Caribbean Basin Proyecto Geology of Honduran Geothermal Sites by Dean B. Eppler ince March 1985 a team of Labora- The geology of Central America is ex- meters apart. Most of these are normal tory geologists has been working tremely complex. The meeting of three faults, developed as a result of stress that is with counterparts from the Empresa tectonic plates in western Guatemala and literally pulling the country apart along an Nacionals de Energia E1ectrica (ENEE) of southern Mexico has resulted in an un- east-west axis. Although Honduras has Honduras and from four American in- usual juxtaposition of structures and rock been spared the devastating earthquakes stitutions on a project to locate, evaluate, types whose geologic history has yet to be that have rocked much of Central and develop geothermal resources in Hon- unraveled. Textbook reconstructions of America, we suspect that deformation is duras. The team, headed by Grant Heiken tectonic-plate motions very often sidestep taking place continually; in some areas and funded by the U.S. Agency for Inter- the problem of how Central America de- faults cut stream gravels that are only sev- national Development, has so far com- veloped through geologic time by never eral thousand years old. The result of this pleted three trips to Central America to showing its existence until the present faulting, as shown in the accompanying study in detail the geology of six geo- time. photo, is rugged topography dominated by thermal spring sites. As shown on the accompanying map, north-south oriented fault basins and adja- Honduras lies on a portion of the Carib- cent fault-block mountains very similar to Basic Geology of Honduras bean tectonic plate called the Chortis those found in the Basin and Range Block. This block, composed of rocks de- physiographic province of the western Honduras, the largest and most rugged posited in a continental environment, is United States. country in Central America, is perhaps the bounded on the north by large strike-slip The rocks of Honduras were deposited least known geologically. Its steep terrain, faults in sourthern Guatemala (the in rapidly changing environments, and the dense vegetation, and paucity of roads Matagua and Polochic faults) that form resulting stratigraphy is as complex as the hampered basic geologic studies until the the boundary between the Caribbean plate structures that modify it. Precise dating is late 1960s. Since then studies sponsored and the North America plate. difficult because of the absence of identi- by American universities, including Ph.D. The continental rocks of the Chortis fiable fossils and the rapid changes in rock dissertations by project collaborators Bob Block are bounded on the south by types over short geographic distances. Fakundiny and Rick Finch, have meshed younger rocks in Nicaragua that were de- However, three distinct age groups are with a greater level of in-country expertise posited in an oceanic environment. The apparent: a basement complex of Paleo- to produce a basic understanding of the western boundary of the Chortis Block lies zoic low-grade metamorphic rocks about geology of the country. Such an under- along the Central American volcanic chain 245 million years of age (Home, Clarke, standing is an essential first step in any and the Middle America Trench, a sub- and Pushkar 1976); an overlying section of geothermal exploration. It has been duction zone where the Cocos plate is Mesozoic limestones and redbeds that is particularly useful in Honduras as we set being thrust under the Caribbean plate. estimated to be between 100 million and out to determine the nature of the geo- The complex geology of Honduras is the 200 million years of age (Mills et al. 1967); thermal heat source and the “plumbing result of its proximity to the intersection and a thick upper sequence of volcanic system” through which the geothermal of the three tectonic plates. In some areas rocks from two distinct episodes of vol- waters reach the surface. of the country, major faults lie less than 10 canism. The Matagalpa Formation, a 86 Fall 1986 LOS ALAMOS SCIENCE Caribbean Basin Proyecto Honduras is positioned on the plate from the Caribbean plate. The termined. We do know that plate Chortis Block near the junction of plate-tectonic and geologic histories movements are continuing to create three tectonic plates: the North of the area are not known well faulting throughout Honduras and American, the Cocos, and the Carib- enough to explain how and when pulling the country apart along an bean. The large arrows indicate the Central America was formed. For ex- east-west axis. Rainwater circulating direction of motion of the plates. The ample, the southern boundary of the through the fault regions has created Cocos plate is being thrust under the Chortis Block, where continental numerous geothermal systems. The Caribbean plate along the Middle rocks end and oceanic rocks begin, map also shows the locations of the America Trench. The Motagua and is indicated by a dashed line be- six geothermal sites now being Polochic faults are large strike-slip cause its exact location in the jungles evaluated as indigenous sources of faults separating the North American of Nicaragua has not been de- energy. LOS ALAMOS SCIENCE Fall 1986 87 Caribbean Basin Proyecto series of early Tertiary interbedded lava flows, pyroclastic flows, debris flows, and interbedded water-laid sediments, is be- tween 40(?) million and 60(?) million years of age (McBirney and Williams 1965). The Padre Miguel Group, the result of the sec- ond episode of volcanism, is a thick se- quence of ignimbrite similar to the Bandelier Tuff and is found throughout the southern half of Honduras; it is be- tween 15 million and 20 million years of age (Williams and McBirney 1969). This bare outline of the geology of Hon- duras will have to be filled in by studies of individual drill holes before we can infer with any confidence the nature of the plumbing system at each geothermal site. Studies of Geothermal Sites In the late 1970s several American firms Fault block mountains on the east side of Lago de Yojoa, Honduras. began preliminary geothermal explora- tions in Honduras but were unable to thermal waters to determine their chem- “Geochemistry at Honduran Geothermal complete them because of economic dif- istry and estimate the temperatures of the Sites.” ficulties. These reconnaissance efforts al- geothermal resevoirs (see “Geochemistry lowed selection of six promising geo- at Honduran Geothermal Sites”). San Ignacio. This site, located on the thermal sites. However, the origin of the north side of the fault-bounded Siria Val- geothermal resource was misunderstood Platanares. This site, located in the west- ley, also appears to be a geothermal system and incorrectly attributed to recent vol- ern portion of Honduras, is similar to of the Basin and Range type. Hot springs canism rather than, as our studies now many being developed in the Basin and are located at the intersection of a young indicate, to tectonic processes. Identifi- Range province of Nevada. That is, water northwest-trending fault scarp with older cation of the nature of the geothermal is heated deep underground and rises to north-trending faults. These faults are also resource is a major contribution to the the surface along faults. The numerous hot extensional, and, again, recently cut de- project. The amazing abundance of hot springs at Platanares are found in lavas, posits of stream gravel suggest recent tuffs, and tuffaceous sediments of the movement. The rocks within the area are thermal resource. Consequently, the proj- Padre Miguel Group. The faults appear to primarily Paleozoic metamorphic schists ect has two goals: selection of two geo- be extensional, and the presence of wedges containing some remnant patches of Padre thermal sites for further development on of gravel perched above the present water Miguel Group tuffs. More than one hun- the basis of detailed studies, by Los Ala- level in the Quebrada del Agua Caliente dred springs were mapped, many of which mos and ENEE geologists, of the six (Gorge of Hot Water) suggests relatively surface in terraces formed in deposits of previously identified sites; and identifi- recent movement on these faults. The hot- silica-cemented gravel. cation of other promising geothermal sites test springs are associated with faults that on the basis of a country-wide inventory of trend mostly northwest and north. Azacualpa. This site, located in highly hot springs by ENEE with technical suD- Thermal energy is being released from faulted sedimentary rocks that bound a port, as necessary, from Los Alamos. boiling springs and numerous fumaroles. major fault basin (the Santa Barbara Detailed geologic studies have so far Since the stream that flows through the graben), also appears to be a geothermal been carried out at three sites: Platanares, gorge is 10 to 15°C hotter in the area of the system of the Basin and Range type. The San Ignacio, and Azacualpa. Concurrently hot springs than it is upstream, additional hot springs and fumaroles are surfacing a team of geochemists from the Labora- energy is probably being released from along segments of the Zacapa fault, which tory, the U.S. Geological Survey, and submerged springs. Estimates of the cuts limestones and redbeds of Cretaceus ENEE has sampled and analyzed the thermal power of this area are given in age. 88 Fall 1986 LOS ALAMOS SCIENCE Caribbean Basin Proyecto Summary flow, and the thickness of the crust. Plans References are also under way to begin drilling Our studies so far suggest that the geo- shallow (about 500-meter) boreholes to Home, Gregory S., Clarke, George S,, and thermal manifestations in Honduras orig- measure the geothermal gradient. By com- Pushkar, Paul.