Radial dike formation on Venus: Insights from models of uplift, flexure and magmatism

Highlights

• First numerical models integrating flexural uplift with magma reservoir rupture.

• When shallow reservoirs fail they feed vertical radial dikes and surface eruptions.

• Resulting edifice growth creates a shallow stress barrier inhibiting dike ascent.

• This shallow stress barrier also helps redirect magma laterally via radial dikes.

• Model results provide one mechanism for giant radial dike swarm formation on Venus.

Abstract

Giant radiating dike swarms on Venus are geomorphic manifestations of deep-seated volcano-tectonic processes. Through an initial series of experiments we assess if these features can be explained as a product of the relationship between magmatism and the flexural stress state caused by lithospheric uplift (due to a rising mantle diapir, plume or underplating processes) using gravitationally loaded, axisymmetric, elastic finite element models. Magma chambers situated in the upper (extensional) lithosphere fail at or near the crest, redirecting magma vertically into radially aligned dikes, but these are expected to feed surface eruptions; conditions promoting lateral propagation of dikes do not occur. Magma chambers located in the lower (compressional) lithosphere are predicted to generate horizontal intrusions (sills) upon midsection failure. When both uplift (from basal loading) and magmatic surface loading (from edifice growth) are introduced in sequence at discrete times, however, conditions that promote shallow lateral propagation of magma within radial dikes are created: (1) as magma from shallow reservoirs ascends and erupts, surface-loading volcanic edifices can form; (2) as the vertical loads increase, flexing the lithosphere downward, high compressional stresses produced within the edifice create a shallow stress barrier that makes it increasingly difficult for magma to complete the process of ascent and eruption; and then (3) stresses acting on dikes ascending from a reservoir toward the surface will thus increasingly favor fracture along their lateral margins, redirecting magma through radial dikes toward distal regions at shallow depth. Our results are consistent with observed patterns of laterally extensive dike systems at volcanic centers such as Mbokomu Mons and Becuma Mons. The models thus provide a robust mechanical explanation that for the first time links lithosphere flexure and reservoir pressurization to the ascent and lateral spreading of magma through radial dike systems on Venus, thereby providing key new spatial and temporal insights into an important magmatic process operating on multiple terrestrial planets.

Introduction

Giant radial lineament systems, thought to form primarily through lateral emplacement of dikes at shallow depth (e.g., Ernst and Baragar, 1992, Grosfils and Head, 1994a, Ernst et al., 1995), are among the most intriguing volcano-tectonic structures on Venus (Fig. 1). Initial reconnaissance mapping at C1-MIDR resolution (Grosfils and Head, 1994b) revealed ∼120 such features with an average radius of ∼325 km, although some examples extend more than 2000 km; however, active global mapping efforts using higher resolution FMIDR images are revealing ∼5-fold more examples (e.g., Ernst et al., 2003, Studd et al., 2010).

Although the presence of radiating dike systems on Venus is likely (inferred through the similarities of graben swarms to dike swarms on Earth), and a link to magma chambers is compelling in many instances (e.g., Grosfils and Head, 1994a, Ernst et al., 1995), we have only limited insight into the mechanical conditions that promote the formation of laterally extensive radial dike systems. More specifically, existing flexural models that predict stress patterns conducive to radial dike formation (through simple domical uplift) do not incorporate interactions with magma chambers (e.g. Stofan et al., 1991, Cyr and Melosh, 1993). On the other hand, simple models using pressurized magma chambers that predict radial dikes propagating laterally from the reservoir do not incorporate flexural effects (e.g., McKenzie et al., 1992, Parfitt et al., 1993, Koenig and Pollard, 1998, Gudmundsson, 2006). While the stress state beneath an edifice insufficient in size to create significant flexure can provide conditions in which radial dikes ascending vertically from a ruptured chamber transition to lateral propagation at shallow depth (Hurwitz et al., 2009), consistent with field observations at many small stratocones (e.g., Poland et al., 2008), this lateral redirection is essentially constrained to the sub-edifice region, and to date no model with appropriate boundary conditions has demonstrated circumstances that promote the growth of giant regional-scale radial dike swarms such as those observed on Venus.

In a previous study, Galgana et al. (2011a) presented an analysis of magma reservoir rupture caused by the interplay between reservoir inflation and flexural effects introduced by edifice loading at the surface. Here, expanding upon these results, we explore how basal uplift and flexure affect the rupture characteristics of inflating magma reservoirs. Guided by our results and those reported previously by Galgana et al. (2011a), we then show how time-dependent introduction of basal uplift followed by edifice loading creates stress conditions within the lithosphere conducive to giant radial dike swarm formation, with a transition from vertical to lateral propagation promoted by the presence of a stress trap at the edifice base.