Local Fault Zones Research Articles

Evaluation of the current in situ stress in the middle Permian Maokou Formation in the Longnüsi area of the central Sichuan Basin, China

Abstract The middle Permian Maokou Formation in the Longnüsi area in the central Sichuan Basin is currently a key formation for exploration and development. The evaluation of the current in situ stress in this area is of great significance for fracture prediction, well pattern deployment, drilling and construction, and fracturing stimulation. This study clarifies the current direction and magnitude distributions of the in situ stress by evaluating the Maokou Formation in the Longnüsi area using finite-element numerical simulation, acoustic emission experiments, and logging data (including data from imaging logging, array acoustic logging, conventional logging, and cross-dipole acoustic logging). Specifically, the current maximum horizontal stress of the Maokou Formation in the Longnüsi area is mainly in the NW‒SE direction, and the stress direction is greatly affected by the local fault zone. The current minimum horizontal stress magnitude of the Maokou Formation obtained by acoustic emission experiments is between 96.29 and 114.36 MPa, the current maximum horizontal stress magnitude is between 126.01 and 145.10 MPa, and the current horizontal stress difference is between 25.59 and 32.58 MPa. The current minimum and maximum horizontal stress magnitudes both decrease from north to south. The current horizontal stress parameters calculated by Huang’s model are not significantly different from those experimentally measured: there is a difference of less than 8% in the current minimum horizontal stress magnitude, a difference of less than 9% in the maximum horizontal stress magnitude, and a difference of less than 15% in the current horizontal stress difference. Therefore, Huang’s model has good applicability in terms of calculating the current horizontal stresses in the Longnüsi area. The current horizontal stress parameters, which are numerically simulated with the finite-element method, are also not much different from those experimentally measured: there is a difference of less than 11% in the current minimum horizontal stress magnitude, a difference of less than 10% in the maximum horizontal stress magnitude, and a difference of less than 20% in the current horizontal stress difference. The numerically simulated current horizontal stress also decreases from north to south. Therefore, the simulated results are highly accurate. This study clarifies the directions and magnitudes of the current in situ stress state of the Maokou Formation in the Longnüsi area of the central Sichuan Basin and provides a basis for the formulation of exploration and development plans for the Maokou Formation reservoir in the study area.

Differential structure of Ordovician karst zone and hydrocarbon enrichment in paleogeomorphic units in Tahe area, Tarim Basin, NW China

Based on a large number of drilling, logging, seismic and production data, the differential structures of karst zone and hydrocarbon distribution in different paleogeomorphic units of the Tahe area, Tarim Basin, are discussed by analyzing the karst drainages and flowing channels. The karst paleogeomorphy of Ordovician in Tahe area is composed of watershed, karst valley and karst basin. The watershed has epikarst zone of 57.8 m thick on average and vadose karst zone of 115.2 m thick on average with dense faults, fractures and medium—small fracture-caves, and 76.5% of wells in this area have cumulative production of more than 5×10 4 t per well. The karst valleys have epikarst zone, vadose karst zone and runoff karst zone, with an average thickness of 14.6, 26.4 and 132.6 m respectively. In the runoff karst zone, the caves of subsurface river are mostly filled by fine sediment, with a filling rate up to 86.8%, and 84.9% of wells in this area have cumulative production of less than 2×10 4 t per well. The karst basin has no karst zone, but only fault—karst reservoirs in local fault zones, which are up to 600 m thick and closely developed within 1 km around faults. Different karst landforms have different water flowing pattern, forming different karst zone structures and resulting in differential distribution of oil and gas. The watershed has been on the direction of oil and gas migration, so medium—small sized connected fracture-caves in this area have high filling degree of oil and gas, and most wells in this area have high production. Most caves in subsurface river are filled due to strong sedimentation and transportation of the river, so the subsurface river sediment has low hydrocarbon abundance and more low production oil wells. The faults linking source rock are not only the water channels but also the oil-gas migration pathways, where the karst fractures and caves provide huge reservoir space for oil and gas accumulation.

Earthquake Source Complexity Controls the Frequency Dependence of Near‐Source Radiation Patterns

AbstractThe spatial patterns of earthquake ground motion amplitudes are commonly represented using a double‐couple model that corresponds to shear slip on a planar fault. While this framework has proven largely successful in explaining low‐frequency seismic recordings, at higher frequencies the wavefield becomes more azimuthally isotropic for reasons that are not yet well understood. Here, we use a dense array of nodal seismometers in Oklahoma to study the radiation patterns of earthquakes in the near‐source region where the effects of wavefield scattering are limited. At these close distances, the radiation pattern is predominantly double couple at low frequencies (<15 Hz). At higher frequencies, the recorded wavefield contains significant isotropic and residual components that cannot be explained as path or site effects, implying complexity in the rupture process or local fault zone structure. These findings demonstrate that earthquake source complexity can drive variability in the ground motions that control seismic hazard.

Pingo drilling reveals sodium–chloride‐dominated massive ice in Grøndalen, Spitsbergen

AbstractDrilling of a 21.8‐m‐deep borehole on top of the 10.5‐m‐high Nori pingo that stands at 32 m asl in Grøndalen Valley (Spitsbergen) revealed a 16.1‐m‐thick massive ice enclosed by frozen sediments. The hydrochemical compositions of both the massive ice and the sediment extract show a prevalence of Na+ and Cl− ions throughout the core. The upper part of the massive ice (stage A) has low mineralization and shows an isotopically closed‐system trend in δ18O and δD isotopes decreasing down‐core. Stage B exhibits high mineralization and an isotopically semi‐open system. The crystallographic structure of Nori pingo’s massive ice provides evidence of several large groundwater intrusions that support the defined formation stages. Analysis of local aquifers leads to suggest that the pingo was hydraulically sourced through a local fault zone by low mineralized sodium–bicarbonate groundwater of a Paleogene strata aquifer. This groundwater was enriched by sodium and chloride ions while filtering through marine valley sediments with residual salinity. The comparison between the sodium–chloride‐dominated massive ice of the Nori pingo and the sodium–bicarbonate‐dominated ice of the adjacent Fili pingo that stands higher up the valley may serve as an indicator for groundwater source patterns of other Nordenskiöld Land pingos.

Characteristics and mechanisms for origin of earth fissures in Fenwei basin, China

The most serious earth fissure disasters in China occur in the Fenwei basin. Since the late 1950s, earth fissure disasters have widely occurred in the Weihe, Yuncheng, Linfen, Taiyuan, and Datong sub-basins in the Fenwei basin, and such disaster has increased progressively in recent years. The current paper summarizes the characteristics of earth fissures in the Fenwei basin. Earth fissures usually occur in an extensional faulted basin, along an active fault zone, and within an area of land subsidence. Earth fissures often have a similar trend, with many branches that intermittently appear along their strike. The main and secondary earth fissures at a shallow depth form rupture belts of different widths, but they merge into a single shear belt and stay connected with the underlying active fault. The throw of two blocks increases with depth along a sedimentary fault. Earth fissures usually show the characteristic of three-dimensional movement. Vertical dislocation is the primary aspect of earth fissure movement. The horizontal tension is smaller and the horizontal twist is the smallest, which is consistent with the movement pattern of underlying active faults. Earth fissures have exhibited activity three or four times since the late Pleistocene. In the past half-century, earth fissures formed during four or five periods of peak activity were influenced strongly by groundwater fluctuation. Our research also indicates that cluster formation of earth fissures is polygenetic and results from a combination of tectonic and human activities. Earth fissures are driven by deep tectonic motions beneath the basin and are controlled by the local tectonic stress of the local fault zones. One of the most important factors contributing to earth fissure formation is groundwater overexploitation. This paper provides a scientific basis for proposed administrative means, mitigation measures and engineering solutions in the field of engineering geology.

Ore bearing factors of breaking-block structures of the Ukrainian shield

Most ore occurrences and mineral deposits are known to be located in tectonic fault zones. In this regard, the article analyzes the ore content of regional and local fault zones developed in the megastructures of the Ukrainian shield: Podolsky, Volynsky, Ingulsky, Dnieprovsky, Priazovsky megablocks and Belotserkovsky-Odessky, Inguletsko-Krivorozhsky, Orekhovo-Pavlogradsky interblock zones. Geological and structural-tectonic features of megablocks and interblock suture zones are characterized; the main stages of their formation are highlighted. Based on the analysis of the distribution patterns of various deposits and ore occurrences in the fault-block tectonics of the Ukrainian Shield, ore-bearing factors and metallogenic features of megablocks and interblock suture zones, which are determined by the geodynamic regime of these structures, are established; specific megablocks and suture zones have their own distinctive features, and the metallogeny of megablocks differs from the metallogeny of interblock suture zones. The metallogeny of the megastructures of the Ukrainian shield and the ore content of regional and local structures is due to a combination of a number of factors of various importance. The main ones are the processes of evolution of mantle fluids and the degree of oxidation of crustal products during geological time. Of great importance was the level of erosion cut of specific structures, which determined the physicochemical conditions for the concentration of specific ore elements.The metallogeny of various fault zones is determined by the combination of a number of favorable factors for the accumulation of metals in geological time and space. The most important ore-concentrating factors are the composition and quantitative ratio of ortho- and para-rocks and the presence in them of above-clark contents of characteristic metals; fault evolution, in which the mobility of metals and their concentration at less deep levels are activated; confinement to faults of igneous rocks; degree of eroded blocks; physicochemical features of fault rocks and uneven distribution of tectonic pressures; conditions and nature of the deformation of structures; lateral structure of faults.

Tomographic imaging of the NW edge of the Hellenic volcanic arc

The broader South Aegean area is characterized by the convergence of the African and Eurasian lithospheric tectonic plates at a rate of approximately 30 mm/year, resulting in a complex and intense deformation of the Aegean (McClusky et al. Geophys. Res. 105: 5695–5719, 2000). The Saronikos Gulf, situated in the NW termination of the Hellenic volcanic arc (Central Aegean Sea), is an area of special geophysical interest due to the variety of the recorded seismicity, regarding both the origin (tectonic, volcanic) and the focal depths. The presence of Plio-Quaternary volcanic centers and the complex seismicity distribution urge the need of detailed research on the geophysical properties such as seismic velocities, revealing possible presence of magmatic bodies or geothermal fluids. In the present study, more than 3000 manually revised events were located using a local 1-D layered velocity model. The main spatiotemporal clusters are concentrated along the Leuces and Methana-Poros E-W striking neotectonic fault zones. A local earthquake tomography (LET) was performed, using body-wave (P and S) travel time data, to investigate small to medium scale (~ 10 km) anomalies that can be related to local neotectonic or volcanic patterns. Major shallow ~ E-W to NW-SE trending discontinuities between positive (to the north) and negative (to the south) VP perturbations, anticorrelated to the respective VS image, are identified south of Aigina Island. These results are mainly related to the principal local stress orientation and fault zones (~ E-W to NW-SE strike), such as the Moni, Anghistri, and Leuces, whereas the respective Vp/Vs ratio values imply the presence of possible magmatic features which can be linked to the Plio-Quaternary NW edge of the Hellenic volcanic arc and hydrothermal activity ENE of Poros Island.

Reativation of Taxaquara Fault and its morphotectonic influence on the evolution of Jordão River catchment, Paraná, Brasil

The Parana basin is one of the major morphotectures of the South American continent. Although its tectono-sedimentary evolution has been widely studied and fairly well understood, this paper aims to fill a gap in knowledge comprising the tectonic evolution after the end of its last sedimentary cycle, in the Upper Cretaceous. In this context, the Jordan River watershed, situated in the surroundings of Guarapuava municipality, south central region of Parana State, was selected for structural and morphometric analysis where the Cretaceous volcanic rocks of Serra Geral Formation are exposed. The Jordan River watershed was studied for the influence exerted by Taxaquara Fault Zone on its morphologic evolution, since Taxaquara Fault Zone is associated to the Brazilian cycle and extends eastwards to the Ribeira belt in the state of Sao Paulo. The morphometric analysis consisted in the interpretation of the Jordan River watershed drainage network and relief elements, considering the distribution of existing knickpoints in the water courses. Structural analysis was based on the calculation of the stress fields responsible for the activation of local fault zones, which were determined by their spatial arrangement and the statistical and mechanical treatments of structural data. During the Oligocene and Miocene, erosional processes developed a planing surface which marks the relief in the central region of the Jordan River watershed and serves as a stratigraphic marker for the associated deformational events. Three events that contributed to the morphological framework of the Jordan River watershed were defined: an oldest one, probably active before the development of the Jordan pediplane in the Paleogene, by a NE-SW maximum horizontal stress (SHmax); and two more recent ones, being one of Plio-Pleistocene age, with a N05W SHmax, and a still active event of transtensive nature showing a N75W SHmax. The paleostress analysis points to a similarity between the Cenozoic evolution of Parana Basin and the tafrogenic basins of southeast Brazil, revealing the amplitude of the deformation events associated to the studied period and ensuring the importance of further studies on the morphotectonic evolutions of intracratonic regions of the South American plate and their correlation to the Andean tectonic cycle.

Large-scale trench-normal mantle flow beneath central South America

We investigate the anisotropic properties of the fore-arc region of the central Andean margin between 17–25°S by analyzing shear-wave splitting from teleseismic and local earthquakes from the Nazca slab. With partly over ten years of recording time, the data set is uniquely suited to address the long-standing debate about the mantle flow field at the South American margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench. Our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations, respectively. The teleseismic measurements show a moderate change of fast polarizations from North to South along the trench ranging from parallel to subparallel to the absolute plate motion and, are oriented mostly perpendicular to the trench. Shear-wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench-parallel but exhibit short-scale variations which are indicative of a relatively shallow origin. Comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement. To infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two-layer models, where the anisotropy of the upper (crustal) layer is constrained by the results from the local splitting. The waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more-or-less perpendicular to the trench. This orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab. The anisotropy within the crust of the overriding continental plate is explained by the shape-preferred orientation of micro-cracks in relation to local fault zones which are oriented parallel to the overall strike of the Andean range. Our results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab.

Variability of cytogenetic disturbances in Lonicera caerulea (blue honeysuckle) population in an active fault zone

Summary: Background. In active fault zones, geophysical and geochemical anomalies may have a genotoxic effect on plants growing there, as one of the factors of evolutionary transformation of plant populations.
 Materials and methods. We applied a cytogenetic analysis to evaluate the genotoxic effect on a Lonicera caerulea L. (blue honeysuckle) natural population in one of the active fault zones in the Altai Mountains.
 Results. We derived principal cytogenetic indices (i.e., mitotic, prophase, metaphase, anaphase, and telophase indices as well as proportion and range of abnormal mitoses) for meristematic cells of Lonicera caerulea seedlings. The increase in the mitotic activity of meristematic cells from the sites in the local fault zone is connected with the occurrence of the prophase-metaphase block to prevent consequences of an increased cell death (as a result of abnormal mitoses in these phases) and to compensate their losses by a greater number of divisions. We observed the increase in the proportion of abnormal mitoses in samples from almost all the test sites, compared with the control site. This demonstrates the increase in the genotoxic effect of geophysical and geochemical anomalies in these sites. The range of abnormal mitoses of samples from all the test sites shows the increase in the proportion of abnormalities in metaphase, compared to the control site where they can be equally found in metaphase, anaphase, and telophase.
 Conclusion. The results demonstrate changes of mitotic activity, frequency of occurrence and the spectrum of mitotic anomalies in the root meristem of blue honeysuckle, which grows in conditions with contrast geophysical characteristics.

Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA

We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.

The Influence of a Local Fault Zone on High Energy Tremor Occurrence During Longwall Mining of a Coal Seam

Underground mining of coal seams in the Upper Silesian Coal Basin in Poland is accompanied by seismic activity of varying magnitude. The investigations which have been performed for several years distinguished high energy mine tremors connected directly with mining or coupled with geological structures, such as large faults. In mined seams, local fault zones occur. Faults in these zones are usually small, with throws comparable with coal seams thicknesses. Local fault zone may be responsible for the occurrence of high energy tremors as well as large faults, as presented in this article. An analysis of source mechanism of high energy tremors generated during longwall mining of the coal seam No. 510, with presence of a local fault zone, in one of the Polish hard coal mines in the Upper Silesian Coal Basin was performed. For this purpose, the seismic moment tensor inversion method was used. In most of foci, the process of shear predominated. Determined nodal plane parameters were correlated with parameters of faults forming the local fault zone. High energy tremors were generated mostly by dislocations on faults of the local fault zone. Weakening of roof rocks in the neighborhood of local fault zone takes an important role too, and was responsible for share of implosion in the focal mechanism.

Fault zone evolution during Cenozoic tectonic inversion in SE Korea

Geometric and kinematic analyses were conducted on a fault zone at a construction site in SE Korea to understand its temporal and spatial evolution. This area is tectonically characterized by the existence of reversely reactivated faults associated with Cenozoic tectonic inversion. Structural elements within fault core such as slip surfaces and gouge zones are examined in a great detail based on grid-mapping analysis in a nearly three-dimension, combining the vertical sections with various directions. The highly deformed zone is composed of multiple gouge zones and geometrically characterized by anastomosing patterns. Kinematic indicators and their cross-cutting relationships indicate that the fault zone evolved by multiple slip events including reverse reactivation of a pre-existing normal fault zone. This implies that tectonic inversion is one of the most important factors controlling the complexity of the architecture of the studied fault zone, and it is important to understand the evolutional history of mature (or evolved) faults. Detailed analysis of geometric and kinematic characteristics of mature fault zones can provide very useful information to various research fields, such as fault zone evolution, interpretation of local tectonics, geotechnical stability, and fault zone modeling associated with fluid flow.

Study of soil gas radon variations in the tectonically active Dharamshala and Chamba regions, Himachal Pradesh, India

Soil gas radon measurements were made in Chamba and Dharamshala regions of Himachal Pradesh, India, to study the correlation, if any, between the soil gas radon, radium activity concentration of soil, and the geology/active tectonics of the study region. Soil gas radon surveys were conducted around the local fault zones to check their tectonic activities using the soil gas technique. Soil gas radon activity concentration at thirty-five different locations in Dharamshala region has been found to be varying from 13.2 ± 1.5 to 110.8 ± 5.0 kBq m−3 with a geometrical mean of 35.9 kBq m−3 and geometrical standard deviation of 1.8. Radon activity concentration observed in the thirty-seven soil gas samples collected from the Chamba region of Himachal Pradesh varies from 5.2 ± 1.0 to 35.6 ± 2.5 kBq m−3, with geometrical mean of 15.8 kBq m−3 and geometrical standard deviation of 1.6. Average radium activity concentrations in thirty-four soil samples collected from different geological formations of Dharamshala region and Chamba region are found to be 40.4 ± 17 and 38.6 ± 1.7 Bq kg−1, respectively. It has been observed that soil gas radon activity concentration has a wide range of variation in both Dharamshala and Chamba regions, while radium activity concentrations in soil samples are more or less same in both the regions. Moreover, soil gas radon activity concentration has a better positive correlation with the radium activity concentration in soil samples collected from Chamba region as compared to Dharamshala region.

Short Baseline Variations in Site Response and Wave-Propagation Effects and Their Structural Causes: Four Examples in and around the Santa Clara Valley, California

Abstract Ground motion records of local and regional events from a portable array are used to investigate the structural causes of variations in ground motion over distances of a few hundred meters to a few kilometers in the sedimentary basin environment of the Santa Clara Valley, California, and its margins. Arrays of portable seismic stations are used to target four study areas with different ground motion patterns: (1) an edge of the alluvial basin extending up onto a marginal ridge (Blossom Hill), (2) a Cenozoic basin with a nearly flat bottom (Cupertino Basin), (3) a long, narrow Cenozoic basin with a steep V profile (Evergreen Basin), and (4) a line perpendicular to the trace of the Hayward fault. Average peak velocities on Blossom Hill from local earthquakes are a factor of 2.5 times higher than nearby valley sites. Three-dimensional (3D) modeling is used to conclude that the majority of the amplification is due to lower shear-wave velocities along a local fault zone (Shannon–Berrocal). Site amplification over the Cupertino Basin in the frequency band 0.5–4 Hz is generally low (less than 2.0 relative to a Mesozoic rock site) and spatially uniform. This response is attributed to the shallow, flat-bottomed shape of the basin and the uniform, flat-laying sedimentary fill. In contrast, site amplification in the Evergreen Basin generally exceeds 3.0 and is attributed to the deep, V-shaped geometry of the basin and younger sedimentary fill. 3D waveform modeling shows the elongated shape of the Evergreen Basin causes more efficient trapping of long-period waves for sources along the long axis of the basin. A low-velocity zone is postulated along the Hayward fault with a width between 100 and 200 m, based on elevated site response along the fault trace and 4.5-Hz fault zone guided waves on the horizontal components of stations near the fault.

A Seiche Hazard Study for Lake Union, Seattle, Washington

Abstract Large seismic waves can be a result of earthquakes on local, regional, or distant fault zones occasionally setting bodies of water into oscillation. Seismic seiching is a recurrent phenomenon in the state of Washington and elsewhere (e.g., Berninghausen, 1969; Barberopoulou et al. , 2004; Cassidy et al. , 2005; Barberopoulou et al. , 2006). To investigate and characterize wave motion in lakes during strong ground shaking, various scenarios of seiche generation are considered for Lake Union in Seattle, Washington, as subjected to a variety of ground motions. Both relatively simple harmonic and more complex earthquake excitation (synthetic and real data) are used in the modeling. To isolate any effects resulting from a predominantly east–west or north–south motion or the contribution from parts of the shoreline, ground motions are first separated into their horizontal components (motions along latitude or longitude) before they are combined. This approach shows that long linear shorelines are the largest contributors to water wave heights. Depending on the shape of the lake, enhanced wave heights can also be a result of focusing. The wave motion in the lake is found to be predominantly east–west even when the ground motions are in the north–south direction. The results of this study suggest that coastal structures can be vulnerable due to resonant excitation initiated from earthquakes on water bodies far from tsunamigenic sources.

AS FONTES EM ITAMONTE, SUL DE MINAS GERAIS – UMA CONTRIBUIÇÃO PARA O ENTENDIMENTO DAS RELAÇÕES ENTRE ÁGUA SUPERFICIAL E SUBTERRÂNEA.

RESUMO O trabalho caracterizou três surgências em microbacia (0,72 km2) situada no município de Itamonte, sul de Minas Gerais. A microbacia é composta por rochas gnáissicas do Complexo Paraíba do Sul, e blastomilonitos do falhamento local. Os dois lineamentos estruturais, N–NE e N–NW, comportam o sistema de drenagem na área de estudo, cujo primeiro é o principal sistema e o segundo apresenta-se intermitente. A execução de 11 perfis pelo método de caminhamento elétrico indicou no sentido N-NE, uma zona de fraturas situada logo abaixo do canal de drenagem, e no sentido N-NW, deslocada 100 metros. As taxas de escoamento básico foram calculadas em 0,29 x 10-3, 0,70 x 10-3 e 5,13 x 10-3 m3/s com base nos hidrogramas das fontes 1, 2 e 3 respectivamente. As águas são de baixa mineralização e de composição bicarbonatadas mistas e cálcicas. O cálculo do balanço hídrico estimou para as fontes 1 e 2, taxa de infiltração de 146,44 e 149,33 mm/ano, em que suas bacias de contribuição são áreas de recarga. Para a fonte 3 estima-se uma contribuição significativa de toda a área de estudo e predomínio de condições de descarga do aqüífero. ABSTRACT This article studied three springs situated in a hydrographic basin (0.72 km2), in the municipality of Itamonte, south region of Minas Gerais State, Brazil. This basin is supported by gneissic rocks, from Paraíba do Sul Complex, and blastomilonites from local fault zones. There are two structural lineaments, N–NE and N–NW directions, supporting the drainage system of the study area. The main system is perennial and has N–NE direction; the second system (N–NW) is intermittent. The geophysics survey consisted by electrical resistivity, using the Schlumberger gradient array, which indicated fractures zones in N–NE direction, below the drainage channel; fractures zones displaced about 100 m in relation to the drainage channel was found in N–NW drainage system. Hydrograms of springs 1, 2 and 3 were used to calculate recessions flows of 0,29 x 10-3, 0,70 x 10-3 e 5,13 x 10-3 m3/s, respectively. These waters have low mineralization and calcium-bicarbonate and calcium-sodium bicarbonate composition. Water balances estimated an infiltration tax of 146,44 and 146,33 mm/year for the areas of springs 1 and 2, where their contribution basins were recharge areas. The spring 3 had aquifer discharge conditions.

Some problems of the tectonics of the central part of the West Siberian Plate

The folded consolidated basement of the central part of the West Siberian Plate is characterized by a complex block structure. In its structure, we may recognize rises, grabens, horsts, and regional and local fault zones that form them. The block nature of the basement has been responsible for the interrupted distribution of the sediments of the petroleum-prospective intermediate complex widely distributed in the troughs and decreasing in thickness sometimes to the point of wedging-out on the slopes of the major uplifts.

Further Notes on the Origin and Nature of the Currie Structure, Navarro County, Texas

Developments in the Mexia fault zone since January, 1923, have shown that the several oil and gas pools in this zone are bounded on the west by distinct faults which overlap one another in a definite manner. The conditions attending these faults suggest that torsional stress operated in their orlgin. In the producing fields the faults have been found to be relatively complicated. Correlation of logs in the Currie pool, described here as an example, shows that more than one fault is involved in the displacement, and that this local fault zone dips more steeply in the Austin chalk and Woodbine sand than in the softer Eagle Ford shale. The relation of the deep Woodbine or Morrow pay at Currie is considered.