PLIOPLEISTOCENE VOLCANIC HISTORY OF THE AHUACHAPAN-GEOTHERMAL-SYSTEM,EL-SALVADOR - THE CONCEPCION-DE-ATACO-CALDERA

The Concepcion de Ataco caldera in the Ahuachapan-Chipilapa region is part of the southern, Plio-Pleistocene volcanic belt that borders the Central Graben of El Salvador. About 17 km(3) of primitive lavas erupt ed between 1.7 and 0.77 Ma, covering basaltic-andesitic lavas and olde r agglomerates forming the Cuyanausul, Apaneca and Empalizada volcanoe s. These lavas preceded the evacuation of 63 km(3) of silicic pyroclas tics (i.e. breccias, surge deposits, and ignimbrites) associated with the main caldera cycle. The eruption of this pyroclastic material led to the collapse of a 5 km by 3.5 km caldera between the times of the y oungest pre-caldera eruptions (0.77 Ma) and the intrusion of the Himal aya dome (0.28 Ma). The caldera collapse was reactivated at its northe astern flank by a new eruption of pyroclastics of limited areal distri bution. These distinctive pyroclastics correspond to laminated tuffs ( ''Cebra tuffs''). Domes of dacitic-to-andesitic composition erupted al ong the caldera border as well as within it. Their magmatism is calc-a lkaline; their ages vary between 0.28 and 0.1 Ma. Three phreatic explo sions, whose craters are in the eastern part of the Concepcion de Atac o caldera structure, only deposited debris locally. A hydrothermal sys tem was emplaced during the final stage of the post-magmatic activity, evidenced by areas of hydrothermal alteration and surface manifestati ons. Four main fault systems, oriented NE-SW, NW-SE, NNW-SSE, and semi circular, have been identified. These faults created relatively high-p ermeability regions within the caldera and nearby grabens, that are fa vorable for the development of geothermal systems. (C) 1997 CNR. Publi shed by Elsevier Science Ltd.