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Mechanics of graben evolution in Canyonlands National Park, Utah
P. Walsh and D.D. Schultz-Ela, Bureau of Economic Geology, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924, USA. Pages 259-270.
Keywords: Canyonlands National Park, extension, faults, grabens, numerical models, salt tectonics.
Long, parallel canyons with strikingly vertical walls, called grabens, dominate the scenery in the Needles District of Canyonlands National Park, Utah. Faulting of rock layers formed the steep canyons over the past 65,000 years. Researchers have been studying these features in detail since the mid-1970’s, but a key geologic process went unnoticed. Combining computer models with land surveying techniques, the authors discovered that erosion by the Colorado River caused rock layers to sink into underlying salt, tilt toward the river, and break apart into the series of canyons observable today.
Early Neogene history of the Central American arc from Bocas del Toro, western Panama
Anthony G. Coates, Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Republic of Panama, et al. Pages 271-287.
Keywords: Neogene, Central America, Panama, Bocas del Toro, paleobathymetry, stratigraphy.
This paper describes a newly discovered sequence of fossiliferous strata in the Bocas del Toro archipelago, Caribbean coast of western Panama. Newly mapped formations are named and described, and their ages and the water depths at which they were deposited are established. These data are then used to interpret timing of the rise of the Isthmus of Panama, its formation as an active volcanic island arc, and the paleogeography of the region as the arc became emergent and began to act as a barrier between the Pacific and the Caribbean. The events described range from about 20 to 8 million years ago.
Controls on localization and densification of a modern décollement: Northern Barbados accretionary prism
G. Wallace, et al., Department of Earth Sciences, University of California Santa Cruz–Santa Cruz, California 95064, USA. Pages 288-297.
Keywords: décollement, image analysis, microstructure, localization.
Understanding the manner in which materials inside of faults deform is critical for interpreting the proceses controlling fault behavior. In this study, we examine patterns of densification and km-scale variations in surface roughness to constrain fault localization mechanisms.
Neoproterozoic allochthonous salt tectonics during the Lufilian orogeny in the Katangan Copperbelt, central Africa
M.P.A. Jackson, Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713, USA, et al. Pages 298-314.
Keywords: allochthons, Congo, Copperbelt, fold-and-thrust belt, nappes, salt tectonics, Zambia.
The Central African Copperbelt, which extends from southern Democratic Republic of the Congo to central Zambia contains the world’s largest concentration of sedimentary cobalt and copper. Because of these resources, this region has been extensively mapped, drilled, and mined over many decades. Despite this intensive exploration, though, some puzzling tectonic questions remain. This paper shows how several striking and bizarre tectonic features in the southern Congo can be explained by concepts of salt tectonics developed in much younger and less-deformed sedimentary basins around the world. Using existing data, the authors infer the former existence of a vast outflow of glacial rock salt mixed with other rocks and the metal ores. This extrusion of salt carried massive pieces of rock up to 10 km wide for up to 65 km. The sheet of weak salt lubricated the later emplacement of thrust sheets as the basin was compressed to less than half its former width in late Precambrian times. During the 650 million years that followed, the salt was dissolved away, leaving behind the stranded fragments and crumpled structures that we see today.
Synextensional Pliocene–Pleistocene eruptive activity in the Camargo volcanic field, Chihuahua, México
José Jorge Aranda-Gómez, Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, P.O. Box 1-742, Querétaro, Querétaro 76001, México, et al. Pages 315-330.
Keywords: Basin and Range province, Chihuahua, Mexico, extension faults, intraplate, Pliocene, slip rates.
The Camargo Volcanic Field is the largest occurrence of mafic alkalic volcanic rocks in the Mexican Basin and Range Province. It is formed from 4.7 to 0.09 Ma intraplate lavas, many of which contain mantle xenoliths. Activity began in the SW part of the field and shifted toward the NE at ~15 mm/year. Volcanism and normal faulting were at least in part coeval. Normal faulting is bracketed between 4.7 and 2.1 Ma. Estimated vertical slip rates on four faults range from 0.03 mm/year, a likely long-term rate, to 1.67 mm/year, interpreted as a short-term rate operative during periods of active faulting. NW-striking faults that cut alluvial fan deposits and Pleistocene lavas in the northern part of the field suggest that central Chihuahua is still extending.
Age, source, and regional stratigraphy of the Roraima Supergroup and Roraima-like outliers in northern South America based on U-Pb geochronology
João Orestes Schneider Santos, Departamento de Geologia, Companhia de Pesquisa de Recursos Minerais (CPRM) Avenida André Araújo 2160, Manaus, Amazonas, 69060-001, Brazil, and Universidade Federal do Rio Grande do Sul (UFRGS) Avenida Bento Gonçalves 9500, Porto Alegre, Rio Grande do Sul, 91500-000, Brazil, et al. Pages 331-348.
Keywords: Amazon craton, baddeleyite, diamond, Guyana Shield, Roraima, U-Pb geochronology, zircon.
The Proterozoic sedimentary rocks related to the Roraima Supergroup cover large areas (about 1,800,000 km2) in northern South America (Venezuela, Guyana, Brasil, Suriname, Colombia). The Roraima landscape is marked by giant table mountains, the highest (up to 3,016 m) in the Guiana Shield, and include the highest water falls in the world. This paper provides a precise sensitive high-resolution ion microprobe (SHRIMP) U-Pb age of 1875 ± 5 Ma on zircon from an interlayered ash-fall tuff from the Roraima Supergroup. The giant sedimentary sequence was deposited in a foreland basin derived from the Trans-Amazonian Orogenic Belt to the north. The Neblina Basin is at least 320 m.y. younger and is interpreted as a continental successor basin deposited during the Mesoproterozoic and derived mostly from the Tapajós-Parima Orogenic Belt to the east. This is a major contribution to the understanding of one of the largest Precambrian sandstone basins in the world, located in the remote Amazon forest region.
SHRIMP U-Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits
Karen Lund, U.S. Geological Survey, Denver, Colorado 80225, USA, et al. Pages 349-372.
Keywords: SHRIMP, U-Pb geochronology, Windermere Supergroup, Idaho, Sturtian glaciation, Rodinia rifting.
In the Late Precambrian (Neoproterozoic), several snowball Earth events may have enveloped the planet due to unique climatic factors as the supercontinent Rodinia broke apart. Newly described rock formations and new ion microprobe ages for metamorphosed sedimentary and volcanic rocks, discontinuously exposed in a northwest-southeast belt across central Idaho, alter models of supercontinent development and disintegration as well as models of global climate change. Studied strata are equivalent to the Neoproterozoic Windermere Supergroup and related rocks found from the Alaska-Yukon region to Death Valley and provide a missing link between northern and southern segments of that belt, redefining the geometry. Interlayered volcanic and sedimentary rocks formed ca. 685 million years ago as a result of rifting along the length of present-day western North America (Laurentia) during breakup of the Rodinian supercontinent. This age clarifies a step-wise history of rifting that ultimately culminated in breakup of Rodinia about 570 million years ago but brings into question the timing of specific continental separations. Very coarse-grained sediments, now exposed as rocks called diamictite, formed by glaciers during the Neoproterozoic Rapitan glacial event. Some workers include this glaciation as part of a worldwide glacial period, internationally called “Sturtian,” hypothesized to have plunged Earth into a global ice age that included glaciers even in equatorial regions. The new ages indicate that the time of rifting and glaciation was as much as 65 million years younger than previously thought. This relatively young age for the Rapitan glacial event raises questions about whether this glaciation was unique to western North America rather than contemporaneous with (less well-dated) Sturtian deposits worldwide and, thus, whether Earth indeed became a planetary snowball at that time.
Historic calcite record from the Finger Lakes, New York: Impact of acid rain on a buffered terrane
C.K. Lajewski, H.T. Mullins, Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244, USA, et al. Pages 373-384.
Keywords: acid rain, Finger Lakes, calcite precipitation, buffered terranes, alkalization.
Results of a study of historical (<200 yr) age sediments cored from the Finger Lakes of central New York State have demonstrated the impact of acid rain in an environment that has high acid neutralizing potential. Most previous studies on the environmental impact of acid rain have been conducted in regions (such as the Adirondack Mountains of New York or in New England) that have a low acid neutralizing capacity because of the igneous/metamorphic nature of their underlying bedrock. In the Finger Lakes region, calcareous shales and limestones make up most of the bedrock. Acid rain falling on this type of terrain dissolves the CaCO3 (calcium carbonate) from the bedrock, which is then transported via streams to adjacent lakes. As a consequence, many of the Finger Lakes seasonally (summer) precipitate calcite (CaCO3) as "whiting" events that reduce water clarity and increase water turbidity. In the case of the Finger Lakes, calcite precipitation has returned after an ~ 4500 year hiatus, due to anthropogenically-induced acid rain. Thus, the environmental impact of acid rain across a "buffered terrain" like the Finger Lakes is to increase the calcium, bicarbonate, and carbonate content of lake waters (i.e. "alkalization"— higher pH) in contrast to the impact of acid rain in areas like the Adirondacks, which results in the "acidification" (low pH) of lakes and detrimental effects on terrestrial and aquatic ecosystems, including fisheries.
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