Hydrothermal Circulation Beneath Mount Pelee Inferred By Self Potential Surveying, Structural And Tectonic Implications

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Journal Article: Hydrothermal Circulation Beneath Mount Pelee Inferred By Self Potential Surveying, Structural And Tectonic Implications

Abstract
Self-potential (SP) surveys were made on Mount Pelee volcano (Martinique Island, French West Indies) in 1991 and 1992 in order to recognize its hydrothermal system, the associated groundwater channeling and the main superficial structures of the massif. Almost 70 km of profiles were carried out with an average sample spacing of 50 m. Measurements essentially reveal negative SP anomalies, down to -1700 mV, with high gradients (-1.83 mV/m) due to the infiltration of meteoric water into the massif. Rims of summit calderas Morne Macouba and Etang-Sec present sharp negative SP anomalies on the western, northern, and eastern flanks. Negative SP anomalies indicate no upward water flow beneath Mount Pelee summit. On the southwestern volcano flank, a 3.5_6 km horseshoe-shaped structure corresponding to a southwest flank collapse event, older than 25,000 years BP, is clearly identified by the SP mapping. High gradients border the inner southern rim from Morne Calebasse to St Pierre town and the Caribbean Sea. Along the northern rim of the horseshoe-shaped structure the negative SP anomalies give place to a positive SP anomaly, up to 200 mV, of SW-NE trend. This zone covers the area of two active hot springs (Sources Chaudes and Puits Chaud: 40-65°C). Marine magnetic surveys and bathymetry show that the horseshoe-shaped structure spreads into the Caribbean Sea up to about 10 km from the coast. Buried structural discontinuities are evidenced inside the flank collapse structure. The upper one deviates the groundwater flow coming from the summit toward the south flank where the flow finds an indentation to expand again downwards. This discontinuity is either an old hypothetical caldera rim partly destroyed by the collapse of the south-southwestern flank and covered by recent pyroclastic deposits, or more probably the trace of a bulge landslide. A circulation model of the hydrothermal waters is proposed. Rainfall (5-6 m/year) is partly drained inside the summital calderas and the flank collapse zone through pyroclastic flows down to an impermeable basement. There the groundwater constitutes perched aquifers at the contact of the bulge landslide, or of the hypothetical old caldera rim. Along the inner northern border of the flank collapse structure the phreatic water is reheated. Warm groundwater flows along the northern avalanche structure rim and discharges near the coast in ground and marine outcrops, of medium temperature. Finally, the main part of the meteoric water is channeled along the old caldera rim, or along the bulge landslide towards the south flank of Mount Pelee, where some gaps in the rim exist. There the groundwater finds again a subhorizontal gravitational circulation along Mount Pelee slopes into the Caribbean Sea.

Authors 
J. Zlotnicki, G. Boudon, J. P. Viode, J. F. Delarue, A. Mille and F. Bruere








Published Journal 
Journal of Volcanology and Geothermal Research, 1998





DOI 
Not Provided
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Citation

J. Zlotnicki,G. Boudon,J. P. Viode,J. F. Delarue,A. Mille,F. Bruere. 1998. Hydrothermal Circulation Beneath Mount Pelee Inferred By Self Potential Surveying, Structural And Tectonic Implications. Journal of Volcanology and Geothermal Research. (!) .