Riccardo Pozzobon

Self-similar clustering distribution of structural features on Ascraeus Mons (Mars): implications for magma chamber depth

Abstract The occurrence and distribution of monogenic eruptive features in volcanic areas testify to the presence of deep-crustal or subcrustal magma reservoirs hydraulically connected to the surface via a fracture network. The spatial distribution of vents can be studied in terms of self-similar (fractal) clustering, described by a fractal exponent D and defined over a range of lengths (l) between a lower and upper cutoff, Lco and Uco, respectively. The computed Uco values for several volcanic fields on Earth match the thickness of the crust between vents and magma reservoirs at depth. This analysis can thus be extended to other volcanic fields and volcanoes on rocky planets in the solar system where features such as vents and dykes occur, and for where complementary geophysical data are currently lacking. We applied this method to the Ascraeus Mons volcano on Mars, which presents hundreds of collapse pits similar to those observed on Earth volcanoes that are most likely related to feeder dykes. Based on structural mapping with High Resolution Stereo Camera data at 12 m/px and Context Camera data at 6 m/px mosaics, more than 2300 collapse pits and dyke traces were analysed, revealing two distinct fractal clustered populations. The obtained Uco values reveal the presence and likely depth of both a deep magma reservoir (c. 60 km deep) and a small shallower chamber (c. 11 km deep). This analysis can help to better constrain the depth and time evolution of volcanic processes on Tharsis, and on terrestrial planets’ volcanoes in general.

Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability

We investigated the equatorial layered deposits (ELDs) of Arabia Terra, Mars, in Firsoff crater and on the adjacent plateau. We produced a detailed geological map that included a survey of the relative stratigraphic relations and crater count dating. We reconstructed the geometry of the layered deposits and inferred some compositional constraints. ELDs drape and onlap the plateau materials of late Noachian age, while they are unconformably covered by early and middle Amazonian units. ELDs show the presence of polyhydrated sulfates. The bulge morphology of the Firsoff crater ELDs appears to be largely depositional. The ELDs on the plateau display a sheet-drape geometry. ELDs show different characteristics between the crater and the plateau occurrences. In the crater they consist of mounds made of breccia sometimes displaying an apical pit laterally grading into a light-toned layered unit disrupted in a meter-scale polygonal pattern. These units are commonly associated with fissure ridges suggestive of subsurface sources. We interpret the ELDs inside the craters as spring deposits, originated by fluid upwelling through the pathways likely provided by the fractures related to the crater formations, and debouching at the surface through the fissure ridges and the mounds, leading to evaporite precipitation. On the plateau, ELDs consist of rare mounds, flat-lying deposits, and cross-bedded dune fields. We interpret these mounds as possible smaller spring deposits, the flat-lying deposits as playa deposits, and the cross-bedded dune fields as aeolian deposits. Groundwater fluctuations appear to be the major factor controlling ELD deposition.

Are terrestrial plumes from motionless plates analogues to Martian plumes feeding the giant shield volcanoes

Abstract On Earth, most tectonic plates are regenerated and recycled through convection. However, the Nubian and Antarctic plates could be considered as poorly mobile surfaces of various thicknesses that are acting as conductive lids on top of Earths deeper convective system. Here, volcanoes do not show any linear age progression, at least not for the last 30 myr, but constitute the sites of persistent, focused, long-term magmatic activity rather than a chain of volcanoes, as observed in fast-moving plate plume environments. The melt products vertically accrete into huge accumulations. The residual depleted roots left behind by melting processes cannot be dragged away from the melting loci underlying the volcanoes, which may contribute to producing an unusually shallow depth of oceanic swells. The persistence of a stationary thick depleted lid slows down the efficiency of melting processes at shallow depths. Numerous characteristics of these volcanoes located on motionless plates may be shared by those of the giant volcanoes of the Tharsis province, as Mars is a one-plate planet. The aim of this chapter is to undertake a first inventory of these common features, in order to improve our knowledge of the construction processes of Martian volcanoes.