The Formation of Widespread Volcanically Filled Crater Floors on Mars: Insights from Modeling and Observations

Abstract:

The identification and mapping of compositionally (olivine-/pyroxene-enriched), thermophysically (rocky), and morphologically (flat floors with lobate margins, no visible ejecta/central peak) distinct infilled craters over the majority of the martian cratered southern highlands coupled with the ancient formation age of floor materials (~3.5-4Ga) recently resulted in the interpretation that volcanic infilling was the likely responsible process. However, the source of this material is enigmatic, as no vents are observed and these deposits do not occur in association with any specific volcanic or geographic provinces, leading to the proposal that impact excavation-induced decompression melting of a mantle source may be responsible for the intra-crater materials.

The conditions under which impact-induced decompression melting occurs, if at all, are not well constrained. In this work, we present the quantitative modeling of this process by coupling the pMELTS thermodynamic model for silicate magmas to a CTH shock physics impact model with realistic rock rheology. Initial conditions are established for early to modern Mars with mantle potential temperatures (1250-1550˚C), surface heat fluxes (20-80 mW/m²) and radiogenic crustal heat production (1x10^-10-1.0x10^-12 W/kg) with a crustal composition of Adirondack class basalts and mantle composition following Dreibus and Wanke.

Early results (~80km lithosphere, 1350˚C mantle potential temperature) show ~5-7% excess melt generated at the lithosphere-asthenosphere boundary for large craters (e.g. 180 km). These impact events also create zones of high shear strain/fracture systems that propagate through the entire brittle lithosphere. In general the thinner lithosphere and higher mantle potential temperature conditions on early planetary bodies should produce more melt at a given crater size and allow for the translation of this process to smaller crater sizes like those observed on Mars. If pre-existing melt is trapped below a stagnant lithospheric lid, it may also be released through fractures created by the impact. Examining these processes through numerical modeling techniques provides the potential to constrain early planetary evolutionary models by placing constraints on mantle potential temperatures, lithospheric thicknesses, etc.