Modeling of dike-induced graben nucleation in the Elysium region, Mars: The role of planetary gravity

Abstract

We have studied the processes of dike-induced graben nucleation through the analytical modeling of the failure sequence, i.e., the timing, location, and mode, of the discontinuities formed between the dike's upper tip and the surface. To this end we use as input parameters dike geometry and the elastic and frictional properties of a homogeneous medium. We applied this analytical model to three dike-induced graben in the Elysium region of Mars; Galaxias, Elysium, and Cerberus Fossae, some of which have been found to show active extensional tectonics. Firstly, we calculated dike aperture and depth from graben topography using an area balance technique. At each site we modeled with different combinations of input parameters which resulted in a total 27 models, of which 14 were found to be compatible with the inferred present-day graben-bounding faults. Based on these results we propose two conceptual models of dike-induced graben nucleation: 1) in a shallow dike intruding a low-compliance host rock, nucleation occurs through the linkage of near-surface mode I cracks and mode II discontinuities propagated from the dike tip, and 2) in a deep dike intruding a stiff host rock, shallow mode I cracks propagate to depth and collapse into normal faults in a dominantly tensile regime. A comparison of equivalent models under Martian versus Terrestrial conditions shows that in the former case failure is more likely to occur in a tension-dominated regime due to the greater weight of the dike induced stresses in a medium with a reduced lithostatic load. On Mars, dense networks of open fractures may have facilitated large fissure eruptions. On Earth, tensile failure occurs at very shallow depths whilst faults accommodate most of the deformation at depth. Our analytical modeling methodology explains intrusion experiments and observed dike-induced deformation. Therefore, the models provide plausible mechanisms of graben nucleation above intruding dikes. These need to be complemented by models which simulate long-term graben subsidence above dikes, which may be key to understand seismicity in tectonomagmatically active regions of Mars.