Buoninfante, Salvatore (2024) Relation between tectonics and crustal structures of Mercury. [Tesi di dottorato]

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Abstract

The main objective of this thesis was to study the relationship between tectonic and crustal structures of the planet Mercury, through the analysis of NASA MESSENGER imagery and gravity data. This project involved the production of a 1:3,000,000 scale geological map of the Michelangelo quadrangle (H12) of Mercury, based on the photointerpretation of the MDIS imagery. The funding for this study was planned to identify targets to be observed at high resolution during the ESA-JAXA BepiColombo mission. Michelangelo appears as a densely cratered quadrangle dominated by degraded crater materials (c2) and intermediate plains. I identified two main regional thrust system trending NW-SE and NE-SW. I found that many lobed scarps developed at the edges of ancient, large impact basins. Clear examples of such tectonic structures in the Michelangelo quadrangle are provided by the Beethoven basin (20.8°S–236.1°E) or by the Vincente-Yakovlev basin (52.6°S–197.9°E). I propose a thick-skinned tectonic model according to which the lobate scarps were formed due to the positive reactivation of previous impact-related normal faults, as a consequence of the tectonic regime deriving from the global contraction. Morphological evidence of thick-skinned tectonics on Mercury is provided by the presence of fault systems reaching out of the basin rim (e.g., Beethoven basin). Beethoven basin is dated as a Tolstojan-Early Calorian (~4.0-3.8 Ga). The tectonic activity affecting Beethoven is more recent than the basin, the smooth plains covering the floor, and the subsequent impact craters. Notwithstanding the low accuracy of XRS data at these latitudes, it appears that the NW-SE faults system in H12 broadly borders the southwestern edge of the high-Mg region (HMR). I also analyzed the relation between volcanic vents and faults. Volcanic vents on Mercury are often associated with impact craters and/or tectonic structures, which can be considered as possible preferential areas for magma uprising. In some cases, vents are arranged along the peak-ring structure in impact craters, probably interacting with faults forming at peak-ring after the impact. I analyzed the MESS160A gravity field model to model crustal heterogeneities in density. According to the flexural isostatic response curve, I assumed a flexural compensation model to first estimate a mean elastic thickness of 30 ± 10 km. I modeled the lithospheric flexure regardless of the gravity field and calculated the isostatic gravity anomalies, showing that considerable lateral density variations occur within Mercury’s crust. I also estimated the crust-mantle interface depth, finding that it varies from 19 to 42 km depth. Isostatic anomalies are mainly related to density variations in the crust: gravity highs mostly correspond to large-impact basins, suggesting intra-crustal magmatic intrusions as the main origin of these anomalies. Isostatic gravity lows prevail, instead, above intercrater plains and may represent the signature of a heavily fractured crust. Bouguer gravity anomaly and crustal thickness data were analyzed to estimate the inner ring and main rim dimensions of peak-ring and multi-ring basins. Lunar impact basins are characterized by a one-to-one relationship between Bouguer diameter and crustal thinning diameter, estimated from regional background values, and the topographic peak-ring diameter. Mercury’s peak-ring basins are instead characterized by a two-to-one relationship between Bouguer diameter and peak-ring size. I evaluated the spatial relationship between mapped tectonic structures and crustal structures in the Victoria and Michelangelo quadrangles. Crustal structures are identified with gravity source edge detection methods: Total Horizontal Derivative (THD) and Enhanced Horizontal Derivative (EHD). Gravity data modelling shows that crustal structures can be spatially correlated with Carnegie, Larrocha systems and Victoria-Endeavour-Antoniadi array (Galluzzi et al., 2019), which partly border the HMR. These results suggest that the main thrust systems on Mercury may be characterized by a thick-skinned tectonic style.

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