Influence of Décollement Friction on Anisotropy of Magnetic

Home , Fold (geology), Strain partitioning

Journal of Structural Geology 144 (2021) 104274

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Influenceof d´ecollement friction on anisotropy of magnetic susceptibility in a fold-and-thrust belt model

Thorben Schofisch¨ *, Hemin Koyi, Bjarne Almqvist

Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Villavagen¨ 16, 752 36, Uppsala, Sweden

ARTICLE INFO ABSTRACT

Keywords: Anisotropy of magnetic susceptibility can provide insights into strain distribution in models simulating fold-and- Analogue modelling thrust belts. Models with layers of sand and magnetite mixture shortened above adjacent décollements with high Anisotropy of magnetic susceptibility and low friction, are used to study the effect of décollement friction on the magnetic fabric. Above high-friction Fold-and-thrust belt décollement, an imbricate stack produced a ‘tectonic’ fabric with magnetic foliation parallel to thrusts. In Basal friction contrast, above the low-friction décollement deformation propagated farther into the foreland, and deformation Décollement intensity is gradual from the foreland to the hinterland by defining a transition zone in between. In this zone, magnetic lineation rotated parallel to the deformation front, whereas in the hinterland the principal axes do not show a preferred orientation due to different deformation mechanisms between “thrust-affected” and “pene trative-strain affected” area. Above both decollement´ types, the principal axes of susceptibility developed tighter clustering with depth. Along the boundary between the two décollements, a deflection zone formed where rotation of surface markers and magnetic fabric reflect the transition between structures formed above the different decollements.´ Through quantifying magnetic fabric, this study reemphasises the clear link between décollement friction, strain distribution and magnitude in fold-and-thrust belts.

1. Introduction results to the salt range in Pakistan, Cotton and Koyi (2000) concluded that the structures that develop in such deflection zones may trend Many models have addressed the influence of basal friction on the parallel to the shortening direction. Strain in such FTB models can be geometric, kinematic and dynamic evolution of fold-and-thrust belts estimated using passive markers at the surface (e.g. Nilforoushan and (FTB) (e.g. Davis et al., 1983; Dahlen et al., 1984; Colletta et al., 1991; Koyi, 2007), topographic change and laser scanning (e.g. Nilforoushan Huiqi et al., 1992; Mulugeta and Koyi, 1992; Willet, 1992; Gutscher et al., 2008) or other optical methods as summarized in Schellart and et al., 1996; Cotton and Koyi, 2000; Koyi et al., 2000; Agarwal and Strak (2016). In addition, taking sections during subsequent stages of Agrawal, 2002; Costa and Vendeville, 2002; Bahroudi and Koyi, 2003; deformation provides further information on internal deformation Koyi and Vendeville, 2003; Koyi and Cotton, 2004; Nilforoushan and across the model and displays the 4D evolution of the model (Colletta Koyi, 2007; Nilforoushan et al., 2008; Vidal-Royo et al., 2009). These et al., 1991; Mulugeta and Koyi, 1992). studies have shown that the structural style of a FTB depends on the The three main components of deformation in a FTB are faulting, décollement friction. For example, above a low-friction décollement (i) folding and layer-parallel shortening. The latter, which includes shortened layers form a wedge that possesses a gentler taper, (ii) the compaction by grain rearrangement and rotation, is not easy to estimate deformation front propagates farther, and (iii) both fore- and back in nature, but is of major significance for understanding the dynamic thrusts develop (Davis and Engelder, 1985; Cotton and Koyi, 2000). In evolution of a FTB, porosity reduction, and creation of balanced cross contrast, above a high-friction décollement a stack of imbricates form, sections (Koyi et al., 2003; Sans et al., 2003). However, anisotropy of that consists of mainly forethrusts with a steeper taper wedge (Mulu magnetic susceptibility (AMS), which is a potentially useful tool to geta, 1988). Cotton and Koyi (2000) showed additionally that a describe details of grain reorientation and deformation in different deflectionzone forms in cover layers at the boundary between adjacent tectonic regimes (e.g. Graham, 1966; Borradaile and Henry, 1997; Pares,´ décollements of contrasting low- and high-friction. Applying their 2015) can also be used to decipher this component of deformation.

* Corresponding author. E-mail addresses: [email protected] (T. Schofisch),¨ [email protected] (H. Koyi), [email protected] (B. Almqvist). https://doi.org/10.1016/j.jsg.2020.104274 Received 1 September 2020; Received in revised form 21 December 2020; Accepted 23 December 2020 Available online 31 December 2020 0191-8141/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). T. Schofisch¨ et al. Journal of Structural Geology 144 (2021) 104274

Recently, Almqvist and Koyi (2018) applied AMS analysis to definethe AMS ellipsoid, which is described by the shape factor T with a analogue models to describe the general strain evolution in a FTB. They spectrum ranges from T = +1 for oblate, T = 0 for neutral to T = 1 for outlined strain partitioning in different shortened models and repro prolate ellipsoids and the corrected degree of anisotropy Pj, which de duced their AMS patterns, which were comparable to AMS fabric evo scribes the degree of anisotropy of the ellipsoid. The mathematical ex lution in natural FTBs (e.g. Kligfieldet al., 1981; Borradaile and Henry, pressions for T and Pj use the natural logarithms (n) of the three 1997; Weil and Yonkee, 2009). In this study, we use the same method principal susceptibility axes (Jelinek, 1981), as shown in the following ology as Almqvist and Koyi (2018), to outline the AMS pattern in a equations: model simulating a FTB shortened above two different frictional sub 2 nint nmax nmin strates in order to evaluate the influence of d´ecollement friction on T = (1) nmax nmin magnetic fabric, strain distribution and intensity at surface and depth. √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅]}̅ Pj = exp { 2 [( n n )2 + (n n )2 + (n n )2 (2) 2. Experimental setup and anisotropy of magnetic susceptibility max mean int mean min mean

= ( ) = ( ) = ( ) = ( + Loose quartz sand (0.124–0.356 mm), which was used to build the with nmax ln kmax , nint ln kint , nmin ln kmin , and nmean nmax + )/ layers of the model, was mixed homogenously with blocky, subangular nint nmin 3. magnetite grains of same size range (<0.1 vol%) to facilitate AMS an Plotting both parameters (T and Pj) provides a distribution of ellip alyses of samples taken from the deformed model. This granular mixture soid shapes, which describes a strain path of the AMS dataset (Jelinek, (cohesion μ = 0.49) is scraped from the backstop (north) to the ‘model 1981; Hrouda, 1982; Borradaile, 1988, 1991; Borradaile and Henry, south’ to a 2.5 cm thick layer within a 67 × 60 cm large sandbox and 1997). shortened above two adjacent substrates simulating different frictional d´ecollements (sandpaper with μ = 0.71, and fibre glass with μ = 0.29) 3. Results (Fig. 1). To decrease the influence of the backstop, a 7-cm long sand ◦ wedge with a 28 taper was built on top of the model next to the 3.1. Structural evolution backstop (Fig. 1). A passive marker of circles with a different colour sand was printed on the surface of the model to monitor surface deformation. Shortening of the model above two different frictional decollements´ After a total of 17 cm (26% of length) bulk shortening, the model was resulted in a differential propagation of the deformation front and pat carefully wetted and sampled systematically for AMS analysis. Wetting terns of different strain (Figs. 2 and 3). Above the high-friction of the model was made through adsorption of water through the porous decollement,´ an imbricate stack formed, which increased the model sand, in order to preserve the fabric and not cause physical disturbance height next to the backstop by a factor of three compared to its initial of the sand. Sequential sectioning was performed, and oriented cubic undeformed state (Fig. 3). Here, forethrusts dip with angles between 15 ◦ samples (volume of 2.2 cm3) were taken at the surface and at depth and 40 towards the backstop. In contrast, above the low-friction along the sections in the wetted model. AMS was measured with a decollement,´ three boxfolds with major backthrusts and several fore ◦ MFK1-FA Kappabridge (Agico Inc.) using an AC fieldstrength of 200 A/ thrusts developed, with dips between 35 and 50 and a gentle wedge m with a frequency of 976 Hz. The measurements provide the average slope. In addition, on this side of the model, deformation propagated orientation and degree of alignment of all grains within a sample, based farther (ca. 10 cm) into the foreland compared to that on the high- ´ on principal axes of susceptibility, kmax ≥ kint ≥ kmin, which are calcu friction decollement (Fig. 3). Ahead of the frontal thrust within the lated from the magnetic susceptibility second rank symmetric tensor. foreland of the low-friction domain, circular strain markers at the sur The orientations of the principal susceptibility axes are plotted on an face were deformed to ellipses indicating the initial compaction of the equal-area lower hemisphere projection using geographic directions in sand prior to the formation of a new forethrust and boxfold (Fig. 2 – grey the model by defining the backstop as north. Also, the principal axes dashed line). Furthermore, at the boundary between these two

Fig. 1. A 3D illustration of the model setup.

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(TZ), hinterland above low-friction d´ecollement (HLF), deflection zone (DZ) with strike-slip fault that developed due to different deformation propagation above each d´ecollement, and a boundary zone in the hin terland (HBZ) where deformation above each different decollement´ interfere without forming a deflection zone. For each zone in the hin terland, the samples can be divided into surface and subsurface (>1 cm below surface) datasets (Fig. 3).

3.3. AMS data

In total, 240 samples were taken from the model, of which 133 were sampled at the surface and 107 from deeper parts (>1 cm beneath surface) where the model was thickened by shortening. The samples were grouped according to their location in the six zones described above.

3.3.1. Foreland fabric Within the FHF and FLF zones, kmax and kint are horizontal, whereas kmin is vertical (Fig. 3a). This fabric defines the initial “depositional” model fabric indicating that deformation had not reached to this part of the model and this is supported by the surface markers which show no deformation in this area. The magnetic fabric is dominantly character ized as oblate (Figs. 4 and 5), with some tendency of kmax grouping (i.e. magnetic lineation) towards the north and south (perpendicular to the backstop) that may arise from scraping of sand layers during model Fig. 2. Photograph of finite model surface after 26% shortening showing the preparation. different zones, based on degree of deformation of the passive strain markers at the surface (see text for details). A, B and C are representative profilesfor each 3.3.2. Transition fabric deformed area above the different frictional decollements.´ In the transition zone (TZ), kmin is clustered around the vertical axis, whereas k is clustered mainly horizontally to the west and k ´ max int contrasting decollements, the deformation front is bent due to the dif distributing subhorizontally in different directions (Figs. 3, 5 and 6). ferential propagation of the deformation front resulting in the formation Notably, the TZ zone is in front of the frontal thrust above the low- of a deflection zone (Fig. 2). Within this area, strike-slip faults and friction décollement, where surface markers depict penetrative strain, boxfolds create a complex pattern of strain. Along the deflection zone, magnetic lineation (clustering of kmax orientations) has an alignment which displays dextral kinematics, the strain ellipses rotate clockwise, perpendicular to the shortening direction parallel to the trend of the from which strike-slip faulting can be inferred (Fig. 2). The strike-slip ◦ frontal thrust (Fig. 6). Overall, this zone is dominated by oblate shapes of faults are slightly oblique (10 ) to the main shortening direction and anisotropy, with some exceptions of prolate fabric at the deeper parts in compensate for the offset in deformation propagation above the front of the frontal thrust and towards the deflection zone (Fig. 4). different décollements. In the hinterland above the boundary between ´ the low-friction and high-friction decollements, faults are characterized 3.3.3. Deflection zone fabric “ ” – ◦ by a transition from steep fore- and backthrusts (dip: 35 50 ) above Across the DZ, the magnetic fabric is characterized by subhorizontal the low-friction décollement towards gently dipping forethrusts (dip: kmax and kint, distributed around the primitive circle, while kmin clusters – ◦ ´ “ 15 30 ) above the high-friction decollement (Figs. 2 and 3). This hin in an E-W trending plane perpendicular to the strike-slip fault (Fig. 3). ” terland boundary zone developed at an early stage of shortening when The magnetic fabric is mainly oblate, but samples next to the strike-slip deflection of the deformation front was not expressed yet. However, fault have a prolate fabric or are close to the transition between prolate interference of different orientation of the structures, enhances the and oblate shapes (i.e. neutral triaxial fabric) (Figs. 4 and 5). complexity of the deformation behaviour within this boundary zone – (Fig. 3 profile B). 3.3.4. Hinterland fabrics The HHF zone is characterized by a difference in clustering of the 3.2. Model division principal susceptibility axes by comparing surface and deep samples, where trends are much more definedin the subsurface (Fig. 3). In both Almqvist and Koyi (2018) divided their models in the direction of datasets from the surface and subsurface, such trends are definedby the shortening into three zones; an undeformed zone (i.e. the foreland), the variety of great circles created by kmax and kint orientations (i.e. mag hinterland, and a transition zone between the hinterland and foreland, netic foliations) with shallow dips to the north and a mean inclination of ◦ which experienced some deformation but with no visible thrusts nor about 32 . kmin axes are spread around a mean axis with inclination of ◦ folds. In this study, we have adapted a similar definition for each ~58 to the south. Confidence ellipses for all three principal axes are décollement side, but a transition zone can only be defined above the narrower for the deep samples. Additionally, the clustering of the low-friction décollement. The deflection zone, i.e. the boundary be principal axes narrows with depth, defininga distinct magnetic lineation ´ tween the two decollement types, has two segments; a segment which is (i.e. clustering of kmax) parallel to the shortening direction. In general, between the hinterlands of high- and low-friction decollements;´ and a compared to the initial fabric, the susceptibility axes rotated within a second segment between the frontal part of the hinterland of the North-South striking plane and most samples within this HHF show an low-friction décollement and the foreland of the high-friction oblate fabric (Figs. 3–5). décollement (Fig. 2). Hence, we have divided the model into six zones In the HLF zone, the AMS fabric type and their orientation show a (Fig. 2): foreland above high-friction decollement´ (FHF), hinterland large variety of orientations (Fig. 3). Here, datasets are not only sepa above high-friction décollement (HHF), foreland above low-friction rated between surface and deep samples, where subsurface samples décollement (FLF), transition zone above low-friction décollement show clearer trends compared to surface samples, but they are also

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Fig. 3. Representative profiles showing the orientation of principal susceptibility axes (kmax ≥ kint ≥ kmin) for each zone, where h indicates model height and l the distance from the backstop at the certain point.

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Fig. 4. a) Jelinek-type plots of different areas above the high-friction d´ecollement, within the deflection zone and above the low-friction d´ecollement showing the distribution of the shape of anisotropy (T) against the corrected degree of anisotropy (Pj). b) Map of the model surface showing the main structural features and how the shape (T) varies throughout the model. The symbols represent a range of T-values within intervals between T = 1 (rotational prolate), T = 0 (neutral) and T = 1 (rotational oblate).

Fig. 5. Shape of anisotropy (T) plotted against the inclination of kmin principal separated by location. Samples taken from areas cut by a thrust, i.e. axes for each zone. “thrust-affected”, are distinguished from samples taken from areas be tween the thrusts, where penetrative strain is assumed to be the main decollements),´ the transition between two frictional decollements´ pro deformation mechanism, i.e. “penetrative-strain affected” (Fig. 3). From duced a complex fault system, which is reflected in the AMS data this division different clusters can be identified, which create a char (Fig. 3). Most samples show a magnetic foliation parallel to the back acteristic fabric for each domain. In the “penetrative-strain affected” thrust, which is comparable with the fabric from the HHF. Additionally, dataset, kmax and kint rotated around a vertical axis, where at the surface some susceptibility axes show different trends, which are similar to the kint shows more variation in inclinations compared to subsurface data fabrics observed in the HLF. For example, some kmax axes have hori “ ” (Figs. 3 and 5). In samples defined as thrust affected , kmax tends to zontal to subvertical orientations towards the west and some kmin axes have steep inclination. However, in the subsurface data, a distribution of plunge subhorizontally to the north as observed in the “thrust- affected” kmax along an East-West oriented girdle can be identified (Fig. 3). In area of in the HLF (Fig. 3). Consequently, the AMS of the HBZ reflect a summary, mainly two clusters of AMS axes can be distinguished above mixture between fabrics produced in the HHF and HLF. Overall, the the low frictional d´ecollement: 1) “thrust-affected” areas with an East- shape of anisotropy varies with no significant trend between predomi West magnetic foliation with kmin plunging subhorizontally and, 2) nantly oblate signature and some prolate fabric (Fig. 4). “penetrative-strain affected” areas, characterized by shallow dipping kmax and almost vertical kmin axes. Overall, the hinterland above the 4. Discussion ´ low-friction decollement produced a variety of kmin inclinations and shapes of anisotropy (Fig. 5). However, compared to the HHF zone, HLF The mechanical characteristics of a d´ecollement definesthe style and zone portrays more prolate pattern, especially in the vicinity of the extent of the deformation within FTBs (Davis and Engelder, 1985; Col deflection zone (Fig. 4). letta et al., 1991; Cotton and Koyi, 2000). The different decollements´ In the HBZ (i.e. above the intersection of both different frictional lead to a different degree of ‘tectonic’ compaction and vergence of

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Fig. 6. Maps of model surface showing the spatial distribution of the declination of a) magnetic lineation (alignment of kmax) and b) magnetic foliation (kmin orientation) at the surface with inclination values (numbers) of the deformed model. structures, leading to a difference in layer-parallel shortening due to between the undeformed “depositional” fabric in the foreland and the variable amount of ‘tectonic’ compaction (Koyi et al., 2003). This is deformed (“tectonic”) fabric in the hinterland is abrupt. In contrast, on expected to be detected in the AMS signature of samples taken from the other side of the model, i.e. above the low-friction décollement, a layers deformed above different decollements.´ However, it has been gradual change in strain is observed. This abrupt versus gradual varia demonstrated in analogue models, that this ‘tectonic’ compaction is tion in strain between two contrasting décollements using surface pas manifested in length, area and volume decrease due to grain repacking sive markers has also been documented by Nilforoushan et al. (2007). In (e.g. Koyi, 1995; Koyi et al., 2003; Nilforoushan et al., 2008). Such grain addition, above the low-friction decollement,´ the AMS fabric indicates repacking in models is detectable by AMS analysis (Almqvist and Koyi, that initial deformation is detectable even when it is not shown by the 2018). Our AMS analysis of models shortened above two different surface strain markers. Above the low-friction décollement, AMS data adjacent décollements shows that this grain repacking results in shows that the area in front of the frontal thrust (i.e. in the TZ), which different fabric, depending on the frictional properties of the does not show any visible structures, is a “transitional” zone between the décollement. hinterland and the undeformed foreland. Such indication of strain Unlike in nature, in models rigid-body rotation and translation of initiation was also recognized by Almqvist and Koyi (2018), in the sand and magnetite grains, i.e. no grain strain, are the main contributors transition zone of their model. This findingis compatible with previous to the AMS fabric. Recrystallisation due to ductile deformation, tem studies where it is has been documented that layer-parallel shortening perature change or additional mineral precipitation as seen in natural can be observed in magnetic fabrics even beyond the frontal thrust to shear zones (e.g. Borradaile and Tarling, 1981; Kligfieldet al., 1981; Hirt wards the foreland (e.g. Pares´ et al., 2015 for review). In the TZ, the et al., 2004; Ferre´ et al., 2014), do not occur in sandbox models and thus magnetic lineation is parallel to the deformation front and perpendicular are not considered in this study. to the North-South shortening direction. Lineation developed parallel to transport direction is also observed in soft sediment deformation in mass transport deposits, where the initial AMS fabric is overprinted by 4.1. Magnetic fabric reorientation above the different frictional layer-parallel shearing (Weinberger et al., 2017; Alsop et al., 2020). décollements Plots of samples from the high-friction décollement show clustering of the principal susceptibility axes Fig. 3 probably due to grain rotation In order to understand the AMS within the deformed zones in the during repacking (layer-parallel compaction), folding of the layers and model, the undeformed foreland fabric in the FHF and FLF zones are displacement along the thrust faults. In the HHF zone, inclination of the considered as an analogue to an original depositional fabric with vertical magnetic foliation has a similar dip and vergence as that of the faults in kmin, and horizontal kmax and kint. In both zones, neither surface defor this zone, i.e. southwards (Fig. 3). It is assumed that within the fault mation nor any rotation of the AMS axes are observed. However, the surfaces, grains rotate with kmax and kint alignment (developing mag magnetic lineation in this zone, which is perpendicular to the backstop netic foliation) parallel to the transport direction along the thrusts and (Fig. 6), is attributed to model preparation, when the sand layers with kmin is perpendicular to the trend of the thrusts (Fig. 5a). As grains rotate constant thickness were scraped from the backstop (north) to the ‘model and align along the faults, reorientation of the principal axes depends on south’. However, a dominantly oblate fabric can still be observed in the the orientation of the fault surfaces and we assume that grains cease FHF and FLF zones (Fig. 4). rotation as soon as they reach alignment with the faults (e.g. Borradaile From the initial fabric in the foreland, the principal susceptibility and Tarling, 1981). Notably, the grains are diverse in shape, which is axes rotate as strain increases towards the hinterland (Weil and Yonkee, reflected in the variation in the shape and degree of anisotropy. How 2009; Almqvist and Koyi, 2018). The impact of décollement on magni ever, principal axes rotation is more pronounced with depth, as we tude and geometry of strain is clearly depicted in the AMS analysis of the observe a tighter clustering of the principal axes compared to those from model samples. Above the high-friction decollement,´ the “boundary”

6 T. Schofisch¨ et al. Journal of Structural Geology 144 (2021) 104274 the surface samples (Fig. 3). This effect can be explained by the decrease have a similar rotation of the principal susceptibility axes as those seen in fault spacing of the imbricates as they merge close to the backstop, in the HHF zone, where kmin axes rotated towards gentle, subhorizontal where several faults can be included in one sample. Additionally, since dip and magnetic foliation strikes East-West. A different trend is penetrative strain increases with depth (Koyi et al., 2003), principal axes observed in the areas that are away from thrust surfaces, where pene rotation towards a distinct fabric with narrower clustering can addi trative strain is assumed to be prevailing. In these areas, clustering of tionally be explained by variation in strain partitioning with depth. axes is still more pronounced with depth showing different strain in However, the hinterland fabric of the model is strongly affected by different parts of the model, as observed by Mulugeta and Koyi (1992) closely-spaced thrust faults, along which grains are aligned. This can be and Koyi et al. (2003). However, kmin maintains mostly its vertical trend compared to the tectonic AMS fabric from shear zones in the field,where and magnetic lineation is rotated around the vertical axis along the the rotation of susceptibility axes is determined by ductile deformation primitive circle relative to the initial “depositional” fabric, which is (Ferre´ et al., 2014) or by the orientation of the cleavage plane with comparable to the fabric produced in the TZ (Fig. 3). brittle deformation and recrystallisation (e.g. Borradaile and Tarling, In general, lower strain is documented above low-friction 1981; Kligfieldet al., 1981; Hirt et al., 2004). Primary orientations of the decollements´ compared to that above high-friction décollements (Cot principal susceptibility axes are expected to be completely reoriented in ton and Koyi, 2000), which coincides with our observations from shear and faults zones. comparing the degree of axes rotation and clustering of the principal Rotation of AMS axes above the low-frictional décollement is more susceptibility axes in samples from both types of décollements. In the diverse than above the high-friction décollement (Figs. 3, 5 and 6). HHF zone, clustering of the AMS axes shows that the initial “deposi However, different clusters of the AMS axes orientations can be differ tional” fabric is overprinted by a ‘tectonic’ fabric. In contrast, defor entiated and attributed to the influence of specific faults. Unlike above mation above the low-friction decollement´ is displayed by development the high-friction decollement,´ where thrusts verge towards the foreland, of ‘intermediate’ and ‘tectonic’ magnetic fabric probably due to grain above low-friction decollement,´ faults verge in opposite directions, i.e. reorientation (Fig. 7). towards the foreland (forethrusts) and hinterland (backthrusts). As such, Towards the DZ, bending of structures caused by the frictional the magnetic fabric is influenced by unequal contributions of shear of contrast between the two decollement´ types results in the rotation and both fore- and backthrusts, which lead to rotation of principal suscep shearing of surface strain markers, which is reflected in the magnetic tibility axes into different directions. Such complex principal axes fabric. At the early stages of model shortening (~10% of bulk short rotation leads to less clustering compared to the fabric produced by the ening), no offset was observed between the structures forming above imbrication above the high-friction décollement. Consequently, the AMS either décollement, i.e. the deformation front did not show any bend. data from the fault zones in the HLF does not strongly reflect the ori Thrusts forming above the high-friction decollement´ intercept with entations of the fault planes that bound the boxfolds and the magnetic thrusts formed bounding boxfolds above the low-friction decollement´ fabric reflectsa higher heterogeneity of strain, which is expected for low resulting in the formation of a boundary zone. This means, that the AMS frictional décollements (Nilfouroushan et al., 2007). data from the HBZ shows a mixture of magnetic fabrics with contribu In more detail, above the low-friction décollement two different tion from the two different décollements (Figs. 3 and 4a). However, fabric trends can be recognized in the hinterland, which are more shearing along thrusts that verge towards the foreland dominate defined at depth similar to the fabrics above the high-friction deformation at depth and is reflected by principal susceptibility axes ´ decollement (Fig. 3). Samples taken from the “thrust-affected” areas (kmax-kint) developing a magnetic foliation, similar to what is observed

Fig. 7. Summary of magnetic fabric in the model.

7 T. Schofisch¨ et al. Journal of Structural Geology 144 (2021) 104274 in the HHF. With further shortening, the offset between the deformation formation of strike-slip faults (Cotton and Koyi, 2000), Bahroudi and fronts above each décollement increased, which led to the development Koyi (2003) related the deformation style of the Zagros FTB and some of of dextral strike-slip faults and bending of thrusts to trend subparallel to the strike-slip faults to a varying extent of the low frictional salt the shortening direction (Fig. 2). Samples from the DZ along the strike- decollement.´ AMS data from around the Kazerun Fault, a surface slip faults show a rotation of kmin away from the fault plane towards expression of a basement-induced strike-slip fault in the Zagros FTB, inclinations perpendicular to the shear plane (Fig. 3), whereas kmax and probably enhanced in the cover by the extent of the Hormuz salt as a ´ kint rotate towards the fault surface. Such principal axes reorientation is weak decollement, shows a bending of the magnetic lineation (cf. Bak characteristic for shear zone experiments inferred from AMS (Borradaile thari, 1998; Aubourg et al., 2010). Furthermore, similar bending of the and Alford, 1988; Borradaile and Puumala, 1989). general trend of the AMS data is also observed in the Wyoming salient (Weil and Yonkee, 2009), and the western Alps (Cardello et al., 2016), 4.2. Differences in the shape and degree of anisotropy where the magnetic lineation follows the curvature of the belts. Such bending of structures and development of magnetic lineation is repro The difference between the deformation above the two contrasting duced in the model AMS data from the deflectionzone between the low- décollements is seen not only in the rotation of the principal suscepti and high-friction décollements (Fig. 6). Moreover, different rotation bility axes, but also in the distribution of the shape and degree of directions (clockwise and anti-clockwise), observed in palaeomagnetic anisotropy (T and Pj) throughout the model. Most of the measured AMS and AMS results in the Zagros FTB, reflect a complex local tectonic samples show oblate fabric, but there is a higher amount of prolate history (Aubourg et al., 2010). In our case, different rotations can be fabric documented within the HLF zone (Figs. 4 and 5). Above the high- confirmed by the variation of the orientations of the principal suscep friction décollement, rotation of principal axes is determined by the tibility axes within the HLF zone (Fig. 3). Similarities between fabric orientation of thrusts, which have a vergence towards a similar direc observed in nature and fabric created in the model, coincides with ob tion. Consequently, a distinct magnetic foliation within the thrust sur servations and interpretations of the role of décollement friction. faces could develop, which creates an oblate fabric. In contrast, fabric Another FTB model with a more complex setup of different frictional above the low-friction décollement is diverse due to different verging decollements´ is represented by Vidal-Royo et al. (2009), who proposed faults. In addition, compaction of the sand layers varies throughout the that a difference in basal friction is reflected by the oblique trend of HLF zone, resulting in formation of both prolate and oblate fabric structures (oblique to main Pyrenean shortening direction) in the regardless of kmin orientation (Figs. 4 and 5). Even when the shortened Southern Pyrenees. AMS and palaeomagnetic data from the Pico de layers above both décollements developed a different distribution of Aguila anticline (Pueyo-Morer et al., 1997; Pueyo et al., 2002; shapes of anisotropy, the degree of anisotropy (Pj) shows less difference. Pueyo-Anchuela et al., 2012) and the thrust oblique zone of the Southern Generally, there is a slight decrease in Pj-values from the foreland to the Central Pyrenees (summarized by Munoz˜ et al., 2013) show a history of hinterland. This trend is valid for both decollements´ (Fig. 4), but further rotation of the magnetic fabric in these zones. Removing the local fold studies and investigations are needed to understand the details of Pj development from the AMS data in these regions, provides a rotational development. history around vertical axes above Triassic evaporites, which act as a low-friction décollement in the Southern Pyrenees. Since the transport 4.3. Model implication and comparison direction along the main thrust system in this area is assumed to be constant (Marcén et al., 2018), structural interpretations from the model Using AMS as strain indicator in models provides a more detailed by Vidal-Royo et al. (2009) and the magnetic fabric from the deflection understanding of strain distribution within sandbox experiments zone of our model demonstrate the significant role of differences in (Almqvist and Koyi, 2018). The results presented here contribute to the decollements,´ while several other options are less favored for this region usage of AMS in analogue models and provide clues for understanding (see discussion by Munoz˜ et al., 2013). the dynamic evolution of FTB above different frictional decollements.´ It Deformation above the high-friction decollement´ in our model is is important to underline that, since AMS is sensitive to local deforma expressed by a ‘tectonic’ fabric, which is determined by sampling across tion, caution should be taken when using magnetic fabric to draw con the closely spaced-imbricates. A similar sampling scenario in nature is clusions on the scale of the orogen (Pocoví et al., 2014). But, scaling our given by the investigation of cleavage zones. In a number of studies, it model to nature, grain orientation within analyzed samples of the cur was shown that with progressive deformation and formation of cleavage rent model represents a summary of data over a larger region in nature. planes, kmax aligns parallel to the bedding-cleavage intersection, where There are many FTBs where sedimentary cover is shortened above larger grains rotate perpendicular to the shortening direction and align adjacent low-friction and high-friction décollement, such as the Potwar with the cleavage plane (e.g. Borradaile and Tarling, 1981; Kligfield region in Pakistan (Cotton and Koyi, 2000; Robion et al., 2007), Zagros et al., 1981; Hirt et al., 2004). In most cases, new smaller magnetic FTB in Iran (Bahroudi and Koyi, 2003), and Southern Pyrenees in Spain crystals form within the cleavage zone, which overprints the overall (Vidal-Royo et al., 2009; Munoz˜ et al., 2013), where the results reported AMS fabric. Since in our models we exclude crystal-plastic deformation, here can be applied and compared to. such as dynamic recrystallisation of minerals, rigid rotation of the larger For example, Robion et al. (2007) conclude that the variations in grains coincides with the rotation of the principal susceptibility axes in AMS data observed in the Potwar region in Pakistan and in Minervois in the hinterland above the high-friction decollement.´ Finally, as suggested the Northern Pyrenees are related to differences in decoupling of the that ‘tectonic’ fabrics are likely to develop above high-friction cover units by their décollements. Based on AMS data, they show that decollements´ (cf. discussion by Robion et al., 2007), our model results basal friction is reflected in the deformation of the sedimentary cover agree with tectonic overprinting of the initial fabric by the deformation above each decollement.´ They interpret the sedimentary and interme above higher friction. diate magnetic fabrics in the Potwar basin and in the Zagros FTB, as results of deformation above a low frictional salt décollement. This 5. Conclusions pattern is reproduced in our model above the low-friction décollement where an ‘intermediate’ magnetic fabric has formed (Fig. 7) and where Results of a shortened sandbox model show that decollement´ friction partly the initial fabric sustained the deformation (Fig. 3). Overall, the has a significantimpact on the development of magnetic fabric in a fold- rotation of the principal susceptibility axes in the FLF zone and HLF zone and-thrust belt. The AMS depicts the difference in deformation above is interpreted to be comparable with the interpretation by Robion et al. each décollement, where the wedge geometry, taper, fault vergence, and (2007). deformation propagation vary with decollement´ friction (Fig. 7). Above As the contrast in friction of adjacent décollements lead to the high-friction décollement, clustering of rotated AMS axes (i.e.

8 T. Schofisch¨ et al. Journal of Structural Geology 144 (2021) 104274 development of magnetic foliation) is determined by closely spaced with palaeostress markers in the western Fars Arc (Zagros, Iran): tectonic – imbricates interpreted as a ‘tectonic’ fabric. In contrast, above the low- implications, 330. Geological Society Special Publication, pp. 97 120. https://doi. ´ org/10.1144/SP330.6. friction decollement, change in fabric and rotation of principal axes is Bahroudi, A., Koyi, H.A., 2003. Effect of spatial distribution of Hormuz salt on less abrupt between fore- and hinterland, defininga transitional zone. In deformation style in the Zagros fold and thrust belt: an analogue modelling – this zone, magnetic lineation parallel to the deformation front (i.e. approach. J. Geol. Soc. 160, 719 733. https://doi.org/10.1144/0016-764902-135. Bakhtari, H.R., Frizon de Lamotte, D., Aubourg, C., Hassanzadeh, J., 1998. 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Geol. 10, 895–904. impact of faulting on deformation. Fore- and backthrusts bounding the Borradaile, G.J., Henry, B., 1997. Tectonic applications of magnetic susceptibility and its boxfolds that formed above the low-friction decollement,´ developed a anisotropy. Earth Sci. Rev. 42, 49–93. https://doi.org/10.1016/S0012-8252(96) differential alignment of susceptibility axes in different directions. 00044-X. Borradaile, G.J., Puumala, M.A., 1989. Synthetic magnetic fabrics in a plasticene Along the deflection zone, which formed between the two contrasting – ´ medium. Tectonophysics 164, 73 78. https://doi.org/10.1016/0040-1951(89) decollements, the kmax axes rotated around the vertical axes towards the 90235-7. slip plane of strike-slip faults, which developed in this zone subparallel Borradaile, G.J., Tarling, D.H., 1981. The influence of deformation mechanisms on – to the shortening direction. Overall, each zone developed its distinct magnetic fabrics in weakly deformed rocks. 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Bull. 112, 351–363. https://doi.org/10.1130/0016-7606 < > 10.17632/j249tyfcnw.2, an open-source online data repository hosted (2000)112 351:MOTFAD 2.0.CO;2. ¨ Dahlen, F.A., Suppe, J., Davis, D., 1984. Mechanics of fold-and-Thrust belts and at Mendeley Data (Schofisch, 2020). accretionary wedges: cohesive coulomb theory. J. Geophys. Res. 89, 10087–10101. https://doi.org/10.1029/JB089iB12p10087. Credit author statement Davis, D.M., Engelder, T., 1985. The role of salt in fold-and-thrust belts. Tectonophysics 119, 67–88. https://doi.org/10.1016/0040-1951(85)90033-2. Davis, D., Suppe, J., Dahlen, F.A., 1983. Mechanics of fold-and- thrust belts and Schofisch,¨ T.: Conceptualization, Methodology, Formal analysis, accretionary wedges. J. Geophys. Res. 88, 1153–1172. https://doi.org/10.1029/ – – & JB088iB02p01153. Writing original draft, Writing review editing, Visualization, Koyi, ´ ´ – & Ferre, E.C., Gebelin, A., Till, J.L., Sassier, C., Burmeister, K.C., 2014. Deformation and H.: Conceptualization, Methodology, Investigation, Writing review magnetic fabrics in ductile shear zones: a review. Tectonophysics 629, 179–188. editing, Supervision, Funding acquisition, Almqvist, B.: Conceptualiza https://doi.org/10.1016/j.tecto.2014.04.008. tion, Investigation, Writing – review & editing, Supervision, Funding Graham, J.W., 1966. Significance of magnetic anisotropy in Appalachian sedimentary rocks. In: Steinhart, J.S., Smith, T.J. (Eds.), The Earth beneath the Continents: A acquisition Volume of Geophysical Studies in Honor of Merle A. Tuve: American Geophysical Union Geophysical Monograph 10, pp. 627–648. https://doi.org/10.1029/ GM010p0627. Declaration of competing interest Gutscher, M.A., Kukowski, N., Malavieille, J., Lallemand, S., 1996. Cyclical behavior of thrust wedges: insights from high basal friction sandbox experiments. Geology 24, The authors declare that they have no known competing financial 135–138. https://doi.org/10.1130/0091-7613(1996)024<0135:CBOTWI>2.3.CO; 2. interests or personal relationships that could have appeared to influence Hirt, A.M., Lowrie, W., Lüneburg, C., Lebit, H., Engelder, T., 2004. Magnetic and mineral the work reported in this paper. fabric development in the ordovician martinsburg formation in the central appalachian fold and thrust belt, Pennsylvania, 238. Geological Society Special Publication, pp. 109–126. https://doi.org/10.1144/GSL.SP.2004.238.01.09. Acknowledgement Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surv. 5, 37–82. https://doi.org/10.1007/BF01450244. The authors acknowledge LKAB Minerals Luleå (Sweden) for Huiqi, L., McClay, K.R., Powell, D., 1992. Physical models of thrust wedges. In: McClay, K.R. 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