Dyke propagation and dynamics during rift initiation

Dykes are tensile fractures that rapidly transport magma from the hot, ductile asthenosphere across the cold, brittle upper lithosphere. They play an important role in tectonic extension settings by drastically reducing the force needed for rifting (Buck, 2004). Yet the balance of mechanisms that drive dyke propagation and how they promote rift initiation remain unclear. Here we investigate the physics of dyke propagation in a two-phase continuum model that can approximate both faults and dykes in an extensional tectonic setting.  

Dykes are fluid-filled fractures, typically modelled as discrete inclusions in an extended elastic continuum.  These models suggest that dyking is dominated by magma buoyancy and that its direction can be altered according to the competition between tectonic stress and the topographic load (Maccaferri et al., 2014). However, this method assumes a constant background stress field in the lithosphere during dyking. Therefore this method cannot capture the interaction between dykes and the long-term deformation of the lithosphere. To resolve this issue, dyking has been prescribed as a weak material in a continuum, one-phase rifting model in which dyking is included in the conservation of mass, momentum and/or energy (Liu and Buck, 2018). This method respects the scale separation between dyking and long-term dynamics, but still neglects the feedback of dyking on the stress field.

We present a geodynamic model that incorporates a novel poro-viscoelastic–viscoplastic rheological formulation with a hyperbolic yield surface for plasticity. With this model, both dyking and faulting can be simulated consistently (Li et al., in review). We validate our theory by comparing the stress field at the tip of the dyke with that from the linear elastic fracture mechanics theory. We then investigate dynamics of dyking in a geodynamic rifting model. We show that dyking assists rifting and its localisation. First, it reduces the yield strength in the brittle layer as the pore pressure balances the compressive stress; second, it promotes the development of near-surface normal faults localised in a relatively narrow rift region near the rift axis. We investigate the physics of dyke propagation with respect to the balance between buoyancy and tectonic forcing, and the effect of topography.

References

Buck, W .R., (2004). Consequences of asthenospheric variability on continental rifting. In Rheology and deformation of the lithosphere at continental margins, chapter 1, pages 1–30. Columbia University Press. doi: 10.7312/karn12738-002.

Maccaferri, F., Rivalta, E., Keir, D., and Acocella, V., (2014). Off-rift volcanism in rift zones determined by crustal unloading. Nature Geoscience 7, 297–300. doi: 10.1038/ngeo2110.

Liu, Z. and Buck, W. R., (2018). Magmatic controls on axial relief and faulting at mid-ocean ridges. Earth and Planetary Science Letters, 491:226–237. doi: 10.1016/j.epsl.2018.03.045.

Li, Y., Pusok, A., Davis, T., May, D., and Katz, R., Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation, in review of Geophysical Journal International.