The attributes and formation mechanisms of Kallistos Vallis, Venus

Kallistos Vallis is a large volcanic channel system located in the Ammavaru volcanic province of Venus. The system extends more than 1200 km downslope from a complex topographic depression, terminating at distal plains characterized by distributary channels that emplaced extensive volcanic flows with estimated total volumes of thousands of cubic kilometers. Some channel reaches of Kallistos Vallis are as wide as 30 km and are in places characterized by the presence of prominent streamlined erosional residuals about which the system anastomoses. Cross-sectional and longitudinal profiles generated for Kallistos Vallis using low-resolution Magellan altimetry data do not constrain the form of the system effectively, beyond demonstrating that topographic relief across channels is generally at least tens of meters, and the kilometer-scale longitudinal slopes that characterize the system are generally well below 1°. The main Kallistos Vallis system has a surface area of ~41,000 km2, roughly 20% of which is comprised of the channel reaches that form the westernmost and central parts of the system, and 80% of which forms the eastern volcanic plains and related distributary channel reaches. The head depression of Kallistos Vallis is interpreted as the surface expression of a deeply-rooted igneous plumbing system that conveyed large volumes of mafic or ultramafic magma to the surface from subcrustal depths. The flow conditions potentially associated with development of the channel system were estimated on the basis of known constraints of the Venusian environment and the nature of volcanic analogs of the inner solar system. Flows with depths of 5 m and 20 m and viscosities of 1 Pa s would have been fully turbulent on essentially all channel slopes, and could have reached velocities of tens of meters per second and discharges of up to ~107 m3/s on longitudinal slopes no greater than 1°. Flows with viscosities of 1 Pa s are expected to have had a ready capacity for thermomechanical incision of at least meters per day, facilitated by the relatively high gravity and hot surface temperatures of Venus, and could have developed the channel system in as little as tens of days. System development by multiple discrete eruptive episodes separated by geological time is also possible. The volumes of effused lava most likely to have been required for development of Kallistos Vallis range from thousands to tens of thousands of cubic kilometers. The eruptive and flow conditions estimated for Kallistos Vallis agree with those previously determined for large ancient volcanic channels of other rocky bodies of the inner solar system, and attest to the remarkable differences between the predominant character of volcanism on modern Earth and that which prevailed on numerous rocky bodies during earlier episodes of solar system history.