Geology of the Ahuachapan-Chipilapa, El Salvador C.A
Total Page：16
File Type：pdf, Size：1020Kb
Gonzalez et GEOLOGY OF THE AHUACHAPAN-CHIPILAPA, EL SALVADOR C.A. GEOTHERMAL ZONE Eduardo Gonzalez Vicente Torres Peter Victor Arellano Alejandro Campos Instituto de Investigaciones Electricas (IIE), Depto. Geotermia, Apto. Postal 475, Cuernavaca, Mor.. 62000 Mexico Comision Ejecutiva del Lempa (CEL), San Salvador, El Salvador C.A. Keywords: El Salvador. Ahuachapan-Chipilapa geothermal field, volcanology. caldera, stratigraphy, tectonics, hydrothermal activity, magma ABSTRACT The caldera Concepcion de Ataco in the region of Ahuachapan- shows characteristics of a calcalkaline series, by an active Chipilapa represents part of the southern, Plio-Quaternary volcanic continental margin The age varies between Middle Pleistocene and belt of Salvador, which one of the borders of the Central Holocene Graben of alkaline lavas were emitted above a basaltic- Three craters of phreatic explosion type. which are mapped in the andesitic basement and older agglomerates, forming the volcanos east of the caldera structure, released debris locally This activity Cuyanausul, Apaneca and Empalizada, which preceeded the corresponds to a postmagmatic hydrothermal phase. representing evacuation of 70 of silicic pyroclastics The pyroclastic the first superficial occurrence of the emplacement of an active sequence is formed by breccias, surges, and ignimbrites Flows hydrothermal system in this region occupy the major part with a lateral distribution of 27 I The most distal pumice and ash fall deposits are distributed over an area of 484 The release of this material caused a caldera collapse INTRODUCTION with a dimension of km x kin. The river carves its bed into this depression El Salvador is one of the countries of Central America that depends The caldera collapse was reactivated at its NE-tlank by a new on geothermal energy for power generation The southern volcanic evacuation, emitting pyroclastics with limited distribution. The belt represents the principal target of exploration. with major pyroclastics correspond to laminated tuff (“Zebra or brown geothermal zones, hot springs, hot wells. fumaroles and tuff of hydrothermal activities (Meyer-Abich, The increased Dome injections of dacitic to andesitic composition partly border interest in geothermal energy has been the of its proven the collapses and the internal parts of the caldera The magmatism economic advantage over all energy (Campos. 1988) N Fig. Localisation of the studied area with symbols (filled circle seismic epicenters, tilled triangles. Quaternary volcanos, filled squares thermal springs and geothermal fields) 683 et al. This paper merges responses fi-om the Ejecutiva Rio Pre caldera Lempa to understand the volcanic arc related to heat sources. Also, it illuminates the geologic pattern The three volcanos Apaneca, and Einpalizada were which determines the of the fluids inapped (Fig.2) first one of an sequence of in the zones of Ahuachapan and Chipilapa The study of the lava, scoria of andesitic to basaltic coinposition. corresponding to a stratigraphy and the structural behaviour is the subject of this stratovolcano type. whose age ranges from work to I 7 0.3 M a features are by faults The Ahuachapan - Chipilapa geothermal field is located kin west and zones of activity affected the northern of San Salvador and kin froin the border to close to producing advanced clay of the city of Ahuachapan and Atiquizaya Turin (Fig. studied Tortugero discharge vapour Low resistivitiec ai-e related to zone covers an area of 200 and comprises parts ofthe counties the graben with the name. which is an deep of Santa Ana, and Sonsonate drilling in the area of Chipilapa is a volcano of the type as Cuyanausul, but is not affected by and tectonics. Its basalt flows include plagioclase phenocrysts RESIJLTS pyroxenes which arc intercalated with scoria The consists of olivine-basalt lava and with an age of I. Volcanic 7 M a is a caldcr-a collapse. marked the bed of On the other tlie bed is The local basement covers the area south of de Ataco covered pyroclastic products of de and the total area of the Sierra de Tacuba Its lithology consists of Ataco The previous to tlie forination of the interlayers of andesitic-basaltic and breccias with the has a calc-alkaline chemical composition and coinposition The age ofthe andesites is 7 37 0 M a Towards estimated to I7 et , ) the north, the Forination fills the Central Graben with tuffs, and ancient pyroclastics Electroconsult, 1082) 322 320 318 3 3 2 310 308 306 304 3 02 300 394 396 398 400 402 406 408 410 412 414 416 418 420 422 424 426 Fig. 2: Geological scheme of the geothermal at El C A 684 Gonzalez et al. Caldera volcanism and fission-track measurement confirm that the caldera released its silicic products within the period from Middle to Morphological features and the interpretation of' satellite images Late Pleistocene Numerous rock samples were taken from the show up clearly the Meseta Concepcion de Ataco-Apaneca. which ignimbrites for studies of reinanent which shows forins a governed by the river normal polarity for the paleomagnetic event (Urrutia et (Fig. 2) The diameter ofthe oval alignment x 4 kin) corresponds , Fission-tracks in volcanic glass of the pyroclastics give to the caldera collapse an age of 4. I93 years The complete stratigraphic column, which crops out between the The extreme NE ofthe caldera was reworked by the formation ofa river (Rio Nejapa) and de Ataco-Apaneca. is shown nested caldera. which released "Zebra Tuff' or brown tuff These in Fig. 3 It is characterized by a sequence of pyroclastic products. new pyroclastics products have an approximate thickness of which represents the petrological phases of an explosive and their spatial distribution is limited. caldera event, defined by et Towards the base of the column. with the volcano 1.3. Post caldera volcanism Empalizada, deposits. with a thickness of about 20 and textures of and veiy abundant pyroclastic The volcanos Cerro de Oro, Las Ninfas, and Laguna Verde closed were observed The pyroclastic tlows are covered by and the cycle related to the calderic evolution of the region ash fall deposits thickest section was found inside the ring The volcanic exposures consist of effusive intercalated with caldera with variatons between 640 and scoria. Petrological and geochemical studies define a basaltic Petrological and geochemical studies indicate a dacitic-rhyolitic composition with calcalkaline characteristics Numerous domes composition with calc-alkaline tendencies and forination surround the calderic collapse and the principle mapped faults (Fig. temperatures of approximately for the evacuation 1). The volume of the magma released is 6 and surface rocks moment of the pyroclastic material data (Tor-res. show an average total density of 2.41 No K-Ar ages were 1991) gives a total density of 0 for the and an achieved, because the rocks are very recent Also. the abundance of average value of I for the Ar is below the detection limit of the method of years The lithological scheme of Fig. 3 the distribution of the The fluid lavas, which were emitted by the volcanos Las Ninfas and pyroclastics. The flows cover an area of 23 I and the distal Laguna Verde at their final stage, extend broadly up to the valley, pumice an area of 484 with the annotations of the indicated which forins the Central Graben They occupy an area of thickness The total volume of the pyroclastic inaterial released by approximately 39 with a variable thickness between 80 and the caldera comprises approximately 70 m Their extension towards the west is limited by the lateral contact with pyroclastics, in which the Ausoles carves its bed The front of the lava, covered by pumice of the LITHOLOGY Coatepeque event, can he at the crossing of the pumice Ahuachapan road The detrital avalanches from Hoyo del Ninfas-Laguna Surface hydrothermal alteration, fumaroles and hot springs Verde correspond to fragmentary deposits which crown the Debris, avalanches: Las Ninfas parts of the volcanos Laguna Verde and Las Ninfas Mapped from del and L. Verde Gonzalez et they represent products from phreatic Monogenetic volcanic activity: explosions an area or with a volume than and dome Hydrothermal activity (Fig. has exposed zones of sui-face Post-Collapse volcanic activity: alteration, generally associated with fumaroles The inineralogy is Las Ninfas, Laguna Verde and characterized by clay mineralization, which retlects an acid medium Cerro de basalt COLLAPSE with temperatures close to indicating the occurrence of Ash and surge to hydrothermal fluids The field relation between principal structures "upper brawn tuff and the of geothermal sites is direct evidence for the importance of secondary permeability in a volcanic medium by means of ascending tluid in with structural Fine ash deposit, graded-ash flow; debility Pyroclastic fall (including ignimbrite, pumice flow and pre-collapse lithic volcanic rock 2. Tectonics and Structural Geology Surqes and volcanic breccia From the tectonic point of view, El Salvador belongs to the Plate, which is subducted by the Cocos Plate the Pacific basalt and andesites: Ocean at an angle of about 40" (Fig. I) The tectonic processes Apaneco (Ap , Empalizada produced recent volcanic fringes, seismic activity and active and Cuyanausul volcanos. hydrotherinal systems. alder agglomerates Four structural system can he detected in the geothermal of Basement andesites racks. Ahuachapan and Chipilapa The NE-SW called Mesas del Llano "Sierra de Tacuba" with tilted blocks to the SE and alinost vertical planes to (Fig. 2) The NW-SE system or El system affects the local andesitic basement of the Sierra de Tacuba and the NE-SW system Fig. 3: Stratigraphic column of the studied The N-S system. which is the youngest. has regional characteristics and is related to faulting of the Graben The fourth one is the semicircular Cuyanausul system with a NNW-SSE tendency. The distended tectonics, based on deformation tensors, impress The determination of rocks sealed by the system is secondary permeability of geothermal interest for the zone.