COMPOSITION AND IMPACT DEFORMATION OF NOACHIAN BASEMENT SURROUNDING THE ISIDIS BASIN, MARS

The Noachian basement exposed west of the 1900-km Isidis basin in the NE Syrtis/Nili Fossae region near candidate landing sites for Mars-2020 provides constraints on the origin and evolution of the oldest Martian crust, including the effects of impacts. We use CTX, HiRISE, and CRISM to investigate the occurrence, composition, stratigraphy, and formation/deformation processes of megabreccia and individual units within the Noachian basement.

We examined all HiRISE coverage and found 171 megabreccia outcrops within a radial distance of ~600-1200 km from the Isidis basin center. Stratigraphic relationships constrain all megabreccia to be of Noachian age, related to Isidis, and pre-date formation of the Olivine-Carbonate unit. Outcrops (n=11) examined in detail for block size statistics each contain 1-100s of blocks with an average size of 25±44 m and outliers up to 982 m across. Block sizes are semi-randomly distributed but with a statistically significant trend of increasing size with radial distance, following a power law relationship.

Five classification categories of megabreccia exposures take into account block packing density (isolated vs. densely packed), the homogeneity/heterogeneity of block albedos (“monomict” vs. “polymict”) and layering preserving internal block stratigraphy. Parameterization of megabreccia block composition through HiRISE color reveals the presence of ≥4 different megabreccia lithologies. Specifically, we find evidence for low-Ca pyroxene (LCP) with variable Ca²⁺/Fe²⁺-content and Fe/Mg-smectite in megabreccia and basement rocks. The composition of megabreccia blocks appears to be correlated with subunits of the Noachian basement as they also contain Fe/Mg-smectite and LCP signatures with variable ~2 μm absorptions. LCP-bearing units with a shorter wavelength ~2 μm absorption overlie those with a longer wavelength absorption and could potentially be related to impact processes such as melting or shock metamorphism; these basement units are the subject of our current ongoing work.

Collectively, the distribution, block size, textures, and composition of Isidis megabreccia further knowledge in impact-related brecciation on Mars and other planets, lend insights to processes affecting Noachian Mars, and provide context for potential future landed exploration.