GEOLOGY, MORPHOLOGY AND TOPOGRAPHY OF SYRTIS MAJOR, MARS: NEW INSIGHTS BASED ON MGS DATA

Based on new MOLA and MOC data we investigated the topography and morphology of Syrtis Major, one of the large volcanic complexes of Mars. It is clearly distinguishable from the surrounding highlands by its smoother surface. In contrast to the later Tharsis Montes, it is characterized by a low relief and gentle slopes of <0.5°. MOLA data show that the height of the structure is ~0.5-1 km, consistent with previous estimates. The volume of lava is ~1.6 - 3.2 x 106 km3. Nili and Meroe Patera are located within a complex large N-S elongated depression and their floors are ~2000 m below the highest point of the shield. MGS data do not show significant differences in crustal thickness underneath Syrtis Major compared to adjacent cratered highland plains. Highlands and Syrtis Major both have a crustal thickness of ~45-60 km. However, the crust is significantly thinner compared to the Tharsis area (>60 km). The Syrtis Major Formation Hs is rougher at all wavelengths (0.6-19.2 km) compared to other studied volcanic units (e.g., Hr, At4, At5, Aop). New crater counts reveal that Syrtis Major is of Early Hesperian age, hence slightly older than previously thought and probably contemporaneous with unit Hr. We also see evidence for resurfacing events. Wrinkle ridge patterns, roughness, and ages suggest that Syrtis lavas flowed into Isidis and are now covered with sediments of the Vastitas Borealis Formation. A similar stratigraphic relationship was proposed for the most widespread Hesperian ridged plains unit Hr. Shallow sills that triggered catastrophic erosion of Noachian rocks were viewed as responsible for the missing western rim of Isidis. We find that volcanic burial is more likely the dominant process. Crater Antoniadi was previously thought to be flooded with Syrtis Major lavas but MGS data show that significant amounts of lava erupted from sources within the crater.