Tectonic Fault Zones Research Articles

Origin of Pan-African mesothermal gold mineralisation at Bin Yauri, Nigeria

In the Bin Yauri area, mesothermal Au-sulphide-quartz-carbonate veins and associated hydrothermal alteration appear to have accompanied late tectonic brittle faulting which cut across the contact zone of a Pan-African granodiorite batholith intruding phyllites. Fluids of magmatic origin were initially suspected to be responsible for the Au mineralisation, due to proximity to the intrusion. However, studies of fluid inclusions, rare earth elements and tourmaline chemistry tend to suggest mineralising fluids of metamorphic origin. The present study involves the use of large ion lithophile elements (K, Rb, Ba, Sr) and stable isotopes (S, C, O) in further constraining the origin of the Au mineralisation. It is found that the hydrothermal alteration (and Au mineralisation) is characterised by a systematic coenrichment of K, Rb and Ba and inter-element ratios (K/Rb, K/Ba, Ba/Rb, Rb/Sr) characteristic of average (metasedimentary) crustal rocks. These are distinctly unlike the K-Rb-Ba-Sr fractionation patterns associated with magmatic-related mineralisation. Similarly, stable isotope evidence, where δ34S = 1.5–9.4‰ and δ13C = −7±0.7‰, are more consistent with crustal sources of S and C, respectively, while δ180 = 13±1.4‰ is within the isotopic range for metamorphic water. The study, therefore, has shown that the direct involvement of magmatic fluids can be ruled out as a source of the Au mineralisation at Bin Yauri, despite the close association with a granodiorite batholith. The fluids involved were probably generated along a regional (transcurrent) fault system and subsequently focused into the metasedimentary country rocks via the late tectonic fault zone.

Experimental study on the mechanism of non-synchronism of seismo-electromagnetic radiation precursors

In order to investigate the physical mechanism of non-synchronism of seismo-electromagnetic radiation precursors, we have made shear fracture and frictional sliding tests of rock samples by loading them in biaxial compression to model the activity of tectonic fault zones in seismogenic region. By installing antennae of different frequency responses and acoustic transducers around the fracture surface, the signals of electromagnetic radiation and acoustic emission in different directions and of different frequencies produced by rock samples during their shear fracturing and frictional sliding have been recorded by an automatic, high-speed and continuous observation system. Some implications given by the experimental results are as follows. The authors hold that electric signals (E) and magnetic signals (M) are of different genetic mechanisms: electric signals from inside the rock sample are produced by the piezoelectric effect of rock-forming crystals and net electric charge produced by the newly created surfaces due to rock fracture, whereas magnetic signals are produced by the high-speed motion of charged rock fragments and the ionization of ambient air excited by electrons emitted by rock in fracturing. Analysis shows that in some moderately strong earthquakes the electric and magnetic signals also showed the phenomenon of non-synchronism: the electric signal was earlier than the magnetic signal. If the same station observes simultaneously the electric field (underground) and magnetic field (air), the efficiency of earthquake prediction by seismo-electromagnetic radiation precursors would be raised effectively.

Stressed state of a rock mass with protective extraction in the wings of tectonic faults

For the October deposit conditions extraction of the gently-sloping deposit of copper-nickel ores within the wings numerous faults intersecting the ore body is connected with increased danger of rock bursts and the requirement for carrying out a considerable amount of labor-consuming driving of dead ends. Mining is carried out in a continuous sequence by layers and chambers with stowing of the worked-out space with consolidating mixtures. Previously by driving and filling parallel cross drifts over the upper contact of the deposit a continuous concrete layer is built. The workings are orientated almost along the strike of tectonics fractures [1]. In seeking the possibility of reducing the volume of driving in zones of tectonic faults industrial experiments were carried out in two areas of the deposit intersected by faults. In contrast to the traditional scheme for constructing a protective slice there was simultaneous erection of sections of a protective slice within the wings of a fault before approaching the extraction front of the main reserves towards the tectonic fault zone. It was assumed that presence of an additional front of a protective slice leads to mutual shifting of the wings of a tectonic fault with loosening of the ore mass. As a result of this treatment of the reserves in the wings of the fault was accomplished without creating a protective slice beneath part of the ore bounded by the overlaying strata. The set of instrument and full-scale observations confirmed this assumption. In test areas both in stopes created beneath the protective slice and in the overworked part of the ore body no signs of increased mine pressure were noted [2]. However, industrial experiments were limited by the specific mining and geological conditions. Observations and measurements made in the test areas give information about the mechanical state of the rock mass at a limited number of points. Transfer of the technology to other conditions and depths requires additional substantiation and studies connected with considerable expenditure and organizational difficulties. In this situation it is possible most simply by means of mathematical modelling to reveal the most common features of stress field formation within the vicinity of a fault. Calculations of the stress-strain state for a rock mass were performed by the finite element method. In the mathematical describing material deformation the rock mass outside the plane of a tectonic fault was represented by elastic materials with corresponding mechanical properties. This assumption is correct in view of the fact that the rocks comprising the rock mass have high elasticity constants and the theological properties appear to an insignificant extent. The deformation of a tectonic fault was described by an elastoplastic material reflecting the main features of fault behavior under load [3, 4]. The original stressed state for the ore and surrounding rock masses is characterized by the presence of tectonic forces. Horizontal compressive stresses are 0.6-1.7"rH, and vertical stresses are 0.6-1.43,H (H is working depth). The most clearly expressed regions of disturbance of the original stressed state are confined to large amplitude faults. According to measured data in them both horizontal and vertical stresses may predominate which may lead to a marked difference in the mechanism of rock mass deformation within the vicinity of a fault on introducing mine working. In the present work the case is considered when within the vicinity of a tectonic fault horizontal stresses predominate. The original stress field in all of the calculations discussed below is adopted as follows: Cry = 3'H, a x = 1.53,H, rxy = 0. Corresponding to this at the outer boundary of the calculation region normal and tangential force components were prescribed. The stress -strained state of the rock mass was determined for the plane strain condition which is fulfilled here due to the considerable extent of a tectonic fault, and stoping and preparatory workings in direction perpendicular to the section in question. Values of mechanical properties for the ore body, the surrounding rocks, and stowing material were taken as typical for the October deposit conditions [5]. The corresponding values of Young's modulus were 5" 104, 8- 104 and 3.104 MPa, and for Poisson's ratio they were 0.25, 0.2, and 0.4. The ore body thickness was 25 m.

Earthquakes in northern Europe in 1375–1989

An updated version of the catalogue of earthquakes in northern Europe is presented. It comprises events inside a region window of about 55–80°N and 10°W–45°E, covering a 615-year period from 1375 to 1989. The data have been compiled using all the available historical publications, catalogues, studies and reports of the region, and for events from 1984 to 1989, a regional seismic parameter database maintained by the Institute of Seismology of the University of Helsinki. The number of events exceeds 5200, of which more than 40% have been registered since 1980. The huge increase of event reports is the result of the installation of new high-frequency networks for microearthquake studies and the upgrade of the seismic instrumentation. Because of the different degree of accuracy and homogeneity of the parameter data, the events were divided into historical (1375–1964) and instrumental (1965–1989) datasets. The source parameters are mainly macroseismic for historical events, and obtained by iterative location procedure for instrumental data. By assuming internal homogeneity between the different regional magnitude scales, we estimated the catalogue to be complete for events with magnitude ≥ 4.5 since the 1880's, magnitude 4.0–4.4 since the 1940's and magnitude 3.5–3.9 since the 1970's. Although the distribution of seismicity exhibits similar patterns in both datasets, the clustering of epicenters and offshore seismicity is more distinct in the instrumental period. The occurrence of the greatest events can be seen more clearly in the historical data. The earthquake epicenters are concentrated in the geologically younger tectonic provinces in the western part of the region and follow, with some exceptions, the tectonic fracture and fault zones. In the Fennoscandian area, 80% of the earthquakes occurred in the upper 20 km of the earth's crust. The deepest epicenters are in the southwestern corner of the region where the crust is thinnest.

The geology of the Madina-Kouta basin (Guinea-Senegal) and its significance for the geodynamic evolution of the western part of the West African Craton during the upper proterozoic period

Abstract The Madina-Kouta basin is a southward extension of the upper Proterozoic Taoudeni basin. This sedimentary basin formed in a SW-NE tectonic fault zone in the Leo shield at ∼ 1000-800 Ma. The deposits indicate a quiet and slightly subsiding area. An important uncorformity allows us to distinguish two lithostratigraphic groups: the Segou Group below and the Madina-Kouta Group above. Sedimentation in this area ended with the Late Proterozoic glaciation 650-620 Ma. The correlations with the Taoudeni basin show a close relationship between the foreland area and the Mauritanides/Bassarides fold belt during the Pan-African I cycle (800-650 Ma). In particular we suppose that the lower part of the Taoudeni basin deposits which correlate with the Madina-Kouta Group (group of Souroukouto) is on the passive margin of the Pan-African I rift, and the post-Madina-Kouta deposits in the Taoudeni basin (Group of Bakoye) correspond to the molassic stage of the Pan-African I fold belt (rift closure stage).

Geochronology of the Aldan Shield, southeastern Siberia

Crystalline Precambrian rocks exposed in the basins of the Olekma and Aldan Rivers represent the southeastern part of the Siberian Platform. To the north and east the basement is covered by Paleozoic and Upper Proterozoic formations. To the south and west lie younger deformed belts.The Aldan Shield can be divided into four regions. The Tcharskaja massif, in the west, is the most ancient block. Pb–U dates on allanite and zircon show a good concordance at 2950 m.y. The prevalent rock type is gneiss and migmatite.In the Aldan River basin three thick series of Lower Proterozoic rocks were deposited during the time interval 2000 to 2700 m.y. Granulite facies metamorphism of these rocks occurred 1800 to 2100 m.y. ago. The time of deposition of the lowest formations (Iengra Series) is given approximately by measurement of the isotopic composition of lead in different syngenetic mineral phases. Using muscovite and magnetite, the 207Pb/204Pb:206Pb/204Pb isochron yields a value of 2550 ± 300 m.y.In the east the crystalline basement of the Aldan complex is covered by thick Proterozoic sedimentary formations. These formations are intruded by alkaline rocks, dated at 1100 m.y. and small ring structures with associated carbonatites, dated at 600 to 650 m.y. The magmatic cycles, combined with glauconite age determinations, indicate a nearly complete, Middle and Upper Proterozoic stratigraphic section. The base of the Middle Proterozoic is established at 1900 ± 50 m.y.The Aldan complex, and the Stanovoi complex to the south are separated by a wide belt of tectonic fault zones. The prevailing metamorphic grade of the Stanovoi complex is amphibolite facies, with only local areas of granulite facies rocks. Pb–U ages from monazites, zircons, and allanites from pegmatites cutting the complex date the amphibolite facies metamorphism at 1900 m.y. Available data suggest an Archean age for the granulite facies rocks. The Stanovoi complex has been affected by at least two subsequent magmatic periods; the first were syenitic intrusions at 1100 m.y., and the second Jurassic granites at 150 m.y. The last magmatic cycle, found in many parts of the Aldan Shield, is an excellent demonstration of the recurrent rejuvenation of Precambrian terrains.