CONSTRAINTS ON THE CHRONOLOGY OF VOLCANISM IN THE THARSIS REGION OF MARS FROM GEOLOGIC MAPPING

Geologic mapping investigations of different parts of the Tharsis region have revealed distinct volcanic sequences within the complex spatial and temporal evolution of Mars’ largest volcanic province. We are conducting 1:1M-scale geologic mapping of a) extensive lava flows fields in Daedalia Planum in southern Tharsis and b) the summit region and western flanks of Alba Mons in northern Tharsis. The combination of detailed maps of volcanic and other geologic features, stratigraphic and cross-cutting relationships, and analyses of crater size-frequency distributions allows reconstruction of volcanic history and derivation of both Martian stratigraphic age and absolute model age estimates.

In southern Tharsis within Daedalia Planum, three main sequences of volcanism have been identified from well-defined exposures of volcanic plains (Hesperian, ~3.4 Ga) at the southern periphery of Tharsis, broad, sheet-like flows (Middle Amazonian, ~900 Ma), and a complex lava flow field with interspersed smooth and rough flow units (Middle to Late Amazonian, ~250-300 Ma). On the basis of both local embayment relationships and crater statistics, Middle Amazonian flows appear to underlie much of the region covered by the youngest flow field. Distinctive buried craters within the smooth flow units show an absolute model age similar to the surrounding broad flows.

In northern Tharsis, maps of Alba Mons show volcanic surfaces of the summit caldera complex, lava flow fields, and lava tube systems that all appear to date predominantly from the Early Amazonian Epoch. Due to surface degradation and mantling in northern Tharsis (and perhaps the older age relative to Daedalia Planum) boundaries of discrete volcanic sequences are difficult to trace systematically such that distinct geologic units can be confidently defined across Alba Mons. Preliminary results for local sequences that do have distinct geologic contacts and for 200 x 200 km grids across the volcano suggest Early Amazonian ages, indicating large eruptive volumes over a duration of several hundred million years.