Role of Water in Conductive Anomalies and Seismic Reflections in the Lower Crust
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概要
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Relationships between electrical conductivity and seismic reflection in the lower crust are discussed in terms of the presence of aqueous fluids. Tectonic/geological interpretations of the lower crustal structures revealed by recent geophysical surveys are given for the region of stable cratons, convergent margins and rift zones, respectively. Most of the lower continental crusts of the stable craton are electrically resistive, and there are little correlations between electrically conductive zones and seismically reflective zones. In the convergent plate margins, the lower crust is conductive and seismic reflectors (or bright spots) are observed at mid-crustal depth. The rift zones are also characterized by both strong anomalies in electrically conductivity and seismic reflectors throughout the lower crustal column. The strong correlation between the conductive zones and the reflective zones observed from the active convergent margins and young rift zones is possibly explained by the presence of the free fluid phase. The distribution of fluid in the lower crust is discussed from the phase relation and wetting properties of constituent minerals. The presence of anhydrous granulites in the outcrops of the lower crustal sections and xenoliths derived from the lower crust indicates that the lower crust is generally dry. In the exposed lower crustal section of the Kohistan Arc hydrous minerals are localized in the vicinity of veins and ductile shear zones developed in the host anhydrous lithology. On the other hand, wetting behavior of plagioclase the most dominant mineral in the lower crust, indicates the absence of interconnecting aqueous and CO2-rich fluids through the equilibrium intergranular channels under lower crustal P-T conditions. It suggests that fluid flow in the lower crust is not pervasive but is localized in cracks and ductile shear zones. Ductile shear zones developed in the lower crust are the most probable sites for water pathways which are responsible for conductive anomalies and seismic reflectors in the continental crust revealed by geophysical observations.
- 2002-03-25
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