Main Fracture Zones Research Articles

Determining the vertical evolution of hydrodynamic parameters in weathered and fractured south Indian crystalline-rock aquifers: insights from a study on an instrumented site

Due to extensive irrigation, most crystalline aquifers of south India are overexploited. Aquifer structure consists of an upper weathered saprolite followed by a fractured zone whose fracture density decreases with depth. To achieve sustainable management, the evolution of hydrodynamic parameters (transmissivity and storage coefficient) by depth in the south Indian context should be quantified. Falling-head borehole permeameter tests, injection tests, flowmeter profiles, single-packer tests and pumping tests were carried out in the unsaturated saprolite and saturated fractured granite. Results show that the saprolite is poorly transmissive (T fs = 3 × 10–7 to 8.5 × 10–8 m2 s–1) and that the most conductive part of the aquifer corresponds to the bottom of the saprolite and the upper part of the fractured rock (T = 1.0 × 10–3 to 7.0 × 10–4 m2 s–1). The transmissivity along the profile is mostly controlled by two distinct conductive zones without apparent vertical hydraulic connection. The transmissivity and storage coefficient both decrease with depth depending on the saturation of the main fracture zones, and boreholes are not exploitable after a certain depth (27.5 m on the investigated section). The numerous investigations performed allow a complete quantification with depth of the hydrodynamic parameters along the weathering profile, and a conceptual model is presented. Hydrograph observations (4 years) are shown to be relevant as a first-order characterization of the media and diffusivity evolution with depth. The evolution of these hydrodynamic parameters along the profile has a great impact on groundwater prospecting, exploitation and transport properties in such crystalline rock aquifers.

Pre-existing oblique transfer zones and transfer/transform relationships in continental margins: New insights from the southeastern Gulf of Aden, Socotra Island, Yemen

Transfer zones are ubiquitous features in continental rifts and margins, as are transform faults in oceanic lithosphere. Here, we present a structural study of the Hadibo Transfer Zone (HTZ), located in Socotra Island (Yemen) in the southeastern Gulf of Aden. There, we interpret this continental transfer fault zone to represent a reactivated pre-existing structure. Its trend is oblique to the direction of divergence and it has been active from the early up to the latest stages of rifting. One of the main oceanic fracture zones (FZ), the Hadibo–Sharbithat FZ, is aligned with and appears to be an extension of the HTZ and is probably genetically linked to it. Comparing this setting with observations from other Afro-Arabian rifts as well as with passive margins worldwide, it appears that many continental transfer zones are reactivated pre-existing structures, oblique to divergence. We therefore establish a classification system for oceanic FZ based upon their relationship with syn-rift structures. Type 1 FZ form at syn-rift structures and are late syn-rift to early syn-OCT. Type 2 FZ form during the OCT formation and Type 3 FZ form within the oceanic domain, after the oceanic spreading onset. The latter are controlled by far-field forces, magmatic processes, spreading rates, and oceanic crust rheology. • Study of pre-existing oblique transfer faults in the Gulf of Aden continental margin • Comparison with other Afro-Arabian rifts • Classification of oceanic F.Z. based on their relationships with syn-rift structures • Segmentation of the Gulf of Aden

Variations in amount and direction of seafloor spreading along the northeast Atlantic Ocean and resulting deformation of the continental margin of northwest Europe

The NE Atlantic Ocean opened progressively between Greenland and NW Europe during the Cenozoic. Seafloor spreading occurred along three ridge systems: the Reykjanes Ridge south of Iceland, the Mohns Ridge north of the Jan Mayen Fracture Zone (JMFZ), and the Aegir and Kolbeinsey Ridges between Iceland and the JMFZ. At the same time, compressional structures developed along the continental margin of NW Europe. We investigate how these compressional structures may have resulted from variations in the amount and direction of seafloor spreading along the ridge system. Assuming that Greenland is rigid and stationary, we have used a least squares method of palinspastic restoration to calculate differences in direction and rate of spreading along the Reykjanes, Kolbeinsey/Aegir and Mohns Ridges. The restoration generates relative rotations and displacements between the oceanic segments and predicts two main periods of left‐lateral strike slip along the main oceanic fracture zones: (1) early Eocene to late Oligocene, along the Faeroe Fracture Zone and (2) late Eocene to early Oligocene and during the Miocene, along the JMFZ. Such left‐lateral motion and relative rotation between the oceanic segments are compatible with the development of inversion structures on the Faeroe‐Rockall Plateau and Norwegian Margin at those times and probably with the initiation of the Fugløy Ridge in the Faeroe‐Shetland Basin during the Eocene and Oligocene. The Iceland Mantle Plume appears to have been in a position to generate differential seafloor spreading along the NE Atlantic and resulting deformation of the European margin.

Low temperature alteration of serpentinized ultramafic rock and implications for microbial life

Alteration (serpentinization) of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water. Hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere. In this study we focus on ongoing low-temperature water–rock processes in partially serpentinized dunite from the Leka Ophiolite Complex, mid-Norway. We aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities. The study is based on the chemical and physical alteration features of a 51m deep rock core as well as the chemical composition of groundwater from the core's borehole. The borehole intersects three main fracture zones. Observations during drilling indicated that water infiltration largely takes place along the deepest zone. The rock in the fracture zones is nearly completely altered to serpentine, brucite, magnetite and minor calcite. An increase in pH and decrease in Mg, Si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water–rock reactions where dissolution of brucite, serpentine and calcite and precipitation of chrysotile, hydromagnesite and iron-hydroxide are most likely to occur. Detection of both H2 and O2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water/rock ratios and to dissolution of Fe(II)-containing brucite and serpentine resulting in the formation and successive diffusion of H2 into the oxic borehole. Oxidation of H2, reduced Fe and Mn, NH4+ and CH4 are all potential energy-yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources. Methane could result from microbial oxidation of H2 and reduction of CO2, or from the degradation of organic matter. The results demonstrate that low-temperature water–rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities.

Integrated geophysics and soil gas profiles as a tool to characterize active faults: the Amer fault example (Pyrenees, NE Spain)

The combined use of geophysical and soil gas composition exploration methods allows to rapidly obtain at relative low cost information that might be related to seismic activity conditions. In this study, we carried out geochemical soil gas sampling (222Rn, 220Rn and CO2), electrical resistivity tomography and seismic refraction profiles in two selected zones near the town of Amer in the Spanish Pyrenees, where the presence of recent fractures is evident in the field. Data analysis clearly reveals anomalous values for each gas at specific positions along the electrical imaging transects. Geomorphologic and hydrogeologic data and the integration of geophysical data and soil gas measurements indicate that: (1) endogene gases radon (222Rn) and carbon dioxide (CO2) are released from the meta-sedimentary basement rocks across the main fractured zones with higher permeability values, while lower Cenozoic detrital sedimentary formations act as an impervious boundary; (2) sites with highest radon concentrations (52 kBq m−3) coincide with the zones in the Amer fault showing more recent geomorphic evidence of activity, and more specifically with those areas covered by thinner surficial formations; (3) the lowest 222Rn values (0.2–0.4 kBq m−3) were recorded just on the master active fault plane. This pattern could be explained by a dilution effect resulting from high rates of soil CO2 efflux (267 g m−2 day−1); (4) soil thoron (220Rn) activity is maximum (143 kBq m−3) in areas with high surficial fracturing; (5) groundwater pumping may cause important distortions in the natural flow dynamics and in the measured concentrations of gases. The agreement between the different data (geochemical, geophysical, and hydrogeological) and field observations (geology and geomorphology) leads us to propose a preliminary tectonic-gravitational model for the study area.

Large-scale numerical simulation of groundwater flow and solute transport in discretely-fractured crystalline bedrock

A large-scale fluid flow and solute transport model was developed for the crystalline bedrock at Olkiluoto Island, Finland, which is considered as potential deep geological repository for spent nuclear fuel. Site characterization showed that the main flow pathways in the low-permeability crystalline bedrock on the island are 13 subhorizontal fracture zones. Compared to other sites investigated in the context of deep disposal of spent nuclear fuel, most deep boreholes drilled at Olkiluoto are not packed-off but are instead left open. These open boreholes intersect the main fracture zones and create hydraulic connections between them, thus modifying groundwater flow. The combined impact of fracture zones and open boreholes on groundwater flow is simulated at the scale of the island. The modeling approach couples a geomodel that represents the fracture zones and boreholes with a numerical model that simulates fluid flow and solute transport. The geometry of the fracture zones that are intersected by boreholes is complex, and the 3D geomodel was therefore constructed with a tetrahedral mesh. The geomodel was imported into the numerical model to simulate a pumping test conducted on Olkiluoto Island. The pumping test simulation demonstrates that fracture–borehole intersections must be accurately discretized, because they strongly control groundwater flow. The tetrahedral mesh provides an accurate representation of these intersections. The calibrated flow model was then used for illustrative scenarios of radionuclide migration to show the impact of fracture zones on solute transport once the boreholes were backfilled. These mass transport simulations constitute base cases for future predictive analyses and sensitivity studies, since they represent key processes to take into consideration for repository performance assessment.

Curie Point Depth Estimates and Correlation with Subduction in Mexico

We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available “Magnetic Anomaly Map of North America”. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2° × 2°. The CPD estimates range between 10 and 40 km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20–30 km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD = 12–16 km) and Oaxaca (CPD = ~16 km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O’Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16–32 km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16–20 km) to east (10–12 km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust’s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico.

Low-cycle fatigue of nickel superalloy single crystals at elevated temperatures

Single-crystal samples of nickel superalloys containing rhenium or rhenium plus ruthenium are subjected to low-cycle fatigue (LCF) tests under rigid cycle conditions at temperatures of 850 and 1050°C. It is found that the single crystals of the alloy containing rhenium with ruthenium have higher LCF resistance at 104 cycles as compared to the alloy containing only rhenium. At a test temperature of 850°C, volume stress concentrators in the form of pores or their clusters represent fatigue crack nucleation zones; at 1050°C, surface corrosion cracks are the main fracture zones. The fatigue microcrack growth rate is anisotropic: it is higher in the [001] direction and lower in the [011] direction.

Evaluating fracture patterns within a resurgent caldera: Campi Flegrei, Italy

Understanding deformation of active calderas allows their dynamics to be defined and their hazard mitigated. The Campi Flegrei resurgent caldera (Italy) is one of the most active and hazardous volcanoes in the world, characterized by post-collapse resurgence, eruptions, ground deformation, and seismicity. An original structural analysis provides an overview of the main fracture zones. NW-SE and NE-SW fractures (normal or transtensive faults and extensional fractures) predominate along the rim and within the caldera, suggesting a regional control, both during and after the collapses. While the NE-SW fractures are ubiquitous in the deposits of the last ∼37 ka, NW-SE fractures predominate in the last 4.5 ka, during resurgence. The most recently (<4.5 ka) strained area lies in the caldera center (Solfatara area), where the faults, with an overall ∼ENE-WSW extension direction, appear to be associated with the bending due to resurgence. Solfatara lies immediately to the east of the most uplifted part of the caldera (Pozzuoli area), where domes form and culminate both on the long-term (resurgence, accompanied by volcanic activity) and short-term deformation (1982–1984 bradyseism, accompanied by seismic and hydrothermal activity). Similar volcano-tectonic behavior characterizes the short- and long-term uplifts, and only the intensity of the tectonic and volcanic activity varies, being related to varying amounts of uplift. Seismicity and hydrothermal manifestations occur during the bradyseisms, with moderate uplift, while surface faulting and eruptions occur during resurgence, with higher uplift. The features observed at Campi Flegrei are found at other major calderas, suggesting consistent behavior of large magmatic systems.

勇払油・ガス田における断層面自動抽出処理とＡＥデータによる断裂帯評価

An automatic fault extraction (AFE) process is expected to contribute to fault interpretation. However, it is important to validate an output of the AFE process because fault patterns extracted by the AFE process change according to extraction parameters.A commercially available AFE process is applied to 3D seismic data acquired in the Yufutsu oil/gas field with an objective to estimate fault-related fractures in a reservoir. To select a suitable output of the AFE process, fracture information derived from Acoustic Emission (AE) data during a massive hydraulic injection is utilized. The AE data delineate fracture zones within the area of six hundred meters by two hundred meters. An output of the AFE process with fault intervals close to those derived by the AE distribution is selected and it is used as an input for Discrete Fracture Network (DFN) modeling to constrain locations and orientations of the fault-related fractures. The dense zones of the fault-related fractures expressed by multiple realizations of DFN models are fairly consistent with main fracture zones delineated by the AE distribution. This consistency indicates that the output of the automatic fault process has been validated by the AE data and that it can be used for estimation of fault-related fractures in the area outside the source location distribution of AE events.

An estimation of groundwater type and origin of the complex karst catchment using hydrological and hydrogeochemical parameters: A case study of the Gacka river springs

Karst aquifers are heterogeneous terrains, where it is hard to assess any hydraulic parameter. Therefore, a multidisciplinary approach is necessary for research on karst aquifers. Catchment area of the Gacka river springs is typical Dinaric karst terrain built of karstified carbonates. Groundwater flow is mostly directed by preferential flow paths usually connected with main faults and fracture zones. In the presented case study structural geological and tectonic characteristics were defined. A recession diagram was created, and the water balance was calculated. Tracer-test data were also used for analysis. All these data were compared with the bulk hydrogeochemical and isotopic analyses of spring and surface waters. For this purpose, samples were obtained every month for one hydrological year at 17 sampling locations. Processing of all these data allowed a tenable definition of the Gacka spring catchment area.

Investigation of Fault Mechanisms during Geothermal Reservoir Stimulation Experiments in the Cooper Basin, Australia

Abstract Subsequent to an initial reservoir stimulation in 2003, a second long-term fluid-injection experiment was conducted in the Cooper Basin, Australia, in 2005 to further enhance and enlarge the existent geothermal reservoir. Approximately 25,000 m 3 of water were injected into the granitic crust at 4250 m depth, thereby inducing about 16,000 seismic events, which were recorded by a local eight-station network. Event magnitudes calibrated by recordings of the permanent network of Geosciences Australia are in the range between M L -1.2 and 2.9 with seismic moments between 10 9 and 10 13 N m. Hypocenter locations could be determined for 8886 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation, which is further extended in the course of restimulation. Early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well. The immediate vicinity of the injection well remains seismically quiet during restimulation. These effects can be explained by a simple Kaiser effect model, where the induced seismicity is controlled by the in situ fluid pressure increase. Compound fault-plane solutions based on P -phase polarities indicate a similar fault mechanism for nearly all events. The dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. A small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimeters of shear displacement.

Large earthquakes during hydraulic stimulations at the geothermal site of Soultz-sous-Forêts

Several deep wells were drilled in the Rhine Graben (Soultz-sous-Forêts, France) to evaluate the geothermal Hot Dry Rock potential of a deep fractured granite reservoir. Three main boreholes, which reached about 5 km depth, intersected a crystalline basement overlain by 1.4 km of Cenozoic and Mesozoic sediments. Stimulations of these three wells were carried out in 2000 for GPK2, 2003 for GPK3 and 2004, and 2005 for GPK4. During these stimulations and other hydraulic activities a seismological surface network was installed in order to monitor the seismicity induced by the massive fluid injection. Here we analyse the seismicity of magnitude larger than or equal to 1.4, which is the lowest magnitude felt by the population. Based on a spectral analysis of the displacement recorded by a Güralp velocimeter at a depth of 200 m, we know that the source dimensions range from tens to hundreds of metres. We analyse several parts of the reservoir where obvious correlation between the fluid path and tectonic features has been pointed out. Based on seismological arguments such as location and focal mechanisms, we show that this activity is linked with tectonic features, or at least with large fractures that control the behaviour of the geothermal reservoir. To constrain the hypothesis, we study 391 events and focal mechanisms to argue in favour of the existence and stability of tectonic features that can be compared to geological data. We show that the largest events recorded on the site occurred after the shut-in. Their spatial distribution appears not to be random within the reservoir, and the focal mechanisms of these events also confirm the non-randomness of their distribution. Given their source dimensions, the largest events can only occur on large structures, such as tectonic ones. Therefore, the behaviour of the reservoir is controlled by these main fractured zones, which either lead the fluid or hinder its path.

Decomposing InSAR LOS displacement into co-seismic dislocation with a linear interpolation model: A case study of the Kunlun Mountain M S=8.1 earthquake

It has always been a difficult problem to extract horizontal and vertical displacement components from the InSAR LOS (Line of Sight) displacement since the advent of monitoring ground surface deformation with InSAR technique. Having tried to fit the firsthand field investigation data with a least squares model and obtained a preliminary result, this paper, based on the previous field data and the InSAR data, presents a linear cubic interpolation model which well fits the feature of earthquake fracture zone. This model inherits the precision of investigation data; moreover make use of some advantages of the InSAR technique, such as quasi-real time observation, continuous recording and all-weather measurement. Accordingly, by means of the model this paper presents a method to decompose the InSAR slant range co-seismic displacement (i.e. LOS change) into horizontal and vertical displacement components. Approaching the real motion step by step, finally a serial of curves representing the co-seismic horizontal and vertical displacement component along the main earthquake fracture zone are approximately obtained.

An Electric Resistivity Study of the Chelungpu Fault in the Tsaotun Area, Central Taiwan

We conducted an electric resistivity survey in the Tsaotun area to investigate the subsurface structures of the complex deformation and rupture of Chelungpu fault system induced by the 1999 Chi- Chi earthquake. Four electric resistivity image profiles with pole-pole array across fault zones and topographic lineation were conducted in this survey. The sounding data were interpreted by the 2-D inversion method. The results show that the electric resistivity structures interpreted from the resistivity sounding data fit well with ruptures and outcrops appeared on the ground surface. We also evaluated the resistivity ranges for different formations, the top layer thickness and the width of the fault zones. Resistivity-layer discontinuities found on the profiles correlate to the fault zones of the Chelungpu fault and the back thrust. The boundary between the Toukoshan Formation and the Chinshui Shale is the characteristic contact of the Chelungpu fault. It is a reverse fault including several fracture zones in the Tsaotun area. Some ruptures developed in weaker layers of the Chinshui Shale have been active since the deposition of the Quaternary terrace gravel bed. The main fracture zones have a width of 640 m and dip eastward at an angle of 800 - 300 that decrease with depth. The fault diverged into V -shape fractures or more branches near the ground surface in some regions. Not all the fractures of the Chelungpu fault system moved during the Chi-Chi earthquake, but they were active previously.

Self-potential chenges associated with volcanic activity: Short-term signals associated with March 9, 1998 eruption on La Fournaise volcano (Reunion Island)

After six years of quietness La Fournaise volcano entered into activity on March 9, 1998. Fissures opened gradually downwards on the northern flank of the cone. Two cones, Kapor and Krafft built, from which lava poured until September 1998. Several other vents opened during this eruption. Mappings, surveys, and continuous recordings of the Self-Potential have been performed on the volcano for twenty years. SP mappings disclose the variability of large scale SP anomalies due to the modification of the hydrothermal system over some ten years. Most of the eruptions take place along a Main Fracture Zone (MFZ) in which ground water flows prevail. SP measurements have also regularly been made on the northern flank of the cone, on a west-east profile crossing the MFZ. Between 1981 and 1992 an enlargement and a shift of the MFZ to the east are evidenced. In particular, the eastern fissural axis trending N35°E could be related to the possible collapse of the east flank of the volcano. After a decrease between 1992 and 1997, the SP anomaly was enhanced again by the 1998 eruption. Short scale, about 250 m wide, 750 mV amplitude anomalies were superimposed on a large scale one, 2500 m wide, and about 250 mV in amplitude. For several years, continuous stations have been measuring the electric field along two directions, with a 20 s sampling, in order to record the genesis of SP signals associated with the volcanic activity. Oscillations belonging to the ULF band were evidenced several days before the 1988 eruption, some of them at 9 km from the summit. Their amplitude reached several tens mV/km. These oscillations sometimes present a phase lag from one station to another; they progressively shift towards the location of the future effusive vents. The polarisation of the oscillations is similar to the polarisation of longer SP variations (1 h period or more) and are correlated with the structural anisotropy. Finally, during the last hours preceding the effusive activity, huge SP signals, up to a few Volts/km, appeared at the stations located on the MFZ, and especially on the branch where the magma migrated. We interpret these SP signals as due to electrokinetic effects generated by fluid flow in cracks opened by the stress field changes.

Three‐dimensional Euler deconvolution and tectonic interpretation of marine magnetic anomaly data in the Puerto Rico Trench

During the summer of 1996 single‐channel‐seismic, magnetic, gravity, Hydrosweep bathymetric and HMR1 sidescan data were collected north of the island of Puerto Rico to constrain the geologic and tectonic setting of the Puerto Rico trench. Magnetic data from this cruise are merged with other available data, then processed and interpreted with the aid of the Euler deconvolution method. The area north of the island of Puerto Rico is divided into three magnetic anomaly zones. Zone 1 is dominated by northwest to southeast trending magnetic anomalies. Zone 2 consists of an east‐west region of relatively low amplitude anomalies and occurs south of zone 1. Zone 3 is dominated by the highest magnetic values in the study area and the source region is roughly centered under the island of Puerto Rico. Respectively, these zones approximate three geologic provinces of the Puerto Rico trench composed of Early Cretaceous ocean crust with sedimentary cover, a blueschist belt and an Oligocene‐Cretaceous island arc with a limestone cap. Also mapped are the Main Ridge fracture zone and the Fourth of July fracture zone, both on the North American plate, which correlate with the Main Ridge and Fourth of July Ridge on the Caribbean plate. The 3‐D Euler deconvolution facilitates the identification of new faults as well as the mapping of known faults also evident in the seismic and bathymetric data. A model concerning the formation of the Puerto Rico trench is proposed that incorporates the existence of strike‐slip faults and the fracture zones associated with the subduction of the North America Plate. Evidence from this study support a tectonic interpretation of subduction followed by more recent strike‐slip faulting that is accompanied by only a minimal amount of subduction.

Structural features of Nigeria's coastal margin: an assessment based on age data from wells

Geological age data from 127 wells drilled for petroleum exploration and production purposes in both the onshore and offshore sections of the Nigerian coastal margin have been examined and correlated from well to well across the margin. By examining the variations in depth to selected key geological marker horizons, the positions of faults and fractures have been inferred between wells and along designated cross-sections. These faults extend across the margin from land to sea, and most of them trend northeast-southwest and north-south. Graben and horst structures dominate the western part of the Nigerian margin. The most prominent of these features on the western part of the margin is the Okitipupa structure, a basement ridge that separates the sediments of the Tertiary Niger Delta to the east from those of the Cretaceous Dahomey Embayment to the west. The Niger Delta basin and the major faults bounding it are the other important tectonic features on the Nigerian coastal margin. It appears that major fractures and transcurrent faults influenced sedimentation rates in different parts of the margin throughout the Tertiary period. A landward prolongation of two main oceanic fracture zones (Chain and Charcot) appears to merge well with some of the inferred faults, especially in the case of the Chain Fracture Zone. It is a well known fact that seismic activities are associated with oceanic fracture zones. The delineation of the extensions of these faults and fractures in the Nigerian continental interior can be regarded as an important basic step towards the assessment of the seismicity of the country's coastal margin. The next logical step would be to determine, through the deployment of a seismograph network, the level of activities associated with these faults.

Boiling and fluid mixing in the chlorite zone of the Larderello geothermal system

The geochemical features of the geothermal fluids produced within the boiling zone in the relatively shallow parts of the Larderello geothermal system (Italy) have been documented as a result of deep drilling which provided samples from 1480 to 2500 m depth. Four wells (Monteverdi 1, Monteverdi 2A, Sasso 22 and Capannoli 2B) have been sampled in the intermediate parts of the Larderello aquifer located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the shallow aquifer at Larderello. Fluid inclusions from recrystallized quartz lenses and quartz veins in samples displaying a predominant quartz–chlorite–(epidote–adularia) paragenesis have been studied by microthermometry and Raman spectroscopy. The inclusions are primary and pseudo-secondary in origin when formed in authigenic quartz, or of secondary origin, when located in fluid inclusion planes related to microfracturing of metamorphic quartz lenses. Several generations of fluids are present and include: H 2O–(CO 2)-dominated vapours and liquids, and a series of aqueous liquids, for the most part of relatively low salinity. The T m-ice of both early and late inclusions are mostly between 0.0 and −4.5°C, indicating that the salinities of the hydrothermal fluid were very low to moderate. However, rare fluid inclusions with lower T m-ice (from −4.9 to −25.0°C) were also observed. These inclusions may record the occasional input of saline fluids, which may be derived from the interaction of the hydrothermal waters with the evaporites present in the shallow part of the Larderello field. At Capannoli 2B, earliest H 2O–(CO 2) liquids were trapped under minimal pressures of 610–645 bars, which bracket the estimated present-day lithostatic pressure (640 bars). In all other samples, the main stage of quartz–chlorite crystallization occurs under boiling conditions attested by the presence of liquid and vapour-rich inclusions, that, in some instances, can be texturally interpreted to be coeval. Their trapping conditions (350–375°C, 160–215 bars) are higher than the present day temperatures at the same depth. Later fluid inclusions attest to a significant cooling of the fluids down to temperatures similar to the present-day temperatures. During this time, pressures were close to hydrostatic conditions. Most fluid inclusions were trapped within the liquid field, this indicating that a significant pressure drop has since affected the main aquifers or fractured zones which are, at present, under vaporstatic conditions.

Self‐potential and magnetic surveying of La Fournaise volcano (Réunion Island): Correlations with faulting, fluid circulation, and eruption

Self‐potential (SP) and magnetic surveys have been performed on the basaltic La Fournaise volcano (Réunion Island, Indian Ocean) between 1989 and 1992. The 1992 SP mapping of the inner caldera (Enclos Fouqué) reveals high relative positive SP anomalies. They are interpreted as related to a large hydrothermal system. A large SP anomaly of 2 V is centered on the terminal cone. A horseshoe‐shaped positive SP anomaly intersects the cone. This anomaly outlines the main arcuate fracture zone (MFZ), oriented north‐south on the cone and extending to the northeast and southeast away from the summit. The zone controls the volcano dynamism. Most of the fissure eruptions occur along this volcanotectonic axis. A second mechanical weakness axis is evidenced by a positive SP anomaly lying along a 120°N trend. Both volcanoes of the island, Piton des Neiges and later La Fournaise, have been built up along it. A large structure with reverse magnetization is found at the southeastern tip of this axis. This 120°N axis is interpreted as an old regional structure associated with a lithospheric weakness zone located beneath Réunion Island. The SP anomalies are thought to be of electrokinetic origin. In the central part of the volcano, along the MFZ, and the 120°N axis, positive SP data suggest hydrothermal upwelling caused by thermal transfer from the magma chamber and the dikes of the last eruptions. Away from the summit, SP data suggest groundwater infiltrates the massif by gravity effect. Changes of the SP with time are related to volcanic activity. The SP anomaly associated with main fracture zone has been generated by the 23 eruptions occurring between 1981 and 1992. Short‐time local SP anomalies that disappear in a few months are associated with individual eruptions.