Independent Project at the Department of Earth Sciences
Självständigt arbete vid Institutionen för geovetenskaper
2017: 1
A 4D Analysis of a Minor Graben Structure at Ekolsund, South Central Sweden
En 4D-analys av en mindre graben-formation vid Ekolsund, södra centrala Sverige
Eric Andin Alexander Sehr
DEPARTMENT OF EARTH SCIENCES
I N S T I T U T I O N E N F Ö R G E O V E T E N S K A P E R
Independent Project at the Department of Earth Sciences
Självständigt arbete vid Institutionen för geovetenskaper
2017: 1
A 4D Analysis of a Minor Graben Structure at Ekolsund, South Central Sweden
En 4D-analys av en mindre graben-formation vid Ekolsund, södra centrala Sverige
Eric Andin
Alexander Sehr
Copyright © Eric Andin and Alexander Sehr
Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2017
Sammanfattning
En 4D-analys av en mindre graben-formation vid Ekolsund, södra centrala Sverige
Eric Andin och Alexander Sehr
Syftet med denna studie är att utföra en kinematisk analys av en
förkastningszon, speciellt där en mindre graben struktur utbildats. Ytan för det studerade området är även det viktigt, det är för att förstå ifall en förkastningsreaktivering har skett i och med en glacial påverkan. Graben strukturen är blottad längs en vägskärning, utmed E18 vid Ekolsund, 14km österut om Enköping. Resultatet av rapporten är baserat på fältstudier med fokus på geometriska relationsmönster hos sprickplan, relativa förskjutningar längs sprickor och en kinematisk analys av rörelser längs sprickor. En
högupplöst terrängmodell var erhållen vid analys av grabenstrukturens formation och för att skapa en 3D sprödtektonisk deformationsmodell.
Modellen var tillämpad vid rekonstruering av deformationsprocessen.
Området innefattar en mer än 1,1km lång dextral NO-orienterad strike-slip förkastning i en polydeformerad Prekambrisk berggrund. Förkastningen är ansluten NO till en av de regionala strukturerna som begränsar
berggrundsblocken. Förkastningen är synlig både på marknivå och från flygfoton. Medan projektet fortlöpte blev det tydligare att området blivit utsatt för en reaktivering. Beläggen för detta påstående grundar sig i de
observerade semi-vertikala sprickplanen med konjugerande subhorisontala striationer som har alternerande stupning och trend. Andra belägg för ett reaktiveringsscenario är de fault gouge strukturerna som innehåller
horisontala striationer såväl som stupande striationer, vilket indikerar på en dip-slip förkastningsrörelse. Huvudzonen av förkastningen uppvisar lägre altitud än kringliggande bergyta. Det här indikerar en asymmetrisk graben struktur som har utsatts för polydeformation.
Vid analys av resultatet, så kan det sammanfattas att en pull-apart basin förkastning har inträffat. Däremot är det mycket möjligt att zonen har
undergått fler modifikationer under dess geologiska historia, vilket har
resulterat i en polydeformerad reaktivering av mindre förkastningar som gett kontur dragen åt graben strukturen.
Nyckelord: Södra centrala Sverige, pull-apart basin, 3D spröd deformationsmodell, reaktivering, striation markörer
Självständigt arbete i geovetenskap, 1GV029, 15 hp, 2017 Handledare: Hemin Koyi och Sven Tirén
Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se)
Hela publikationen finns tillgänglig på www.diva-portal.org
Abstract
A 4D Analysis of a Minor Graben Structure at Ekolsund, South Central Sweden Eric Andin and Alexander Sehr
The objective of this project is about a kinematic analysis of a shear zone, specifically a graben structure. The surface of the study area has also been investigated, this is to understand if a faulting reactivation has occurred due to glacial impact. The graben structure is exposed along a road cut, next to the E18 highway at Ekolsund, 14km east of Enköping. The results of the report are based on field studies of the geometrical relationship pattern of fracture planes, relative displacement along fracture and a kinematic analysis of movements along fractures. A high resolution terrain model was acquired in the analyzing of the graben structure formation and to create a geometrical 3D brittle
deformation model. This model was applied to reconstruct the deformation process.
The area contains a more than 1,1km long dextral NE-trending strike-slip fault feature in a polydeformed Precambrian bedrock. The fault is connected NE to one of the regional structure that delimits the bedrock blocks. The fault is visible both as a topographical feature from the ground and from aerial photos.
While the project followed through it was clear that the area had been subjected to reactivation. The evidence for this claim stems from an observed semi-
vertical fracture plane with conjugating subhorizontal striations markers that contain an alternating plunge and trend. Other evidence for a reactivation scenario is the fault gouges that contain horizontal striation as well as steep plunging striation, indicating a dip-slip faulting movement. The main zone of faulting displays lower altitude than the surrounding rock. This indicates an asymmetric graben structure that has been subjected to polydeformation.
When analyzing the results, it is concluded that a pull-apart basin event has occurred. It’s highly possibly that it has been undergoing modifications during its geological history, resulting in a polydeformed reactivation of the minor faults outlining the graben structure.
Key words: South central Sweden, pull-apart basin, 3D Brittle deformation model, reactivation, striation markers
Independent Project in Earth Science, 1GV029, 15 credits, 2017 Supervisors: Hemin Koyi and Sven Tirén
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se)
The whole document is available at www.diva-portal.org
Table of Contents
1. Introduction ... 1
2. Objective ... 1
3. Geological setting ... 2
3.1 Regional ... 2
3.2 Local ... 2
4. Method ... 3
4.1 Field studies ... 4
4.2 3D Terrain model ... 4
5. Results ... 5
5.1 Foliation and fracture planes analysis ... 5
5.2 Kinematic indicators ... 9
5.3 Compilation of kinematic data ... 10
5.3.1 Shear indicators ... 10
5.3.2 Fault gauge ... 12
6. Discussion ... 14
7. Conclusions ... 18
8. Acknowledgments ... 19
References ... 20
Internet sources... 21
Appendix: Striation identification ... 22
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1. Introduction
In the early 2000s Geosigma carried out extensive research at Ekolsund, 14 km east of Enköping (figure 1). The bedrock area is exposed along four extensive road cuts along the E18 high, the height is up to 18m and length 500m. Geosigmas research was about studying how lineaments are correlated into the bedrock structures and also how to correlate structures between the four roads cut sections.
At a large-scale, the geology of the area is governed by block faulting that is related to the regional shear zone that lies alongside the northern part of a regional E-W trending Sörmlands horst (Tirén & Beckholmen, 1990). It is this shearing and associated block fault that gives Lake Mälaren its shape (figure 2).
The goal of this study is to obtain field data concerning how to correlate the intrinsic bedrock structures and kinematics. This data will then be interpreted to construct a viable development of the graben structure. The field studies comprise characterization of investigated and measured fault gouges, foliations, fracture
planes and with an emphasis on kinematic indicators. The field data are then used to do a reconstruction of the fault movements in the area. The surface morphology has also been investigated; this was done to determine whether or not the area had been subjected to a glacial/post-glacial faulting process that could’ve distorted the
measured striation orientations. This was done by measuring the glacial striation across the studied site. If previously mentioned would have been the case, the glacial striations would be deviating from each other.
To contribute with the research, Geosigma has kindly provided with different types of maps, articles and a detailed topographical 3D-model of the site. The model is based on precision levelling of reference points and detailed photographs of the road cut. The accuracy and precision of the model is in millimeters and is operated
through ArcGIS Scene, a 3D-model program. The model covers the asymmetric graben area.
2. Objective
The objective of this report is to interpret a section of a fault zone containing a graben-like structure and to study the kinematics of the latter. This is done by performing:
● Field studies
● Examining glacial and postglacial influence
● Compile a 3D brittle deformation model
● A 3D block model with movement indications for different fracture/fault segments
Field studies will help to include and exclude theories about the geological settings of the zone. The study site shows difficulties in relating different indications of
movements with different deformation phases. These relationships display different suggestions of solutions to what has happened, such as dip-slip, strike-slip and oblique-slip components in the form of striation. The measured striations were sorted into different classes to be distinguishable from each other.
Analysis of glacial and post glacial processes is performed to see if a faulting event has occurred due to decompression forces acting upon the bedrock. If post glacial faulting would have occurred, it is possible that different segments in the bedrock could have moved and would therefore create distortion in the acquired data.
The glacial striation will be measured to see if the measurements are deviating from each other.
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Creating the previously mentioned models of the area is done by utilizing the acquired 3D terrain model provided by Geosigma. These models will then be used as material to reconstruct the deformation process of the area.
3. Geological setting 3.1 Regional
The middle part of Sweden involves complex geological structures. The studied shear zone is considered to be a part of a larger block tectonic region (Tirén &
Beckholmen, 1990). At a large-scale, the geology of the area is governed by block tectonic movements that are a part of the regional dextral shear zone. The regional shear zone reaches along the Sörmlands horsts northern part and builds up the Lake Mälaren (Tirén & Beckholmen, 1990).
Previous studies suggest that ridge push from the Mid-Atlantic Ridge have been the highest regional horizontal stress field that has affected the central Fennoscandia (Stephansson et al., 1991). The stress field is believed to have act in a NE-SW
direction. This implies that movements by the Atlantic rift system have created the stress which triggered the shearing of the regional shear zone, which has been classified as a riedel shear reaching E-W (Tirén & Beckholmen, 1990).
3.2 Local
No valid conclusions have been done regarding the lithology and age of the bedrock at Ekolsund. Regional association proposes that it is granitoids belonging to the Trans-Scandinavian Igneous Belt (Larsson and Tullborg, 1993). The bedrock has also been interpreted as weakly foliated (Geier, 2004).
No major regional stress field measurements have been performed in this region, but Stephansson’s description of the Fennoscandia stress field is thought to be valid (Geier, 2004).
Figure 1 Overview picture of the main study area. Photo S.Tirén
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The presence of fault gauges within the area is a direct consequence of shearing events. Its
components are made up of granular material formed by fragmented material from the host rock.
Fragmentation is due to shearing motion that is occurring within the fault surfaces (Twiss, 2000).
Further erosion and weathering have continued after the formation of the shear zone. Glacial striae is today one of the most visible traces of erosion at the rock surface. Records of ice movements at the area have been described as SSE oriented (Geier, 2004).
A change in orientation of glacial striations would indicate that there would have been a post-glacial reactivation of the shear zone.
4. Method
Each measurement was drawn and marked with an ID on an overview picture of the area to help clarify the location of the features; these measurements were later marked onto the 3D model. A photo of each measurement was taken to improve the visual aspect and examining for later stages in the evaluation process. The fractures and faults were specified to be longer than 1m whilst mapping all visible striation, measuring all fractures would have taken too much time. All measurement
parameters could not be registered due to lack of visual input; some fissures were exposed to water movement which soaked the crushed rock in the fault gauges.
The different fracture ID’s and their respective strike/dip and trend/plunge
properties were transferred into an excel sheet to simplify the transformation of data to stereonets.
Stereonet is a tool for geologists to evaluate data by stereographic projection of e.g. strike and dip (Groshong, 2006). The tool allows for the possibility to detect
Figure 3 Illustration of the similarities between Lake Mälaren and a common strike-slip fault system.
Figure 2 Overview picture of the researched area with markers. The red lines define the specific road cut study area. The square shows the position of the half graben where the 3D-model was made and most measurements were taken from.
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stacks of planes with similar strike trend. By using stereographic projections it is easy to compare and divide parts of the studied area. When data is plotted it provides a pole and a great circle representing the plane, at the lower hemisphere. Lineation is being represented as a dot on the line representing the plane, showing the direction of movement. The stereonet program that was utilized is called
Stereonet32(Referens - http://visiblegeology.com/SNet/).
4.1 Field studies
The mapping of the Ekolsund area was prepared and executed within 2 months. The first field study was carried out with help from our supervisors. The purpose of this excursion was to obtain a general impression of the tectonics in the area. This was done by extensive field mapping across the fault area and scanning for indications or evidence of tectonic-/multi tectonic events. The different parameter and properties that were sought after in the field was:
● Foliation
● Fault plane markings as displacement sense indicators – here called fault sense
indicators
● Fracture parameters - strike, dip, fissure form and fissure length.
● Crevasse ending - how the fissure terminates (towards another fracture, below surface or within blind terminations)
● ID collisions, as in what fissures correlate with each other.
● Angle of opening - if it is open or closed and the width of the opening
● Fissure properties - fissure mineral, weathering level, fault rocks.
● Kinematic indicators - trend, plunge, relative movement (I.e. sinistral or dextral) and age correlation between the different fractures.
4.2 3D Terrain model
A 3D terrain model was applied for further analyze of the study site.
The model generates a detailed view over the area and provides an accurate way of understanding the relationship between
fractures and structural features due to a better overview. The model provided a base when constructing the 3D brittle deformation mode.
The terrain model was constructed by Cedric Pieterse at Geosigma. Construction of the 3D model was done by collecting over 800 pictures
of the rock wall feature. The angle of perspective for the photos needs to differ to complete a fully functional model of the area. Therefore pictures showing the same surface were taken several times from different angles. The overburden area was first captured and the work continued until the bottom area also had been covered.
To achieve good GPS accuracy in the model it was important to have reference points (figure 4). Reference positions were created by using a GPS sensor. The points helped the 3D--modelling and rendering software to correlate the pictures and particles with each other. The deviation in the model is about 2,5 cm.
Figure 4 Reference points used to construct the 3D-model.
5 A 3D terrain model was applied for further analyze of the study site
ArcScene was used to handle and view data from the model. The coordinate system that is used is SWEREF99.
5. Results
5.1 Foliation and fracture planes analysis
Field studies confirmed a weak foliation of the dioritic country rock in the area. It showed schistose foliation, which is composed of larger minerals with platy minerals as the dominant. A mylonite zone was observed, but no major studies were
performed on it.
Foliation measured in the area shows three different sets of orientations (figure 6 and 7). Most of the measured foliations are almost perpendicular to the NE-SW going shear zone and are not bound to have the same direction as the sense of shear.
When approaching the shear zone, the foliation will become more or less parallel to the sense of shear.
Figure 5 Elevation data over the studied area.
Figure 6 All measurements of foliation.
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Construction of a rose diagram shows the dominant dip direction of measured fractures. Orientation of fractures is widely distributed and shows no tendency of conjugating fractures (figure 8). All measured fractures contribute a high angle of dip, with an average range between 70-78 degrees. The distance between the fractures varied a lot. Some open fractures were detected and had a gap of 1-10 cm. The largest gaps were found within fault gauges.
A second rose diagram was constructed to illustrate fractures frequency in classes of 10° (figure 9). The diagram shows three main sets of fractures orientation, NNW, NW and NE.
Figure 7 Poles to plane projection of all measured foliation.
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Figure 8 Illustration of the dominated dip directions.
Figure 9 Rose diagram of measured fracture planes.
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A stereographic projection illustrates the orientations even better. Figure 10a is based on all the collected field data, as strike and dip of fractures. By analyzing the data you will find that three sets of fractures are overrepresented. Almost 60% of all measurement will tend to follow one of these orientations of strike, NNW, NW and NE.
Figure 10 A) Pole to plane of all measured fractures across the area. The majority of the
measured planes contribute with a steep angle of dip. Some of the measured surfaces belong to subhorizontal fractures with a lower dip angle. B) All of the NW-oriented fracture planes. C) All fracture planes with a dominantly an N-strike direction. The angle of dip tends to have a high value within the stack. D) Is showing planes within the zone, striking in a NE direction.
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5.2 Kinematic indicators
The measured kinematic indicators were in all cases different forms of striations, except for one displacement feature. The striations were specifically tectonic generated slickensides.
Slickensides are created by fault movement, it’s a surface created by friction movement along two surfaces in a fault plane. The various types of slickensides are: polished surfaces, coated fault plane, fibrous mineral striae, streaked fault gauge and ridges and grooves (Dehandschutter, 2001).
The general descriptions of the examined fracture surfaces in the area are the following:
● In most cases fiber growth parallel to the striation grooves.
● Exhibits well defined step features
● The striation covers in most cases the whole fracture plane
● Glassy surfaces
● Both symmetric and asymmetric striation steps on the fracture planes
Thetwo sets in (figure 11) indicate that there have been multiple movements upon conjugating planes containing different striation directions. To the right of the orange line, the two sets conjugate. The shear direction that is shown, the one that overrides the other, is the youngest generation of striation because its origin is from a later stage during the faulting process. Thus making it the ‘youngest’ striation
marker and implies that the stress field has changed direction during deformation phase (Fossen, 2010). In figure 11 the sinistral striation is the dominant one on the right side of the orange line.
Displacement of gneissic mineral band at the western edge of the graben area is shown in figure 12. It was found within a plastic deformed zone. Estimation of how the movements have affected the rock is hard to predict. Data provided by measuring movements close to the source shows a possible oblique movement on a fault surface. Many marks of lineation can be found nearby supporting the
believed movement. The plunge of the movement may be very small and infers a greater strike-slip than vertical slip component. This implies that a vertical displacement of 5 cm requires a larger horizontal displacement. This is due to measurements of plane with similar orientation has a rather small plunging angle. The movement can enclose a lot larger displacement due to the fact that no indications of dip-slip have been found nearby.
Figure 12 Displacement of gneissic mineral band at the western edge of the graben area.
Figure 11 Arrows indicating different striation directions upon conjugating planes.
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5.3 Compilation of kinematic data
5.3.1 Shear indicatorsLineation found at the fracture planes gives the opportunity to show the general movement of the shearing. The data of shear indicators was collected by measuring the trend and plunge of movements. The information was then plotted with the strike and dip of the lineated plane. Collected information shows both dextral and sinistral movement along measured planes. Field observations showed that the main
direction of movements within the shear zone have been dextral. Characteristic for almost all striations found was that they were situated on steeply dipping plane. Many of the measured striations found therefore had a small angle of plunge for various directions. The projection shows that the angle of plunge is independent from the sense of movement, meaning that the low plunging angle applies for both dextral and sinistral movements.
Rare cases of dip-slip and subhorizontal striations could be seen at the location.
Dip-slip movements were exclusively found in fault gauges.
Sinistral and dextral measurements were divided into separate stereonets (figure 13). This did not result in any trend of movement within the area. The stereonet do show that planes with a SW strike tend to have sinistral movement and a lack of dextral (figure 13A).
Figure 13 A) Displays all measured sinistral movements with respect to its fracture plane. B) Displays all the measured dextral movements with respect to its fractured plane.
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Striation data of the most dominant strike direction with station marks was separately plotted to be able to detect any trend of the sense of movement. The data shows that the orientation of the fractures are independent from the sense of movements along the fracture planes. No direct trend can be defined regardless of the orientation of fractures.
Figure 15 Movements along NW striking planes, where green dots represent sinistral movements and black dextral movements.
Figure 14 Movements along NE striking planes, where green dots represent sinistral movements and black dextral movements.
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Figure 16 Movements along NNW striking planes, where green dots represent sinistral movements and black dextral movements.
5.3.2 Fault gauge
At the studied area there were two major fault gouges (figure 17). The gouge material had similar properties to what is expected within a granular fault gouge, which is assumed because the country rock in the area is constituted of diorite and tonalite. The first fault gauge stretched about 4-5m. The fracture had a
straight form and contained two striation marks.
The plane had highly pulverized parts with a yellow/brownish colour. The second fault gouge had similar pulverized properties but also
contained bigger rock fragments in the form of flat elongated rocks. These rocks were aligned in the same direction as the fault and were wedged into the granular country rock. The
gouges measurements are following: Figure 17 The image shows two different faults gauges. The circled areas indicate the location of the described striations that can be found in the table below. The second striation of fault gauge 2 lies outside the picture.
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● Fault gouge 1 - 210/63
○ Striation 1# 310/17, dextral
○ Striation 2# 70/60
● Fault gouge 2 - 20/75
○ Striation 1# 345/20, dextral
○ Striation 2# (situated 1m to the east of the fault gouge) 250/82
Figure 18 The location of the two fault gauges.
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6. Discussion
A model of the evolution for the graben structure was created from observations and investigation of field data. During the analysis of the result data, it was concluded that a pull-apart basin had been created during the shearing of the area. Different
acquired evidence, confirming the theory of setting that is believed to be the main explanation of the study area will be discussed. The results will be discussed in a sequence, with its first arguments covering a wide spectrum and then successively decreasing its focus to a more detailed study. The theory for the faulting event will be supported by a block diagram and finalized by a brittle deformation model of the block movements.
The early stages of the shear zone involved depths that incorporated high temperature and pressure. At this depth, the shearing is exposed to ductile deformation. This ductile shearing is believed to be the origin of the foliation and mylonite within the study-site.
When analyzing the foliation in the stereograms, the foliation examines curvature that becomes increasingly parallel as you approach the main shear zone, the
asymmetric graben. The opposite occurs when moving away from the shear zone;
foliation becomes increasingly perpendicular to the main shear zone. Field
observation of later striation marks implies that we have a dextral shear zone and gives a hint about how the curvature of the foliation will be.
Previous research suggests that the maximum horizontal stress field during the early evolution of the shear zone must have acted in a NE-SW orientation. It requires a change in the stress field to generate dextral movements in a shear zone oriented NNE-SSW. A possible explanation of the orientation change may be the influence of block movements along the major shear zone that builds up Lake Mälaren. Possible movements could have generated a local stress field strong enough to initiate the smaller shear zone at Ekolsund.
Development of fractures within in a shear zone highly depends on the applied stress field. It has been concluded that the development of shear zone structures are scale independent (Tchalenko, 1974). First fractures to develop within a shear zone are in the form of R-shears (Tchalenko, 1974). An exact estimation of which fractures that developed first is hard to prove, but the three overrepresented sets of fracture orientation is believed to have formed in the beginning of the brittle deformation.
Growth simulations of shear zones imply that R-shears is the first to develop (Tchalenko, 1974). R-shear features form under conditions associated to brittle deformation as a consequence of the applied stress field. Together they will start to form an en echelon formation (figure 19). Further shearing results in the development of new fractures in the form of P-shear, linking the R-shears together (figure 20). This generates a parallel principal displacement in the general direction of movement. But the step-over also causes the strike-slip fault to deviate from a straight course,
meaning it creates irregularities within the shear zone. Further shearing will generate a space problem, in this case an transextension phase (figure 21). The generated space problem resulted in faulting blocks that fell down in an asymmetrical fashion and gave rise to striation marks on the blocks. The process of blocks falling down did not only result in dip-slip markers, but also involved rotation and stepwise movements (figure 22). Rotation and stepwise movements would explain the diverse trend and plunge angles of the measured striations. The conjugating fault planes containing different orientation of the striation is one example that may be answered by the theory. Both are thought to be of the same deformation phase because the
weathering, steps and appearance are similarly looking. The upper plane is a sinistral
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Figure 19 R-shear developing an en echelon series.
Figure 20 Development of P-shears which ultimately links the R-shears together.
movement whilst the bottom is dextral (figure 11). The striation marker overprinting the other implies that the stress field has changed direction during deformation phase (Fossen, 2010). It is therefore believed that the conjugating subhorizontal striations is a consequence of reactivation of the system.
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Figure 22The space problem is solved by faulting blocks falling down in an asymmetrical fashion, creating what is called a negative dextral flower structure.
Figure 21 Further shearing generates a space problem in between two linked R-shears.
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As the blocks starts to fall down, they rotate and abrade against the fault wall, resulting in the formation of fault gouges. Movements within the shear structures are necessary in the development of fault gauges. The supra crustal fault zones is not linear with a flat planar contact between each surface wall. Instead a finite layer of damaged fault rocks was displayed in between the fault surfaces (Dehandschutter, 2001). Having fault gouges indicates an environment with low temperature and can be associated with hydrothermal conditions. Usually they are formed at shallow depth of >5km. At this point, the shearing is in form of brittle deformation due to decreased temperature and pressure. The striations found in fault gouge are due to a
reactivation in the zone. The reactivation probably occurred during and after the Cambrian epoch.
Data of glacial striations did not show any changes of trend within the studied area, therefore no visible evidence of a reactivation of the fault zone due to previous glacial forces. The whole area contributes with visible striae with very little variation of orientation and maintains more or less the same orientation of 160°. Striae within the graben area were only found on high elevated areas and there were no indication of glacial movements at the lower parts. Glacial influences may instead be seen by decompression fractures. These possible decompression fractures are believed to have been formed as a consequence by the geologically rapid melting of the ice sheet, which resulted in a faster uplift of the upper part of the crust.
Figure 23Cross section of the graben structure.
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7. Conclusions
The fault is connected NE to one of the regional structure that delimits the bedrock blocks. While the project followed through it was clear that the area had been subjected to reactivation. The evidence for this claim stems from an observed semi- vertical fracture plane with conjugating subhorizontal striations markers that contain an alternating plunge and trend. Other evidence for a reactivation scenario is the fault gouges that contain horizontal striation as well as steep plunging striation, indicating a dip-slip faulting movement. The main zone of faulting displays lower altitude than the surrounding rock. This indicates an asymmetric graben structure that has been subjected to polydeformation.
It is concluded that a pull-apart basin event has occurred which led to the
formation of our negative dextral flower structure (figure 24). It’s highly possibly that formation has been undergoing modifications during its geological history, resulting in a polydeformed reactivation of the minor faults outlining the graben structure.
Figure 24 Illustration of extrapolated minor faults outlining the graben structure.
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8. Acknowledgments
We would like to thank our supervisor Hemin Koyi for guidance and valuable opinions. We would also like to thank Sven Tirén at Geosigma for contributing with valuable information and knowledge to us. A big thanks to Geosigma for creating a 3D-model over the study site.
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22
Appendix: Striation identification
The identification of the striations was done according to (Petits, 1987) classification system.
For most cases the striation exhibits clear visible steps in a subperpendicular pattern that are compiled in
segments along the striated plane. Fiber growth had also been developed on the breaking of the step. This is due to a pressure shadow in the boundary of the step. A
decrease of pressure makes it more suitable for crystal growth in these areas. The fiber growth were exceptionally well developed along the striation steps that possessed a plunge of <60° whilst the striation with a plunge of
horizontal degree had more weathered steps and less fiber growth.
While the striations with a steep plunge follow one specific tendency, the subhorizontal plunging striation follows another. These striations have a mean plunge of 15° (43 measurements) and are generally situated on subvertical fracture planes. They’re all following the smoothness criterion which means that you can tell the sense of slip direction by touching the surface. This is possible due to their plucking step character (figure 25A) fiber growth direction (which in most cases follow the plucking step direction) and that they have grooves engraved in their surface (Tjie, 2014).
Figure 25B, shows a differentiating dark and light colored scheme, with light plagioclase crystal formed on the tip of the steps and the darker crystals elongated on the plane. The plagioclase steps forms Bruised Steps (Tjie, 2014) which are elongated step risers along the fault plane. The visible step shavings may be recrystallized during a faulting process.
Figure 25C exhibits accretionary steps (Tjie, 2014).
This happens when the fault gouge manifests elongated crystals, this gives rise to a systematic parallel accretion step pattern.
Sinistral striation with clearly visible steps is seen in figure 26. The plunge is intermediate with an angle of 60°.
The movements have caused the rock to reach the failure envelope and fracture in a brittle way. Prominent plucking steps are seen with deep grooves and intermediate step fiber growth.
Figure 25 Striation markers, the arrow indicated movement of visible plane. A: Plucked steps (PS), B: Bruised Steps (BS) and C: Accretionary steps (AS), the white lines indicate the step feature.
23 Figure 26 Striation marker.
