ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2015

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1223

Superimposition of ContractionalStructures in Models and Nature

HONGLING DENG

ISSN 1651-6214ISBN 978-91-554-9160-4urn:nbn:se:uu:diva-243235

Dissertation presented at Uppsala University to be publicly examined in Hambergsalen,Geocentrum, Villavägen 16, Uppsala, Friday, 27 March 2015 at 10:00 for the degree ofDoctor of Philosophy. The examination will be conducted in English. Faculty examiner:Professor Sudipta Sengupta (Department of Geological Science, Jadavpur University).

AbstractDeng, H. 2015. Superimposition of Contractional Structures in Models and Nature. DigitalComprehensive Summaries of Uppsala Dissertations from the Faculty of Science andTechnology 1223. 45 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9160-4.

Superimposition of contractional structures is widely observed in different scales in the world.Superimposed structures form due to different processes: change in strain accommodation fromone type of structure to another during a single progressive shortening; successive coaxialshortening phases separated by an unconformity; superimposition of different non-coaxialshortening phases. Using results of a series of systematic analogue models and detailed fieldstructural mapping, this thesis focuses on the geometry and kinematics of such superimposedstructures that are formed by these three processes. During a single progressive folding, thrustsdevelop within a fold to accommodate stain variations in different regime of the fold. Limiteddisplacement along these thrusts does not significantly modify the geometry of the fold.However, during multiple shortening phases (coaxial or non-coaxial), early formed structuresare modified by the later phase ones. The later thrusts can cut and displace the pre-existingstructures. The early folds are tightened or interfered by the later folding phase. Pre-existingthrusts may be reactivated either in dip direction and/or along strike during the later shortening.The pre-existing structures in turn influence development of the later structures, which resultsin change in structure spacing. An angular unconformity between two shortening phases clearlytruncates the early phase structures and separates structures of different levels. Unlike inthe post-erosional layers, in the layers below the unconformity, complicated superimposedstructures are visible. This thesis shows that geometry and sequence of structures formed duringone progressive shortening or multiple shortening phases strongly depend on the mode of thesuperimposition (coaxial, orthogonal or oblique) and the orientation of pre-existing structures.

Keywords: superimposition, progressive deformation, multiple phases, coaxial, non-coaxial,analogue modelling, fold-and-thrust belts

Hongling Deng, Department of Earth Sciences, Solid Earth Geology, Villav. 16, UppsalaUniversity, SE-75236 Uppsala, Sweden.

© Hongling Deng 2015

ISSN 1651-6214ISBN 978-91-554-9160-4urn:nbn:se:uu:diva-243235 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-243235)

To my parents献给我最亲爱的父母

邓家文，黄红群

Home is behind, the world ahead, And there are many paths to tread.

Through shadows to the edge of night, Until the stars are all alight.

Then world behind and home ahead, We’ll wander back to home and bed. Mist and twilight, cloud and shade, Away shall fade! Away shall fade!

Fire and lamp, and meat and bread, And then to bed! And then to bed!

J.R.R. Tolkien

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals. Reprints were made with permission from the respective publisher, Journal of Structural Geology, Elsevier.

I. Deng H., Zhang C. and Koyi H.A., 2013. Identifying the character-

istic signatures of fold-accommodation faults. Journal of Structural Geology, 56, 1-19.

II. Deng H., Koyi H.A. and Froitzheim N., 2014. Modelling two se-quential coaxial phases of shortening in a foreland thrust belt. Jour-nal of Structural Geology, 66, 400-415.

III. Deng H. and Koyi H.A., 2014. Mega arrowhead interference pattern in the Central part of the Yanshan Orogenic Belt, North China. Jour-nal of Structural Geology, under review.

IV. Deng H., Koyi H.A. and Nilfouroushan. F., 2015. Superimposed folding and thrusting by two phases of non-coaxial shortening in analogue models. Journal of Structural Geology, under review.

For paper I, the initial concept and design, field study and partly analyses, were supervised by Prof. Zhang from China University of Geosciences (Bei-jing), China. Prof. Koyi, in addition to contributing to the text, reinterpreted some of the examples and helped me to add displacement plots. My contri-bution is 65%.

The idea of paper II was initiated with Prof. Koyi who helped me to con-struct the initial models, which I continued with. We analysed model results and I wrote a first draft which was modified and improved by my supervisor. Prof. Froitzheim from University of Bonn, Germany wrote the field descrip-tion of one of the natural examples. My contribution is 65%.

Paper III was the result of detailed field mapping and data collection by my-self and partly my supervisor. Field data were analysed in Uppsala and I wrote a first draft of the manuscript, which was modified and revised 3-4 times by my supervisor. My contribution is 65%.

The idea of paper IV was initiated during our field trip to the YOB where two non-coaxial folding phases has occurred. We designed a series of sys-tematic models, which I ran. We analysed the results and I wrote the first draft which was modified and revised 3-4 times by my supervisor. Dr. Nil-fouroushan contributed to this work by helping with the physical experi-ments and commenting on the final version of the manuscript. My contribu-tion is 65%.

In addition, I have contributed to the following papers, which were published during my PhD study time but not included in this thesis.

Zhang, C., Li, C., Deng, H., Liu, Y., Liu, L., Wei, B., Li, H. and Liu, Z., 2011. Mesozoic contraction deformation in the Yanshan and northern Tai-hang mountains and its implications to the destruction of the North China Craton. Sci China Earth Sci 54, 798-822.

Zhang, C., Deng, H., Li, C. Liu, Z., Deng H. and Teng, F., 2012. An out-of-syncline thrust model for the ‘Chengde Thrust Sheet’ in central intraplate Yanshan Orogenic Belt, northern North China Craton (in Chinese with Eng-lish abs.). Earth Science Frontiers. 19(5), 27-40.

Deng H., Zhang C., Deng H. and Ouyang, J., 2012. Deformation mechanism for the development of fold-accommodation faults: Examples of outcrop-scale structures from central intraplate Yanshan Orogenic Belt, North China (in Chinese with English abs.). Earth Science Frontiers, 19(5), 61-75.

Contents

1. Introduction ........................................................................................... 9 1.1 Superimposition of contractional structures ......................................... 9 1.2 Effect of erosion and sedimentation ................................................... 11 1.3 Thesis objectives ................................................................................ 12

2. Methodology ........................................................................................ 14 2.1 Physical experiments .......................................................................... 14 2.2 Field mapping ..................................................................................... 18

3. Summary of papers .............................................................................. 21 3.1 Identifying the characteristic signatures of fold-accommodation faults ......................................................................................................... 21 3.2 Modelling two sequential coaxial phases of shortening in a foreland thrust belt ................................................................................... 24 3.3 Mega arrowhead interference patterns in the Central part of the Yanshan Orogenic Belt, North China ....................................................... 28 3.4 Superimposed folding and thrusting by two phases of non-coaxial shortening in analogue models ................................................................. 31

4. Conclusions ......................................................................................... 35

5. Sammanfattning på svenska ................................................................ 37

Acknowledgements ....................................................................................... 39

References ..................................................................................................... 41

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1. Introduction

1.1 Superimposition of contractional structures Superimposition of contractional structures that is formed during a single progressive deformation or by multiple shortening phases (coaxial and non-coaxial) is widely reported from different parts of the world (e.g. Suppe, 1983; Mitra, 1990; Wrede, 2005; Ramon and Rosero, 2006; Misra and Gupta, 2014). Superimposition related to a single progressive deformation induces a close relationship between folding and thrusting. During develop-ment of a fault, the hanging wall layers may fold when they ride over the fault bend (fault-bend fold), or slip is consumed by folding in the fault tip (fault-propagation fold), or the relatively competent layers are folded in re-sponse to slip on a décollement (detachment fold) (Suppe, 1983; Jamison, 1987; Mitra, 1990; Suppe and Medwedeff, 1990; Erslev, 1991; Homza and Wallace, 1995; Allmendinger and Shaw, 2000; Cristallini, 2002; Johnson and Johnson, 2002; Cardozo et al., 2003; Mitra, 2003; Suppe, 2011). In con-trast, during progressive folding, faults may develop to accommodate strain variations related to structural and stratigraphic position within a fold. Such faults are called fold-generated faults, fold-accommodation faults, and fold-related faults (Johnson, 1980; Morley, 1994; Mitra, 2002; Wu et al., 2007). When superimposition occurs where later deformation phases overprint pre-existing structures (coaxial and non-coaxial), the early formed structures are modified by a later phase shortening (i.e. they are cut by the later phase faulting or folded by later phase folding) (Steiger, 1964; Roy, 1995; Saha, 2002; Weinberger et al., 2009; Dario et al., 2010; Cashman et al., 2011; Der-ryberry, 2011; Li et al., 2012; Shah et al., 2012; Sen and Pfander, 2013). In some cases, the early phase structures may be reactivated during the later shortening (Jayangondaperumal and Dubey, 2001; Corredor, 2003; Ramon and Rosero, 2006; Soto et al., 2007; Cashman et al., 2011; Lim and Cho 2012).

Many researchers have been studying the superimposition related to a single progressive deformation focusing on fault-related folds using analyti-cal and experimental investigations, and natural example studies (Suppe, 1983; Jamison, 1987; Mitra, 1990; Suppe and Medwedeff, 1990; Erslev, 1991; Homza and Wallace, 1995; Storti et al, 1997; Bonini, 2003; Bernard et al., 2007), Their works indicated that in fault-related folds formation of fold is geometrically and kinematically controlled by slip along a fault surface.

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Furthermore, some other investigations also documented that during devel-opment of fault-related folds, parameters such as variable mechanical strati-graphy, initial fault dip, inter-layer detachments, multi-ramps and friction along fault ramp may affect the way faults propagate and thus fold geometry (Medwedeff and Suppe, 1997; Maillot and Koyi, 2006; Albertz and Lingrey, 2012; Albertz and Sanz, 2012). In contrast, studies on fold-related faults or fold-accommodation faults are relatively few. Mitra (2002) firstly provided a systematic investigation to such structures, where he defined fold-accommodation faults and summarized the common types for these struc-tures. The concept of fold-accommodation faults has been later applied to interpret the tectonic evolution of some regions (Wrede, 2005; Muk-hopadhyay, 2008). A primary question is though how to recognize fold-accommodation faults in nature. Some researchers have attempted to address this question by providing identifying characteristics for fold-accommodation faults (Mitra, 2002; Wrede, 2005; Wu et al., 2007; Deng et al., 2009). However, based on these published studies, fold-accommodation faults could still not be distinguished from the general reverse faults and thrusts in fault-related folds in an applicable and efficient way.

In the case of superimposition by multiple phases of shortening, reactiva-tion of pre-existing thrusts during the later shortening phases has been re-ported from some areas, either during coaxial superimposition (Ramon and Rosero, 2006), or non-coaxial shortening phases (Jayangondaperumal and Dubey, 2001; Corredor, 2003; Lim and Cho, 2012). Superimposed folding and thrusting have been widely reported in nature, where pre-existing thrusts may be reactivated; fold-interference patterns develop; and pre-existing structures are displaced by later phase thrusting (Corredor, 2003; Abu Sharib and Bell, 2011; Lim and Cho, 2012; Shah et al., 2012; Crespo-Blanc, 2013). During non-coaxial superimposition, when the later shortening direction is oblique to the trends of pre-existing structures, reactivation may result in both dip- and strike-slip along the pre-existing thrusts (Soto et al., 2007; Lim and Cho, 2012). Superimposition of folding could result in tightening of the pre-existing fold during coaxial shortening (Corredor, 2003; Derryberry, 2011) and formation of fold interference patterns during non-coaxial short-ening (Aller and Gallastegui, 1995; Simon, 2004; Abu Sharib and Bell, 2011; Lim and Cho, 2012; Crespo-Blanc, 2013; Tian et al., 2013). Fold-interference patterns are also investigated analytically and experimentally (Ramsay, 1962, 1967; Ghosh and Ramberg, 1968; Skjernaa, 1975; Thiessen et al., 1980; Watkinson, 1981; Ramsay and Hube, 1987; Ghosh et al., 1992, 1993, 1995, 1996; Grujic, 1993; Grujic et al., 2002; Grasemann et al., 2004; Sengupta et al., 2005). In such studies, geometry and kinematics of superim-posed folds are well illustrated. However, the published analytical and ex-perimental studies related to fold interference patterns have not considered the effect of thrusting. Such common features, i.e. superimposition of thrust-related folding need more analytical and experimental investigations, where

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the interaction between folding and thrusting during superimposition could be studied.

Published investigations on superimposed buckle folding indicate that orientation and geometry of the early phase folds have significant influences on the later phase folds and the final geometry of the fold-interference pat-tern (Ghosh and Ramberg, 1968; Skjernaa, 1975; Watkinson, 1981; Odonne and Vialon, 1987; Ghosh et al., 1992, 1993; Grujic, 1993; Sengupta et al., 2005). However, how the pre-formed structures and fault reactivation affect the geometry, spacing and propagation of the later phase structures during multiple phases of shortening (coaxial and non-coaxial) remains poorly un-derstood.

1.2 Effect of erosion and sedimentation Erosion and sedimentation commonly occur either synkinematically (i.e. during development of structures) under one progressive shortening phase (Hessami et al., 2001; Barrier et al., 2002; Bonnet et al., 2007; Malavieille, 2010; Sieniawska et al., 2010), or in an interval between two deformation phases to form an unconformity (Corredor, 2003; Cashman et al., 2011). Many published experiments have focused on syn-tectonic surface processes during one progressive shortening (e.g. Storti and McClay, 1995; Mugnier et al., 1997; Koyi et al., 2000; Persson and Sokoutis, 2002; Saura et al., 2004; Konstantinovskaya et al., 2007; Cruz et al., 2010; Konstantinovskaya and Malavieille, 2011). These studies illustrated that syn-tectonic erosion and sedimentation could affect the number, geometry, activation and reactivation of thrusts, and the propagation sequence of thrust wedges. However, few studies have investigated the effect of an angular unconformity on the devel-opment of the newly formed structures and the reactivation of a set of pre-existing thrusts during multiple shortening (both coaxial and non-coaxial).

Studies of natural examples of fault reactivation and different structures shown in the layers above and below an unconformity are usually conducted based on the structures deciphered from seismic profiles (Corredor, 2003; Cashman et al., 2011). Structures shown in the layers below an unconformity may not be well displayed due to the presence of younger sediments above the unconformity, and parts of structures may be missing due to later ero-sion. As such, experimental studies on multiple shortening phases separated by an unconformity are needed. In such experiments, the entire geometry of structures shown in the layers both above and below the unconformity can be observed in either vertical or horizontal sections. In addition, reactivation of the pre-existing structures can also be monitored during experiments.

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1.3 Thesis objectives The general objective of this thesis is to investigate superimposition of struc-tures formed during either progressive deformation, or multiple phases of shortening (coaxial and non-coaxial). The structures were studied in ana-logue modelling and in nature. Through this thesis, the following problems are discussed and better understood: superimposition of faulting to the pro-gressive folding phase (i.e. formation of fold-accommodation faults), reacti-vation of early phase structures by a later coaxial and non-coaxial shortening phase, effect of pre-existing structures on development of later phase struc-tures (e.g. spacing of the later phase structures), fold interference pattern observed in natural examples and produced by experiments, interaction be-tween folding and thrusting during multiple shortening, impact of erosion and sedimentation between two shortening phases on development of new structures, and reactivation of the pre-existing structures.

The detailed aims of the thesis are described as followed: 1) Study the geometric and kinematic relationship between fold-

accommodation faults and their associated folds by detailed analyses of structural and spatial distribution of faults within a fold, displacement distri-bution along the faults, and the change of cut-off angle between the faults and folded layers in different fold regimes. (Paper I)

2) Summarize the characteristic signatures of fold-accommodation faults in order to identify them from the general reverse faults and thrusts in fault-related folds. (Paper I)

3) Study the geometric features of structures formed by two phases of shortening (coaxial and non-coaxial), shown in vertical and horizontal sec-tions. The focus has been on the following elements: feature of folds, thrusts, fold interference patterns, and the relationship between folds and thrusts. (Paper II and IV)

4) Study the development of structures during a later shortening phase by monitoring structure spacing. To evaluate the effects of the pre-existing early phase structures on the spacing of later phase structures. (Paper II and IV)

5) Investigate the reactivation of early phase structures (thrusts) during a later shortening phase, by monitoring strike- and dip-slip along these thrusts in profile and map-view. (Paper II and IV)

6) Study the effect of an angular unconformity on reactivation of pre-existing structures, by changing the depth of erosion and the thickness of post-erosional deposit above the unconformity, and addition of a weak layer on the unconformity surface. (Paper II)

7) Study “large-scale” fold interference patterns formed during two phases of non-coaxial shortening (orthogonal and oblique) where folds and thrusts are superimposed, and comparing them with “pure” fold superimpo-sition without the effect of thrusting. (Paper IV)

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8) Study fold interference patterns from two kilometer-scale examples exposed in the central part of the Yanshan Orogenic Belt, North China from their map-view geometry and overprinting sequence of the two folding phases and detailed field structural mapping. (Paper III)

9) Investigate the structural relationship between superimposed folds and their bounded thrusts. The aim here has been to understand the time se-quence of multiple shortening phases and the main shortening directions in the study area, the central part of the Yanshan Orogenic Belt, North China. (Paper III)

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2. Methodology

2.1 Physical experiments Scaled physical modelling (i.e. sandbox and centrifuge modelling) has been widely used for simulating the progressive evolution of thrust and fold sys-tems (Koyi, 1997; McClay, 2004). Some researchers have used sandbox models to investigate development of thrusts that are influenced by syn-tectonic erosion and sedimentation during one continuous shortening process (e.g. Storti and McClay, 1995; Koyi et al., 2000; Persson and Sokoutis, 2002; Konstantinovskaya and Malavieille, 2005, 2011; Bonnet et al., 2007; Malavieille, 2010; Sieniawska et al., 2010). These studies have shown that erosion and sedimentation can promote activity of internal structures, reacti-vate inactive thrusts, and result in out-of-sequence propagation in a fold-and-thrust belt. Reactivation of faults during two separate deformation phases has been studied by Soto et al. (2007) who experimented that a series of Riedel faults formed by an initial pure strike-slip phase were reactivated as oblique thrusts during later shortening. Fold interference patterns that are formed by two phases of orthogonal shortening have also been studied using viscous materials such as plasticine, modelling clay, silicone putty, and paraffin waxes (e.g. Ghosh and Ramberg, 1968; Skjernaa, 1975; Ghosh et al., 1992, 1993; Grujic et al., 2002). Superimposed buckling in single or multilayers was achieved in these studies, where fold interference patterns such as dome-and-basin, mushroom- or arrowhead-shaped interference patterns were displayed.

In this thesis, in order to investigate development and reactivation of structures that have formed by multiple shortening phases, two types of sandbox modelling were carried out, where the two shortening phases were coaxial or non-coaxial. Two different granular materials were used in the experiments. Quartz sand with grain size of 80-120 µm, density of 1.53 g/cm3, and internal friction angle of 30-33° (Maillot and Koyi, 2006) was used to simulate the brittle upper crustal rocks. Another material, micro glass beads, with the grain size of 50-100 µm, density of 1.46 g/cm3, and internal friction angle of 19-23° (Maillot and Koyi, 2006) was used to simulate weaker sedimentary rocks (shale and marl). The relatively weak glass beads were used as a potential décollement horizon in the coaxial modelling.

In experiments with two phases of coaxial shortening, models were set up in a rectangular sandbox (length 60 cm, width 39 cm). The initial thickness

15

of sand layers differs from model to model. All models were shortened from one side to form a wedge (Figure 2.1). In some models, part of the wedge was eroded and new sand layer were placed on the top of the erosional sur-face. A second deformation phase was then carried out by further shortening the models in the same direction. A 0.5 cm thick glass beads layer was placed on the top of the erosional surface in some models.

Figure 2.1 Simplified sketches of the different model setups.

In the experiments with two phases of non-coaxial shortening, all the sand-box models were built up with a uniform dimension (length 52 cm, width 48 cm, and sand thickness 1.5 cm). Basically, the models were shortened ini-tially from one direction during the first deformation phase. During the sec-ond phase, some models were shortened from another direction that was perpendicular to the first shortening phase and orthogonal superimposition was achieved. Some other models were shortened in an oblique way, i.e. the second shortening direction was oblique to the trend of the first phase struc-tures. In order to carry out oblique superimposition, either a triangle back-stop was used in the first shortening phase (Figure 2.2), or during the second phase the model was shortened with a backstop that made an acute angle with the first phase shortening direction.

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Figure 2.2 a) Photography of the model setup for an oblique superimposition ex-periment. b) Schematic drawing of the model setup. The inbuilt triangular backstop can induce structures striking parallel to the oblique boundary.

Two centrifuge models were carried out to investigate fold interference pat-terns excluding the effect of thrusting that developed in the sandbox models. Pure plasticine (competent material) and mixture of plasticine with silicone

17

putty and barite powder (relatively less competent material) were used in these centrifuge experiments (Figure 2.3). The pure plasticine has the density of 1.64-1.65 g/cm3, and the viscosity is 2 to 6×108 Pa s, while the plasticine-putty mixture had a density of 1.66-1.67 g/cm3 and the viscosity of 2×107 to 2×108 Pa s. The analogue materials were rolled out to 1 mm thick layers. The two models were constructed of multiple layers with the dimensions of 15 ×15 ×1 cm and 18 ×16 ×1 cm, respectively. Two phases of folding was achieved by two sequential orthogonal shortening phases during spinning in the centrifuge at 700 G.

Figure 2.3 a) Oblique view of a centrifuge model before deformation showing the plasticine and plasticine-putty layers. b) Pure-shear box used to shorten models in the centrifuge. The back-stop is driven by a small motor attached to a worm screw. In this image, model is shortened in one direction to 20%.

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2.2 Field mapping Model investigation has been combined with detailed structural mapping of superimposed structures in the Yanshan Orogenc Belt (YOB). This belt is generally trending E-W and NE-SW, and extends across the northern edge of the North China crustal block (Figure 2.4). The characteristic basement-involved thrusts, basement-cored anticlines and nappes were mainly formed due to Mesozoic multiple phases of shortening (Davis et al., 2001; Cope et al., 2007; Zhang et al., 2011). The Chengde-Pingquan area in the central part of the YOB displays a number of thrusts, strike-slip faults, buckle folds that are dominantly trending E-W, NE-SW and NW-SE. Two kilometer-scale folds show fold interference patterns, which is resulted from two phases of non-coaxial shortening, and hence suitable to compare with the superim-posed structures shown in the models. Detailed field mapping was conducted to decipher the geometry and kinematics of the two superimposed structures that we called Daheishan and Pingquan folds in this thesis.

Based on the published geological map (HBG-SGSG, 1976), stratigarphic and structural boundaries were checked by tracing their strikes and determin-ing the lithologies and dips on either side of the boundaries. According to our field data, several stratigarphic and structural boundaries have been modified. Our mapping results are significant to better understand the ge-ometry and kinematics of the Daheishan and Pingquan folds. The small-scale structures related to folding such as cleavage, schistosity, crenulation, and flexural-slip striations are poorly developed in these two superimposed struc-tures. However, outcrop-scale folds and faults are abundantly observed within the well-bedded strata. About 50 outcrop-scale folds were observed, photographed, sketched and measured in the field. Measurements of bed dips and fold axes were plotted using a Stereonet Program (version 9.1.0 by R.W. Allmendinger). These stereonet plots of outcrop-scale folds were related to the large-scale folds, and helped in interpreting the kinematics of regional-scale folding.

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3. Summary of papers

3.1 Identifying the characteristic signatures of fold-accommodation faults In Paper I, we investigate hand-specimen and outcrop-scale examples of folds to identify the characteristic signatures of fold-accommodation faults. The geometric and kinematic relationship between thrusts and folds that they reside in were described and analysed in detail.

The hand-specimen scale folds are formed in thin layers of limestone and argillaceous limestone (Figure 3.1). They are generally open folds and have round hinge zones. These hand-specimen scale folds are concentric where individual layers display constant thickness throughout the fold. The out-crop-scale folds are seen in various lithologies. Some deformed layers are composed of 5 to 15 cm thick siltstone and thin silty shale and/or shale in-terbeds (Figure 3.2). Other folds are formed in medium-thick beds of marble or limestone. In most cases, the relatively competent layers have constant thickness throughout the folds. These outcrop-scale folds have an angular, chevron-like or round hinge zone. They are exposed in the scales varying from several to dozens of meters. In these fold examples which are of differ-ent scales, geometries and lithologies, fold-accommodation faults are devel-oped. Among them, out-of-syncline thrusts, into-anticline thrusts and wedge thrusts are commonly observed.

Out-of-syncline and into-anticline thrusts are formed as the result of space reduction in the core of a fold. In general, these thrusts originate in one limb of the fold, and their tips are usually located at the hinge zone. In some cases, the thrust can also cut through the hinge zone and tip in another limb with a large cut-off angle. Displacement along out-of-syncline and into-anticline thrusts decrease toward the core of the fold, which is quantitatively shown in a separation-distance plot. Wedge thrusts, which are another type of fold accommodation faults, are commonly developed in interbedded strata with competence contrast. Wedge thrusts usually display an flat-ramp ge-ometry as they gradually cut through the competent layers up-section, and slip parallel to bedding within the relatively incompetent layers. A wedge thrust is commonly moving in the same direction as the sense of flexural-slip. A wedge thrust that forms in the hinge zone of a fold is called hinge wedge thrust, while it also can develop in the limb of a fold, where it is called limb wedge thrust. A series of hinge wedge thrusts can be developed

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as imbricate thrusts with the same dip and moving sense, or as overlapping wedges dipping and moving in opposite directions to each others. Formation of overlapping wedges causes a significant thickening of the hinge zone in comparison to the limbs of the fold. It is noticed that not all wedge thrusts are fold-accommodation faults. Some thrusts may predate folding, and are involved into a fold later. In this case, these thrusts are not necessarily mov-ing in the same direction as the sense of the flexural-slip during folding.

Figure 3.1 Scanned images of the hand-specimen folds (left column) and their corre-sponding line drawings (right column). Out-of-syncline and into-anticline thrusts, e.g. fault F5 (a) and fault F4 (b); Limb wedge thrust, e.g. faults F9 and F14 (c).

Based on observations from hand specimen, outcrop-scale structures, and published literatures, several characteristic signatures that are summarized in

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order to identify fold-accommodation faults from other types of thrusts. Fold-accommodation faults extend both upwards and downwards within a fold. Limited displacement along fold-accommodation faults renders the fold relatively intact. Displacement along the out-of-syncline thrust decreases from the limbs toward the hinge zone of the fold, and displays diagnostically negative slope (separation value decreasing away from the upper fault tip) in the separation-distance graph. Observations indicate that fold-accommodation faults usually have small cut-off angles in the limbs of the fold, but in some cases the cut-off angle could be large when thrusts cut through the hinge zone. Spatial distribution of fold-accommodation faults displays a close relationship with the axial surface of the fold. Our analyses also indicate that fold-accommodation faults display kinematic consistency with flexural-slip folding.

Figure 3.2 Photography and line drawing of one outcrop-scale fold asscoaited with fold-accommodation faults. Faults F1, out-of-syncline thrust; F2, into-anticline thrust; F3, limb wedge thrust.

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3.2 Modelling two sequential coaxial phases of shortening in a foreland thrust belt Paper II presents the results of modelling the formation and reactivation of structures during two sequential coaxial phases of shortening in a fold-and-thrust belt. In these models, a sufficient hiatus between the two sequential coaxial phases is considered. Erosion and sedimentation were conducted after the first phase, which created an angular unconformity that was subse-quently deformed during further coaxial shortening. Results of the analogue models were compared with two examples of foreland thrust belt, one from the northeastern Cordillera in Colombia and another from the Variscan fron-tal zone in Western Europe. In these two areas, an angular unconformity separates at least two phases of shortening.

Two kinds of analogue materials were used in the experiments, dry and loose quartz sand, and micro glass beads. A thin layer of glass beads was placed on the erosion surface in some models to act as a potential décolle-ment horizon, since glass beads has a relatively lower frication than quartz sand. The initial thickness of shortened sand layers in the first phase and the thickness of sediment on the erosion surface differed between models. After sedimentation the total thickness of the shortened sand layers during the second phase is 4 cm in some models and 4.5 cm in the others. In order to discuss how thickness variation in the part of model above and below the unconformity affects the second phase structures, a parameter of thickness ratio [T2-T1]:T1 is introduced here, where T1 and T2 are the total thickness of sand package after erosion and sedimentation, respectively. One standard model was prepared, in which the 4 cm thick sand layers was shortened con-tinuously without applying erosion or sedimentation. Structures formed dur-ing one continuous shortening in the standard model was compared with structures formed by two sequential coaxial shortening phases separated by an unconformity. All the models were shortened above the same basement with a relatively low basal friction (µ = 0.23).

Vertical sections of the models show that the unconformity separates the models into two parts (Figure 3.3). Above the unconformity, structures are dominated by the reactivated first phase thrusts, second phase thrusts and relatively broad second phase box-folds. Below the unconformity, superim-posed coaxial shortening produces a complex structural style, where more folds and thrusts are shown than above the unconformity. The first phase folds are generally tighter, and their crests are truncated by the unconform-ity. Some of the first phase thrusts are inactive during the subsequent short-ening, as they terminate at and do not displace the unconformity. Some oth-ers are reactivated and propagate up-section into the upper sediments dis-placing the unconformity. Along these reactivated thrusts, total displacement abruptly increases downwards across the unconformity where these thrusts accumulate two phases of displacement in the layers below the unconform-

25

ity. This feature could assist in distinguishing thrust reactivation in the field. The second phase thrusts cut the layers below the unconformity. Some of them just develop in the undeformed foreland of the first shortening phase. Other second phase thrusts are formed in area containing the first phase structures (hinterland), and hence cut across and displace the pre-existing folds and thrusts.

Figure 3.3 Photograph and line drawing of one model which has experienced two phases of coaxial shortening. a) A series of folds and thrusts formed after the first shortening phase. b) After the second phase of shortening, newly thrusts formed (e.g. T2 and T6); some early phase thrusts remained inactive (e.g. F3-5); whereas, some early phase thrusts were reactivated (e.g. F12/T4).

Models results demonstrate that reactivation of pre-existing structures has significant effect on structure spacing during the later shortening phase. In the models which experienced two coaxial shortening phases, spacing of the second phase structures is less than spacing of the structures of the standard model, even though thickness of the shortened layers in both models is simi-lar. Presence of a weak glass-beads layer at the unconformity level did not actually act as an intermediate décollement. Reactivation of pre-existing structures offsets the weak layer and hence, prevents it from acting as an intermediate décollement. Model results also show that when the thickness ratio is large, i.e. the proportion of post-erosional layers increases in the en-tire shortened package, relatively fewer first phase thrusts are reactivated

26

during the second shortening phase. Meanwhile, along the reactivated thrusts less displacement occurs in models with a large thickness ratio.

Comparison between model results and the two regional examples display good agreements. As in the models, both natural examples had relatively tight folds below the unconformity. The crests of these folds were eroded and incorporated into the later phase folding that folded both the unconform-ity boundary and the overlying upper sediments (Figure 3.4). In the Cordil-lera example, a series of imbricates was truncated by the unconformity and remained inactive during the later phase of deformation. In the Variscan frontal zone, a number of early phase thrusts were reactivated during the later phase. They propagated through the unconformity, and had signifi-cantly larger displacement within the stratigraphy below the unconformity than within those above it.

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Figure 3.4 Seismic profile across the Los Deseos syncline in the Eastern Cordillera fold belt, Colombia, and the corresponding line drawing (from figure 6 in Corredor, 2003).

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3.3 Mega arrowhead interference patterns in the Central part of the Yanshan Orogenic Belt, North China In paper III, we investigated two kilometer-scale fold interference patterns from the Central part of the Yanshan Orogenic Belt (YOB), the Chengde-Pingquan area, North China. We call these two examples Daheishan and Pingquan folds here. Detailed geological mapping and measurements of outcrop structures were conducted to decipher the geometry of the two su-perimposed structures. Age of the two folding phases that formed the Dahe-ishan and Pingquan folds and their overprinting sequence were also dis-cussed in this paper.

The E-W trending YOB has formed due to Mesozoic multiple shortening, which has been suggested by other researchers (Zheng et al., 2000; Davis et al., 2001; Deng et al., 2005; Cope et al., 2007; Zhang et al., 2007; Dong et al., 2008). The Chengde-Pingquan area in the central part of the YOB is characterized by a number of thrusts, strike-ship faults, buckle folds, where some structures are trending E-W to NE-SW, and others are trending NW-SE on the geological map (Figures 3.5 and 3.6). The Daheishan and Pingquan folds which are exposed in this area display fold interference pat-terns (Figures 3.5 and 3.6). In general, two sets of fold axial traces can be recognized in map-view, where one axial trace trends NE-SW to ENE-WSW, and the other is roughly NW-SE. In the Daheishan and Pingquan folds, small-scale structures, e.g. planar cleavage, schistosity, crenulation, and flexural-slip striations are poorly developed. However, outcrop-scale folds and faults are abundantly seen in the field. Stereonet plots of bedding in these outcrop-scale folds are used to analyse kinematics and overprinting relationship of the large Daheishan and Pingquan folds. Field mapping and stereonet plots indicate that both the Daheishan and Pingquan folds show arrowhead interference pattern in map-view (Figure 3.7). The NW-SE trend-ing F1 folds were refolded by a later ENE-WSW F2 folding, and hence dis-play the current crescent geometry. Bedding plots also show that the inter-limb angles of the two phases folds are slightly larger than 90°, and the folds have steeply inclined axial surfaces but no overturned limb.

Steronet plots for some outcrop-scale folds show that some of these out-crop-scale folds are roughly trending ENE-WSW, plunging sub-horizontally, and have cynlindrical or sub-cynlindrical geometries. We interpret that these outcrop-scale folds were formed during F1 folding phase. However, stereonet plots for some other outcrop folds indicate that the trends of their axial traces vary from SSW, WSW to NW, and their plunge is variable (10-30°). Addi-tionally, these outcrop-scale folds are spatially located in the limbs and/or close to the hinge zone of the large-scale F1 folds. We interpret that these outcrop-scale folds were formed during F1 folding, and probably rotated with the large-scale F1 folds and even refolded during the F2 folding. The Dahe-ishan and Pingquan folds are cut by faults with different trends. Restoration

29

of the later faults displayed two mega arrow-head interfernce patterns indi-cating the superimposition of two phases of folding. These restored map-views of the Daheishan and Pingquan folds show that an arrowhead interfer-ence pattern (here called modified type-2 pattern) also can form, when the early F1 folds have an interlimb angle slightly larger than 90° but their axial surfaces are inclined.

Figure 3.5 Geological map of the Daheishan folds and the related thrusts. CDF-Caoniangou-Dajikou fault; SPF- Sangyuan-Pingquan fault; XPF-Xiadian-Pingquan fault; XSF-Xiadianzi-Shangyuan fault; YSF-Yuzhangzi-Shangyuan fault (modified after HBG-SGSG, 1976).

Figure 3.6 Geological map of the Pingquan folds and the related thrusts. DSF-Dongshuiquan-Songzhangzi fault; GPF-Gubeikou-Pingquan fault; LLF-Lijialiangzi fault; WZF- Wanzhangzi fault (modified after HBG-SGSG, 1976).

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Figure 3.7 Restored map view geometry of the Pingquan area. An arrowhead inter-ference pattern is shown. Apparent displacement along later faults is removed.

Several kilometre-scale faults are shown in the Chengde-Pingquan area. The strikes of these faults are generally NW-SE and NE-SW, sub-parallel to the axial traces of F1 and F2 folds, respectively. The geological map and our field observations show that these faults clearly cut the interference patterns of the Daheishan and Pingquan folds. This structural relationship together with the unconformable relationship implies that these faults postdate the two folding phases that formed the interference pattern. Based on our analy-ses of field data and reinterpretation of the findings of previous studies, we suggest five deformation phases in the Chengde-Pingquan area, the central part of the YOB. Superimposition of the roughly NW-SE trending F1 folds and the ENE-WSW trending F2 folds resulted in the formation of the arrow-head interference pattern during the first two shortening phases, which had occurred earlier than Late Triassic. Proterozoic and Paleozoic units were folded during the two folding phases, and later unconformably covered by upper Triassic units. The NE-SW trending faults were formed during a third deformation phase (thrust SPF in Figure 3.5, and thrust DSF in Figure 3.6), from late Triassic to early Jurassic. These faults were later displaced by the NW-SE trending faults of the fourth deformation phase in about Middle Jurassic. The fifth deformation phase formed another set of faults that are trending ENE-WSW to NE-SW and cut SPF and DSF, and YSF and LLF structures of the third and fourth deformation phases.

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3.4 Superimposed folding and thrusting by two phases of non-coaxial shortening in analogue models In paper IV, superimposition of folds and thrusts were studied using results of a series of systematically designed analogue models. The models simulate superimposed structures that were formed by the second phase folds and thrusts overprinting the first phase ones, either orthogonally or obliquely. The surface of the models was monitored by a high-resolution laser scanner. Centrifuge models were also run to help better understanding fold interfer-ence patterns by excluding the effect of thrusting, since in sandbox models folds and thrusts developed together. Pure quartz sand was used in the sand-box models, while in the centrifuge models two analogue materials were used, pure plasticine and plasticine-silicone putty mixture. After experi-ments, horizontal sections were taken at different levels in each model, from which the 3D geometry of the structures can be observed.

In the sandbox modelling, after the first shortening phase, a wedge formed and thickened hinterland, whereas towards the foreland the sand layers remained undeformed keeping their original thickness. During the second shortening phase, the newly formed structures superimposed the early phase wedge orthogonally or obliquely. As a result, in the area where two phases of hinterlands superimposed, the highest topography was created and deepest layers were exhumed. On the contrary, the lowest topography was created in the part of the models where the forelands of the two shorten-ing phases superimposed. During the second shortening phase, thrusts later-ally propagated from the first phase thin foreland to the thickened hinterland. As such, structure spacing is larger in the thickened hinterland areas and smaller where such thickening is limited. However, the increase in structure spacing in the thickened hinterland is not as large as expected in thick mod-els where no earlier shortening has taken place. During oblique superimposi-tion, the pre-existing first-phase thrusts were reactivated during the second shortening phase, where dip- and/or strike-slip were observed along these thrusts.

Model results seen in both topography and horizontal sections show that the trends of structures that were formed during two orthogonal shortening phases are approximately orthogonal to each other (Figure 3.8). In oblique superimposition, the second phase structures are striking obliquely to the first phase ones in the thin foreland areas, while the two structural trends tend to be orthogonal to each other where the layers were thickened by the first phase (hinterland) (Figure 3.9). Presence of an unconformity between the two shortening phases causes formation of different structures at differ-ent structural levels. At shallow levels, if erosion removes part of the struc-tures above the unconformity boundary, only the later phase structures can be seen in the post-erosional layers. At deeper levels below the unconform-ity, the superimposed and the firsts phase structures are shown.

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In the horizontal sections (map-view), dome-and-basin fold interference pattern is commonly seen in the sandbox and centrifuge models. Arrowhead-shape interference pattern is also seen in the centrifuge models. Where an anticline overprints an anticline, a dome structure is formed and the deep structural level is uplifted. Bedding is curved outwards in the dome, and bed dip and fold plunge are outwards. A syncline overprints a syncline to form a basin, where bedding is curved outwards in map-view, and bed dip and fold plunge are inwards. A relatively shallow structural level is exposed in the basin. In areas where an anticline overprints a syncline and/or a syncline overprints an anticline, bedding is curved inwards, and anticlines plunge inwards whereas synclines plunge outwards. In the arrowhead-shaped inter-ference pattern developed in the centrifuge models, the overprinting se-quence of two folding phases could be determined according to the map- view geometry. In the neck of the arrow, the early phase folds clearly show the inwards curved bedding and outwards plunging closures for an early syncline and inwards plunging for an early anticline.

Model results are compared with three natural examples from the Chung-nam Basin, Korea, the Mount Isa Inlier, Australia, and the central part of the Yanshan Orogenic Belt. All these tectonic areas have undergone multiple phases of non-coaxial shortening. As shown in the models, dome-and-basin and arrowhead-shaped fold interference patterns are displayed on an orogeny-scale in these areas. The pre-existing early phase thrusts can be reactivated by the later shortening phase. In each shortening phase deforma-tion might not develop throughout everywhere. As such, superimposed struc-tures can be seen in some areas, whereas, only one phase structure is shown in other places. In the models, thrusts and folds are formed together in each shortening phase. Therefore, development of thrusts does not modify the fold interference patterns. However, in some natural examples the map- view geometry of fold interference patterns are modified by the later phase thrust-ing.

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35

4. Conclusions

In this thesis, superimposition of structures which are resulted from progres-sive deformation (fold-thrust relationship) or multiple shortening phases i.e. coaxial and non-coaxial (orthogonal or oblique) are investigated in models and nature. Analogue models are designed to simulate two phases of short-ening, where the early phase structures are superimposed by a later coaxial or non-coaxial phase. The main findings of this thesis are summarized be-low:

1) During progressive folding, thrusts can develop in either the hinge zone or the limbs of the fold, which accommodates strain variations during fold evolution. These thrusts, which are called fold-accommodation faults, are secondary structures, and their superimposition does not significantly modify the geometry of the fold. However, such thrusts can cut through the fold axial plane or thicken its hinge zone. Displacement along the thrusts is relatively limited, and usually decreases towards the hinge zone of the fold. Our studies also suggest kinematic consistency between fold-accommodation faults and the flexural-slip folding, which has been reported previously.

2) During coaxial superimposition, more folds and thrusts are seen in the layers below the unconformity. The first phase folds are truncated by the unconformity and are tightened during the later coaxial shortening phase. Some early formed thrusts are inactive during the later shortening and also truncated by the unconformity. Some of the pre-existing structures are cut and displaced by the later thrusting. These features are clearly seen in natural examples from the northeastern Cordillera (Colombia) and the Variscan frontal zone in western Europe, where tightened and partly eroded early phase folds and truncated early phase thrusts indicate two phases of coaxial superimposition.

3) During non-coaxial superimposition, hinterland overprinting hinterland creates the highest topography, where superimposed structures and deepest units are shown in map-view after erosion. Formation of an unconformity between two non-coaxial shortening phases separates different structures. Below such an unconformity, superimposition is only seen in the deeper layers. In contrast, layers above the unconformity show only the later phase structures.

4) Dome-and-basin (type 1) and arrowhead-shaped (modified type 2 here) interference patterns are formed by non-coaxial superimposition. Both in the

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domes and the basins, bedding is curved outwards in map-view. In the dome, layers are dipping and anticlines are plunging outwards, whereas, in the ba-sin the dip of layers and the plunge of synclines are inwards. Where an anti-cline is superimposed by a syncline or a syncline is superimposed by an anti-cline, bedding is curved inwards, and the anticlines plunge inwards and the synclines outwards. The latter feature could be used to determine the over-printing sequence for the arrowhead-shaped interference pattern. Such oro-genic-scale dome-and-basin and arrowhead-shaped interference patterns are shown in nature where two non-coaxial shortening phases have occurred, for example the Chungnam Basin, Korea, and the central part of the Yanshan Orogenic Belt, North China.

5) Reactivation of early phase thrusts during the later shortening phase is commonly observed in coaxial and oblique non-coaxial superimposition. In coaxial superimposition, displacement increases downwards across the un-conformity along reactivated thrusts. This feature is also seen in the profile of a reactivated thrust from the Variscan frontal zone in western Europe, where significantly larger offset in the rocks below the unconformity is seen than in those above the unconformity.

In models with oblique non-coaxial superimposition, pre-existing thrusts are reactivated in both dip direction and along strike. Such reactivation is seen in the Chungnam Basin, Korea, where some segments of the early phase thrusts are reactivated in strike-slip during the later deformation phase.

6) Spacing of later phase structures is significantly affected by pre-existing structures. Basically, structure spacing is much less in models with pre-existing structures than in models where the layers have the same thick-ness but no earlier shortening has taken place. In the case of non-coaxial superimposition, structure spacing is variable being larger in areas where a hinterland overprints a hinterland, and smaller towards the area where a fore-land overprints a foreland.

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5. Sammanfattning på svenska

Överprägling av geologiska kompressionsstrukturer i olika skalor är vanligt förekommande i naturen. Överpräglade strukturer bildas på grund av olika processer: ändring i deformation-anpassning från en struktur till en annan under enstaka progressiva hopskjutningar; successiva coaxiella hopskjut-ningsfaser separerade av en unconformitet; överprägling av olika icke-coaxiella, hopskjutande faser. Genom att använda serier av systematiska analoga modeller kombinerat med detaljerad strukturell kartering i fält, fo-kuserar denna avhandling på geometrin och kinematiken hos överpräglade strukturer som bildas av dessa tre processer. Under en progressiv veckning bildas överskjutningar i ett veck för att anpassa variationer av deformation i olika delar av vecket. Veckets geometri påverkas inte i någon större ut-sträckning av begränsade förskjutningar längs dessa överskjutningar. Under multipla hopskjutningsfaser (coaxiella eller icke-coaxiella),kan däremot de senare överskjutningarna skära av, förskjuta, eller trycka ihop de redan exi-sterande strukturerna. Tidigare existerande överskjutnngar kan reaktiveras i stupningsriktningen och/eller längs strykningen under senare hopskjutning. Tidigare existerande strukturer influerar i sin tur utvecklingen av senare strukturer, vilket resulterar i variation av strukturernas inbördes avstånd. En vinkel-unconformitet mellan två hopskjutande faser skär tydligt tidigare strukturer, och separerar strukturer på olika nivåer. Komplicerade överpräg-lade strukturer är synliga i lagren under unconformiteten, i motsats till i post-erosionslagren . Denna avhandling visar att geometrin och struktur-sekvensen som bildas under en progressiv eller multipel hopskjutning är starkt beroende på om överpräglingen är coaxiell, ortogonal eller i i vinkel, samt av de tidigare strukturernas orienteringen.

Denna studie har kommit till följande slutsatser: 1) Under progressiv veckning kan överskjutningsförkastningar bildas, i

veckets omböjning eller veckben, vilket anpassas efter deformations-variationer under förkortningen. Dessa överskjutningar, vilka kallas veckan-passade förkastningar, är sekundära strukturer, och deras överprägling modi-fierar inte veckets geometri i någon högre grad. Sådana överskjutningar kan dock skära genom veckaxelplanet eller förtjocka veckomböjningen. För-skjutning längs överskjutningarna är relativt begränsad och avtar vanligtvis mot veckets omböjning. Våra resultat är kongruenta med tidigare studier som visar på en kinematisk koppling mellan veckanpassade förkastningar och ”flextur-slip” veckning.

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2) Under coaxiell öveprägling syns fler veck och förkastningar i lagren under unconformiteten. Den första fasens veck skärs av unconformiteten och blir tätare under den senare coaxiella hopskjutningfasen. En del tidigare bil-dade överskjutningar är inaktiva under den senare hopskjutningen och skärs också av unconformiteten. Andra tidigare existerande strukturer skärs och förskjuts av senare överskjutning. Dessa fenomen kan tydligt observeras i de naturliga exemplen från den nordöstra Kordilleran i Colombia och från den variskiska frontalzonen i Västeuropa, där förtätade och delvis eroderade veck och avklippta förkastningar indikerar två faser av coaxiell överprägling.

3) Under icke-coaxiell överprägling, bildas den högsta topografin då hin-terland överpräglar hinterland, där överpräglande strukturer och de djupaste enheterna kan ses i kartvy efter erosion. Bildning av en unconformitet mel-lan två icke-coaxiellt avkortande faser separerar olika strukturer. Under en sådan unconformitet syns överprägling bara i djupare lager. I contrast till detta syns bara de sena strukturer ovanför unconformiteten.

4) Dom-och-bassäng (typ 1) och pilspetsformade (modifierad typ 2) inter-ferensmönster bildas av icke-coaxiell överprägling. Både i domerna och i bassängerna är bäddningen kurvad utåt i kartvyn. I domen stupar lagren utåt, liksom antiklinalerna, medan både lagren och synklinalerna stupar inåt i bassängen. Där en antiklinal överpräglas av en synklinal eller en synklinal är överpräglad av en antiklinal, kurvar bäddningen inåt, och antiklinalen stupar inåt och synklinalen utåt. Det senare fenomenet kan användas för att be-stämma överpräglingssekvensen för det pilspetsformade interferensmönstret. I naturen kan man se sådana dom-och-bassäng samt pilspetsformade interfe-rensmönster i orogen skala, där två icke-coaxiella förkortningsfaser har in-träffat, till exempel Chungnambassängen i Korea, och den centrala delen av Yanshan orogenesen i norra Kina.

5) Reaktivering av tidiga överskjutningar under den senare förkortnings-fasen observeras ofta i coaxiella och vinklade, icke-coaxiella överpräglingar. I coaxiella överpräglingar ökar förskjutningen nedåt över unconformiteten längs reaktiverade överskjutningar. Detta fenomen kan också ses i profil i en reaktiverad överskjutning från den variskiska frontalzonen i Västeuropa, där signifikant större offset kan ses i bergarterna under unconformiteten än de som ligger över den. I modeller med vinklad, icke-coaxiell, överprägling, reaktiveras tidigare existerande överskjutningar, både i stupningsriktningen och längs strykningen. Sådan reaktivering kan ses i Chungnambassängen (Korea), där några segment av den tidigare fasens överskjutningar är reakti-verade i strike-slip under den senare deformationsfasen.

6) Skalan på sena strukturer är avsevärt påverkade av tidiga strukturer. I grunden är skalan på strukturerna mycket mindre i modeller med redan exi-sterande strukturer än i modeller där lagren har samma tjocklek utan att nå-gon tidigare avkortning har skett. I fallet med icke-coaxiell överprägling, är strukturskalan variabel, större i områden där hinterland överpräglade ett hin-terland, och mindre i området där foreland överpräglade ett foreland.

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Acknowledgements

For the past few years, I met many people; many things happened to me; a lot of happy memories I keep in my heart; while a lot of sorrow inevitably also accompanied. However, I have been given a lot during these four years of my life. For all of these, I want to give my thanks, and I will never forget.

First and foremost, I sincerely thank my main supervisor Prof. Hemin Koyi for all the help he has given to me. I am grateful for being given the opportunity to come to Sweden and work with Hemin. Hemin is such a great supervisor who always gave me his constant support, encouragement, advice and guidance. I appreciate the way he expanded my mind when we did map-ping in the field. I thank him for introducing me into the fantastic world of analogue modelling. I will never forget the times when we discussed a struc-ture in the field or modelling, revised each manuscript and debated the rele-vant scientific problems. Every piece of knowledge I learned from Hemin helps me getting close to be a structural geologist.

I would like to thank my co-supervisors, Prof. Changhou Zhang from China University of Geosciences (Beijing) and Dr. Faramarz Nilfouroushan. Prof. Zhang who was my supervisor during my MSc study continued sup-porting the field surveys in China every summer. Without his arranging a nice vehicle and the excellent divers, I couldn’t have worked in the field so efficiently. I appreciate Prof. Zhang for his advice and support to my study and life in Uppsala. I thank Faramarz for helping me a lot in the lab and be-ing friendly to me all the time. I also want to thank my co-author Prof. Niko-laus Froitzheim from University of Bonn. Thanks to his contributions and positive comments, so that we had a good publication.

Thanks to the MSc students Hongdan Deng, Jiasui Ouyang, Xiaolong Shi and Yipeng Zhang from China University of Geosciences (Beijing) for assis-tance and friendly company in the field. Thanks also go to the Yong Zhang and Zhangsheng Wang for their safe driving and help with practical things during the field trips. I would like to thank the local people whom we met in the field. They were all friendly to us even when we drove a jeep into their villages and hammered rocks by the roadside.

Many thanks to the lovely colleagues at the department: Abigail Barker, Steffi Burchardt, Michael Krumbholz, Frances Deegan, Lara Blythe, Peter Dahlin, Ester M. Jolis, Kirsten Pedroza, Börje Dahren, Lukas Fuchs, David Budd, Iwona Klonowska, Tobias Mattsson, Harri Geiger, Petar Lazor, Håkan Sjöström, Karin Högdahl, Valentin Troll, Jochen Kamm. I thank all

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of you who gave me the good times over the past years. I am particularly grateful to Börje whom I had the pleasure to share the office with. Thanks also to the technical and administration staff at Geocenter for their help and support: Taher Mazloomian, Anna Callerholm, Tomas Nord, Ingegerd Ohls-son, Siv Petterson, Jeanette Flygare, Susanne Paul, Fatima Ryttare-Okorie, Zara Witt-Strömer, Leif Nyberg. Many thanks go to Örjan Amcoff for trans-lating the Swedish summary and Börje Dahren for correcting the translation.

Thanks to all my Chinese friends who have shared wonderful times and exciting experiences with me: Zhina Liu, Ting Zhang, Yan Wang, Chunling Shan, Zuo Xu, Fengjiao Zhang, Peng He, Peng Yi, Ping Yan, Shunguo Wang, Lichuan Wu, Fei Huang, Na Xiu, Liang Tian, Haizhou Wang, Zhibing Yang, Lebing Gong, Pianpian Wu, Zheya Sheng, Tianqing Zhu, Long Zhou, Huiting Jia, Jie Song, Le Gao.

Speicial thanks go to the China Scholarship Council (CSC) for supporting my PhD study in Sweden. My study was partly financially supported by the Swedish Research Council (VR) through Hemin Koyi, and Department of Earth Sciences at Uppsala University. I also want to thank Mr Wei Wang from Chinese Embassy in Sweden and Fan Zhang from China University of Geosciences (Beijing) who helped me a lot during the time I was on a sick leave.

Last but not the least I would like to give me deepest gratitude to my par-ents for their endless love, support, understand and respect. No language in the world is enough to express my love and gratitude to my parents. I appre-ciate every member in my big family in China for their concern, support, and looking after my parents. Finally I thank my dearest friends in China and in other countries for their precious friendship, which always encourages me to keep moving on.

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