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.     

The use of geologic and seismologic information to reduce earthquake Hazards in California

Five examples illustrate how geologic and seismologic information can be used to reduce the effects of earthquakes Included are procedures for anticipating damage to critical facilities, preparing, adopting, or implementing seismic safety studies, plans, and programs, retrofitting highway bridges, regulating development in areas subject to fault-rupture, and strengthening or removing unreinforced masonry buildings. The collective effect of these procedures is to improve the public safety, health, and welfare of individuals and their communities.     

Evolution of oceanic gabbros from DSDP Leg 82: influence of the fluid phase on metamorphic crystallizations

Gabbro breccias were recovered from an anomalously shallow level of the ocean crust during DSDP Leg 82. The rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater-derived fluid under static conditions. Secondary phases consist of plagioclase, amphibole and minor clinopyroxene, ilmenite, sphene and chlorite. Petrological study indicates that deformation took place at high temperature, under anhydrous conditions, and was followed by hydrothermal circulation. The compositions of secondary minerals (i.e. strong zonations, presence of chlorine in amphiboles, varying compositions of secondary plagioclase) indicate that reactions of the gabbros with the fluids occurred at a low water/rock ratio. Relations between Cl, Na and K in amphiboles suggest penetration of at least two distinct fluids of different compositions. Metamorphic crystallization stopped when greenschist facies conditions were reached( 350°C), probably because hydrothermal circulation faded out.     

Summer meroplanktonic prosobranch larvae occurring off Beaufort,North Carolina

The occurrence of planktonic prosobranch larvae identified during the summers of 1977, 1979 and 1981 off Beaufort, North Carolina is given. Forty-two species were recorded; 29 of these were found each summer. Observations and identifications were made on 12 newly described larvae: Lacuna sp, Epitonium sp. 1 (apiculatum), Epitonium sp. 2, Strombus alatus, Erato maugeriae, Simnia acicularis, Natica sp. 1, Natica sp, 2, Natica sp. 3, Natica sp. 4, Olivella adelae, Mangelia sp. and Sp.? Turridae. A key for the identification of planktonic prosobranch veligers occuring in summer months is included.     

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.     

Trialling tools using hand-weeding,weed mat and artificial shading to control nuisance macrophyte growth at multiple scales in small agricultural waterways

ABSTRACT

Introduced aquatic macrophytes can dominate small agricultural waterways in summer and autumn becoming a significant management problem. Excessive growth can clog waterways, causing drainage issues and reducing agricultural productivity while in-stream velocities are reduced and sedimentation increased. Consequently, water managers remove them by mechanical clearance, chemical spray and cutting which can be costly and have negative impacts on in-stream habitat and ecological health. We trialled three tools to reduce macrophytes: hand-weeding, weed mat and artificial shading, at a reach-scale (50?m) and larger-scale (200–400?m). Hand-weeding reduced cover in the short-term, however macrophytes recovered to pre-treatment levels within one season. Weed mat along the banks was effective at reducing emergent macrophytes, particularly Erythranthe guttata (monkey musk) and Nasturtium microphyllum (watercress). Weed mat lasted for several growing seasons and continued to be effective. Shading over the waterway using polythene markedly reduced submerged macrophytes indicating that heavy shading by riparian plantings could reduce submerged macrophytes in the longer-term. These results indicate that in the short-term, weed mat could be used to limit sprawling emergent macrophytes. In the longer-term, weed mat used in conjunction with riparian planting could provide shading so that macrophyte cover in these small waterways could be reduced and controlled.