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

Sammanfattning: 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.