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Volume 41 Issue 4
Aug.  2020
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Wengang He. Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China. Journal of Earth Science, 2020, 31(4): 795-807. doi: 10.1007/s12583-020-1281-2
Citation: Wengang He. Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China. Journal of Earth Science, 2020, 31(4): 795-807. doi: 10.1007/s12583-020-1281-2

Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China

doi: 10.1007/s12583-020-1281-2
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  • The eastern Sichuan-western Hunan and Hubei belt (ESWHHB) is an important fold-thrust belt in the Middle-Upper Yangtze region of China, and it is also an important area for petroleum and gas prospect in China. The influence of mechanical stratigraphy on the deformation evolution of the ESWHHB is a hot problem that has received widespread attention. However, due to the complexity of geological conditions, this issue has not been sufficiently addressed. Previews geological exploration studies show that the deformation evolution of the belt is closely related to the mechanical stratigraphy. Physical simulation has proven to be effective for studying the deformation evolution of fold-and-thrust belt. Based on the geological conditions of the ESWHHB, six groups of physical models were designed to analyze the influences of the ductile layer and overlap configuration on the structural deformation of the ESWHHB. The results show that the mechanical stratigraphy has significant control on the deformation evolution of the fold-thrust belt. The ESWHHB evolution is related to the lower viscosity of the ductile layer and the larger thickness of the ductile layer, while only gradual propagated fold-and-thrust belt can be resulted from the higher viscosity of the ductile layer and the smaller thickness of the ductile layer. Additionally, the overlap between the stratigraphy at various structural belts leads to significant differences in their mechanical properties, and it critically influences the structural patterns of the ESWHHB.
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Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China

doi: 10.1007/s12583-020-1281-2

Abstract: The eastern Sichuan-western Hunan and Hubei belt (ESWHHB) is an important fold-thrust belt in the Middle-Upper Yangtze region of China, and it is also an important area for petroleum and gas prospect in China. The influence of mechanical stratigraphy on the deformation evolution of the ESWHHB is a hot problem that has received widespread attention. However, due to the complexity of geological conditions, this issue has not been sufficiently addressed. Previews geological exploration studies show that the deformation evolution of the belt is closely related to the mechanical stratigraphy. Physical simulation has proven to be effective for studying the deformation evolution of fold-and-thrust belt. Based on the geological conditions of the ESWHHB, six groups of physical models were designed to analyze the influences of the ductile layer and overlap configuration on the structural deformation of the ESWHHB. The results show that the mechanical stratigraphy has significant control on the deformation evolution of the fold-thrust belt. The ESWHHB evolution is related to the lower viscosity of the ductile layer and the larger thickness of the ductile layer, while only gradual propagated fold-and-thrust belt can be resulted from the higher viscosity of the ductile layer and the smaller thickness of the ductile layer. Additionally, the overlap between the stratigraphy at various structural belts leads to significant differences in their mechanical properties, and it critically influences the structural patterns of the ESWHHB.

Wengang He. Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China. Journal of Earth Science, 2020, 31(4): 795-807. doi: 10.1007/s12583-020-1281-2
Citation: Wengang He. Influence of Mechanical Stratigraphy on the Deformation Evolution of Fold-Thrust Belts: Insights from the Analogue Modeling of Eastern Sichuan-Western Hunan and Hubei, South China. Journal of Earth Science, 2020, 31(4): 795-807. doi: 10.1007/s12583-020-1281-2
  • The eastern Sichuan-western Hunan and Hubei belt (ESWHHB) is an important fold-thrust belt that is located in the Middle–Upper Yangtze region of China (Fig. 1). The structural patterns of the ESWHHB are apparently different from classical fold-thrust belts in the world, although the deformation characteristics in the fold-thrust belts of typical regions in the worldwide also have been controlled commonly by the weak décollement layers (He et al., 2018; Bonini, 2007). The influence of mechanical stratigraphy on the deformation evolution of the fold-thrust belt is an important geological problem and this issue has received great attention in the last decades owing to its importance relate to structural deformation, petroleum, ore deposit generation etc. Geological exploration has been conducted in this belt for several years (Liu S G et al., 2018; Luo et al., 2018; Zhang et al., 2013; Liu S et al., 2010; Mei et al., 2010); structural research exhibits that the tectonic deformation evolution of the belt is closely related to the mechanical stratigraphy. This area is considered to be ideal for studying the influence of the mechanical stratigraphy on the deformation evolution of the fold-thrust belt which is located with typical weak layers, differ shortening and distinctive fold patterns (Zhou et al., 2019; Liu et al., 2015; Zhang et al., 2013; Yan et al., 2003).

    Figure 1.  Regional geological map and study scope of eastern Sichuan-western Hunan and Hubei belt ((a) and (b), after He et al., 2018). (c) is regional balanced cross section across the ESWHHB from interior Sichuan Basin to the eastern of the Xuefeng uplift.

    Physical modeling is an effective way of investigating the deformation evolution and formation mechanisms of fold-thrust belts. Related studies show that the deformation of the crust and lithosphere is controlled by various factors, including the boundary conditions, which control the deformation geomorphological features, and the distribution of the ductile layer and its overlap, which control the geometric characteristics of the fold-thrust belt (Santolaria et al., 2015; Marques and Cobbold, 2006; Sherkati, 2006; Costa and Vendeville, 2004, 2002). The difference between the strengths of the substrates and the basal strength difference of the strata affect the shear stress of the basement, which significantly controls the tectonics deformation characteristics of fold-thrust belt (Costa and Vendeville, 2002; Bonini, 2001). Furthermore, various factors, such as erosion, sedimentation, convergence rate, compressive time, and lateral friction, influence the structural characteristics of the fold- thrust belt (Zhou et al., 2016; Cruz et al., 2011; Marques and Cobbold, 2006, 2002; Bonini, 2001; Gutscher et al., 2001; Rossetti et al., 2000). For example, sedimentation has effect on the deformation patterns of the thrust belt, leading to deformation patterns characterized by the outward fold propagation to the passive-roof duplex style (Bonini, 2001). Overall, it is found that various factors influence the tectonic deformation, posing great difficulties in the analysis of the deformation evolution of orogenic belts and basins.

    Previous studies on the deformation evolution of orogenic belts and basins have achieved meaningful results and have formulated the critical taper theory and the bulldozer model (Graveleau et al., 2012; Rossetti et al., 2000; Dahlen et al., 1984; Davis et al., 1983). However, some physical simulation studies that investigate the effects of the mechanical stratigraphy on deformation are observed to be insufficient owing to the complex geological process; and the deformation evolution need in-depth research.

    The structural deformation characteristics of the ESWHHB has been the subject of decades-long research by structural geologists (Wang et al., 2018; Li et al., 2015; Liu et al., 2015; Zou et al., 2015; Guo and Tong, 2014; Xie et al., 2013; Zhang et al., 2013; Hu et al., 2009; Ni et al., 2009; Tang et al., 2007; Jin et al., 2006; Ma et al., 2005; Yan et al., 2003). However, the effects of mechanical stratigraphy on the structural deformation evolution are not very clear in this study area (He et al., 2018). Therefore, based on previous research, this study investigates the influence of mechanical stratigraphy on the deformation evolution of fold-thrust belts thanks to a two-series protocol comprising six groups of physical models. The results of this study denote that mechanical stratigraphy influences the deformation evolution and the structural patterns.

  • The ESWHHB is bounded by the Dabashan fold-thrust belt to the northeast, the Youjiang fold-thrust belt to the southwest, the Dayong fault to the southeast, and the Huayingshan fault to the northwest. The entire belt has an area of approximately 2.4×105 km2 (Fig. 1).

  • The ESWHHB is divided into the following two regions by the Qiyueshan fault: the southeast side of the western Hunan and Hubei region with a box-shaped fold (located in thick- skinned belt) and the northwest side of the eastern Sichuan region with a low-amplitude fold (located in thick-skinned belt) (Fig. 1). The stratigraphic features of the western Hunan and Hubei region are characterized by the Neoproterozoic schist and phyllite, Sinian siltstone, shale, argillaceous rock and limestone, Cambrian limestone and shale, Ordovician limestone and dolomite, and Mesozoic and Cenozoic strata have been eroded by structural activity. The basement is considerably involved in deformation, forming a typical thick-skinned structure. The weak layers mainly comprise Neoproterozoic schist- phyllite and Sinian shale (Fig. 2; Mei et al., 2010; Yan et al., 2003). The stratigraphic features of the eastern Sichuan region are characterized by the Neoproterozoic to Paleozoic strata, which mainly comprise limestone, sandstone, and shale. The weak layer mainly comprises Cambrian shale and gypsum, and the basement strata are observed to be relatively stable. However, the Mesozoic, Permian and Triassic occur during the deformation. Finally, previous studies have documented that the Sinian, Cambrian, Silurian, and Triassic in the region exhibiting numerous weak layers which contribute to generate thin-skinned structure (Li et al., 2015; Yan et al., 2003, and Fig. 1c).

    Figure 2.  Comprehensive stratigraphic column of the eastern Sichuan-western Hunan and Hubei regions (modified from Yan et al., 2000 and Li et al., 2015).

  • A major difference can be observed between the thicknesses, viscosities, mechanical strengths, and geothermal gradients of the continental crust in the study area and its surroundings (Burchfiel et al., 2008; Luo, 1998; Qiu et al., 1998). The crust thickness of the Xuefeng belt is observed to be approximately 30 km, which is slightly more than the crust thickness (27 km) of the Sichuan Basin. The crustal viscosities of the ESWHHB and Sichuan basins are 1.0×1022 and 1.0×1024 Pa·s, respectively (Lu et al., 2014). The crustal viscosity of the study area is smaller than that of the Sichuan Basin. Moreover, the drilling data indicate that the geothermal gradient of the ESWHHB is 20 ℃/km, which is smaller than the geothermal gradient of 30 ℃/km inside the Sichuan Basin (Qiu et al., 1998). Furthermore, seismic tomography reveals that the velocity of the seismic P and S waves in the Sichuan Basin basement is large, with rigid cratonic properties (Burchfiel et al., 2008). In summary, the stratigraphy properties are different between the ESWHHB and its southeast and northwest neighbors.

  • The sedimentary strata with ages of Sinian–Cenozoic in the ESWHHB have a total thickness of ~10 km and lie on Proterozoic basement (Yan et al., 2003, He et al, 2018). The strata with Jurassic and Cretaceous ages as well as a few Cenozoic ages occur mainly in the eastern Sichuan region (Fig. 1 and Fig. 2). Cambrian strata in the eastern Sichuan region are the main ductile décollement controlling the deformation evolution of the eastern Sichuan region. In the western Hunan-Hubei region, ductile layers of Proterozoic schist form the main ductile décollement, which controlled the deformation evolution of the region (Fig. 2; Yan et al., 2003; Li et al., 2015).

    According to the strength profiles of strata (Fig. 1c and Fig. 2), we adopted silicone putty to simulate the basal and interbedded décollement composed of Cambrian and Triassic ductile layer under the eastern Sichuan region and the basal décollement composed of Proterozoic schist under the western Hunan- Hubei region (Table 1). Dry quartz sands were used to simulate the brittle strata in both the eastern Sichuan and western Hunan- Hubei regions. Although the deformation characteristics of ESWHHB has been studied by some previous (Liu et al., 2013), it lacks the deeper discussion of the influence of mechanical stratigraphy on the deformation evolution of fold-thrust belts with 3D sand box. I therefore designed two sets of models considering these deformation evolutions (Table 1, Fig. 3).

    Experiments Eastern Sichuan region Western Hunan and Hubei region Stratigraphic structure of ESWHHB
    Quartz sand layer (mm) Silicone layer (mm) Viscosity
    (Pa·s)
    Quartz sand
    (mm)
    Silicone
    (mm)
    Difference viscosity A01 8 4 8 300 14 2 Overlay
    A01 8 4 18 300 14 2 Overlay
    Silicone and quartz sand overlay B01 8 4 18 300 14 2 Overlay
    B02 8 4 18 300 14 4 No overlay
    B03 8 4 18 300 14 2 No overlay
    B04 8 4 18 300 2.8 4 No overlay

    Table 1.  Physical model parameters of ESWHHB

    Figure 3.  Experimental apparatus and model geometry ((a) series of difference viscosity model). The silicone viscosity in the A01 model is 8 300 and the silicone viscosity in the A02 model is 18 300 Pa·s; (b) series of stratigraphic difference overlay model: the strata of B01 is overlap, and the strata of B02, B03, and B04 is no overlap.

  • Models were built in an Acrylic squeeze box with internal dimensions 820 mm×735 mm×60 mm. Loose quartz sand and silicone are the similar materials that have been extensively used in physical modeling. In our modeling approach, the brittle layers are replaced by the quartz sand, whereas the weak layers are replaced by silicone. Because the features of the ductile layers and the mechanical stratigraphy are unclear, the ductile layers of the stratigraphic in the model are replaced by silicone viscosities of 18 300 and 8 300 Pa·s (Fig. 3). Furthermore, the similarity of the model is 1.0×10-6, so that 1 cm in the model represents 10 km in nature. The scaling ratios between model and nature for cohesion and stress (i.e., c* and σ*) were calculated to be 2×10-6 and 6×10-7, suggesting that our models fulfill a dynamic similarity criterion (He et al., 2018; Bonini et al., 2012; Weijermars and Schmeling, 1986). Grain size of sand is 0.3-0.45 mm, cohesive of sand is 80 Pa, density of sand is 1 430 g/cm3, and density of silicone is 940 g/cm3 at room temperature (measured with numerical Brookfield-type viscometer). The physical properties of materials cited from He et al. (2018) (Table 2).

    Parameter Model Nature Model/nature ratio (*)
    Brittle layer density, ρb (kg·m-3) 1 430 2 400 ρb*=0.6
    Ductile layer density, ρd (kg·m-3) 940 2 200 ρd*=0.43
    Ductile layer lower viscosity, η (Pa·s) 8 300 7.75×1020 η*=1.07×10-17
    Ductile layer higher viscosity, η (Pa·s) 18 300 7.75×1020 η*=2.36×10-17
    Length, l (m) 0.01 1×104 l*=1×10-6
    Internal friction coefficient of brittle layer, μ 0.6 0.6–0.85 μ*=0.7–1
    Brittle layer cohesion, c (Pa) 80 4×107 c*=2×10-6
    Gravity acceleration, g (m·s-2) 9.81 9.81 g*=1
    Stress, σ (Pa) 112 1.89×106 σ*=ρ*×g*×l*=6×10-7
    Time, t (s) 5.4×104 3.2×1015 t*=1.75×10-11
    Strain rate, ε (s-1) 1.8×10-4 3.1×10-16 ε*=5.8×1010
    Displacement velocity, v (m·s-1) 2.5×10-6 vn=4.3×10-11 v*=5.8×104

    Table 2.  Scaling parameters (modified from He et al., 2018)

    Based on the calculation of the model and nature, the geometric, kinematic, and dynamic similarities are observed to satisfy the principles of physical simulation that have been proposed by a previous study (Hubbert, 1937). Based on the geological characteristics of this study area and in accordance with the affecting factors for deformation evolution, two series of physical models have been designed by the different silicone viscosities, the thickness of the sand and silicone layers is overlapped between the upper brittle and ductile layers (Figs. 3A01-A02, 3B01-B04). Further, the six experiments are done in 2 groups to study the tectonic deformation patterns of the ESWHHB (Table 1 and Fig. 3). The shortening velocity of the deformation experiments is observed to be 0.9 cm/h. Additionally, the total thicknesses of the B04 model and the other models are 3.2 and 1.6 cm, respectively. All the experiments were performed at the Tectonophysics Laboratory of the China University of Petroleum in Beijing.

  • In the A01 model, the silicone viscosity was the lowest, with 8 300 Pa·s. At the beginning of the shortening, the Qiyueshan and Dayong faults began to rise (Figs. 4A01 a-c); further, the deformation range was observed to be large. With an increase in compression strain, a large amount of deformation propagates leftward and was distributed in the equivalent eastern Sichuan. Deformation was characterized by low-amplitude folding (Figs. 4A01 d-g). In the A02 model, the silicone viscosity was the highest, with 18 300 Pa·s. During the early stage of shortening, the Dayong fault initially formed in the middle of the ESWHHB (Figs. 4A02 h-k). With an increase in compression strain, the shortening was mainly distributed in western Hunan and Hubei (Figs. 4A02 i-m). During the later stage of the compression, a few low-amplitude folds were formed in Eastern Sichuan (Figs. 4A02 n-o). At the same time, box-shaped folds with large amplitude were formed in western Hunan and Hubei (Fig. 4A02).

    Figure 4.  Experiment results of different viscosity silicone layers in sand-box modeling (models A01 and A02), and the (a–g) and (h–o) is surface photographs of the sequential deformation stage. ESR. Eastern Sichuan region; WHHR. western Hunan-Hubei region; XFU. Xuefeng uplift; QYF. Qiyueshan fault.

  • The silicone and quartz sand and their overlap were observed to be significantly different in this models, and the viscosity of the silicone layer was the highest, with 18 300 Pa·s. In the B01 model, the thickness of the basal silicone layers in equivalent western Hunan and Hubei, and equivalent eastern Sichuan both was 2 mm, and the thickness of interbedded silicone layer is 4 mm in the eastern Sichuan. Furthernore, the interbedded silicone layer was overlapped between the eastern Sichuan and western Hunan and Hubei regions. During the early stage of deformation, the shortening initially began to increase in the southeastern Hunan and Hubei (Figs. 5B01 a-c). As the amount of shortening increased, the strain crossed the Qiyueshan fault and gradually passed to the Eastern Sichuan belt (Fig. 5B01 e-g). In the B02 model, the basement of equivalent eastern Sichuan was set as 2 mm thick rigid acrylic plate. The thickness of the interbedded silicone layer is 4 mm in equivalent eastern Sichuan and the thickness of the basal silicone layer of equivalent western Hunan and Hubei was set as 4 mm.

    Figure 5.  Experiment results of strata difference overlap or no overlap in sand-box modeling (models B01, B02, B03, and B04), and (a)-(g) surface photographs of the sequential deformation stage. BS. Shortening; ESR. eastern Sichuan region; WHHR. western Hunati-Hubei region; XFU. Xuefeng uplift; QYF. Qiyueshan fault.

    The silicone layer was not overlapped. During the early stage of compression, deformation initially began to rise in the southeastern Hunan and Hubei region (Figs. 5B02 a-c). As the amount of shortening increased, the strain did not cross the Qiyueshan fault and was mainly distributed in the Hunan and Hubei region (Fig. 5B02 e-g). In the B03 model, the thickness of the basal silicone layer in western Hunan and Hubei was 2 mm, whereas that the interbedded silicone layer in eastern Sichuan was 4 mm. The basal silicone layer was not overlapped. During the early stage of compression, the deformation initially began to increase in southeastern Hunan and Hubei (Figs. 5B03 a-c). As the amount of shortening increased, the strain crossed the Qiyueshan fault and the deformation was transmitted to the eastern Sichuan belt, which resulted in the formation of several low-amplitude folds (Fig. 5B03 e-g). In the B04 model, the basement of eastern Sichuan was rigid, with thickness of acrylic plate (2 mm). However, the total thickness of the strata was 3.2 cm. Furthermore, the thickness of the basal silicone in Hunan and Hubei and eastern Sichuan was 4 mm, and no overlap was observed in the basal silicone. During the early stage of compression, the deformation initially began to increase in Southeastern Hunan and Hubei (Figs. 5B04 a-c). With an increase in shortening, the strain was mainly distributed in the western Hunan and Hubei regions, resulting in the formation of box-shaped folds with a large scale and high amplitude (Figs. 5B04 e-g).

    The comparison of the model results (A01, A02, B01, B02, B03 and B04), shows that the results of model B03 are reasonable and more similar to the nature owing to appearing the low-amplitude folds in eastern Sichuan and the box-like folds in western Hunan and Hubei.

  • The viscosities of the silicone weak layer in the A01 and A02 models were different and the model results exhibited different deformation patterns. The simulation results exhibit that the mechanical stratigraphy of the ESWHHB had an important controlling effect on the deformation pattern (Fig. 4). In the A01 model, the basement of the equivalent eastern Sichuan was weak since the interbedded silicone viscosity was 8 300 Pa·s. The shortening of eastern Sichuan was observed to be faster than that in western Hunan-Hubei region during the shortening process. In the A02 model, the viscosity of the basal silicone layer was the highest, with 18 300 Pa·s, which was one order of magnitude larger than that of the A01 model. Consequently, propagation of deformation toward the left was slower. This denotes that the mechanical stratigraphy strongly influenced the deformation evolution, which was also confirmed by some previous studies (Rossetti et al., 2002, 2000; Guscher et al., 2001). The experimental of Rossetti's studies (2002) exhibited that the deformation rate was large, which resulted in the formation of a narrow and steep thrust wedge in fold-thrust belt. In contrast, the deformation rate as small, a gentle fold was formed (Rossetti et al., 2000). In our physical models, the difference between the stratigraphic properties is produced by in the velocity of shortening and the viscosity of the strata. A large viscosity will lead to a slow deformation rate. However, when the viscosity decreased, deformation propagation increased. Therefore, the deformation rate is closely related to the viscosity of the strata, and the deformation evolution also was controlled by the deformation time (Guscher et al., 2001).

    The model B03 contains a low-viscosity interbedded ductile layer in the eastern Sichuan region, which causes the strain to transfer rapidly during the shortening process, resulting in the formation of a large number of low-amplitude folds; further, the high-viscosity basement in eastern Sichuan leads to a large amount of shortening accumulations in the western Hunan and Hubei region. Simultaneously, it was difficult to expand the strain to the eastern Sichuan belt. Obviously, the mechanical stratigraphy has a crucial effect on deformation evolution.

    Geological data of the eastern Sichuan has revealed several sets of basement weak layers in the Neoproterozoic, Sinian, Cambrian, Silurian, and Triassic slate, mud shale, and gypsum rock in eastern Sichuan, and the strata exhibit high flows (Fig. 2). However, the weak layers in the western Hunan and Hubei mainly comprise the Neoproterozoic and Sinian schist-phyllite and Sinian shale. Furthermore, the effective viscosity of the mantle lithosphere in western Hunan and Hubei is different from those in the eastern Sichuan tectonic belt (Lu et al., 2014). Based on the comprehensive geological features and the three-dimensional (3D) physical models, the structural style of the ESWHHB is closely related to the distribution of the weak layers and the mechanical stratigraphy in the area.

  • The results of the B01–B04 models confirm that the overlap difference of basal and interbedded viscous strata has an important influence on the structural deformation, as previously demonstrated (Santolaria et al., 2015). Four groups of physical modeling experiments denote that box-shaped folds are formed in the western Hunan and Hubei region, whereas low-amplitude folds are formed in the eastern Sichuan region. However, the structural styles also vary because of the different structural combinations of the strata. The B01 model, the thickness of silicone layer was very small, with 2 mm in western Hunan and Hubei. The brittle layer of quartz sand is set as 14 mm thick, and the thickness ratio of the brittle-ductile layer (7.0) is relatively large; therefore, the strain from the compression section is directly transmitted to eastern Sichuan. At the same time, the strain transmission is fast because eastern Sichuan displays a weak interbedded layer of 4 mm in thickness. In the B02 model, for western Hunan and Hubei as well as for eastern Sichuan, the thickness of the weak layer silicone is 4 mm. During model shortening, the thick silicone in western Hunan and Hubei causes the strain to easily propagate in this structural belt. Therefore, a large amount of shortening is observed to be mainly concentrated in western Hunan and Hubei. In the B03 model, the basal silicone thickness is 2 mm in the equivalent WHHB, whereas the interbedded silicone thickness is 4 mm in the equivalent eastern Sichuan. The forward propagation of deformation is easier than that model B02; however, the deformation of this forward propagation to the eastern Sichuan belt is difficult because of the rigid structure of this area (Burchfiel et al., 2008). In the B04 model, the thickness of the basement silicone layer in the equivalent western Hunan and Hubei and eastern Sichuan is 4 mm but the thicknesses of the overlaying brittle layer is much thicker than that in B01, 02 and 03. It is 0.8 cm in eastern Sichuan and 2.8 cm in western Hunan and Hubei. This condition considerably increases the thickness ratio of the brittle-ductile layer (6.0) in western Hunan and Hubei, and a large amount of shortening is mainly concentrated in western Hunan and Hubei.

    Although the aforementioned four models exhibit the same viscosity values, differences of deformation pattern can be observed in these models. The results of 3D physical simulation suggest that the strata combination and its overlap lead to the differences of the mechanical properties and formation of the difference of the strain distribution and fold patterns which are observed from our models.

    Compared with other similar physical models, strata overlap and its effect have been previously studied (e.g., Santolaria et al., 2015; Bonini, 2007, 2003; Marques and Cobbold, 2006; Sherkati et al., 2006; Costa and Vendeville, 2002; Rossetti et al, 2000). For example, the physical simulation study conducted by Bonini (2001) exhibits that a 0.2 cm thickness of the basal ductile layer and a thickness ratio of less than 3 of the brittle-ductile layer is conducive for outward fold propagation. Conversely, a passive-roof duplex is formed. In western Hunan and Hubei, the thickness ratio of brittle-ductile layer of our models (model B01=7.0, model B02=3.5, model B03=7, model B04=6) also form a passive-roof duplex style. In eastern Sichuan, the thickness ratio of brittle-ductile layer of our models (model B01=2.0, model B02=2.0, model B03=2.0) also form an outward fold propagation style, and the thickness ratio of brittle-ductile layer of model B04(=4.5) form a passive- roof duplex style. Combined with past study, our physical models reveal that the difference in the combination of the strata results in various mechanical properties of the rock and has an important influence on the structural deformation patterns.

    The balance restoration analysis of the western Hunan and Hubei strata denotes that the Mesoproterozoic basement ductile layer is thin with a thickness of approximately 2 km, while the overlying brittle layer is thick with a thickness of approximately 14 km. The thickness ratio of the brittle-ductile layers in this region is observed to be large (=7). In contrast, the thickness of the brittle layer in the eastern Sichuan region is 6–8 km, and the thicknesses of the Cambrian, Silurian, and Triassic gypsum rock and shale is approximately 4 km, with a small thickness ratio for the brittle-ductile layer (=1.5-2.0). Furthermore, the NW distal layer of ESWHHB, and the basement of the Sichuan Basin exhibits rigid properties. The overlap of the upper brittle layer and lower ductile layer affects the mechanical properties of the strata; therefore, box-shaped and low-amplitude folds are formed in the western Hunan and Hubei and eastern Sichuan, respectively. Therefore, the 3D physical simulation results reveal that the stratigraphic rheology characteristics and the overlapping relation of strata have an important effect on the deformation patterns in the ESWHHB.

  • From our physical models, the B03 model exhibits a deformation evolution that is similar to that observed in nature (Fig. 6). The similarities can be presented as follows. The wavelength and amplitude of the fold and the planar distribution characteristics of physical model are similar to the present tectonic characteristics. The box-shaped fold was distributed in the western Hunan and Hubei region, whereas the low-amplitude fold was distributed in the eastern Sichuan region (Fig. 7).

    Figure 6.  Comparison of regional geological map and model B03 (is more similar to the model-3 of Fig. 8 in He et al. (2018); box-shaped fold in western Hunan and Hubei region, and low-amplitude fold in eastern Sichuan region).

    Figure 7.  Along strike variation of deformational feature of model B03. (a) The top view and (b) the cross-section (from B03-1, B03-2 to B03-3).

    The differences can be presented as follows. The Huayingshan fault and the horsetail-shaped pattern in eastern Sichuan are observed to lack clarity. Furthermore, the effect of erosion and sedimentation is not considered in the current experiment. Therefore, the simulation results are inconsistent with the distribution characteristics of the Huayingshan fault belt. However, the results of the rheological and strata overlap experiments in 3D physical simulation provide a new perspective for understanding the deformation evolution and main controlling factors of the study area.

  • This study investigates influence of mechanical stratigraphy on the deformation evolution of fold-thrust belts with sand-box of eastern Sichuan-western Hunan and Hubei, South China. The results of physical modeling show that there are apparent differences in mechanical stratigraphy between the eastern Sichuan region and the western Hunan and Hubei region, which is favorable for the formation of fold patterns in ESWHHB. At the same time, the comparison of four group models with the same weak viscosity layer and different brittle- ductile overlap layers denotes that various stratigraphic combinations lead to different mechanical properties of the strata, which exhibits an important influence on the tectonic deformation pattern that is formed in ESWHHB. When the thickness ratio of the brittle-ductile layer increases, which is considered positive for the formation of fold patterns, the shortening can be easily transferred to the front of the structural belt. In contrast, when the thickness ratios of the brittle-ductile layer decrease, the deformation propagation process is considered negative for the formation of fold patterns. Furthermore, it is difficult to extend the compression strain toward the front edge of the structural belt.

  • This study was supported by Project of the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing (No. PRP/Open- 1903), and the Zunyi Normal University Project (No. BS[2018]04). I thank Prof. Jianxun Zhou, associate Prof. Graveleau and two anonymous reviewers for their helpful reviews and suggestions. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1281-2.

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