Structural analysis of Alba Patera, Mars : from deep-seated to shallow events.
Today, the planet Mars is in a phase of volcanic and tectonic quiescence. However, the 11.000 m high and 8000 km wide volcanic dome of the Tharsis region, transected by giant tectonic features, bears witness to past intense activity on the red planet. At the northernmost boundary of the Tharsis dome, the volcanic edifice of Alba Patera forms a widespread shield of 2700 km diameter and 7.000 m maximum elevation. The tectonics on Alba Patera has been the particular object of attention. The volcano and its environment are intensely breached by lineaments, which are interpreted as being fault-type structures. The orientation of those faults is in general north-south, radial to the Tharsis dome. On the flanks of Alba Patera, however, the faults curve around and are arranged in concentric pattern, encircling the summit of the edifice. In this thesis, the structural information of Alba Patera was reviewed and newly analysed based on Viking Mosaic data. Due to the high resolution of 60 m pro pixel, details about the fault development on Alba Patera are observable. Based on cross-cutting relationships of the observed lineaments, it becomes obvious that concentric fracturing of Alba Patera formed progressively, after radial fracturing. The further focus of this thesis is to study the implications of observed structures for hidden subvolcanic processes and to infer coupled genetic mechanisms that generate tension within the crust and the volcanic edifice. It is demonstrated that the lineaments record the dynamic past of Alba Patera, enlightening that this region of the red planet suffered a multifaceted history of crustal and volcanic evolution. Modelling the observed structures is used as a crucial key to understand their formation. Two modelling methods were used. Most of the analysis was performed by numerical models using the Finite Element code TEKTON. Semi-automatic Fortran subroutines were programmed to accelerate Finite Element data input and allow modelling with a large variety of unknown mechanical parameters. The numerically predicted surface deformation and fault arrangements are compared to the Viking Mosaic lineaments, and finally reproduced qualitatively by analogue experiments in a sand-box. Variations of subsurface source characteristics have been investigated specifically their interplay with the volcanic loading, and their superposition with a regional extension representative of the Tharsis doming. This study shows that the observed radial structures, i.e. the Tantalus Fossae, that extend from the flanks of Alba Patera toward large northern distance, reflect the extensional surface effect of local uplift. This supports the idea that Alba Patera was an autonomous hotspot and structural dome. The following tectonic phase on Alba Patera is dominated by the formation of concentric grabens on the western and eastern mid and upper flanks of the volcano. The orientation and position of these grabens are found to be best reproduced by local crustal subsidence, superposed with a regional extensional stress field. Simulating these mechanisms in analogue sand-box models also produced surface-structures of arrangements that almost perfectly mimic the observed lineaments on Alba Patera. As seen from the mosaic image analysis, the formation of the circumferential grabens was incrementally and increasingly pronounced towards the final stages of volcanic shield construction. Since this spans a time period probably on the order of a billion years, this suggests long-term geodynamic processes to be responsible for subsidence of the central Alba Patera area. The progressive change toward higher concentricity likely resulted from an increase of density by cooling and accumulation of intrusive material during the construction of the volcano – and thus subsidence of the region above this volcanic root. The reader will find within various parts of the thesis implications for the potential mechanical properties of Alba Patera and the Martian crust. Also the absence of lineaments can be used, to infer new information about the mechan ical properties of the planet’s upper lithosphere. For instance, the absence of bending signatures surrounding the Martian volcano provides information about the mechanical properties of the lithosphere. The methodical work presented in the second part of this thesis is devoted to the modelling of lithospheric flexure under the load of Alba Patera. It is shown that the lithospheric viscosity together with the load emplacement, the planet curvature and the crustal strength are primordial parameters to reduce the flexural stresses, and thus explaining the absence of flexural fractures surrounding the volcano. Summary and perspective: Due to the minor crustal plate movement and erosion, a very long-term creation history of Alba Patera is reconstructable. The formation of lineaments, as analysed by Viking photo-mosaics, was simulated by numerical and analogue methods to elucidate the dynamic history of this volcanic edifice. The proposed vertical tectonic processes may have been not only of importance on Alba Patera, probably on Earth similar mechanisms exist on similar dimensions that are however obscured by plate tectonics.