ABSTRACT:

This work illustrates how the 3D Finite Element (FE) method can be used to gain information on the reactivation potential and recent behaviour of faults. The Upper Rhine Graben (URG), Germany is used as a case study to demonstrate the proceedure. The URG is an intra-plate graben that developed during the Tertiary in an area with a complex pre-Tertiary (Variscan) crustal architecture. Several fault trends characteristic of Variscan and Tertiary tectonic phases can be distinguished within the highly faulted graben. The URG is presently characterized by relative low deformation rates and seismicity. Since the area is highly populated and industrialized, knowledge on the reactivation potential of the numerous faults is of high societal importance. The northern part of the URG is represented as a series of fault blocks bounded by frictional contact surfaces. Using the commercial FE solver Abaqus, the 3D state of stress for this model is obtained. The modelled stress state serves as a basis for the calculation of slip tendency and dilation tendency parameters, which describe the likelihood of the faults to slip or dilate respectively. The slip tendency results enable distinction between fault segments that are relatively prone to shear reactivation in the present-day stress field. The dilation tendency provides information on tensile failure of these segments. Additionally, the relative displacements of the fault blocks within the model show fault segments with large displacements as well as locked segments.

1.INTRODUCTION

In this paper, the potential for reactivation of upper crustal faults in an abandoned continental rift within the present-day stress field is investigated. The contemporary state of stress is approximated using the method of 3D finite element modelling. The geometrical complexity and rheological heterogeneities modelled induce a highly variable (i.e. non-Andersonian; Figure 1) stress field within the model domain as a function of the boundary conditions applied.

Figure 1. The 3 Andersonian stress regimes and the associated faulting types. Close to the surface, which can carry no shear stresses, the principal stress vectors are parallel to the vertical and horizontal stress components. Figure modified after [1].(available in full paper)

The state of stress obtained from the modelling is then used to calculate the Slip Tendency [2] and Dilation Tendency [3] for the upper crustal fault surfaces included in the models. These static risking parameters are then compared to predicted slip magnitudes along the faults and geological reference data to evaluate their reactivation potential.

2.MODELLING APPROACH

The in-situ state of stress affecting the URG area was approximated using a series of simplified mechanical earth models (MEMs). The upper crust is defined as two rheological domains describing the graben interior (i.e. Caenozoic sediments) and it's shoulder regions (i.e. Palaeozoic and Mesozoic rocks). Within the models, individual fault-blocks are separated by frictional contact surfaces which enable slip to occur during the analysis [4]. A uniform coefficient of friction of µ= 0.6 was assigned to all fault surfaces considered [5]. Linear elastic properties (i.e. density, Young's modulus and Poisson's ratio) for each element in the two rheological domains are defined as a function of lateral density variations of the crustal domains and their depth.

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