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The Qualibou caldera, St. Lucia, West Indies

The Qualibou caldera, St. Lucia, West Indies,10.1016/0377-0273(86)90081-8,Journal of Volcanology and Geothermal Research,KENNETH WOHLETZ,G HEIKEN,M AN

The Qualibou caldera, St. Lucia, West Indies   (Citations: 9)
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Wohletz, K., Heiken, G., Ander, M., Goff, F., Vuataz, F.-D. and Wadge, G., 1986. The Qualibou caldera, St. Lucia, West Indies. J. Volcanol. Geotherm. Res., 27: 77--115. Recent geological, geophysical, and hydrogeochemical studies conducted in the Qualibou area of St. Lucia, West Indies, provide new data for reevaluation of the geo- thermal resource and recommendation of sites for renewed drilling activities. This work supports the original hypothesis of Tomblin that the Qualibou depression is a caldera. Precaldera volcanic activity was concentrated along faults associated with regional NE- and NW-trending structures. Basaltic lavas, dated at 5.5 Ma, crop out along the western coast and are overlain by andesitic composite cones, dated at 1.2 and 0.9 Ma, which form the highest ridges of the island. Superimposed upon the andesitic cones are dacitic domes (0.25 Ma), the eroded plugs of two of these form the spectacular Pitons. The major event in this volcanic field was the intermittent eruption of the Choiseul Pumice concurrent with the formation of the Qualibou caldera (32,000 to 39,000 yrs ago). About 6 km 3 (dense rock equivalent) of lithic-crystal andesitic tephra was erupted mainly as nonwelded to welded pyroclastic flows and surges. Some of these tuffs have been identified in geothermal drill holes within the 12-kin ~ caldera. Postcaldera eruption of dacitic tephra and dome lava (20,000 to 32,000 yrs ago) occurred from vents within the caldera and appear to be a result of magmatic resurgence. A 5.2-km-long dipole-dipole DC resistivity survey, measured along a north--south- trending line through the caldera gave apparent resistivity results similar to those obtained in previous studies. These results are compatible with a caldera substructure where low apparent resistivities (< 10 ohm-m) correspond in location to thermal upwellings along major caldera faults at depths of 1 km or more. Analysis and interpretation of hydrogeochemical data from the Qualibou caldera indicate that a geothermal reservoir underlies the Sulphur Springs area and consists of three layers: (1) an upper steam condensate zone; (2)an intermediate two-phase (vapor) zone; and (3) a lower brine zone. Measured temperatures at depth of 212°C are comple- mented by estimated temperatures of 250°C in the brine layer. The water chemistry of various thermal springs indicates upwelling primarily near the caldera center at Sulphur Springs, which feeds steam to steamcondensate hot springs along the northern caldera wall.
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