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