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Hellas basin acts as a major sink for the southern highlands of Mars and is likely to have recorded several episodes of sedimentation and erosion. The north-western part of the basin displays a potentially unique Amazonian landscape domain in the deepest part of Hellas, called “banded terrain”, which is a deposit characterized by an alternation of narrow band shapes and inter-bands displaying a sinuous and relatively smooth surface texture suggesting a viscous flow origin. Here we use high-resolution (HiRISE and CTX) images to assess the geomorphological interaction of the banded terrain with the surrounding geomorphologic domains in the NW interior of Hellas to gain a better understanding of the geological evolution of the region as a whole. Our analysis reveals that the banded terrain is associated with six geomorphologic domains: a central plateau named Alpheus Colles, plain deposits (P1 and P2), reticulate (RT1 and RT2) and honeycomb terrains. Based on the analysis of the geomorphology of these domains and their cross-cutting relationships, we show that no widespread deposition post-dates the formation of the banded terrain, which implies that this domain is the youngest and latest deposit of the interior of Hellas. Therefore, the level of geologic activity in the NW Hellas during the Amazonian appears to have been relatively low and restricted to modification of the landscape through mechanical weathering, aeolian and periglacial processes. Thermophysical data and cross-cutting relationships support hypotheses of modification of the honeycomb terrain via vertical rise of diapirs such as ice diapirism, and the formation of the plain deposits through deposition and remobilization of an ice-rich mantle deposit. Finally, the observed gradual transition between honeycomb and banded terrain suggests that the banded terrain may have covered a larger area of the NW interior of Hellas in the past than previously thought. This has implications on the understanding of the evolution of the deepest part of Hellas. [ABSTRACT FROM AUTHOR]

Hellas basin is a large impact basin situated in the southern highlands of Mars. The north-western part of the basin has the lowest elevation (−7.5 km) on the planet and contains a possibly unique terrain type, which we informally call “banded terrain”. The banded terrain is made up of smooth-looking banded deposits that display signs of viscous behavior and a paucity of superimposed impact craters. In this study, we use newly acquired high spatial resolution images from the High Resolution Imaging Science Experiment (HiRISE) in addition to existing datasets to characterize the geomorphology, the morphometry and the architecture of the banded terrain. The banded terrain is generally confined to the NW edge of the Alpheus Colles plateau. The individual bands are ~3–15 km-long, ~0.3 km-wide and are separated by narrow inter-band depressions, which are ~65 m-wide and ~10 m-deep. The bands display several morphologies that vary from linear to concentric forms. Morphometric analysis reveals that the slopes along a given linear or lobate band ranges from 0.5° to 15° (average ~6°), whereas the concentric bands are located on flatter terrain (average slope ~2–3°). Crater-size frequency analysis yields an Amazonian-Hesperian boundary crater retention age for the terrain (~3 Gyr), which together, with the presence of very few degraded craters, either implies a recent emplacement, resurfacing, or intense erosion. The apparent sensitivity to local topography and preference for concentrating in localized depressions is compatible with deformation as a viscous fluid. In addition, the bands display clear signs of degradation and slumping at their margins along with a suite of other features that include fractured mounds, polygonal cracks at variable size-scales, and knobby/hummocky textures. Together, these features suggest an ice-rich composition for at least the upper layers of the terrain, which is currently being heavily modified through loss of ice and intense weathering, possibly by wind. [ABSTRACT FROM AUTHOR]

In the vicinity of Lake Thun at the front of the Bernese Alps (Switzerland), we performed a multidisciplinary neotectonic study combining onshore and offshore geological data and geophysical measurements in order to identify potentially active fault structures. Paleoseismic reconstructions on the northern margin of the Alps have documented several strong earthquakes with moment magnitudes ≥6 during the Late Quaternary, which have long recurrence intervals of 1,000 to 2,000 years. Such earthquakes are expected to produce surface ruptures. In this light, we investigated the study area located near Lake Thun primarily for on-fault evidence, to date still a shortcoming in Switzerland. We detected several features indicating potential fault activity, such as aligned subaquatic morphological depressions, offset horizons observed in reflection seismic profiles of lake sediments and in ground-penetrating radar images, all delineating a fault trace. Observations of fluvial deposits in a nearby gravel pit in the prolongation of the inferred structure supports these findings. A narrow zone with rotated long axes of pebbles (inclining at ∼60°) is clearly distinguishable and crosscuts the original bedding with predominantly horizontal orientation of pebble axes. This zone further shows an apparent 1.1 m offset of oxidized horizons and is therefore considered as a potential fault plane in a normal faulting regime. A dated radiocarbon age of ∼11,000 years BP of the gravel deposits hence suggests a younger fault activity during the Holocene. The Einigen Fault Zone (EFZ), proposed on the basis of these observations, is considered as a complex fault system with a combination of dextral strike-slip and normal faulting, as suggested by GPR images. Observations in the gravel pit and radar data independently show that it includes at least two fault strands. However, while five earthquakes with epicentral intensities I 0 ≥ VI and numerous smaller seismic events are known within less than 30 km epicentral distance to Lake Thun over the past 400 years, none of these seem to coincide with the location of the EFZ. [ABSTRACT FROM AUTHOR]