First deep electrical resistivity section of Costa Rica

摘要

First long-period magnetotelluric investigations were conducted in early 2008 along a profile extending from the coast of the Pacific Ocean to the Nicaraguan border, traversing the volcanic arc in northwestern Costa Rica. The aim of this study is to gain insight into the electrical resistivity structure and thus fluid distribution at the continental margin formed by the subduction of the Cocos plate beneath the Caribbean plate. Data near the coastline of Nicoya peninsula show a remarkably large coast effect with tipper values exceeding 1.1 due to the proximity of the trench and the resistive ultramafic complexes of the peninsula. This effect decreases rapidly farther inland due to the conductive Tempisque Basin. Beneath the backarc basin a bad conductor at moderate depths images the basement with a clear termination in the south; this gap is interpreted to map the Santa Elena suture zone. The volcanic arc shows no pronounced anomaly at depth, but a moderate conductor underlies the backarc with a possible connection to the upper mantle. A stable feature of the model is a conductor at deep-crustal levels in the forearc, which may reflect fluid release from the downgoing slab, leading to serpentinization of.the forearc mantle.