Rio Grande Rift Geothermal Resources in Colorado and New Mexico

By Darrel Dunn, Ph.D., PG, Hydrogeologist 

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Introduction to Rio Grande Rift Geothermal Webpage

This webpage describes the Rio Grande Rift and its geothermal resources in Colorado and New Mexico, USA.  It is written in nontechnical language with parenthetical elements that may be more technical.  It 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  This literature has evolved for decades and some interpretations have been superceded, some interpreateions are not in complete agreement, and uncertainties persist.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.

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 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 to form magma, and (4) rise of magma to the surface to be extruded as lava.  This process is diagrammatically illustrated in Figure 2.  The asthenosphere in Figure 2 is a more mobil part of the mantle that can slowly flow over geologic time and cause disruption in the lithosphere.  Some crustal wedges moved downward more than others, forming distinct basins within the Rio Grande Rift.

The major rifting probably began about 25 million years ago (Oligocene, Ricketts, 2021), but the transition to rifting varied from place to place and probably occurred over a period of several million years.   The compressional folds and faults of the Rocky Mountains were already present (end of Laramide deformation was about 40 Ma) and the region was rising (post-Laramide uplift began about 28 Ma and continues today).  The Rio Grande Rift has been inferred to develop from south to north (Lueth and others, 2005).  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 the opening of the Rio Grande Rift postdates the beginning of the Basin and Range Province.  Both are extensional, but the Rio Grande Rift is 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 alluviul surfaces and recent seismic activity indicate that the activity has continued to modern time.  Values given for current east-west stretching across the Rio Grande Rift have ranged from about 0.6 to 2.5 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 (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 moveing 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]).

Rio Grande Rift Basins

The Northern Rio Grande Rift contains three major basins, San Luis, Espanola, and Albuquerque (Figure 3).  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 normal faults on at least one side and contain buried horsts, grabens, and tilted fault blocks.  They are asymmetrical and may be characterized as complexly faulted half-grabens.)  In addition, there is a narrow trough that extends north of the San Luis Basin to the continental divide north of Leadville.  It has been called the Upper Arkansas Graben (aka Arkansas Basin) and is regarded as an extension of the Rio Grande Rift.  The Sawatch Range is located along the western edge of this basin (which is a normal fault downthown on the east side to form a half-graben).  The Arkansas Graben is separated from the San Luis Basin by high bedrock in the Poncha Pass area.  Likewise, the San Luis Basin is separated from the Espanola Basin by high bedrock northwest of the Picuris Mountains.

The southern Rio Grande Rift contains a series of north-trending basins that are graben and half-graben systems.  The transition between the narrow Northern Rio Grande Rift and the wider Southern Rio Grande Rift is also the location of the Soccorro 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.  The broadening between the Northern Rio Grande Rifth 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 separated by mountain ranges and hills.  The north-south string of basins along the Rio Grande River from the Mesilla Basin to the Socorro Basin are interconnected.  Connection between the Jornada and Mesilla basins has also been reported.  The Socorro Basin is separated from the La Jencia  Basin by the Lemitar Mountains intrarift horst.  The Tularosa Basin is separated from the Jornada Basin by the Organ Mountains and San Andres Mountains (an intrarift horst, Chapin, 1971).  The Jornada Basin is separated from the interconnected basins along the Rio Grande River by mountian ranges that include the Portillo, Sierra de Las Uvas, Fra Cristobal, and Caballo mountains (also an intrarift horst).  

The raised and outward-dipping eastern shoulder of the Rio Grande Rift includes several mountain ranges including the Sangre de Cristo, Sandia, and Sacramento mountains.

The basins of the Rio Grande Rift contain sediment derived from the adjacent mountain ranges and hills.  The sediment is composed of gravel, sand, silt and clay (Santa Fe Group).  In some basins, lava is interlayered with the sediment.  The Santa Fe Group is overlaing by Rio Grande River alluvium in parts of some basins.  The basins vary in depth and some are asymmetric.  The deepest part of the northern Rio Grande Rift is on the eastern side of the San Luis Basin just northwest of the Great Sand Dunes where the basin fill is approximately 21,000 feet thick.  The maximum thickness in the Albuquerque Basin is approximately 20,000 feet near the eastern side.  Both basins are bounded on the east side by fault zones with large displacements (downward movement of a section of the Earth's crust on the west side of long north-south breaks in the rock).  Conversely, the Espanola Basin is deepest and has the largest fault displacement on the west side.  Rock layers are generally tilted to the east in the San Luis and Albuqueque basins and to the west in the Espanola Basin.   Accommodation zones at the north and south ends of the Eapanola basin contain so-called transfer faults.  The transfer faults accommodate the change in tilt of the rocks.  A further complication is that basins (like the San Luis Basin, the Albuquerque Basin, and the Mesilla Basin) contain buried rock structures that form subbasins that are not evident at the surface.  (The Albuquerque Basin is bounded on almost all sides by normal faults, and it is composed of multiple sub-basins.  Large subbasins are the Albuquerque Subbasin in the north part of the basin and the Belin Subbasin in  the south part.)

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 basaltic lava flows, 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 aligned in a southwest-northeast lineament called the Jemez Lineament.  This lineament crosses the Rio Grande Rift near the border between the Espanola and Albuquerque basins.  The Jemez Volcanic Field is the volcanic area near 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 agoe.  The Valles Caldera is within the Jemez Volcanic Field.  It was created about 1.25 million years ago 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.  The volcanic rocks (lava and ash) of the Taos Plateau are underlain by sedimentary basin deposits (Santa Fe Group).  Some of the volcanic deposits of 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 Falt zone in northen New Mexico probably began forming about 20 million years ago.

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 ranges) 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 to the land surface relatively rapidly through conduits following breaks in the rock (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 sprngs are at the rift margin, rather than in interior of the rift.

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).  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).  Such temperatures may require construction of deep thermal wells.