|30 Nov. 2011
GSA Release No. 11-81
Director of Education, Communication, & Outreach
LITHOSHERE Highlights: DECEMBER ISSUE ONLINE
Boulder, CO, USA –Highlights for LITHOSPHERE articles published online in the December 2011 issue are provided below. Research explores the age of continental crust, an updated paleomagnetic pole, seismic ambient noise in Patagonia, a previously unrecognized age province in the southwestern U.S., and hypotheses about the linkages between the growth of the Tibetan Plateau and global climate.
Keywords: Sweden, rapakivi intrusions, skewness, chron 32, Chile Ridge Subduction Region, Farmington Zone, Tibetan Plateau.
View abstracts for all LITHOSPHERE papers in the current issue at http://lithosphere.gsapubs.org/content/current.
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Ancient and juvenile components in the continental crust and mantle: Hf isotopes in zircon from Svecofennian magmatic rocks and rapakivi granites in Sweden
U.B. Andersson et al., Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden. Posted online 17 October 2011; doi: 10.1130/L162.1.
Recently published global compilations of isotopic data relating to the age and evolution of the Earth's crust reveal that the much of the continental crust is significantly older than previously understood, with more than 60% originating in the Archean, more than 2.5 billion years ago. Subsequent injection of mantle-derived magmas (plus melting, tectonic dissection, and sedimentary cannibalization of the older crust) have given rise to the current situation where most rocks of the upper crust are much younger. Using U-Pb age and Hf isotopic data from zircons, U.B. Andersson of the Swedish Museum of Natural History and colleagues focus on the 1.5- to 1.9-billion-year-old crust of central and southern Sweden as a natural laboratory to understand these processes. Their results indicate that the upper crust of the Bergslagen region of southern Sweden, dominated by magmatic rocks formed in a plate margin environment, has only a small component of Archean crust, having experienced multiple injections of fresh mantle magmatism between 2.2 and 1.9 billion years ago. In contrast, central Sweden has an upper crust dominated by metasediments cut by several generations of granitic rocks, implying a tectonic behavior as a rigid micro-continental block. Analyses of rapakivi intrusions comprising the youngest suite indicate a significant contribution from Archean sources, in the order of up to 40%, consistent with the presence of an Archean lower crust. Together this work indicates that Archean crust is most likely to be preserved within micro-continental blocks, rather than within adjacent active margins.
The spreading-rate dependence of anomalous skewness of Pacific plate magnetic anomaly 32: Revisited
E. Koivisto et al., University of Helsinki, Dept. of Geosciences and Geography, Institute of Seismology, P.O. Box 68 (Gustaf Hallstromin katu 2b), Helsinki, FI-00014, Finland. Posted online 17 October 2011; doi: 10.1130/L167.1.
E. Koivisto of the University of Helsinki and colleagues present an updated high-quality Pacific paleomagnetic skewness pole for chron 32. This updated paleomagnetic pole successfully corrects for the spreading-rate dependence of anomalous skewness, a correction that has not been applied to Pacific skewness poles before. Here, Koivisto and colleagues remedy this deficiency by explicitly correcting the skewness estimates for spreading-rate dependence of anomalous skewness by using predictions determined from a model originally formulated for data in the Arctic, Atlantic, and Indian oceans. In this study, the spreading-rate dependence of anomalous skewness in the Pacific is found to be consistent with that in other ocean basins and with the model for spreading-rate-dependent anomalous skewness. The presence of anomalous skewness is one of the main factors limiting the accuracy of paleomagnetic poles determined from the skewness data. Thus, successfully correcting for the anomalous skewness, as was done in this study, significantly improves the reliability of the skewness poles. The results also confirm the earlier assertions that the Hawaiian hotspot has shifted southward relative to the spin axis by 13 degrees since about 72 million years ago.
Azimuthal anisotropy in the Chile Ridge Subduction Region retrieved from ambient noise
A. Gallego et al., University of Florida, Geology, 241 Williamson Hall, Gainesville, FL 32611, USA. Posted online 8 November 2011; doi: 10.1130/L139.1.
A. Gallego of the University of Florida and colleagues analyze seismic ambient noise recorded in Northern Chilean Patagonia to calculate preferential fast velocity directions of surface waves. These waves tend to travel faster in directions that coincide with geologic structures and alignment of minerals. They found that these structures were formed due to stresses generated during the subduction of the Nazca and Antarctic tectonic plates and the collision of an active oceanic ridge beneath South America.
Paleoproterozoic Evolution of the Farmington Zone: Implications for Terrane Accretion in southwestern Laurentia
P. Mueller et al., Dept. of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. Posted online 8 November 2011; doi: 10.1130/L161.1.
Research presented by P. Mueller of the University of Florida and colleagues reveals the presence of a previously unrecognized age province in the southwestern United States that represents a time on Earth for which geologists have scant information. The tectonic and magmatic (volcanic) activity about 2.45 billion years ago is recorded in rocks in the Wasatch and Uinta Mountains of Utah and few other places in the world. Mueller and colleagues completed the analyses that confirmed this age using the sensitive-high resolution-ion microprobe (SHRIMP-RG) at the Stanford University-U.S. Geological Survey laboratory. Using this instrument to obtain U-Pb ages on about 30 micrometer diameter spots of the mineral zircon also reveals that all of these rocks were buried to great depth and metamorphosed about 1.67 billion years ago. This later event is thought to record the final addition of continental crust to the supercontinent Laurentia, which now makes up most of North America.
Late Miocene-Pliocene range growth in the interior of the northeastern Tibetan Plateau
W. Craddock et al., Dept. of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Posted online 8 November 2011; doi: 10.1130/L159.1.
The Tibetan Plateau occupies a land-surface area similar to the western United States, but on average, it stands about 17,000 feet above sea level (about 5,000 meters). As such, the Tibetan Plateau is the world's foremost site at which to investigate the processes that govern how mountains are created during continental collision. Given its size, the Tibetan Plateau influences atmospheric circulation in the northern hemisphere and helps shape its climate; it follows that increases in the height and surface area of Tibet in the past may have been linked to past environmental changes. Thus, in an effort to evaluate hypotheses about the linkages between the growth of the Tibetan Plateau and global climate, a community of earth scientists has worked to understand the timing and patterns of mountain building across this vast region. In this study, W. Craddock of Penn State and colleagues provide data from sedimentary deposits that provides relatively precise constraints on the timing of a mountain building associated with faults in the interior portion of the northeastern Tibetan plateau. From a regional perspective, a key finding of the study is that many of the large fault networks of the northeastern Tibetan plateau initiated at approximately the same time, between about 7 and 12 million years ago. Collectively, a growing body of research in northeastern Tibet suggests a significant expansion of high topography in the region at ca. 10 million years ago, and this event may be tied to a number of environmental changes.