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 imposed by vertically displacing the salt layer (elevation head) or the weight of overburden sediments (pressure head). Basement faulting in salt-bearing extensional basins is widely acknowledged as a potential trigger for hydraulic heads and the growth of salt structures. In this study, scaled analogue experiments 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 experiments viscous silicone putty simulated ductile rock salt, and a PVC-beads/quartz-sand mixture was used to simulate a brittle supra-salt layer. In order to derive 2-D incremental displacement and strain patterns, the analogue experiments were monitored using an optical image correlation system (particle imaging velocimetry). By varying layer thicknesses and extension rates, the influence of these parameters on the kinematics of salt flow were tested. Model results reveal that significant flow can be triggered in the viscous layer by small-offset basement faulting. During basement extension downward 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 viscous 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 eventually leads to the formation of pillow structures adjacent to the basement faults (primary pillows). The subsidence of peripheral sinks adjacent to the primary pillow causes the formation of additional pillow structures at large distance from the basement fault (secondary pillows). The experimentally obtained structures resemble those of some natural extensional 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.
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 gradients in a salt layer can be described by the concept of hydraulic 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).
Conceptual model showing the evolution of a salt diapir
induced by basement faulting:
Previous scaled analogue experiments investigating salt diapirism driven by thick-skinned extension demonstrated that at the beginning discrete basement faulting is balanced by flexural bending of the overburden and decoupled by diffuse 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; Vendeville 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 footwall 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 hanging wall side in a mature stage (Burliga et al., 2012; Koyi et al., 1993). However, in most of these experimental studies finite displacement of the basement faults was large compared to the thickness of the viscous layer. This obscures incremental 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 basement 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 sensitivity study was carried out to test the role of characteristic parameters, namely salt thickness, cover thickness and extension rate, in affecting flow patterns and post-extensional structural evolution. A 2-D optical particle tracking system (PIV) was applied to observe incremental particle displacements and strain patterns in both the brittle and the ductile layer during the 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
For representative quantitative and qualitative information, analogue models
have to be scaled geometrically, kinematically and dynamically (Hubbert,
1937; Ramberg, 1967). This requires dimensionless ratios relating the rheologies
and stresses of nature to be similar to those for experiments. Referring to previous
studies, we employ a geometric scaling factor
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). Nevertheless, 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 (
In this generic experimental study, we intend to simulate conditions of salt structure evolution during the onset of basement 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 sedimentation. 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 basement 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”.
Scaling parameters. Material properties (density, viscosity, friction and cohesion) are stated for analogue materials used in this experimental study (Appendix A).
Appropriate experimental values for e.g. layer thicknesses, displacement of
the basement faults or the duration of extension are adapted to the
conditions of the Central European Basin system, which contains prominent
extensional segments (e.g. Central Graben, Horn Graben, Mid-Polish Trough)
(Ziegler, 1982) and numerous salt structures (Maystrenko et al., 2013).
Here, the pre-extensional overburden thickness
Experiments were performed in a 90
Each experiment starts with a displacement of the basal plate (extensional
phase), whereby the middle block is pulled down
In total, 12 experiments were carried out (Table 2). In order to examine sensitivity on the boundary conditions and reproducibility 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 applied 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 structural evolution and kinematics (Table 2). In Exp. 4b, sand accumulation was applied simultaneously with displacement of the basal plate.
Experiments and key parameters, where
Explanation of lateral strain observed with PIV in top view
(see results).
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 using 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 strain in the granular and viscous layers,
respectively, during 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;
Restoration of final cross-section of Exp. 1b.
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 representative 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
Strain monitoring with PIV is non-intrusive (Adam et al., 2005), i.e. only the strain at the model surface can be observed. 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 displacement, which takes place in the middle of the box (Warsitzka et al., 2013). We reduced the effect of lateral friction of viscous 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 distance from the glass walls.
Cross-sections through the centre of the box of reference experiments 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 downthrown 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 basement 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.
Two types of pillow-like structures can be distinguished in the experiments. The first (primary pillows) are situated adjacent to the basement fault above the footwall block. The second type (secondary pillow) is located on the footwall platform 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 secondary 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 silicone layer beneath the HPS is not completely depleted.
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 displacement. Successive growth beds (red–white) reflect downwarping 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 centre 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 (displacement rate
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.
Results of PIV monitoring during syn-extensional phase of
the reference experiments 1b and Exp. 1c.
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 (
In the side view of Exp. 1b, horizontal displacement
Results of PIV monitoring during the post-extensional phase
of the reference experiments 1b and Exp. 1c 15 min after basement
extension had ceased.
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 period, patterns of the lateral strain
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
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
displacement
The summarizing sketch (Fig. 7d) explains how the zones of silicone
expulsion correspond to areas of lateral cover compression (
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.
One hour after filling the surface depressions, zones of extension
(
Time series of PIV displacement patterns of the reference
experiment Exp. 1b during syn-sedimentary phase.
Comparison of horizontal displacement
The displacement pattern revealed in side view (Fig. 8c) indicates that the
viscous material flows from the downthrown 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
(
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 viscous layer is predominantly compensated by vertical movement 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 induces surface uplift adjacent to the basement step. Anticlinal uplift above the footwall blocks (yellow areas) eventually 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 during the post-extensional phase, which is a result of burial under sand in the centre of the cover graben.
During ongoing accumulation of sand, the horizontal flow pattern in the silicone layer changes gradually (Fig. 9). Initially, 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 affected by viscous flow above the fault tip widens and migrates 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). Although 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 influx 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 directed displacement (blue) at the left edge of the box increases. 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 structures 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.
After 11 days, a second phase of rapid extension with 6 mm of displacement
was applied in reference experiment 1b (displacement rate
The structural development and flow kinematics are basically similar in all
experiments presented above. However, systematic variations depending on the
thickness of the viscous layer
Summarized results of the structures, the kinematics, and
the displacement rates
Slower basement extension causes a wider and smoother bending of the
monoclines above the fault tips (Exp. 3; Fig. 11a). Furthermore, the
position of the cover graben is located in a greater distance from the
basement fault. If
During the syn-extensional phase, the zone above the fault tip, which is
affected by downward flow, increases in width, if
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
In summary, decreasing the displacement rate of the basement fault
A thicker viscous layer
The zone affected by viscous flow increases in width in all phases of the
experiment, if the thickness of the silicone layer is larger (Exp. 5; Fig. 11b). During basement extension, flow velocities are higher for a thick
viscous layer (Exp. 5:
A thicker cover layer
Varying
Our experimental procedure involved a short pulse of basement extension
followed by a long-lasting phase of post-extensional sedimentation, which is
different from most previous 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.
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
material is squeezed beyond the basement fault and that the primary pillows
develop further away from the basement 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). The pillows of the second type (secondary pillows) are located further
on the footwall block. In our experiments, 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 produced by the peripheral sink of the neighbouring
primary 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).
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 during (post-extensional) sediment accumulation in the HPS. We interpret this reversal of flow direction in our experiment 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 upward 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 gradually increasing pressure head. During later stages of the experiments, lateral spreading of regions affected by viscous flow across the footwall platform (Fig. 9) suggests that an additional 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 material 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 vertically 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 affected by channel flow were observed above the
basement step. This demonstrates that small displacements of the basement
(
Varying the layer thicknesses and extension rates provides insights into the
reliability of the observed material flow patterns and conditions for pillow
formation. By increasing the thickness of the viscous layer
In experiments with a thicker cover layer, flow velocities were generally lower and the formation of pronounced pillow structures was suppressed. This results from enhanced normal stresses at the top of the viscous layer, if the overburden 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 viscous material accumulated over a larger region leading to subtle, but broad uplifts.
The basement extension rate
All experiments have been set up with a pre-kinematic overburden layer before the onset of basement extension. However, 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).
Comparison of two experiments with equal initial
conditions, but
Decoupled cover grabens and secondary pillows developed 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 minibasin in a laterally extended salt layer (e.g. Peel et al., 2014).
Basement displacement and syn-kinematic sediment accumulation were
artificially separated in our experiments, although these processes are
contemporaneous in nature. In some preliminary experiments, e.g. Exp. 4b, additional sand was sieved into the subsiding HPS during basement
extension. In these experiments downward flow above the basement 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 simulating 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 was
Conceptual model of the formation of a pillow and
corresponding flow patterns in nature and experiment.
In nature, overburden density increases due to compaction and exceeds that of salt at depths of 600–1500 m (Jackson and Talbot, 1986). This compaction process is difficult to simulate in analogue models. Assuming a relatively thin pre-kinematic overburden layer (< 1000 m), the average density of the natural overburden is lower than that of salt. However, 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 increasing subsidence of the HPS. For comparison of our experimental 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 during 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).
Both components of the hydraulic head (elevation head and pressure head), modelled separately in our experiments, temporally and spatially overlap in natural salt-bearing extensional 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 overburden 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 subsidence (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 sufficiently 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 uplifted. Overall, this is a feasible mechanism explaining salt pillow formation and corresponding peripheral sinks in extensional settings. Other processes may modify the evolution 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 following hypotheses for
salt structure evolution in extensional settings:
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 experiments. Therefore, we suggest that small-offset basement
faults ( 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 basement step.
This reduces material supply from the hanging wall side (Burliga et al.,
2012), but promotes reactive diapirism above the basement fault tip without
the occurrence of a pillow stage. In experiments involving large initial overburden thicknesses
(> 1 cm), no distinct pillow structure evolved. Scaled to nature,
this implies that pillows are suppressed when the overburden is thicker than
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 (Krzywiec, 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 (Midland Valley). Different from our simplified experimental set-up, the present-day basement of the salt layer exhibits a complex fault pattern. Nevertheless, differentially subsided basement 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 resemblance 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 evidence for the onset of basement faulting. During a phase of intensified extension in Middle Keuper time (Late Triassic) 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 differences 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 extension during Late Keuper (Late Triassic) and Early to Middle Jurassic times reactive diapirism took place.
Extension in the Mid-Polish Trough (Fig. 14d) had already begun during Early Triassic or might have continued since Late Permian (Krzywiec, 2004b). For this reason, the pre-kinematic (pre-extensional) overburden above the Upper 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 generate significant differential loading and to support an upward directed salt flow at the beginning (Early Triassic). During the Middle Triassic, a second phase of basement extension 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 reveal 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 progressively expelled towards the footwall block, where it accumulated 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.
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.
Early flow patterns within evolving salt structures cannot be retrieved from seismic interpretations, but can be proposed 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 downwards 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 loading between the basin centre and the crest of the salt structure (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 reconstructions. Furthermore, examples from salt mines in the 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. Bornemann, 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.
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.
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 sink).
Secondary pillows develop above the footwall block at considerable distance from the
basement fault. These pillows are predominantly caused by the subsidence of the peripheral
sink flanking the primary pillow on the side facing away from the basement fault
(footwall peripheral sink). The sensitivity study revealed that the growth of salt
pillows is facilitated by a large thickness of the viscous layer, a small thickness
of the overburden and fast basement extension with a small offset of the basement fault. Monitoring the deformation patterns within the analogue 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 differential sedimentary loading forces upward flow. We propose that
the downward and subsequent upward flow also occur successively in nature. At the
beginning, average density of the overburden is lower than that of salt. Consequently,
differential loading is low and the elevation head dominates. During further basement
subsidence, 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 upward 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.
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 expulsion 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 thicknesses or displacement
rates. However, the zone affected by viscous flow above the fault tip is wider
if the ductile layer is thicker, or if the basement displacement 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. The comparison of our experimental results with natural salt-bearing
extensional basins demonstrates the similarity of structural geometries, e.g.
hanging wall peripheral 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 evolution of salt structures in basins that
experienced multiple extensional phases followed by post-extensional thermal subsidence.
The mechanical behaviour of granular material like sand is described by the
coefficient of internal friction
The values of average frictional properties and bulk density 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
variability of densities of the sifted granular materials is of the order of
Material parameters (density
The silicone oil (Polydimethylsiloxane; Momentive Performance 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
Physical properties of silicone putty used in the study presented here.
In order to record material movement with PIV, the transparent silicone was
mixed with PVC beads. The viscosity of the mixed silicone is only slightly
higher (
To ensure the significance of experimental results, experiments have to be
properly scaled geometrically, kinematically and dynamically. Models are
considered as geometrically scaled if ratios of linear dimensions in the
model are proportionally similar to length ratios in nature (Hubbert, 1937).
We used a length ratio
In order to fulfil dynamic scaling, the strength ratio between brittle cover
The mixture of quartz sand and PVC beads possesses a slightly lower angle of
internal friction (
Shear stresses in a Newtonian fluid can be calculated by (Turcotte and
Schubert, 2014)
In our experiments, the displacement rate of the simulated basement
extension is
The dimensionless strength ratios are calculated by
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). Experiments 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. Krzywiec and an anonymous reviewer that significantly improved this paper. Edited by: F. Rossetti