Exploring the pre-Noachian Crust in the Terra Cimmeria-Sirenum Region

In situ observations of possible remnants of continental crust by MSL Curiosity (Sautter et al. 2015) and the recent finding of differentiated crustal clasts in the NWA7533 martian meteorite (Hewins et al. 2017) shed new lights on the crustal growth/formation in the early periods of Mars. In this context, a recent study reconstructed the ancient crustal thickness of Mars and revealed the existence of a crustal block which was thicker than the rest of the southern Noachian highlands before the impact basins formation (Bouley et al. 2020). This block, located in the region of Terra Cimmeria-Sirenum, was identified after the removal of large impact basins as well as volcanic provinces like Tharsis. It is also characterized by regional enrichment in potassium/thorium (Boynton et al. 2007) and exhibits the strongest magnetic anomalies of the martian surface (Langlais et al. 2019). These unusual properties of Terra Cimmeria-Sirenum make it an attractive region for investigating the dynamics of the primitive crust. In this study, we achieved a new geological map of this area, using CTX, HiRise, Themis imagery, MOLA, pre-Tharsis DTMs (Bouley et al. 2016) and geophysical data (Crustal thickness and magnetic anomalies). This study highlights the occurrence of a series of adjacent basins scattered across the region, filled with late-noachian to early Hesperian deposits, some of these deposits are linked to paleolake activities (Irwin et al. 2002, Adeli et al. 2015). Furthermore, several basins in the eastern part of Terra Sirenum are associated with a mound composed of tilted layers, which was previously mapped as a volcano by Scott & Tanaka (1986). However, a detailed analysis based on more recent data on this structure reveals that the mound is associated with pre-Noachian tectonic features, which are much older than the regional deformation event that has occurred in middle to late Noachian (Karasozen et al. 2016). These observations raise new questions about the mechanism(s) behind the formation of the various basins across Terra Cimmeria-Sirenum and the past dynamics of the region. Several hypotheses of formation, including impact processes, vertical deformation (sagging of upper units as in some of the Precambrian dynamics on Earth), followed by erosion processes, will be discussed in light of available data and observations.