EXTENSIONAL TECTONICS AT ALBA MONS: A CASE STUDY FOR REGIONAL AND LOCAL STRESS FIELDS

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 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 (where one type of structure grades into the other).

Here we present a structural survey of five highly fractured study areas, each with a different proximity to Alba Mons (spanning 240–270° E and 14–50° N), to determine the orientations and formational history of these structures. For each structure population we recorded proximity to the volcano, map-view orientation (and any changes in such; gradual or abrupt), morphological properties, and spatial and temporal (i.e., cross-cutting) relationships with adjacent volcanotectonic landforms.

Graben in the study region typically trend N–S, but each of study areas shows notable secondary trends, some of which overprint or cross-cut earlier structures. Graben adjacent to Alba Mons (e.g., the North Ceraunius and Tantalus Fossae systems) generally strike N–S but begin to trend northeast on the eastern margin of the volcano. Progressively south from the volcano, graben maintain a ~N–S trend but with a considerable presence of graben with incongruent orientations (such as those within the South Ceraunius and Tractus Fossae systems). Graben northeast of Alba Mons (i.e., Alba Fossae) seemingly curve around the volcano before assuming a northeast trend and continuing across the dichotomy boundary. The Alba Patera Formation middle member, a unit composed of flows originating from the Alba Mons volcanic center, also hosts structures that show a gradual shift in strike from 160 E to 030 E. And some of the greatest density of faulting and highest variability in fault orientation is situated within three elevated patches of exposed ancient bedrock with frequently crosscutting faults that trends from 005 E to 030 E, approximately 150 km to the south of the volcano. These observations suggest that local stresses from Alba Mons formed northwest-strikes normal faults, whereas the widespread north—northeast-oriented faults in the region are likely associated with the formation of the Tharsis Rise.