Quantifying strain in analogue models simulating fold-and-thrust belts using magnetic fabric analysis

Applying the anisotropy of magnetic susceptibility to analogue models provides detailed insights into the strain distribution and quantification of deformation within contractional tectonic settings like fold-and-thrust belts (FTBs). Shortening in FTBs is accommodated by layer-parallel shortening, folding, and thrusting. The models in this research reflect the different deformation processes and the resulting magnetic fabric can be attributed to thrusting, folding and layer-parallel shortening. Thrusting develops a magnetic foliation parallel to the thrust surface, whereas folding and penetrative strain develop a magnetic lineation perpendicular to the shorting direction but parallel to the bedding. These fabric types can be observed in the first model of this study, which simulated a FTB shortened above two adjacent décollements with different frictional properties. The different friction coefficients along the décollements have not only an effect on the geometric and kinematic evolution of a FTB, but also on the strain distribution and magnitude of strain within the belt.  The second series of models performed in this study show the development of a thrust imbricate and the strain distribution across a single imbricate in more detail. Three models, with similar setup but different magnitudes of bulk shortening, show strain gradients by gradual changes in principal axes orientations and decrease in degree of anisotropy with decreasing distance to thrusts and kinkzones. These models show that at the beginning of shortening, strain is accommodated mainly by penetrative strain. With further shortening, formation of thrusts and kinkzones overprint the magnetic fabric locally and the degree of anisotropy is decreasing within the deformation zones. At thrusts, an overprint of the magnetic fabric prior deformation towards a magnetic foliation parallel to the thrust surfaces can be observed. A rather complex interplay between thrusting and folding can be analysed in the kinkzones. In general, this thesis outlines the characteristics of magnetic fabric observed in FTBs, relates different types of magnetic fabric to different processes of deformation and provides insights into the strain distribution of FTBs

Revealing invisible strain : Magnetic Fabric Analysis as Strain Indicator in Analogue Models and Nature

Strain is accommodated by folding, thrusting and an “invisible” component, known as penetrative strain. Magnetic fabric analysis allows for identification and quantification of this imperceivable strain. In this thesis, magnetic fabric analysis is applied to quantify strain in analogue sandbox models. Several cases are simulated by the outlined models, such as the development of fold-and-thrust belts, single thrust-imbricates and a basin with subsequent inversion. These models developed characteristic sets of magnetic fabric that are comparable with observations from nature. The main results observed in the models can be summarized as follows. The initial fabric is affected by model preparation and subsequent deformation is accommodated by penetrative strain, folding and thrusting. Pouring and scraping the model material creates a horizontal magnetic lineation (axis of maximum susceptibility) parallel to the scraping direction. In contrast, sieving produces a fabric similar to a sedimentary fabric in nature, with a random magnetic lineation in the bedding plane. Penetrative strain overprints the initial magnetic fabric and compensates initial differences that are created during model preparation. The observed penetrative strain-induced fabric is classified by a clustering of the principal axes of magnetic susceptibility with magnetic lineation oriented mainly horizontally, perpendicular to the shortening direction. With the development of faults, the magnetic foliation aligns parallel to the fault surface. It is noted that thrusting is more efficient in aligning the magnetic foliation in contrast to normal faulting. However, the development of such a magnetic fabric depends on the maturity of a thrust. Moreover, with increasing strain, the magnetic fabric shows gradual changes in reorientation of the principal axes and degree of anisotropy. In detail, such gradual changes are observed from the foreland towards the hinterland and correlate with distance to a thrust within a thrust imbricate. This thesis demonstrates the use of magnetic fabric analysis as strain indicator in analogue models and provides insights in the development of magnetic fabric in nature. In fact, the results presented in the thesis barely scratches the surface of a potential rich research subject, which could be extended to tackle various questions in structural geology, tectonics and geodynamics.

Revealing invisible strain : Magnetic Fabric Analysis as Strain Indicator in Analogue Models and Nature

Strain is accommodated by folding, thrusting and an “invisible” component, known as penetrative strain. Magnetic fabric analysis allows for identification and quantification of this imperceivable strain. In this thesis, magnetic fabric analysis is applied to quantify strain in analogue sandbox models. Several cases are simulated by the outlined models, such as the development of fold-and-thrust belts, single thrust-imbricates and a basin with subsequent inversion. These models developed characteristic sets of magnetic fabric that are comparable with observations from nature. The main results observed in the models can be summarized as follows. The initial fabric is affected by model preparation and subsequent deformation is accommodated by penetrative strain, folding and thrusting. Pouring and scraping the model material creates a horizontal magnetic lineation (axis of maximum susceptibility) parallel to the scraping direction. In contrast, sieving produces a fabric similar to a sedimentary fabric in nature, with a random magnetic lineation in the bedding plane. Penetrative strain overprints the initial magnetic fabric and compensates initial differences that are created during model preparation. The observed penetrative strain-induced fabric is classified by a clustering of the principal axes of magnetic susceptibility with magnetic lineation oriented mainly horizontally, perpendicular to the shortening direction. With the development of faults, the magnetic foliation aligns parallel to the fault surface. It is noted that thrusting is more efficient in aligning the magnetic foliation in contrast to normal faulting. However, the development of such a magnetic fabric depends on the maturity of a thrust. Moreover, with increasing strain, the magnetic fabric shows gradual changes in reorientation of the principal axes and degree of anisotropy. In detail, such gradual changes are observed from the foreland towards the hinterland and correlate with distance to a thrust within a thrust imbricate. This thesis demonstrates the use of magnetic fabric analysis as strain indicator in analogue models and provides insights in the development of magnetic fabric in nature. In fact, the results presented in the thesis barely scratches the surface of a potential rich research subject, which could be extended to tackle various questions in structural geology, tectonics and geodynamics.

Magnetic fabric analyses of basin inversion : a sandbox modelling approach

A magnetic fabric analysis is a useful tool to dis- play deformation in nature and in models. In this study, three sandbox models represent basin inversion above a veloc- ity discontinuity (base plate). After complete deformation of each model, samples were taken in different parts of the models (along faults and areas away from faults) for mag- netic fabric analysis. Model I, which simulates basin for- mation during extension, shows two kinds of magnetic fab- ric: an “undeformed”/initial fabric in areas away from faults and a normal fault-induced fabric with a magnetic foliation that tends to align with the fault surface. Models II and III were extended to the same stage as Model I but were sub- sequently shortened/inverted by 1.5 cm (Model II) and 4 cm (Model III). Both inverted models developed “thrusts” during inversion. The thrusts show an alignment of magnetic folia- tion parallel to the fault surfaces that depends on the maturity of the thrust. Our results highlight that thrusting is more ef- ficient in aligning the magnetic fabric along them compared to normal faults. Moreover, models II and III reveal a mag- netic fabric overprint towards a penetrative strain-induced fabric (magnetic lineation perpendicular to shortening direc- tion) with increasing strain in areas away from thrusts. Such overprint shows a gradual transition of a magnetic fabric to a penetrative strain-induced fabric and further into a thrust- induced fabric during shortening/inversion. In contrast, ex- tension (Model I) developed distinct magnetic fabrics with- out gradual overprint. In addition, pre-existing normal faults are also overprinted to a penetrative strain-induced fabric during model inversion. They define weak zones within the main pop-up imbricate and steepen during model inversion. Steepening influences the magnetic fabric at the faults and,in general, the strain propagation through the model during inversion. The magnetic fabric extracted from the models presentedhere reflect the different stages of basin development and in- version. This study is a first attempt of applying magnetic fabric analyses on models simulating inverted basins. This study illustrates the possibility of applying a robust tool, i.e. magnetic fabric analyses, to sandbox models, whose initial, intermediate, and final stages are well documented, to under- stand fabric development in inverted tectonic regimes

Magnetic fabric signature within a thrust imbricate : an analogue modelling approach

In this study, we report results from three analogue models with similar initial setup and different amounts of bulk shortening, to simulate a development of a pop-up structure in fold-and-thrust belts at different stages. Samples are taken in different places of the deformed models for analysis using anisotropy of magnetic susceptibility. Shortening of the models resulted in the formation of a pop-up structure, which is bounded by backthrust(s) and complex forekink zone(s). Several forethrusts at different degrees of maturity developed in front of the pop-up structure. Three distinct types of magnetic fabric can be identified throughout the models: (i) a compactional oblate fabric that changes as function of distance towards a localized deformation zone (e.g., thrust or kinkzone), (ii) a thrust-induced fabric with magnetic foliation parallel to the thrust surface, and (iii) a complex forekink zone fabric with broad girdle distributions of principal axes and magnetic lineation perpendicular to shortening direction. The latter indicate interplay between folding and thrusting of the shortened sand layers. Additionally, a decrease in degree of anisotropy with appearance of a quantitatively more prolate fabric can be observed towards the thrusts and kinkzones. Additionally at thrusts, a variation in strain is reflected by the magnetic fabric and can be inherited in a thrust-induced fabric. In conclusion, strain is changing as function of distance towards localized deformation zones with characteristic fabric, and differences in magnetic fabric are distinct between data away and within deformation zones as deformation zones mature

Influence of décollement friction on anisotropy of magnetic susceptibility in a fold-and-thrust belt model

Anisotropy of magnetic susceptibility can provide insights into strain distribution in models simulating fold-and-thrust belts. Models with layers of sand and magnetite mixture shortened above adjacent décollements with high and low friction, are used to study the effect of décollement friction on the magnetic fabric. Above high-friction décollement, an imbricate stack produced a ‘tectonic’ fabric with magnetic foliation parallel to thrusts. In contrast, above the low-friction décollement deformation propagated farther into the foreland, and deformation 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 “penetrative-strain affected” area. Above both décollement 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 décollements. Through quantifying magnetic fabric, this study reemphasises the clear link between décollement friction, strain distribution and magnitude in fold-and-thrust belts