Preliminary Results from Geologic Mapping of the Volcano Atira Mons, Beta Regio, Venus

We are currently completing a geologic map of the volcano Atira Mons (diameter ~600 km; height ~1.1 km) at 1:400,000 scale (Fig. 1), in order to reconstruct its stratigraphy and geological history. Our study area is centered at 52.2°N, 92.4°W in the NE portion of the Beta-Atla-Themis (BAT) region. Regional analysis shows that Atira Mons and its apron of flows occupy an area of ~350,000 km². The apron partially overlies older regional plains on the N and terminates against topographic barriers on the E and W. A field of shield volcanoes is located NE of the Atira Mons' flows. These small volcanoes and their flows appear to be younger than the regional plains, but have an uncertain age relationship with Atira Mons. On the south and southwest, Atira Mons' flows partially cover a cluster of coronae and corona-like features. SW-trending flows can also be traced ~800 km away from the Atira Mons summit, where they partially cover older plains with wrinkle ridges. Most of the mapped lava flow units initially extend radially away from Atira Mons' summit region. However, most of the W-, NW- and N- trending flows eventually swing ~40-50º to the W and converge towards a local bowl-shaped depression of unknown origin ~300 km NW of the summit. The E flank flows are fed from the E side of the caldera interior, crossing its low rim. However. the sources for the W and SW flows are more likely buried circumferential dykes outside the caldera. Radar bright landslides partly cover the assumed sources of the W-trending flows on the W and NW flanks of the caldera. The mapping of the summit region suggests the following sequence of events: 1) caldera formation with a greater collapse on the E; 2) emplacement of flows on the floor of the caldera, which were later deformed by polygonal fractures; 3) emplacement of flows covering part of the summit floor and fed from small shield volcanoes; 4) formation of a set of wrinkle ridges on the youngest flows. A major arcuate fracture system was identified on the E flank of Atira Mons. This suggests that late in its history the volcano started to develop corona-like characteristics. Since this system cuts across the majority of the E-trending radial flows, we interpret it to be younger than the E-trending flows, but older than the NE flows that cover it.