New and Inexpensive Method to Evaluate Region for Geothermal Energy Exploration

To offset the need for fuel imports, to decrease greenhouse gas emissions, and to increase U.S. energy independence, geothermal energy has emerged as an important part of the U.S. energy portfolio. This well-illustrated study, published in Geosphere this week, presents a new and inexpensive method using Geographic information system (GIS) and National Geothermal Data System data to evaluate a region for geothermal energy exploration.

Authors Anna Crowell and Will Gosnold gathered and analyzed free-access GIS data for trends that could help geoscientists assess whether a sedimentary basin could be economically utilized for geothermal power production. In their article, they identify several counties in the states of Colorado, Illinois, Michigan, and North Dakota where geothermal energy could be used for different energy production scenarios.

In particular, they find that the Denver-Julesberg Basin (which spans Wyoming, Nebraska, and Colorado, and has a surface area of approx. 155,000 square kilometers) has the highest capacity for large-scale, economically feasible geothermal power production. They write that, "assuming an adequate, sustainable water supply," high-population areas west of Denver, near the depocenter of the basin and the Golden fault along the Front Range of the Rocky Mountains, are of greatest interest because costly infrastructure is already in place.

FEATURED ARTICLE

Integrating geophysical data in GIS for geothermal power prospecting

Anna Crowell and Will Gosnold, University of North Dakota, Harold Hamm School of Geology and Geological Engineering, Grand Forks, North Dakota 58202, USA. This article is online at http://geosphere.gsapubs.org/content/early/2015/10/02/GES01161.1.abstract. Themed issue: Geothermal Energy from Sedimentary Basins: Challenges, Potential, and Ways Forward.

Other newly released GEOSPHERE articles are highlighted below:

Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions

David L. Kimbrough et al., Dept. of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA. This article is online at http://dx.doi.org/10.1130/GES00982.1. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II.

The path of any possible pre-Grand Canyon Colorado River and how the modern river became established through the western Grand Canyon ~5 million years ago are matters of long-standing contention. New detrital zircon U-Pb ages presented here together with a published database from supracrustal rocks deposited atop the Colorado Plateau allow the authors to interpret the evolution of the Colorado River's source region over the past 5 million years in terms of progressive incision into Cenozoic and Mesozoic deposits in the eastern and northeastern regions of the Colorado River catchment. The Kolmogorov-Smirnov statistic is adapted to ternary mixing calculations that allow construction of readily visualized ternary diagrams that show the compositional range over which end-members can be mixed to produce a given age distribution at 95% confidence. Reference age distributions for the Holocene and early Pliocene Colorado River provide a necessary starting point to future studies in assessing the path of possible pre-Grand Canyon Colorado River drainages.

An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic field, Colorado

Peter W. Lipman et al., U.S. Geological Survey, Menlo Park, California 94025, USA. This article is online at http://dx.doi.org/10.1130/GES01184.1.

Among large ignimbrite calderas, Bonanza in the Southern Rocky Mountain volcanic field displays diverse depth and structural levels that provide special insights concerning explosive eruptive processes and caldera development. Bonanza, source of a compositionally complex regional ignimbrite sheet erupted at 33.12 million years plus or minus 0.03 million years is a subequant structure ~20 in km diameter that subsided at least 3.5 km during explosive eruption of ~1,000 cubic kilometers of magma, then resurgently domed its floor a similar distance vertically. Bonanza provides a rare site where intact caldera margins and floor are exhumed and exposed, providing valuable perspectives for understanding younger similar calderas in some of the world's most active and dangerous silicic provinces.

Australia going down under: Quantifying continental subduction during arc-continent accretion in Timor-Leste

Garrett W. Tate et al., Chevron Corporation, 1500 Louisiana Street, Houston, Texas 77002, USA. This article is online at http://dx.doi.org/10.1130/GES01144.1.

Subduction zones often feature dense oceanic crust subducting below a buoyant continental plate. Subduction of a continent below an ocean plate, however, tends to be inhibited by buoyant forces. It is therefore often supposed that when a continent and an ocean plate collide in such a way that the continent is part of the subducting plate, little of the continent will be subducted into Earth's mantle. In this article, Garrett Tate and colleagues test this supposition in Timor-Leste (East Timor), where the Australian continental margin has been subducted beneath the Banda volcanic arc. By mapping the rock strata and faults exposed on the surface in central Timor-Leste, they reveal key aspects of the geometry of mountain building during collision at Timor. Results document the amount of shortening due to faulting and folding during mountain building, and characterization of the rock units involved in this shortening allow for estimation of the magnitude of subducted continent. This research suggests that 215-219 km of Australian continental lithosphere have subducted at Timor, significantly greater than traditional estimates would suggest.

Polyphase Proterozoic deformation in the Four Peaks area, central Arizona, and relevance for the Mazatzal orogeny

Calvin A. Mako et al., Dept. of Geosciences, University of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, Massachusetts 01003, USA. This article is online at http://dx.doi.org/10.1130/GES01196.1.

The basement rocks in central Arizona were affected by two major mountain building events about 1.65 and 1.45 billion years ago, known as the Mazatzal and Picuris orogenies. We have demonstrated that both of these events can be observed in the geologic record in the Four Peaks area in the southern Mazatzal Mountains. The large fold known as the Four Peaks syncline formed during the Picuris orogeny, rather than the Mazatzal orogeny as was previously thought. This work helps us to understand how the core of North America was formed and what geologic events affected its growth.

Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

M.P. Searle et al., Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. This article is online at http://dx.doi.org/10.1130/GES01199.1.

The emplacement of large thrust sheets of dense oceanic crust and upper mantle rocks (ophiolites) onto more buoyant continental margins has been a long-standing puzzle for geologists. In the mountains of Oman-United Arab Emirates a vast thrust sheet of Cretaceous ophiolite rocks has been emplaced onto the Arabian continental margin. We studied highly deformed and metamorphosed rocks preserved along the base of the ophiolite that record high temperatures during emplacement. Our dating has proven that the ophiolite and the metamorphic sole formed at the same time (96-95.5 million years ago), and that the high temperature sole rocks beneath were stacked up in the subduction zone and the continental margin was too buoyant to descend.

Paleostress directions near two low-angle normal faults: Testing mechanical models of weak faults and off-fault damage

Gary J. Axen et al., Dept. of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. This article is online at http://dx.doi.org/10.1130/GES01211.1.

Current understanding of the strength and mechanics of faulting relies heavily upon laboratory friction tests on rock samples of about 2 to 10 cm in size. However, many observations of large active and inactive faults suggest that they are much weaker than standard laboratory friction values. Such faults are the San Andreas plate-boundary fault (system) in California and gently dipping normal faults, along which rocks above the fault move down a gentle slope over rocks below. This apparent contradiction has led to a variety of mechanical models to explain slip on "weak faults." These models generally predict (among other things) the orientation of the stress field near the faults. We test some of these models, and others that predict minor fracture orientations near large faults, by inverting orientations of fractures in the rocks immediately below two low-angle normal faults for the stress fields that were active during slip on the main faults. We find that a model that uses standard lab friction values and moderately elevated pore-fluid pressure fits our results well in the brittle upper crust, and that the stress field was rotated by about 45 degrees descending from the upper, brittle crust to the brittle-plastic mid-crustal transition. Models not favored predict a large rotation of the stress field near the master fault, extremely elevated pore-fluid pressure in the fault zone, or appeal to low-friction materials on the main fault; granular shear in the fault zones is only weakly supported. Near-fault damage predicted by elastic models of sinusoidal faults fit our results well but those based on fault- or earthquake-rupture propagation are not fit well.

Detrital-zircon geochronology and provenance of the Ocloyic synorogenic clastic wedge, and Ordovician accretion of the Argentine Precordillera terrane

William A. Thomas et al., Geological Survey of Alabama, 420 Hackberry Lane, P.O. Box 869999, Tuscaloosa, Alabama 35486-6999, USA. This article is online at http://dx.doi.org/10.1130/GES01212.1.

From the abstract: The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle-Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge.

All GEOSPHERE articles are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, [email protected].

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