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International audience; Chars are useful proxies in many disciplines such as the geosciences and archaeology. Raman spectrometry is frequently used in the characterization of such materials. However, when they are ancient and potentially altered, chars have the particularity of being oxidized, which can strongly disturb the spectra and seriously distort their interpretation. In this study, we highlight the Raman parameters affected by oxidative weathering based on comparisons between the spectra of experimental and ancient chars, of vegetal and animal origin. This makes it possible to differentiate between

International audience

The study of geomagnetic excursions is key for understanding the behavior of the magnetic field of the Earth. In this paper, we present the geomagnetic record in a 2.29-m-long continuous core sampled in a flowstone in Liguria (Italy) and dated to the Lower Brunhes. The cored flowstone developed from Marine Isotopic Stage (MIS) 13 to MIS 7, according to 21 U-series dates. The mean growth rate is closely related to glacial and interglacial isotopic stages. Magnetic remanence was measured using u-channel and deconvolved. Four geomagnetic excursions were recorded at the same location, in a single flowstone, during interglacial MIS 11 and 13; Basura 1, 2, 3 and 4, at depths of 213 cm, 181, 160 and 92 cm, respectively. Due to the uncertainties of U-Th dating, the timing of the three events, namely Basura 1, 2 and 3 overlaps. The Basura 4 is well-dated to 417 + −7/8 ka and is clearly distinguishable from the others. It should therefore be considered as a possible excursion.

U-series disequilibrium measurements carried out on thermogenic travertine samples from a 12.6 m-long core and a 10 m-thick section from southeastern Morocco yielded finite ages ranging from 500 ka to the present-day, as well as two clusters determined to be older than 500 ka. The calculation of initial 234U/238U activity ratios in all samples younger than 500 ka shows high, reasonably constant values, with an average of 5.172 ± 0.520 (one standard deviation). Assuming that this value prevailed for periods older than 500 ka, we derived ages of up to approximately 1.2 Ma using the initial 234U excess decay. Our results indicate that the two older clusters have ages of 776 ± 14 ka for samples from between 8 and 10.1 m and 1173 ± 22 ka for deeper samples respectively. The palaeomagnetic record of the core shows normal polarity inclinations from the surface to around 9 m followed by reverse polarity inclination and antipodal declinations. The inversion is attributed to the Brunhes-Matuyama transition. 234U excess ages for the interval corresponding to the part of the core where the polarity inversion occurred are in the range of 735 ± 51 to 794 ± 54 ka, with an arithmetic mean value of 776 ± 14 ka for the B-M transition. This age is in good agreement with that determined previously using other dating methods.

Resume Etablir un cadre chronologique precis commun pour l’ensemble des archives climatiques reste un defi important. Afin de modeliser les differents mecanismes qui gouvernent les variations climatiques en Afrique du Nord-Ouest, il est indispensable d’etablir avec rigueur la chronologie des depots calcitiques (speleothemes et travertins) de ces archives continentales et de caler les enregistrements dans une echelle stratigraphique isotopique basee sur les variations des isotopes de l’oxygene. Les archives travertineuses sont potentiellement datables par de nombreuses methodes de datation dont les plus performantes sont le carbone 14, les series de l’uranium, la resonance de spin electronique et la stratigraphie magnetique (methode de datation indirecte). Les resultats des datations obtenues sur des travertins hydrothermaux ou fluviatiles preleves sur 24 sites repartis sur l’ensemble du territoire marocain sont presentes. Les sites ou l’occupation humaine est attribuee aux premieres periodes du Pleistocene moyen sont privilegies. Les principaux resultats montrent que les depots travertineux, independamment de leur origine, se sont principalement formes a partir de la fin du Pleistocene inferieur, aux alentours de la « Middle Pleistocene Transition » (MPT), qui s’est mise en place entre 1,2 et 0,7 Ma, jusqu’a l’Actuel. La deuxieme information majeure montre que les periodes de croissance de ces formations travertineuses se sont developpees aussi bien au cours des stades isotopiques pairs qu’au cours des stades isotopiques impairs. Cette relation est etablie a la fois dans les travertins hydrothermaux et fluviatiles, a partir du debut de la periode Brunhes grâce aux donnees paleomagnetiques. Dans ce travail, nous presentons principalement les resultats obtenus a partir des methodes des series de l’uranium et de la magnetostratigraphie.

There is growing evidence that the magnetic assemblage of claystones, illustrated by low-temperature magnetic transitions at ~120 K and ~35 K, may be representative of the peak burial temperature in the so-called oil-window (60-150°C). In previous studies, it was proposed that this magnetic assemblage is characterized by fine-grained pyrrhotite (Fe7S8) and magnetite (Fe3O4). However, evidence of pyrrhotite was not so obvious and the magnetic transition near 35 K of fine-grained pyrrhotite has similarities with those of siderite (FeCO3) or rhodochrosite (MnCO3). Here, we propose some diagnostic tests to distinct

Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer. In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m − 1 /day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m − 1 /day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m − 1 /day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.

SUMMARY The Siberian traps rank among the largest flood basalt provinces in the world. Major magmatic activity is restricted to several short periods of less than 1 Myr around the Permo-Triassic boundary. We measured the anisotropy of magnetic susceptibility (AMS) of 183 basaltic samples from 28 individual lava flows belonging to the northwestern part of the flood basalt province, in order to test the reliability of such data in structural interpretation. These preliminary AMS data show a remarkably consistent pattern of magnetic lineations, that likely reflect the primary flow-related texture of the basalt. Our results also emphasize that AMS data are consistent with the well-documented petrological and geochronological data. The inferred flow directions are also consistent with flow directions expected from isopach maps. Our results suggest that the entire 2 km-thick lava pile was fed by fissure eruptions located in the rifted basin that bound the Siberian platform, and was controlled by topographical features, particularly major rift zones.

A detailed palaeomagnetic and magnetostratigraphic study of the Permian–Triassic Siberian Trap Basalts (STB) in the Noril'sk and Abagalakh regions in northwest Central Siberia is presented. Thermal (TH) and alternating field (AF) demagnetisation techniques have been used and yielded characteristic magnetisation directions. The natural remanent magnetisation of both surface and subsurface samples is characterised by a single component in most cases. Occasionally, a viscous overprint can be identified which is easily removed by TH or AF demagnetisation. The resulting average mean direction after tectonic correction for the 95 flows sampled in outcrops is D=93.7°, I=74.7° with k=19 and α95=3.3°. The corresponding pole position is 56.2°N, 146.0°E. Unoriented samples from four boreholes cores in the same regions have also been studied. They confirm the reversed–normal succession found in outcrops. The fact that only one reversal of the Earth's magnetic field has been recorded in the traps can be taken as evidence for a rather short time span for the major eruptive episode in this region. However, there is evidence elsewhere that the whole volcanic activity associated with the emplacement of the STB was much longer and lasted several million years.

Hematite reduction in the presence of pyrite and CaO led to the formation of anhydrite and magnetite at 30 MPa and 723 K under hydrothermal conditions in a sealed gold container. For the first time, hydrothermal magnetite production was monitored magnetically at the run conditions. Sample magnetic susceptibility was found to follow an Avrami kinetic law with k = 1.5 10 −5 /s and n = 2.07. Application of up to two external field reversals in the course of the magnetite production reaction shows that partial Chemical Remanent Magnetizations (CRM) are recorded by the sample. The intensity of each partial CRM acquired within a given time interval (under a given field intensity) is proportional to the amount of magnetite produced during that time interval. Alternating magnetic field demagnetization failed at separating these partial CRMs, which are believed to superpose in the ( V , Hc ) diagram of Neel. Additional TEM, SEM, and magnetic characterizations suggest that continuous nucleation and fast magnetite growth (to a maximum size of ca. 10 μm), compared to the field polarity duration, can account for the evolution of the sample CRM with time. This study demonstrates that one should be cautious when using natural CRM apparent polarity to infer timing of the corresponding chemical event.

International audience; The thermal evolution of sediments in the shallowest part of sedimentary basins is challenging to investigate as most of the geothermometers are at their application limits. This is typically the case of the Callovian-Oxfordian claystones at the eastern border of the Paris Basin. A recent 2000m deep well penetrated the entire Jurassic and Triassic sedimentary series and permitted coring of sediments never reached by previous drilling in this part of the basin. Thanks to the use of several independent geothermometers (apatite fission track thermochronology, fluid inclusion microthermometry, clay mineralogy and sedimentary organic matter evolution) the burial history of the Mesozoic series has been reconstructed, modeled and calibrated using kinetic algorithms relevant to the various paleothermometers used. In addition to these conventional methods, an innovative approach was developed using the temperature-dependence of the magnetic mineral assemblage within clay-rich sediments ("MagEval"). Thermal modeling indicates that the Callovian-Oxfordian and the Lower Triassic experienced maximum burial temperatures of 50±5°C and 90±10°C respectively, i.e., about 25°C higher than present-day. This temperature offset implies the erosion of several hundred meters of sediments which, depending on the thermal flux and conductivity considered, most likely constituted Upper Cretaceous chalk deposits. Falling sea level resulted in the exposure and subsequent dismantling of the chalk cover at the beginning of Cenozoic times. The eastern border of the basin was then subjected to slow erosion and discrete tectonic deformation.

The role played by plume-generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the East Greenland volcanic margin. We investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the Tertiary opening of the North Atlantic Ocean. The thickness of the sampled dykes ranges from 3 to 37 m. The dykes are of doleritic texture and contain up to 12% of opaque minerals and 35% of plagioclase laths. Dykes showing a magnetic foliation plane within the dyke plane, i.e. a magnetic fabric usually attributed to magmatic processes, represent 40% of the samples from the studied swarm. These dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate. Inverse magnetic fabrics, where the maximum principal susceptibility axis lies near the pole to the dyke account for 60% of the data. We interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy. The flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation. Analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations. The inferred flow directions are predominantly horizontal, throwing a new light on volcanic margin development.

The French Cadomian belt (northern Armorican Massif) constitutes an excellent area for studying Panafrican orogenic processes in Europe for which geophysical methods are useful for obtaining a realistic interpretation of the geological structures at depth. Magnetic modelling of the area to the west of Saint-Brieuc Bay was carried out to determine the geometry of the Cadomian geological units. Modelling was computed along profiles extracted from the regional aeromagnetic map. The shape of the magnetic bodies was determined with the help of interpretative geological sections and the total magnetization of the magnetic bodies was determined from a data base of about 350 rock sample measurements. The measured susceptibility (χ) ranges from 0.15×10 −3 SI to 221.5×10 −3 SI (the induced magnetization IM =40 A/M ×χ. ) The natural remanent magnetization (NRM) is weak and shows varied directions; it is therefore negligible at the formation scale, and only the magnetic susceptibility was taken into account for modelling. A computed mean IM was attributed to each magnetic body in the model. Measurements of the magnetic properties and thin-section observations of field samples show that magnetite is the main carrier of the magnetization. The strong susceptibilities are consistent with ferromagnetic behaviour of multidomain grains, whereas the weak susceptibilities accord with dominantly paramagnetic behaviour. Sample measurements show that the Saint-Quay intrusion, the acid part of the Lanvollon Formation and one of the three lenses of Squiffiec metagabbro are the most highly magnetic formations. Magnetic modelling made it possible to estimate the long-wavelength fold shape of the Binic formation. The basin, within this formation, reaches about 2.0–2.5 km depth. The modelling also indicates that the Saint-Quay intrusion cuts the Binic basin at its centre with vertical contacts. Modelling shows that the intrusion is apparently composed of two imbricated magnetic bodies in its eastern part, and of homogeneous magnetic bodies in its western part, which is consistent with the geological observations. The Lanvollon Formation exhibits an heterogeneous magnetic behaviour consistent with its lithology of intercalated acid and basic metavolcanic bodies. Magnetic modelling indicates a difference in thickness of the acid part of the Lanvollon Formation between the profiles of the Binic basin area and those of the Squiffiec–Plouha area; this we interpret as reflecting an initial variation in the thickness of the acid volcanic bodies. The Binic and the Squiffiec–Plouha profiles show that the southern boundary of the basic part of the Lanvollon Formation is probably vertical or steeply dipping to the north.

International audience; Streaming potential and electric conductivity have been measured in a laboratory on 11 consolidated samples coming from five deposits of the different evolutionary stages of Mount Pelde volcano. The streaming potential coupling coefficient ranges from-35 to-4905 mV MPa-1 and increases with increasing permeability. This increase is mainly due to the dependency of rock effective conductivity with permeability. The permeability of the samples varies from 0.146x10-12 to 34x10-12 m 2. The zeta potential, at pH-7 and water conductivity of 2.1x10-4 S m-l• is relatively small for the majority of the samples. It ranges from-4 to-19 reV. According to water conductivity analysis on Mount Pel•e, streaming potential coupling coetficients of-25 to-406 mV MPa-1 can be expected for this volcano.

We present a preliminary study of magmatic flow in dykes of San Jorge Island, Azores, using anisotropy of magnetic susceptibility (AMS). Oriented samples were taken from dyke margins. All dykes show well-grouped minimum susceptibility axes near the pole of the dykes. In addition magnetic foliation planes present an imbricated arrangement relative to each dyke margin. Magmatic flow direction is determined independently, from each margin, and is the result of relative orientations of the magnetic foliation plane and the dyke margin plane. Digital processing of thin sections has shown that maximum and intermediate susceptibility axes may be correlated with alignments of large crystals. Moreover, in most cases, orientation of the intermediate susceptibility axes is in agreement with the flow sense derived from analysis of the imbricated magnetic foliation planes. Overall results show a predominance of horizontal or sub-horizontal flow over vertical flow.

A new fossil locality is reported from the argiles a lignite du Soisonnais (Early Ypresian, MP7) of the Oise region (France). After the preliminary survey of the flora and the vertebrate and arthropod faunas, we propose a reconstruction of a fluvio-lacustrine palaeoenvironment with a forest, under a warm and wet seasonal climate. This site is outstanding because of the richness, diversity and the state of preservation of the fossils. The present discovery opens a unique window on terrestrial life during the Earliest Eocene.

SUMMARY Precise time and facies correlations between drilled holes are fundamental for a better understanding of the geological evolution of sedimentary basins. A downhole magnetic measurement device called the geological high-sensitivity magnetic tool (GHMT) has been run within two wells drilled by Gaz de France in the Landes oil-field (southwest France) as part of a gas storage exploration program. The method of interpretation of downhole magnetic measurements yielded a magnetostratigraphy within each well, allowing absolute dating and time correlations between the wells. Magnetic susceptibility and natural gamma ray intensity are useful parameters for establishing high-resolution lithological correlations at a basin scale. We present a correlation parameter established from a simultaneous analysis of the susceptibility and the gamma ray logs within each well. The correlation parameter appears to provide a new tool for delineating lithological elements when local lithological changes are too subtle to show clear well-to-well correlations either from susceptibility logs or from gamma ray logs. This new approach is interpreted as a sensitive way to detect relative variations between the detrital and clay content of the penetrated sediment.

High-resolution magnetic records have been obtained by downhole measurement with the Geological High Resolution Magnetic Logging Tool (GHMT) in a well drilled in the North Sea for hydrocarbon exploration. This tool enables a continuous measurement of the polarity of the magnetic remanence and of the magnetic susceptibility throughout the drilled formations. A high-resolution magnetic chronostratigraphy between 2.6 and 1.6 Ma is deduced from the tie points given by the downhole magnetostratigraphy and from a detailed spectral analysis of climatically influenced variations of susceptibility. The orbital frequencies identified by spectral analysis of the magnetic susceptibility in the depth domain allow a refined estimation of the sedimentation rate and determination of a high-resolution chronology of the sequence. Characteristic features of the onset of Northern Hemisphere glaciation and its fluctuations during the Late Pliocene are identified through this analysis of the susceptibility record. Our analysis indicates that massive discharge events of non pelagic deposits — similar, but with a higher amplitude, to Heinrich events that are observed during the last glaciation — occurred between 2.5 and 2.0 Ma and were probably related to instabilities of the Laurentide ice sheet. The relative power of the orbital forcing factors is compared with that documented at other North Atlantic Ocean sites. Comparison with an insolation model does not indicate direct correlation but suggests a strong influence of the Northern Hemisphere ice cover and its local variations.

The palaeomagnetic studies carried out on Permian to Triassic formations from the north-west part of the Great Caucasus and from southwest Turkmenistan question the existing palaeogeographic models of evolution of this Central Asia region. The impressive differences in latitude between the Scythian and Turan plates, and the East European Platform during the Permo-Triassic, lead us to suppose that the Scythian-Turan blocks moved away from Eurasia during the Permian in a large transversal westward movement. The docking of this block onto the southern border of Eurasia probably occurred at the end of the Triassic.

SUMMARY The deep drill hole SG6 in western Siberia (66°N, 78.5°E) penetrated 1.1 km of lower Triassic basalts, which are possibly an extension of the central Siberian Permo‐ Triassic flood basalt province. About 300 samples of these basalts were progressively demagnetized and measured. Principal component analysis often shows multiple magnetizations carried by haematite and magnetite. The corrected mean inclinations are +77° and ’77° for the haematite component. A magnetostratigraphic scale was derived and showed a N‐R‐N‐R‐N succession. This is quite diVerent from the Noril’sk and Taimyr typical polarity scale, R‐N. The basalts found in the SG6 deep drill hole are slightly younger than those of central Siberia and Taimyr. They correspond to middle‐upper Induan age, whereas the Noril’sk and Taimyr sections correspond to an uppermost Permian and lower Induan age. Altogether they indicate that, after a high output rate of volcanic material near the Permo‐Triassic boundary, this activity slowed down drastically on the Siberian platform and Taimyr, but persisted for several million years in western Siberia.

Paleomagnetists dealing with deep drill samples cannot use the classical Fisher statistics when they don't know the declinations of the magnetization of their samples. The arithmetical mean of the inclinations gives a strongly biased estimation of the true mean inclination when these inclinations are steep and scattered. By fitting cumulative distribution curves, we show that we can obtain an unbiased (or at least less biased) estimation of the true mean inclination and of the k value. The method is tested on artificial and real cases.

Streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments (Fontainebleau sandstones). The streaming potential ∃V was found to be proportional to the driving pore pressure ∃P. Pulses of amplitude 15-40 mV in the frequency range of 0.1 to 0.5 Hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low, corresponding to a Darcian velocity of 17 to 30 cm/h. The amplitudes of these pulses are 47% to 133% of the corresponding steady components of the ∃V values. Such geochemically induced effects may possibly be responsible for the frequency signals from 0.1 to 0.5 Hz that were sometimes observed before an earthquake.

Calc-alkaline volcanic deposits from the south-west of the Turan plate, near the city of Turkmenbasi (40°00'N, 52°58&E) in Turkmenistan, were studied paleomagnetically. These rocks have been affected by a greenschist-facies metamorphism, possibly of regional extent, that has been K/Ar-dated as 200 to 227 Ma old. A low-blocking-temperature component (D = 349°, I = 64°), close to the present field direction and probably of viscous or recent chemical origin, was isolated by a negative fold test at three sites. The mean direction of a high-blocking-temperature component isolated at 15 sites, mainly carried by magnetite, is scattered before and after tectonic correction and is therefore difficult to interpret. A group of seven sites with low inclinations before and after tectonic correction was isolated. The mean inclination of these sites (31 ± 8°), syn-folding or post-folding, corresponds to a paleolatitude of 17 ± 8° which is lower than the conventional Eurasian paleolatitudes for post-Permian times. The paleomagnetic data from the Turan and Iran plates constrain this low paleolatitude to the Late Triassic and Jurassic period. This requires a shortening of at least 7° between the Turan plate and Eurasia during this time.

A continuous record of the geomagnetic field intensity from 4.7 to 2.7 Ma has been obtained, together with a precise magnetostratigraphy, from downhole magnetic measurements at Site 884 of ODP Leg 145 in the North Pacific. The record confirms the saw-tooth pattern of geomagnetic field intensity proposed by Valet et Meynadier [10]. Reversals are characterized by a steep intensity decrease followed by a quick regeneration. Over each polarity interval, rapid variations are superimposed over a progressive decrease in the mean intensity. We find that the duration of each polarity interval is inversely proportional to the mean rate of decrease in the field intensity over this period, and thus this duration seems to be pre-determined.

The Siberian traps were emplaced near the Permo-Triassic boundary and are among the largest continental igneous provinces. A 1750 m thick section of traps from the western Taimyr (72.9°N, 84°E) has been sampled. This section is equivalent to the lower and middle part of the total Siberian traps section. Thirty-one oriented hand samples were studied and demagnetised by alternating fields and heating cycles. A stable characteristic component passes the fold test and is thought to be primary. The mean direction is D = 84 °, I = 75 ° with α 95 = 10 ° ( N = 29) and the corresponding pole is 59°N, 150°E. The bottom part (660 m) is of reversed polarity, the middle part (900 m) is normal and the uppermost level (150 m) is mixed. Correlations with reference Permo-Triassic magnetostratigraphic timescales are discussed. The most probable correlation indicates an age quite close to the Permian/Triassic boundary and a duration for the volcanism of less than 1 m.y. The data are therefore consistent with the hypothesis that this volcanism triggered the Permian/Triassic crisis.

Geological processes, such as burial, can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume, called blocking volume. In this study, we designed a heating experiment for claystones obtained from the Paris Basin (France), in the 50-130°C temperature range, in order to simulate 20 nm and ∼20 nm minerals respectively. This result led us to propose a conceptual model of nucleation-and-growth process of magnetite during low grade burial from ∼2 to ∼4 km depth. Ultrafine magnetite (≤20 nm) was predominant over single domain magnetite (>20 nm) for

International audience; We investigate the broad lines of magnetic mineral formation for non-metamorphic claystones. More particularly, we focus on the formation of magnetite from c. 20 day-experiments aiming to reproduce burial conditions. It is shown that the sole action of temperature from 50 °C to 250 °C leads to the formation of magnetite. The neoformed magnetites carry a chemical remanent magnetization which equates at least the natural remanent magnetization. We propose a schematic of burial with three magnetic windows where a chemical remanent magnetization superimposed the natural remanent magnetization. These are the greigite window (subsurface), the magnetite window (depth > 2 km) and the pyrrhotite window (depth > 6 km). The formation of magnetic minerals has profound consequences for the magnetostratigraphy record. We propose a conceptual model that shows that the continuous production of magnetite during burial may result in magnetozones that have no relation to the age of the sediment.

Permeability is the critical factor governing fluid flow in accretionary prisms. Accretionary wedges are highly deformed, so permeability changes in an indurated claystone sample from the Nankai accretionary prism were measured during a triaxial stress experiment by the pulse decay method. Three zones were identified from the loading test. In zone I, the sample has deformed less than 1.3% and its permeability was 5.10−20 m². In zone II, the deformation reached 1.5% and the permeability was unstable with time. In zone III, the sample deformation reached 2% and its permeability reached 3.8 10−18 m². The permeability strongly decreased (from 3.2 10−18 m² to 1.7 10−19 m²) with increasing effective pressure (from 2.3 to 10.8 MPa) after the sample was well deformed (zone III). The streaming potential was not measurable when the sample had a low permeability (zone I), but clearly occured when the permeability increased (zone III) : the streaming potential measured was 6 mV when the pore pressure pulse applied was about 1.17 MPa and the permeability 3.8 10−18 m². Our experiments suggest that shear deformation under low effective pressure increased the vertical permeability of sediments above the decollement. This increase in permeability may be detected by measuring the streaming potential. Variations of flow rate of expelled fluid in accretionary wedges may be detected by monitoring changes of electrokinetic potential, giving new insights on the state of stress related to the seismic cycles.

The magnetostratigraphy of weakly magnetized sediments is determined for the first time from direct downhole measurements. Experimental results were obtained in both scientific and oil-exploration drillholes penetrating Jurassic sediments of the Paris basin, France. The magnetic polarity of the remanence is deduced from the total magnetic induction, combined with the magnetic susceptibility. The magnetic induction is measured in depth by a Proton Precession Magnetometer with Overhauser effect designed by the Laboratoire d'Electonique de Technologie et Instrumentation (Commissariat a I' Energie Atomique). This method gives a continuous record with an accuracy of 0.1 nT, and an excellent stability. In order to cancel out the transient component of the Earth's magnetic field, the magnetic induction is computed as the difference between the induction measured in the borehole and that obtained at a surface reference point located near the drillhole. The dipole-dipole susceptibility probe gives a continuous record with an accuracy of a few 10−6. The four total magnetic induction logs recorded in oil exploration holes show a tight correlation, although distant from up to 80 km. This technique is therefore excellent for precise stratigraphic correlation. Data processing allows us to isolate the component of induction due to remanent magnetization and to find the magnetic polarity. A sequence of magnetic polarity is proposed for Kimmeridgian and Oxfordian stages. The variation of the sedimentation rate is analyzed with a very high resolution in time, and compared with the mean sedimentation rate deduced from a lithologic log. Cores were collected in the lowermost part of two holes. The correlation between the magnetostratigraphy deduced from paleomagnetic measurements performed on 240 standard plugs and that obtained by logging is good. A study of the components of magnetization found in the core has shown that the remagnetization occurred mainly during the coring operation itself or during the storage of the core. Consequently, downhole magnetostratigraphy proves to be reliable, besides providing the advantage of a continuous investigation performed over a large volume of rock scrutinized in situ.

Dans les régions arides, la datation des travertins conduit à établir une chronologie de périodes humides. Des travertins de cascade, fluviatiles et hydrothermaux ont été échantillonnés à la bordure nord du Sahara marocain et datés par la méthode des isotopes de l’Uranium, à la fois par spectrométrie alpha et spectrométrie de masse (TIMS). Les résultats indiquent que les constructions de travertins se produisent avant le stade isotopique marin (SIM) 11, pendant le SIM 9-8, secondairement durant le SIM 5 et principalement durant le SIM 3 et le SIM 2. La relation entre les travertins et les phases humides se vérifie à l’échelle du Quaternaire. À l’échelle plus détaillée des périodes récentes, les constructions s’effectuent autour de 50, 30 et 15 ka BP, apparemment en relation avec des refroidissements de l’Atlantique subtropical oriental. Mais le dispositif géomorphologique induit dans chaque cas des différences dans la construction travertineuse : réponse plus tardive des tertres hydrothermaux à l’accroissement pluviométrique, aridification plus précoce des travertins fluviatiles. Les travertins des régions arides continentales sont ainsi des dépôts discontinus incluant des réponses climatiques à la fois planétaires et régionales. In arid countries, the dating of travertines has led to establish a chronology of humid phases. Waterfall, fluvial and hydrothermal Quaternary travertines were sampled at the northern border of the Moroccan Sahara and dated by the U-series and TIMS methods. The results indicate that travertine formation occurred prior to MIS 11, during MIS 9-8, scarcely during MIS 5, and mainly during MIS 3 and MIS 2. The relationship between travertines and wet phases is verified at the Quaternary time scale. At a short recent scale, the constructions occurred about 50, 30 and 15 ka BP, seemingly linked to subtropical eastern Atlantic cooling. But the geomorphological setting induces in each case differences in travertine growth, such as a later response of the hydrothermal mounds to the rainfall increase, or an earlier drought in the fluvial deposits. As a result, travertines are discontinuous deposits in arid continental environments, including both global and regional distinct signals.

International audience; We investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation: immature (burial temperature ~40°C) of Bure Callovo-Oxfordian claystones in the Basin of Paris (France); early mature (burial temperature ~85°C) of Opalinus Lower Dogger claystones from the Mont Terri anticline in front of the Jura fold belt (Switzerland); and mature to overmature (burial temperature

The Magsat data have yielded a global cartography of long-wavelength anomalies of lithospheric origin. The magnetizations needed to model these anomalies are much higher than those measured on samples by laboratory experiments. An enhancement of both induced and viscous magnetization at high temperature has often been put forward to account for the “missing magnetization”. Experimental measures of viscous magnetization Mv at high temperature are presented for oceanic deep crustal- and mantle-rocks. At room temperature, Mv hereafter defined in experiments as the magnetization acquired after 400 s in a weak field, is negligible in comparison with the room temperature induced magnetization Mk0. At higher temperatures, between 200°C and 450°C, Mv represents about 20% of Mk0 and is further enhanced near 500°C, ranging from 25 to 130% of Mk0. An extrapolation of these results to geological times shows that the viscous magnetization acquired since the last reversal of the Earth magnetic field may be of the order of magnitude of the induced magnetization Mk0, above 200°C and possibly predominant near 500°C. However, a magnetic uppermost mantle is still needed to explain the magnetizations of oceanic lithosphere derived from Magsat data.

A stable and well defined magnetization withD = 350°,I = 55.5° andα95 = 4°, carried by hematite, is isolated in the red beds of the Tajik basin. The fold test is positive (99% probability) and this magnetization predates the Late Eocene-Early Oligocene folding phase in the Tajik basin. The coordinates of the VGP are: 82°N, 323°E,dp = 4,dm = 6 with a paleolatitude of Dushanbe (38°38N, 68°51E) of35.6 ± 5°. A high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of Ragoon, but it has not been isolated. This component predates the Long Cretaceous Normal Period and is compatible with the older stratigraphic age attributed to the formation (Valanginian); these arguments indicate that this component is likely of primary origin. The other component, of higher intensity and clearly isolated in all the sites, is a pre-folding remagnetization. The process of remagnetization is attributed to the tectonic events, such as fluids migration and slight heating during a few million years, which took place just before the first Late Eocene-Early Oligocene phase of folding An anticlockwise rotation of 26° encountered by the region of Dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation West of the Indian wedge and clockwise rotation to the east. A comparison of the paleolatitude of Dushanbe, computed from the direction of magnetization, and the paleolatitude expected from the apparent polar wander path for Eurasia, gives an estimate of the post-collision continental shortening north of the Tajik basin. This shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km. An estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of Eurasia at 45 Ma is proposed. The paleolatitude deduced from the prefolding remagnetization of the Red Beds of the Tajik basin is then compared with the results obtained by Soviet workers in Tertiary sediments of the same area, revealing strong contradictions in interpretation. This contradiction emphasizes the problem of the rigidity of European plate.

We acknowledge the Geophysical Journal International and the Association/Society and Blackwell Publishing. The definitive version is available at www.wileyinterscience.com. The reference is : Bordes, C., L. Jouniaux, S. Garambois, M. Dietrich, J.-P. Pozzi, and S. Gaffet, Evidence of the theoretically predicted seismo-magnetic conversion, G.J.I., 174, issue 2, 489-504, doi:10.1111/j.1365-246X.2008.03828.x; International audience; Seismo-electromagnetic phenomena in porous media arise from seismic wave-induced fluid motion in the pore space, which perturbs the equilibrium of the electric double layer. This paper describes with details the original experimental apparatus built within the ultra-shielded chamber of the Low Noise Underground Laboratory of Rustrel (France). We measured seismo-magnetic conversions in moist sand using two induction magnetometers, and a pneumatic seismic source to generate the seismic wave propagation. We ensured to avoid the magnetometer vibrations, which could induce strong disturbances from induction origin. Interpretation of the data is improved by an analytical description of phase velocities for fast (P_f) and slow (P_s) longitudinal modes, transverse mode (S) as well as the extensional mode due to the cylindrical geometry of the sample. The purpose of this paper is to provide elements to measure correctly co seismic seismomagnetic fields and to specify their amplitude. The seismic arrivals recorded in the sample showing a 1200$-1300 m/s velocity have been associated to P and extensional waves. The measured seismo-magnetic arrivals show a velocity of about 800 m/s consistent with the calculated phase velocity of S waves. Therefore we show that the seismo-magnetic field is associated to the transverse part of the propagation, as theoretically predicted by Pride (1994), but never measured up to now. Moreover, the combined experimental and analytical approaches lead us to the conclusion that the measured seismo-magnetic field is probably about 0.35 nT for a 1 m/s2 seismic source acceleration (0.1 g).

Paleomagnetic analysis of marine formations in the northwestern Sahara (Algeria) yields the following results: three sites in the lower Devonian Orthoceras-bearing limestones of Beni-Abbes (29.67° N, 2.15° W) indicate a mean direction of remanence: Dm = 149.6°; Im = 1.6°; α95 = 6.8°. This typical Permian direction implies a complete remagnetization of the sites. The Famennian griotte limestones of the Ben-Zireg anticline (31.92° N, 1.82° W) show high remanent magnetization intensities. In this structure, which has undergone polyphase tectonism, multicomponent analysis evidences a high unblocking temperature (HTb) component carried by haematite leading to a positive fold test (after partial unfolding) at the 95% confidence level with a mean direction D = 156.4°, I = 47.6°, α95 = 0.9° and attributed to the Early Carboniferous (355 Ma) with a paleopole at 25.3°S, 21.1°E, dp = 0.8°, dm = 1.2°. An intermediate unblocking temperature (ITb) component of magnetization carried probably by magnetite, also present, satisfies a positive fold test (after tilt correction), with a mean direction D = 154.9°, I = 54.3°, α95 = 3.7°. This pre-folding direction of magnetization is attributed to the Famennian. It gives a paleopole at 19.2°S, 19.8°E, dp = 3.7°, dm = 5.3°. Comparison with previous results shows a significant S-N crustal shortening of about 2500 km during Carboniferous times accompanied by a clockwise rotation of the Saharan region of about 25°. These new results give information about the tectonic phenomena involved in the collision between Gondwanaland and Laurasia: they argue for a Pangea A2 dominating model, and define mean values for shortening rate and plate velocity comparable to the present ones (3 cm/yr).

SUMMARY The aim of the Couy experiment in the 'Gtologie Profonde de la France' programme was to reach the source body of the magnetic anomaly of the 'Bassin de Pans'. The drilling stopped at 3500 m. Magnetic field logging and susceptibility logging were undertaken to obtain as complete a description as possible of the magnetic parameters of rocks crossed by the borehole. Remanent magnetization and susceptibility measurements on the core were also carried out. The entire set of results is presented in this paper for both sedimentary layers and the basement. The values obtained for magnetic field variations and for susceptibility lead to the conclusion that the magnetic source body of the AMBP was not reached.

[1] Seismic wave propagation in fluid-filled porous materials induces electromagnetic effects due to small relative pore-fluid motions. In order to detect the seismo-magnetic couplings theoretically predicted by Pride (1994), we have designed a small-scale experiment in a low-noise underground laboratory which presents exceptional electromagnetic shielding conditions. Our experiment included accelerometers, electric dipoles and induction magnetometers to characterize the seismo-electromagnetic propagation phenomena. To assess the electrokinetic origin of the measured electric and magnetic fields, we compared records obtained in dry and fluid-filled sand. Extra care has been taken to ensure the mechanical decoupling between the sand column and the magnetometers to avoid spurious vibrations of the magnetometers and misinterpretations of the recorded signals. Our results show that seismo-electric and seismo-magnetic signals are associated with different wave propagation modes, thus emphasizing the electrokinetic origin of these effects.

Suite aux résultats de l’étude préliminaire réalisée sur l’affleurement de travertin d’Irdi (Errachidia) (Boudad et al., 2003), un carottage a été fait en novembre 2005 sur ce site afin de déterminer l’épaisseur des formations travertineuses. L’étude a été ensuite étendue à d’autres affleurements travertineux marocains en ayant recours à des méthodes d’analyse complémentaires (palynologie, paléomagnétisme) pour identifier les différentes phases de concrétionnement et pour mieux cerner le cadre paléoclimatique de ces formations. Des encroûtements travertineux actuels qui se forment aujourd’hui à Irdi dans l’oued Ziz ont été analysés afin demieux appréhender les processus physico-chimiques qui président à la cristallisation de ces structures ; en particulier, la mesure du rapport d’activité 234U/238U montre que l’eau responsable de la formation des travertins fossiles avait la même origine que celle d’aujourd’hui. Plusieurs phases de concrétionnement, qui se répartissent du Pléistocène supérieur au Pléistocène moyen, ont été identifiées. Ces phases devront être intégrées dans l’évolution climatique du Maroc et corrélées aux stades glaciaires et interglaciaires déterminés en milieu marin. After a preliminary study of the travertine outcrop of Irdi (Errachidia province), this site was drilled and the survey was extended to other exposures in Morocco. Complementary methods of investigation were used (palynology, paleomagnetism) together with U-series analyses in order to identify the successive deposition phases and to precise the associated paleoclimatic framework. For a better understanding of the travertine formation, we have studied the travertine formations which are presently forming at Irdi. The respective 234U/238U activity ratios of the present water and fossil travertines are similar, i.e. physical and chemical conditions of travertinization were more or less similar today as in the past. Several crystallization phases were identified from Upper Pleistocene to Middle Pleistocene. These phases must be integrated into the climatic evolution of Morocco and correlated to glacial and interglacial stages defined in marine environment.

SUMMARY A record of two magnetic polarities during the acquisition by heating of a laboratory chemical remanent magnetization (CRM) in a natural pyrite-rich claystone was investigated. The samples were heated for 22 hr at 250 ◦ Ci na controlled magnetic field (1.0 mT) under an argon atmosphere. We interpret the origin of CRM as the surface oxidation of pyrite to magnetite, which in turn is progressively oxidized into haematite. We carried out experiments under a constant-polarity magnetic field and under two opposite polarities. The resulting CRM was measured after cooling in zero field: it is parallel to the applied field and has the direction of the last polarity. Thermal demagnetization under an argon atmosphere isolates an unexpected lowunblocking-temperature component (T UB < 220 ◦ C). This component probably results from thermal alteration of magnetic carriers during subsequent thermal demagnetization. In the onepolarity experiments, thermal demagnetization of CRM above 220 ◦ C isolates a well-defined component parallel to the imparted field direction. In the experiments with two magnetic polarities thermal demagnetization of CRM reveals two components of opposite polarity. The component which is parallel to the direction of the last applied field is well defined, while the other component, which has the polarity of the first applied field, is ill defined. Oppositely directed components are also detected by using alternating field demagnetization.

[1] The understanding of the streaming potential in partial water saturation conditions in porous media is of great interest for the interpretation of spontaneous polarization observations. We built a device which allows us to quantify the streaming potential at various saturation conditions using a sand column of 1-m height and 8-cm diameter. This is the first time that such a quantification has been performed. Different gases such as argon, nitrogen, and carbon dioxide are injected into the sand to decrease its water saturation, and to make the fluid flow within the sand. The measured electrokinetic coupling coefficient in partial saturation is either constant or decreases by a factor ∼3 with decreasing water saturation from 100 to 40%, whereas the sand electrical resistivity is enhanced by a factor of ∼5.

— In order to investigate how a streaming potential coefficient measured in the laboratory, at a typical scale of 10 cm, can be incorporated into a field model, with a typical scale of 1 to 10 km, we measured the electric field induced by water flows forced at 150 m depth through a 10-m wide granite fractured zone. The water flows were obtained by pumping cyclically 10 m of water from a borehole that cut the fractured zone at depth, and contemporaneously reinjecting it into another borehole located 50 m away. After one day a steady-state fluid flow regime was reached, with pumping cycles lasting 45 minutes, indicating a hydraulic conductivity of 10−5 m s−1 and a specific storage coefficient of 3.25×10−6 m−1. The expected self-potential at the surface was an anomaly with two maxima of opposite sign and 2μV amplitude each, both located 160 m away from the middle of the borehole heads, the signal being divided by two 500 m away from the middle of the borehole heads (in agreement with Wurmstich and Morgan, 1994). Instead, we observed an electrical signal of 8 mV midway between the borehole heads, and smaller than 5 mV, 33 m away from the borehole heads. The discrepancy observed between the data and the model can be explained by fluid flow leakages that occurred close to the water-table head, represented about 20% of the total water flow, and activated smaller but closer electric sources. This experiment thus illustrates the practical difficulty of detecting streaming potentials generated at depth. It shows in particular that in fractured zones, and hence in the vicinity of a major active fault small water flows located distantly from an energetic targeted source, but close to some of the electrodes of the network, can sometimes drastically distort the shape of the expected anomaly. Models of possible electrical earthquake precursors therefore turn out to be more speculative than expected.

[1] We conducted a series of electrokinetic measurements on partially saturated sandstone samples. Both steady and transient electric signals were observed. We recorded transient electric signals with frequencies of 0.5–2Hz and amplitudes 10–100 times smaller than the quasi-DC streaming potential, which we associate with fluid flow instabilities in the unsaturated porous rocks samples. Some of these electric signals were correlated with fluid pressure variations, delayed by one to a few seconds.

The streaming potential due to fluid flow in rocks was measured on saturated sandstones. During triaxial failure test, the electrokinetic coupling coefficient was increased at about 72–86% of the failure stress. This increase is thought to be due to an increase of the macroscopic zeta potential in the shear zone where new surfaces are created and connected, allowing fluid to flow through these new cracks. The streaming potential variations could therefore be used as a precursor of the rupture. Measurements were also performed on volcanic samples, with the aim of using electrokinetic phenomena to monitor volcanic activity. Streaming potential was found to depend on the transport properties i. e., permeability and formation factor. Theoretical consideration suggest that such a dependence should be caused by surface conductivity, but this is not the case here.

International audience; The streaming potential, due to fluid circulation in rock, was measured on saturated sediments (Fontainebleau sandstones). The electrokinetic coupling coefficient, which is the ratio of the streaming potential and the excess pore pressure, is proportional to the fluid resistivity. Additionally, for a fluid conductivity of 10-3 S/m, the electrokinetic coupling coefficient varies from 10 mV/0.1 MPa to 6642 mV/0.1 MPa for sample permeability in the range of permeabilities from 0.15x10-15 m2 to 1220x10-15 m2. The different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes non negligible compared to fluid conductivity for low permeability. When the sample is deformed under triaxial stress up to failure, the vertical permeability (along the principal stress) drops about by 0.20% per 0.1 MPa when failure occurs. The typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75% of the yield stress, and stopping at the failure. This increase of the electrokinetic coupling coefficient is due to an increase of Ζ potential in the shear zone when new surfaces are created and connected. Possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake. It is shown that in some cases self-potential anomalies reported in the deformed zone preceding an earthquake occurence, could be due to an increase of the electrokinetic coupling coefficient from 75% of the yield stress to rupture, in the vicinity of one of the electrodes. Any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient, inducing a surface electrokinetic potential anomaly. As regards to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow, for instance in a shallow borehole. An electrokinetic potential anomaly up to 30 mV, for a fluid conductivity of 0.01 S/m and a rock permeability of 10-12 m2, could be observed with a change of the underground water table level as slight as 50 cm (50 mbar). Moreover if the permeability between the electrodes is increased by a factor of 8 10+3, the electrokinetic coupling coefficient could be enhanced by a factor up to 650.

Streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities. The electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity. A model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity. When fluid resistivity is below 1 Ω.m permeability and streaming potential are no more related.

The mechanical stability of the wall of a magma chamber depends on the value of the stresses likely to be developed in the immediate vicinity of this boundary in relation to the conditions for failure to occur. Various structural and physical factors contributing to these stresses are systematically analyzed. Models of volcanic systems having an axial symmetry around a vertical axis with an isolated reservoir, filled with magma under pressure, in a homogeneous elastic medium are investigated. Spheroidal magma chambers of different aspect ratios (0.7–2) and approximately the same volume (13.5–14 km3), with tops at 3.5–4.5 km depth, are considered. Stress distributions including the effect of gravity, free surface of the Earth, and remote stresses increasing with depth are calculated by using a numerical finite element method. The development of tensile tangential stresses greater than 10 MPa in the elements adjacent to the wall of the chamber is assumed to be a sufficient condition for its instability. The standard state of stress assumed for the crust (i.e., the boundary conditions imposed at large horizontal distance from the chamber) is varied in a continuous range having as lower limit a state of uniaxial strain in the vertical direction and as upper limit a state of hydrostatic stress. Calculations are first performed by making the assumption that the magma pressure P acting at the top of a chamber equals the lithostatic pressure of the overlying rocks. Alternatively, the upper limit of the values of P (critical pressure Pc) for which the initial shape of a chamber may be stable, according to the assumed necessary conditions for stability, is determined. For boundary conditions corresponding to uniaxial strain and Poisson's ratio v = 0.25, Pc turns out to be approximately half of the lithostatic pressure of the overlying rocks. A simple criterion is proposed to estimate if a chamber may evolve toward a new stable shape when P > Pc. This is highly improbable if P equals the lithostatic pressure of the overburden. Larger values of Poisson's ratio (0.30–0.35) favor stability, yielding critical pressures exceeding the lithostatic pressure. Stability is extremely sensitive to the standard state of stress assumed for the crust. Boundary conditions progressively approaching a hydrostatic state of remote stresses yield increasingly higher critical pressures. It turns out that the density contrast between magma and host rocks is not crucial for stability (for Δρ/ρ < 11%). For the rather large spheroidal chambers investigated, the shape is not a critical factor either. The effect of topographical details (e.g., a volcanic edifice, 1.5 km high and 6 km in radius) is practically irrelevant.

The Xigaze ophiolite (29.2°N, 89.5°E), which outcrops in the Yarlung Zangbo suture zone, represents the remnants of an oceanic lithosphere formed in middle Cretaceous times between the Lhasa block to the north and the Indian plate to the south. In an attempt to define the paleo-orientation and latitude of the spreading center at which it has been created, a paleomagnetic study has been done on three sites in volcanics and overlying (or interbedded) radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the Xigaze Group flysch which stratigraphically overlies the volcanics to the north. In each site, hand-blocks carefully oriented both with sun and magnetic compass have been sampled. The paleomagnetism data, combined with structural data on the ophiolite dolerite intrusives, allow a partial reconstruction of South Eurasia at the time of formation of the Xigaze ophiolite. The paleolatitude of accretion and deposition of the Xigaze ophiolite and overlying sediments is found to be 10–20°N. Both ophiolite and basin have encountered a 85 ± 20° anti-clockwise rotation. The corresponding ridge was close to the southern margin of the Lhasa block and was oriented N175 ± 25°.