Friday, November 13, 2015

Mapping the Geology of the Hellas Basin on Mars

Photogeologic mapping and the geologic history of the Hellas basin floor, Mars


Bernhardt et al


The Hellas basin on Mars is the second-largest topographically well-defined impact structure in the Solar System and has repeatedly been interpreted as a major sink of volcanic, glacio-fluvial and eolian materials. Based on established guidelines for planetary mapping, we compiled a comprehensive photogeological map of Hellas Planitia, i.e., the Hellas basin floor (1:2,000,000; ∼1.8 × 106 km2; see Supplementary online material), using the Thermal Emission Imaging System (THEMIS-IR) day-time mosaic as basemap (supplemented by several other datasets). We identified 33 units, which were categorized into a “Rim Assemblage”, containing “Dissected units”, a “Layered rim sequence”, and “Other basin rim units”, as well as a “Floor Assemblage”, containing the “Honeycomb formation”, an “Interior formation”, and a “Plains sequence”. Relative dating of units was performed wherever contacts revealed stratigraphic relationships and was complemented by absolute model ages (AMAs) of all units that lend themselves to reliable crater-size frequency distribution (CSFDs) measurements. On the basis of our results, as well as AMAs of circum-Hellas volcanic provinces by previous authors, we compiled a chronostratigraphic model of the Hellas basin floor. The northern basin rim shows evidence (vast layered, hydrous mineral-bearing deposits containing meandering, channel-like valleys), that the early history of the basin until ∼3.8 Ga ago experienced extended periods of low-energy fluvial, and possibly lacustrine, activity. Superposing the layered rim sequence, the majority of the Hellas basin infill (∼1.5–1.7 × 106 km3) consists of volcanic material (a lower and an upper wrinkle-ridged plains unit), which was shortened by a compressive stress field relatively soon after its emplacement. Based on their ages and stratigraphic considerations, we identified Malea Patera (and possibly also Tyrrhena Patera) as a suitable source for the older, lower plains (∼3.8 Ga), and Hadriaca and/or Amphitrites Paterae for the younger, upper plains (∼3.7 Ga). Shortly after these volcanic episodes, the entire basin floor was covered by large amounts of deposits (greater than 106 km3) containing aqueously altered mafic materials. We interpret the deposits to have originated from Hesperia Planum and the Hesperia-Hellas trough, where they might have been removed by glacio-fluvial processes, which also sculpted the Hellas rim sections along northern Promethei Terra and Malea Planum around the same time. Such an apparent correlation between nearby volcanic activity and increased erosion/deposition is in agreement with previous models, which suggest outgassing by a maximum in global volcanism at that time caused warmer episodes enabling repeated subaerial water run-off. After the emplacement of these deposits, intense erosion, likely deflation along a rim-parallel annulus, began to exhume the outer parts of the wrinkle-ridged plains again, forming an interior formation of more elevated erosional remnants including the Alpheus Colles. Since ∼3.7 Ga ago, fluvial activity of Dao/Niger and Harmakhis Valles dissected the eastern plains and emplaced pancake-shaped deposits in the basin center. Later, deflation and abrasion by katabatic winds moving through the Hellas basin in a clock-wise direction parallel to the basin floor outline carved out the northwestern Hellas Planitia trough (called Peneus Palus), possibly within a time span of few hundreds of Ma or less within the Amazonian. Being consistent with erosional patterns we observed within the basin, circulation models predict such winds to be ongoing today and might, thus, have recently exhumed the so called “honeycomb” formation on parts of Peneus Palus. This enigmatic, unique terrain was possibly formed by ductile deformation of pre-3.8 Ga material and appears to be partially covered by another unique landform, the “banded terrain”. Although we cannot rule out that it is the surface expression of a thicker unit, the banded terrain might represent flows of volatile-rich airfall deposits emplaced along the edges of the Hellas wind alley by turbulent, marginal wind currents, which currently/recently prevail on both sides of the Hellas Planitia trough. In summary, our investigations enabled us to draw an updated, comprehensive and self-consistent picture of the geologic evolution of the Hellas basin floor, including volcanic, (peri-)glacial, fluvial and eolian processes, their possible interactions, and the implications on the climatic and geologic development of the circum-Hellas region and the entire planet.

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