V. Ansan

Sinuous gullies on Mars: Frequency, distribution, and implications for flow properties

Recent gullies on Mars are suspected to be the result of liquid‐water‐bearing flows. A formation from wet flows has been challenged by studies invoking granular (dry) flows. Our study focuses on the sinuous shapes observed for some of the recent Martian gullies. Sinuous gullies are found in locations and slopes (of 10°-15°) similar to straight gullies, and they are therefore related to the same formation processes. Numerical simulations of granular flows are performed here by introducing topographic variations such as obstacles, roughness, or slope changes that could possibly generate flow sinuosity. None of these simulations was able to reproduce sinuous shapes on a slope lower than 18° with friction angles typical of dry granular material. The only way to simulate sinuous shapes is to create small‐amplitude periodic variations of the topography of the deposit, an origin not supported by current Martian imagery. Given the presence of sinuosity in natural terrestrial debris flows, we have concluded that sinuous Martian gullies are better reproduced by liquid‐water‐bearing debris flows. Sinuous shapes in leveed flows are used to derive mechanical parameters from several Martian gullies using photoclinometry. Values in yield strength of 100-2200 Pa, velocities of 1.1-3.3 m s−1, and viscosities from 40 to 1040 Pa s are found, which are all within the range of values for terrestrial debris flows with various proportions of liquid water (20%-40%).

Why is the Central Trans-Mexican Volcanic Belt (102°–99°W) in transtensive deformation?

Abstract Using well-defined focal mechanisms, new poles of rotation, and accurate bathymetry, it will be shown that the N34°E oblique convergence between the Cocos and North America plates, along the N290°E Middle American Trench, in front of Central Mexico (102°–99°W), is associated with slip partitioning. In the same way, it will be shown that the forearc sliver will move rigidly eastward along a trench parallel left-lateral strike-slip fault. By reviewing the literature and analysing seismicity, the E–W Central Trans-Mexican Volcanic Belt (TMVB) is the only active zone in Central Mexico. It is the best candidate to accommodate slip partitioning at the trench where mainly extensive E–W-trending normal active faults with left-lateral component are reported. Fault striae inversions, derived from the literature and our focal mechanisms inversion, confirm that the state of stress in the Central TMVB is transtensive since Middle Quaternary with Shmin (σ3) trending NW–SE and Shmax (σ1 or σ2) trending NE–SW. The oblique convergence and slip partitioning at the trench induce this state of stress. The N290°W trench-parallel component, which should be accommodated in the upper plate, is, however, not parallel to the E–W arc-parallel component, but differs by about 20° clockwise. This implies that the former must be partitioned along the E–W faults of the Central TMVB. If we assume that the trench-parallel slip rate of 8 mm year−1 (102°–99°W) is fully accommodated along the Central TMVB, we should expect maximum horizontal extensional and left-lateral slip rates along E–W normal faults of 2 mm year−1 and less than 7 mm year−1, respectively. Strain partitioning may also be accommodated partially by the accretionary wedge or/and by normal faulting that affects the deeper part of the upper plate around 18°N of latitude.

A sedimentary origin for intercrater plains north of the Hellas basin: implications for climate conditions and erosion rates on early Mars

Understanding the origin (volcanic or sedimentary) and timing of intercrater plains is crucial for deciphering the geological evolution of Mars. We have produced a detailed geological map of the intercrater plains north of the Hellas basin, based on images from the Mars Express High-Resolution Stereo Camera, the Mars Reconnaissance High-Resolution Imaging Science Experiment, and Context. Erosional windows and fresh impact craters provide a way of studying the lithology of intercrater plain units. They are composed predominantly of light-toned sedimentary rocks with subhorizontal bedding over a broad extent (greater than tens of kilometers), showing cross-bedding stratifications locally. The broad extent, geometry, and flat topography of these sediments favor a formation by aqueous processes (alluvial and lacustrine) rather than airfall (eolian and volcaniclastic). The Late Noachian (~3.7 Ga) sedimentary plains are locally covered by dark-toned, rough-textured lava flows of Late Hesperian age (~3.3 Ga). Fe/Mg phyllosilicates were detected within sedimentary rocks, whereas volcanic rocks contain pyroxene and lack signatures of alteration, in agreement with interpretations made from texture and morphology. In erosional windows, the superimposition of sedimentary rocks by younger volcanic flows enables the estimation of an erosion rate of ~1000 nm yr−1 during the Hesperian period (3.3–3.7 Ga). Thus, our study shows that an intense sedimentary cycle occurred on the northern rim of the Hellas basin before and during the Late Noachian, leading to the formation of widespread sedimentary plains, which were then eroded, in agreement with a gradual change in the climatic conditions in this period, and later covered by volcanic flows.

Tectonic interpretations of central Ishtar Terra (Venus) from Venera 1516 and Magellan full-resolution radar images

Abstract For more than a decade, the mapping of Venus has revealed a surface that has had a complex volcanic and tectonic history, especially in the northern latitudes. Detailed morphostructural analysis and tectonic interpretations of Central Ishtar Terra, based both on Venera 15 16 and Magellan full-resolution radar images, have provided additional insight to the formation and evolution of Venusian terrains. Ishtar Terra, centred at 0°E longitude and 62°N latitude, consists of a broad high plateau, Lakshmi Planum, partly surrounded by two highlands, Freyja and Maxwell Montes, which have been interpreted as orogenic belts based on Venera 15 and 16 data. Lakshmi Planum, the oldest part of Ishtar Terra, is an extensive and complexly fractured plateau that can be compared to a terrestrial craton. The plateau is partially covered by fluid lava flows similar to the Deccan traps in India, which underwent a late stage of extensional fracturing. After the extensional deformation of Lakshmi Planum, Freyja and Maxwell Montes were created by regional E-W horizontal shortening that produced a series of N-S folds and thrusts. However, this regional arrangement of folds and thrusts is disturbed locally, e.g. the compressive deformation of Freyja Montes was closely controlled by parallel WNW-ESE-trending left-lateral shear zones and the northwestern part of Maxwell Montes seems to be extruded laterally to the southwest, which implies a second oblique thrust front overlapping Lakshmi Planum. These mountain belts also show evidence of a late volcanic stage and a subsequent period of relaxation that created grabens parallel to the highland trends, especially in Maxwell Montes.