above the footwall
block at considerable distance from the basement fault.
These pillows are predominantly caused by the subsi-
dence of the peripheral sink flanking the primary pillow
on the side facing away from the basement fault (foot-
wall peripheral sink). The sensitivity study revealed that
the growth of salt pillows is facilitated by a large thick-
ness of the viscous layer, a small thickness of the over-
burden and fast basement extension with a small offset
of the basement fault.
2. Monitoring the deformation patterns within the ana-
logue materials using PIV reveals that during basement
extension, viscous material above the basement fault tip
moves downward driven by elevation head. During the
syn-sedimentary stage, the pressure head due to differ-
ential sedimentary loading forces upward flow. We pro-
pose that the downward and subsequent upward flow
also occur successively in nature. At the beginning, av-
erage density of the overburden is lower than that of
salt. Consequently, differential loading is low and the
elevation head dominates. During further basement sub-
sidence, syn-kinematic sedimentation and compaction
in the hanging wall peripheral sink lead to an increased
overburden density. Differential loading increases until
the pressure head exceeds the elevation head and up-
ward flow is induced. Viscous material is now squeezed
out from beneath the hanging wall peripheral sink to
form pillow-like structures on the footwall side. During
further post-extensional phases, an additional pressure
head is applied due to the development of a peripheral
sink above the footwall block. This initiates the growth
of the secondary pillow.
3. The flow regime in the viscous material can be mainly
characterized as squeezed channel flow. Subsidence and
uplift of the overburden are spatially correlated with ex-
pulsion or inflation of the viscous material. Compared
to focused, small displacements at the basement faults,
the region affected by viscous flow is widespread and
widens further during post-extensional sedimentation.
As shown by the sensitivity study, strain patterns in the
viscous layer are basically independent of layer thick-
nesses or displacement rates. However, the zone af-
fected by viscous flow above the fault tip is wider if
the ductile layer is thicker, or if the basement displace-
ment rate is smaller. Flow velocities in the viscous layer
above the basement fault tip are higher if the ductile
layer is thicker, if the overburden layer is thinner, or if
the basement displacement rate is higher.
4. The comparison of our experimental results with natural
salt-bearing extensional basins demonstrates the sim-
ilarity of structural geometries, e.g. hanging wall pe-
ripheral sinks surrounded by pre-diapiric salt pillows.
Despite the limitations of the experimental set-up, e.g.
simple basement geometry or the temporal separation
between extension and sedimentation, our experimental
procedure provides a generic model for the early evolu-
tion of salt structures in basins that experienced multiple
extensional phases followed by post-extensional ther-
mal subsidence.
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M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 27
Appendix A
A1 Frictional-plastic material behaviour
The mechanical behaviour of granular material like sand is
described by the coefficient of internal friction µ and the
cohesion C (Lohrmann et al., 2003). These material prop-
erties were measured in two-cycle dynamic friction tests at
variable normal loads and variable shear rates with the ring-
shear tester RST-01.pc (Schulze, 1994). During a first shear
load cycle, peak friction was determined. This defines the
strength during material failure and shear zone formation. In
a second shear load cycle, the static stable frictional strength
was measured. This represents the strength during reactiva-
tion of shear zones within the material. Following material
failure after exceeding peak strength or static stable strength,
respectively, a constant dynamic-stable strength in a stage of
ongoing fault activity is reached (Panien et al., 2006).
The values of average frictional properties and bulk den-
sity of the materials used in our experiments are listed in
Table A1. Similar to our experimental procedure sand was
sifted in the measurements of the ring shear tests to achieve
optimal grain package (Lohrmann et al., 2003). The variabil-
ity of densities of the sifted granular materials is of the or-
der of 1ρ = 30 kg m−3. Accuracy of the estimation of cohe-
sion is quite low (∼ 50 %). Due to electrostatic forces, the
cohesions between PVC beads might be significantly higher,
especially under tension. Nevertheless, the mixture of PVC
beads and quartz sand visibly reduces electrostatic accumu-
lation of the particles.
A2 Viscoelastic material behaviour
The silicone oil (Polydimethylsiloxane; Momentive Perfor-
mance Materials Baysilone® SE30) used in our experiments
is characterized by a power-law viscous rheology. In creep
and relaxation tests a low power-law stress exponent of
n= 1.3 had been determined by Rosenau et al. (2009) for
the same silicone material used in our study. Additionally, we
measured the viscosity with a cone-plate rheometer RC20.1-
CPS-P1 with C25-2 measurement system (RheoTec). During
measurement a thin film of silicone is sheared between the
plates of the rheometer at controlled shear rates and room
temperature (20 ◦C). The viscosity is calculated as the ratio
of measured shear stress and strain rate. The thickness of the
silicone film was increased stepwise from 1 to 3 mm. Mea-
surements were repeated three times for each thickness. Av-
erage values for viscosity are listed in Table A2.
In order to record material movement with PIV, the trans-
parent silicone was mixed with PVC beads. The viscosity
of the mixed silicone is only slightly higher (1η = 240 Pas)
than that of pure silicone (Table A2). The bulk density of the
mixed silicone remains unaffected since the PVC beads pos-
sess a similar grain density compared to the density of pure
silicone.
Appendix B
To ensure the significance of experimental results, experi-
ments have to be properly scaled geometrically, kinemati-
cally and dynamically. Models are considered as geometri-
cally scaled if ratios of linear dimensions in the model are
proportionally similar to length ratios in nature (Hubbert,
1937). We used a length ratio l∗ between model and nature
of 10−5 (1 cm in the model corresponds to 1 km in nature),
which is derived from the ratio of cohesion of the brittle cover
(e.g. Koyi et al., 1993; Nalpas and Brun, 1993; Vendeville et
al., 1995; Weijermars et al., 1993):
l∗ = (C0/gρ)∗ (B1)
in which C0 is the cohesion, g the gravitational acceleration,
ρ the density and ∗ represents the ratio between model and
nature. Kinematic scaling is achieved if the ratios between
strain and time are similar for model and nature (Hubbert,
1937). The time ratio considers the viscosity ratio η∗ between
the silicone and the rock salt and the stress ratio σ ∗ (Table 1):
t∗ = (η∗/σ ∗). (B2)
Since we simulate a tensile stress regime, lithostatic pressure
is the main principal stress (Twiss and Moores, 1992):
σN = g · ρb ·hb, (B3)
where g is the gravitational acceleration, ρb the bulk density
and hb the thickness of the salt cover layer. Implying val-
ues for viscosities, length and density of Table 1, the time
ratio becomes t∗ =∼ 1.3× 10−9. Consequently, 1 h in ex-
periments equals approximately 1 Ma. The strain rate ratio
ε∗ (εm / εn) is the inverse of time ratio 1/ t∗.
In order to fulfil dynamic scaling, the strength ratio be-
tween brittle cover Sb and ductile layer Sd has to be equal for
nature and experiment (Weijermars et al., 1993). Sb equals
the differential stress between maximum and minimum prin-
cipal stresses:
Sb = σ1− σ3. (B4)
As mentioned above, the maximal principal stress σ1 equals
the overburden pressure due to gravity σN, whereas the min-
imal principal stress σ3 is horizontal and in direction of ex-
tension. Both natural sediment and granular analogue mate-
rial are supposed to deform according to the Mohr–Coulomb
failure criterion which is given by (Byerlee, 1978; Dahlen,
1990)
τm = σm sinφ+C0 cosφ (B5)
in which σm = normal stress, τm = shear stress, ϕ is the angle
of internal friction and C0 the cohesion. Translating to an
expression of maximum and minimum stresses gives
σ1− σ3
2
= σ1+ σ3
2
sinφ+C0 cosφ. (B6)
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28 M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth
Table A1. Material parameters (density ρ, coefficient of internal friction µ and the cohesion C) of used granular material measured with ring
shear tester RST-01.pc.
Material Grain size [mm] Preparation peak ρ [kgm−3] Peak µ Peak C [Pa] Stable static µ Stable Static C [Pa] Stable dynamic µ Stable dynamic C [Pa]
PVC beads 0.36–0.5 sifted 750± 10 0.33± 0.006 109± 44 0.32± 0.003 119± 25 0.34± 0.030 121± 196
Quartz sand G23 0.02–0.63 sifted 1740± 17 0.70± 0.008 88± 45 0.61± 0.005 92± 36 0.55± 0.002 103± 10
Mixture PVC : quartz 0.02–0.63 sifted 1250± 10 0.51± 0.006 107± 44 0.50± 0.003 72± 25 0.45± 0.030 72± 196
sand 1 : 1
Table A2. Physical properties of silicone putty used in the study
presented here.
Material Weight of sand Density ρ Strain rate ε Shear stresses τ Viscosity η
added to silicone [kg] [kgm−3] [s−1] [Pa] [Pa s]
Silicone PDMS 0 970 0.01–0.08 < 2000 23 000± 1000
Silicone PDMS 0.01–0.03 970 0.01–0.08 < 2000 23 240± 1000
mixed with sand
With separation of σ3, one obtains
σ3 =−σ1 (1− sinφ)
(1+ sinφ) + 2C0
√
(1− sinφ)
(1+ sinφ) . (B7)
Finally, brittle strength Sb can be expressed by
Sb = σ1+ σ1 (1− sinφ)
(1+ sinφ) − 2C0
√
(1− sinφ)
(1+ sinφ) . (B8)
For upper crustal rocks, a suitable value for angle of inter-
nal friction is ϕ = 30◦ and for cohesion is C0 = 5× 106 Pa
(Byerlee, 1978). Generally, density of clastic sediments usu-
ally exceeds the density of rock salt (of ρSalt = 2200 kg m−3)
at depths of 650 to 1500 m (Baldwin and Butler, 1985).
Assuming a pre-kinematic cover thickness of hb = 1000 m
we applied a cover density of ρb = 2300 kg m−3 for the
calculation (Baldwin and Butler, 1985). Therefore, σN =
σ1 = 2.26× 107 Pa and σ3 = 0.297× 107 Pa. Hence, the brit-
tle yield strength at the base is Sb (nature) = 1.96× 107 Pa.
The mixture of quartz sand and PVC beads pos-
sesses a slightly lower angle of internal friction (ϕ =
∼ 27◦). The density contrast between granulate and silicone
(1ρ = 300 kg m−3) is only slightly higher than the natu-
ral salt-cover density contrast, which is an advance com-
pared to previous analogue models using pure quartz sand
(1ρ = 500–700 kg m−3). Using values of Table 1 Sb (model)
= 60–160 Pa.
Shear stresses in a Newtonian fluid can be calculated by
(Turcotte and Schubert, 2014)
Sd = η
(
u
/1
2
hd
)
(B9)
in which η is dynamic viscosity, u the displacement rate
and hd the thickness of the viscous layer. The extension
rate for continental rifting varies between 0.1 and 1 mm a−1
(Allen and Allen, 2005), which gives strain rates in the vis-
cous layer of ε = 10−14 to 10−15 s−1 for a salt thickness
of 2000 m. Assuming an average viscosity for natural rock
salt of η = 1018 Pa s (Nalpas and Brun, 1993; van Keken et
al., 1993) and a salt thickness of 2000 m, Sd (nature)= 104–
105 Pa.
In our experiments, the displacement rate of the simu-
lated basement extension is e = 4 mm h−1 (except for one ex-
periment with e = 0.6 mm h−1 and another experiment with
e = 20 mm h−1, Table 2). Consequently, strain rates in the
viscous layer reach values of ε =∼ 10−5 s−1. Incorporating
a silicone layer of a thickness of hd = 1.5 cm and a viscosity
of η = 2.3× 104 Pas, Sd (model) =∼ 1 Pa.
The dimensionless strength ratios are calculated by
Sb/Sd(nature)=∼ 100–1000 (B10)
and
Sb/Sd(model)= 50–180. (B11)
The model ratio lies within the lower range of the natural
ratio. Hence, the models can be taken as dynamically scaled.
Solid Earth, 6, 9–31, 2015 www.solid-earth.net/6/9/2015/
M. Warsitzka et al.: Analogue experiments of salt flow and pillow growth 29
The Supplement related to this article is available online
at doi:10.5194/se-15-9-2015-supplement.
Acknowledgements. This experimental study was funded by the
German Research Foundation (DFG). Additional funding for
Michael Warsitzka came from the Federal Ministry of Education
and Research (BMBF) (grant Nr. 03IS2091A INFLUINS). Exper-
iments and measurements of material properties were carried out
at the Helmholtz Centre Potsdam GFZ German Research Centre
for Geosciences. We thank the GFZ technicians for assistance in
the laboratory as well as M. Rosenau, M. Scheck-Wenderoth, C.
von Nicolai and F. Jähne for constructive discussions. The authors
gratefully acknowledge the critical reviews by C. J. Talbot, P. Krzy-
wiec and an anonymous reviewer that significantly improved this
paper.
Edited by: F. Rossetti
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