Volcano dynamics vs tectonics on Mars: evidence from Pavonis Mons

Highlights

• Narrow grabens/pit chains penetrate up to the sub-crustal magma reservoir

• Fractal clustering of features quantify the penetration depth to 20 and 100 km

• Dyke-induced grabens on Pavonis Mons

• Pavonis Mons experienced active rifting at early stages of formation followed by volcano growth

Abstract

Volcanic activity is widespread within the inner Solar system and it can be commonly observed on rocky planets. In this work, we analyse the structures of Pavonis Mons in the Tharsis volcanic province of Mars by performing structural mapping, azimuth, and topographic distribution of linear features on the flanks of Pavonis, such as grabens and pit chains. We tested whether their formation is to be ascribed to the volcano dynamics and magmatic activity or the tectonics. Through the length size distribution and fractal clustering analyses of the structural features, we found that large grabens are vertically confined in the upper mechanical layers of the brittle crust whereas pit chains penetrate the whole crust up to the magmatic source, indicating that they can be considered the main feeders of Pavonis Mons. We inverted the topography with dykes and faults models to test whether grabens at the surface are the expression of intrusions at depth and we suggest that thin dykes inducing normal faulting are the most likely mechanism. Furthermore, two azimuthal distribution of the grabens are identified: concentric grabens occur on the volcano summit while linear grabens at its base show NE-SW trend as the Tharsis Mons volcanos alignment. The occurrence of linear grabens suggests that Pavonis likely experienced a phase of active rifting with the formation of such structures, followed by a phase of volcano growth and concentric magma intrusions when volcano and magma chamber dynamics prevailed.

Introduction

Planetary grabens, identified as linear topographic lows bordered by normal faults, have been widely recognized on Mars and other rocky planets as well as on the Moon (Ernst et al., 2003; Watters et al., 2012; Klimczak, 2014). However, the origin of these structures is debated (Mège et al., 2003). 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. The formation and evolution of the Tharsis volcanic province is still a matter of debate and several hypotheses have been suggested for the formation of such a huge volcanic province, including the presence of a plume on a stationary plate (Meyzen et al., 2014) or on a decoupled lithosphere-mantle with differential rotation (Zhong, 2008), a giant impact resulting in the formation of a large igneous province (Reese et al., 2004), episodic rollback of a subducting slab (Yin, 2012).

Here we explore whether volcano dynamics or tectonic processes control deformations at Pavonis Mons by analysing in detail i) the variation of graben azimuth distribution from its base to the summit, ii) the spatial clustering of grabens and pit chains in the volcano and iii) the volcano-tectonic structures on the volcano summit. We find that the azimuthal distribution of grabens shows two main patterns: concentric grabens occur on the volcano summit and linear grabens at its base. We interpret the linear grabens at the base of the volcano as evidence of active rifting followed by a phase of volcano growth and magma resurfacing when concentric grabens formed due to the volcano dynamics.