Formation of a terraced fan deposit in Coprates Catena, Mars

We have studied a terraced fan deposit with unique characteristics located within a trough of Coprates Catena. The fan has an average length of 6.8 km, and is approximately 44 km² in area and 18 km³ in volume. The fan's broad contributing valley is approximately 35 km long and it noticeably increases in depth about 12.8 km before it intersects the trough, where a rounded knickpoint marks the transition between flat-floored upstream and V-shaped downstream cross-sections. A 14-km-long channel with no apparent source enters the contributing valley from the south. A much smaller sinuous channel has incised along a smaller V-shaped valley in the uppermost eastern portion of the fan deposit. We explored several possible origins for the terraced fan, including mass wasting, volcanic flow, alluvial fan, and delta. We propose that water sourced from volcanic melting of ice eroded and transported material along the contributing valley. This material was then deposited as a delta in a lake within the trough. The concentric terraces are most likely the result of shoreline or ice cover erosion during drops in lake level. A light-toned layered deposit to the east of the fan deposit along the floor of the trough may represent a sedimentary unit formed during the terminal stages of the lake. Although other terraced fans have been identified on Mars, the Coprates Catena fan is unique because it has many more terraces and its surface was incised by a channel and associated valley. The identification of several other valleys to the east suggests that volcanic melting of volatiles during the Hesperian Period created favorable conditions for water flow along the plains in this region.     

Homogénéisation et validation des corrections de terrain gravimétriques jusqu'à la distance de 167 km sur l'ensemble de la FranceUpdating and validating the French gravity terrain corrections out to a distance of 167 km

For regional and national study purposes, there is a high need for updating the terrain corrections (TC) in the French gravity database. We have recomputed the TC for all the French gravity stations from 50 m out to a distance of 167 km. We compute the TC with a flat-top-prism algorithm and three DEM with grid spacing of 50, 250 and 1000 m, used in the zones 53 m/3 km, 3 km/10 km et 10 km/167 km, respectively. Analysing the DEM/station Δz and comparing our results to the ones previously obtained in the Alps area, we estimate the accuracy of our TC to be better than 1 mGal. To cite this article: G. Martelet et al., C. R. Geoscience 334 (2002) 449–454.     

Simulating strong ground motion with the `k -2 ' kinematic source model: An application to the seismic hazard in the Erzincan basin, Turkey

We present a seismic hazard application of a kinematic broad-bandrupture model. This model is based on the k-square dislocation distribution concept (Herrero and Bernard, 1994).Synthetic seismograms are calculated in the far-field approximation with alayered velocity medium for the 13 March 1992 Erzincan earthquake.With a parametrization of the source constrained by other studies,the far-field contribution correctly fits the recorded strong ground motion, which presents a 0.5 to 2 Hz dominant frequency range.As the k-square model is particularly well adapted to synthetize realistic strong-motion at short distances from the fault,it is a reliable tool for calculating seismic hazard maps around active faults. We thus present a synthetic peak ground acceleration map associatedwith the 13 March 1992 activated fault, for a 60 km × 60 km regionaround the epicenter taking into account a smoothed velocity structure ofthe basin in agreement with the absence of significant site effects related to1D resonance deduced from the aftershock records study.This map is compared with several post seismic reports: macroseismicintensities, detailed distribution of damage, and soil cracking andliquefaction. Our model shows that the values of the peak acceleration andvelocity can explain the dominant spatial distributionof these effects, which concentrates in a narrow band along theactivated segment fault, and in particular at its southernextremity. These results enable us to present such maps forhypothetical future earthquake ruptures, located on the major visible orinferred active fault segments in and around the basin. The effects ofthese potential sources are analyzed in relation to the 1992 eventeffects in order to eliminate unknown site responses. We show that thesouthern part of the basin is particularly exposed because of thepresence of strike-slip faults,and that the western part of the basin would suffer a significantlyhigher strong motion levelthan during the 1992 event with the activation of moderate sizednormal faults evidenced on thesouthwestern edge of the basin.     

The destruction of the past: Nonrenewable cultural resources

Destruction of cultural resources has increased dramatically in the last quarter of the 20th century as a result of two forces: first, development and construction, and second, the astonishing explosion of demand for primitive and prehistoric art. On the cover: Rock art of the Mimbres Culture at the Pony Hills site in south-west New Mexico. Similar rock art nearby has been destroyed by pistol shots or chipped out and illegally sold. Photo by Andrew Gulliford, Public History Program, Middle Tennessee State University.For the purposes of Legacy, the term cultural resource refers to both archeological and architectural resources. For archaeology, it includes, but is not limited to, traditions, lifeways, cultural and religious practices, and other institutions to which a community, neighborhood, Native American tribe, or other group ascribes cultural significance, together with any artifacts and real property associated with such elements. For architecture, it includes, but is not limited to, buildings, sites, districts, structures, or objects, landscapes, and vistas. In addition, the term encompasses historic documents and relics.     

Little Ice Age and neoglacial landforms at the Inland Ice margin, Isunguata Sermia, Kangerlussuaq, west Greenland

Neoglacial and Little Ice Age (LIA) limits occur within 2km of the Inland Ice margin in the Kangerlussuaq area on west Greenland. The LIA limit is clearly demarcated by ice-cored and non-ice-cored moraines, out-wash surfaces and trimlines. Rhizocarpon sp. thalli of ≥16mm on these landforms indicate a 1-2km retreat of the Inland Ice in the past c. 100 years, coincident with peripheral thinning of the ice. An older neoglacial moraine host of Rhizocarpon sp. thalli <40 mm indicates a minimum limiting age of <400 BP, whereas Optically Stimulated Luminescence (OSL) ages on aeolian silt capping the moraine yield close limiting ages of c. 2000 BP. Aeolian silt deposition beyond neoglacial limits yields OSL ages of c. 3000 BP, potentially coeval with advance of the Inland Ice. Aeolian sedimentation and the inferred age of the moraine are coincident with pronounced cooling inferred from palaeolimnological records from west and south Greenland. This neoglacial event at c. 2000 BP is probably of similar extent to the LIA maximum, because of the paucity of preserved moraine remnants.     

The Vulnerability of Canada to Volcanic Hazards

Western Canada lies in a zone of active tectonics and volcanism, but thedispersed population has witnessed few eruptions due to the remoteness of the volcanoes and their low level ofactivity. This has created a false perception that Canada's volcanoes are extinct.There are more than 200 potentially-active volcanoes in Canada, 49of which have erupted in the past 10,000 years. They occur in five belts, with origins related totectonic environment. The minimum annual probability of a Canadian volcanic eruption is approximately 1/200;for an effusive (lava) eruption the probability is about 1/220, and for a significant explosive eruptionit is about 1/3333. In-progress studies show that there have been earthquakes associated with at least 9 ofthe youngest Canadian volcanoes since 1975. A scenario of an eruption of Mt. Cayley (50.1°N,123.3°W) shows how western Canada is vulnerable to an eruption. The scenario is basedon past activity in the Garibaldi volcanic belt and involves both explosive and effusive activity.The scenario impact is largely a result of the concentration of vulnerable infrastructure in valleys.Canadian volcanoes are monitored only by a regional seismic network,that is capable of detecting a M > 2 event in all potentially-active areas.This level of monitoring is probably sufficient to alert scientistsat or near eruption onset, but probably insufficient to allow a timelyforecast of activity. Similarly the level of geological knowledge about the volcanoes is insufficient to createhazard maps. This will improve slightly in 2002 when additional monitoring is implemented in theGaribaldi volcanic belt. The eruption probabilities, possible impacts, monitoring limitations and knowledgegaps suggest that there is a need to increment the volcanic risk mitigation efforts.     

Intercomparison of Boron Isotope and Concentration Measurements. Part II: Evaluation of Results

The Istituto di Geoscienze e Georisorse (IGG), on behalf and with the support of the International Atomic Energy Agency (IAEA), prepared eight geological materials (three natural waters and five rocks and minerals), intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials were distributed to twenty seven laboratories - virtually all those performing geochemical boron isotope analyses in the world -which agreed to participate in the intercomparison exercise. Only fifteen laboratories, however, ultimately submitted the isotopic and/or concentration results they obtained on the intercomparison materials. The results demonstrate that interlaboratory reproducibility is not well reflected by the precision values reported by the individual laboratories and this observation holds true for both boron concentration and isotopic composition. The reasons for the discrepancies include fractionations due to the chemical matrix of materials, relative shift of the zero position on the δ¹¹B scale and a lack of well characterized materials for calibrating absolute boron content measurements. The intercomparison materials are now available at the IAEA (solid materials) and IGG (waters) for future distribution.     

The thermal signature of subducted lithospheric slabs at the core–mantle boundary

We study the thermal structure around a cold deformable lithospheric slab as it sinks to the core–mantle boundary and migrates along it. We present analytical results for the steady thermal structure established by a steady but spatially varying motion. The analysis gives a time-like criterion for the thermal signature of a cold slab to persist by the time that the slab moves along the core–mantle boundary. The model is used to assess the feasibility of a purely thermal origin for some of the observed seismic reflectors near the core–mantle boundary. Calculations of the time-like criterion show that the dynamical conditions in our model, namely the velocity and the thickness of the descending slab, are hard to reconcile with observations of subduction and seismic features. Seismic reflections and refractions from anomalously fast regions above the core–mantle boundary could be explained as thermal slabs if the thickness of the slab at subduction was larger than 200 km or somewhat less if the slab did not split at the core–mantle boundary. A simple thermal model also predicts from mineral physics a certain correlation between S- and P-wave velocity anomalies, which is not observed. However, a purely thermal origin cannot be ruled out if the slab is buckling. This process could be in agreement with the observations: the amplitude of the seismic anomalies, the vertical extent of high-gradient zones and the P versus S comparisons. Chemical heterogeneities and phase transformations remain alternative or complementary explanations.     

The northwestern Ethiopian Plateau flood basalts: Classification and spatial distribution of magma types

The extensive, complex, continental flood basalt (CFB) province which occurs in Ethiopia and Yemen consists of Oligocene prerift volcanism related to the Africa–Arabia continental break-up. Basalts from the northwestern Ethiopian Plateau exhibit a particularly large range of compositions and, for the first time in the Afro-Arabian CFB province, low-Ti basalts have been encountered. Major and some trace element data have been used to identify distinct geochemical groups and evaluate the role of differentiation processes. Three magma types have been distinguished: two high-Ti groups (HT1 and HT2) and one low-Ti group (LT). The transitional to tholeiitic LT suite exhibits low TiO₂ (1–2.6%), Fe₂O₃^* (10.5–14.8%), CaO/Al₂O₃ (0.4–0.75), Nb/La (0.55–0.85) and high SiO₂ (47–51%). In contrast, the HT2 suite exhibits high TiO₂ (2.6–5%), Fe₂O₃^* (13.1–14.7%), CaO/Al₂O₃ (0.9–1.43), Nb/La (1.1–1.4) and low SiO₂ (44–48.3%). The HT1 series is intermediate between the LT and HT2 groups. These three groups of lavas originated from different parental magmas. They display distinct differentiation trends, either controlled by the removal of a shallow level gabbroic (Pl+Ol+Cpx) assemblage (LT and HT1 suites) or by deeper Ol+Cpx fractionation (HT2 suite). Most of this thick continental flood lava pile was emplaced over a short time interval (about 1–2 Ma). The three contrasted magma types do not reflect a temporal evolution of their sources but rather a strong spatial control. Indeed, the northwestern Plateau may be subdivided into two different subprovinces as all the low-Ti basalts are located in the northern part of the plateau, and the high-Ti basalts are exposed in the eastern and southern parts. The LT and HT1 basalts display compositional ranges similar to those of the low- and high-Ti groups from other main CFB provinces (e.g. Parana, Deccan, Karoo, Siberia, …). However, the HT2 group exhibits extreme OIB-like compositions. This unusual geochemical signature suggests the involvement of deep mantle in the genesis of the HT2 magmas. The LT compositions rather reflect the participation of the continental lithosphere, through mantle derived melts and/or crustal contamination.