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Home , Rio Grande Rift

summer 2012

Our Growing Understanding of the The Rio Grande rift is a narrow, documented that the valleys of the Rio north-striking belt of en echelon Grande depression are bounded by

(slightly overlapping, east or west 42°N northerly striking normal faults, which stepping), elongated basins that bisects WY form when an area is under tension, southern and New . causing the valleys to drop down and The rift formed when the earth’s crust the flanking mountains to rise relative stretched and thinned during westerly to one another. The down-dropped directed extension, starting about Steamboat Springs blocks along the Rio Grande depression, 36–37 million years ago. This has 40°N called , contain accumulations allowed hot mantle to move upward, of washed from the surround- creating volcanoes, hot springs, and Leadville CO ing highlands that are much thicker mineral deposits, forming the topo- Buena Vista than comparable sedimentary deposits graphically low area along which the in the neighboring Basin and Range Rio Grande flows. Geoscientists have province in and southwestern 38°N long recognized the importance of the New Mexico. Kelley suggested that Rio Grande rift in localizing water and Alamosa CREST the depression along the Rio Grande economic resources used by Native, structural belt was due to many miles of Hispanic, and American inhabitants of horizontal, east-west directed, tensional this region. Taos displacement. In 1952 he was the first to Kirk Bryan from Harvard 36°N propose that “deep-seated rifting in late University, in his classic 1938 work Tertiary time probably is the underlying relating water resources to the geol- Santa cause of the en echelon basins and ogy of New Mexico, was the first Albuquerque Fe uplifts which constitute the Rio Grande to describe the basins and flanking NM depression.” The latter part of Kelley’s mountains of the Rio Grande rift. career coincided with the acceptance 34°N Socorro Bryan made the important observation of the concept of plate by that the basins along the Rio Grande most geologists. Plate tectonic theory between central Colorado and El Paso, LA RISTRA provided a framework for Kelley’s later Texas, were filled with , publications about the Rio Grande rift. siltstone, and conglomerate of the Las Cruces In 1971 Charles Chapin, who later same general age. These 32°N Ba became director of the New Mexico sin El Paso sedimentary deposits were named an Rio Grande Bureau of Geology and Mineral d R the Santa Fe Formation (now the ang TX Resources, was one of the organizers ) for exceptional e of the annual New Mexico Geological MEXICO exposures of these deposits north of Society fall field conference in the San Santa Fe. Bryan referred to the aligned Luis Basin of Colorado. Chapin was 108°W 106°W 104°W elongated valleys along the river as the encouraged to write a paper about the Rio Grande depression. He realized Physiographic expression of the Rio Grande rift, initially rift by Kelly Summers, a hydrologist that the basins are commonly called the Rio Grande depression. interested in placing the geothermal bounded, and he noticed an asym- tectonics, and no formation mechanism for resources of the state into a regional metry in some basins, particularly the San the depression was proposed. tectonic context. In typical fashion, Chapin Luis Basin and the basins near Socorro. Vincent Kelley, a legendary geologist dug deeply into the literature about Bryan presented his observations before who worked at the University of New worldwide and drew upon his extensive the development of the concept of plate Mexico between 1940 and 1970, later experience as a field geologist in New

Published by the New Mexico Bureau of Geology and Mineral Resources • A Division of New Mexico Tech Mexico and Colorado to write a definitive later well up to replace the thinned crust. development. Kirk Bryan relied on fossils paper that outlined the boundaries of the In either case, thinned crust and the pres- in sedimentary rocks to assign rock ages, Rio Grande rift. One of his illustrations ence of hot mantle at shallow depths cause whereas Vincent Kelley had access to early was an idealized cross section that high- basalt-rhyolite volcanism in the vicinity of radiometric dating techniques. Charles lights many characteristic features of the rifts. Much of the early work, both in the Chapin facilitated the construction of Rio Grande rift. This cross section is a clas- Rio Grande rift and worldwide, was based a modern 40Ar/39Ar laboratory at New sic example of a conceptual model, which on geologic mapping and construction of Mexico Tech. As a result, hundreds of is a simple diagram that portrays complex cross sections using surface observations. high precision dates for volcanic rocks relationships among geologic processes to Our view of the rift basins along the in Colorado and New Mexico are now explain multifaceted and often disparate Rio Grande changed in the 1970s with available, which have helped constrain the data sets. Such a model provides a broad the addition of new technology and new timing of rifting processes. framework for data analysis and can be data. First, Shell Oil Company drilled What have we since learned about the used as a roadmap to guide future research. several deep petroleum exploration wells Rio Grande rift? Drill hole, seismic, and This sketch sparked consid- gravity data have revealed erable new research. that the basins of the Rio Chapin noted that the Colorado Rio Grande High Plains Grande rift are deep, in Plateau rift eastern margin of the rift is places as much as 20,000 generally higher in elevation feet (6.1 km) deep, and than the west side, and W asymmetric. The asymmetry volcanic centers and hot E varies from basin to basin. springs are more abundant For example, the San Luis on the west side. In synthe- Basin and the Albuquerque sizing these observations, Crust Basin are deepest and have Chapin suggested that the the largest fault displace- continental plate west of ments on the east side of the the rift is drifting west faster Mantle basin, although the Española than the continental inte- Basin is deepest and has the Idealized cross section of the Rio Grande rift from Charles Chapin’s 1971 paper. rior. Consequently, the east largest fault displacements side of the rift developed on the west side. As a result, greater structural relief by riding up on in the Albuquerque Basin, providing the rock layers are generally tilted to the east the mantle bulge, whereas the west side of first direct evidence of the rock types, unit in the San Luis and Albuquerque Basins the rift was relatively subdued because of thicknesses, and structure in this basin. and to the west in the Española Basin. crustal stretching as the crust pulled away Although we learned a lot from these dry The alternating tilt of the Española Basin from the mantle bulge. Later, Chapin holes (no oil), drilling wells is expensive and the basins to the north and south is and co-author William Seager pointed and provides limited coverage. Thus, a accommodated by northeast-striking faults. out that the Rio Grande rift is generally number of physics-based tools that are These transfer faults are referred to as superimposed on older north-striking more cost effective have since been used to accommodation zones. Furthermore, broad structural zones associated with earlier indirectly image the basins and the crustal basins like the San Luis Basin and the Pennsylvanian–Permian Ancestral Rocky structure of the Rio Grande rift. Seismic Albuquerque Basin appear at the surface Mountain and Late Cretaceous–early refraction experiments were designed to to be a single basin, but gravity, magnetic, Cenozoic Laramide deformation. measure crustal thickness. Seismic reflec- and seismic data indicate that each of these Chapin’s research inspired increased tion data were collected to detect buried basins actually consists of at least three sub- interest in understanding the mechanisms rock layers, igneous intrusions, and faults. basins. These revelations about the deep driving rift formation worldwide, in part Estimates of basin depth and the location architecture of the rift basins have vastly motivated by the fact that water and of buried faults were determined with improved our ability to locate, character- geothermal resources, as well as mineral gravity data. Subtle variations in the earth’s ize, and manage water resources in New deposits (fluorite-barite), are commonly magnetic field associated with the presence Mexico and Colorado. associated with modern rifts. The East of certain iron-bearing rocks were used to The Rio Grande rift has experienced African rift, the Baikal rift in Russia, identify buried faults and volcanoes. The quite modest extension (7 to 22 percent), and the Rhine in Germany are amount of heat emitted in the vicinity of although locally in the Socorro area, the among the extensional features subjected the rift has been calculated from precise extension is as high as 170 percent. The to much scrutiny during the latter part measurements of temperature as a function amount of regional extension based on of the twentieth century. A great debate of depth combined with measurements of geologic evidence is about 50 percent, or ensued concerning the origin of rifts. Some the relative ability of rocks to conduct heat. 14 miles (22km), over the last 32 million researchers thought that rifts form by active These heat flow measurements are used years near the latitude of Socorro. Dates upwelling of the mantle into the crust, to identify geothermal areas and young on volcanic rocks indicate that rifting which causes the overlying crust to bulge igneous intrusions. Finally, advances in began about 36 million years ago near Las upward and crack under tension. Other the technology used to determine when Cruces and around 37 million years ago in scientists thought that rifts form more a lava flow erupted or when central Colorado. The timing of initiation passively due to far-field extensional stresses was deposited have been key in figuring of rifting roughly coincided with the late breaking the crust, causing the mantle to out when rifting began and the rate of its Eocene– peak of Cenozoic new mexico earth matters 2 summer 2012 volcanism, particu- dense and more slowly larly in southern New Rio Grande Great Plains through parts of the rift 50 45 40 35 30 25 20 15 Mexico, where -3 10 5 Seismic stations mantle that are less clusters become 0 dense. The density of Crust progressively younger Moho (crust-mantle boundary) the mantle is affected to the southwest. 50 31 by chemical composi- In northern New Mantle tion and temperature. 100 62 Mexico, the early rift When geophysicists

basins were broad and 150 93 identify a part of the shallow. The rift nar- mantle where the rowed and the basins 200 124 seismic waves slow deepened dramatically down, that area is between 14 and 16 250 155 usually colored red million years ago. The on maps or cross sec- 186 amount of rifting has 300 tions; areas of higher 110˚W 109˚W 108˚W 107˚W 106˚W 105˚W 104˚W Longitude decreased in the last 5 velocity are colored

million years. 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5 blue. Interpreting the Third, seismic Seismic shear wave velocity (km/s) physical characteristics data indicate that the of the seismically crust beneath the Rio Tomographic cross section of the Rio Grande rift along the LA RISTRA seismic line. The slow mantle is tricky Grande rift near the orange and yellow areas depict parts of the mantle that are seismically slow. These areas are because we cannot go latitude of Socorro, interpreted to represent hot, upwelling mantle material. Image from Wilson et al. (2005). The to the earth’s interior New Mexico, is 22 interpretation that upwelling is focused along the eastern margin of the rift is discussed in Reiter to collect a sample. miles (35 km) thick, and Chamberlin (2011). Is the mantle volume much thinner than seismically slow the crust beneath the Great because it is hot, or does the Plains to the east and the W E mantle volume contain partially Southern Rocky Mountains Colorado Plateau to the Sangre de Cristo molten rock or other fluids? west, where the crust is San Luis Basin Mountains Is the seismically slow mantle Colorado Rio Grande rift 28–31 miles (45–50 km) Plateau UT CO Great Plains made up of a low-density rock thick. Seismic methods type? In geologically active have detected a thin, 330- areas like the Rio Grande rift, 50 31 foot (100-m) thick we tend to interpret the red, body beneath the rift at low-velocity areas on maps and 100 62 a depth of 12 miles (19 cross sections as hot or partially km) that is 25 miles (40 molten rocks. A deep-imaging 150 93 km) wide at the latitude of seismic program known as LA Socorro. Although the heat RISTRA (Colorado Plateau/Rio 200 124 flow is high along the Rio Grande Rift Seismic Traverse Grande rift, anomalously Depth (km) Experiment) has used seismic high heat flow in Colorado 250 155 instruments located across and northern New Mexico the Colorado Plateau, the Rio is not restricted to the 300 186 Grande rift, and the Great rift. The latter two sets of Plains to produce a tomographic 217 observations have a bearing 350 image. The results, which are on our evolving models of illustrated as a cross section of rift development. 400 249 differential shear-wave velocity, Our perceptions of rift- clearly show that the crust is 450 280 ing are shifting again as we 109˚W 108˚W 107˚W 106˚W 105˚W 104˚W thinned in the rift and appears acquire new data using an to show that hot mantle is imaging technique called moving upward under the seismic tomography, which Rio Grande rift. According to uses seismic energy from another interpretation of these natural to map Tomographic cross section showing low relative P-wave velocity beneath the data, the main zone of upwell- the velocity structure of the and the Sangre de Cristo Mountains in southern Colorado. ing is focused along the eastern Note that the Rio Grande rift is not clearly defined in the mantle on this cross interior of the earth, par- section; instead the red, low-velocity mantle is present west and east of the rift margin of the rift adjacent to ticularly the below the high mountain peaks of Colorado. The arrows depict a possible < 10 the Great Plains near longitude to depths of 250 miles million year disturbance of the 250 mile (410 km) boundary in the mantle that 106°W. The upwelling is (400 km). Seismic waves has caused hot material to rise and overprint the mantle signature of the Rio restricted to the upper mantle travel quickly through Grande rift. The diffuse - boundary zone is shown as at depths of 30–186 miles dashed lines. parts of the mantle that are (50–300 km). new mexico earth matters 3 summer 2012 to tectonic problems, mapping Recent GPS surveys, volcanic fields, and evaluating the which detect small changes geothermal potential of the Rio in horizontal and vertical Grande rift in New Mexico. distances between geographic points, indicate that the North Richard Chamberlin is an American continent is generally emeritus senior field geologist moving west due to broad-scale at the New Mexico Bureau of plate tectonic forces, but the Geology and Mineral Resources. area on the west side of the rift Basin & Range He has extensive experience in

is moving west faster than the 11 and knowledge of the geology of m 0 Colorado Plateau m/y ear R i

Great Plains to the east, as pre- o the Socorro area.

Plate G

boundary zone r 0 dicted by Chapin in 1971. This a 7 ear n y d m/ m differential velocity between the e r i Suggested Reading f t CARIBBEAN east and west sides of the rift is PLATE causing the rift in New Mexico Tectonics of the Rio Grande and southern Colorado to Vector scale depression of central New Mexico widen at a measured rate of one 50 mm/year by V. C. Kelley, 1952. In: Rio Grande Country, New Mexico inch (2.5 cm) every 40 years. zones The widening is not limited teeth on over-riding plate Geological Society, Guidebook to the rift, but also affects the 3, pp. 93–105. Mid-ocean ridges SA western Great Plains and the The Rio Grande rift, part I: eastern Colorado Plateau. modifications and additionsby This observation emphasizes Map showing the relative velocities of the North American plate and the Pacific plate, shown as red arrows. The orange area highlights parts of the C. E. Chapin, 1971. In: San another shift in the conceptual North American plate that accommodate the difference in direction and Luis Basin (Colorado), New model of the Rio Grande rift. velocity of motion between the North American and Pacific plates. The San Mexico Geological Society, During the 1970s and ‘80s the Andreas fault accounts for 80 percent of the divergence. GPS measure- Guidebook 22, pp. 191–201. Rio Grande rift was viewed ments show that the western United States moves 1 to 4 mm/yr faster as an intraplate rift. Now the (total velocity 71–74 mm/yr) compared to the central United States (total Tectonics, by E. Moores and R. velocity 70 mm/yr). Data from Moores and Twiss, 1995. western United States between J.Twiss, 1995; W.H. Freeman California and New Mexico is Grande rift but are located to the east and and Co., ISBN 0-7167-2437-5. viewed as a broad zone of related deforma- west of the accepted boundaries of the rift. tion. Examination of global seismic Researchers are puzzling over the new data. Alternative Perspectives of Crustal and Upper tomographic data by many geoscientists A possible explanation is that pieces of Mantle Phenomena Along the Rio Grande reveals one fundamental characteristic that lithosphere below southwestern Colorado rift, by M. Reiter and R. M. Chamberlin, is present in the North American plate in detached starting 25 million years ago 2011. In: GSA Today, v. 21, no. 2, pp. 4–9, all interpretations of the data: the western and slowly descended into the deep layers doi: 10.1130/GSATG79AR. U.S. is underlain by seismically slow and of mantle. These lithospheric pieces hit a Geology and groundwater conditions of boundary in the mantle at about 250 miles presumably hot mantle, and the interior the Rio Grande depression in Colorado (410 km), causing deep mantle material of the continent is seismically fast and and New Mexico, by Bryan Kirk, 1938. presumably cold mantle. The Rio Grande to move up during the last 10 million In: Regional Planning, Part 6, Upper Rio rift is at the boundary. The combination of years, overprinting the shallow mantle Grande: Washington National Resources both local and global seismic tomography signature of the rift. Low velocity zones Committee (Planning Board), pp. 197–225. and recent GPS data suggests that the Rio within the crust are associated with the rift Grande rift is forming because the region in Colorado. These new observations are http://cgsdocs.state.co.us/Docs/ERC/ to the west of the rift travels west more causing us to formulate new strategies for geology and ground-water quickly than the region to the east, and geothermal exploration and seismic and conditons of the rio grande the mantle is passively upwelling into the volcanic hazards evaluation. Stay tuned as depression-bryan 1938.pdf thinned, stretched lithosphere. our thoughts about rifting advance as new Mantle-driven dynamic uplift of the Rocky Just when we think we have it all figured data sets are gathered. Mountains and Colorado Plateau and its out, we collect another batch of data that —Shari Kelley and surface response: Toward a unified hypothesis, causes us to rethink our interpretations. Richard Chamberlin by K.E. Karlstrom, and the CREST New seismic tomography data from working group, 2012. In: Lithosphere; v. 4; another experiment called CREST We thank Charles Chapin for sharing his no. 1; pp. 3–22, doi: 10.1130/L150.1 (Colorado Rockies Experiment and Seismic story with us and for his helpful comments on Transects) in southwestern Colorado reveal the article. Imaging the Seismic Structure of the Crust a complex image of the mantle and indi- and Upper Mantle Beneath the Great Plains, cate that the crust is thinned across a broad Shari Kelley is a senior geophysicist and field Rio Grande Rift, and Colorado Plateau region outside the surface expression of the geologist with the New Mexico Bureau of Using Receiver Functions, by D. Wilson, Rio Grande rift. The seismically slow parts Geology and Mineral Resources. Her research R. Aster, et al., 2005. Geophys. Res., 110, of the mantle are NOT beneath the Rio interests include applying thermochronology B05306, doi:10.1029/2004JB003492. new mexico earth matters 4 summer 2012 Bureau News Geology Hall of Fame ing and appreciation for the earth sciences, Shari Kelley, the bureau’s senior geophysi- and to encourage proper stewardship of cist and field geologist, was honored by the our planet. Celebrated October 14–20, Department of Geological Sciences at her 2012, the theme for this year is “Discover- alma mater, New Mexico State University, ing Careers in the Earth Sciences.” with the 2011 Alumni Hall of Fame Award The New Mexico Bureau of Geology in recognition of outstanding professional and New Mexico Tech participate in this achievement. After completing her event each year. Our governor, Susana bachelor’s degree at NMSU, Shari earned Martinez, has issued a proclamation declar- a Ph.D. in geophysics from Southern ing October 14–20 as Earth Science Week Methodist University. Her research has in New Mexico. Free toolkits containing posters, DVDs, brochures, bookmarks, fact included geologic mapping, applications Volume 12, Number 2 of apatite fission-track thermochronology, sheets, postcards, an activity calendar, and Published twice annually by the and geothermal exploration. much more are available to teachers. Two- New Mexico Bureau of Geology minute radio broadcasts recorded by NMT and Mineral Resources professionals will air at noon each weekday L. Greer Price GSA Fellow during Earth Science Week on KUNM. Director and State Geologist a division of Steve Cather, Senior Field Geologist, These broadcasts, and those recorded in New Mexico Institute of was made a Fellow of the Geological previous years are posted on our web site Mining and Technology Society of America, an honor accorded at: http://geoinfo.nmt.edu/events/esweek/ Daniel H. López to fewer than fifty geoscientists each year. home.html. For more information, please President Steve has worked in New Mexico for contact Susie Welch at the Bureau of 801 Leroy Place Socorro, New Mexico 87801-4750 over thirty years. His research interests Geology at: [email protected], or visit AGI’s (575) 835-5420 include large-scale structural evolution and web site at: http://www.earthsciweek.org/ index.html Albuquerque Office sedimentation processes in the Southwest. . 2808 Central SE His research is based on detailed field Albuquerque, New Mexico 87106 mapping. Steve has published extensively 20th Anniversary for Lite Geology (505) 366-2530 on his work. Visit our main Web site This year we celebrate the twentieth geoinfo.nmt.edu anniversary of Lite Geology. First published Rockin’ Around New Mexico in 1992, the periodical originated when Board of Regents Ex Officio This year’s teachers’ workshop, “Rockin’ then-director Chuck Chapin assembled a Susana Martinez Around New Mexico,” returned to the team of staff members to create an informa- Governor of New Mexico National Preserve Science tive yet entertaining publication focused José Z. Garcia Education Center in Jemez Springs and on earth science topics and current issues Secretary of Higher Education visited places inaccessible last summer due in New Mexico, one that would appeal to Appointed to the Las Conchas fire. From July 9 to 12, a broad audience and serve as an outreach Richard N. Carpenter teachers representing grades K through 12 tool for educators and the public. Early President participated in field exercises and classroom issues of Lite Geology contained whimsical 2011–2014, Santa Fe activities that focused on water quality, cartoons, creative short features, and articles Jerry A. Armijo fire ecology, the volcanic history of Valles on topics ranging from mineral resources to Secretary/Treasurer 2009–2014, Socorro caldera, current and future seismic risks geologic hazards to New Mexico’s enchant- on local faults, and safety in ing geology. Over the years the original Deborah Peacock 2011–2016, Albuquerque schools. Rockin’ Around New Mexico quarterly print publication evolved into relies on the New Mexico Bureau of Geol- a semi-annual online publication. Today Abe Silver, Jr. ogy/New Mexico Institute of Mining and Lite Geology is available as a free download 2007–2012, Santa Fe Technology, New Mexico Department of from the New Mexico Bureau of Geology Omar Soliman Homeland Security and Emergency Man- and Mineral Resources website, along with 2011–2012, Socorro agement, New Mexico Geological Society, the entire collection of back issues. Each Editors and other agencies and private companies issue now includes short articles, classroom L. Greer Price for funding and instructor support. For activities, puzzles, and links to online Jane C. Love, Gina D’Ambrosio information on the 2013 workshop, resources. Some of the early editors are still Layout, Graphics, and Production contact Susie Welch at [email protected]. on the job and have been joined in recent Gina D’Ambrosio years by other bureau staff. We welcome Stephanie Chavez Earth Science Week 2012 your input for future issues. Check out Earth Matters is a free publication. For subscription information please call The American Geological Institute has the latest issue at http://geoinfo.nmt.edu/ publications/periodicals/litegeology/home. (575) 835-5490, or e-mail us at sponsored Earth Science Week each year [email protected] html since 1998. This national and international Cover photo of Ship Rock, New Mexico event is designed to help the public, teach- © Gary Rasmussen ers, and students gain a better understand- new mexico earth matters 5 summer 2012 New Mexico Institute of Mining & Technology NONPROFIT ORGANIZATION New Mexico Bureau of Geology and Mineral Resources U.S. Postage 801 Leroy Place Socorro, New Mexico 87801-4750 PAID Return service requested permit no. 1888 Albuquerque, NM

Publications New Mexico Geology. Geology of the Warm New Mexico Geology, the Springs Region. This is the 63rd bureau’s technical quarterly in a series of annual guidebooks journal, has been publishing of the New Mexico Geological peer-reviewed research papers for Society, published for the 2012 34 years. A 1-year subscription is fall field conference. ISBN: 978- $15; a 2-year subscription is $25. 1-58546-098-4. $65.00 book, Single issues are $4 plus shipping spiral-bound road logs only $25. and handling. Majestic mountain sky islands, The current August 2012 deep dark canyons, and hidden issue includes the third part of a springs interrupt the vast desert year-long special series of articles landscape of the Warm Springs honoring the 100th anniversary region in south-central New of New Mexico’s statehood, Mexico. This guidebook examines “The geology of New Mexico as the spectacular geology that led to the formation of these landforms understood in 1912.” Also included is a paper on middle Turonian in what may well be the most geologically complex part of New stratigraphy, “Evolution of the Late Cretaceous oyster genus Mexico. Rock exposures in this region represent almost the entire Cameleolopha Vyalov 1936 in central New Mexico.” Visit the New record of geologic time, from granites over a billion years old to Mexico Geology web pages at geoinfo.nmt.edu/publications/period- modern river deposits of the Rio Grande. Six road logs explore dif- icals/nmg to subscribe online, to view past issues, and to download ferent portions of the Warm Springs region, 26 minipapers discuss published papers and guidelines for manuscript submissions. specific geologic features along the way, and 30 scientific papers provide details on geologic features and processes. These articles discuss a wide range of geological topics including regional geology, impact structures, igneous and sedimentary petrology, stratigraphy, paleontology, and earth resources. Available in October.

For more information about these and other bureau publications: • Visit our Web site at http://geoinfo.nmt.edu • Write or visit our Publications Office on the campus of New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801 • Call (575) 835-5490 or e-mail us at [email protected] • Publication prices do not include shipping and handling or taxes where applicable. new mexico earth matters summer 2012