January 26, 2012
Lithosphere Highlights – February 2012
The new issue of LITHOSPHERE is online now. Papers present evidence for the on-going re-shaping of the Rocky-Mountain—Colorado Plateau region by young uplift driven from below (mantle buoyancy), research in the Aegean Sea that documents a newly defined extensional fault system, and study of the hydrologic heterogeneity of faulted and fractured sediment layers with implications for similar rocks to affect the flow of moisture downward toward the spent nuclear fuel geologic repository at Yucca Mountain.
Highlights are provided below. Representatives of the media may obtain complimentary copies of LITHOSPHERE articles by contacting Christa Stratton at the address above. Abstracts for all LITHOSPHERE articles are available at http://lithosphere.gsapubs.org/.Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Christa Stratton for additional information or assistance.
Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: toward a unified hypothesis
K. Karlstrom et al., University of New Mexico, Earth and Planetary Sciences, Albuquerque, New Mexico 87131, USA. v. 4, no. 1, p. 3—22; doi: 10.1130/L150.1.
The high elevations of the Colorado Rocky Mountains include 55 peaks greater than 14,000 feet (4 km) and regions with average elevation above 10,000 feet (3 km). Yet, the Rockies are more than 1000 miles (1600 km) from the plate boundary and provide an enigma for plate tectonic theory, which postulates that mountain belts usually form near collisional plate margins. New results from the CREST project (Colorado Rockies Experiment and Seismic Transects) show that the Colorado Rockies are underlain by relatively thin (26 mile; 43 km) crust and that the mountains are not buoyed up by a low density crustal root, like the Himalayas are, for example. Hence the Rockies are “rootless.” Instead, new high-resolution seismic images (like a catscan of the Earth) reveal zones of hot and buoyant mantle under the San Juan and Aspen regions that are lifting-up the Rockies. Geologic data have allowed the CREST team to determine that the Rockies have been uplifted in several stages and that the youngest uplift component, which may account for up to half of the total uplift of the region, accelerated about 6—10 million years ago. Detailed studies of the incision history of the continental- scale Colorado River system by the CREST team indicate that mantle-driven uplift of the Rockies is still ongoing. Similarly, at the downstream end of the Colorado River system, buoyant mantle is driving uplift of the Grand Canyon region. The overall model presented by Karlstrom et al. is that the Rocky-Mountain—Colorado Plateau region is being re-shaped by young uplift that is being driven from below (mantle buoyancy). This young uplift is interacting with top-down erosional processes to produce our rugged mountain and canyonland landscapes.
Miocene bivergent crustal extension in the Aegean: evidence from the western Cyclades (Greece)
B. Grasemann et al., Department of Geodynamics and Sedimentology, University of Vienna, A-1090 Vienna, Austria. v. 4, no. 1, p. 23—39; doi: 10.1130/L164.1.
The Earth´s tectonic plates, the outer most solid shell of the planet, undergo long-term cyclical pushing and pulling along their margins. During tectonic extension, the pulling forces rupture the plate and cause widespread thinning of the crust. The research of Grasemann et al. in the Aegean Sea of the eastern Mediterranean documents a newly defined extensional fault system, and modifies the models for extension in the region. Grasemann et al. propose that extension and thinning is accommodated by two opposing directions of faulting, focused along major, subhorizontal crustal-scale structural discontinuities exposed on the islands of the Cyclades. Radiometric age data from the islands indicates the onset of extension was about 23 million years ago, and continues today as testified by the highest concentration of modern-day seismic activity in Europe.
Hydrogeologic heterogeneity of faulted and fractured Glass Mountain bedded tuffaceous sediments and ashfall deposits: The Crucifix site near Bishop, California
C. Dinwiddie et al., Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, San Antonio, Texas 78238, USA. v. 4, no., 1, p. 40—62; doi: 10.1130/L179.1.
Dinwiddie et al. examined layered volcanic sediments and found that fault-enhanced irregularities affect seepage through the formation. The researchers used many field and laboratory methods to quantify the hydrologic heterogeneity of faulted and fractured sediment layers over length scales ranging from micrometer (one-millionth of a meter) to dekameter (ten meters) in order to better understand the potential impact that structural deformation features can have on groundwater flow paths through permeable rock units at this site and elsewhere. The rocks that they studied are similar to rocks within the non-welded Paintbrush Tuff, which lies in the variably saturated zone above the proposed high-level radioactive waste and spent nuclear fuel geologic repository at Yucca Mountain, Nevada. Fault-enhanced heterogeneities that affect porosity and permeability may direct the flow of moisture downward toward the repository.
Constraints on exhumation and extensional faulting in southwestern Nevada and eastern California, U.S.A., from zircon and apatite thermochronology
D. Ferrill et al., Southwest Research Institute, Department of Earth, Material, and Planetary Sciences, 6220 Culebra Rd., San Antonio, Texas 78238-5166, USA. v. 4, no. 1, p. 63—76; doi: 10.1130/L171.1.
Eastern California and southwestern Nevada is an area of active extensional and transtensional deformation of the Earth's crust. New data were collected by Ferrill et al. to study regional patterns of cooling of rocks that was caused by large-scale thinning by extensional faulting of Earth's brittle crust related to this tectonic deformation. These data were derived by analyzing fission tracks in zircon and apatite grains from some of the oldest sedimentary and metasedimentary rocks in the region, which were brought to the Earth's surface by slip along faults. An area of relatively young ages indicates progressively younger ages (from 14 to 5 million years) westward into Death Valley. This pattern indicates west-northwest migration of the cooling front, consistent with well-documented Miocene extension of the Basin and Range. The active trailing edge of the hanging wall of this system generally coincides with Death Valley. Migration rates of the cooling front are on the order of 10—11 mm/yr. Extrapolation of apatite fission-track closure ages suggests that exhumation along the eastern margin of the system continues beneath Death Valley today.
Basin formation near the end of the 1.60-1.45 Ga tectonic gap in southern Laurentia: Mesoproterozoic Hess Canyon Group of Arizona and implications for ca. 1.5 Ga supercontinent configurations
M. Doe et al., Colorado School of Mines, Geology and Geological Engineering, 1516 Illinois Street, Golden, Colorado 80401. USA. v. 4, no. 1, p. 77—88; doi: 10.1130/L160.1.
The history of the Earth has involved the assembly and break up of supercontinents about every 800 million years. Well before the most recent and familiar supercontinent (Pangaea), North America was part of earlier supercontinents: Rodinian (1000 million years ago) and Nuna (1800 million years ago). Supercontinents are composed of smaller “plates” which scientists try to put together using various geologic clues. Although they all generally agree that there were former supercontinents, scientist often differ on the plate configurations. New data by Doe et al. from southern Arizona add a piece towards resolving this global puzzle. A fragment of a 1.4—1.5-billion-year-old basin, called the Yankee Joe Basin, has been discovered in the upper Salt River Canyon of central Arizona. Detrital zircon ages reveal abundant 1.5—1.6-billion-year-old grains within these sandstones. These new ages fill a “magmatic gap” essentially unknown from southern North America indicating that the grains were washed in from an exotic/external source terrane. The preferred plate configuration model is that the grains were derived from Australia, but other candidate source regions include Antarctica and South America. This new piece of the puzzle provides new information for a time when little is known about North America: the so-called 1.60—1.5-billion-year-old magmatic gap, and supports the hypothesis that a long-lived plate margin in southern North America may have been linked to Australia and other once-connected continents within the supercontinent of Nuna.
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