Magnetotelluric and geomagnetic modelling reveals zones of very high electrical conductivity in the upper crust of Central Java

We present results of two- and three-dimensional modelling of magnetotellur ic and geomagnetic data from Central Java, Indonesia. We adopt a hypothesis testing approach, where we examine the influence of individual structural elements of competing models on the total model responses. The models show that ocean anomalies do not overwhelm the signal due to strong electrical c onductivity anomalies within Java. These highly conducting regions with res istivities of below 1 Omega m are located in the central and northern parts of the island. Fitting of the geomagnetic data requires that the innermost parts of both regions consist of material with extremely low resistivity ( 0.2 Omega m). Young sediments, tephra and their alteration products can be responsible for some of the low resistivity regions at shallow depths. The two zones of extremely high conductivity, however, are best explained as co nsequences of geothermal activity in the vicinity of active volcanism (Mt. Merapi) in Central Java and by fluid or graphite enrichment along shear zon es in northern Java. Subduction zone related processes in a depth range of 70-400 km cannot be resolved because the highly conductive regions at shall ow depths effectively screen deeper structures. (C) 2001 Elsevier Science B .V. All rights reserved.