­TERRESTRIAL LAVA-RISE PLATEAUS AS ANALOGS FOR LOBATE FLOW UNITS NEAR HRAD VALLIS, MARS

Hrad Vallis is an Amazonian-age outflow channel located within the Elysium Volcanic Province of Mars. It is associated with a variety of flow units, some of which may be the products of catastrophic aqueous floods, while others may result from effusive volcanism. For the three youngest flow units near Hrad Vallis, we employ geological mapping, geomorphological observations, numerical simulations of viscous flow processes, and thermal modeling of lava cooling rates to distinguish between these formation mechanisms. Additionally, we compare these units to inflated lava flows on Earth, such as the McCartys flow in New Mexico. The McCartys flow includes lava-rise plateaus, lava-rise pits, and fractured margins that closely resemble the characteristics of the middle flow unit associated with Hrad Vallis, but are inconsistent with the properties of the other two units. Based on these lines of evidence, we infer that the lower and upper flow units are the products of catastrophic aqueous floods, whereas the middle unit is a pāhoehoe-like lava flow formed through the process of inflation over the course of decades (16–36 years). The middle flow unit is similar to terrestrial lava-rise plateaus, but its emplacement may have been complicated by interactions with surface ice, which affected its flow path and resulted in a range of lava–water interactions. Evidence of well-developed inflation features near Hrad Vallis implies the occurrence of long-lived effusive volcanic eruptions on Mars during the geologically recent past. Such flows differ substantially from the short-lived and turbulent lava flows associated other young outflow channels on Mars like Athabasca Valles.