MAGNETOSTRATIGRAPHY OF THE AHUACHAPAN-CHIPILAPA GEOTHERMAL-FIELD, EL SALVADOR

The volcanic stratigraphy for the Ahuachapan-Chipilapa geothermal fiel d is defined on the basis of the magnetostratigraphic results on 156 o riented samples from 33 sites. The magnetostratigraphic sequence shows that the major volcanism associated with the Concepcion de Ataco cald era and the Cuyanausul volcano took place during the middle Brunhes ch ron (Quaternary). Pre-caldera activity of small centers such as Empali zada and Apaneca in the southern sector of the held occurred during th e early Brunhes (0.77 +/- 0.07 Ma). Basaltic-andesitic activity associ ated with the Cuyanausul volcano took place earlier, i.e. during the M atuyama chron, possibly around 1.3 +/- 0.15 and 1.7 +/- 0.3 Ma. The lo cal igneous basement is composed of Late Miocene-Pliocene andesites, i gnimbrites and volcano-sedimentary deposits. Normal polarities and a K -Ar date of 7.37 +/- 0.73 Ma indicate that the volcanic activity in th e study area extends beyond the Gauss chron. The polarity of some of t he units in the post-caldera sequence and in the Concepcion de Ataco a nd Cuyanausul sequences suggests that they may have recorded short pol arity subchrons. (C) 1997 CNR. Published by Elsevier Science Ltd.