Hydrothermal system mapped by CSAMT on Karthala volcano, Grande Comore Island, Indian Ocean

Controlled source audio-magnetotelluric (CSAMT) has been used to investigat e the resistivity structure of the summit region of Karthala volcano. The m ajor purpose of this CSAMT survey is to locate the active hydrothermal syst em. The presence of the hydrothermal system had already been inferred from surface evidence of hydrothermal activity and from self-potential (SP) mapp ing of the Karthala summit zone. The results of the ID CSAMT inversion indi cate the following: (1) a highly resistive (500-5000 Ohm m) 200- to 400-m t hick surface layer, that is characteristic of dry basaltic rocks, and made up of lava flows and/or tuff breccias; (2) a 300- to 1200-m thick layer of intermediate resistivity (20-400 Ohm m), thought to he representative of th e groundwater body, and (3) a deep conductor with a resistivity of less tha n 2 Ohm m, which might be related to the active hydrothermal system. This hydrothermal system appears to be bounded by caldera edges and shows t he same north-south trend. Its depth ranges from more than 1 km to less tha n 0.7 km in the northern part of the caldera and its resistivity ranges fro m 2 to 0.5 Ohm m in the nor-them part of the caldera. Less resistive zones and the shallowest depths of this conductive layer are well correlated with the largest SP positive anomalies and are assumed to be generated by hot f luid circulation. The most active hydrothermal zone is situated in the nort hern part of the caldera. (C) 2001 Elsevier Science B.V. All rights reserve d.