Deciphering the deformation history of Ulysses Fossae, Mars, using fault pattern analysis
Authors: Shahrzad Stephanie , Bramham Emma , Thomas Mark , Byrne Paul K. , Mortimer Estelle , Piazolo Sandra 
Published: July 2021

AbstractWe mapped and analyzed the morphology and strike of a large population of faults observed in Ulysses Fossae, Mars, to determine the sequence and character of deformation in the area. Preliminary results based on distinct fault populations suggest a structural history of Ulysses Fossae featuring activity at several different volcanic centers, regional extension, and later reactivation of older fault populations.1. IntroductionThe heavily faulted terrain of Ulysses Fossae lies at 10&#176;N, 123&#176;W, amongst the volcanoes of the Tharsis Rise (Figure 1). Olympus Mons is situated ~800 km to the west, the Tharsis Montes to the east, and the Ulysses Patera volcano immediately to the south. Ulysses Fossae has been the target of only a few dedicated studies [e.g., 1, 2], but is a distinct location in that it displays faults which appear radial to Olympus Mons. Further investigation of this area is therefore expected to help illuminate the complex volcano-tectonic history of the Olympus Mons volcano and the Tharsis volcanic province more generally.2. MethodsWe used Context Camera (CTX) images, with a resolution of 6 m/pixel [3], to map all the faults in the Ulysses Fossae study site at a scale of ~1:250,000; mapping from Tanaka et al. [4] providing outlines for the three Hesperian geological units in the study area. Mars Orbiter Laser Altimeter (MOLA) data were used to create topographic profiles of select areas in Ulysses Fossae. The MOLA data was complemented by images from the High-Resolution Stereo Camera (HRSC), creating a Digital Elevation Model (DEM) with a resolution of 200 m/px [5]. Crater statistics derived from these three units with the &#8220;Cratertools&#8221; ArcMap tool and the Craterstats v.2 program [6, 7] were used to calculate absolute model ages.3. Preliminary Results and DiscussionIn total, we mapped 5,165 faults in Ulysses Fossae (Figure 2), with fault patterns ranging from linear to curved. Detailed analysis of the mapped faults, including investigating cross-cutting relationships, morphology, and azimuths, allowed us to divide the faults to into eight main fault populations (see Table 1). On the basis of strike, the majority of these populations reflect influence from local volcanic centers, as they display radial fault patterns to Olympus Mons, Pavonis Mons, and Ulysses Patera. One of the older populations, Population 2 (Table 1), dated to be Early Hesperian, appears to solely be the result of regional extension associated with the growth of Tharsis. The youngest fault population, Population 7, from the late Hesperian, consists of slightly curved grabens with a sigmoidal geometry and en echelon patterns, and appears to have formed by reactivation of the two oldest mapped fault populations.Additionally, we acquired crater statistics for the geological units in the study area to calculate absolute model ages to aid in our understanding of the geological history of Ulysses Fossae. This analysis resulted in giving two of the units a potentially significantly younger, Amazonian age, than previously determined [4] see Fig. 2.4. Summary and ConclusionsOur mapping resulted in the recognition and analysis of eight fault populations, which together suggest several stages of structural activity in the Ulysses Fossae region. Results from relative and absolute model age analysis so far indicate a complex history involving radial fault growth from the several surrounding proximal volcanoes (Olympus Mons, Pavonis Mons, and Ulysses Patera), later reactivation of pre-existing fault populations and some influence from the regional extension of the Tharsis rise.Further investigations based on this initial work will include measuring extensional strain for the eight fault populations, and maximum displacement/length (Dmax/L) relationships for select study faults, with the data ultimately being intended to help in comparisons to other volcanic areas on Mars and other terrestrial planets. &#160;&#160;Acknowledgements To NASA for providing CTX and MOLA data, and ESA for HRSC data. Funding for this project was provided by the UK Research and Innovation (UKRI) Natural Environment Research Council&#8217;s (NERC) Doctoral Training Programme.References[1] Scott, D. H., &amp; Dohm, J. M. (1990). 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