INTEGRATIVE MODEL OF AN ACTIVE HYDROTHERMAL SYSTEM, MASAYA VOLCANO, NICARAGUA

We discuss the relationship between geologic structure, SP and CO2 anomalies in a small area (about 150 m square) on the flank of an active volcano in Central America. In August 2007 we carried out a ground magnetic survey at a fumarole area on the flank of Masaya Volcano in Nicaragua. The structural model was estimated from the result of this survey and compared to SP and CO2 flux measurements recorded in May 2006 in the same area. The ground magnetic measurements were taken every second with a Geometrics, Inc. Portable Cesium Magnetometer G858 along roughly parallel lines. The sensitivity of the magnetometer is 0.01nT. Measurement locations were recorded using a GPS (Leica Geosystems Inc. GS20 Professional Data Mapper) and compared with another base station GPS data to improve the data accuracy. SP and CO2 flux profiles were completed in May 2006 using 1 m grid spacing for SP measurements and 2 m grid spacing for CO2 flux measurements. SP measurements were made using non-polarizing Pb-PbCl2 electrodes and a high impedance voltmeter and CO2 flux measurements were made using a LI-COR, Inc. Li-800 portable gas fluxmeter. Modeling the magnetic data indicates that topographic offsets correspond to SE dipping normal faults that form a massive system transecting the volcano. A NE trending positive SP anomaly about 30 m in width corresponds to the footwall of these faults. In contrast the SE side of the map shows a clear negative anomaly with a sharp boundary which corresponds to the topographic offset at the surface. CO2 flux profiles show similar changes, although the changes are more abrupt than those seen in the SP profiles. However, high CO2 flux was detected in the foot-walls of faults. Together, these data allow us to construct a 3D model of fluid flow in the fault system.