This chapter contains sections titled: Introduction Definition of Fracture-Vein System Morphological Features of Fluid Feed Zones in Exploited Geothermal Fields Characterization of Productive Fractures Principle of Brittle Failure and Fluid Injection Formation Mechanism of Productive Fractures Conclusion
Sr isotope ratios are analyzed for voluminous acidic pyroclastics erupted at 1-3 Ma in five areas on the volcanic front of Northeast Japan. The initial values of 87Sr/86Sr ratios range from 0.7040 to 0.7055. There is no significant difference in ratios between 1-3 Ma acidic pyroclastics and 0-1 Ma andesitic volcanics in each area in spite of differences in age and in mean SiO2 content. On the other hand, the ratios in both of 1-3 Ma and 0-1 Ma volcanics vary along arc in the same manner. The changes of Sr and Rb contents in each area are consistent with systematic changes by fractional crystallization. The low 87Sr/86Sr ratios and chemistry suggest that 1-3 Ma acidic pyroclastics of Northeast Japan formed by a high degree of fractional crystallization from basic magma which is common in genesis with young andesitic volcanism. The mechanisms of the formation of the basic magma and the character of mantle source in each area have not changed for the past 3 Ma. Degrees of fractional crystallization changed with changes of the tectonic condition of shallow magma chamber from a weak horizontal compression stress field to a strong one.
A geothermal resources map of the Tohoku volcanic arc was compiled at 1:1,000,000 scale to allow a better understanding of the geothermal resources and geothermal heat sources of the area, from geology and hot spring geochemistry. The map was created using ARC/INFO. Some interesting features in the distribution pattern of the geothermal resources are indicated. Most high temperature resource areas (hot spring temperature > 90°C) are in and around Quaternary volcanic terrains with few exceptions. The volcanic front is delineated by the N-S trending eastern margin volcanoes. High potential geothermal resources are related to Quaternary volcanoes and/or concealed intrusive bodies, the most likely resource for geothermal electric generation. However, medium temperature resources areas (hot spring temperature; 42-90°C, geochemical temperature > 150°C) occur outside of Quaternary volcanic terrains, and are roughly arranged on NW-SE trending zones extending west from the volcanic front. This implies that these areas are heated by deep circulation of meteoric water along deep fracture zones from conductive heat transfer through highly conductive rock units. These areas could be regarded as identified potential fields for further geothermal exploration. However, geothermal exploration has already proved that most of these areas contain only medium enthalpy hydrothermal water (around 200°C), which is not enough for electricity generation. Medium to low temperature resources areas occur in late Neogene to Quaternary sedimentary basins, and are categorized as deep-seated hot water resources in sedimentary basins of non-volcanic terrain. This resources map is correlated with the distribution maps of heat discharge by hot springs, and depths to basement from gravity. Developed high temperature resources areas correspond to high heat discharging areas of hot springs and shallow basement. This suggests that these areas are heated actively by fluid circulation in Tertiary and Quaternary formations, and also heated by conductive heat transfer in pre-Tertiary basement.
Groundwater level lowering in the North China Plain (NCP) has been developing during past several decades by overuse in populated cities and industrial districts as well as by irrigation. To interpret groundwater level lowering in terms of aquifer-bearing beds having relatively high porosity, contour maps and cross sections of water level lowering based on monitoring data are compared with subsurface Quaternary geologic map. The comparison revealed that the extents of productive shallow and deep aquifers are generally restricted in specifi c regions where sand and gravel occur in lenticular form. The groundwater resources are regarded as fossil resource because groundwater of deep aquifer is dated as older than 10,000 years BP. Therefore, if overproduction of groundwater continues at the same rate as at present, it should result in depletion or severe lowering of water level and induce ground subsidence and water quality deterioration. To overcome the problems, effective use of irrigation and recycling of used water from industrial and domestic uses by water purifi cation are essential.
This paper briefly reviews the history and present status of magnetostratigraphy, and re-examines the usefulness of magnetostratigraphy in absolute chronology. At present, two different kinds of time scale are used as standards for magnetostratigraphic age determination. The first is that time scale developed from a combination of paleomagnetic polarity data and radiometric age dates derived from volcanic rock sequences on land. The second is based on the interrelationships between the sequence of geomagnetic reversals and marine planktonic microfossils. Usually, these time scales are assumed to be equivalent.Based on the collection of radiometric dates made on magneto- and biostratigraphically-studied sedimentary sections, the relationships between the geomagnetic-radiometric and geomagnetic-microbiostratigraphic time scales were examined. A distinct discrepancy was noticed between the radiometric age and the estimated absolute age from the magnetic-microbiostratigraphic scale, as indicated in Fig. 5. This discrepancy is worthy of serious consideration by uses of the geomagnetic time scale in absolute chronology. Further examination of various methods of measurement is needed to increase the reliability of magnetostratigraphy, microbiostratigraphy and radiometric chronology.In Japanese marine sedimentary sections, there exist many pyroclastic intercalations that may provide suitable materials for such examinations.
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