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

The near “one-plate” planet evolution of Mars has led to the edification of long-lasting giant shied volcanoes. Unlike the Earth, Mars would have been a transient convecting planet, where plate tectonic would have possibly acted only during the first hundreds of million years of its history. On Earth, where plate tectonic is active, most of them 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 Earth’s deeper convective system. Many geodynamical features of the young volcanism (less than 30 Myr old) of these motionless intraplate oceanic island environments resemble those of Mars, where plate tectonic plays no meaningful role in shaping planetary geodynamics. As observed on Mars, in these environments, volcanoes do not show any linear age progression (at least for the last 30 Ma), 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. Here, the near stationary absolute plate motion probably exerts a primary control on volcanic processes, and more specifically, on the melting ones. The residual depleted mantle, that is left behind by the melting processes, cannot be swept away from the melting locus. Over time, the thickening of this near-stationary depleted layer progressively forces the termination of melting to higher depths, reducing the melt production rate. Such a process gradually leads both to decreasing efficient melt extraction and increasing mantle lithospheric-melt interactions. The accumulation of this refractory material also causes long-term fluctuations of the volcanic activity, in generating long periods of quiescence. The presence of this residual mantle keel induces over time a lateral flow deflection, which translates into a shift of future melting sites around it. This process gives rise to the horseshoe-like shape of some volcanic islands on slow-moving plates (e.g. Cape Verde, Crozet). Finally, the pronounced topographic swells/bulges observed in this environments may also be supported both by large scale mantle upwelling and their residual mantle roots. Most of these processes are likely similar to those observed on Martian giant shield volcanoes. The goal of this presentation will be to describe the essential characteristics of intra-oceanic plumes on slow moving plates on the Earth and to point out their similarities with those of the large shield volcanoes from the Tharsis region. Mars can effectively be considered as a one-plate planet, where geodynamics processes occur within a stagnant-lid regime. Several features of volcanism both on Earth and Mars can be related to the (near-) absence of plate tectonic motion (Table 1). The dividing into several distinct chemical mantle provinces, as observed on Mars and over Antarctic and Nubian plates on Earth, reflects a more efficient intracell rather than cross-cell convection mixing, allowing preservation of large-scale mantle chemical heterogeneities. Such a pattern reflects a mantle flow regime dominated by laminar flow, as expected in the (near-) absence of plate tectonic motion. In these geodynamical environments, poor residual mantle lateral flowing traction from the melting site will lead to the formation of a near-stationary depleted layer, which will thicken with time. The low buoyancy of upwelling in enhancing conductive heat loss during mantle rising will also lead to the formation of a thicker lithosphere relative to that of overlying higher-buoyancy plumes such as Hawaii. These two thickening effects add to that due to planetary cooling (Baratoux et al. 2011). Both planetary cooling and depleted lid formation will lead to a cessation of melting, progressively forced toward greater depths over time, while the average extent of melting will be reduced. In both environments, low melt supply will lead to the development of a two-level fractionation architecture of the plumbing system, with most of the fractionation occurring in the uppermost mantle. The widespread distribution of the Tharsis volcanoes, as well as the horseshoe-like shape of some volcanic islands on slow-moving plates (e.g. Cape Verde, Crozet), could be inherited from lateral flow deflection induced by the presence of a residual mantle keel, shifting future melting loci around the keel. The pronounced topographic swells/bulges observed in this environments may also be supported both by large scale mantle upwelling and their residual mantle roots. Another similarity between volcanoes on slow-moving plates and those in Tharsis is their great longevity, which is due to their stationary position relative to the melting source. Both volcanoes also have a history of protracted activity punctuated by long periods of quiescence. The accumulation of refractory material at the rim of their melting zone might play a role in the fluctuations of their long-term volcanic activity. Until now, volcanism on Mars was defined as predominantly of tholeitic composition. However, recent in situ analyzes at Gale Crater have identified a mugearite-like rock. This raises the possibility that both intraplate volcanism on Earth at slow-moving plates and that on Mars would be instead of an alkali nature. Our knowledge of terrestrial volcanoes from motionless plates can thus help us to better understand the nature and significance of large-scale melting and differentiation processes at Mars volcanoes. However, the extent of this knowledge is still insufficiently detailed in many respects, and requires further investigation. New data from the Martian planetary record will help to provide new perspective on the processes and evolution at volcanoes on near-stationary plates. On Mars, the great range of scales of upwelling, the influence of crustal and lithospheric thicknesses in space and time are thus relevant to studies of hotspots on motionless plates.