SYRTIS MAJOR AND HESPERIA PLANA, MARS: TWO LIPS (LARGE IGNEOUS PROVINCES) TELLING DIFFERENT STORIES

Syrtis Major Planum (9.2°N, 67.1°E) and Hesperia Planum (21.4°S, 109.9°E) are both ridged plains that stratigraphically mark the base of the Hesperian system on Mars. Although their similarities outnumber their differences, these volcanic plana experienced unique evolutionary paths. Comparing and contrasting their origins and evolution may help us to understand how LIPs form and evolve on the terrestrial planets.

Both Syrtis Major and Hesperia Plana cover >10⁶ km² and contain >10⁶ km³ of mafic lava. Syrtis Major Planum is adjacent to the 1200-km-diameter Isidis impact basin; Hesperia Planum is adjacent the 2300-km-diameter Hellas impact basin. Both plana contain paterae, interpreted to be collapse calderas: Nili and Meroe Paterae comprise the summit caldera complex of Syrtis Major; Tyrrhenus Mons, embayed and almost surrounded by Hesperia Planum material, is capped by Tyrrhena Patera. Both plana display intersecting, near-orthogonal, mare-type wrinkle ridges, revealing that both plana have experienced an almost 90° shift in the orientation of the most compressive stresses through time. Ridge rings (interpreted to be the location of impact crater rims buried beneath layered lava flows), along with partially flooded impact craters around the plana margins, provide local constraints for the thickness of the lava pile. Total lava thicknesses are likely <3 km (and locally as thin as <0.5 km) for both Syrtis Major and Hesperia Plana.

Wrinkle ridges within Hesperia Planum generally trend either concentrically or radially to the center of the Hellas basin; those within Syrtis Major Planum tend to trend radially or concentrically to the elongate summit caldera complex that contains Nili and Meroe Paterae. This suggests that Syrtis Major Planum’s compressional forces were dictated by an underlying magma chamber, whereas Hesperia Planum’s wrinkle ridges were generated by Hellas basin-related tectonics.