Controls on structural styles and decoupling in stratigraphic sequences 1 with double décollements during thin-skinned contractional tectonics: 2 insights from numerical modelling

6 Six series of particle-based numerical experiments were performed to simulate thin-skinned 7 contractional tectonics in stratigraphic sequences with double décollements during horizontal 8 shortening. The models were assigned with varying rock competence, depth and thickness of the 9 upper décollement, which resulted in significantly different styles of deformation and decoupling 10 characteristics above and below the upper décollement. The models composed of the least 11 competent material produced distributed sinusoidal detachment folds, with many shallow 12 structures profoundly decoupled from the deep-seated folds. The models composed of a more 13 competent material are dominated by faulted, diapir-cored box folds, with minor disharmonic folds 14 developed in their limbs. Differently, the results of models composed of the most competent 15 material are characterised by localised piggyback thrusts, fault-bend folds and pop-up structures 16 with tensile fractures developed in fold hinges. Depth of the upper décollements also plays an 17 important role in controlling structural decoupling, i.e. the shallower the upper décollements, the 18 higher the degree of decoupling becomes. Thicker upper décollements can provide sufficient 19 mobile materials to fill fold cores, and contribute to the formation of secondary disharmonic folds, 20 helping enhance structural decoupling. Our modelling results are comparable to the structural 21 features exhibited in the Dezful Embayment of the Zagros Fold-and-Thrust Belt with the Miocene 22 Gachsaran Formation acting as the shallow upper décollement, and the Fars with the Triassic 23 Dashtak Formation as its intermediate décollement. This study demonstrates that rock competence, 24 depth and thickness of the upper décollements can jointly affect the structural styles and 25 decoupling. Our modelling results are instructive for structural interpretation of deep zones in fold- 26 and-thrust belts that exhibit distinct structural decoupling features. 27


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Thin-skinned deformation styles are typical of many fold-and-thrust (FAT) belts in the foreland of 32 a collisional zone (Chapple, 1978). In such systems, a basal weak layer (e.g. shale and evaporite) 33 serves as the main décollement that allows the deformed overburden to be detached on during 34 shortening (Davis and Engelder, 1985). Two or multiple décollements have been reported in many tests, synthetic rock samples were created and loaded in a strain-controlled fashion by displacing 137 the boundary walls at a sufficiently slow rate, so as to attain a quasistatic solution. The stresses 138 and strains experienced by the rock sample were determined in a macro-fashion by summing the 139 forces acting upon walls and tracking the relative distance between the walls. The test results reveal 140 that the Young's Modulus for the three types of materials is 21.14 MPa, whilst the unconfined 141 compressive strength (UCS) for materials 1, 2 and 3 is 1.87, 3.51 and 3.54 MPa, respectively.

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The particles were packed by allowing randomly-generated particles to settle to the bottom of 144 model under gravitational force. The system was considered to have reached static equilibrium 145 when the mean unbalanced forces have been reduced to a negligible value. The particle assembly 146 was then trimmed to the desired thickness, which gave rise to a small amount of vertical elastic 147 rebound and surface uplift. This was followed by repeated trimming processes that allowed the  The three elastic walls served as the confined boundaries for the particle assembly, and the upper 151 surface was free. The left wall advanced at a controlled, uniform rate to the right, i.e. towards the 152 foreland direction, to yield horizontal shortening and tectonic deformation in the system (Fig. 1). 153 The models were gravitationally loaded by 1 g.

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We present six snapshots of each model during the sequential shortening and deformation process, The modelling result of model 1 is characterised by successive formation of multiple short-164 wavelength detachment folds that spread across the entire section (Fig. 2a). The first-order folds 165 individually consist of multiple second-order parasitic folds that exhibit a much smaller fold 166 wavelength and amplitude. The early-formed, symmetrical fold F1 is composed of two minor 167 disharmonic folds in the inner units that have a vergence toward the hinge zone, and grew by 168 consistent fold tightening with an increasing fold amplitude. F2 started to appear as a symmetric 169 detachment fold since T4, and continued to grow till T7. At T6, a forethrust began to be initiated 170 when the fold was right-verging, which was accompanied with a diapir rising from the basal 171 décollement. During T6 to T7, F2 evolved into a fault-propagation fold as a result of propagation 172 of the forethrust and a clock-wise rotation of its forelimb. Notably, the stratigraphic units above

Model 2 182
The deformation structures formed in model 2 are characterised by three similar-sized box folds, 183 and five distinct diapirs originated from the basal decollement (Fig. 2b). Fold F1 was initiated as 184 a symmetrical box fold with oppositely dipping axial surfaces and a sub-vertical diapir as its core.    (Table 1). The accommodation of shortening was jointly achieved by vertical 197 fold growth and diapirism. The modelling result of model 3 is characterised by the formation of piggy-back thrusts (Fig. 2c).   Model 5 successively produced three dominant asymmetric folds towards the foreland direction 231 (Fig. 3b). F1 was initiated at T2 as a detachment, diapir-cored fold and subsequently evolved into 232 a fault-propagation fold with a backthrust. The fold continued to grow from T2 to T5, which was 233 accompanied with the growth of its dispir. A forethrust was generated in its forelimb at T4. At T5, 234 a symmetric box fold F2 was initiated, which later became asymmetric by clockwise rotation of 235 its forelimb. F3 was formed at T6, and evolved into fault-propagation fold with a backthrust. In 236 this model, horizontal shortening was predominantly accommodated by folding and coeval 237 diapirism. The undeformed foreland is 2.33 km long (Table 1).  (Table 1).  (Table 1).

Model 9 273
The modelling result of model 9 is characterised by the formation of a single dominant fault-bend 274 fold in the foreland (Fig. 4c). Initially, the shortening was accommodated by uplifting and   (Table 1). The length of the unreformed foreland is longer than the other two models in 289 this series. Generally, the modelling result of model 10 is rather similar to that of model 1, which is 294 characterised by successive formation of five dominant sinusoidal folds with numerous second-295 order parasitic folds developed in the limbs of larger folds (Fig. 5a). A strong decoupling occurs 296 between the layers above and below the upper décollement. This is represented by the widespread 297 folding in the lower units, whilst the superficial layers remained flat and smooth. Crustal 298 thickening occurred throughout the system (Table 1) The modelling result of model 11 is characterised by the formation of three dominant detachment 304 fold and five diapirs (Fig. 5b). Initially, a minor fault-propagation fold with a forethrust was  Meanwhile, F5 was formed as a rather symmetric box fold, whose fold limbs were later cut by 314 minor thrusts.
The modelling result of model 15 is characterised by in-sequence development of a fault-bend fold 361 and a fault-propagation fold (Fig. 6c). Initially, a fault-bend fold was developed on the leftmost 362 side of the model. The hangingwall fragment overrode the right layers, and became overturned as Model 16 successively produced a series of sinusoidal detachment folds towards the foreland 376 direction (Fig. 7a). The folds below the upper décollement exhibit a lower amplitude and 377 wavelength than the upper folds. Disharmonic folds occurred in layers above the upper 378 décollement as a result of vergence of neighbouring minor folds that constitute a larger fold. Other 379 than the disharmonic folds, the fold traces below and above the upper décollement are largely 380 parallel, and structural decoupling is less significant than that exhibited in model 10 and 13. Crustal 381 thickening occurred throughout the system (Table 1), which results in a downslope angle of 13.1°.

Model 17 384
The modelling result of model 17 is characterised by the formation of four dominant box folds and 385 seven diapirs (Fig. 7b). F1 was initially formed as a symmetric box fold with its core consisting of 386 a sub-vertical diapir. The fold became tightened as shortening continued, resulting in thrust faults 387 that propagated along both the fold axial traces. Later on, the axial plane of F1 had a clockwise 388 rotation, resulting in the asymmetric geometry of F1. This was accompanied with the growth of its 389 core diapir along the backthrust. Secondary folds were cut by reverse faults in the forelimb of F1.

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F2 was initiated at T3, which evolved to become a fault-propagation fold with a backthrust at T4. propagation fold with a backthrust, and a later fault-bend fold with a forethrust (Fig. 7c). The   This section is focused on the discussion of the controls of rock mechanical property, depth and strain rather than vertical growth of existing structures, which resulted in crustal thickening across 498 the entire section (Table 1)

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Although the models presented are not aimed for directly simulating structures of any natural 519 prototypes, here it is attempted to compare the modelling results to the Zagros FAT Belt with multidécollements. The Zagros FAT Belt is a NW-SE-trending, 1800 km long segment of the 521 Alpine-Himalayan orogenic belt (Fig. 9a), which has been extensively studied not only because 522 that it is one of the most active collisional belts worldwide (Sella et al., 2002;Pirouz et al., 2017), 523 but also due to its specular fold trains developed in a thick multilayer of Paleozoic to Cenozoic 524 sediments that serve as the host to one of the world's largest hydrocarbon provinces (Cooper, 2007).

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The three major décollements significantly affected the mechanical stratigraphy and rock rheology 534 profile of Zagros (Sepehr et al., 2006) (Fig. 9b).  It should be noted that the numerical models presented are highly simplified and only show the 564 first order structural similarities to the Zagros FAT Belt. To better reproduce the structures developed in this area, it is suggested that future models should incorporate more comprehensive 566 regional geological data.