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GEOLOGY
Isotopic dating of the migration of a low-grade metamorphic front during orogenesis
Birger Rasmussen, University of Western Australia, School of Earth and Geographical Sciences, Crawley, Perth, Western Australia 6009, Australia; et al. Pages 773-776.
The migration of fluids in the Earth's crust controls the distribution of mineral and petroleum resources. Tracing ancient fluid flow has proven notoriously difficult, but now, researchers using recently developed dating methods have mapped the former passage of hot, aqueous fluids over vast regions (>100,000 km2) of an ancient continent. Researchers estimate that fluids were driven outwards at ~5.0 millimeters per year by an advancing mountain chain that developed during continental collision ~2.2 billion years ago. The dating method is likely to be widely applicable and will provide important new insights into the history and evolution of the Earth's crust.
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Central Andean rotation pattern: Evidence from paleomagnetic rotations of an anomalous domain in the forearc of northern Chile
Graeme K. Taylor, University of Plymouth, School of Earth, Ocean and Environmental Sciences, Plymouth, Devon PL4 8AA, UK; et al. Pages 777-780.
The Andes are one of the world's great mountain chains, and the major bend (or orocline) in the chain in central South America is one of its most outstanding features. The way in which the rotation of the limbs of the bend occurred has been studied to detect the rotation of the crust since it formed. The overall pattern of rotations confirms that the bending is in response to crustal shortening during the past 20 million years. In northern Chile, however, Taylor et al. have found an anomalous zone that shows much greater rotations (up to 50 degrees in a clockwise sense) than predicted. They argue that this rotation pre-dates the creation of the bend and postulate that it relates to a period of markedly oblique convergence between the Nazca and South American tectonic plates between 55-40 million years ago. This, in turn, will help researchers' understanding of the tectonic evolution of this mineral-rich region of the world.
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Granite emplacement during tectonic exhumation: The Adirondack example
Bruce Selleck, Colgate University, Geology Department, Hamilton, NY 13346, USA; et al. Pages 781-784.
An ancient fault zone forms the boundary between the Adirondack Highlands and Adirondack Lowlands of New York State. Since the fault extended to great depth, the rocks within the fault zone were plastically deformed like warm taffy rather than being crushed into fine powder. Molten granite magma intruded the fault zone at the same time the rocks within the zone were deformed at depths of 25-35 km. Uranium-lead dating of zircon crystals from the granite demonstrate that the fault was active around 1.045 billion years ago. Rocks on the northwest side of the fault, which form the modern Adirondack Lowlands, were moved downward over more deeply buried rocks of the Highlands. The intrusion of granite during motion along deep fault zones occurs in modern mountain belts like the Himalayas and Andes.
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Amplitude and timing of sea-surface temperature change in the northern South China Sea: Dynamic link to the East Asian monsoon
Delia W. Oppo, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA 02543-1541, USA; and Youbin Sun, University of Tokyo, Department of Earth and Planetary Science, Tokyo, Japan. Pages 785-788.
Oppo and Sun present 145,000-year-long proxy records for surface temperature and salinity changes from a sediment core from the northern South China Sea (SCS). Abrupt temperature changes during the past 15,000 years appear synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern SCS surface temperature. Oppo and Sun suggest that this linkage persisted for the previous 130,000 years, when orbital-scale 2–3 °C temperature changes and several small abrupt events occurred in the northern SCS. The results provide evidence of a redistribution of moisture from the East Asian landmass when the summer monsoon is strong, to the western Pacific during times of weaker summer monsoon, during this time interval. Finally, the results weigh in on the controversy related to the timing of the penultimate deglaciation. If northern SCS warming during penultimate deglaciation was also synchronous with the well-dated change in East Asian summer monsoon rainfall, the suggested timing of a proxy for ice volume change, measured in the same sediment core, indicates that ice sheet demise during penultimate deglaciation did not precede Northern Hemisphere summer insolation increase, as other investigators have argued.
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Orbitally forced Lofer cycles in the Dachstein Limestone of the Julian Alps (northeastern Italy)
Andrea Cozzi, Eidgenössische Technische Hochschule Zürich, Institute of Geology, Department of Earth Sciences, Zürich 8092, Switzerland; et al. Pages 789-792.
The long-debated origin of the m-scale peritidal carbonate cycles of the Dachstein Limestone Formation are investigated using time-series analysis of gray-scale pixel scans of field photographs and detailed sedimentological observations. Cozzi et al. show that the depositional pattern of the Lofer cyclothems have a distinctive, periodic pattern, with time-frequency characteristics that are specific to orbital forcing from the Precession index. While such a forcing has been suspected for nearly half a century, no one has been able to detect a clear signal to confirm it; this has led to a protracted debate on the exact nature of the origin of the cyclothems. In this respect, Cozzi et al. report a novel result that contributes significantly to the Lofer debate.
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Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield
J. Wilkinson, Imperial College London, Department of Earth Science and Engineering, South Kensington Campus, London SW7 2AZ, UK; et al. Pages 805-808.
Wilkinson et al. report how their analyses of microscopic droplets of 350-million-year-old hydrothermal solutions trapped inside quartz crystals provide new insights into the formation of zinc and lead ore deposits. They studied samples from ores that were precipitated in limestones laid beneath the tropical seas covering Ireland in the Carboniferous period. Their results show that the solutions that transported and deposited the metals were derived from evaporated seawater that had circulated to depths of at least 5 km into the Earth's crust via faults and fractures in the rock.
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Variability of Southwest Indian summer monsoon precipitation during the Bølling-Ållerød
Kevin Cannariato, University of Southern California, Earth Sciences, Los Angeles, CA 90089, USA; et al. Pages 813-816.
Cannariato et al. shed light on how Indian Monsoon rainfall varied over a 4.5-thousand-year interval when the Earth was emerging from the last ice age (~15,000 years ago). The precipitation reconstruction indicates that the strength of the Indian Monsoon varied over hundreds to thousands of years synchronously with variations of the East Asian Monsoon. Because a portion of the stalagmite grew very rapidly, the scientists have been able to document Indian Monsoon rainfall variations over this interval at unprecedented temporal resolution (<2.5 years) for the deglaciation. These high-resolution results indicate that multi-decadal rainfall variability was, in fact, an important aspect of Indian Monsoon behavior in the past. The similarity of the frequencies of multi-decadal rainfall variability during the deglaciation to those of climate variability in the recent past suggests that similar climate dynamics may have been responsible even though the boundary conditions of these two time intervals were very different.
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Mercury isotope fractionation in fossil hydrothermal systems
Christopher Smith, University of Michigan, Geological Sciences, Ann Arbor, MI 48109-1064, USA; et al. Pages 825-828.
Mercury is a volatile, toxic element that is highly mobile within Earth reservoirs. Smith et al. present the first data on the fractionation of stable isotopes of mercury in fossil hydrothermal systems. Significant fractionation is demonstrated within individual systems indicating that low-temperature geochemical processes occurring in the near surface environment can affect mercury isotopic compositions. Indeed, the range of fractionation recorded at the sites studied is comparable to that of much lighter metal isotope systems, presenting the potential for mercury isotopes to become a useful tool for exploring the cycling of this element in the Earth.
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Grain growth control of isotope exchange between rocks and fluids
Michihiko Nakamura, Graduate School of Science, Tohoku University, Institute of Mineralogy, Petrology and Economic Geology, Aoba, Sendai, Miyagi 980-8578, Japan; et al. Pages 829-832.
Minerals in rocks and meteorites contain much useful information--such as flow of groundwater and ascent of magma--about Earth and other planets. Nakamura et al. propose a new mechanism by which this information is recorded in the minerals. Such topics are very basic and instrumental in understanding the genesis and evolution of Earth. Even though the mechanism investigated in the current study is found to be more efficient than the traditional ones, it has been overlooked for a long time.
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(226Ra)/(230Th) excess generated in the lower crust: Implications for magma transport and storage time scales
Josef Dufek and Kari Cooper, University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195. USA. Pages 833-836.
Excesses of radium over its parent isotope of thorium in arc lavas have previously been interpreted to reflect deep mantle processes; this, in turn, implies very rapid (1000 m/yr) ascent rates of melt from the source region. Previous investigations suggested that lower-crustal melting produced insignificant Ra-excesses. Dufek and Cooper demonstrate that, when the timescale of melting is explicitly incorporated into the model, incongruent melting of amphibolite in the lower continental crust can produce significant Ra-excesses and therefore reduce inferred vertical transport rates.
GSA TODAY Science Article
Geologic processes in sedimentary basins inferred from three-dimensional seismic imaging
Richard J. Davies, 3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, UK, and Henry W. Posamentier, Anadarko Canada Corporation, Calgary, Alberta T2P4V4, Canada. Pages 4–9.
New views of inner Earth: The same technology that the medical industry uses to gain views of the human body (CAT scans) is used by the geoscience community to gain new insight and understanding of Earth. Termed "three dimensional seismic reflection profiling" and widely used by the petroleum industry, the technology uses sound waves to produce images of Earth's interior, both on land and at sea. In the October issue of GSA Today, Richard Davies and Henry Posamentier present spectacular images derived from formerly proprietary data sets that show striking examples of deep sea channels in the Gulf of Mexico, of how igneous intrusions are emplaced in the North Sea, and of how natural gas moves in the subsurface. This is just a glimpse of the high tech world the petroleum industry has at its fingertips, offering new advances both in petroleum exploration and in understanding how Earth works.
To view the complete table of contents for the September issue of GEOLOGY, go to
http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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