A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia

The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.     

Geochemistry and petrogenesis of basalts from the West Siberian Basin: an extension of the Permo–Triassic Siberian Traps, Russia

New major and trace element data for the Permo–Triassic basalts from the West Siberian Basin (WSB) indicate that they are strikingly similar to the Nadezhdinsky suite of the Siberian Trap basalts. The WSB basalts exhibit low Ti/Zr (50) and low high-field-strength element abundances combined with other elemental characteristics (e.g., low Mg#, and negative Nb and Ti anomalies on mantle-normalised plots) typical of fractionated, crustally contaminated continental flood basalts (CFBs). The major and trace element data are consistent with a process of fractional crystallisation coupled with assimilation of incompatible-element-enriched lower crust. Relatively low rates of assimilation to fractional crystallisation (0.2) are required to generate the elemental distribution observed in the WSB basalts. The magmas parental to the basalts may have been derived from source regions similar to primitive mantle (OIB source) or to the Ontong Java Plateau source. Trace element modelling suggests that the majority of the analysed WSB basalts were derived by large degrees of partial melting at pressures less than 3 GPa, and therefore within the garnet-spinel transition zone or the spinel stability field.

It seems unlikely that large-scale melting in the WSB was induced through lithospheric extension alone, and additional heating, probably from a mantle plume, would have been required. We argue that the WSB basalts are chemically and therefore genetically related to the Siberian Traps basalts, especially the Nadezhdinsky suite found at Noril'sk. This suite immediately preceded the main pulse of volcanism that extruded lava over large areas of the Siberian Craton. Magma volume and timing constraints strongly suggest that a mantle plume was involved in the formation of the Earth's largest continental flood basalt province.     

Sr and Nd isotopes as tracers of clastic sources in Lake Le Bourget sediment (NW Alps, France) during the Little Ice Age: Palaeohydrology implications

Geochemical methods (major elements and Sr, Nd isotopes) have been used to (1) characterize Lake Le Bourget sediments in the French Alps, (2) identify the current sources of the clastic sediments and estimate the source variability over the last 600 years. Major element results indicate that Lake Le Bourget sediments consist of 45% clastic component and 55% endogenic calcite. In addition, several individual flood levels have been identified during the Little Ice Age (LIA) on the basis of their higher clastic content (> 70%).Potential sources of Lake Le Bourget clastic sediments have been investigated from Sr and Nd isotope compositions. The sediments from the Sierroz River and Leysse River which are mainly derived from the Mesozoic Calcareous Massifs are characterised by lower ⁸⁷Sr/⁸⁶Sr ratios and slightly lower ?_Nd(0) ratios than the Arve River sediments which are derived from the Palaeozoic Mont-Blanc External Crystalline Massifs. The Rhône River appears to have been the main source of clastic sediments into the lake for the last 600 years, as evidenced by a similar Sr and Nd isotopic compositions analyzed in core B16 sediments (⁸⁷Sr/⁸⁶Sr = 0.719, ?_Nd(0) = − 10) and in the sediments of the Rhône River (⁸⁷Sr/⁸⁶Sr = 0.719, ?_Nd(0) = − 9.6).The isotopic signatures of flood events and background samples from core B16 in Lake Le Bourget are also similar. This indicates that prior to ∼ 1800, the inputs into the lake have remained relatively homogeneous with the proportion of clastic component mainly being a function of the palaeohydrology of the Rhone River. Early human modification (deforestation and agriculture) of the lake catchment before the 1800s appears to have had little influence on the source of clastic sediments.     

Microanalysis of carbon isotope composition in organic matter by secondary ion mass spectrometry

An analytical procedure has been developed for the in situ measurement of carbon isotope composition of organic matter, with a spatial resolution of 20-30 μm, using a Cameca IMS 1270 ion microprobe. Instrumental mass fractionation (IMF) of carbon isotopes was observed to be independent of primary ion beam intensity and sputtering time, but did depend on vacuum conditions and on the chemical composition of the sample. To evaluate such “matrix effects”, a set of 9 standards representative of the natural chemical variability of organic matter was prepared, with H/C atomic ratios and organic carbon contents (C_org) ranging between 0.04 and 1.74 and between 41 and 100 wt.%, respectively. Under the analytical conditions tested, IMF was not found to be influenced by the presence of silicate mineral impurities in the organic matter, but variations in IMF up to 5‰ were observed over the set of standards with the magnitude of IMF negatively correlated to the H/C ratios of samples. Aliphaticity ratios determined using Fourier transform infrared microspectroscopy provided an in situ estimation of H/C ratios with a spatial resolution barely exceeding that of the ion microprobe and permit a correction for matrix effects with a standard error of ± 0.2‰ (1σ). Taking into account all sources of uncertainty, ion microprobe δ¹³C were accurately determined with a ± 0.7‰ (1σ) total uncertainty. The mechanism for the matrix effect of H/C ratios upon IMF is still to be determined but it is likely related to the difference in proportion of atomic vs. molecular carbon ions observed between samples of different H/C ratios.     

Les placers à silice ultrapure français : étude gîtologique comparative

The comparative study of two French high-purity silica placers of European class suitable for the electrometallurgy of silicon and ferrosilicon, located at Boudeau and Thédirac, in the Dordogne and Lot departments respectively, indicates strong similarities as regards the sources and the transportation of quartz, as well as a same process of undercover karstification as a determining factor in the formation of the deposits. The main difference lies in the number of the stages necessary to the formation of the trap. This comparative study allows the constitution of dynamic models for the genesis of these economic quartz concentrations. To cite this article: L. Désindes et al., C. R. Geoscience 337 (2005).     

The identification of an active fault by a multidisciplinary study at the archaeological site of Sagalassos (SW Turkey)

The archaeological site of Sagalassos (SW Turkey) is located in a region characterized by the absence of any significant recent seismic activity, contrary to adjacent regions. However, the assessment of earthquake-related damage at the site suggests that the earthquakes that have been demonstrated to have struck this Pisidian city in ca. AD 500 and in the middle or second half of the 7th century AD are characterized by an MSK intensity of at least VIII and occurred on a fault very close to the city. Different investigation techniques (archaeoseismology, remote sensing and geomorphology, surface geology and structural data, 2D resistivity imaging and palaeoseismological trenching) have been applied at the archaeological site and its direct surroundings in search for the causative fault of these earthquakes. This multidisciplinary approach shows that each of the different approaches independently provides only partial, non-conclusive information with respect to the fault identification. Integration is imperative to give a conclusive answer in the search for the causative fault. This study has, indeed, revealed the existence of a to date unknown active normal fault system passing underneath ancient Sagalassos, i.e. the Sagalassos fault. A historical coseismic surface rupture event on this fault could be identified. This event possibly corresponds to the devastating Sagalassos earthquakes of ca. AD 500 and the middle or second half of the 7th century AD. Finally, this study demonstrates that in the particular geodynamic setting of SW Turkey archaeological sites with extensive earthquake-related damage form an important tool in any attempt to asses the seismic hazard.     

Seismicity wedge beneath the Upper Rhine Graben due to backwards Alpine push?

The distribution of hypocentres in the Upper Rhine Graben area is re-examined, and discussed with respect to the present day tectonic framework. Most earthquakes occur within a N60° striking wedge, located on top of a Moho dome. This wedge is limited by the surface and at depth, by a plane which, in the south of the dome, coincides with the SE dipping Conrad discontinuity. In depth, the seismicity shows a normal distribution the maximums of which are located on a surface dipping 6° towards SE, parallel to the south-eastward dipping Conrad and Moho. This surface outcrops along the north-western edge of the uplifted crystalline Vosges and Black-Forest. The main shocks in earthquake swarms in the region often occur in the vicinity of this surface and along pre-existing N–S to NE–SW Variscan or Tertiary faults and show focal mechanisms of strike-slip. In contrast, part of the aftershocks show focal mechanisms of reverse faulting associated with SE–NW striking compression. The seismic wedge and the north-westward rising seismic surface suggest initiation of crustal ramp, which starts at the south-eastern rim of the Conrad dome and which may become a thrust plane if SE–NW compression continues. In the south-eastern edge of the graben and above the south-eastern ridge of the Moho dome, where evidences for extension have been found, we identify clustering of hypocentres along a surface that strikes N150°, parallel to the main compression and dipping towards NE. Dominant normal faulting mechanisms along this surface suggests initiation of a normal, probably listric fault. At depth, the onset of the future fault plane is located on top of the NW–SE striking ridge of the lower crust and Moho, which act as a an indenter. In addition to thrusting of the whole wedge towards NW, increasing of NW–SE compression would lead to the formation of a half graben at the place of the present Sierentz depression.     

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Inference of the structure of karst conduits using quantitative tracer tests and geological information: example of the Swiss Jura

Karst aquifers are known for being particularly heterogeneous with highly transmissive conduits embedded in low permeability volumes of rock matrix. Artificial tracer experiments have been carried out in a complex karst aquifer of the folded Jura Mountains in Switzerland with the aim of deciphering the conduit organisation. It is shown that tracer experiments with multiple injection points under different flow conditions can lead to useful information on the conduits’ structure. This information has been combined with data from structural geology, spring hydrology, and speleological observations. A conceptual model of the conduit network shows that a detailed inference of the conduit organisation can be reached: geology controls conduit location and orientation; spring hydrology, including temporary springs, constrains conduit elevations and relative hydraulic heads in the aquifer subsystems; and tracer tests identify major flow paths and outlets of the system and dilution caused by non-traced tributaries, as well as the presence of secondary flow routes. This understanding of the Aubonne aquifer structure has important implications for the future management of the groundwater resource. Similar approaches coupling geological information, spring hydrology, and multi-tracer tests under various flow conditions may help to characterise the structure of the conduit network in karst aquifers.