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[sup 40]Ar/[sup 39]Ar single-grain laserprobe dating of detrital white micas from early Oligocene to middle Miocene (31–14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third Tertiary age cluster reflecting late Alpine cooling is restricted to sediments younger than 20 Myr old. Micas with cooling ages < 30 Myr are interpreted to originate from the footwall of the Simplon detachment fault, thus representing formerly exposed upper levels of the present-day Lepontine metamorphic dome. Erosion of these levels is reflected by an increase of low-grade metamorphic lithic grains in the sandstones. This interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the Central Alps. [ABSTRACT FROM AUTHOR]

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]