Gravity and magnetic assessment of the Tenorio area, Costa Rica

Abstract

The Tenorio area is located about 20 km from the Miravalles geothermal field which is a high temperature productive geothermal area, and is related to a major volcanic system referred to as the Tenorio volcano. This report partially represents the terminal stage of the pre-feasibility study done in 1989. It deals with the analysis of gravity and ground magnetic data over 550 km². The study starts with the density determination, followed by an anomaly separation from a statistically determined regional trend surface and ends with a frequency analysis of the Bouguer anomaly thus obtained. The analysis is carried out by means of directional filtering and frequency filtering, enhancing or removing linear features, and enhancing or removing spatial frequency ranges. The potential methods can be referred to as structural methods, regarding them as an extension of the geological mapping. Therefore, a structural synthesis map with the structures suggested in this report and the ones found previously by other means, is offered and briefly discussed within a geothermal context. A mass estimation of one of the larger anomalies is also offered. Two main fault systems are proposed: 1) A NW-SE system correlated with the oldest system whose signature seems to be traceable all over the southern part of the area; 2) An almost perpendicular NE-SW system, apparently much more recent, forming a pattern of blocks confined by these and the previous set of faults. A caldera border is extended after the analysis of the gravity map low-pass filtering. A negative magnetic belt is considered with a NE-SW strike overcoming the anomalies of the south. A possible association of gravity highs with magnetic lows is suggested.