Structural Analysis of the Victoria Quadrangle (H2) of Mercury based on NASA MESSENGER Data

Objective of this thesis is the mapping and structural analysis of the H2 quadrangle, “Victoria”, and a reconnaissance study of the geometry and kinematics of lobate scarps on Mercury. To this end, I produced a 1:3,000,000 geologic map of the area using the images provided by the NASA spacecraft MESSENGER, which has been orbiting the planet since March, 2011. The geologic map shows the distribution of smooth plains, intermediate plains, intercrater plains units and a classification of crater materials based on an empirical distinction among three stages of degradation. Structural mapping shows that the H2 quadrangle is dominated by N-S faults (here grouped into the Victoria system) to the east and NE-SW faults (Larrocha system) to the west, with the secondary existence of NW-SE-trending faults (Carnegie system) in the north-western area of the quadrangle. A systematic analysis of these systems has led to the following results. 1) The Victoria system is characterized by a main array of faults located along Victoria Rupes – Endeavour Rupes – Antoniadi Dorsum. The segmentation of this array into three different sectors changes from north to south and is spatially linked to the presence of three volcanic vents located at the boundaries between each sector and at the northern end of the Victoria Rupes sector, suggesting that volcanism and faulting are interrelated. 2) The main array of Carnegie system is kinematically linked and antithetical to the Victoria system. Both systems have arguably controlled the growth of a longitudinal, fault-free, crustal and gravimetric bulge in the central area of the Victoria quadrangle, which is interpreted as a regional contractional pop-up. 3) The Larrocha system is interrupted against the central bulge and thus is probably older than the Victoria and Carnegie systems. Buffered crater counting performed on the Victoria system confirms the young relative age of its fault segments with respect to the map units. The faults of the Victoria system post-date the smooth plains, even though the morphological evidence suggests a probable syndepositional fault activity. The structural analysis was supplemented by an innovative method to calculate fault slip data using craters cross-cut by lobate scarps. This method permits to fully constrain remotely-sensed fault kinematics, and it was applied on 16 craters found across 30% of Mercury, covered by stereo-DTM data. Six of the faulted craters are located within the H2 quadrangle and reveal that the Carnegie system and the Victoria – Endeavour array have near-dip-slip kinematics. The former dips 30° eastward, the latter dips 15°-20° westward. Inversion of fault slip data allows estimation of the orientation of the stress field pertaining to the Victoria-Carnegie-systems, whose σ1 trends 71° N. At a global scale, the application of the method developed to constrain fault kinematics documented that Mercurian faults have a wider range of dips (7° to 57°) than that predicted by mechanical models. Moreover, this analysis revealed that lobate scarps on Mercury have rakes ranging from 40° to 141° demonstrating the presence of oblique-slip kinematics, differently from what is assumed by the global contraction model that allows only pure or near dip-slip kinematics. Fault slip data were plotted on dip-rake, strike-rake, dip-latitude and strike-latitude diagrams. When more data will be available, these diagrams will help in evaluating the different tectonic models proposed for Mercury and individuating the probable reactivation of ancient tidal despinning structures. However, analysing these diagrams with the available data suggests that neither global contraction nor tidal despinning can satisfactorily explain the oblique-slip kinematics recorded by some faulted craters. Thus the contribution of additional models, such as mantle convection, should be incorporated in a global explanation of Mercurian tectonics