2011 Analogue modelling of extensional tectonics with a curved basal plate
A.P. van Nunen
1615734
VU Amsterdam
07-09-2011
Supervisors:
Dr. F. Beekman
Dr. D. Sokoutis
A.P. van Nunen
1 Picture on the cover: Cross section made of model 1 Masterthesis VU Amsterdam 07-09-2011
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
ABSTRACT
This report is based on an analogue modeling study done at the tectonic laboratory at the Vrije
Universiteit Amsterdam, the Netherlands. The starting point for this research is an analogue modeling
study by Corti et al. (2007), in which the western branch of the East African Rift System was
investigated.
The research presented in this report consists of two parts; first repeating a part of the analogue
modelling done by Corti et al. (2007) and second to extend their study by changing some of the
modeling parameters like the amount of extension and the strain rate.
The objective is to look at the effects on the extensional structures when changing these parameters.
To reach these goals, four analogue models are made; one to compare with the model made by Corti
et al. (2007), one model with a different amount of extension and two models in which the strain rate
is changing. The differences and similarities between the models will be observed, described and
explained.
From comparing the model made by Corti et al. (2007) with the model made during this research it
appears that a higher power law (n-value) will result in a wider rift zone with more grabens and a
decrease in grain size results in steeper faults and narrow rift zones, due to an increase in cohesion.
A higher amount of extension result in more and more pronounced fault zones in which the small
faults are linked together to form larger faults. At second a higher amount of extension favors block
rotation and a decrease in dip angle, because planar rotational faults can accommodate a higher
amount of extension in comparison with planar non-rotational faults.
An increase in strain rate results in a wider deformation zone with several grabens due to an increase
in silicon putty strength and therefore an increase in brittle-ductile coupling.
2
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3 Masterthesis VU Amsterdam 07-09-2011
CONTENT INTRODUCTION ............................................................................................................................................. 4
DESCRIPTION OF THE RESEARCH AREA .......................................................................................................................... 4
Location and morphology .............................................................................................................................. 4
Relation between landforms and tectonics ................................................................................................... 5
Main tectonic features .................................................................................................................................. 7
Kinematics ..................................................................................................................................................... 7
Transform and transfer fault zones ............................................................................................................... 8
Timing and rift propagation .......................................................................................................................... 8
STRUCTURES DEVELOPING DURING EXTENSIONAL TECTONICS ............................................................................................ 9
CHAPTER 1 MODEL SET‐UP AND MATERIALS ........................................................................................... 11
1.1
MODEL CORTI ET AL. (2007) .................................................................................................................... 11
1.2
MODELS MADE DURING THIS RESEARCH ....................................................................................................... 12
CHAPTER 2 RESULTS ................................................................................................................................ 14
2.1
COMPARING THE MODEL CORTI ET AL. (2007) MADE WITH MODEL 1 ................................................................ 14
2.1.1
Short description of model CRS21 ................................................................................................. 14
2.2.2
Comparing model CRS21 with model 1 ......................................................................................... 15
2.3
COMPARING THE MODELS WITH DIFFERENT AMOUNTS OF EXTENSION ................................................................ 17
2.3
COMPARING THE MODELS WITH INCREASING STRAIN RATE ............................................................................... 21
CHAPTER 3
DISCUSSION AND CONCLUSIONS ........................................................................................... 25
ACKNOWLEDGEMENTS ................................................................................................................................ 28
REFERENCES…. ............................................................................................................................................ 29
APPENDIX 1 MEASURING THE VISCOSITY AND DENSITY OF THE SILICON PUTTY LAYER .............................. 31
VISCOSITY MEASUREMENT ....................................................................................................................................... 31
DENSITY MEASUREMENT ......................................................................................................................................... 33
APPENDIX 2
MODEL 1
MODEL 2
MODEL 3
CALCULATING THE CRITICAL STRESS ...................................................................................... 34
4 CM/HOUR ...................................................................................................................................... 34
1.64 CM/HOUR ................................................................................................................................. 35
7.5 CM/HOUR ................................................................................................................................... 35
APPENDIX 3 TOP VIEW PHOTOS OF THE DIFFERENT MODELS .................................................................... 36
APPENDIX 4 CROSS SECTIONS OF THE DIFFERENT MODELS ........................................................................ 40
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
INTRODUCTION
This report describes an analogue modeling study done at the Vrije Universiteit Amsterdam. Different
extensional models are made. The basis of this research is an analogue modeling study by Corti et al.
(2007), in which the Western Branch of the East African Rift System was investigated. The models
they made have a curved basal plate, based on the geometry of the East African Rift System.
All models described in this report use this curved basal plate. The research project is made up of two
parts; first repeating a part of the analogue modelling done by Corti et al. (2007) and second to
extend their study by changing some of the modeling parameters like the amount of extension and
the strain rate.
The objective is to look at the effects on the extensional structures when changing these parameters.
The differences and similarities between the models will be observed, described and explained.
DESCRIPTION OF THE RESEARCH AREA
LOCATION AND MORPHOLOGY
The East African Rift System (EARS) is located in the eastern part of Africa; it originates in the north in
the Afar Triangle where it is linked to the Red Sea Rift towards the northwest and to the Carlsberg
Ridge by way of the Gulf of Aden in the east (figure 1). An excellent overview of this area is given by
Chorowicz (2005).
FIGURE 1 OVERVIEW OF THE DIFFERENT RIFTZONES. EARS= EAST AFRICAN RIFT SYSTEM, CR= CARLSBERG RIDGE, RSR= RED SEA RIFT North of Lake Victoria, the EARS splits into two branches; the Eastern branch has a length of 2200 km
and starts at the Afar triangle in the north, then following the main Ethiopian rift, the Omo-Turkana
lows, the Kenyan (Gregory) rifts, and the branch ends in the basins of the North-Tanzanian
divergence in the south. The Western branch has a length of 2100 km and follows Lake Albert, Lake
Edward, Lake Kivu and Lake Tanganyika (Chorowicz, 2005). South of Lake Victoria the two branches
cross each other again (Nolet & Mueller, 1981). Lake Victoria is situated in the northern part of the
Tanzanian Craton, which is surrounded by the two branches of the EARS (figure 2). This report is
mainly focused on the Western branch.
FIGURE 2 THE LEFT FIGURE SHOWS THE MAIN STRUCTURAL FEATURES OF THE EASTERN AND WESTERN BRANCHES OF THE EARS. (MODIFIED AFTER CORTI ET AL., 2007). THE RIGHT PICTURE SHOWS A RELIEF MAP OF THE EARS. AAE=ADDIS ABABA, LWI =LTIRO. NAI =NAIROBI (AFTER NOLET & MUELLER, 1981). 4
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5 Masterthesis VU Amsterdam 07-09-2011
At the surface the EARS is well visible as an aligned successions of individual tectonic basins, with a
total length of several thousands of kilometres. Each single basin is controlled by faults and has typical
dimensions of about hundred kilometers long and tens of kilometres wide and is bordered by uplifted
shoulders. The basins can be empty or filled with sediments and/or volcanic rocks (Chorowicz, 2005).
The western branch of the EARS includes 3 successive rift segments, each with its own structural
trends, types of basement and magmatic versus non-magmatic contexts (Albaric et al., 2008). These
basins are all bordered on both sides by high relief. Figure 3 shows the different lakes on a simplified
structural map.
1. The northernmost segment has a length of ca. 500 km and a width of 50 km. It includes Lake
Albert, Lake Edward and Lake Kivu basins. This segment follows a dominant NE-SW trend and
is composed of en echelon asymmetrical half-graben basins with an upper Miocene age.
2. The central rift segment has a length of 300 km and an NNW-SSE trend. This segment starts
in the north at Lake Kivu en ends at the northern extremity of Lake Tanganyika. Its structure
is directly controlled by the reactivated fault zones from the Ubendian orogeny (1110 Ma).
3. The southern rift segment extends from Lake Tanganyika in the north to Lake Malawi in the
south. According to Albaric et al. (2008) this segment “corresponds to a pronounced
cartographic anti-clockwise deflection of the roughly NS Cenozoic rift axis along NW–SE
oriented inherited structures, initiated during the Eburnean orogeny (ca. 2100 Ma) and later
rejuvenated as steep Ubendian strike-slip shear zones.” During the Jurassic, these zones of
weakness were reactivated as extensional master faults, which formed, example, the halfgraben of Lake Rukwa.
FIGURE 3 SIMPLIFIED STRUCTURAL MAP OF THE CENTRAL AND SOUTHERN EAST AFRICAN RIFT SYSTEM. (MODIFIED AFTER ALBARIC ET AL., 2008) RELATION BETWEEN LANDFORMS AND TECTONICS
The basins of the northern segment are characterized by an arched succession of half-grabens, most
of the basins have a lake inside. They are developed in the Kivu dome with the Virunga NE-trending
group of large volcanoes in the centre. When looking at the lakes that are encountered in this
segment, it can be seen that the shoulders at the western side of the lake are higher than the
shoulders at the eastern side of the lake. In the north, Lake Albert has a rift floor elevation of 618m
with eastern shoulders more than 1300 m high and western shoulders more than 2200 m high. Going
to the south Lake Albert has a rift floor at 200 m and shoulders at 2300 m (west) and 1600 m (east).
The southernmost basin of this segment, Lake Kivu, has a rift floor of 1420 m and is bordered by
3000 m high shoulders.
Moving from Lake Kivu to Lake Tanganyika the rift floor drops from 1420 m to 773 m, still with well
developed shoulders; 3400 m in the west and 2600 m in the east. The 700 km long and maximal 70
km wide Tanganyika rift is almost entirely covered by Lake Tanganyika (650 km long). Lake
Tanganyika can be divided in two main basins, separated by a strike slip fault (figure 4). The northern
basin is again divided by NW-striking faults into three asymmetric half-graben sub-basins; the
Bujumbura (350 m deep), Rumonge (1150 m deep) and Kigoma sub-basins (1310 m deep). The
western side of these basins is formed by a major listric normal fault with curved trace and a
maximum vertical throw of 10000 m. The southern basin is also divided into sub-basins by NW-
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
striking faults, each more than 300 km in length. In contradiction to the northern sub-basins, these
basins are not always bordered by high shoulders, but by plains and narrow hills with an elevation of
about 1000-2000 m.
FIGURE 4 GENERAL MAP OF THE LAKE TANGANYIKA REGION SHOWING THE SEVEN STRUCTURAL SUB‐BASINS THAT FORM THE TANGANYIKA TROUGH AND THE MAIN RIVERS THAT PARTICIPATE TO THE LAKE TANGANYIKA HYDROLOGY. 1=BUJUMBURA SUB‐BASIN; 2=RUMONGE SUB‐BASIN; 3=KIGOMA SUB‐BASIN; 4=KALEMIE SUB‐BASIN; 5=MOBA SUB‐BASIN; 6=MARUNGU SUB‐BASIN; 7=MPULUNGU SUB‐BASIN. TRM=TANGANYIKA‐RUKWA‐MALAWI FAULT ZONE. (MODIFIED AFTER PFLUMIO ET AL., 1994). In the southern part of the western branch Lake Rukwa and Lake Malawi can be found. The strike-slip
faults that form the boundaries of Lake Rukwa are connected to the strike-slip faults of Lake
Tanganyika. The lake has a length of more than 200 km and is 60 km wide with the water surface at
around 800 m. The last rift discussed in this part is the Malawi rift (650 km long and 60 km wide).
This rift is mostly covered by Lake Malawi (500 km long and a water surface at 472 m). The shoulders
that are bounding this lake are highest in the central part (more than 2000 m) and are descending
towards the south to 1500 m. The rift is composed of four half-graben basins which are separated by
NW-striking faults. The eastern and western border faults have an arc shape.
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7 Masterthesis VU Amsterdam 07-09-2011
MAIN TECTONIC FEATURES
Linking the morphotectonics of the EARS to tectonics, it can be concluded that the morphotectonics
are under control of divergent movements, including localized extensional strain in the continental
lithosphere. The main tectonic features of the EARS can be observed both in brittle crust and in
Lithospheric mantle: the brittle crust is deformed by faulting and subsidence, resulting in elongated
and narrow rifts while the lithospheric mantle show some sharply defined ductile thinning which
includes ascension of asthenospheric mantle
The most characteristic features in the rift system are narrow elongated zones of thinned lithosphere
which are related to deep intrusions of asthenosphere in the upper mantle. These deep intrusions of
asthenosphere are expressed by thermal uplift and the creation of rift shoulders (figure 5).
FIGURE 5 GENERALIZED E‐W SECTION ACROSS A CONTINENTAL RIFT SHOWING LATE STAGES OF ITS FORMATION (AFTER OMENDA, 2005).
The main tectonic features visible at the surface are normal faults. But there are also strike-clip,
oblique slip and some reverse faults visible. The extension is resulting in widespread, syn-depositional,
open fractures comprising tension gashes. The volcanism which occurs at some time periods is closely
related to tectonics.
When looking at the fault characteristics, three components can be distinguished: first the vertical
component, which is dominant, at second the horizontal strike-slip component and a last horizontal
transversal component. The faults are listric and at a depth of 20-30 km connected with low-angle
detachment levels. The largest faults have a length of several tens of kilometers and a vertical throw
of several kilometers. They dip at the surface with an angle of ca. 65°. Per graben there is only one of
these major normal and detachment faults, forming asymmetric roll-over structures. The other faults
in the graben are much smaller with a length of hundreds of meters or less.
The listric faults may be open in their upper part and filled with sedimentary breccias which may
transform into tectonic breccias due to subsequent movement of the fault. In general it seems that
normal faults do not offer opening that would permit magma ascension and therefore volcanoes along
normal faults are infrequent. The transcurrent faults have a dominant strike slip component and their
length can be more than 500 km. in some places folds occur, which are related to strike-slip faults,
these folds are also syn-depositional. The tension fractures, in which the horizontal transversal throw
component predominates, are widespread in rift zones. They may look like valleys with floors mad of
breccias and olistolite fill. These fractures are the first structures to form and are immediately filled
with breccias or magma.
KINEMATICS
There are large uncertainties about the kinematics of the EARS. According to local earthquake
mechanisms Fairhead & Girdler (1972) proposed an E-W to NE-SW extension. A couple of years later
Boccaletti et al. (1998) came with a two directions model for the Ethiopian rift, with NW-SE extension
before the beginning of the Quaternary and E-W extension after. And in approximately the same
period Bosworth & Strecker (1997) came with a more complex pattern, based on analysis of the
paleostress fields. They concluded that the rift extensions in Kenya vary from E–W to ENE–WSW and
NE–SW direction between 12 and 0.6 Ma, to turn to a NW-SE direction since 0.6 Ma. All directions
mentioned above are based on local paleostress analysis, but the regional scale kinematics is not
clear.
So far, Chorowicz (2005) concluded that there are two types of movement that affect the same faults,
alternating through time and space.
1. NW-SE drifting movement of large continental blocks, which is coherent with the overall
geometry of the rift system.
2. E-W local extension, especially in the eastern branch. This is mainly gravity sliding along the
major border faults, triggered by high relief.
A combination of these two types is also possible according to Clifton et al. (2000).
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
8
TRANSFORM AND TRANSFER FAULT ZONES
The transform fault zones related to the EARS are NW-striking faults, which are linking rift basins
together and are parallel to the main extensional movement. These faults have a major strike-slip
component. The eastern branch and the two main segments of the western branch are linked by
these intracontinental transform zones, forming a unique breaking line in eastern Africa that looks like
oceanic ridges linked by oceanic transform faults. In the western branch of the EARS two main
transform fault zones can be observed. First the Aswa transform fault zone, located at the northern
part of the western branch and continues to the southeast to end south of the Kenyan rift. This zone
consists of several large left-lateral NW-striking faults. According to several parameters, this transform
zone represents early stages of a right-lateral transform fault. The second transform zone is the
Tanganyika-Rukwa-Malawi fault zone. This right-lateral zone connects the two main segments of the
western branch. It forms complex left-stepping en-echelon fault and fold zones. The folds are
developed in the narrow zones between dextral strike-slip faults and will affect the basement. These
folds also explain some of the morphology (e.g. synclines forming low plains in the border lands
instead of high shoulders). The exact quantity of movement of these rift zones is not well known, but
might be ca. 13 km.
In contradiction to the transform faults that are located outside the rift basins, the transfer faults are
located inside the rift basins and correspond to a change in fault geometry. The transfer fault is
parallel to the movement of extension in a basin. The zone in which this change occurs is called a
accommodation zone. Both concepts can be observed in the western branch of the EARS. In the
Malawi rift, the main boundary fault is changing its position along the basin as a result of transverse
NW-striking faults, resulting in a succession of different basins, separated by topographic heights and
each with a length of about 70 km and a width of about 40-60 km. This type of transfer faults (NWtrend and located inside the basin) is very common in the EARS. The southern end of Lake Kivu
illustrates the concept of an accommodation zone. In this area NE-striking faults were reactivated as
reverse faults by NW-trending movement, causing local compression. These NE-striking faults are
submitted to strike-slip deformation because they deduce a NE-trending movement.
The combination of transform, transfer and accommodation zones divide the EARS into individual
basins with a length of 50-100 km.
TIMING AND RIFT PROPAGATION
The different stages of rifting in the EARS can be observed in figure 6. This timeline is based on the
information given by Chorowicz (2005).
FIGURE 6 TIME LINE SHOWING THE MAJOR STAGES IN EARS RIFTING. THE PERIOD BETWEEN 10 MA AND 5 MA IS ENLARGED TO FIT ALL THE STAGES IN. A.P. van Nunen
9 Masterthesis VU Amsterdam 07-09-2011
The EARS has a progressive development to the south with mean rates between 2.5 and 5 cm/year.
The rift fracturing and subsidence occurred during the late Miocene and started in the north in the
Afar and Kenyan rift and moved to the south to the Virunga and central Tanganyika. The rift system is
still moving.
Different evolutionary stages can be observed at the various rift segments, depending on the age of
rift initiation and distance to the Eulerian pole.
1. Pre-rift stage: this stage is characterized by horizontal movements resulting in en-echelon
tension gashes and many strike-slip and oblique-slip faults, all with a small offset. The stress
field is that of a strike-slip fault with σ1 and σ3 horizontal. The volcanism has a tholeiitic
hyperalkaline composition. The morphology is characterized by swamps and shallow lakes.
The intrusion of asthenosphere is discrete. This stage can not be observed in the western
branch of the EARS.
2. Initial rift stage: this stage is related to subsident and divergent movements with frequent
earthquakes. The fault density is lower and the faults are most of the time extensional
oblique-slip faults. The stressfield is tensional with a vertical σ1 and a horizontal σ3. The
volcanism consists of contaminated alkaline magma. The resulting basins are composed of
halfgraben throughs with a roll-over geometry and pronounced shoulders. These basins can
be tens of kilometres in length with a width of 30-40 km. The basins are separated from each
other by transform, transfer or accommodation zones. The asthenospheric intrusions in the
lithosphere are pronounced. This stage can be observed in the Malawi rift.
3. Typical rift stage: characterized by a well defined rift valley and thick graben deposits. Also in
this stage (low-energy) earthquakes occur. This stage is dominated by normal faults with
most of the deformation concentrated along the major border faults of each graben. Under
the influence of gravity local tension can occur. The smaller basins from the previous stages
are now connected to form longer basins with most of the time a great lake inside, the
shoulders of these basins are high. The intrusion of asthenosphere in the crust starts. A good
example of this stage is the northern part of the Tanganyika rift.
4. Advanced-rift stage: the asthenosphere is intruding into the crust, considered as the first
appearance of oceanic-type material. Alkaline volcanism is driven by large tension fractures.
This stage is also not visible in the western branch of the EARS.
5. Oceanic rift stage: the Afar region, with oceanic crust.
STRUCTURES DEVELOPING DURING EXTENSIONAL TECTONICS
According to van der Pluijm & Marshak (2004), there are two main models of rifting. First the model
of symmetric horsts and grabens (figure 7a), in which pairs of normal faults dipping towards each
other outline grabens, while pairs of normal faults dipping away from each other outline horsts. At
depth these faults simply die out. The second model is the asymmetric rift model with tilted fault
blocks and half grabens, as shown in figure 7b. In this model extension is accommodated by
displacement on arrays of sub parallel normal faults, most of which dip in the same direction. These
faults are listric and merge at depth with a regional sub horizontal basal detachment.
FIGURE 7 MAIN RIFT MODELS. (A) CROSS SECTION ILLUSTRATING THE CONCEPT OF SYMMETRIC HORSTS AND GRABENS. (B) CROSS SECTION ILLUSTRATING THE CONCEPT OF TILTED FAULT BLOCKS AND HALF GRABENS. (AFTER VAN DER PLUIJM & MARSHAK, 2004) Normal faults can have different geometries as displayed in figure 8; the fault planes can be planar
which means the dip remains constant with depth, or listric in which the dip decreases with depth.
The listric faults can be further dived in listric faults with a tilted surface or listric faults with a roll over
anticline. Both structures form under gravity.
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
FIGURE 8 GEOMETRY OF NORMAL FAULT ARRAYS. (A) PARALLEL ROTATIONAL FAULTS. BEFORE FAULTING (TOP), THE FAULTS ARE PARALLEL AND NOT CURVED. AFTER FAULTING (BOTTOM), FAULT BLOCKS ARE TILTLTED. (B) LISTRIC FAULTS. BEFORE FAULTING (TOP), THE FAULTS SHALLOW WITH DEPTH AND MERGE WITH A DETACHMENT. AFTER FAULTING (BOTTOM), THE BLOCKS HAVE MOVED. THE BLOCK TO THE RIGHT CURVES DOWN TO MAINTAIN CONTACT WITH THE FOOTWALL. (C) A ROLLOVER ANTICLINE. THE SAME PRINCIPLE AS SHOWN IN (B) BUT ONLY ONE LISTRIC FAULT. (MODIFIED AFTER VAN DER PLUIJM & MARSHAK, 2004)
The secondary faults which form after the formation of the breakaway can be divided in antithetic and
synthetic faults; an antithetic fault is a fault that dips in the direction opposite to that of the major
fault and a synthetic fault is a fault that dips in the same direction as the major fault (figure 9).
FIGURE 9 ANTITHETIC AND SYNTHETIC FAULTS
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A.P. van Nunen
11 Masterthesis VU Amsterdam 07-09-2011
CHAPTER 1
1.1
MODEL SET-UP AND MATERIALS
MODEL CORTI ET AL. (2007)
The models Corti et al. (2007) made were crustal-scale models using both pure and simple shear
deformation. They consist of a basal ductile layer made of a mixture of corundum sand and silicone
overlain by a brittle upper crust analogue made of sieved Qz sand (tables 1 and 2). The models have
a width of 110 cm, and a height of 3.5 cm, extensional deformation was achieved through
displacement of a basal thin acetate sheet (with a maximal width of 25 cm (figure 10)) fixed to a
mobile wall, driven by a stepper motor. Motion of the basal sheet induced a central velocity
discontinuity (VD), whose displacement was transferred by viscous coupling at the base of the sand
layer. The VD localized the extensional deformation within the overlying crust and reproduced the
pervasive fabric corresponding to the weak zones at the craton’s western border. The velocity of the
extensional deformation was fixed at 5 cm h-1. During the deformation top-view photos are made to
monitor the model evolution and after the deformation the models were soaked in water and
sectioned to observe internal structures.
The authors build two different reference models; the first one (CRS21) was characterized by a
homogenous crustal rheology, the second model (CRS22) should simulate discrete zones of weakness
in the upper crust, achieved by inserting pieces of cardboard, with a dip of ~90° and a width of ~5
mm, through the sand layer down to the top of the silicone layer. The trend of these pieces was
varied to match the main Proterozoic belts. The introduction and removal of the cardboard results in
grain rearrangements, creating a zone of dilation; this zone obeys a Coulomb’s frictional slip criterion
and represents a zone of weakness because the coefficient of friction is lower than the internal friction
of the undisturbed sand. In this report only model CRS21 is used to compare with the models made
during this research.
FIGURE 10 SET UP OF THE MODELS CORTI ET AL. (2007) BUILD. THE UPPER FIGURE REPRESENTS THE TOP VIEW AND THE LOWER FIGURE REPRESENTS A SIDE VIEW IN WHICH THE PINK LAYER CONSISTS OF QZ‐SAND AND THE GREY LAYER CONSISTS OF A MIXTURE OF SILICONE AND SAND Composition
Brittle layer, dry quartz sand (>99% SiO2) with a
rounded grain shape and a grain size <250 μm
Ductile layer, mixture of silicone Mastic
Rebondissante 29 + corundum sand (100:60, %
weight)
Density,
kg m-3
Angle of
Internal Friction
Cohesion,
Pa
~1550
~40°
~65
n: 1.4
A: 3x10-7 Pa-n s-1
~1600
TABLE 1 CHARACTERISTICS OF EXPERIMENTAL MATERIALS (CORTI ET AL., 2007)
Models
nature
ρd, kg m-3
1600
2950
ρb, kg m-3
1550
2700
g, m s-2
9.81
9.81
hd, m
1x10-2
1x104
Power Law
Parameters
Hb, m
2.5x10-2
2.5x104
μ
0.8
0.6
τc, Pa
65
60x106
TABLE 2 SCALING PARAMETERS FOR BRITTLE AND DUCTILE DEFORMATION (CORTI ET AL., 2007) V, m s-1
1.4x10-5
~6.5x10-11
Rm
1.6
3.3
Rs
1.1
2.0
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
1.2
MODELS MADE DURING THIS RESEARCH
The first goal of this research is to repeat the experiment done by Corti et al. (2007). Because the
original model is very large (110 cm) and the resulting graben is supposed to be symmetrical, we
chose to shorten the model (figure 11). In these models extensional deformation is also achieved
through displacement of a basal thin plastic sheet (with a maximal width of 25 cm) fixed to a mobile
wall, driven by an engine.
The materials used in this research are partly different than the originally used materials: the brittle
material is the same: quartz sand, but instead of silicone + corundum sand, silicone putty is used
(table 3). All models have the same set-up: from the bottom to the top 1 cm silicone putty, 0.8 cm
white sand, 0.8 cm blue sand, 0.3 cm pink sand, 0.3 cm white sand, 0.3 cm blue sand.
FIGURE 11 MODEL SET UP. THE BRIGHT PART IS THE MODEL USED IN THIS RESEARCH AND THE WASHED OUT PART IS THE MODEL USED BY CORTI ET AL. (2007) Composition
Brittle layer, dry quartz sand (>99%
SiO2) with a rounded grain shape and
a grain size <250 μm
Ductile layer: silicon putty
Density,
kg m-3
Angle of Internal
Friction
Cohesion,
Pa
1520
60°
30
1574
Power Law
Parameters
n: 1.2279
TABLE 3 MATERIAL PROPERTIES USED IN THE ANALOGUE MODELLING. FOR THE CALCULATIONS OF THE DUCTILE LAYER PROPERTIES SEE APPENDIX 1 The second step is to expand the modelling done by Corti et al. (2007) There are 2 parameters that
are investigated:
1) The amount of total extension: the models made by Corti et al. (2007) have a total extension
of 1 cm. During the research described in this report one model is made with a total extension
of 1 cm and the other models are made with a total extension of 2 cm, this because the
structures are much more clear when using 2 centimeters of extension.
2) The velocity of extensional deformation: Corti et al. (2007) used a velocity of 5cm/hr. In this
research velocities varying between 1.64 and 7.5 cm/hr are used.
During the deformation top-view photos (figure 12) are made to monitor the model evolution and
after the deformation the models are sprinkled with water till they are soaked and afterwards
sectioned to observe the internal structures (figure 13).
All experiments have been done at the ISES Tectonic Laboratory of the Institute of Earth and Life
Sciences, Vrije Universiteit, Amsterdam.
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13 Masterthesis VU Amsterdam 07-09-2011
FIGURE 12 EXAMPLE OF SOME OF THE TOP VIEWS MADE DURING DEFORMATION OF MODEL 4
FIGURE 13 EXAMPLE OF SOME OF THE CROSS SECTIONS MADE AFTER DEFORMATION OF MODEL 4 A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
CHAPTER 2
RESULTS
This chapter contains the results of the analogue modeling. First an overview is given about the
different structures which can develop during extensional tectonics, followed by a summary of the
results Corti et al. (2007) obtained. Finally a description of the results obtained during this research.
Appendix 3 shows the large images of the top views of all models, and appendix 4 shows the cross
sections of all models).
Table 4 shows the amount of extension and the deformation velocity of all different models described
in this report. Model CRS21 (from Corti et al., 2007), model 1 and model 2 should have the same
deformation velocity, but because of problems with the engine this is not the case.
Name of the model
Model CRS21
Model 1
Model 2
Model 3
Model 4
Amount of extension
1 cm
1 cm
2 cm
2 cm
2 cm
Velocity of deformation
5 cm/hr
4 cm/hr
4 cm/hr
1.64 cm/hr
7.5 cm/hr
TABLE 4 AN OVERVIEW OF THE DIFFERENT MODELS DESCRIBED IN THIS REPORT 2.1
COMPARING THE MODEL CORTI ET AL. (2007) MADE WITH MODEL 1
2.1.1 SHORT DESCRIPTION OF MODEL CRS21
First looking at the overall structures visible, it can be observed that normal faults are bounding a
subsiding depression localized above the velocity discontinuity (VD) (figure 14b).
The model (CRS21) has a deformation pattern which was primarily controlled by the curvilinear shape
of the VD and the lateral variation in rift kinematics. A contradiction can be observed between the
central part where the linear normal faults with dominantly dip-slip kinematics and basins are
perpendicular to the direction of extension, and the lateral edges of the model were the structures are
increasingly oblique to the extension direction resulting in short, en echelon oblique-slip boundary
faults (Figures 14a–14b). The cross sections (figure 14c) show a typical basin asymmetry in which the
location of the border faults and the basin tilting is alternating over the length of the rift, caused by
the curvature of the rift system.
FIGURE 14 RESULTS OF ANALOGUE MODEL CRS21 (WITH HOMOGENOUS UPPER CRUST. (A) TOP‐VIEW PHOTO FOR 1 CM (=10 KM IN NATURE) OF EXTENSION. (B) LINE DRAWING OF STRUCTURES IN THE MODEL. (C) SCHEMATIC CROSS SECTIONS OF THE MODEL. ONLY A REPRESENTATIVE PORTION OF THE MODELS IS ILLUSTRATED (MODIFIED AFTER CORTI ET AL., 2007).
14
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15 Masterthesis VU Amsterdam 07-09-2011
2.2.2 COMPARING MODEL CRS21 WITH MODEL 1
Model CRS21 made by Corti et al. (2007) and model 1 made during this research both have an
extension of 1 cm and an extensional velocity of ca. 5 cm/hr, and therefore are expected to produce
similar structures.
FIGURE 15 TOP VIEW OF BOTH MODEL CRS21 AND MODEL 1 When comparing the top view of both models (figure 15) there are indeed similarities. Firstly, both
models have three main fault zones which are located on both sides of the VD. Secondly, both models
show a small bend near the center in the left boundary fault (figure 16, red circle), although it is not
very clear in model 1.
FIGURE 16 THE RED CIRCLE INDICATES A SMALL BEND VISIBLE NEAR THE CENTER IN THE LEFT BOUNDARY FAULT. THE GREEN CIRCLE INDICATES A SMALL FOURTH FAULT ZONE WHICH STARTS TO DEVELOP. THE LEFT FIGURE SHOWS MODEL CRS21 AND THE RIGHT FIGURE SHOWS MODEL 1 Another similarity between the two models can be seen at the main border faults, which are
composed of smaller en echelon boundary faults near the edges of the model, while the faults in the
central part of the model mainly consist of a few large faults. But in contradiction to model CRS21 the
en echelon faults in model 1 are not convincingly oblique to the extensional direction.
Besides these similarities there are also differences visible between the two models. A first difference
is the width of the rift zone (chart 1); the average width in model CRS21 is 8.11 cm while the average
width in model 1 is 7.01 cm.
CHART 1 WIDTH OF THE RIFT PLOTTED AGAINST ALONG‐STRIKE DISTANCE A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
While model CRS21 consists of mainly large faults which are all connected. Model 1 consist of many
smaller faults of which most are not connected very well, especially on the left side of the VD. Both
models have a small fourth fault zone which starts to develop very close to the VD, but in model
CRS21 this fault zone is dipping to the left and in model 1 this zone is dipping to the right (figure 16,
green circle). A fourth large difference can be seen in the location of the VD, in model CRS21 the VD
is more centered between the faults, in model 1 the VD is more to the right, with the faults extending
further to the left. A last difference between the two models can be observed in the lower left part of
the figure where model CRS21 has a minor fault dipping to the right and model 1 has a minor fault
dipping to the left.
FIGURE 17 SCHEMATIC CROSS SECTIONS OF THE MODELS. LEFT MODEL CRS21 AND RIGHT MODEL 1 The next step is to compare the cross sections made after the deformation (figure 17).
It can be seen that both models have an asymmetric graben structure in which the geometry of the
grabens varies with the length of the rift. But the geometry of the basins is different in both models,
model CRS21 only shows one single graben bounded by planar fault planes while model 1 also shows
a horst-graben structure with a few planar faults at the sides. The smaller grabens in model 1 mainly
consist of one major fault with an antithetic fault, this is clearly visible in cross section 1 (figure 18).
FIGURE 18 CROSS SECTION 1 MADE OF MODEL 1 SHOWING AN ANTITHETIC FAULT
The faults in both models are mainly planar with some somewhat listric faults in model 1. Another
large difference is the dip angle of the faults (chart 2), the faults in model CRS21 all have
approximately the same dip angle (between 61° and 72°) with a most common angle of 65-66°, while
the faults in model 1 have a variation in dipping angle; between 54° and 74°, and a most common
dipping angle of 67-68°. It appears if the offset in model CRS21 is larger than the offset in model 1. CHART 2 THE FAULT DIP ANGLES OF BOTHE MODEL CRS21 AND MODEL 1 16
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17 Masterthesis VU Amsterdam 07-09-2011
Although both models should show the same structures, there are differences visible as can be seen
above. These differences can be explained first by a difference in ductile material. While Corti et al.
(2007) used silicon putty with an n-value of 1.4 and a density of 1600 kg m-3, the silicon putty used in
this research has an n-value of 1.23 and a density of 1574 kg m-3. According to several authors
(O’Lenick et al., 2004; Pongjanyakul & Puttipipatkhachorn, 2007; Sokoutis, 1986) an increase in nvalue is related to an increase in viscosity, and a higher viscosity results in a stronger material. This
principle will explain the wider rift zone in model CRS21 and the geometry of the rift zone (one single
graben bounded by planar fault planes in model CRS21 and horst-graben structures with a few planar
faults in model 1). A second difference between the two models is the grain size of the brittle sand
layer. Model CRS21 used a grain size of <300 μm, while the sand layer of model 1 consists of grains <250
μm. According to Geyer (2007) and McClay (1990) a decrease in grain size results in an increase in
cohesion of the material, therefore the rifts are wider with an increase in grain size. This cohesion will
also affect the steepness of the faults, as a higher cohesion results in steeper faults. The small
difference in extension rate is negligible, as also can be noticed later on during the discussion of the
models with different strain rate.
2.3
COMPARING THE MODELS WITH DIFFERENT AMOUNTS OF EXTENSION
After comparing model CRS21 and model 1, we will now compare two models with a different amount
of extension, model 1 has 1 cm of total extension and model 2 has 2 cm of total extension, all other
parameters are kept the same.
The first photos to look at are the top view photos made during the deformation (figures 19 and 20).
FIGURE 19 TOP VIEW PHOTOS MADE DURING DEFORMATION OF MODEL 1. FROM LEFT TO RIGHT THE PHOTOS ARE TAKEN AFTER 4 MIN, 7 MIN, 10 MIN AND 13 MIN. FIGURE 20 TOP VIEW PHOTOS MADE DURING DEFORMATION OF MODEL 2. FROM LEFT TO RIGHT THE PHOTOS ARE TAKEN AFTER 0.5 CM, 1 CM, 1.5 CM AND 2 CM
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
In both models the first fault zone which starts to develop is the fault zone on the right side of the
model, this fault zone is located on top of the VD. In model 2 both fault zones, on the right and left
side, seems to start developing at the same time, but also in this case it seems if the fault zone at the
right side is somewhat more developed than the fault zone at the left side. Together with this fault
zone the left boundary fault also starts to develop, but the development of this fault zone is not as
quick. In the next “deformation phase” the small boundary faults starts connecting more and more to
form a few large boundary faults, at the same time an inner fault zone starts to develop at the left
side of the rift, this zone also starts with several small faults which are connected later on, this is well
visible in model 2; in model 1 this fault zone is not well connected. The last stage is the development
of a second inner fault zone, this time on the right side of the rift. In model 1 this fault zone is at the
very beginning and not very well visible, but in model 2 this fault zone is well visible, especially in the
lower half of the rift zone.
FIGURE 21 TOP VIEW OF THE DIFFERENT MODELS. LEFT MODEL 1 AND RIGHT MODEL 2 The top view figures made after deformation of both models (figure 21 and appendix 3) do show
some similarities. First it is visible that both models have three distinct zones of faulting, two zones at
the left side of the VD that form a graben structure and one zone at the right side of the VD. The
distance between the left boundary fault and the VD stays approximately the same in both models,
meaning that at a larger amount of extension the rift is expanding to the right, causing a
centralization of the VD. With 1 cm of extension the deformation zone has an average width of 7.01
cm (chart 3), while an extension of 2 cm causes a deformation zone with an average width of 8.23 cm
(chart 3).
CHART 3 WIDTH OF THE RIFT PLOTTED AGAINST ALONG‐STRIKE DISTANCE
In both models the lower part of the faults, located left of the VD, jumps to the right (figure 22).
FIGURE 22 THE RED CIRCLE INDICATES A SHIFT IN FAULTS TO THE FIGHT. LEFT MODEL 1 AND RIGHT MODEL 2. 18
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19 Masterthesis VU Amsterdam 07-09-2011
When looking at other differences between the two models, it can be seen that the boundary faults of
model 1 consist of many smaller faults, most of the time not connected or forming en echelon
structures while the boundary faults of model 2 consist of a few large faults that extend along a large
part of the rift and very less minor faults. These large faults also have more offset in comparison with
the smaller faults. In both models a fourth fault zone starts to develop very close to the VD, but this
zone is much more pronounced and has more offset in model 2. A last difference between the two
models can be observed towards the edges, the rift zone is longer and more extended towards the
edges in model 2.
FIGURE 23 SCHEMATIC CROSS SECTIONS OF THE MODELS. LEFT MODEL 1 AND RIGHT MODEL 2 The cross sections of both two models (figure 23 and appendix 4) show the same overall patterns:
asymmetric basins with horst and graben structures and some planar faults near the sides. Because of
the curvature in the basal plate the geometry of the basins varies through the rift. A difference is the
dipping angle of the faults (chart 4), the angles of model 1 range between 54° and 74° with a
common dip angle of 67-68° while the angles of model 2 range between 49° and 71° with a common
dip angle of 57-58°.
CHART 4 THE FAULT DIP ANGLES OF BOTH MODEL 1 AND MODEL 2
Another difference can be observed in the geometry of the grabens; the grabens of model 1 are
composed of major faults with smaller antithetic faults, while the grabens in model 2 are mainly
composed of two faults with the same size ending in the silicon layer at the base (figure 24). The
offset along the faults is another difference between the two models; the faults in model 2 have a
larger offset than the faults in model 1. In common the faults in model 2 are also more curved than
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
the faults in model 1, and the fault blocks between two planar faults are more tilted in model 2 in
comparison with model 1.
FIGURE 24 THE LEFT FIGURE (MODEL 1) SHOWS A GRABEN COMPOSED OF ONE MAJOR FAULT AND AN ANTITHETIC FAULT WHILE IN THE RIGHT FIGURE (MODEL 2) THE GRABEN IS COMPOSED OF TWO FAULTS WITH THE SAME SIZE AND ENDING IN THE SILICON It is expected, that the effect of more extension is that the structures become more pronounced and
more fault zones develop. This is also visible in this research; at 1 cm of extension, the two boundary
faults are visible, but they consist of many small faults partly connected, with increasing extension,
these small faults link together to form large faults and a sharp fault zone develops. As can be
observed in the top views made through time of both models, first the two boundary faults form, next
the inner fault at the lower side and at last the inner fault at the upper side starts to develop. The VD
is located in between the two inner fault zones, This location is also the reason why the lower inner
fault develops earlier, here the putty is pulled towards the north because of the moving plate and
below the upper inner fault the putty is on top of the moving plate so therefore relative quit and
stable. The rift structure is expanding towards both sides with ongoing extension, because the putty is
pulled apart both on the stable plate and on the moving plate. According to Buiter et al. (2006) a
larger amount of extension leads to a more pronounced block rotation and a decrease in fault dip.
This has to do with the difference between planar non-rotational faults and planar rotational faults
(Brug, 2011). Planar non-rotational faults can only accommodate 30% extension. Planar rotational
faults can accommodate more extension because rigid body rotation significantly increases the
horizontal component of slip on each fault (figure 25). The change in block tilting and dip of the faults
is also visible in the models made during this research.
FIGURE 25 FAULTING ON PLANAR NORMAL FAULTS (AFTER BRUG, 2011) 20
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21 Masterthesis VU Amsterdam 07-09-2011
2.3
COMPARING THE MODELS WITH INCREASING STRAIN RATE
The last comparison can be made between the models with increasing strain rate. Model 3 has a
strain rate of 1.64 cm/hr, model 2 has a strain rate of 4 cm/hr and model 4 has a strain rate of 7.5
cm/hr.
FIGURE 26 TOP VIEW PHOTOS MADE DURING DEFORMATION OF MODEL 3. FROM LEFT TO RIGHT THE PHOTOS ARE TAKEN AFTER 30 MIN, 40 MIN, 50 MIN AND 70 MIN. FIGURE 27 TOP VIEW PHOTOS MADE DURING DEFORMATION OF MODEL 2. FROM LEFT TO RIGHT THE PHOTOS ARE TAKEN AFTER 0.5 CM, 1 CM, 1.5 CM AND 2 CM
FIGURE 28 TOP VIEW PHOTOS MADE DURING DEFORMATION OF MODEL 4. FROM LEFT TO RIGHT THE PHOTOS ARE TAKEN AFTER 5 MIN, 8 MIN, 10 MIN AND 12 MIN. A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
22
The first photos to look at are the top view photos made during the deformation (figures 26-28).
In the first top views, both boundary fault zones are already developing, the zones consist of many
small faults, not well connected. In the second top views it can be observed that in both fault zones
the smaller faults starts connecting to form large ongoing boundary faults, on the right side this
process is more developed than on the left side. In all three models during this step a first inner fault
zone starts to develop on the left side of the rift, and in both model 2 and 4 even a second inner fault
zone starts to form on the right side of the rift. In the third step it is visible that the boundary fault
zones more and more consist of a few large faults which run along a large part of the rift instead of
many small faults. The left inner fault zone is expanding throughout the model and also the fourth
fault zone is growing. During the last stage, the inner fault zones are developing further and the offset
along the boundary faults becomes larger.
FIGURE 29 TOP VIEW OF THE DIFFERENT MODELS. FROM LEFT TO RIGHT STRAIN RATE IS INCREASING. Figure 29 and appendix 3 show the top view images made after deformation of these models. Looking
at fault zones, the two boundary faults are visible in all three models, model 3 and 2 show a clear
third fault zone at the left side of the VD, however in model 4 this third fault zone is well visible only
in the lower part of the model. Towards the top this fault zone almost disappears in a network of
many smaller faults. Instead of a pronounced fault zone left of the VD, model 4 has a clear fault zone
very close to this line. In the other two models, this fourth fault zone is also evolving but is sill in a
beginning stage. In model 4 it might be possible to see a fifth fault zone developing at the lower and
upper parts of the model. The average width of the rift basin increases with an increase in strain rate
(chart 5); the rift width at a speed of 1.64 cm/hr is 7.91 cm, the width at a speed of 4 cm/hr is 8.23
cm and the width at a speed of 7.5 cm/hr is 8.29 cm.
CHART 5 WIDTH OF THE RIFT PLOTTED AGAINST ALONG‐STRIKE DISTANCE The geometry of the fault also changes with a changing strain rate, from model 3 to model 2, thus
with increasing strain rate, the boundary faults grow larger and the amount of both major and minor
faults becomes less. When comparing model 2 to 4, with even higher strain rate, it can be seen that
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23 Masterthesis VU Amsterdam 07-09-2011
the amount of faults grow again, the major faults decrease in size while the minor faults increase in
size. The offset visible in the lower part of the left faults in model 2 (figure 29) has disappeared in the
both other models. A last difference can be observed between models 3 & 2 and model 4. While in
both first models the grabens are mainly unfaulted, in model 4 both grabens (left and right of the VD)
are highly faulted.
FIGURE 30 SCHEMATIC CROSS SECTIONS OF THE MODELS. FROM LEFT TO RIGHT: MODEL 3, MODEL 2 AND MODEL 4 Figure 30 and appendix 4 display the cross sections made of all 3 models. Again similarities and
differences can be observed between the different models. A major similarity which can be seen at all
the models is the asymmetric graben structure, and like in all other models seen so far, this geometry
is changing throughout the rift as a result of the curved basal plate. And also the planar faults near
the edges of the basin are returning features in all models. There are also some major differences
between the models. In model 3 and 2 the right side of the rifts is mainly bounded by planar faults
with some horst-graben structures, while model 4 has mainly horst graben structures with
occasionally a planar structure. The higher the strain rate, the larger the amount of faults visible in
the cross sections, in model 4 even the basins are faulted, most of the time with high angle faults.
The dip angle of the faults also change (chart 6); in model 3 the angles range between 51°-79° with a
common dip angle of 57-60°, in model 2 the angles range between 49°-71° with a common dip angle
of 57-58° and in model 4 the angles range between 54°-82° with common dip angles between 61-62°
and 67-68°. A last difference between the models is the tilting of the fault blocks; these are more
tilted in model 3 and 2 than in model 4.
CHART 6 THE FAULT DIP ANGLES OF BOTHE MODELS 2, 3 AND 4
Several authors have studied the effects of a changing strain rate ((Brun & Tron, 1993; Brun, 1999;
Corti et al., 2003; Gueydan et al., 2008; Michon and Merle, 2000). It appears that a low strain rate
results in a lower strength of the silicon putty (chart 7) and therefore a low brittle-ductile coupling,
which leads to a more localized deformation and the formation of a single graben. A high strain rate
results in a higher strength of the silicon putty (chart 7) and therefore in an increase of the brittle-
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
ductile coupling, which leads to a wider deformation zone with the formation of several grabens.
According to Brun (1999) block tilting increases as a direct function of strain rate.
CHART 7 DISPLAYING THE STRENGHT PROFILES OF BOTH BRITTLE AND DUTILE LAYERS. THE STRENGTHVALUES OF THE DIFFERENT STRAIN RATES ARE MENTIONED ABOVE THE AXIS FOR A BETTER VIEW ON THE DIFFERENCES BETWEEN THE MODELS. Also in the models made during this research an increase in average rift width is visible when
increasing the deformation speed.
The amount of grabens is also consistent with the explanation given above. The models run with
lower deformation speed mainly show a single graben with planar structures near the sides while the
model run with high deformation speed show 2 graben structures with planar faults towards the
outside of the rift.
24
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25 Masterthesis VU Amsterdam 07-09-2011
CHAPTER 3
DISCUSSION AND CONCLUSIONS
Based on the results and possible explanations, some conclusions can be made. They are listed per
research topic.
1. Comparison model CRS21 and model 1
a. A higher n-value is related to an increase in viscosity and therefore to an increase in
strength. An increase in silicon strength will cause a wider rift zone with more
grabens.
b. A decrease in grain size results in an increase in cohesion of the sand, which will
cause smaller rift geometries and steeper faults.
2. Different amounts of extension
a. An increase in amount of extension results in more and more pronounced fault zone.
The numerous small faults in the model with 1 cm of extension are linked together to
form large and sharp boundary faults in the model with 2 cm of extension.
b. The first boundary fault which forms is located above the VD, the second boundary
fault forms to the left, above the stable, not moving table. The first inner fault to
develop is located above the stable, not-moving table, because here the putty is
pulled apart because of the moving plate. This is visible in all four models made
during this research.
c. The rift structure is expanding towards both sides with ongoing extension, because
the putty is pulled apart both on the stable plate and on the moving plate.
d. With increasing extension block rotation becomes more pronounced and fault dip
decreases. This is caused because the planar non-rotational faults are replaced by
planar rotational faults to be able to accommodate a higher amount of extension.
3. Different strain rate
a. A higher strain rate results in a higher strength of the silicon putty and therefore in an
increase of the brittle-ductile coupling, which leads to a wider deformation zone with
the formation of several grabens instead of one.
b. Only a small difference in strain rate is necessary to see the effects taking place. The
rift becomes wider at a higher strain rate. The expected change in geometry like more
grabens at a higher strain rate is also visible with a small change in strain rate.
Although it was not the main goal of this research, it is possible to compare the models with nature.
Here the rift geometry and cross sections made in the area of interest are described. Figure 32 shows
the fault patterns in nature. Next to this map some schematic cross sections of the rift area are
displayed.
As mentioned in chapter 1, Corti et al. (2007) made two different models (figure 31): their model
CRS221 with a homogeneous crust; is compared with model 1 in this project. The second model is
model CRS22, the difference between the two models is the introduction of discrete zones of
weakness in the upper crust to simulate the main Proterozoic belts (Corti et al., 2007), this model is
not mentioned so far. The authors used this second model to compare with nature, the large scale
fault patterns developed during the modeling of that model look very similar to the structures
developed in nature; especially in the central part of the western branch, the fault zones become
more oblique to the direction of movement, with a NNE-NE orientation.
Because the models made in this project do not include these discrete zones of weakness it is very
hard to compare the top views of the models with nature because the main structures are not the
same. All models developed fault zones running from north to south without large interventions and
mainly parallel to the VD.
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
FIGURE 31 MODELS CORTI ET AL. (2007). A & B SHOW MODEL CRS21 AND C & D SHOW MODEL CRS22 Although the overall structures observed in the analogue can not always be compared easily with
nature, the models 1, 2 and 3 show a shift in fault zones at the lower left part of the rift, which is also
observed in the northern part of Lake Tanganyika. Another similarity between the analogue modeling
and nature can be observed in model 4. The upper part of the left boundary fault show a series of enechelon faults while the right boundary fault mainly consist of one fault.
FIGURE 32 A) TECTONIC INTERPRETATION OF THE RIFT BASINS (AFTER CORTI ET AL., 2007). THE FAULTS SHOWN AS SOLID LINES INDICATE THE AREA CORRESPONDING WITH THE MODELS MADE DURING THIS RESEARCH. THE RED LINES CORRESPOND TO THE LOCATION OF THE CROSS SECTIONS GIVEN IN C AND THE RECTANGLE IS GIVING THE LOCATION OF B. B) SCHEMATIC CROSS SECTIONS OF LAKE TANGANYIKA (AFTER CORTI ET AL., 2007) C) SCHEMATIC CROSS SECTIONS. OF NYANZA LAC BASIN, MOBA BASIN AND MARUNGU BASIN (AFTER ERBINGER, 1989) 26
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27 Masterthesis VU Amsterdam 07-09-2011
Comparing the cross sections made from both the analogue modeling and nature, some observations
can be made. Section A-A` in figure 32 corresponds to the lower sections in model 2 and 3. The
sections of these models show a graben on the left side of the section with a planar fault on the right
side (figure 33), simplifying the structures of section A-A`, the same structures are visible: a graben
on the left side of the section with planar faults bounding the rift on the right side.
FIGURE 33 UPPER FIGURE REPRESENTS CROSS SECTION 1 MODEL 3 AND THE LOWER SECTION RESPRESENTS SECTION A‐A`.
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
ACKNOWLEDGEMENTS
First of all I would like to thank my supervisor Fred Beekman for his support and help. And at second I
would like to thank the people of the Teclab for their help with the analogue modeling, especially Inge
van Gelder, Ernst Willingshofer and Dimitrios Sokoutis.
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29 Masterthesis VU Amsterdam 07-09-2011
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Michon L. & Merle O. (2000). Crustal structures of the Rhinegraben and the Massif Central grabens;
An experimental approach. Tectonics. 19, 896-904
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Mooney M, & Ewart R.H. (1934). The Conicylindrical Viscometer.
Physics, Volume 5, Issue 11. Physics 5, 350 (1934); doi:10.1063/1.1745219
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& Toiletries. Vol. 119 No. 5. May 2004
Omenda P.A. (2005). The geology and geothermal activity of the East African Rift System. Paper to be
presented at Workshop for Decision Makers on Geothermal Projects and Management, organized by
UNU-GTP and KengGen in Naivasha, Kenya, 14-18 November, 2005
Pflumio C., Boulègue J., Tiercelin J.J. (1994). Hydrothermal activity in the Northern Tanganyika Rift,
East Africa. Chemical Geology 116 (1994) 85-109
Pongjanyakul T & Puttipipatkhachorn S. (2007). Sodium Alginate–Magnesium Aluminum Silicate
Composite Gels: Characterization of Flow Behavior, Microviscosity, and Drug Diffusivity. AAPS
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30
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31 Masterthesis VU Amsterdam 07-09-2011
APPENDIX 1
MEASURING THE VISCOSITY AND DENSITY OF
THE SILICON PUTTY LAYER
VISCOSITY MEASUREMENT
The viscosity of the silicon putty is measured by using a coni-cylindrical viscometer (figure 34),
developed at the institute following the design by Mooney & Ewart (1934).
The model material is initially placed in a cylindrical cup. A weight attached to a fine chord and
suspended over pulley generates the force necessary to rotate the inner coni-cylindrical viscometer
body and hence shear at the material. Several measurements are made, each with an increasing
weight. For each weight the inner body has to rotate an x-amount of degrees (w in the tables below)
the time this rotation takes (in seconds) is measured and written down. When all measurements for
one weight are done, the putty has to relax for a few hours. After these hours, the process described
above is repeated. The next step is to calculate the shear stress and shear strain rate for each weight:
Shear stress = 8.92*(weight-0.78) and Shear strain rate = 0.095*angular velocity
At last the viscosity is given by the shear strain divided by the shear strain rate.
FIGURE 34 VISCOMETER
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Masterthesis VU Amsterdam 07-09-2011
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Log Strain Rate vs. Log Shear Stress
y = 3E-05x 1,2279
R2 = 0,9974
Log Shear Strain Rate
10
1
Series1
Power (Series1)
0,1
0,01
1
10
100
1000
10000
Log Shear Stress
CHART 8 PLOT OF SHEAR STRAIN RATE VS. SHEAR STRESS. The formula y=cxn gives the power law relationship. In this formula c is a constant and n is the power
law exponent. The n-value is equal to the inverse of the slope on the graph (chart 8) of strain rate
against stress (Sokoutis, 1986). In this case the n-value is 1.2279.
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33 Masterthesis VU Amsterdam 07-09-2011
DENSITY MEASUREMENT
The density of the viscous media is determined with a pycnometer, an apparatus that enables to
measure the weight of a constant volume. ρ(Material) = (weightPycnometer-filled – weightPycnometer-empty)
 weight pycnometer


 material  
 weight pycnometer empty 


11.17

filled
 116.58  99.00 
3
  1.574 gr / cm 11.17


The density of the silicon putty used in this experiment is 
A.P. van Nunen
Masterthesis VU Amsterdam 07-09-2011
APPENDIX 2 CALCULATING THE CRITICAL STRESS
To measure the critical stress for the different models we use two different formulas: one for the
brittle part of the model, which is represented by the sand and another formula for the ductile part of
the model, which is represented by the silicon putty. These formulas are listed and explained below.
For extensional conditions the critical stress for the brittle sand can be calculated with the following
formula (Brun, 1999; Michon and Merle, 2000):
(σ1-σ3)=2/3 ρgh
Where ρ is the density of the quartz sand (1520 kg/m3), h is the thickness of the sand layer (0.025 m)
and g is the acceleration due to gravity (9.81 m/s2)
The ductile strength of the silicon putty can be calculated from the ratio between the extension rate
(the velocity) and the thickness of the ductile layer (Michon and Merle, 2000):
σ = μ (v/z)
Where σ is the strength, μ is the viscosity, v the velocity of deformation and z the depth (0.01 m) of
the ductile layer.
MODEL 1
4 CM/HOUR
 1   3   2/3*1520*0.025
 1   3   25.33 Pa*m

 1.111  10 5
  
 6093.52 
0 .5


  0.135 Pa*m
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35 Masterthesis VU Amsterdam 07-09-2011
MODEL 2
1.64 CM/HOUR
 1   3   2/3*1520*0.025
 1   3   25.33 Pa*m
 0.000004556

 6093.52 
0.5


 
  0.056 Pa*m
MODEL 3
7.5 CM/HOUR
 1   3   2/3*1520*0.025
 1   3   25.33 Pa*m
 0.000020833

 6093.52 
0.5


 
  0.254 Pa*m
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Masterthesis VU Amsterdam 07-09-2011
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APPENDIX 3
TOP VIEW PHOTOS OF THE DIFFERENT MODELS
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Masterthesis VU Amsterdam 07-09-2011
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39 Masterthesis VU Amsterdam 07-09-2011
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Masterthesis VU Amsterdam 07-09-2011
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APPENDIX 4
CROSS SECTIONS OF THE DIFFERENT MODELS
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43 Masterthesis VU Amsterdam 07-09-2011