Topographic Signatures of Extensional Tectonic Landforms at Alba Mons, Mars

Alba Mons is a large shield volcano on Mars, the development of which appears to be responsible for tectonic landforms oriented radially and circumferentially to the shield. These landforms include those interpreted as extensional structures, such as normal faults and systems of graben (e.g., Alba, Ceraunius, Tractus, and Tantalus Fossae), pit crater chains (likely caused by dilational faults and/or dike intrusion), and combinations of both (i.e., where one type of structure grades into the other). Here, we present structural and statistical analyses for several graben populations proximal to Alba Mons (in a region spanning 240-270° E and 14-50° N) to determine the geometry and formational history of these structures. Distinguishing traits for each structure population include fault geometry, vertical displacement (i.e., throw) distribution profiles, displacement-length scaling, geomorphological observations, and spatial (i.e., cross-cutting) relationships with adjacent volcanotectonic landforms. Topographic profiles across representative graben in each study area provide information on the location of primary and secondary faults for each landform. Additionally, measurements of graben width and depth at 5 km intervals help characterize along-strike changes in geometry of each structure. These data are also used to determine maximum displacement (Dmax) versus length (L) for representative graben-bounding faults in each study area. Preliminary results show that grabens in study areas adjacent to Alba Mons largely have east-dipping primary faults, whereas graben in more distant areas have generally west-dipping primary faults. The deepest graben occur in the Ceraunius Fossae graben system, with those that host pit crater chains being about three times deeper than typical flat-floored graben in our study region. We also find a proportional increase in graben depth with width in all study areas, including those graben that are home to pit crater chains. Preliminary Dmax/L ratios for representative graben in the North Ceraunius system have a range of 3-6.7 for flat-floored graben, and 29 for graben in which pit crater chains occur. Our results indicate that the Alba Mons region is both mechanically variable spatially and that it may have undergone multiple tectonic events.