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2 STRUCTURAL TECTONC ANALYSS OF AHUACHAPAN-CHPiAPA GEOTHERMAL FELD, EL SALVADOR. GARDUSO l4.v.t GONZALEB P.E.8 VERMA MOP., RENTERA T.D., TORRES RoVop NEVA Do AND FAS P o Geotermia, nstitute de nvestigaciones Electricas, A.P Cuernavaca, Morelos 62000, Mexico. ABSTRACT The Cocos plate is subducting under the Caribbean plate with an angle -40". The subduction has produced a graben (Central graben) and volcanic chain (Southern volcanic chain) in El Salvador. The area of Ahuachapan-Chipilapa geothermal field is highly fractured. Four important fault (fracture) systems have been identified: the Llano System (NE-SW), the Molino System (NW-SE), Chipilapa System (N-S) and Semicircular Cuyanausul System. NTRODUCTON The regional tectonics of El Salvador has been studied by many workers to define the position of the Cocos plate with respect to the Caribbean plate (Benioff, 1954, Servicio Geologico Nacional, 1960; Schulz, 1960, 1963; Dengo, 1968, 1983; Molnar and Sykes, 1969; Malfait and Dinkelmant, 1972; Ladd, 1976; and Schwartz and Cluff, 1979). The Quaternary faults in the region are related to the movement of Caribbean plate (Schwartz and Cluff, 1979). The National Geological Survey of El Salvador published a map of Bouguer's anomaly for the entire country in 1960, in which the negative anomaly regions correspond to young volcanic structures. n this article we present an analysis of satellite images in order to understand tectonic movement of the region. The interpretation is focused on the local and regional geodynamics and the distribution of tectonic stress in the region which may allow an understanding of the regional and local fractures. REGONAL TECTONCS The studied area is located in the west of El Salvador and is part of a volcanic chain situated on the western border of the central graben of the country (Figure 1). El Salvador forms an extreme occidental part of Honduras tectonic block (Dengo, 1968) and is part of the Caribbean plate (Malfait and Dinkelmant, 1972). The detachment of Honduras block started in late Cretaceous from south of Mexico and coupled with Caribbean plate in early Oligocene. t produced a transverse deformation to 'Sierra Chiapaneca', Mexico and 'Sierra Mesozoica', Guatemala. The Cocos plate, which is responsible of volcanism and seismicity in El Salvador, is actually located at the eastern edge of Pacific ocean to the west of Central America (Molnar and Sykes, 1969). The geometry of this plate has produced a zone which contains seismic epicenters, and geothermal activities and is about 150 km from the trench of Central America (Figure 1) Both the Caribbean plate and Cocos plate are moving in the same direction, but their velocities are different. This has produced a graben and a volcanic chain. The Central Graben of El Salvador and Nicaragua and Southern Volcanic Belt (Plio-Quaternary) are formed as a consequence of actual subduction of Cocos plate along the Middle Amereica trench. The distribution of volcanism in the region in time and space indicates that the angle of subduction of the Cocos plate has increased since early time of subduction. This may be due to the cooling and consequently increase in density of the Cocos plate with time. The Northern Volcanic Belt formed in the late Tertiary. The formation of the two volcanic belts is related to change in subduction angle of the Cocos plate and is explained by the'mapping of hypocenters to define the Benioff zone (Figure 1). The seismic studies in the area by Molnar and Sykes (1969) also show the present angle of subduction of Cocos plate as -40". The movement of the Caribbean plate towards the east along 'Motagua-Polochic- Jocotan' transverse fault has changed from 4 cm/yr in the Paleogene to 2 cm/yr about 2 m.y. ago and the movement produced various faults and formed a graben, 297

3 Gardufio, et.al 0 SECTON E Y f n Q) hypocen[er\. 200 NORTHERN VOLCANC BELT A + c tocean > o Continent Km 0 +i ( AFTER: MOLNAR AND SYKES 1969) Symbol SOUTHERN VOLCANC BELT, 0 Seismic hypocenter 0 Seismic epicenter A Quaternary volcanoes Thermal spring and geothermal area Figure 1 Tectonic location map of study area in El Salvador. The Section 1-1' shows the seismic activity along the line. t also explains the formation of Northern and Southern volcanic belts. 298

4 Garduifo, et. a1. Figure 2 The rosette diagrams to represent trends of faults and fractures in the region. The faults and fractures are obtained with satellite images. 1: Graben N-S, 2~Central depresion, 3.-Aligement, 4, Normal fault, 5.- Lateral fault, 6.- Volcanic domes, 7.- Colapse 299

5 Garduco, et.al. 'Graben de palal with north-south orientation (Figure 2). The orientation of the graben supports this hypothesis. Figure 2 presents the principal trends of fractures and faults which are inferred from the satellite images. t can be observed that 'Motagua-Polochic- Jocotan' system is a shear fracture. The movement of the Carribbean plate along the fracture has produced a complementary system (Rl- R2; Figure 3) which is usually termed as Riedel shears. A rosette indicates the structural morphological features of the region (Figure 2). The dark broad lines in a rosette represent quantity and orientation of the systems in the region. This helps in dividing the region in blocks. A block is the area containing similar rosettes. A structral map has been constructed, which divides the region in blocks (Figure 3). The blocks are segments of Caribbean plate and are formed due to movement of the plate across the 'Jocotan fault', subduction of the Cocos plate and movement of the Caribbean plate. The vectors of movement of Guatemala block are related to the Jocotan fault which is the limit between Caribbean plate and North American plate. The block of El Salvador has an extensive system. This is a result of a faster movement of the Carribean plate in the same direction as the subduction of the Cocos plate. The block of Honduras moves in the same direction as that of the Caribbean plate. The formation of the pala graben is associated with this block. LOCAL STRUCTURAL GEOLOGY Figure 4 shows the four principal geological structural systems of Ahuachapan-Chipilapa geothermal system: o u 2 pj3 H4 B 5 pj *g Figure 3 Principal movements of the blocks between Motagua -Polochic- Jocotan and the Central depression of El Salvador and Guatemala. 1.- Graben ( N-S ), 2.- Central depression, 3.-Fracture, 4.-Normal fault, 5.-Volcanic domes, 6~Collapse, 7r Movement vector of Honduras block, 8: Movement vector of Motagua - Polochic - Jocotan, 9.- Stress in the Central depression. The 'Llano' System (NE-SW This structure is dominant in the studied area and corresponds to normal faults: it is covered with basaltic lava in the geothermal field of Ahuachapan. t is in pyroclastic rocks, which forms part of the production zone of the geothermal reservoir. The 'Molino' System (NW-SE The fault system presents blocks which are falling almost vertical (angle of 80" to 89') towards NE. t affects basically the andesitic basement of 'Sierra de Tacuba' and the caldera of 'Ahuachapan-Concepcion de Ataco'. ChiPilana System (N-S The system is related to uplift of domes in the region and is youngest in the area. t is located between the systems, Molino and Cuyanausul, and is active. 300

6 Semicircular Cuvanausul System The Cuyanausul system corresponds to the ring structure of volcanic caldera of Ahuachapan-Concepcion de Ataco. The caldera of Cuyanausul contains volcanic edifices. A study of microseismicity in the area supports the existence of a shallow geothermal reservoir (Fabriol et al, 1990). The principal geothermal manifestations are related to the local faults and fractures. There are good geothermal prospects in the graben of Cerro-Blanco and the Cuyanausul-Tortuguero graben. CONCLUSONS A young volcanic belt has formed in the occidental margin of Central America. This belt contains most of the geothermal fields in the area. The Ahuachapan- Chipilapa geothermal system is part of this volcanic belt which is formed by the subduction of Cocos plate under the Caribbean plate at an angle of 40". n the zone of Ahuachapan-Chipilapa geothermal field four important geological faults are identified: The Llano System (NE-SW), The Molino System (NW-SE), Chipilapa System (N-S) and Semicircular Cuyanausul System. The zone defined as the NE border of the caldera of Ahuachapan-Concepcion de Ataco and its intersection with the graben of Cerro-Blanco, and the Cuyanausul graben has good possibility for geothermal energy exploitation. This is associated with secondary permeability caused by intense fracturing. ACKNOWLEDGEMENTS This research work has been carried out under the project, "Estudios Geocientificos y de igenieria de reservorios del campo geotermico de Chipilapa", which was funded by CEL, El Salvador. We wish to thank Dr S P Verma for reading the manuscript and the Editor for comments and suggests to improve it. REFERENCES Garduiio, et.al. BENOFF H Orogenesis and Deep Crustal Structure-Additional Evidence from Sismology. GEOL. SOC. AM. BULL., V. 26, 96p. DENGO G Estructura Geologica, Historia Tectonica y Morfologia de America Central. nstituto Centroamericano de nvestigaciones y Tecnologia ndustrial (CAT), Guatemala. DENGO G Mid America: Tectonic Setting for the Pacific Margin from Southern Mexico to Northwestern Colombia. Centro de Estudios Geologicos de America Central, Guatemala, Guatemala. FABROL H., BEAUCE, A. Y LE MASNE D Seismic Monitoring of Chipilapa Geothermal Area (El Salvador). J. VOLC. GEOTHERM. RES., V. 43. pp LADD J.N Relative Motion of South America with respect to North America and Caribbean Tectonics: GEOL. SOC. AM. BULL. V. 17 pp MALFAT S.T. and DNKELMANT M.G Circum-Caribbean Tectonic and gneous Activity and the Evolution of the Caribbean Plate: GEOL. SOC. AM. BULL., V. 83, pp MOLNAR P. and SYKES L.R Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity: GEOL. SOC. AMER. BULL., V. 80, pp SCHULTZ R Actividad Sismica en El Sa1vador:ENERG. GEOTERM., V. 1, 32 p. SCHULTZ R Estudio sobre la sismicidad en la region Centro-Americana: BOL. BBLO. GEOFS. Y OCEANOGRAF. AM. 111, GEOFSCA, MEXCO, pp SCHWARTZ, D.P. and CLUFF, D Quaternary Faulting along the Carribbean- North America Plate Boundary in Central America: TECTONOPHYSCS, V. 52, PP SERVCO GEOLOGC0 NACONAL Energia Geotermica. nforme No. 1, Ministerio de Obras Publicas, El Salvador. 301