FORMATION OF KASEI VALLES THROUGH INCISION BY LOW-VISCOSITY LAVA FLOWS

Kasei Valles is the largest outflow channel on Mars, extending more than 3000 km from its head at Echus Chasma to the lowlands of Chryse and Acidalia. The system descends ~3 km in elevation over its full length, and the floors of the most prominent channels are 1-3 km below adjacent plateaus. The system is characterized by complexly anastamosing reaches and the presence of streamlined erosional residuals, chaotic terrain, cataracts, inner channels, terraces, longitudinal grooves, and extensive volcanic mantles. The channels of Kasei Valles are widely interpreted to have formed as a result of catastrophic aqueous outbursts from the subsurface, possibly in conjunction with periodic releases of ponded surface waters and the action of glaciers, debris flows, and sapping processes. However, strong evidence in support of such aqueous origins is lacking. Instead, channel origins involving mechanical or thermal erosion by low-viscosity mafic lavas are most consistent with available data. The volcanic hypothesis for development of the Kasei Valles system is congruent with a wide range of considerations including: 1) the volcanotectonic context of the system; 2) the striking absence of clear depositional units of fluvial or diluvial character; 3) the widespread mantling of component channels by volcanic flows including those erupted from the system’s head; 4) the existence of extensive volcanic plains at the terminal basin; 5) strong correspondence between properties of the associated Viking 1 landing site and those of lunar volcanic plains; 6) consistency with mineralogical evidence for extraordinarily dry conditions through much of the Hesperian and Amazonian; 7) consistency of proposed mechanisms with geochemical predictions for low Martian lava viscosities and deep volcanic sources; and 8) availability of lunar and Venusian analogs for development of large channel systems through eruption of low-viscosity lava. Volcanic development of the Kasei Valles system is estimated to have involved an erupted volume of 9 million cubic kilometers, or approximately 50 times the total volume of the terrestrial Columbia River Basalt Group.