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1.
Nine basaltic lava-flows, which vary in thickness between 60 feet and 300 feet, were established in the NW Rajmahals. The flows were, at places, laid down one above the other and, at others, were found to contain intervening intertrappean horizons. All the flows are essentially of basaltic composition and are made up of labradoritie plagioclase, pigeonitic and augitic pyroxene, opaque ore, primary glass and secondary minerals (palagonite, secondary silica, calcite and zeolite). The phenocrystic plagioclase ranges in composition between An72 and An62, while the constituents of the groundmass range between An50 and An17. The microphenocrysts of pyroxene are mainly augitic and occasionally pigeonitic while the constituents of the groundmass are essentially pigeonitic. The opaque minerals are magnetite and ilmenite. Petrographically, the lava-flows are more or less similar to one another. The first three flows are, however, more remarkably porphyritic and a little coarser in grain size than the six overlying flows. The eighth flow is devoid of palagonite. Calcite occurs only in certain portions of the second flow. There is a gradual increase in the percentage of primary glass from the first to the ninth flow with a corresponding fall in the total percentage of plagioclase and pyroxene. Statistical analysis of the grain size variation in the plagioclases was carried out and the results were found to be directly related to the prevailing rates of cooling in the different flows and also in the different horizons of the same flow. Modal analysis of the nine flows (in all, 98 samples) was carried out and this brought out some interesting results. Samples from three of the flows were analysed chemically and the corresponding norms were calculated. The order of crystallisation of the primary constituents was established from petrographic and petrological studies. The basaltic magma, which gave rise to the lava-flows of this region, does not appear to have undergone any significant differentiation during the course of its cooling and consolidation. The only discernible effect of crystallisati on differentiation was an enrichment of silica (and, perhaps, alkalis) in the residual liquids and no noteworthy enrichment of iron appears to have occurred at any stage.  相似文献   

2.
Concentrations of Cd, Cr, Cu, Ni and Pb were determined in filtered water, suspended particulate matter, and bottom sediments from a 2000 km section of the Ob and Irtysh Rivers. Dissolved Cd, Cr, Cu and Ni concentrations are similar to, or higher than, results from other Russian Arctic and large world river-estuaries. Concentrations of Cd, Cr, Cu, Ni and Pb in suspended particulate matter are generally comparable to results from other Russian Arctic and large world rivers and estuaries. Comparison of trace metal ratios in crustal material and suspended particulate matter and bottom sediment suggests that the source of Cr, Cu and Ni is continental weathering. Particulate Cd and Pb are elevated relative to their crustal abundance, suggesting a source of these metals to the Ob-Irtysh in addition to continental weathering.  相似文献   

3.
Summary A possible formal approach to a closed steady-state theory of the mean axially-symmetric variables is outlined. The approach involves alternating iterative solutions of the energy and momentum equations. In these equations the effects of transient eddy phenomena of all frequencies are assumed to be parameterized in terms of the mean symmetric variables.  相似文献   

4.
Deep (> 5 m) sheeting fractures in the Navajo sandstone are evident at numerous sites in southern Utah and derive from tectonic stresses. Strong diurnal thermal cycles are, however, the likely triggers for shallow (< 0.3 m) sheeting fractures. Data from subsurface thermal sensors reveal that large temperature differences between sensors at 2 and 15 cm depth on clear summer afternoons are as great as those that trigger sheeting fractures in exposed California granite. Extensive polygonal patterns in the Navajo sandstone are composed of surface-perpendicular fractures and were produced by contractile stresses. Numerous studies have shown that porewater diminishes the tensile strength of sandstone. Based on our thermal records, we propose that cooling during monsoonal rainstorms triggers polygonal fracturing of temporarily weakened rock. On steep outcrops, polygonal patterns are rectilinear and orthogonal, with T-vertices. Lower-angle slopes host hexagonal patterns (defined by the dominance of Y-vertices). Intermediate patterns with rectangles and hexagons of similar scale are common. We posit that outcropping fractures are advancing downward by iterative steps, and that hexagons on sandstone surfaces (like prismatic columns of basalt) have evolved from ancestral orthogonal polygons of similar scale. In lava flows, fractures elongate intermittently as they follow a steep thermal gradient (the source of stress) as it rapidly moves through the rock mass. In our model, a steep, surficial thermal gradient descends through unfractured sandstone, but at the slow pace of granular disintegration. Through time, as the friable rock on stable slopes erodes, iterative cracking advances into new space. Hexagonal patterns form as new fractures, imperfectly guided by the older ones, propagate in new directions, and vertices drift into a configuration that minimizes the ratio of fracture length to polygon area. © 2020 John Wiley & Sons, Ltd.  相似文献   

5.
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6.
We describe the stratigraphy, chronology, and grain size characteristics of the white trachytic tuff (WTT) of Roccamonfina Volcano (Italy). The pyroclastic rock was emplaced between 317 and 230 Ma BP during seven major eruptive events (units A to G) and three minor events (units BC, CD, and DE). These units are separated by paleosol layers and compositionally well-differentiated pyroclastic successions. Stratigraphic control is favored by the occurrence at the base of major units of marker layers. Four WTT units (1 to 4) occur within the central caldera. These are not positively correlated with specific extracaldera units.The source of most of the WTT units was the central caldera. Units B and C were controlled by the western wall of the caldera, whereas units D and E were able to overcome this barrier, spreading symmetrically along the flanks of MC. The maximum pumice size (MP) of units increases with distance from the caldera, whereas the maximum lithic size (ML) decreases. MP and ML of the marker layer of unit D (MKDa–MKDp) do not show any systematic variations with respect to the central caldera. In contrast, the thickness of surge MKDa decreases with distance from the source, and MKDp accumulates to the north of MC probably controlled, respectively, by mobility-transport power and by wind blowing northwards.The grain size characteristics of the WTT deposits are used for classifying the units. There is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units. Large variation of components in subunit E1, with repetitive alternation of pyroclastic flow to surge through fallout vs. surge deposits, suggests that the process of eruption took place in a complex or piecemeal fashion.Pumice concentration zones (PCZ) occur at all WTT levels on the volcano, but they are much thicker and pumice clasts are much larger within the central caldera. These were probably originated by the disruption of lava (flow or dome) to pumice fragments and fine ash due to sudden depressurization and interaction with lake waters of the molten lava. Local basal PCZ are, in some cases, similar to the lapilli-rich “layer 1P” that has been described elsewhere, and may have been deposited from currents transitional between pyroclastic surge and flow. Other basal PCZ formed in response to small undulations in the substrate, or can be originated by fallout. Lenticular PCZ within ignimbrite interiors and tops are interpreted to record marginal pumice levees and pumice rafts, some of which were buried by subsequant pyroclastic flows.Lithic concentration zones (LCZ) also occur at various stratigraphic height within the extracaldera ignimbrites, whereas intracaldera LCZ are absent, probably due to the fact that ignimbrite currents are strongly energetic and erosive near vent. LCZ at the top of basal inversely graded layers are formed by mechanical sieving or dispersive pressure in response to variable velocity gradients and particle concentration gradients (a segregation process). Coarse LCZ and coarse lithic breccias (LB), that reside in the interior or tops of pyroclastic flows and that occur in medial to distal areas, are interpreted to be the result of slugs of lithic-rich debris introduced by vent collapse or rockslides into the moving pyroclastic flows along their flow paths. These LCZ become mixed to varying degrees due to differential densities and velocities relative to the pyroclastic flows (desegregation processes).  相似文献   

7.
The basaltic rocks of the Hut Point area contain a diverse suite of inclusions. The xenolithic nature of many sedimentary, metamorphic and igneous inclusion types is clearly displayed, but previous studies have left considerable doubt as to whether the ultrabasic inclusions are noncognate xenoliths, and possible relicts from the mantle brought to the surface in a magma originating in this zone; or rather of cognate and/or cumulative origin. Olivine grains in the dunite and enstatite bearing peridotite inclusions commonly display undulose extinction and strain bands (translation laminae), and preferred indicatrix orientations expressed as B γ girdles. Enstatite frequently contains clinopyroxene exsolution laminae, and many grains are deformed and/or disrupted by shear zones. Weak B γ olivine fabrics have also been detected in the titanaugite peridotites and pyroxenites, but no evidence of post-crystallization deformation was detected. Titanaugite also occurs as phenocrysts (xenocrysts?) and in the groundmass of the host basalt chemical and optical data suggest that the titanaugite grains in the basalt and the inclusions are genetically related. These preliminary findings suggest that the titanaugite bearing inclusions may be of cognate origin, and that the dunite and enstatite bearing peridotite fragments are more likely noncognate xenoliths. Field and laboratory studies of this problem are continuing as supported by a grant from the National Science Foundation.  相似文献   

8.
Thermal waters hosted by Menderes metamorphic rocks emerge along fault lineaments in the Simav geothermal area. Thermal springs and drilled wells are located in the Eynal, Çitgöl and Na a locations, which are part of the Simav geothermal field. Studies were carried out to obtain the main chemical and physical characteristics of thermal waters. These waters are used for heating of residences and greenhouses and for balneological purposes. Bottom temperatures of the drilled wells reach 163°C with total dissolved solids around 2225 mg/kg. Surface temperatures of thermal springs vary between 51°C and 90°C. All the thermal waters belong to Na–HCO3–SO4 facies. The cold groundwaters are Ca–Mg–HCO3 type. Dissolution of host rock and ion-exchange reactions in the reservoir of the geothermal system shift the Ca–Mg–HCO3 type cold groundwaters to the Na–HCO3–SO4 type thermal waters. Thermal waters are oversaturated at discharge temperatures for aragonite, calcite, quartz, chalcedony, magnesite and dolomite minerals giving rise to a carbonate-rich scale. Gypsum and anhydrite minerals are undersaturated with all of the thermal waters. Boiling during ascent of the thermal fluids produces steam and liquid waters resulting in an increase of the concentrations of the constituents in discharge waters. Steam fraction, y, of the thermal waters of which temperatures are above 100°C is between 0.075 and 0.119. Reservoir pH is much lower than pH measured in the liquid phase separated at atmospheric conditions, since the latter experienced heavy loss of acid gases, mainly CO2. Assessment of the various empirical chemical geothermometers and geochemical modelling suggest that reservoir temperatures vary between 175°C and 200°C.  相似文献   

9.
Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.  相似文献   

10.
In the course of studying the Deccan Trap Hows around Igatpuri (latitudes 19°38′ and 19°45′: longitudes 73°30′ and 73°42′), picrite-basalts, not hitherto reported from this area, have been found occurring associated with basaltic flows. Thirty-eight flows, of a total thickness of 2200 feet, have been delimited. Of these, 8 flows may be termed picrite-basalts with a thickness varying from 25 to 75 feet. A feature of these basic flows is the abundance of olivine phenocrysts, with a complete absence of pyroxene phenocrysts in two of the flows. Two flows may be termed oceanite, two ankaramite, while four flows have phenocrysts of olivine, pyroxene and lelspar of An 65–70 %. In the oceanite flows the olivine phenocrysts constitute 20 to 30 per cent of the rock. They are mostly fresh, but some are altered to iddingsite. As regards the basaltic flows, about half have scanty olivine phenocrysts, the remainder being devoid of olivine. Eight are coarse enough to be termed olivine-dolerites. In the picrite-basalts the pyroxene phenocrysts have an optic axial angle of from 55° to 60°. In the basaltic flows the angle varies from 45° to 52°. The olivine phenocrysts of the picrite-basalts are highly magnesian, whereas those of the basalts are more ferruginous, as determined by optical methods. West’s view that the origin of these picrite-basalts is due to differentiation by crystal settling followed by freezing and extrusion, seems to be supported by this study.  相似文献   

11.
A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al2O3, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.  相似文献   

12.
Athens experienced a devastating earthquake on September 7, 1999, centred about 10km beneath the outskirts of the city, with peak accelerations in the meizoseismal area estimated above 0.5 g. While the magnitude of the earthquake was moderate (MS=5.9), damage and loss of life were extensive, numbering over 143 fatalities, 90 collapsed buildings, and over 100,000 rendered homeless. The most severe losses took place within 8 km of the fault, and peak rock accelerations in excess of 0.35 g were recorded 12 km from fault in downtown Athens. Local geologic effects played an important role, as demonstrated by concentrations of building damage on pockets of soft soil and near river canyons in the northwest part of the city. The earthquake was the first to severely damage Athens in over 2500 years. This study examines the engineering characteristics of 12 triaxial strong motion accelerograms recorded during the main shock. Properties investigated include peak ground acceleration and velocity, bracketed duration, Housner intensity, and response spectra. Spectral values at different orientations in the horizontal plane (“planar spectra”) are computed to infer the interaction of source directivity effects and building period, on damage potential. The role of rupture directivity close to the fault is investigated by means of idealized triangular pulses. Inelastic effects are examined using ductility spectra and sliding block analyses. Evidence is presented that low-rise structures in the area may have higher strength and longer natural period than those anticipated by building codes. The implications of the damage potential calculations for earthquake hazard assessment in Eastern North America and Northern Europe are also addressed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
The Vulsinian volcanic area in central Italy is made up of several volcanic complexes: the older east part of the area, with the large volcano-tectonic depression — in our opinion not simply a caldera — of the « Lago di Bolsena », and the younger Latera volcano are the major features. With the aid of field studies, paleomagnetic measurements, absolute age determinations and petrologic data, a scheme of the volcanic evolution has been drawn up, dividing the volcanic activity into six stages. Volcanicity started about 0.92 M.y. ago and lasted until subrecent. Three rock series are distinguished: a trachytic-phonolitic magma, mainly found as ash flows, a potassic mafic magma with its salic derivatives. mainly as lavas, scoria and pyroclastics, and a trachybasaltic rockseries, mainly found as lavas and scoria.  相似文献   

14.
The devastating earthquake on 26 January 2001 at Bhuj, India, resulted in large-scale death and destruction of properties of several million US dollars. The moment magnitude of the earthquake was 7.7 and its maximum focal intensity exceeded X in MM scale. The rate of aftershocks of this earthquake, recorded at Gauribidanur seismic array station (GBA), shows a monotonic decay with time superposed with oscillations. For the Indian continent the Lg phase is a prominent arrival at regional distances. The estimate of Lg amplitude is obtained by optimally fitting the Lg wave train to a exponential decay curve. The logarithm of these amplitudes and logarithm of root mean square (rms) value of actual amplitudes of the Lg are calibrated with USGS mb to create a local mbLg magnitude scale. The energy released from these aftershocks is calculated from the rms value of Lg phase. The plot of cumulative energy release with time follows the power law of the form tp, superposed with oscillations. The exponent of the power law, p, is estimated both by a time-window scanning method and by an interpolation method. The value of p is 0.434 for time-window scanning method and 0.432 for the interpolation method. The predominant periods found in the oscillatory part of the cumulative energy, obtained by differencing the observed from the power law fit, are 10.6, 7.9, 5.4, 4.6 and 3.5 h for time-window scanning method. The corresponding periods for interpolation method are 13.4, 11.5, 7.4, 4.2, 3.5, 2.6 and 2.4 h.  相似文献   

15.
Two new records of glacial stratigraphy obtained from borecores collected in southcentral Saskatchewan are compared to previously published records from Wellsch Valley and Swift Current Creek in southern Saskatchewan. The comparisons are based on magnetostratigraphy and tephrochronology, and describe preglacial, glacial, and interglacial deposits. The new stratigraphy is compared to the composite global marine oxygen isotope record. Although the ages and extents of late Pleistocene continental glaciations are relatively well constrained, they are less well defined for the middle and early Pleistocene. Data presented here highlight recent studies carried out from borecores and outcrops of extensive pre-Illinoian (pre-Saalian) glacial deposits. Based on this new data, at least seven Laurentide (continental) glaciations are recognized in Saskatchewan, and these records are the most extensive to date in the Northern Interior Plains of Canada. The magnetostratigraphic records from the Sutherland Overpass and Wascana Creek sites provide the first evidence of reversely magnetized glacial deposits in the Canadian Prairies. These deposits can be assigned to the latest Matuyama (MIS 20) and indicate that Laurentide (continental) glaciations did not impact southern Saskatchewan until the late Early Pleistocene.  相似文献   

16.
The eruptions, seismicity, and deformations, the properties of the magma feeding, and the mechanism of the activity of Klyuchevskoy, a giant basaltic volcano in Kamchatka, are considered. Twenty-eight author’s papers on the above subjects, published from 1985 to 2006, are reviewed. The activity of Klyuchevskoy the adventive and summit eruptions of Klyuchevskoy from 1986 to 2005 is described. The seismicity of Klyuchevskoy from 1986 to 2005 and its relation to eruptions are considered. It was inferred from geodetic measurements that the center of the magmatic pressure beneath the volcano moved in the depth range from 3 to 25 km during the period from 1979 to 2005. Based on previously developed models and observations from 1986 to 2005, the main properties of the Klyuchevskoy magma feeding system and the magma ascent in five major parts of the system are described and characterized: near the top of the plunging Pacific plate (with a depth of approximately 160 km), in the asthenosphere (160 to 40 km), in the region of the intermediate magma chamber where the magmas coming from below are accumulated (40 to 20 km), in the crust (20 to 5 km), and in the upper part of the system (from a depth of 5 km under the volcanic edifice to the crater at an altitude of 4.75 km). A comparison between the outputs from the summit and adventive eruptions on Klyuchevskoy as functions of time for the period from 1978 to 2005 shows that the probability of adventive eruptions should increase in the future.  相似文献   

17.
Laboratory measurements for compressional and shear wave velocities (Vp and Vs, respectively) and porosity were conducted with core samples from the Nobeoka Thrust Drilling Project (NOBELL) under controlled effective pressure (5–65 MPa at 5 MPa intervals) and wet conditions. Samples were classified according to deformation texture as phyllite, foliated cataclasite, or non‐foliated cataclasite. Measured values of Vp, Vs, and porosity are within a range of 5.17–5.57 km/s, 2.60–2.71 km/s, and 2.75–3.10 %, respectively, for phyllite; 4.89–5.23 km/s, 2.46–2.57 km/s, and 3.58–4.53 %, respectively, for foliated cataclasite; and 4.90–5.32 km/s, 2.51–2.63 km/s, and 3.79–4.60 %, respectively, for non‐foliated cataclasite, which are all consistent with the previous laboratory experiments conducted with outcrop samples under dry conditions. However, our results also indicate higher Vp and Vs and lower porosity than those measured by the previous studies that adopted the wire‐line logging methods. The variations in Vp, Vs, and porosity are controlled by deformation structure and are greater for phyllite and foliated cataclasite than for non‐foliated cataclasite.  相似文献   

18.
In this study, we preliminarily investigated the dynamic rupture process of the 1999 Chi-Chi, Taiwan, earthquake by using an extended boundary integral equation method, in which the effect of ground surface can be exactly included. Parameters for numerical modeling were carefully assigned based on previous studies. Numerical results indicated that, although many simplifications are assumed, such as the fault plane is planar and all heterogeneities are neglected, distribution of slip is still consistent roug...  相似文献   

19.
Over 180 springs emerge in the Panamint Range near Death Valley National Park, CA, yet, these springs have received very little hydrogeological attention despite their cultural, historical, and ecological importance. Here, we address the following questions: (1) which rock units support groundwater flow to springs in the Panamint Range, (2) what are the geochemical kinetics of these aquifers, and (3) and what are the residence times of these springs? All springs are at least partly supported by recharge in and flow through dolomitic units, namely, the Noonday Dolomite, Kingston Peak Formation, and Johnnie Formation. Thus, the geochemical composition of springs can largely be explained by dedolomitization: the dissolution of dolomite and gypsum with concurrent precipitation of calcite. However, interactions with hydrothermal deposits have likely influenced the geochemical composition of Thorndike Spring, Uppermost Spring, Hanaupah Canyon springs, and Trail Canyon springs. Faults are important controls on spring emergence. Seventeen of twenty-one sampled springs emerge at faults (13 emerge at low-angle detachment faults). On the eastern side of the Panamint Range, springs emerge where low-angle faults intersect nearly vertical Late Proterozoic, Cambrian, and Ordovician sedimentary units. These geologic units are not present on the western side of the Panamint Range. Instead, springs on the west side emerge where low-angle faults intersect Cenozoic breccias and fanglomerates. Mean residence times of springs range from 33 (±30) to 1,829 (±613) years. A total of 11 springs have relatively short mean residence times less than 500 years, whereas seven springs have mean residence times greater than 1,000 years. We infer that the Panamint Range springs are extremely vulnerable to climate change due to their dependence on local recharge, disconnection from regional groundwater flow (Death Valley Regional Flow System - DVRFS), and relatively short mean residence times as compared with springs that are supported by the DVRFS (e.g., springs in Ash Meadows National Wildlife Refuge). In fact, four springs were not flowing during this campaign, yet they were flowing in the 1990s and 2000s.  相似文献   

20.
Seismotectonic position of the Kaliningrad September 21, 2004, earthquake   总被引：1，自引：1，他引：0  
The paper presents an alternative consistent seismotectonic model of the Kaliningrad (Russia) September 21, 2004, earthquake according to which source zones of the two strongest shocks were confined to a N-S fault off the Sambiiskii Peninsula in the Kaliningrad region. A left-lateral deformation fractured a local crustal zone between the town of Yantarnyi and the settlement of Bakalino. The model was constructed with the use of a method developed by the authors for structural analysis of gravity and magnetic data. Initial materials are revised in terms of the EMS-98 macroseismic scale, and modified maps showing the shaking intensity in the NW part of the Sambiiskii Peninsula are compiled.  相似文献   

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