Two mechanisms of graben nucleation above dikes based on elastic and frictional models applied at three locations in the Elysium Rise, Mars

Introduction

We modeled the sequence of discontinuities forming in the subsurface due to dike intrusion with the objective of understanding the possible mechanisms of dike-induced graben nucleation. We did this at three sites in the Elysium Mons region of Mars: Galaxias Fossae (GF), Elysium Fossae (EF) and Cerberus Fossae (CF). From our results we propose to general models of graben nucleation above dikes under Martian conditions.

Methodology

Firstly, we used cross-section area balancing on MOLA topographic profiles across one graben in each site to obtain approximations of dike geometry, mainly, aperture (a) and top-dike depth (D_d) (Rivas-Dorado et al., 2020). Because these parameters are sensitive to fault angle, for each location we used three values: 55, 60 and 65º. Then, for each of these we used three sets of mechanical properties, corresponding to a weak, intermediate, and strong host rock, as inputs for the dynamic models. In each model, driving stress is increased step wise and at each step the following processes take place. First, the composite dike-induced stress field is calculated considering both the dike’s driving stress and the lithostatic load (Pollard and Segall, 1987). Then, the principal stresses ( and ) are calculated from the total dike-induced stresses. Finally,  and  are used in the modified Griffith failure criterion to determine which points in the grid are at failure, and if this is the case, their mode and orientation. At the end of the model, a sequence of discontinuities forming between the dike tip and the surface has been recorded. This methodology was applied to all three sites, which resulted in 27 models.

Results

14 models were found to be compatible with the nucleation of dike-induced faults, from which we propose two general models. We define compatible models as those in which the discontinuities form through a sequence, and at an orientation and location, which are compatible with the inferred present-day faults.

The first generic model corresponds to narrow and relatively shallow dikes emplaced in a low-compliance host rock (Fig. 1a). In this case, fault nucleation occurs through the coeval propagation of mode-I tensile cracks from the surface to depth, and of mode-II discontinuities formed under general compression or tension-compression, from the dike tip to the surface. In this scenario, the dike driving stresses are relatively small and tension cracks dominate over 50-65 % D_d. The second model corresponds to wider and deeper dikes emplaced in a higher-compliance host rock (Fig 1b). In this scenario, fault nucleation occurs through the propagation of near-surface mode-I cracks to depth, and mixed-mode I-II plus mode-I discontinuities from the dike tip to the surface, which lead to the formation of high-angle faults. The required dike driving stresses in these models are larger, and the section between the dike tip and the surface is dominated by tensile structures, which occupy >65% D_d.

We also ran equivalent models under Martian versus terrestrial conditions to explore the differences in the style of growth of the discontinuities. We found that under terrestrial conditions the extent of the tension-dominated section is narrower and restricted to the near surface, <20% D_d, and that fault nucleation requires a larger participation of mode-II discontinuities. This is because of the greater contribution of the lithostatic stresses which, in general, make tensile failure at depth more difficult.

The results of our models are consistent with several numerical and analogue models, and with multitude of observations. For example, mapping of the displacements along dike-induced graben in the Exmouth plateau, through high-resolution 3D seismic, frequently reveals two points of maximum slip along many faults. This indicates that these have grown from two nucleation points, one shallower and one deeper, as proposed in both our generic models (Magee and Jackson, 2020, 2021). Although comparisons with terrestrial models and observations must be made with caution, since they respond to dike emplacement under different pre-diking conditions, the observed similarities support our model results.

Conclusions

We performed several dynamic models of the discontinuities formed during the emplacement of dikes under different conditions, at three locations in the Elysium Mons region of Mars. From these we propose two generic models of dike-induced graben nucleation: 1) shallow, narrower dikes emplaced in a weaker host rock in which both tensile cracks and faults participate in fault nucleation, and 2) deeper, wider dikes emplaced in a stronger host rock, which generate faults in a dominantly tensile regime. These results can be reconciled with models of dike-induced deformation, and with observations in real case studies. However, our models only address the question of early fault nucleation. Further research and modelling efforts are required to understand later dike-induced graben development and the processes associated with it, such as long-term post-diking seismicity. This may be key to understand the origin of the marsquakes currently being detected by InSight in the Elysium region, and specifically, in the vicinity of Cerberus Fossae.

We constructed dynamic models of the fractures and faults formed around an intruding dike by considering both the dike and lithostatic stresses. From these we propose two distinct conceptual models of graben nucleation with different participation of faults and fractures; one in which shallower and narrower dikes nucleate graben through both faults and tensile cracks, another in which deeper and wider dikes generate graben mostly through the linkage of mode-I fractures. 

References

Magee, C., Jackson, C., 2020. Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm, offshore NW Australia. Solid Earth 11, 579–606. https://doi.org/10.5194/se-11-579-2020

Magee, C., Jackson, C.A.L., 2021. Can we relate the surface expression of dike-induced normal faults to subsurface dike geometry? Geology 49, 366–371. https://doi.org/10.1130/G48171.1

Pollard, D.D., Segall, P., 1987. Theoretical displacements and stresses near fractures in rock: with applications to faults joints, veins, dikes and solution surfaces, Fracture Mechanics of Rock. Academic Press Inc., London. https://doi.org/https://doi.org/10.1016/B978-0-12-066266-1.50013-2

Rivas-Dorado, S., Ruiz, J., Romeo, I., 2020. Subsurface Geometry and Emplacement Conditions of a Giant Dike System in Elysium Fossae, Mars. J. Geophys. Res. Planets n/a, 2020JE006512. https://doi.org/https://doi.org/10.1029/2020JE006512