Radial dike formation on Venus: Insights from models of uplift, flexure and magmatism
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.
Section snippets
Method
Following methodology described previously in Grosfils (2007) and Galgana et al. (2011a), we use COMSOL Multiphysics (COMSOL AB, 2008) to build axisymmetric finite element models of volcanic centers within the Venusian lithosphere (Fig. 2); most are purely elastic, but a few are viscoelastic. In either case, the models extend to 900 km in radius, a sufficient distance to avoid edge-related effects. The gravitationally loaded (lithostatic prestress) lithosphere is assumed to have a basaltic
Lithospheric stresses due to uplift
Models of buoyant subsurface loading produce a flexural stress state with high σD in the upper and lower parts of the lithosphere, separated by a σD minimum in the neutral plane (Fig. 3). The upper lithosphere is predominantly under extension, while the lower lithosphere is under compression. The principal stress axis patterns show σE = σθ (out of plane) and σC = σz (vertical) throughout the upper lithosphere, predicting the formation of radially aligned structures in the upper lithosphere, either
Predicted lithosphere intrusions and magma ascent
Our models have important implications for the formation of radial dike swarms from magma chamber pressurization. In the upper portions of an uplifted lithosphere, a pressurized chamber fails at the crest area (Fig. 10), and predicted intrusion orientations are radial (Fig. 3). Thus, initial magma transport from the top of an upper lithosphere chamber will occur via vertically propagating, radially-oriented dikes (Fig. 9), a result similar to those derived from recent half-space models (
Conclusions
We present volcanologically motivated mechanical models that for the first time (to our knowledge) can explain the genesis of giant radiating dike systems found on the Venusian lithosphere, though we caution that additional explanations must continue to be sought as only about half of the radial systems on Venus are affiliated with elevated central topography (Grosfils and Head, 1994b). Our finite element models show that lithosphere fracture orientations and magma reservoir failure modes are
Acknowledgments
We thank Oded Aharonson (editor), Lionel Wilson and an anonymous reviewer for their thoughtful reviews and constructive comments that significantly improved the manuscript. This work is supported by NASA Grant NNX08AL77G from the Planetary Geology and Geophysics program and NASA CAN grant to the Lunar and Planetary Institute, USRA, with additional student involvement funded by the Summer Undergraduate Research Program of Pomona College. This is LPI Contribution No. 1728.
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2021, Journal of Volcanology and Geothermal ResearchCitation Excerpt :The directions of the principal stresses control dike orientation, with dikes propagating perpendicular to the least compressive stress (σ3) and parallel to the maximum compressive stress (σ1; e.g., Nakamura, 1977; Rubin and Pollard, 1987). Normal stress gradients or unfavorable principal stress orientations may result in the arrest of ascending dikes near the base of an edifice and potentially a transition to sub-horizontal propagation (Pinel and Jaupart, 2000; Pinel and Jaupart, 2004; Poland et al., 2008; Hurwitz et al., 2009; Galgana et al., 2013). However, it remains unclear to what extent the overpressure (combined excess magma pressure and magma buoyancy minus the normal stress; Gudmundsson, 2002, 2012) within a dike, or host rock heterogeneities within the CIC, contributes to these mechanisms.
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2021, Journal of Volcanology and Geothermal ResearchCitation Excerpt :On Earth linear grabens may form due to dyke intrusions (Gudmundsson and Loetveit, 2005; Gudmundsson et al., 2008; Koehn et al., 2019; Wyrick and Smart, 2009), normal faulting (Melosh and Williams, 1989; Koehn et al., 2019) or a combination of both -dyke-induced normal faulting above a dyke- (Gudmundsson and Loetveit, 2005) in extensional tectonic regimes (Mège et al., 2003; Acocella and Neri, 2009). Besides, concentric dykes can also originate from a pressurized magma chamber without any active tectonics (Acocella and Neri, 2009; Bistacchi et al., 2012) or, at the regional scale, they can be caused by interaction between magmatism and lithospheric flexure due to loading (Galgana et al., 2013; Grosfils et al., 2015). Therefore, both extensional tectonics and volcano dynamics can have played an important role in the formation of Mars grabens.
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2020, Earth and Planetary Science LettersCitation Excerpt :Lessons learned from Earth applied volcanology have been transferred to planetology with success. In particular, similar giant dike swarms observed on Venus have been interpreted either in terms of local plumbing system or large scale rising diapir effects (Ernst et al., 1995; Galgana et al., 2013). More recent studies of magma transport on Earth have relied not only on the magmatic intrusion shape and orientation but also on vent location to gain quantitative information on the stress state, otherwise difficult to constrain, within volcanic edifices (Maccaferri et al., 2017) or rifting areas (Maccaferri et al., 2014).
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2017, Physics of the Earth and Planetary InteriorsCitation Excerpt :Therefore, if magma transport through the lower to middle Venusian crust is dominated by diapirism, then a lower fraction of crust-situated melt can reach the surface and erupt, relative to Earth. Occasionally however, a sill may form that is large enough to generate enough uplift to initiate faulting in the brittle crust, creating a set of concentric vertical faults (see Galgana et al., 2013). If the magma reaches these faults (or vice versa) then melts can propagate upwards, forming ring-dikes or arachnoids (Head et al., 1992; Donahue and Russell, 1997; Basilevsky and Head, 2003; Wilson, 2009).