Source: https://sp.lyellcollection.org/content/early/2019/01/22/SP487.5
Timestamp: 2019-04-21 18:54:04+00:00

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A series of analogue models were run to investigate oblique inversion of pre-existing grabens when overprinted by later shortening and the effect of these grabens on development of contractional structures. Obliquity angle (α) defining the initial trend of pre-existing grabens relative to the shortening direction, was systematically changed from 0°, 10°, 20°, 30°, 40°, 50°, 65° and 90°. Different structural styles are shown in different models and also in sections cutting across different parts of the models. Model results show that existence of multi-grabens enhances lateral discontinuity of overprinted thrusts in map view. With increasing the obliquity angle, more and longer lateral ramps developed sub-parallel to the graben trends. The pre-existing grabens were apparently rotated from their initial trends during shortening. Some of the normal faults bounding the grabens were partially inverted and resulted in bulging of the syn- and post-rift graben fill sediments. Most normal faults were displaced and rotated by thrusting, and provided relatively weak zones for propagation of thrusts. By comparing with observations from Qingxi graben in western China and from the SW Taiwan fold-and-thrust belt, where oblique inversion occurred, model results can be used to interpret unclear relationships between thrusts and pre-existing extensional structures during superimposed deformation.
Ramsay (1967) stated in his book that rocks behave in a complex manner in response to stress. Most rocks deform under the case of their initial heterogeneous and anisotropic properties (Ramsay 1967). Pre-existing structures (e.g. faults and/or folds) provide the heterogeneity or anisotropy that could play a significant role during superimposed deformation and may significantly influence the geometric, kinematic and dynamic evolution structures and their frequency and distribution. Fold-interference patterns are a very well-known example. Another example is basin inversion where sedimentary basins bounded by normal faults experience uplift and reverse reactivation of faults during a superimposed shortening (Cooper et al. 1989; Turner & Williams 2004; Buiter et al. 2009; De Vicente et al. 2009; Bonini et al. 2012). Basin inversion has been widely documented in a variety of tectonic settings, where uplift of basin may be accompanied or not by reactivation of normal faults in reverse slip and strike-slip (e.g. Letouzey et al. 1990; Withjack et al. 1995; Kelly et al. 1999; Giambiagi et al. 2009; Glen et al. 2005; Carrera et al. 2006; De Paola et al. 2006; Ghisetti & Sibon 2006; Roca et al. 2006; Koyi et al. 2008; Farzipour-Saein et al. 2009; Scisciani 2009; Di Domenica et al. 2012; Fernández-Lozano et al. 2012; Mescua & Giambiagi 2012). In many natural cases, geometries of extensional basins and their inversion during shortening play a controlling role on the geometric and kinematic signature of the contractional structures, e.g. orientation and localisation of thrusts, along-strike variation in width of the structural domains and mechanical mode of fault-related folds (Zanchi et al. 2006; Calamita et al. 2012; Likerman et al. 2013; Carrera & Muñoz 2013; Surpless et al. 2015).
Basin inversion and fault reactivation have been studied by means of analogue modelling with the aim of understanding the mechanism of deformation. Two types of ‘set-ups’ are used to simulate the formation of extensional basins and their later inversion. In one set-up, one or two rigid fault blocks representing a basement were used to form grabens with either listric or planar major normal fault detachment. Subsequently, reverse movement along the major detachment has been shown to induce inversion and reactivation of the pre-existing normal faults mainly visible in the graben fill (Koopman et al. 1987; Buchanan & McClay 1991, 1992; Yamada & McClay 2003; Burliga et al. 2012). In the second set-up, both footwall and hanging wall were free to deform. In this set-up, a mobile basal plate was placed at the base of the model to create a velocity discontinuity; as a result basin bounding faults are induced in the analogue material (e.g. McClay & Ellis 1987; Mitra & Islam 1994; Eisenstadt & Withjack 1995; Nalpas et al. 1995; Gartrell et al. 2005; Roca et al. 2006). Set-ups using a mobile basal plate could be subdivided into two categories: the mobile basal plate acts as a velocity discontinuity during both extension and shortening phases (e.g. Brun & Nalpas 1996; Dubois et al. 2002; Konstantinovskaya et al. 2007; Pinto et al. 2010); and the basal plate is unattached from a pushing wall or stays inactive during overprinted shortening (e.g. Panien et al. 2005; Del Ventisette et al. 2006; Yagupsky et al. 2008; Bonini et al. 2012). With these different set-ups, a set of problems could be addressed, such as variation in distribution, orientation and geometry of an extensional basin, and inheritance and reactivation of pre-existing extensional structures during the later shortening phase (Sassi et al. 1993; Nalpas et al. 1995; Brun & Nalpas 1996; Dubois et al. 2002; Panien et al. 2005; Del Ventisette et al. 2006; Gutowski & Koyi 2007; Yagupsky et al. 2008; Bonini et al. 2012; Burliga et al. 2012; Likerman et al. 2013).
In this work, we use a different approach where both hanging wall and footwall are deformable to investigate the interaction between the pre-existing basin and a superimposed fold-and-thrust belt. A systematic change in graben orientation with respect to the shortening direction was examined, i.e. graben-parallel, graben-oblique (different angles) and graben-orthogonal inversions were tested. However, some previous studies have focused on two types of oblique inversion: (a) changing orientation of one graben in each model (e.g. Del Ventisette et al. 2006; Yagupsky et al. 2008); and (b) simulating two grabens with the same orientation (e.g. Dubois et al. 2002). In this study, we tested inversion on two or three grabens with different orientations. In order to focus on the effect of initial orientation of the grabens on later compressional structures, we exclude other parameters that have been suggested to influence reactivation of extensional structures (e.g. basal velocity discontinuity during shortening and the presence of a ductile layer within model stratigraphy; Amilibia et al. 2005; Panien et al. 2005; Del Ventisette et al. 2006; Konstantinovskaya et al. 2007; Mandal & Chattopadhyay 1995; Marques & Nogueira 2008; Pinto et al. 2010; Yagupsky et al. 2008; Bonini et al. 2012). In this study, a series of sandbox experiments were carried out. The aim of this paper is to study the effect of distribution and orientation of pre-existing grabens on the geometry and propagation of the compressional structures that form during tectonic inversion. We will specifically focus on the reactivation and uplift of pre-existing grabens and bounding faults under changing orientation of graben geometry and distribution, and whether the grabens affect each other during inversion. Model results were compared with two natural cases, one from the Northern Qilian belt in western China, where a series of Mesozoic half-grabens with slightly different orientations were obliquely overprinted by Cenozoic shortening. Another example is from the SW Taiwan fold-and-thrust belt, where pre-existing normal faults of a continental margin were overprinted by an oblique collision.
The Quilian Orogen represents the western extension of the Central Orogenic Belt in China. It is located between the Tarim Craton, the North China Craton, the Qaidam Block and the Alxa block (Xu et al. 2010; Song et al. 2013; Fig. 1a). The Qilian Orogen consists of three secondary belts: the Northern, the Central and the Southern Qilian belts. The Northern Qilian belt is suggested to have formed in the Late Silurian and Devonian, and was reactivated in the Cenozoic owing to the Indian–Eurasian collision (Xiao et al. 2012). As a result, the Northern Qilian thrust system, trending about NW–SE, was formed and has been propagating northwards in sequence from the Northern Qilian Mountains (hinterland) into the Neogene to Quaternary foreland basins (the Hexi corridor; Yang et al. 2007; Xiao et al. 2012). The western part of the Hexi corridor is named the Jiuquan foreland basin, and is subdivided into Jiuxi basin to the west and Jiudong basin to the east (Fig. 1a).
(a) Geological map of the western part of the Northern Qilian belt. Hanxia–Dahuanggou fault is one of the Northern Qilian faults that separate the Northern Qilian Mountains (hinterland) and the Neogene to Quaternary foreland basins (Jiuquan foreland basins in this area). (b) Restored Jurassic–early Cretaceous half-grabens in the Jiuquan region. (c) Three half-grabens (B1, B2, and B3) simulated in this study. Cenozoic thrusts obliquely overprinted on the pre-existing half-grabens.
Prior to Cenozoic shortening, the Northern Qilian belt, including the current mountain and foreland basin regions, underwent regional erosion to form a peneplain, and then regional extension during the period of Jurassic to early Cretaceous (Li 2006). During extension, a series of NE-trending half-grabens were formed in this area. They are generally bounded by NE–SW-striking normal faults in the Jiuquan region (Fig. 1b) (Yang et al. 2002; He et al. 2004; Cheng 2006; Pan et al. 2012). The Mesozoic half-graben system has been obliquely overprinted by the NW–SE-striking Cenozoic Northern Qilian thrust belt (Fig. 1c). It has been deduced that about 20–50 km of the Mesozoic graben system has been shortened within the Northern Qilian Mountains (hinterland; He et al. 2004; Cheng 2006). There are some major characteristics shown by frontal thrusts of the Northern Qilian thrusts belt. For example, thrusts are apparently segmented by the development of transfer faults along their strikes; and each segment of the thrusts shows a difference in frontal thrust propagation, bulk shortening and structural style (Yang et al. 2002; Zhao et al. 2004; Li et al. 2005; Shen et al. 2010). Some researchers have hence proposed that development of the Cenozoic Northern Qilian thrust belt might have been controlled by the pre-existing half-graben and half-horst configuration (Zhao et al. 2004; Li et al. 2005; Pan et al. 2012; Hu et al. 2014).
Sedimentary rocks involved in the deformation of the Jiuxi foreland basin mainly consist of Ordovician to Quaternary strata (GBG-SGSG 1969). Marine sedimentation occurred before Carboniferous. Deposits during Carboniferous indicate a shift from marine to terrestrial environment. From Permian onwards, terrestrial sedimentation dominates in this region. Prior to Mesozoic extension, the lithology of Ordovician to Triassic sedimentary rocks mainly includes conglomerate, sandstone and siltstone interbedded with relatively thin layers of shale. The thickness of pre-extensional strata is estimated to be 8000 m maximum (GBG-SGSG 1969). However, since the Northern Qilian belt has experienced a single phase of shortening during Late Silurian to Devonian, the measured thickness of pre-extensional strata may not represent the original sedimentary thickness. During Jurassic to early Cretaceous, sandstone, siltstone, dark mudstone and pelite were mainly deposited within grabens/half-grabens, although a thin layer of the late early Cretaceous sediments also covered the horst areas. The thickness of the graben sediments varies from 4500 to 5000 m. Tertiary conglomerate and sandstone and Quaternary unconsolidated deposits cover the whole area, with a thickness of 2600–3600 m (GBG-SGSG 1969).
A series of sandbox models have been used in this study. All models experienced two deformation phases, phase 1 extension and phase 2 shortening. The models were built in a rigid PVC-sided sandbox. Depending on the model, two or three sides of the box were mobile in order to be able to apply extension and later shortening from different directions (Fig. 2a). The initial dimensions of the models were variable in order to enable the arrangement of multiple grabens with different orientations. Dry loose quartz sand was used in the models to simulate sedimentary rocks, including pre-rift units, synrift graben sediments, post-rift and syn-shortening covers. The sand was 80–120 µm in grain size, 1.53 g cm−3 in density and 30–33° in internal frictional angle (Maillot & Koyi 2006). White sand was scraped to 2–5 mm-thick horizontal layers and interstratified by coloured sand (thinner than 1 mm) acting as passive markers between layers. Models were scaled accordingly. On the basis of modelling area in the Jiuxi basin of the Northern Qilian belt, a length ratio of 1.6 × 10−5 was applied to all models, where 6 mm in the models represents about 1 km in nature.
(a) Top view of modelling deformation gear before depositing sand layers. Plastic sheet at the base was intentionally cut into pieces. The mobile plastic sheets were pulled with the mobile walls to initiate formation of grabens trending parallel to the cuts. (b)–(f) Map view sketch of all models after phase 1 extension. Grabens with different initial orientations are shown in dark grey. Grabens are labelled B1, B2 and B3 in each model.
During extension, a hard plastic sheet with oriented cuts was placed at the bottom of models (Fig. 2a). Different pieces of plastic sheet were attached to the mobile walls and pulled away in opposite directions to form two or three grabens with specific orientation in the overlying pre-rift sand layers. By changing the orientation of each cut, various graben orientations could be obtained in each model (Fig. 2b–f). In this study, the initial orientation of a graben (obliquity angle α) is defined as the angle between the trend of the graben relative to the shortening direction. As such, in the models, the initial orientation of grabens varied from 0° (parallel inversion) to 10°, 20°, 30°, 40°, 50°, 65° (oblique inversion) and 90°(orthogonal inversion) (Table 1). This means that graben trends are changing from low-angle oblique to perpendicular to the shortening direction.
Six sets of models were performed. Models 1 and 2 were designed based on the geology of the southern part of the Jiuxi region in the Northern Qilian belt. The three grabens formed in the models were distributed and orientated as their natural prototypes (Figs 1c & 2b). In all models, the sand layers mainly represented three sedimentary sequences: pre-rift, synrift and post-rift strata. In models 1 and 2, in order to be consistent with the geology of the Jiuxi foreland basin, most of the synrift strata were deposited within the grabens only. During subsequent shortening, synkinematic erosion and sedimentation were conducted on these two models in three periods. At the early stage, after each 1 cm shortening, a 3 mm-thick sand layer was deposited in the foreland of the model and any topography higher than the new deposition top was removed in the hinterland. Owing to this procedure, a package of syn-shortening strata is presented in models 1 and 2. In other models, synrift sediments covered the entire models. The development of structures and the change in topography during both extension and shortening phases were monitored using a high-resolution Laser scanner. At the end of each experiment, sections were cut parallel to the shortening direction in most models (e.g. models 1, 3, 4 and 5), and perpendicular to the 20°-orientated graben in model 2. We sectioned different parts of model 6 in different orientations, which can help to gain insight into graben inversion during subsequent shortening with different orientations. We also performed a series of horizontal sections for model 4 to show the relationship between graben sediment, normal faults and thrusts in map view.
During the first phase of extension, two mobile walls were simultaneously pulled away from each other, which dragged the basal plastic sheets to form grabens oriented along their edges in the overlying sand package (Fig. 2). Grabens developed by a major normal fault on one side and a series of progressively formed antithetic normal faults on another side (Fig. 3). The major normal faults show a relatively straight fault trace with a dip angle of about 60° in profiles. They were active during the whole process of extension. However, the normal faults that bounded the other side of the grabens were formed progressively, with the older fault ceasing activity after a new one emerged within the graben (Fig. 3c, d). These faults were listric with their dip angle decreasing towards the base of the model.
(a) and (b) Map views of models 5 and 6 after extension, respectively, showing the locations of extension sections. (c) and (d) Cross-sections of model 5 and 6, showing initial geometries of the grabens. The models were extended from two sides in total by 4 cm. Synkinematic layers of sand were deposited within and outside the grabens at regular intervals of each 1 cm extension.
After shortening, model topography is mainly characterized by a geometry wedge. In the hinterland of all the models, owing to strong thrusting and thickening, a high and relatively flat topography is shown (Fig. 4). However, the topography becomes lower and laterally uneven towards the foreland, which is more noticeable in models 3 and 4. Towards the foreland the relatively high topography is associated with uplifting of the underlying pre-existing grabens (Fig. 4a, b and c). In model 5, a high topography was manifested by two large pop-up structures simulating two highlands, one close to the pushing wall and the other far away from the foreland (Fig. 4d). The two pop-up structures trend sub-parallel to the orientation of pre-existing grabens.
Laser scanned images of the model topographies at the final stage of shortening. Outlines of deformed grabens are projected on the model surfaces shown by white dashed lines.
In map view, the geometry of the thrust is significantly affected by both the pushing wall and the pre-existing grabens. In the hinterland of all models, thrusts are roughly trending sub-parallel to the pushing wall (Fig. 5). However, towards the foreland, traces of thrust are curved and segmented, and overlap each other along their strikes, particularly shown in the oblique inversion models 1–4 (Fig. 5a–d). Some segments of thrusts tend to follow the trend of pre-existing grabens, which trend oblique to the backstop (10°, 20°, 30°, 40°, 50° and 65°; Fig. 5a–d). In model 5, where the pre-existing grabens are orientated orthogonal to the shortening direction, thrust traces are roughly parallel to graben trends from hinterland to foreland (Fig. 5e). Thrust traces are slightly convex forwards in the middle part of model 6. The sequence of thrust formation was monitored through surface photography for models 3–6 (Fig. 5c–f). A general in-sequence propagation of thrusting towards the foreland is seen in model 6 (Fig. 5f). However, in models 3–5 thrusts were formed in an out-of-sequence manner. Some later-formed thrusts are seen in the hanging wall of the older one, e.g. thrusts T16 and T17 developed in the hanging wall of T15 in model 3 (Fig. 5c); in model 5, three right-stepped thrusts, T13, were formed in the hanging wall of T7 and later connected by thrusts T4 (Fig. 5e).
Line drawing of thrust traces in map view. Dashed lines indicate reactivated graben margins. Black solid lines with triangular symbols are thrust traces. Blue solid lines show outlines of the grabens projected on the model surfaces after shortening. Red solid lines indicate locations of cross-sections that shown in Figures 6, 7, 8, 9, 10, 11.
Horizontal sections and cross-sections of model 4 showing the signatures of structures. Horizontal section (a2) cut at a shallow level and (b2) cut at a deep level. Assuming the model is shortened from the south, cross-sections (a1) and (b1) were cut along the western side of the model, while cross-sections (a3) and (b3) were cut along the eastern side. Light blue lines in cross-sections indicate the levels of horizontal sections HS14 and HS18, respectively. Section cross locations are shown in Figure 5d. Rectangles in the cross-sections outline the bulge of the inverted grabens shown in detail in Figure 13.
Cross-sections of model 1 cut parallel to shortening direction. Section locations are seen in Figure 5a. Grabens B3, B2 and B1 have the initial orientations (i.e. obliquity angle α) of 10°, 20° and 30° relative to shortening direction, respectively. Cross-sections are oblique to the trend of the grabens. Rectangle outlines the bulging area of the inverted graben shown in detail in Figure 13.
Cross-sections of model 2 cut perpendicular to the trend of graben B3. Section locations are seen in Figure 5b. Grabens B3, B2 and B1 are initially orientated (i.e. obliquity angle α) 10°, 20° and 30° relative to the shortening direction, respectively. Cross-sections are oblique to the strikes of thrusts.
Cross-sections of model 3 cut parallel to the shortening direction. Section locations are seen in Figure 5c. Grabens B2 and B1 are initially orientated (i.e. obliquity angle α) 40° and 50° relative to the shortening direction, respectively. Cross-sections are oblique to the trend of the grabens. Rectangle outlines the bulging area of the inverted graben that is shown in detail in Figure 13.
Cross-sections of model 5 cut parallel to shortening direction and perpendicular to the trend of grabens, i.e. obliquity angle α is equal to 90°. Section locations are seen in Figure 5e. Rectangles outline the bulging area of the graben that is shown in detail in Figure 13.
(a) Lateral section of model 5 perpendicular to the shortening and also graben orientation, showing graben vertically repeated by several thrusts. (b) Extensional and contractional structures shown in a 3D section. The left side of the section is 45° to the shortening direction and graben orientation, whereas the right side is parallel to the shortening direction. Traces of normal faults (dashed yellow lines) at the right side are recognized according to the thickness variation of sand layer. (c) An oblique section (45° to the shortening direction) showing both thrusts and displaced graben.
In models where the thrust wedge experienced erosion, curved thrust traces were also observed from the eroded surface, e.g. horizontal sections from model 4 (Fig. 6). Since the angle between the trend of the grabens and the shortening direction is 65°, grabens (B1 and B2) are closer to the backstop in the eastern part of the horizontal sections and getting away from the backstop in the western part. Traces of thrusts are sub-parallel to the backstop in the western part of the model (e.g. thrusts T1, T3, T5 and T12 in Fig. 6). However, in the eastern part of the model, thrusts tend to originate within graben B1 and are curved to follow the trend of grabens. Towards the foreland, conjugated thrusts T17 and T18 are trending sub-parallel to the strike of normal faults of graben B2. The lateral overlapping between the thrusts (e.g. T8 and T3, T9 and T8) can be identified from topographic image, but not from the horizontal sections owing to the erosion of parts of marker layers (Figs 5d v. 6).
Cross-sections of the models show major extensional and compressional features (Figs 6, 7, 8, 9, 10, 11, for section locations see Fig. 5). All cross-sections cut through one or two grabens in each model. For models 1, 3, 4 and 5, cross-sections are parallel to the shortening direction. These cross-sections are, therefore, oblique to the trend of grabens of models 1, 3 and 4, and thus normal faults show gentle apparent dips in the sections. Cross-sections in model 2 are cut perpendicular to the 20° oblique graben (B3), and thus shows oblique profiles for shortening structures and the other grabens (B1 and B2) (Figs 5b & 8). Sections with different orientations were taken for model 6 (Fig. 8f). Three types of cross-sections for showing relationship between grabens and thrust wedge are displayed here: e.g. section 31 is orthogonal to shortening direction; section 7 is orientated 45° to the shortening direction; and section 16 consists of two profiles, one making an angle of 45°and the other parallel to the shortening direction (Figs 5f & 11).
In the oblique and parallel inversion models (models 1–4 and 6), some similar signatures of thrusts are seen in the cross-sections (Figs 6, 7, 8, 9 & 11). In general, a series of dominantly close-spaced fore-thrusts with close spacing dominantly originated in front of the backstop and gradually stacked up in the hinterland. Towards-foreland conjugate fore- and back-thrusts are seen in some sections of the models (e.g. Figs 6a1, b1, 7a, b, 8b, 9a, c & 11b). However, in the orthogonal model 5, conjugate fore- and back-thrusts are characteristic in both hinterland and foreland (Fig. 11). It is noticed that, owing to lateral discontinuity of thrust traces and oblique angle between graben orientation and shortening direction, sections cutting through different parts of an oblique inversion model could display differences in structural features (Figs 6, 7, 8, 9). For example, the graben B2 of model 3 is shown in all three sections (Fig. 9). However, the graben B2 is located far in the foreland and partly displaced by thrust T15 in section 10, but entirely modified and uplifted in the hinterland in section 33, owing to the graben B2 being 40° oblique to the shortening direction. Because of the lateral discontinuity of thrust traces, large displacement along thrusts T3 and T9 shown in section 33 are laterally accommodated by the thrusts T4 and T10 in section 18, and by thrusts T4 and T11 in section 10 (Fig. 9). In model 4, grabens B1 and B2 trend 65° oblique to the shortening direction so that they are seen located close to the backstop in section 9, but further towards foreland in section 14 (Figs 5 & 6). Consequently, in section 14 a series of fore-thrusts and a relatively steep wedge slope are shown in the hinterland where the graben B1 is only slightly affected by T12 and T21 (Fig. 6a1, b1). However, in section 9 the graben B1 is entirely uplifted by a pair of conjugate fore- and back-thrusts in the hinterland, and the graben B2 is also deformed by thrusts T17 and T18. Therefore, the wedge slope is relatively flat as shown in section 9 (Fig. 6a3, b3).
In model 5, where two pre-existing grabens were oriented orthogonal to the shortening direction, major thrusts are relatively continuous along-strike and display similar features in all sections (Figs 5e & 10). Close to the hinterland, the first large structure actually consists of two pairs of pop-up structures. The first pop-up structure was formed in front of the backstop, which is confined by the fore-thrusts T2 and T4 (or T6) and a blind back-thrust. Next to this pop-up structure a second one is constricted by the fore-thrusts T7 and back-thrusts T8, T16 and T17 that are generated from graben B1. The back-thrusts of the second pop-up structure cut through the fore-thrusts of the first one. Far towards the foreland, the third pop-up structure originated within graben B2, bounded by back-thrust T20 and fore-thrusts T19 and T22 in section 16 and fore-thrusts T21 and T23 in sections 5 and 10.
In models 1 and 2, syn-shortening erosion and sedimentation prolonged activities of thrusts in the hinterland. As a result, the early formed thrusts were significantly uplifted and the deep-level sand-layers, i.e. the pre-rifting strata, were exhumed to the surface (Figs 7 & 8). Without the effects of syn-shortening erosion and sedimentation, less exhumation of deep pre-rifting strata occurred in the hinterland of models 3–6 (Figs 6 & 9, 10, 11). Oblique sections in models 2 and 6 show similar features where a series of fore-thrust are dominant in the hinterland area with apparent gentle dips (Figs 8 & 11b, c). In a lateral section of model 6, newly formed thrusts are shown at relatively deep level, whereas the older ones are shown at a shallow level (Fig. 11a). The newly formed thrusts propagate upwards by forming lateral ramps and connect with an old one. For example, the thrust T7 propagates upwards to join T6, and T2 joins T1. The feature of newly formed thrusts propagating upwards and connecting with the old ones in the lateral section is consistent with the fact that thrust traces are discontinuous and overlap each other in map view. If the thrusts are relatively straight and continuous in map view, it is expected to observe a series of straight fault traces that are sub-parallel to each other in a lateral section.
During the subsequent shortening, pre-existing grabens were deformed by thrusting and folding. Modification of graben geometry can be seen from map view and sections. In order to evaluate graben deformation in map view, we overprinted the map-viewed geometries of grabens before (dotted–dashed lines) and after (blue solid lines) shortening to the model map views at their final deformation stages (Fig. 12). It is shown that the boundaries of grabens are relatively straight and smooth before shortening. However, in the oblique inversion models, graben boundaries change trend during shortening (Fig. 12a–c). During shortening, grabens are rotated and shifted away from their initial trends. Two parameters affect the degree of rotation, i.e. the location of grabens and their obliquity angles. For example, the parts of grabens closer to the backstop have been shortened more and thus show more distinctive rotation and shift than those parts towards the foreland (Fig. 12a–c). In model 3, the obliquity angle of the two grabens are 40° and 50°, where maximum rotation (15–20° of rotation) and shift are observed in the hinterland of the model (Fig. 12b). In the orthogonal model 5, the grabens are entirely pushed forwards during shortening and show no rotation (Fig. 12d). Similar situations are seen in the parallel inversion (model 6), where the grabens do not show rotation but widen by compression towards the hinterland (Fig. 12e).
Line drawing of graben outlines and thrust traces in map view. Dashed and dotted lines indicate initial orientation of each graben after extensional phase. Blue solid lines show outlines of the grabens projected to the model surface after shortening. Black solid lines with triangular symbols are thrust traces. An apparent rotation of grabens is shown by comparing their initial and final orientations.
In the horizontal sections of model 4, graben B1 that is located close to the backstop is obliquely cut into several blocks by thrusts (Fig. 6a2, b2). Each block of the graben is displaced forwards and upwards by different thrusts. The apparent shift between each block in horizontal section is actually caused by oblique displacement of each block along-thrusts. However, towards the foreland graben B2 is relatively intact. Shortening is not as strong as in the hinterland and also strikes of fore-thrust T17 and back-thrust T18 are sub-parallel to the boundary-normal faults of graben B2.
The relationship between deformed grabens and the thrusts that cut through them seen in horizontal sections of model 4 can also be observed in cross-sections (Figs 6 v. 7, 8, 9, 10, 11). After shortening, some grabens were strongly displaced and vertically thickened by thrusting. For example, graben B1 (obliquity angle of 10°) in model 1 was significantly uplifted and exhumed owing to a stack of imbricate thrusts and synkinematic erosion and sedimentation (Fig. 7c). In model 3, close to the backstop, both of the grabens B1 and B2 (obliquity angles of 50° and 40°, respectively) were largely displaced forwards and upwards by several thrusts (Fig. 9). Uplift of grabens by thrusts is clearly observed in oblique and lateral profiles cut in models 2 and 6 (Figs 8b & 11a, c). For example, the graben B1 in model 2 and the grabens in model 6 are vertically repeated several times by a series of fore-thrusts. However, in the orthogonal model 5 the grabens are entirely uplifted by conjugate fore- and back-thrusts that originated from the base of the grabens (Fig. 10). Therefore, the grabens in model 5 show less vertical repetition and thickening than other grabens in the oblique and parallel inversion models (models 1–4 and 6).
In our models, most graben boundary-normal faults have large dip angle of about 60° (e.g. Fig. 3c, d). After subsequent phases of shortening, these normal faults were clearly sheared and displaced by thrusts (Figs 6, 7, 8, 9, 10, 11). Fault traces were folded and rotated by thrusts as sedimentary bedding was deformed. These sheared and rotated normal faults are clearly seen in most sections that were sliced parallel to the shortening direction. For example the normal fault on the left-most side of section 8 of model 1 was entirely overturned by thrusting (Fig. 7c). In model 3 the boundary-normal faults of the graben B1 were strongly sheared by thrusts T2, T4 and T10, and their lower parts were rotated to be sub-parallel to the dip angles of the thrusts (Fig. 9b). Reactivation of some normal faults was observed from both topographic images and cross-sections (Figs 5 & 13). However, reactivation of the normal faults was slight and only observed along some segments of the normal faults. In map view the reactivated segments of the normal faults are highlighted by dashed lines, e.g. T1 and T5 in model 3, T14, T15 and T16 in model 4, T1 and T5 in model 5 (Fig. 5c–e). In cross-sections, subtle bulge of the syn- and post-rift layers of the grabens indicates the reactivation along the normal faults (Fig. 13). For example, the major normal faults are slightly reactivated to be reverse faults (T5 in Fig. 13b, T18 in Fig. 13e, T18 in Fig. 13g). Reactivation can also occur by short displacement along several adjacent normal faults, e.g. T14 in section 14 of model 4, T6 in section 16 of model 5 (Fig. 13d, f).
Close-up line drawings showing the details of graben fill bulging in models 1, 3, 4 and 5, with the oblique angles of grabens at 20°, 40°, 65° and 90°, respectively. The purple lines indicate the levels along the base of post-rift layers and/or the same layer of synrift fill. Above these lines (levels) bulges of graben are highlighted.
Many published articles have suggested that pre-existing extensional grabens could control later development of overprinted thrust belts. Such cases are particularly observed during an oblique inversion where later border reverse faults trend parallel to the underlying rift margins and oblique to shortening direction (Dubois et al. 2002; Amilibia et al. 2005; Panien et al. 2005; Del Ventisette et al. 2006). Some previous analogue modelling studies investigated the role of one or more ductile layers (e.g. silicone putty) at the base of or within the shortened layers, on inversion, and concluded that such layers promote inversion to take place along high-angle normal faults, significantly influencing the orientation of contractional structures (Bonini et al. 2012). Others used micro glass beads to play the role of a weak layer in synrift graben fill, which also favours reactivation of the major graben-bounding normal faults and localize formation of the main forward thrusts (Panien et al. 2005; Bonini et al. 2012). However, in our models, which consisted of only loose sand, we focus on the effect of variable orientations of pre-existing normal fault/grabens on the trend and location of subsequent contractional structures.
In map view, the complicated thrust systems that formed in the models are mainly characterized by laterally discontinuous and out-of-sequence forward-propagating structures, influenced by pre-existing grabens of variable orientations (Fig. 5). Yagupsky et al. (2008) discussed the signatures of thrust propagation during oblique basin inversion by changing the basin orientation in each model (Fig. 14c–e). Similar to our models, in Yagupsky et al. (2008)’s models, thrusts were striking perpendicular to the shortening direction before they propagated into the pre-existing basins. During superimposed shortening, some thrust segments developed from the inversion of normal faults and thus were striking parallel to the graben margins, which were oblique to the shortening direction. However, away from the inverted grabens, thrust traces tend to strike perpendicular to the shortening direction. The length of thrust segments that are controlled by reactivated normal faults increases with increasing the obliquity angle (from 45° to 60° to 75° to the shortening direction) (Fig. 14c–e). Similarly, in this study, towards the foreland, more and longer thrust segments form, striking sub-parallel to the grabens when the obliquity angle increases, e.g. thrusts T9 and T15 in model 3, thrusts T17, T18 and T20 in model 4 (Fig. 14a, b). In Yagupsky et al.’s (2008) models, where one graben was simulated in each model and was filled with micro glass beads, thrust segments formed by reactivation of normal faults are relatively long and straight. However, in this study, thrusts were formed in relatively short segments and overlap each other by forming lateral ramps. Owing to the presence of more than one graben, lateral propagation of a thrust ceases with its lateral ramp near the boundaries of a graben, and is overlapped by the formation of another thrust that may cease propagating laterally at another graben margin (Fig. 5a–d). This lateral propagation mode of thrust is clearly seen in all of our oblique inversion models, which indicates the significance of orientation and distribution of pre-existing grabens on the trend of thrusts.
(a) and (b) Map view line-drawings showing the relationship between thrust traces and orientation of pre-existing grabens in models 3 and 4. (c–e) Model results from part of figure 14 in Yagupsky et al. (2008) showing close similarity with our model results.
The effects of variable orientations of pre-existing grabens on thrust system can not only be reflected in map view but are also seen in cross-sections. Based on the published studies, basal friction is one the principal parameters to govern the geometry of a thrust system (Davis & Engelder 1985; Cotton & Koyi 2000; Koyi & Cotton 2004; Bahroudi & Koyi 2004; Graveleau et al. 2012). With low basal friction, successive pop-up structures are easy to form, whereas imbricate thrusts stack up with high basal friction (Mulugeta 1988; Cotton & Koyi 2000). In addition, the geometry of a thrust system also depends on the amount of inversion and the symmetry of the pre-existing grabens (Roca et al. 2006). In Roca et al.’s (2006) models, where grabens were orthogonally inverted above a ductile substrate, a pop-up structure with broad crest formed during the inversion of symmetric graben. In the current study, relatively symmetric grabens were formed during extension, and all models were shortened on a metal substrate providing a basal friction of μ = 0.23 (Deng et al. 2014). However, depending on the orientation of the pre-existing grabens relative to the superimposed shortening, different structural styles of thrust systems were developed. Fore-thrusts are dominant contractional structures when the obliquity angle is low to moderate (e.g. models 1–3 and 6 where the angles are 0–50°; Figs 7, 8, 9 & 11). With increasing the obliquity angle to 65° and 90° in models 4 and 5, respectively, pop-up structures are inclined to form, bounded by conjugate fore- and back-thrusts (Figs 6 & 10). These observations demonstrate that, in addition to the symmetry of the pre-existing grabens, their orientation also has a significant influence on the geometry and trend of the superimposed thrusts.
As mentioned above, lateral discontinuity of the thrust is a major characteristic in map view of the oblique inversion models, which implies that bulk shortening was accommodated by different structures across different parts of the models. For example, combining the map view and cross-sections in model 3, it can be seen that close to the pushing wall the laterally continuous thrust T2 is visible in all three sections (Fig. 5c & 9). However, since the grabens had different orientations, thrust T2 did not cut through graben B2 in section 33, but modified graben B1 in section 18 and had not yet reached graben B1 in section 10 (Fig. 9). When shortening reached the middle zone of the model, thrust T9 terminated laterally and was overlapped by thrust T10 when approaching the boundary of graben B2. Thrust T10 was in turn replaced by T11 before propagating into graben B1 (Fig. 5c). Correspondingly, thrust T9 was generated from the lower part of graben B2 to accommodate shortening shown in section 33. However, in sections 18 and 10 thrusts T10 and T11 developed in the middle area of the model and cut through graben B1 (Fig. 9). Towards the most frontal part of the model, development of fore-thrust T18 had no connection to any grabens shown in section 33, whereas T15 cut through graben B2 in sections 18 and 10. These observations indicate that complicated contractional structures form during inversion of oblique grabens with different orientations, and thus shortening is progressively accommodated by different structures in the shortening direction and also along the strikes of structures. Therefore, one or two profiles can hardly be representative of the whole signature of structures in regions with pre-existing oblique grabens.
Graben deformation during model inversion is mainly characterized by change of graben orientation and bulging of syn- and post-rift sediment along reactivated normal faults. During the superimposed shortening of the models, the pre-existing grabens were partially inverted. Reverse displacement could be seen along the pre-existing normal faults bounding the grabens in sections parallel to the shortening direction (Fig. 13). Progressive shortening led to the formation of a series of folds and thrusts, which progressively cut through the pre-existing grabens, i.e. the normal faults and graben fills were displaced and incorporated into the hanging wall of the thrusts (Figs 6, 7, 8, 9, 10, 11). This led to tectonic thickening of the graben fills by repetition (Figs 8b & 11a, c). This graben fill repetition is strongly influenced by the initial trend of the basin relative to the direction of the superimposed shortening. Comparing cross-sections cut perpendicular to the graben orientations in models 2 and 6 illustrates the difference in graben repetition. In model 2, where the graben trends 10° to the superimposed shortening, upward repetition of graben B1 is not along an exactly vertical axis, but shows a gradual shift to the right from the bottom to the surface (Fig. 8b). However, in model 6, where the graben trends 0° to the superimposed shortening, this upward shift is not observed from the repetition of graben B2 (Fig. 11a). Based on the different modes of graben fill repetition caused by variable angles between graben trends and the superimposed shortening, an apparent rotation of graben orientation is shown after shortening in the inverted models with oblique grabens (Fig. 12a–c). Comparing initial and final orientations of grabens shows that a maximum apparent rotation could be obtained for grabens with initial obliquity angle 40–50° relative to the shortening direction (Fig. 12). In the orthogonal (α = 90°, model 5) and parallel (α = 0°, model 6) inversion models, pre-existing grabens showed no apparent rotation during superimposed shortening. Rotation of pre-existing basement faults during subsequent oblique shortening has also been suggested and modelled in the Zagros fold-and-thrust belt (Koyi et al. 2016).
Amilibia et al. (2005) and Panien et al. (2005) reported that shortening of a graben is initially accommodated by upward bulging shown by the graben fill and post-rift sedimentary cover, regardless of the composition of sediment or graben orientation. Our study supports this claim. In the models presented here, some parts of the grabens were slightly bulged at a relatively early stage of shortening, which is well illustrated by the topographic relief in the scanned images (Fig. 15). With progressive shortening, the upward bulge of grabens was successively exhibited from the hinterland towards the foreland. Graben bulging happened at different stages in different models of variable graben trends. For example, in model 1, where the obliquity angles are 10–30°, graben bulge is clearly shown at 30% bulk shortening, which is near the final stage of bulk shortening (33%). In contrast, in models 3 and 4 (α = 40–50° and 65°, respectively), graben bulge started to show from 10% and 11% bulk shortening, respectively (Fig. 15). However, in model 5 (α = 90°), graben inversion started at 4% bulk shortening (Fig. 15). These observations document that graben bulging happen at the earlier stage of models shortening when grabens trend at a high angle to the shortening direction (e.g. α = 40°, 50°, 65° and 90°). After the early and subtle bulge, active thrusting accommodated a large amount of shortening (solid lines with triangular symbols in Fig. 5). It is clearly seen in cross-sections that subtle bulging is associated with minor amount of reverse displacement along the steeply dipping, pre-existing boundary normal faults and also along small faults within the grabens. Even though minor reverse displacement along each single reactivated normal fault is hard to detect, the accumulated uplift of the basin fill can clearly be seen (Fig. 13). Model results also show that pre-existing normal faults that dip towards the hinterland induce more distinctive reverse displacement than those dipping towards the foreland (Fig. 13).
Topographic laser scanned images of models during progressive shortening (three stages for each model). Numbers within the rectangle indicate the bulk shortening at each stage, e.g. BS18%, BS30% and final BS33% in model 1. Original outlines of grabens are projected to the model surfaces shown by white dashed lines.
Instead of large amounts of reverse movement long the pre-existing normal faults, partial reactivation of the pre-existing normal faults, their displacement by the later thrusts and their rotation and folding during the superimposed shortening are features that are commonly observed in our models as well as in some other experimental and natural studies (Koopman et al. 1987; Buchanan & McClay 1991; Krantz 1991; Eisenstadt & Withjack 1995; Brun & Nalpas 1996; Tavarnelli 1996; Cloke et al. 1997; Erickson et al. 2001; Giambiagi et al. 2009; Eisenstadt & Sims 2005; Panien et al. 2005; Del Ventisette et al. 2006; Ghisetti & Sibon 2006; Kato et al. 2006; Burliga et al. 2012; Carrera & Muñoz 2013). Comparing parts of cross-sections in our models with some other studies, we can see that some hinterland-dipping normal faults were partially reactivated at their lower parts, and slip propagates upward along a shallow-dip footwall short-cut thrust into the post-rift layers (Fig. 16b–d and h). However, the normal faults dipping towards the foreland were sheared, folded and rotated by fore-thrusting (Fig. 16a, c–f and i). When either fore-thrusts or back-thrusts cut through a series of close-spaced normal faults, it seems that thrusts tend to utilize the adjacent normal fault ramps to propagate upwards (Figs 16a–d, f & i). This feature is well illustrated in two restored sections for models 3 and 5 (Fig. 17). The two cross-sections were restored using line-length restoration. Displacement along thrusts was removed and folded layers were unfolded layer by layer from the top of the model to the bottom. The initial geometry and location of the grabens were restored based on the map view photographs and cross-sections cut after phase 1 extension. As shown in the restored sections, by connecting the adjacent normal fault ramps, thrusts have relatively shallower dips, and thus can more easily propagate forwards and upwards. Tavarnelli (1996) reported similar features when studying the Umbria–Marche foreland fold-and-thrust belt. He simplified the relationship between normal faults and thrusts in a diagram that illustrates how a thrust truncates the pre-existing normal fault and the thrust ramps coincide with the location of the normal faults (Fig. 18). However, Tavarnelli (1996) only focus on one major normal fault and the associated thrust. In our models, the close-spaced normal faults are not only truncated by thrusting and incorporated from the footwall to the hanging wall of the thrust system, but also provide weak zones for propagation of the thrusts.
Line drawings of cross-sections of analogue model results and natural case from our models and published works, showing modification of grabens and normal faults by later thrusting. (a) to (d) Parts of a model cross-sections from this study. The dark grey colour shows the synrift sediments within and outside the grabens. (e) and (f) Sand model results modified from part of figure 8, Panien et al. (2005). The darkest grey colour with a thick white marker indicates the micro glass beads layer of graben fill. NGwm, normal graben filled with weak middle; NGab, normal graben filled with loose sand. (g) Cross-section of clay inversion model, from figure 15 in Eisenstadt & Withjack (1995). (h) Schematic 3D block diagram showing reactivation of a normal fault and its footwall short-cut thrust, from figure 10, Kato et al. (2006). (i) A local cross-section of the southern Cordillera Oriental showing inverted basins in the Pucará area (from Carrera & Muñoz 2013).
(a) Cross-section 33 of model 3 and (b) the corresponding restoration to the stage before shortening. (c) Cross-section 10 of model 5 and (d) the corresponding restoration to the stage before shortening. The cross-sections and their restorations show the relationship between pre-existing normal faults and thrust traces that displace them during either oblique (model 3) or orthogonal inversion (model 5).
(a) and (b) Kinematic models (numbers 1–3 indicating the stages of progressive shortening) showing the effects of pre-existing normal faults on development of thrust ramp, normal fault dipping either towards (a) foreland or (b) hinterland (from figure 8, Tavarnelli 1996).
The Qingxi graben was formed during the early Cretaceous. The basin fill was later covered by Paleogene to early Neogene strata and then syncontractional deposition during the period of late Neogene to Quaternary. The Qingxi graben is inferred to have initially orientated about 10–20° relative to the later shortening direction (graben B3 in Fig. 1c), which is simulated in our model 1 (Fig. 2a, graben B3). In the hinterland of the natural cross-section (Fig. 19a), a series of imbricates are considered to be transported from further south and transported the deep pre-rift strata (Paleozoic and pre-Cambrian) upwards to the surface/shallow levels. Syn-shortening erosion has removed most of the Mesozoic and Cenozoic sediments from the hinterland (Yang et al. 2007). Towards the foreland, two pop-up anticlines are shown in the natural section. The most frontal one is interpreted to be related to a reactivated pre-existing normal fault (Fig. 19a). The geometry of the Qingxi half-graben and its inverted boundary normal fault are interpreted from a seismic profile (Fig. 19b). Cretaceous graben fill is thickened towards the syndepositinal normal fault. This normal fault has been reactivated during later superimposed shortening, which is definitely proved by the kink of graben fill and the upper younger sediments (Fig. 19b).
(a) Cross-section cut through the Qingxi graben, the Northern Qilian belt in the west of China (modified from figure 2, Yang et al. 2007). The Cretaceous extensional graben sediment is highlighted by the grey infill. (b) Seismic profile cut through the Qingxi graben (modified from figure 11, Yang et al. 2007). Early Cretaceous strata (K1, green lines) deposited within the Qingxi half-graben, and covered by the upper Cenozoic sediments (E3b to Q4). A reactivated normal thrust is indicated by a thick solid line with a two-way arrow. Another thrust (thick dashed line) developed and seems to cut upwards the half-graben.
Structural signatures shown in the natural section and the seismic profile from the Qingxi half-graben area are also seen in our model results. A comparable contractional process was conducted in model 1, where syn-shortening erosion occurred in the hinterland and sedimentation in the foreland (Fig. 7a). A series of imbricate fore-thrusts were formed in the hinterland of the model, and pre-rift strata were exhumed to the surface, which is similar to the hinterland imbricates in the natural section (Figs 19a v. 7a). In the foreland area of model 1, a pop-up structure is formed, which is bounded by pairs of fore- and back-thrusts that are initiated within a graben, resulting in uplifting and slight bulge of its infill (Fig. 7a).The inverted boundary normal fault is more clearly observed in other models, e.g. in section 18 of model3 where the graben is trending 40° relative to the shortening direction (Fig. 13b). The boundary normal fault is reactivated during the shortening phase, and thus the upper post-rift layers are folded by the inverted normal fault, similar to the graben fill and the upper Cenozoic sediments in the Qingxi half-graben (Figs 19c v. 13b). Newly formed thrusts cut through the other boundary of the graben in model 3 (Fig. 13b, left side). Similarly, in the seismic profile, a thrust has developed and seems to cut upwards the half-graben. Model results are generally comparable with the natural case, in regard to the sequence of tectonic events and mainly structural style either in the hinterland or in the foreland areas. Model results indicate an internal deformation and graben bulging during oblique inversion, which could help in interpreting the detailed structures and tectonic process that are still controversial in this region.
Another example where oblique inversion is documented is the SW Taiwan orogen, which is an active fold-and-thrust belt forming on the outer shelf and slope of the Eurasian continental margin (Biete et al. 2018). The Philippine Sea Plate is colliding with the Eurasian continental margin in a NW direction at a rate of about 8.2 cm a−1 since the late Miocene (Fig. 20a). Prior to the collision, several extensional phases have occurred on some part of the Eurasian continental margin in the Eocene, resulting in the formation of a number of NE–SW oriented basins developed by an array of roughly ENE-striking normal faults. A number of these faults can be traced on land (e.g. the Yichu fault system, the Chiali fault system; Fig. 20a). The subsequent collision from the SE caused the development of a series of NNE-striking thrust sheets, bounded by the Chaochou fault to the east and by the buried tip line of the Changhua thrust to the west (Biete et al. 2018). The overprinting shortening has also probably induced oblique inversion of the pre-existing extensional faults, with an obliquity angle of 40–50° between the trend of normal faults and the Philippine Sea Plate collision direction (Fig. 20a).
(a) Structural units of the SW Taiwan fold-and-thrust belt. The inset shows the main tectonic and stratigraphic units of the Taiwan orogen. PSP, Philippine Sea Plate; LF, Lishan fault; SKF, Shuilikeng fault; ChF, Chauchou fault; CP, Coastal plain; WF, Western foothills; HR, Hsuehshan range; CR, Central range; CoR, Coastal range (modified from figures 1 and 5, Biete et al. 2018). (b) Structural contour line map of the basal thrust, indicating two oblique ramp zones (modified from figure 9a; Biete et al. 2018). (c) Geological cross-section across CC′ the interpreted basement high area between two basal thrust oblique ramps, showing the imbricate thrust sheets overprinting the pre-existing extensional continental margin (modified from figure 6 section CC′; Biete et al. 2018). (d) Restored section of CC′ showing that basal thrust might develop correlating with the pre-existing faults (modified from figure 7 section CC′, Biete et al. 2018).
Structural architecture of the SE Taiwan fold-and-thrust belt shows a significant along-strike change across a roughly east–west-trending zone (Biete et al. 2018). To the north of the zone, five thrust sheets are dominant, whereas in the south, major structures are replaced by the broad Changhua thrust sheet and the Xuxian antiform and the Kuanglin synform (Fig. 20a). This east–west-trending zone is called the Hsinhua transverse zone, which happens to be located along the land projection of the shelf-slope break (Biete et al. 2018). Beneath the Hsinhua transverse zone, the basal thrust is inferred to form a Hsinhus oblique ramp documented by the depth contours. Another basal thrust oblique ramp is the Yichu oblique ramp in the northern part of the belt (Fig. 20b). In these two oblique ramp zones, basal thrust contours are trending roughly parallel to the trend of the adjacent extensional faults, which is a similar to what is shown in our model results where lateral ramps of thrusts are sub-parallel to the trend of the pre-existing grabens (Figs 14a v. 20b).
Based on Biete et al. (2018), structural characteristics of the fold-and-thrust belt, e.g. dextral strike-slip faulting, along-strike change in trend of the thrusts, were formed owing to the presence of the two basal thrust oblique ramps. Formation of the two oblique ramps can be correlated with the basement highs and lows that were inherited from the extensional continental margin (Fig. 20c). Biete et al. (2018) suggested that there are some uncertainties in the interpretation of the basement–cover interface. However, our modelling results support their interpretations that shortening across the pre-existing oblique grabens could have induced the formation of lateral oblique ramps of thrusts trending slightly oblique to the graben margin. A change in the trend of overprinting contractional structures (folds and thrusts) is also observed in the Zagros fold-and-thrust belt, which has been interpreted to be due to oblique reactivation of pre-existing basement strike-slip and normal faults during Zagros collision. Model results simulating such scenarios have shown the formation of oblique ramps along thrust trends along the trace of the underlying basement faults (Koyi et al. 2016). Similar to the features observed in restored model sections, the basal thrust of the fold-and-thrust belt in the SW Taiwan may have utilized the pre-existing extensional faults to propagate upwards and forwards. Consequently, the reactivated parts of the pre-existing normal faults formed the oblique ramps along the overprinting thrusts (Figs 17 v. 20d).
The results of models simulating inversion of basins with different trends show that the pre-existing grabens influence the development of later, superimposed contractional structures. On map view, thrust traces are discontinuous, segmented and overlap each other, resulting in their discontinuity along-strike. Lateral ramps are inclined following the trend of the pre-existing grabens. Styles of contractional structures are affected by the variable trends of pre-existing grabens. Sections cut through different parts of the oblique inversion models (α = 10–65°) show that the contractional structures exhibit different features. In contrast, in orthogonal and parallel inversion models (α = 90° and 0°, respectively), structures show similar features in all sections.
Pre-existing grabens experience an apparent rotation during oblique inversion. This is caused by displacement and transport of the part of the grabens in the hanging wall of the superimposed thrusts. Most rotation occurs where grabens are initially trending 40–50° relative to the shortening direction. Subtle bulge of graben fill and upper post-rift layers are observed in the oblique and orthogonal inversion models at an early stage of shortening. The slight bulging is caused by minor reverse reactivation of the pre-existing normal faults. This reactivation does not only occur along the border major faults, but also induces internal deformation along the normal faults within the graben. The graben bulge and fault reactivation are more obvious when the obliquity angle is larger (40–90°). In addition, most normal faults are displaced, folded and rotated during later shortening. A series of closely spaced normal faults within the grabens provide weak zones for thrust propagation with even shallower dips.
Comparison between model results and the inverted Qingxi graben in the foreland area of the Northern Qilian belt, western China, indicates that syn-shortening erosion in the hinterland removes the syn- and post-rift strata and exhumes the deep-level pre-rift strata to the surface. Similarly, model results show that the deeper units are tectonically exhumed to shallow levels during subsequent shortening. A natural-case example from the SW Taiwan fold-and-thrust belt supports that the along-strike change in trend of structures can be correlated with basement topographic highs and lows that are inherited from the pre-existing rifting continental margin. Formation of the oblique ramps and the change in trend of the superimposed thrusts are reproduced in the model results presented here. As such, model results assist in better interpretation of seismic profiles of areas with superimposed shortening, where pre-existing extensional structures are overprinted obliquely by later contractional structures.
This study was supported by the China Geological Survey through Professor Jinjiang Zhang (grant number 201306010046). Hemin Koyi acknowledges financial support from Swedish VR. Thanks go to Associate Professor Bo Zhang, from the School of Earth and Space Sciences, Peking University, who helped in understanding the tectonic setting of the Northern Qilian belt. We also acknowledge the constructive comments and suggestions of two anonymous reviewers.
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