Essay: Salt tectonics in strike slip settings: Application to the North Sea

INTRODUCTION
A strike slip system is a deformation mechanism that occurs in continental as well in oceanic crust at different scales (Burg, 2011). These narrow systems contain faults between the two adjacent blocks where the sense of direction is parallel to the strike of sub vertical fault plane (Jaeger, et al., 2009). The deformation formed by this type of fault results in complex structures, which are difficult to interpret (Dooley & Schreurs, 2012). For example, seismic data which is used by the petroleum industry, is two dimensional and faulting is a 3 dimensional system. Analogue modelling of this complex system is therefore essential for researching the evolution of these structures through time and for understanding the cross sections observed on the seismic sections. The first analogue strike slip experiments were made by Cloos (1928) and Riedel (1929). They investigated the deformation structures made by a reactivated vertical basement fault on an initially undeformed system. Later researchers created a model that was more true to nature (Faugere, et al., 1986): the use of a ductile basement which represents the ductile lithosphere, produced a better distribution of the horizontal strain.
The formation of salt diapirs is only possible when the driving forces for salt flow exceed the resistance of flow. Salt diapirs have three evolutionary stages: active, reactive and passive (Vendeville & Jackson, 1992). The active stage of a diapir happens when there is external tectonic influence, like compression, which lifts the salt layer to the surface. In the reactive phase the salt layer ascends along normal faults formed by regional extensional faulting (Weijermars & Jackson, 1993). This is the consequence of the increase in pressure due to the falling of the hanging wall into the salt layer. Hudec & Jackson (2007) do not distinguish the active and reactive phase. The passive stage takes place when the salt diapir has reached the surface and spreads syn-sedimentation under its own weight, forming a salt sheet (Hudec & Jackson, 2007). In this case the extrusion rate is fast enough to keep the salt elevated above its surrounding area (Weijermars & Jackson, 1993). In this thesis we use the classification of Weijermars & Jackson (1993).
Salt tectonics is already researched with analogue modelling in a variety of tectonic processes, such as compressional (Brun & Fort, 2004; Letouzey, et al., 1995), extensional systems (Vendeville & Jackson, 1992; Vendeville, et al., 1995), pull-apart basins (Smit, et al., 2008) and releasing bends (Koyi, et al., 2008). Until today, no specific research is performed that combines salt tectonics with analogue pure strike slip experiments.
In this thesis, we investigate the evolution and the internal and external structure of salt tectonics in pure strike slip systems. To get a general analysis of salt tectonics in strike slip systems, we first explain the basics of analogue modelling and then the specifically designed laboratory experiments will be presented. In the experiments we variate a few parameters, like the size of the basement, to comprehend its effects on the model and thereby also on nature. After the right construction of the model is found, we variate the speed of the experiment and in the use of erosion, sedimentation and resting. The results will be put in broader context of the literature, more specifically to data of the North Sea (originally from EBN).
ANALOGUE MODELING PROCEDURE
In nature, only the most recent phase of the deformation history of rocks can be found. Analogue modeling is an instrument to study the mechanics of (salt) tectonic deformation, but its aim is not to rebuild nature. All experiments were executed in the TecLab at Utrecht University, that provided the required instruments and material.
Material
The choice of analogue material has a large control on the structures that form in the model (Dooley & Schreurs, 2012).
Brittle
Brittle rocks deform according to the Mohr-Coulomb criterion (Byerlee, 1978; Handin, 1969):
”=C+(tan”)” (2.1)
where ” represents the shear stress, C the cohesion, ” the angle of internal friction and ” the normal stress. Following this formula the maximum differential stress in the brittle layer increases linearly with depth and is independent of strain rate. Because the experiments show pure strike-slip where ” = ”2, the maximum differential stress is (Smit, 2005):
”_1 – ”_3=C+”gT_b (2.2)
where ”1 and ”3 are relatively the maximum and minimum principal stresses, ” the sand density, g the acceleration due to gravity and Tb the thickness of the brittle layer.
Dry feldspar sand, a Mohr-Coulomb material (Sokoutis, et al., 2005; Weijermars & Jackson, 1993), is used to represent this brittle behavior in the experiments. The physical properties of dry feldspar sand are: density ” 1.3 g/cm3 (Luth, et al., 2010), sieved to a grain size d 100 ‘ 350 ”m and a coefficient of internal friction ”fric of 0.6 (Sokoutis, et al., 2005).
Ductile
Rock salt is generally represented by silicone putty, which is a Newtonian viscous fluid (Weijermars & Jackson, 1993). Its resistance is linearly dependent on strain rate. The shear stain rate (‘) in the silicone putty can be described by (Brun, 2002; Smit, 2005):
‘= V/T_d (2.3)
where V is the velocity and Td the thickness of the ductile layer.
In these experiments the rock salt is represented by the transparent silicone putty SGM-36 that belongs to the group of PDMS, a poly-dimethyl siloxane manufactured by Dow Corning Inc.. The physical properties of this silicone putty are: density ” is 0.970 g/cm3 (Anma & Sokoutis, 1997; Weijermars & Jackson, 1993), no yield strength and the viscosity ”vis 5 . 104 Pa.S at room temperature (20 degree Celsius) (Weijermars, 1986).
Construction
Scaling
In analogue modelling the physical parameters such as rheologies, distributions of stresses and densities, need to be exactly balanced with respect to the area of interest in order to derive meaningful interpretations (Hubbert, 1937; Brun, 2002; Luth, et al., 2010; Weijermars & Jackson, 1993). The power-law of Goetze & Evans (1979) describes the ductile flow of rocks:
” ”=Aexp(-Q/RT)'(”_1- ”_3)’^n (2.4)
where ” ” is the deviatoric strain, Q the activation energy, R the universal gas constant, T the absolute temperature, A a material constant and n the stress component. This equation of dynamics is used to describe the correct scaling of the strength of ductile layer with respect to the brittle layers and gravitational forces (Smit, 2005). Brun (2002) found two conditions that respect the equation of dynamics:
”^*=”^* g^* L^* (2.5)
”^*=g^* ‘(t^*)’^2 (2.6)
where, ” represents the stress, ” density, g gravity acceleration, L length, ” deformation, t time and exponent * describes the ratio between model and nature. Hubbert (1937) discovered that the inertial forces can be ignored in geological processes, which implies that only equation 2.5. has to be verified.
The materials used for the experiments have a range in density between 0.970 to 1.3 g/cm3 and for the rocks in nature the densities lie between 2.3 to 3.0 g/cm3, which gives a density ratio close to 1. The experiments are performed under normal gravity (g* = 1), which results in a simplified equation:
”^*’ L^* (Brun, 2002) (2.7)
One centimeter on the vertical scale of the models represents one kilometer in nature.
Set up
Two thin plastic plates (0.2 mm) were horizontally placed on a table next to a metal bar, one plastic plate was kept stationary by clamps. On top of the mobile plate a rectangular piece of wood was fixed with a screw. The screw thread was attached to the motor that pulled the non-fixed plate at the requested velocity. Soap was placed under the plate to minimize the friction between the mobile plate and the table.
Multiple layering set-ups were made to discover the model parameters that gave the right internal and external structures. The models are constructed by stacking metal bars with variating thicknesses. Between every layer a thin layer of sieved black feldspar sand for distinguishing purpose. The two experiments were made to determine the most efficient construction. The main focus lied on the position of the silicone putty layer. The cross sections of the two different constructions is given in figure 2a and 2b. For the comparison of the results, see section silicone putty placement on page 11.
In later experiments the position of the silicon putty layer was shifted from bottom to one of the top layers, where there is less weight of the feldspar sand layers on the silicon putty. The cross section of the final construction is given in figure 3.
480 gram of silicon putty was placed in a mold of metal bars with the dimension 20 x 60 cm, which over time resulted in a flat layer with a thickness of 0.4 cm. It was ensured that the putty contained no bubbles to decrease errors. The putty was placed on 2.2 cm of multicolored sieved sand. On the top blue layer a thin layer of blue sand was sieved for a better contrast of the top view photos. A grid of black quartz sand (0.5 cm apart) was made for visualizing the amount of deformation.
The variable parameters of the experiments were the amount of distance the mobile plate had moved, resting and if syn- / post-kinematic sedimentation and/or erosion took place and the regularity of the process. In the resting phase the apparatus no longer sheared the experiment, but the experiment was not yet blocked. In this way we can determine if the silicone putty can rise further as the pure strike slip movement has stopped. We used a vacuum cleaner with a small suction nozzle for eroding the uplifted areas of the model. Erosion is applied until the silicone putty was visible and the thickness of the syn-/ post-kinematic sedimentated layers is roughly 1-2 mm. Note that the erosion/sedimentation process is only used for distributing the mass.
Establishing data
Photos of the top view of the experiments were made on a regular basis during the deformation. The movies made of these images contributed to the understanding of the evolution of the external structures. When the experiment was finished, water was poured over the sand resulting in a cemented experiment. The experiment was cut into strips of 1 cm perpendicular to the shear zone for researching the internal structures in the cross sections. Photos were made of the cross sections.
Limitations
The experiments are simplified versions of nature itself. The brittle and ductile layers in nature are not homogenous, which is in contrast with the analogue experiments, for example the temperature gradient of the crust and the change in the pore fluid pressure, viscosity and density is missing in the analogue models. Only constant strain rates are used in the experiments of this thesis, which is also dissimilar to nature. These limitations must be taken into account when interpreting the models.
Analogue modeling is still an important method to examine certain geological systems. There is complete control on the parameters, changing one parameter while keeping the others consistent. ‘
Table 1 Conditions for the seven series of experiments
Total slip (cm) and speed (cm/hr) Series 0: Placement of silicone putty layer PURE BRITTLE MODEL SERIES 1:
NO EROSION, NO SEDIMENTATION NO RESTING SERIES 2:
EROSION, NO SEDIMENTATION, NO RESTING SERIES 3:
EROSION, RESTING, NO SEDIMENTATION SERIES 4:
EROSION, SEDIMENTATION, NO RESTING SERIES 5:
EROSION, SEDIMENTATION, RESTING
8 cm &
2 cm/hr –
6.9 cm & 2.5 cm/hr Model 2: –
10 cm & 2.5 cm/hr Model 4: Erosion (3 cycles)
12.5 Cm & 2.5 cm/hr Model 1: Erosion (3 cycles) Model 3: – Model 5: Erosion ( 3 cycles) Model 6: Erosion (4 cycles), Sedimentation (2 cycles) Model 7: Erosion (4 cycles), Sedimentation (2 cycles)
EXPERIMENTAL RESULTS
Series 0: Placement of the silicone putty layer
This section compares two experiments based on their cross sections. The models have a similar construction: they have the same height, but the placement of the ductile layer is different. The first model (seen on fig 2a and fig 5) has the silicone putty layer located on top of the plastic plates, while the same layer in the other model is roughly 2/3 of the total height above these plastic plates (seen on fig 2b and fig 6).
The cross section of model 1 consists of a variation of a positive flower structure. The faults incline towards each other, but they never meet above the brittle-ductile boundary and instead crash into the silicone putty layer. A pop-up structure is formed by these faults. The silicone putty layer follows the reverse faults for a small distance, but it never penetrates further than the bottom white layer.
The cross section of model 2 has a small positive flower structure where the faults splay up from the basement fault, which causes a small pop-up structure beneath the silicone putty layer. Above the pop-up structure the silicone putty is slightly deformed: on the flanks of the pop-up structure the thickness of the ductile layer is decreased and on the highest point of elevation the silicone putty tries to intrude the overlying brittle layers.
Further constructions of the experiments are based on the results of model 2, because the silicone putty tries to rise through the experiment.
‘
‘
Brittle model
This model is used as a reference model, for comparison with the brittle-ductile models. The 0.4 cm pink layer symbolizes the silicone putty layer of the other models. The results are representative for pure strike-slip faulting in brittle rocks (Cloos, 1928; Tchalenko, 1970; Richard, et al., 1995; Le Guerroue & Cobbold, 2006; Riedel, 1929). Formula 2.2 from Smit (2005) implies that the speed of the deformation has no influence at the total deformation (as the model travels the same distance).
In the first 2 cm displacement, Riedel faults (R-faults) form at the boundaries of the model above the basement fault. When deformation further occurs, the R-faults develop en echelon towards the center of the model. These faults make an angle of roughly 18 degrees clockwise with the basement fault (Fig 7.a). This development of structures is also described by Riedel (1929).
After 4 cm displacement, various lengths of R-faults are found along the entire length of the model. A new type of fault, P-fault, connects the R-faults. Together they form pop-up structures (Fig 7.b).
”Photos are missing between 4 cm displacement and the final state of the brittle model (8 cm displacement) due to technical difficulties. Thereby there is no Fig 7.c that shows the external structure formed at 6 cm displacement.
At 8 cm displacement, more P-faults have formed and Y-shears have developed parallel to the basement fault through the pre-existing features (Fig 7.d). The average shear zone width of the final state is approximately 5.5 cm.
The cross sections of the pure brittle model show that all faults splay up from the basement fault. This network of faults form a positive flower (Fig 8a and 8b).
‘
Series 1: No erosion, sedimentation and resting
In the first 2 cm of deformation of model 3, the R-faults started to form at the boundaries of the model and continued to develop in the direction of the center of the model. These faults formed en echelon with an angle of 10 degrees with the basement fault and were closely spaced. Figure 9.a shows a uplift with the same orientation as the R-faults. Faults have formed on the uplift, which are almost perpendicular to the R-faults.
After 4 cm displacement, the R-faults have grown in length and started to show small extension along the faults. The uplift has grown in area size (Fig 9.b). There is no other uplift like the previous formed.
As deformation continued, P-faults formed between the R-faults, which progressed the extension along the R-faults. The uplift followed the sense of direction of the basement fault and its elevation was decreased. The P-faults grew over time and thereby also the pop-up and pop-down structures formed by the combination of the R-faults and P-faults, however these pop-ups do not look like the first uplift (Fig 9.d). As the displacement increased, the Y-faults formed an extra boundary along the pop-up and pop-down structures. The extension along the R-faults did not grow since the formation of these Y-faults. The R-faults rotated to an angle of 14 degrees clockwise with the basal cut.
In the final state of the model (at 12 cm displacement) the combination of R-fault, P-faults and Y-faults form an anastomosing network of faults. The uplift described in the first part of this section is completely swallowed by this network (Fig 9.f). The average width of the shear zone is 2 cm.
In the cross section all of the faults beneath the silicone putty have their origin in the basement fault. The positive flower structure forms two significant pop-ups under the silicone putty. Above these pop-ups the silicone putty is stretched out along the flanks of the pop-ups and compressed at the highest point of the elevation (fig 10). A small uplift is formed at the right side of the cross section due to a far-researching reverse fault.
Above the silicone putty layer the faults are difficult to interpret due to the rising silicone putty (begin stage of a salt diapir). Along the silicone putty boundaries there are faults.
‘
Series 2: Erosion, no sedimentation and no resting
The deformation in the first 2.5 cm displacement of model 4 is the same as in series 1, after that erosion is applied to the model.
After 2.5 cm of displacement after the first erosion, which was at 5 cm displacement in total, the R-faults reappeared en echelon with an angle of 8 degrees clockwise with the basement fault, on the surface of the model. These R-faults were closely spaced and had a small component of extension along the fault planes. In the sections where there was no erosion applied, the amount of extension was larger. Two uplifted locations can be distinguished and are indicated with the number 1 and 2 in Fig 11.a , with had an initial length of respectively 3.5 and 5 cm. After that moment the experimental model undergoes a second cycle of erosion, which is applied on the topography. The lower white layer became visible by that action.
When the model was deformed with a total displacement of 7 cm, the contours of the silicone putty became noticeable at the location of the previous two uplifts (Fig 11.c). The R-faults between these former uplifts became more closely spaced and the Y-faults surrounded the P- and R-faults, stopping the extension along the R-faults.
After 7.5 cm of displacement, a third cycle of erosion was performed on the former uplifts, whereby the silicone putty ridge became fully distinguishable (Fig 11.d). The silicone putty is orientated at an angle of 8 degrees counterclockwise with the basement fault. The previous formed R-faults have changed the angle with the basement fault into 19 degrees clockwise, which means that the angle between the R-faults and silicone putty is 27 degrees.
From this point on to the final state of the model (total displacement of 10 cm), putty zone 2 became wider and moved with the sense of direction of the basement fault while putty zone 1 remained stationary. The two silicone putty zones grew from initially 3.5 and 5 cm to respectively 4 and 6 cm, which gives the average growth ratio of 116 %. The average width of the shear zone at the final state of the model is 2 cm (Fig 11.f).
The cross sections of both the putty zones show roughly symmetrical positive flower structures that splay out of the basement fault. This structure forms one pop-up structure with an average width of 1 cm under the silicone putty layer. The cross sections differ in the appearance of the silicone putty: whereas cross section of silicone putty zone 2 (fig 12.b) shows the same thickness all over the ductile layer, the silicone putty layer of cross section of silicone putty zone 1 is stretched out along the flanks of the pop-up structure and compressed at the highest point of elevation and splits into two separate begin phases of diapirs (fig 12.a).
‘
Series 3: Erosion, resting and no sedimentation
The first 10 cm displacement gives the same deformation in model 5 as observed in the model of series 2 (Fig 13.a to 13.c). On this model there was only one silicone putty zone traceable and the R-faults made an angle 10 degrees clockwise with the basement fault. This model deforms further for 2.5 cm.
After 11 cm displacement the R-faults rotated and from that moment on they made an angle of 17 degrees clockwise with the basement fault. In this series the silicone putty also made an angle with the faults: 10 degrees counterclockwise with the basement fault and 27 degrees with the R-faults. Also another small uplift began to form on the model. The growth of this thin silicone putty ridge is difficult to trace and that is why it is not indicated on Fig 13.
At 12.5 cm displacement the width of the shear zone was approximately 3 cm (Fig 13.f). The silicone putty ridge has moved in the sense of the bulk shear. At that moment the machine was stopped and the model underwent six hours of resting.
After resting the size of the silicone putty zone was increased from 6 cm before the last erosion cycle to 8 cm after six hours of resting, which indicates a growth rate of 133%. No growth or movement of the silicone putty was identified during the resting process.
The cross section shows a nearly symmetrical positive flower structure, which produces a pop-up feature underneath the silicone putty. Along the flanks of the pop-up structure the silicone putty is slightly stretched, while the silicone putty is thicker on the maximum of the elevation. At the thicker part the silicone putty splits up, one more developed than the other (Fig 14).
‘
Series 4: Erosion, sedimentation and no resting
The deformation of model 6 in the first 5 cm displacement is the same as described in series 2, even though the erosion cycle happened more frequently. There was a difference in the rising of the silicone putty and that is that there was only one significant uplift and thereby there was only one silicone putty zone traceable in this model (Fig 15). The R-faults made an angle of 10 degrees clockwise with the basement fault. At that moment the silicone putty was already visible and the erosion process stopped and the first sedimentation cycle was applied to the model.
After a total of 7 cm of total deformation the contours of the silicone putty ridge reappeared on the surface. While the length of the visible silicone putty was still the same as in fig 15.b , the total area of the uplift increased. The lower situated R-faults and P-faults also grew through the sedimentated layer. The Y-faults became more detectable above the basement fault. Together these faults formed an anastomosing network right above the basement fault (Fig 15.c).
Over time the length of the silicone putty zone grew and rose further out of the sedimentated layer. The silicone putty ridge made an angle of 7 degrees counterclockwise with the basement fault, which means that the angle between the R-faults and the silicone putty ridge was 17 degrees. The small elevations caused by R-faults and P-faults became progressively more deformed by the increase of Y-faults.
From the beginning of the sedimentation cycle to its end (total displacement in this cycle was 5 cm) the silicone putty zone grew from 4 cm in length to 7 cm, which gives a growth ratio of 175% (Fig 15). This zone was stationary during deformation. The average width of the shear zone at the end of this sedimentation cycle of the model is circa 2 cm.
Cross section 16.a and 16.b are found in the silicone putty zone that is discussed above and cross section 16.c is located further in the sense of the direction of movement. All cross sections show a positive flower structure with its origin in the basement fault. This structure forms one pop-up at cross sections 16.b and 16.c, while cross section 16.a has two small pop-ups. A small-scale pop-up structure is developed at the right side of all of the cross sections due to a far-reaching reverse fault.
The two types of cross sections differ in the shape of the silicone putty. The cross sections situated in the distinguished silicone putty zone are stretched on the left side of the pop-up structure and thicker on the right side. The other type of cross section shows that the silicone putty has penetrated down into the positive flower structure and has squeezed the pop-up structure. The rise of the diapirs is roughly the same: a small elevation of the putty approximately above the basement fault. Cross section 16.a is further developed: on the top of the elevation the silicone putty has spread.
Series 5: Erosion, sedimentation and resting
The 12.5 cm displacement of model 7 gave the same deformation as observed at model 6 of series 4. The two silicone putty zones moved a little in sense of the bulk shear. From that moment on the machine stopped and sedimentation is applied to the model.
At the beginning of the six hour long resting phase the average width of the shear zone is roughly 2 cm.
The two silicone putty zones grew from 3.5 and 4 cm before the first sedimentation phase to respectively 5.5 and 6.5 cm after the resting phase, which gives the average growth ratio of 160 %. No growth of the silicone putty was detected during the resting process.
The cross sections consist of a relative wide positive flower structure with its origin in the basement fault. One symmetrical pop-up is formed by this flower structure and one small-scale uplift on the left side is made by a far-reaching reverse fault. The cross sections differ in the shape of the silicone putty layer: cross section 18.a shows that the silicone putty is lightly stretched out along the flanks of the large pop-up and on the highest point of elevation the silicone putty pierces through the sand layers. Cross section 18.b has a narrow pop-up structure and along the right flank of the pop-up structure the silicone putty has undergone large stretching, decreasing the thickness to 1 mm.
DISCUSSION
The analogue models are used to determine the evolution of the internal and external structure of salt tectonics in pure strike slip systems. First the models are compared to each other and then they are interpreted and put in a broader context of the literature.
Summary of model results
Difference structures on the surface of the reference model and the salt models
The en echelon R-faults in the pure brittle model are regularly distributed and make an angle of 18 degrees with the basement fault, while the salt models show much more closely spaced en echelon R-faults that make on average an angle of 10 degrees with the basement fault in the beginning of deformation. As deformation continues these R-faults rotate clockwise to an average angle of 14 degrees (Table 2). The R-faults of series 4 and 5 do not change in angle.
At the final state of the pure brittle model average shear zone width is around 5.5 cm, which is significantly larger than the average shear zone width of the salt deformation series (around 2.2 cm). There is no significant difference in the shear zone width between the series.
Table 2: Overview of the orientation of the external structures and the average shear zone width
Begin angle R-faults with basement fault End angle R-faults with basement fault Angle between silicone putty ridge and
R-faults Average shear zone width (cm)
Pure brittle model 18” 18” Not applicable 5.5
Series 1: no erosion, sedimentation and resting
10”
14”
Cannot be determined
2.0
Series 2: erosion, no sedimentation and resting
8”
19”
27”
2.0
Series 3: erosion, resting and no sedimentation
10”
17”
27”
3.0
Series 4: erosion, sedimentation and no resting
10”
10”
17”
2.0
Series 5: erosion, sedimentation and resting
10”
10”
17”
2.0
All of the salt models show a small extensional component along the R-faults (Fig 19) (best described in series 1 as the rest of the experiments underwent an erosion process). As deformation continues the amount of extension grows until the formation of Y-faults. These Y-faults cut the R-faults off and thereby end the growth of the extension.
Table 3: Overview of the evolution in length of the silicone putty zones
Initial length silicone putty zone (cm) Final length silicone putty zone (cm) Growth ratio silicone putty zone
Pure brittle model Not applicable Not applicable Not applicable
Series 1: no erosion, sedimentation and resting
Cannot be determined
Cannot be determined
Cannot be determined
Series 2: erosion, no sedimentation and resting
3.5 & 5.0
4.0 & 6.0
116 %
Series 3: erosion, resting and no sedimentation
6.0
8.0 (no growth in resting phase)
133 %
Series 4: erosion, sedimentation and no resting
4.0
7.0
175 %
Series 5: erosion, sedimentation and resting
3.5 & 4.0
5.5 & 6.5 (no growth in resting phase)
160 %
Effect of erosion, sedimentation and resting
The silicone putty in series 1 is not detectable in the shear zone, while the models with an erosion process have one or two traceable silicone putty zones. The visible salt ridges found on the models make a counterclockwise angle with the basement fault (Table 2). These ridges grow as the deformation continues (Table 3) and some remain stationary while others move in the sense of direction of the basement fault.
The models with sedimentation cycles, series 4 and 5, have a larger growth ratio of the silicone putty zone than the series 1 to 3.
Series 3 and 5 have a resting phase of six hours where the silicone putty did not advanced further above the surface.
Cross sections
Model 1 and 2 of series 0 are used to determine the preferred location of silicone putty layer in the construction of the rest of the experiments. Model 1 has the silicone putty layer directly on the two plastic plates, while model 2 has it in the middle of the experiment. The silicone putty layer of model 1 forms right above the basement fault little ridges in a wavy pattern, which means the vertical displacement of the silicone putty is limited to a minimum. Under the silicone putty layer of model 2 a positive flower structure with vertical thrusting forms. This lifts the ductile layer and therefore the weight of the overlying brittle layers decreases, which allowed the silicone putty to rise up towards the surface.
All of the cross sections of the experiments consist of a positive flower structure where the vertical reverse faults splay up from the basal cut. The amount of faults and of positive vertical displacement varies between the series. Above the basement fault the positive flower structure forms one or two pop-ups under the silicone putty layer. This resembles a smaller scale version of the structure of the reference experiment has (Fig 8). Due to the rapid uplift the silicone putty stretches along the flanks of the pop-up structure, while it is compressed at the highest point of elevation. Only in series 1 where there is no erosion, the silicone putty does not rise towards the surface and forms ridges in a wavy pattern. If the silicone putty rises the ductile structures are very narrow (between one and five millimeters) and sometimes split into two separate begin phases of diapirs (Fig 12.a; Fig 14; Fig 16.a).
The upper brittle layer (0.6 cm) is so thin that the downward propagation of the surface faults is difficult to detect and therefore the effect of erosion and sedimentation on these faults cannot be determined.
Interpretation
The structures formed by pure strike slip in a pure brittle model are complex and difficult to interpret. As these structures have influence on the geometry of the salt layer, the total deformation of the system becomes even more complex: the structures of the underlying brittle part of the model become trapped underneath the salt layer. The pop-ups beneath the salt layer are needed to reduce the weight of the overlying sand what triggers the formation of diapirs. It also causes the salt layer to stretch to a minimum thickness of one millimeter along the flanks of the uplift (Fig 18.b) and therefore the supply of new salt decreases heavily. This produces narrow diapir structures as the salt rises along the faults of the shear zone through the brittle layer. These diapirs are relatively thinner than the diapirs formed by the analogue models with thin-skinned extension of Vendeville & Jackson in 1992.
The ease of ascending of the salt layer highly improves as the model undergoes an erosion process. The sedimentation process pushes the salt towards the surface and therefore improves the growth in length of the visible salt on the surface, while resting has no effect on the salt, even if the salt was already above the surface. This can be explained by the two factors that control the diapir growth: the reduced supply of salt and the thickness of the diapirs.
A plausible explanation for the remarkable orientation of the salt ridges is that the helicoidal geometry of the positive flower structure (Fig 20) pushes the salt layer towards the surface, which forces the salt to flow over the fault plane and therefore making the counterclockwise angle with the basement fault. This observation can be used to determine the shear sense of a pure strike slip fault, however this fragile structure can easily be deformed in nature, making it difficult to detect this ridge at all.
‘
COMPARISON TO DATA OF THE NORTH SEA
The stratigraphy of the North Sea consists of a thick sequence of recurrent marine evaporates (like gypsum and halite) of the Zechstein group (Late Permian age) that buries the Rotliegend group (Kombrink, et al., 2011) (Fig 21). When the Zechstein group was deposited, the evaporates remobilized across the central and southern North Sea (Davison, et al., 2000). This had a large effect on the petroleum reservoirs within the North Sea (seal, trap, reservoir quality) and therefore the dynamics of salt tectonics must be fully understood by the hydrocarbon industry (Harding & Huuse, 2015). In various parts of the North Sea large dextral strike slip faults are found underneath the Zechstein group.
Until today, the dynamics of salt tectonics in the North Sea are still not fully understood. 3D seismic data and well log interpretation allows us to better understand the interaction between salt tectonics and sedimentation (Harding & Huuse, 2015; Kombrink, et al., 2011). With the results of this thesis we can reanalyze some of these seismic sections. The precise location of the discussed seismic section is found on figure 22.
The red block on the seismic section (Fig 23) is positioned right above a strike slip fault and surrounds the Zechstein group and a small amount of the overlying and underlying groups. Under the Zechstein group a significant cluster of reflectors can be distinguished. The reflectors display a vertical displacement, more specifically a positive flower structure. The influence of this structure on the lower boundary of the Zechstein group is very small, but in the middle the reflectors curve upwards indicating ascending salt. The upper boundary of the Zechstein group is slightly folded.
The analogue models and this seismic section show some similarities, but also some differences. In both cross sections a positive flower structure positioned beneath the salt layer and right above the strike slip fault the salt tries to ascend towards the surface. The difference between the laboratory model and nature is the missing pop-up structure beneath the salt of the seismic section. It is possible that there is a small pop-up, but the analogue models show a trapped pop-up that largely elevates the salt. The thin diapir structure is also missing in the seismic section. These differences can be explained by the thickness of the Zechstein salt layer. All of the analogue models have had the same salt thickness and therefore the effect of a variating thickness is still unknown.
The salt ridges seen in the analogue models are difficult to distinguish in figure 22. This can be due to the consistent deformation of the area, while the analogue models are homogenous and only underwent one deformation phase.
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CONCLUSIONS
From the seven analogue experiments on deformation of salt in pure strike slip settings, we can conclude the following statements:
The pop-up structure formed by a positive flower structure beneath the salt layer is essential for the formation of diapirs. It helps to reduce the overlying weight of the sand.
As erosion and sedimentation are absent (series 1), the salt will not grow towards the surface. Reducing the weight of the overlying sand by erosion helps the salt to rise through the brittle layers. The salt rises along the faults of the shear zone, but the final width of the diapir is very small.
Sedimentation and resting processes have less effect of the salt diapir formation than the erosion process. As deformation continues the salt diapir can rise through the sedimentated layer as long as the layer is thin enough,
The salt ridges found on the surface make a counterclockwise angle with the basement fault, while the R-faults make a clockwise angle. These ridges grow in length as deformation continues and they can move in the direction of shear.
RECOMMENDATIONS
For future studies that investigate salt tectonics in strike slip settings more information on the structures below the salt layer and on the effect of the thickness of the salt layer can be obtained. The salt ridges found on the surface of the model all make a counterclockwise angle with the basement fault. An explanation is given in this thesis, but it needs further research.
ACKNOWLEDGEMENTS
I would like to thank my supervisors Prof. Dr. Dimitrios Sokoutis and Dr. Jeroen Smit for their supervision and their motivation. EBN and Marten ter Borgh are thanked for providing the data of the North Sea and finally Stefan Peeters is thanked for his help with constructing the models.
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