Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter ...cones is the only accepted meso‐ to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features.
•Artisanal and small-scale gold mining is a serious threat to the environment.•In Ivory Côte d’Ivoire, this activity has developed very quickly recently.•The lack of geographical information is an ...obstacle to the regulation.•The monitoring of artisanal mining from space can help governements.
Recent development of small-scale gold mining activities in Côte d’Ivoire is a serious threat to the environment because of deforestation, soil scrapping, pit mining, over-use of water resources and pollution of surface and ground waters by mercury, cyanides, and acids. The challenge for the governance of this activity is to resolve the tension between the desired development of a small-scale mining activity, which may benefit the local and national economy, and the impacts of this activity on the environment. The regulation of the exploration and exploitation of mining sites and the promotion of best practices was part of the National program for the rationalization of gold-panning in Côte d’Ivoire. The capacity of the government to monitor the expansion of numerous mining sites disseminated all over the country is one of the key aspects for successful implementation of these policies. This study explores the potential value of computer-assisted mapping of artisanal mining sites based on Sentinel-2 imagery. The detection method, using artificial intelligence and training data sets generated during field campaigns, was inspired from a previous experience in Senegal. It was applied to a region of about 600 km2 in Central Côte d’Ivoire. Annual maps of areas affected by the mining activities were produced for the period 2018 – 2021. The areas affected by artisanal mining activities expanded from 3.39 km2 to 8.80 km2 in December 2021, corresponding to an average growth rate of 0.24 km2/month. The temporal and spatial resolution of the Sentinel satellite imagery proved to be useful to map and quantify the expansion rate of artisanal mining sites in Côte d’Ivoire. Recommendations are made for the integration of these tools into plans for the development of small-scale mining activities in Côte d’Ivoire that would be more respectful of the environment and societies.
Reconstruction of the geological history of Mars has been the focus of considerable attention over the past four decades, with important discoveries being made about variations in surface conditions. ...However, despite a significant increase in the amount of data related to the morphology, mineralogy and chemistry of the martian surface, there is no clear global picture of how magmatism has evolved over time and how these changes relate to the internal workings and thermal evolution of the planet. Here we present geochemical data derived from the Gamma Ray Spectrometer on board NASA's Mars Odyssey spacecraft, focusing on twelve major volcanic provinces of variable age. Our analysis reveals clear trends in composition that are found to be consistent with varying degrees of melting of the martian mantle. There is evidence for thickening of the lithosphere (17-25 km Gyr−1) associated with a decrease in mantle potential temperature over time (30-40 K Gyr−1). Our inferred thermal history of Mars, unlike that of the Earth, is consistent with simple models of mantle convection.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Tharsis region is the largest volcanic complex on Mars and in the Solar System. Young lava flows cover its surface (from the Amazonian period, less than 3 billion years ago) but its growth ...started during the Noachian era (more than 3.7 billion years ago). Its position has induced a reorientation of the planet with respect to its spin axis (true polar wander, TPW), which is responsible for the present equatorial position of the volcanic province. It has been suggested that the Tharsis load on the lithosphere influenced the orientation of the Noachian/Early Hesperian (more than 3.5 billion years ago) valley networks and therefore that most of the topography of Tharsis was completed before fluvial incision. Here we calculate the rotational figure of Mars (that is, its equilibrium shape) and its surface topography before Tharsis formed, when the spin axis of the planet was controlled by the difference in elevation between the northern and southern hemispheres (hemispheric dichotomy). We show that the observed directions of valley networks are also consistent with topographic gradients in this configuration and thus do not require the presence of the Tharsis load. Furthermore, the distribution of the valleys along a small circle tilted with respect to the equator is found to correspond to a southern-hemisphere latitudinal band in the pre-TPW geographical frame. Preferential accumulation of ice or water in a south tropical band is predicted by climate model simulations of early Mars applied to the pre-TPW topography. A late growth of Tharsis, contemporaneous with valley incision, has several implications for the early geological history of Mars, including the existence of glacial environments near the locations of the pre-TPW poles of rotation, and a possible link between volcanic outgassing from Tharsis and the stability of liquid water at the surface of Mars.
New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a ...conservative range of density values (2700–3100 kg/m3). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100–3700 kg/m3), igneous rocks at Gusev crater (3100–3600 kg/m3) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma‐Ray Spectrometer on board Mars Odyssey (3250–3450 kg/m3). In addition, the density of mineral assemblages resulting from low‐pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100–3300 kg/m3). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid‐to‐topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.
Key Points
Reappraisal of the Martian crust densityMartian basalt density ranges from 3100 to 3300 kg/m3Existence of buried felsic component in the Noachian highlands
Multiple temperature-age constraints for eight H-chondrite samples have been used to provide insight into the thermal history of their parent-body through combination with numerical models of thermal ...evolution assuming internal heating by 26Al and conductive cooling. The effect of spreading accretion out over time is the principal focus of this work. A wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo-chronological data. We conclude that the H-chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric ‘onion-shell’ internal structure. That body had a radius no larger than 130km, and accretion most probably took place over a time interval on the order of 0.0–0.2Myr, approximately 2Myr after CAI condensation. In any case, the time interval of accretion is unlikely to have been more than 0.5Myr supporting evidence in favour of rapid accretion, possibly through reassembly of the fragments of an earlier generation of bodies. Furthermore, the H-chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism, indicating that preservation of the onion-shell structure is unlikely. The presence of an insulating regolith does not modify this conclusion, as appropriate thermal histories for the three H6 samples considered cannot be reproduced at depths near the surface. Asteroid 6-Hebe may be the parent body of the H-chondrites, but the high bulk density of the latter is difficult to reconcile with a ‘rubble-pile’ structure of pure H-chondrite material. Finally, optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types (800, 1000, and 1140K for the H3/4, H4/5, and H5/6 boundaries respectively). In detail, the type 6 samples studied here are inferred to have experienced a similar peak temperature (on the order of 1180K), but spent different times at that temperature, while samples of lower metamorphic grade experienced different peak temperature, but spent more or less the same time within 10K of peak temperature (on the order of 1Ma). These results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted.
In geological mapping, maps of K, Th, and U concentrations provided by airborne radiometric surveys are widely used to delineate geological units in tropical regions from the few rare outcrops. ...Indeed, thanks to their specific geochemical properties and behaviors, K, Th, and U allow us to trace geological processes. However, the combination of the concentrations of these radioelements does not allow us to determine the lithology in a unique way. We examined the potential of delineating the statistical parameters of K, Th, and U concentrations for geological mapping using the purpose airborne radiometric data in eastern Senegal. The mean, standard deviation, skewness, and kurtosis were calculated and mapped at a baseline of 3000 m. We noted the narrow dispersion of skewness and kurtosis values away from the expected curve for the log-normal distribution, implying that log-normal distributions dominate at the scale of analysis. The higher moments (kurtosis and skewness) varied more over shorter distances than lower order moments (mean and standard deviation). Mixtures of log-normal distributions across some lithological contacts with large differences in statistical parameters may account for this behavior. The area covered by the airborne radiometric data was classified into eight units according to the statistical parameters. The eight clusters do not show obvious correlations with geological units, but they may be interpreted in terms of the superposition of lithology and recent superficial processes (erosion and weathering).
A review of volatiles in the Martian interior Filiberto, Justin; Baratoux, David; Beaty, David ...
Meteoritics & planetary science,
November 2016, Letnik:
51, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Multiple observations from missions to Mars have revealed compelling evidence for a volatile‐rich Martian crust. A leading theory contends that eruption of basaltic magmas was the ultimate mechanism ...of transfer of volatiles from the mantle toward the surface after an initial outgassing related to the crystallization of a magma ocean. However, the concentrations of volatile species in ascending magmas and in their mantle source regions are highly uncertain. This work and this special issue of Meteoritics & Planetary Science summarize the key findings of the workshop on Volatiles in the Martian Interior (Nov. 3–4, 2014), the primary open questions related to volatiles in Martian magmas and their source regions, and the suggestions of the community at the workshop to address these open questions.
Geologically recent small gullies on Mars display morphologies consistent with erosion by water or by debris flows. Suggested formation models are divided into two main categories: (1) groundwater or ...(2) melting of near‐surface ice/snow sourced from the atmosphere. We have measured location and orientation and recorded the local contexts of gullies to constrain the likely models of gully formation. More than 22,000 Mars Orbiter Camera Narrow Angle (MOC NA) and >120 Mars Express High Resolution Stereo Camera (HRSC) images in the southern hemisphere were searched for gullies. Discrete gullied slope sections with consistent orientation were recorded rather than individual gullies. Slope setting (impact crater, valley wall, etc.), location, and orientation were recorded for each slope section. More than 750 MOC images with gullies (>900 distinct gullied slope sections) and more than 40 HRSC images (>380 distinct gullied slope sections) were identified. From both MOC and HRSC, gullies were found to be most common between −30 and −50 degrees latitude and to have an overall pole facing preference. The preferred gully orientation for HRSC is southeast rather than south in MOC, owing to illumination effects that make gullies difficult to detect on south‐ to southwest‐facing slopes in HRSC. In both MOC and HRSC surveys, higher‐latitude gullies show less preference for pole facing than those at mid latitudes. Both data sets produced similar results, demonstrating that our data are reliable. We suggest that the observed latitudinal and orientation distributions of gullies show that insolation and atmospheric conditions play a key role in gully formation.
The onset of India‐Asia contact can be dated with both biostratigraphic analysis of syn‐collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating ...of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction‐collision stages. These data, together with paleomagnetic data, show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intraoceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo‐Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo‐Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto‐Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50–45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm yr−1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening versus amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.
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