Table of Contents for Rio Grande Rift Geothermal Energy Resources in Colorado and New Mexico

This webpage describes the Rio Grande Rift and its geothermal resources in Colorado and New Mexico, USA. It is written in nontechnical language with technical terms in parentheses. Some technical words and concepts are introduced and subsequently used. This webpage summarizes the relevant geology and hydrology, but does not present all of the detailed information available in the extensive literature on the Rio Grande Rift. This literature has evolved for decades and some interpretations have been superceded, some interpreateions are not in complete agreement, and uncertainties persist. I have tried to select and summarize interpretations in a relatively simple manner that can provide a general understanding of Rio Grande Rift and its geothermal resources for the layman. Literature used to develop this webpage is listed in the bibliography at the bottom of the page. Some of the references in the bibliography are cited in the webpage.

Rio Grande Rift Geology

The Rio Grande Rift is a narrow continental rift that extends from near Leadville, Colorado, to El Paso, Texas, and on into Mexico (Figure 1). A continental rift is an extensional feature of the Earth's crust (Figure 2) in which the crust breaks and the two sides of the break separate very slowly during millions of years. This disturbance of the crust is due to movement in the subjacent super hot rock (mantle) deep below the Earth's surface. The extension is accompanied by (1) downward movement of blocks of the crust separated by breaks where rocks on one side of the break have moved up or down relative to the other side (faults), (2) upwelling of the mantle, (3) melting of mantle constituents (minerals) to form magma, and (4) rise of magma to the surface to be extruded as lava. This process is diagrammatically and simplistically illustrated in Figure 2. The asthenosphere in Figure 2 is a more mobile part of the mantle that can slowly flow over geologic time and cause disruption in the lithosphere. The actual configuration of the crustal wedges is more complex than indicated in Figure 2. The irregular configuration of the crustal wedges in the Rio Grande Rift formed distinct basins. Most of the extrusive volcanic activity occurred outside of the rift but near its boundaries.

Rio Grande Rift, Colorado and New Mexico

Figure 1. Rio Grande Rift location. NR is Northern Rio Grande Rift. SR is Southern Rio Grande Rift. RM is Rocky Mountains. CP is Colorado Plateau. BR is Basin and Range. S is Sacramento Section of Basin and Range. Boundaries are approximate.

Figure 2. Generalized diagram of the Rio Grande Rift. (From U.S. Geological Survey.)

The major rifting probably began about 25 million years ago (Oligocene, Ricketts, 2021), but the beginning of rifting varied from place to place and probably occurred over a period of several million years. The Rio Grande Rift has been inferred to develop from south to north (Lueth and others, 2005). The maximum extension rate probably was during the period from 23 to 5 million years ago (Miocene Epoch). The faults produced in the Miocene now commonly have a northwest orientation. Later faults are oriented north-south, which implies a rotation of crustal stress direction. The compressional folds and faults of the Rocky Mountains were already present when the Miocene extension began (end of Laramide deformation was middle Eocene, about 40 Ma) and the region was rising (post-Laramide uplift began about 28 Ma and continues today). The southern part of the rift separates the Mexican Highland Section of the Basin and Range Province from the Sacramento Section (Figure 1). The Basin and Range Province began forming about 35 million years ago (late Eocene), so most opening of the Rio Grande Rift postdates the beginning of the Basin and Range Province. Both are extensional, but the Rio Grande Rift has become more active than the adjacent part of the Basin and Range Province. This activity includes changes in basin depth and more recent faulting and volcanism (Ricketts, 2021). Fault scarps cutting young alluvial surfaces and recent seismic activity indicate that the activity has continued to modern time. A value given for the modern east-west stretching rate across the Rio Grande Rift is 0.12 millimeters per year (Ricketts, and others, 2014). Other estimates have ranged up to 2.6 millimeters per year. These values may be larger than the long-term rate of extension. The rate of extension has varied with location and time. It may be influenced by pressure in the groundwater moving through the fault systems. The extension is westerly directed in that the the Earth's crust is moving westward on both sides of the rift, but he west side is moving faster than the east side. The crust beneath the Rio Grande Rift is thinner than the crust beneath the Great Plains and the Colorado Plateau. Seismic data indicates the crust beneath the rift near Socorro, New Mexico, is 22 miles thick. Whereas the crust beneath the Great Plains and and the Colorado Plateau is 28 to 31 miles thick (Kelley and Chamberlin, 2012).

The Rio Grande Rift may be subdivided into two parts, (1) the Northern Rio Grand Rift, and (2) the Southern Rio Grande Rift (Figure 1). The parts differ in width and distribution of the basins they contain. The Northern Rio Grande Rift is relatively narrow and contains a north-south string of basins (Figure 3). The Southern Rio Grande Rift is broader and contains a north-south string of basins along the Rio Grande River that are flanked by other basins (Figure 4). The basins of the Rio Grande Rift contain sediment derived from the adjacent mountain ranges and hills (basin-fill, aka rift fill). The sediment is composed of gravel, sand, silt and clay (Santa Fe Group). In some basins, layers of lava are present within the basin-fill. The Santa Fe Group is overlain by Rio Grande River alluvium in parts of some basins. The Rio Grande River may have been formed when uplift of the Sangre de Cristo Mountains deflected an east-flowing stream southward along the Rio Grande Rift. The Rio Grande River deposited alluvium until about 0.7 to 0.5 million years ago. Then it became erosional and cut deeply into the basin-fill. The basins of the Rio Grande Rift vary in depth and most are asymmetric.

Northern Rio Grande Rift Basins

The Northern Rio Grande Rift contains three major basins: San Luis, Espanola, and Albuquerque (Figure 3). In addition, there is a narrow trough that extends north of the San Luis Basin to the continental divide near Leadville. It has been called the Arkansas Basin (aka Upper Arkansas Graben) and is regarded as an extension of the Rio Grande Rift. These basins are caused by relative downward movement of blocks of the Earth's crust as the sides of the Rio Grande Rift separated. They are bounded laterally by faults on one side where the basement rocks have moved downward to form the basins. They are asymmetrical and the basement rocks are tilted toward the bounding faults (half-grabens), but they contain internal fault blocks. Most of the internal fault blocks are buried beneath basin-fill. The basins may be characterized as complexly faulted half-grabens. (A graben is an elongated block of the Earth's crust that is displaced downward relative to blocks on both sides. The displacement is along faults. A half-graben is displaced downward along a fault on only one side and the bedrock is tilted toward the fault.) A brief description of each basin in the Northern Rio Grande Rift follows. Geographic locations in the descriptions are shown on satellite images in Figure 8, Figure 9, and Figure 10.

Arkansas Basin of the Northern Rio Grande Rift

The Arkansas Basin is a narrow north-tapering, west-tilted half-graben that extends toward the continental divide near Leadville, Colorado. It is bounded on the west by the (granitic) Sawatch Mountain Range at the Sawatch Range Fault. It is bounded on the east by the Mosquito Mountain Range. Basin-fill deposits in the half-graben (silt, sand, and gravel) reach a depth of over 6,500 feet. The Arkansas Basin is separated from the San Luis Basin by a transfer fault in the Poncha Pass area. Transfer faults are breaks in the Earths' crust between basins where the bedrock tilt is opposite. The bedrock dips westward in the Arkansas Basin and eastward in the San Luis Basin. The bedrock is also relatively shallow in the Poncha Pass area forming a restriction between the basins.

San Luis Basin of the Northern Rio Grande Rift

The San Luis Basin is a large east-dipping half-graben bounded on the east by faults on the west side of the Sangre de Cristo Mountain Range. It is bounded on the west by the San Juan Mountains, which are composed mainly of volcanic rocks. Seismic evidence indicates that the main bounding fault at the Sangre de Cristo Range curves to a nearly horizontal attitude at a depth of about 17 miles (listric fault). Seismic data also indicates the presence of a buried up-faulted block trending northwestward in the middle of the San Luis Basin. It is called the Alamosa Horst. The Alamosa Horst has been confirmed by a test well that encountered basement rocks (Precambrian) at 5,400 feet. The Baca Graben lies to the east of the Alamosa Horst and contains about 21,000 feet of sediment at a location just north of the Great Sand Dunes. The shallower Monte Vista Graben (half-graben) lies to the west. The part of the San Luis Basin north of the Rio Grande River has relatively flat topography with internal drainage. The San Luis Basin is filled with gravel, sand, silt and clay derived from the adjacent mountains. The basin also contains layers of volcanic rock derived from volcanism in the San Juan Mountain area. Most, if not all, of the basement rocks below the basin-fill in the San Luis Basin are very old (Precambrian).

The flat topography of the San Luis Basin south of the Rio Grande River is interrupted by the San Luis Hills, which rise three hundred feet above the adjacent terrain. They are upfaulted blocks capped by volcanic rock (andesitic) which is equivalent to some of the older volcanics extruded in the San Juan Mountains about 27 million years ago. The Taos Plateau is a thick deposit of volcanic flows (basalt) that extends from the south end of the San Luis Hills just north of the Colorado-New Mexico state line to the south border of the San Luis Basin. The Costilla Plains are the area of the San Luis Basin east of the San Luis Hills in Colorado and the Taos Plateau in New Mexico. The Rio Grande River is at the boundary between the Taos Plateau and the Costilla Plains in New Mexico.

The San Luis Basin is separated from the Espanola Basin by the Embudo Constriction which is a bedrock high located between Taos and Espanola, New Mexico. Taos is near the southern end of the San Luis Basin, and Espanola is in the Espanola Basin. The Embudo Constriction contains the Embudo fault zone, which is the transfer fault zone between the eastward tilted bedrock of the San Luis Basin and the westward tilted bedrock of the Espanola Basin. Connection between the basins through alluvial and volcanic deposits in the San Luis Hills, the Costilla Plains,the Taos Plateau, and the Embudo Constriction is restricted, although basin-fill alluvium is present beneath the volcanic rocks of the Taos Plateau, and volcanic rocks of the Costilla Plains are covered with alluvial deposits that form an upper aquifer.

Espanola Basin of the Northern Rio Grande Rift

The Espanola Basin is a half-graben with bedrock dipping westward. The basin is bounded on the east side by the Sangre de Cristo Mountains. No major faulting is present at this eastern boundary which is south of the Embudo transfer fault zone. The Espanola Basin is deepest and has large fault displacement on the west side. The southwestern part of the Espanola Basin is hidden by volcanic deposits of the Jemez Volcanic Field (see below) which overlie basin-fill sediments. The northwestern boundary of the Espanola Basin is bordered by a fault zone with major displacement down on the east side. The Tusas Mountains are west of this fault zone. Basin-fill deposits are greater than 6000 feet thick at the west edge of the basin. The Espanola Basin is separated from the Albuquerque basin by a narrow bedrock constriction that is concealed by the Jemez Volcanic Field. Transfer faulting is in this constriction.

Albuquerque Basin of the Northern Rio Grande Rift

The Albuquerque Basin is bounded on almost all sides by faults, and it is composed of multiple sub-basins. Similar to the San Luis Basin, listric faults and low angle extensional faults occur on the deeper eastern side of the Albuquerque Basin, but these faults level out at shallower depths (about 8 miles). The basin is bordered on the east by the Manzano Mountains and Sandia Mountains, which are tilted blocks dipping eastward from major faults. The western border of the basin is more subdued and follows the course of the Rio Puerco and the south edge of the Nacimiento Mountains. Rock layers in the basin are generally tilted to the east. Large subbasins are the Albuquerque Subbasin in the north part of the basin and the Belen Subbasin in the south part. Dry travertine mounds and springs precipitating travertine are present along much of the western margin of the Albuquerque Basin. The maximum thickness of basin-fill in the Albuquerque Basin is more than 21,000 feet near the eastern side.

The Albuquerque Basin is separated from the Socorro Basin in the Southern Rio Grande Rift by a narrow bedrock constriction. This transition between the narrow Northern Rio Grande Rift and the wider Southern Rio Grande Rift is also the location of the Socorro magma body. This magma is about 12 miles deep and 37 miles wide. It is a region of anomalously high seismicity where the land surface is currently rising at 2 to 5 millimeters per year.

Figure 3. Northern Rio Grande Rift basins. SL is San Luis Basin. E is Espanola Basin. ALB is Albuquerque Basin (aka Middle Rio Grande Basin).

Southern Rio Grande Rift Basins

The southern Rio Grande Rift contains a series of north-trending basins. The broadening between the Northern Rio Grande Rift and the Southern Rio Grande Rift represents a southward increase in crustal extension. The locations of the basins within the Southern Rio Grande Rift are shown in Figure 4. The basins are mainly separated by mountain ranges and hills. The north-south string of basins along the Rio Grande River from the Socorro Basin to the Mesilla Basin are interconnected through restrictions. A brief description of each basin in the Southern Rio Grande Rift follows. Geographic locations in the descriptions are shown on satellite images in Figure 11 and Figure 12.

Socorro Basin of the Southern Rio Grande Rift

The Socorro Basin is the northernmost basin of the string of basins along the Rio Grande River. It is bounded on the west side by the Socorro-Lemitar Mountains fault block (horst), which separates it from the La Jencia Basin. It is bounded on the east by the Loma de las Canas-Cerro Colorado uplift (aka Quebradas). Crustal blocks on the western margin in the basin have downward displacements perhaps as great as 11,000 feet. These depressions rise gradually to the east, probably by a combination of step faulting and tilting. The Socorro Basin is connected to the San Marcial Basin by a narrow bedrock constriction containing alluvial sediments of the Rio Grande River valley.

San Marcial Basin of the Southern Rio Grande Rift

The San Marcial Basin is an east tilted half-graben bound on the east by a major fault that extends northward from the Fra Cristobal Mountains and by a sub-alluvial platform surmounted by lava flows. The San Mateo Mountains are west of the basin. The basin contains intrabasin faults that were active within the last million years. The San Marcial Basin is connected to the Engle Basin by a constriction (Pankey Channel) between the San Mateo Mountains and the Fra Cristobal Mountains.

Engle Basin of the Southern Rio Grande Rift

Like the San Marcial Basin, the Engle Basin is an east tilted half-graben bound on the east by the Hot Springs Fault at the west edge of the Fra Cristobal Mountains. The basin-fill thickness along the east edge of the basin is estimated to be 2,300 feet or more. The Engle Basin extends westward to the Black Range. The Cuchillo Negro fault zone occupies the central part of the Engle Basin, and forms a series of aligned subbasins. The Engle Basin is separted from the Palomas Basin at a constriction east of the Mud Springs Mountains and the north end of the Caballo Mountains (Cachillo Channel) where several faults intersect.

Palomas Basin of the Southern Rio Grande Rift

Like the San Marcial Basin and Engle Basin, the Palomas Basin is an east tilted half-graben that contains intrabasin faults. The Palomas Basin is bound on the east by a major fault at the west edge of the Caballo Mountains and Red Hills. The Palomas Basin extends westward to the Black Range, Animas Hills, Salado Hills, and southern Sierra Cuchillo. The basin contains up to 7,000 feet of basin-fill along its deep eastern margin. The Truth or Consequences Geothermal Site (see below) is located at the faulted north end of the basin. The Palomas Basin is separated from the Mesillia Basin by a constriction between the Dona Ana Mountains and the Robledo Mountains.

Mesilla Basin of the Southern Rio Grande Rift

The Mesilla Basin is bounded on the east by the Franklin Mountains and the Jornada fault zone located between Las Cruces and the Organ Mountains. However, the northeastern basin boundary is a restricted connection with the Jornada del Muerto Basin. The Potrillo Mountains (East Potrillo Fault Zone) , Aden Hills, Sleeping Lady Hills, and Rough and Ready Hills form the western edge of the Mesilla Basin north of the Mexican Border. The Mesilla basin-fill deposits thickness is affected by buried fault blocks and ranges to greater than 5,000 feet. The southern part of the Mesilla Basin is in Mexico.

Central Mimbres Basin of the Southern Rio Grande Rift

The Central Mimbres Basin is bounded on the east by the Portillo Mountains. The basin is bounded on the west by the Tres Hermanos Mountians, Florida Mountains, and Cooke Mountains. The basin-fill deposits are more than 2000 feet thick southeast of the Florida Mountains and more than 1500 feet thick east of the Cooke Mountains. The southern part of the Mimbres Basin in is Mexico.

La Jencia Basin of the Southern Rio Grande Rift

The eastern edge of the La Jencia Basin is at the Socorro-Lemitar Mountains fault block (horst), which separates it from the Socorro Basin. The separation occurred about 10 million years ago when the Socorro-Lemitar fault block was formed. The western boundary of the La Jencia Basin is at the Magdalena Mountains. The basin contains about 10,000 feet of basin-fill deposits.

Jornada del Muerto Basin of the Southern Rio Grande Rift

The Jornada Basin is separated from the interconnected basins along the Rio Grande River by mountain ranges that include the Fra Cristobal and Caballo Mountains (intrarift up-faulted blocks) and a sub-alluvial bedrock high surmounted by local basalt flows (San Pascual Platform). The basin is bounded on the east by Chupadera Mesa and the Oscura Mountains and San Andres Mountains. The Jornada Basin is a relatively shallow bedrock downwarp (syncline) between uplifts that are fault blocks tilted into the basin. Minor faulting is present along the basin's west margin. The basin deepens to the south and the basin-fill sediments are generally less than 350 feet thick. A fault zone is located in the center of the southern part of the syncline (Jornada Draw fault zone). The south end of the basin is a connection with the northeast end of the Mesilla Basin by a buried bedrock high between the Dona Ana and Tortugas Mountains east of Las Cruces.

Tularosa Basin of the Southern Rio Grande Rift

The Tularosa Basin is separated from the Jornada Basin by the San Andres Mountains (an intrarift, west-tilted up-faulted block), and from the Mesilla Basin by the Franklin Mountains. The San Andres-Organ-East Franklin Mountains fault system trends along the mountain ranges that bound the west edge of the Tularosa Basin. This fault system has been active during the last 0.5 million years. The Tularosa Basin is bordered on the east side by faulting (Pliocene and Pleistocene) at the east-tilted Sacramento Mountains.

South Rio Grande Rift Basins, New Mexico

Figure 4. Location and approximate boundaries of basins within the southern Rio Grande Rift. T is Tularosa Basin. JM is Jornada del Muerto Basin (aka Jornada Basin). M is Mesilla Basin. P is Palomas Basin. E is Engle Basin. L is La Jencia Basin. SM is San Marcial Basin. S is Socorro Basin. C is Central Mimbres Basin. T or C is the town Truth Or Consequences. The blue line is the Rio Grande River.

Igneous Rock Associated With the Rio Grande Rift, Colorado and New Mexico

As mentioned above, the development of the Rio Grande Rift was accompanied by the rise of magma from the mantle to the surface in and near the rift. The magma was extruded as volcanic deposits. Figure 5 shows the approximate location of most of the larger volcanic deposits in and near the Rio Grande Rift. The deposits are discontinuous within the outlined areas, and small deposits occur outside the outlined areas. Most of the deposits are lava flows (basalt and andesite), but the older, lower deposits contain much solidified volcanic ash (ignimbrites). Some of these older, lower deposits predate the Rio Grande Rift. Much of the volume of the volcanic deposits is in the Mogollon-Datil and San Juan volcanic fields. A large portion of the remainder is in the smaller volcanic areas (basalt and ryolite) aligned in a southwest-northeast lineament called the Jemez Lineament. This lineament crosses the Rio Grande Rift along the Embudo fault zone, which separates the Espanola and Albuquerque basins. The Jemez Volcanic Field is the volcanic area at the border between the Espanola and Albuquerque basins near Los Alamos. It contains a variety of extrusive igneous rocks from eruptions that began about 16.5 million years ago. The Valles Caldera is within the Jemez Volcanic Field at the western edge of the Rio Grande Rift. It was created about 1.25 million years ago (Quaternary) by collapse of a magma chamber during volcanic eruption. It is about 14 miles in diameter.

The southern extension of the San Juan Volcanic Field in New Mexico is called the Taos Plateau Volcanic Field. The surficial volcanic rocks (lava and ash) of the Taos Plateau are underlain by basin-fill deposits (Santa Fe Group). Some of the volcanic deposits beneath the Taos Plateau predate the Rio Grande Rift (Drenth and others, 2019). Also, some volcanic deposits in the Sangre de Cristo Mountains in New Mexico predate the southern Sangre de Cristo Fault zone, which forms the eastern boundary of the Rio Grande Rift in that area. The Sangre de Cristo Fault zone in northen New Mexico probably began forming about 20 million years ago.

Volcanic deposits near the Rio Grande Rift, Colorado, New Mexico

Figure 5. Approximate location of some volcanic deposits (brown areas) in and near the Rio Grande Rift in Colorado and New Mexico.

Origin of Rio Grande Rift Thermal Springs

The origin of thermal springs associated with the Rio Grande Rift is illustrated in Figure 6. Rainfall and snowmelt (meteoric water) percolates to the groundwater in the topographically high area (mostly mountain range first order water table features) at the margins of the rift. Some of this water migrates downward thousands of feet into the hot rock above the mantle. The water is heated and returns toward the land surface relatively rapidly through conduits following faults. The geologic conditions associated with thermal springs varies. Figure 6 is not meant to demonstrate optimal conditions because it indicates mixing with cooler water from basin-fill. Optimal conditions would be where mixing with cooler water does not occur and the flow system brings hot water to the surface with relatively low heat exchange with cooler shallow rock. Some thermal springs are at the rift margin, rather than in interior of the rift, and the deep circulation may be parallel to the rift axis rather than normal to it. There may also be some springs that discharge thermal water that is diverted laterally through permeable layers in volcanic rocks. The nature of the faulting shown in Figure 6 shows how a hot spring might occur, but it does not accurately represent the faulting in any particular basin, which varies greatly (as described above); and the nature of the faults at great depth is interpreted by geophysical methods and is indefinite.

Groundwater flow system of Rio Grande Rift

Figure 6. Generalized cross-section of the Rio Grande Rift illustrating the nature of the groundwater flow system. Modified from a diagram in Lueth and others (2005).

Location of Rio Grande Rift Thermal Springs and Wells in Colorado and New Mexico

Figure 7 shows the location of thermal springs and wells with water temperatures above 860F (300C). The figure only shows springs and wells located in or close to the Rio Grande Rift in Colorado and New Mexico. There are many springs and wells located in the map area farther from the rift that are not shown. The color of the stars that represent the wells and springs is not significant. Some stars are red or green so they will show. The wells and springs shown in Colorado are from a map distributed by the Colorado Geological Survey (Barret and others, 1976), and those shown in New Mexico are from a map distributed by New Mexico State University (Witcher, 1995). Additional thermal springs and wells may exist. The water temperatures reported range up to 181oF (83oC). Such water is low-temperature geothermal water and is suitable for direct use. Direct use includes heating buildings, swimming pools, spa facilities, greenhouses, and aquaculture ponds. These temperatures are too low for generation of electricity, which usually requires temperatures greater than 347oF (175oC). Temperatures as low as 25ooF might be used for small electric power plants. Such temperatures may require construction of deep thermal wells.

The thermal wells are ones that produce hot water. Any well that is drilled deep enough will encounter bottom hole temperatures greater than 347oF due to normal geothermal gradient. For example, a well drilled by a petroleum company south of Albuquerque measured 434oF at its total depth of 21,266 feet. However, such wells are not thermal wells because they do not produce beneficial amounts of hot water.

Thermal springs and wells - Rio Grande Rift, Colorado, New Mexico

Figure 7. Thermal springs and wells in and near the Rio Grande Rift, Colorado and New Mexico.

Valles Caldera Deep Thermal Test Wells

Deep thermal test wells have been drilled in the Valles Caldera. The Valles Caldera is a 14 mile diameter caldera in the Jemez Mountains west of Los Alamos, New Mexico. Much of the caldera is within the Valles Caldera National Preserve, managed by the U.S. National Park Service. The caldera formed during volcanic eruptions about 1.25 million years ago, when it collapsed over at least one older caldera. Since that time, there have been volcanic eruptions within the caldera. The last eruption was about 42,000 years ago. Very few earthquakes originate beneath the Valles Caldera. Seismic investigations suggest the presence of magma at a depth between about 3 and 9 miles. The caldera contains active thermal springs and fumeroles. The test wells encountered temperatures ranging to 648°F (342° C), but the geothermal resource was judged too small to be economically feasible for generating electricity. One test well within the caldera was drilled to basement (Precambrian) at a depth of 10,300 feet.

Fenton Hill Geothermal Energy Test Site

The Los Alamos National Laboratory operated a dry rock geothermal test site at Fenton Hill from 1973 to 1995. The site is located near the southwestern edge of the Valles Caldera. It involved constructing wells and surface facilities to test the feasibility of extracting geothermal energy from fractured hot rock. The deepest rock tested was 14,764 feet with a temperature of 617°F. Fractures in the rock were enhanced by pressurization (hydraulic fracturing). Water was circulated under pressure from an injection well to a production well through the fractured rock and energy was extracted from the water before reinjectiing the cooled water. Power production ranged up to 10 megawatts, enough to power about 10,000 homes. The boreholes were subsequently plugged and abandoned.

Sulphur Springs Geothermal Site

Sulphur Springs is the site of an abandoned resort with hot springs and mud pots. The site is in the Valles Caldera on the west side of the Valles Caldera National Preserve. It also has fumaroles and gaseous cold springs. Temperatures of the springs range up to the boiling point. The gas is mostly carbon dioxide, but contains hydrogen sulfide. Several types of data suggest that the hot gases are in equilibrium at depth with reservoirs temperatures of 392°F to 572°F (200°C to 300 °C). The Sulphur Springs site is at the intersection of a northeast-trending fault and one or more cross faults. This intersection of faults likely produces the conduit that serves the geothermal features. The surface geology at the site consists of caldera volcanic rocks (ryolite) and caldera-fill deposits. The main fumerole is in a landslide deposit. Enough sulfur has been deposited in the area that it was once mined.

Jemez Springs Geothermal Site

Jemez Springs is located about a half mile south of the Valles Caldera in a village of the same name on the Jemez River. The flow of the springs is on the order of 400 gallons per minute. Jemez Springs issue from alluvium on the west side of the river. Alignment of the springs suggests control by faults in the underlying bedrock. The maximum temperature of the spring discharge is 162°F, although there is an early report (1875) of 180°F. A thermal well drilled to 837 feet found the base of the alluvium to be on limestone at 70 feet. Below the alluvium, the well penetrated 700 feet of sedimentary rock, mostly limestone, and 60 feet of crystalline rock (gneiss) below the limestone. The hottest water encountered in the well was 162°F at the base of the alluvium. Thermal water was also found at about 500 feet in a fractured shale layer within the limestone. No major water was yielded by the crystalline rock. Thermal water conduits may include solution widened vertical fractures in the limestone offset by bedding plane solution features. Porous layers in the alluvium might also affect the thermal water flow paths. A subsurface electrical resistivity study indicated that thermal water is present at the base of the alluvium in the area near the Jemez Springs that is occupied by the village.

Chalk Creek Geothermal Site

The Chalk Creek Geothermal Site is located in the Chalk Creek Valley about nine miles south of Buena Vista, Colorado, at the base of Mount Princeton. It contains the Mount Princeton Hot Springs and Hortense Hot Spring. Hortense Hot Spring is about a mile west of Mount Princeton Hot Springs. In addition, there are thermal wells in the area between the two hot springs and a few thermal wells along a line extending southward along Chalk Creek. The thermal wells are used for heating and recreational purposes. Mount Princeton Hot Springs serve the resort of the same name. Hortense Hot Spring has also been used for recreational purposes.

The main east-west line of thermal springs and wells crosses the projection of the Sawatch Fault into the alluvial valley of Chalk Creek from the north. The Sawatch Fault is the western boundary of the Arkansas Basin (half-graben) at this location. The fault is offset about 2 1/2 miles where it crosses Chalk Creek so that the fault line is farther west on the south side of the Chalk Creek valley (dextral offset). There is probably a complex fault zone beneath the Chalk Creek alluvium, and thermal water conduits are in this zone. The bedrock beneath the alluvium is Mount Princeton Batholith, a granitic intrusive igneous rock (quartz monzonite) 34 to 38 million years old. Some wells drilled into the quartz monzonite beneath the Chalk Creek alluvium have encountered thermal water in fractures

Hortense Hot Spring is the hottest thermal spring in Colorado at 180°F. The Mount Princeton Resort hot springs are reported to be in the range of 110°F to 150°F. Domestic wells in the Chalk Creek Geothermal Area have produced water that is above 100°F. Total upwelling of thermal water into the Chalk Creek alluvium through conduits in the quartz monzonite has been estimated at 1000 gallons per minute. The combined discharge of Mount Princeton Hot Springs has been measured at 175 gallons per minute. The discharge of Hortense Hot Spring has been reported to be 18 gallons per minute. The total dissolved solids in the thermal water is low. The age of the thermal water is estimated (tritium analysis) to be 20 to 50 years. The suggested origin is from snowmelt in the high Mount Princeton area circulating down deep into the bedrock system and then discharging upward into the Chalk Creek Valley. Estimates of the temperature deep in the geothermal system have ranged from 300°F to 400°F. The Chalk Creek Geothermal Site has a geothermal gradient greater than 5.5°F per 100 feet, which is the highest gradient in Colorado.

Truth or Consequences Geothermal Site

The Truth or Consequences Hot Springs (T or C ) are located in the Palomas Basin near the boundary with the Engle Basin. Several faults intersect near T or C and separate the two basins. T or C is at the northwestern edge of the Caballo Mountians near the Caballo fault system which contains numerous small high-angle faults. Numerous shallow thermal wells discharging hot water that comes from alluvium at depths of 49 feet to 249 feet exist in the area of T or C. They have been used for decades for spas and space heating. The hot water in the alluvium comes from the faulted crystalline basement rock. Temperatures of the thermal well water ranges from 100°F to 115°F. The aggregate flow is estimated at about 800 gallons per minute. Rough calculations suggest that the a temperature of 356°F is at a depth of 11,920 to 15,325 feet.

Rio Grande Rift Basins Satellite Images

This section of the Rio Grande Rift Geothermal Resources webpage contains Google Earth satellite images with geographic sites mentioned in the text shown as labelled placemarks.

Figure 8. Arkansas Basin placemarks.

Figure 9. San Luis Basin and Espanola Basin placemarks.

Figure 10. Albuquerque Basin and Espanola Basin placemarks.

Figure 11. Socorro Basin, San Marcial Basin, Engle Basin, Palomas Basin, La Jencia and Jornada Basin placemarks.

Figure 12. Mesilla Basin and Central Mimbres Basin placemarks.

RIO GRANDE RIFT GEOTHERMAL ENERGY RESOURCES BIBLIOGRAPHY

Baldridge, W. S., K. H. Olsen, and J. F. Callender (1984): Rio Grande Rift - Problems and Perspectives: New Mexico Geological Society 35th Annual Fall Field Conference Guidebook.

Barret, J. K. (1976): Map Showing Thermal Springs, Wells, and Heat-Flow Contours in Colorado; Colorado Geological Survey, Information Series 4.

Belcher, R. C. (1975): The Geomorphic Evolution of the Rio Grande; Baylor Geological Studies, Bulletin No. 29, Baylor University, Waco, Texas.

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Published 2/3/2023. Last revision 8/5/2023.