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Filling in gaps in the history of earth's magnetic field

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Geological Society of America

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IMAGE: This is a picture of paleomagnetic poles from paleomagnetic directions: Comparison of paleopoles from components A, B, and C from the Marcellus Formation with the apparent polar wander path for... view more

Credit: GSA Bulletin and D. Minguez et al.

Boulder, Colo., The Marcellus Shale is famous as a formation being explored for natural gas resources. That exploration has also offered some insight into an age-old problem: where was the continent of North America 400 million years ago?

In a research partnership between Lehigh University and The Pennsylvania State University, subterranean rock samples have been used to help fill a gap in the history of Earth's magnetic field, and, as a corollary, places some constraints on the position of North America when the Marcellus Formation was deposited.

Geologists use the magnetizations of rocks, acquired during their formation to determine the positions of continents relative to Earth's magnetic field. The magnetic record for the Devonian Period (ending roughly 360 million years ago) is rather patchy. This study shows that the Marcellus Formation (of Devonian age), and others like it may be able to fill in those gaps.

FEATURED ARTICLE

Paleomagnetism of the Oatka Creek Member of the Marcellus Formation: A Devonian paleopole for North America

D. Minguez et al., Department of Earth and Environmental Science, Lehigh University, 1 W. Packer Avenue, Bethlehem, Pennsylvania 18015, USA. This paper is online at http://dx.doi.org/10.1130/B31291.1.

Other GSA BULLETIN articles (see below) cover such topics as

1. Ediacaran large body and trace fossils

2. Ice sheet and landscape evolution in Antarctica

3. Magma storage at Lake City caldera, Colorado, USA

4. A hot and high Eocene Sierra Nevada, USA.

Oxygen, facies, and secular controls on the appearance of Cryogenian and Ediacaran body and trace fossils in the Mackenzie Mountains of northwestern Canada

E.A. Sperling et al., Harvard University, Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, Massachusetts 02138, USA. This article is online at http://dx.doi.org/10.1130/B31329.1.

Geochemical data do not record an oxygenation event associated with the appearance of large body fossils and trace fossils in Ediacaran rocks of Northwestern Canada. These organisms were periodically colonizing a predominantly anoxic and iron-rich basin. Thus, hypotheses that directly connect an Ediacaran rise in oxygen with the rise of large organisms need to be reconsidered.

An early to middle Miocene record of ice sheet and landscape evolution from the Friis Hills, Antarctica

Adam R. Lewis and Allan C. Ashworth, Department of Geosciences, North Dakota State University, Fargo, North Dakota 58108, USA. This paper is online at http://dx.doi.org/10.1130/B31319.1.

Adam Lewis and Allan Ashworth present a unique record of terrestrial glaciation and tundra communities that existed in Antarctica 20 million years ago. The record captures the transition from small alpine glaciers flowing toward the interior of the continent to a large ice sheet that inundated the Transantarctic Mountains and flowed toward the sea. During times of alpine glacier retreat, tundra vegetation consisting of shrubs, herbs, and mosses recolonized valley floors. Summer temperatures based on tundra plants were probably 20 degrees Celsius warmer than today. The study provides a challenge for ice sheet modelers because it shows that the first overriding of the Transantarctic Mountains did not take place until 10 to 15 million years after super-sized ice sheets reportedly covered the continent. The study also sheds light on the age of the long-controversial Sirius glacial deposits, which have to be at least 20 million years old.

A unique annually laminated maar lake sediment record shows orbital control of the Southern Hemisphere mid-latitudes across the Oligocene/Miocene boundary

B.R.S. Fox et al., School of Science, University of Waikato. This article is posted online at http://dx.doi.org/10.1130/B31349.1.

Earth has experienced episodes of climate warming in the past, but understanding these episodes requires a temporal resolution not generally available from the geological record. In this paper, Fox and colleagues present an annually resolved record from the South Island, New Zealand, deposited during the Mi-1 event, a period of rapid glaciation and subsequent deglaciation of Antarctica 23 million years ago. Their results indicate a rapid response of mid-latitude climate to Antarctic deglaciation through a strong ocean-atmosphere coupling in the Southern Hemisphere and influence from both low and high latitudes.

Magma storage, differentiation, and interaction at Lake City caldera, Colorado, USA

Ben Kennedy et al., Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, H3A 0E8, Canada. This paper is online at http://dx.doi.org/10.1130/B31305.1.

This paper by Ben Kennedy and colleagues shows how magma chambers are assembled suitable for a super-eruption. It details the specific arrangement of the magma chambers and how this affects the style of a subsequent super-eruption. Conversely, the paper is among the first to describe the dynamic feedback of the super-eruption on the magma that is left behind underground.

Provenance, age, and tectonic evolution of late Variscan flysch, southeastern France and northeastern Iberia, based on zircon geochronology

F.J. Martínez et al., Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA. This article is online at http://dx.doi.org/10.1130/B31316.1.

When continents collided 340-330 million years ago to make the vast supercontinent Pangea (the climax of the Variscan orogeny), Himalaya-scale mountains were created across Europe. The rise and fall of these mountains is recorded by the U-Pb ages of tiny sand grains of zircon in sediments eroded off of the mountains. These sediments are preserved in a series of small basins preserved across southeastern France and northwestern Spain. Unlike the Himalaya today, where similar sand can travel from the mountains far into the India Ocean along an interconnected network of rivers, the ages of hundreds of analyzed zircon grains show that the sediments eroded off the Variscan mountains did not travel far from their source areas. In the Variscan mountains, eroded rock and sediment fell into isolated basins. The mountains, far from any ocean, must have been high and dry, a bleak, little vegetated, but impressive landscape.

The oldest Paleo-Tethyan ophiolitic mélange in the Tibetan Plateau

Q.-g. Zhai et al., Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China. This article is posted online at http://dx.doi.org/10.1130/B31296.1.

The Paleo-Tethys Ocean was a Paleozoic ocean located between the Gondwana and Laurasia supercontinents. It is commonly considered to have opened in the early Paleozoic due to the rifting of the Hun superterrane from Gondwana. However, the detailed time of this rifting is still unclear. Recently, an Early Paleozoic ophiolitic mélange was documented in the north-central Tibetan Plateau. It is composed of serpentinite, isotropic and cumulate gabbros, basalt, and plagiogranite. Geochemical features, rock assemblages, and their field relationships suggest these rocks represent an ophiolite suite. Zircon U-Pb dating yields ages of 437 to 501 million years old. Therefore, Zhai Qing-guo and colleagues suggest that the Paleo-Tethys Ocean began to open in the Middle Cambrian; it continued to grow throughout the Paleozoic and closed in the Late Triassic. The Paleo-Tethys Ocean was formed by the breakup of the northern margin of Gondwana, with southward subduction of the Proto-Tethys oceanic lithosphere along the northern margin of the supercontinent.

A hot and high Eocene Sierra Nevada

H.T. Mix et al., Department of Environmental Studies and Sciences, Santa Clara University, Santa Clara, California 95053, USA.

This article is online at http://dx.doi.org/10.1130/B31294.1.

The topographic and climatic evolution of California's Sierra Nevada remains an issue of great importance, interest, and active discussion within the tectonics, earth-surface processes, terrestrial paleoclimate, and climate modeling communities. In addition, the general public holds the Sierra Nevada in special regard given its iconic landscape, historical importance, and natural resources. In this study, Hari T. Mix and colleagues reconstruct the temperature and elevation of the Early Eocene Sierra Nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral Yuba and American Rivers that drained the windward (Pacific) flank of the Sierra Nevada. Their findings support prior hydrogen isotope studies suggesting the northern Sierra stood as tall by the Eocene. Further, their paleo-elevation reconstructions support the development of a complex drainage network of deeply-incised paleovalleys and warm conditions in the Sierra during the Eocene greenhouse world.

Late Cenozoic evolution of high-gradient trough mouth fans and canyons on the glaciated continental margin offshore Troms, northern Norway -- Paleoclimatic implications and sediment yield

T.A. Rydningen et al., Department of Geology, University of Tromsø, The Arctic University of Norway, N-9037 Tromsø, Norway. This article is online at http://dx.doi.org/10.1130/B31302.1.

The spectacular scenery of Norway is to a large degree a result of the action of glaciers under climate conditions very different from today. Little is however known on the paleoenvironment of this period and the rate at which the glacial erosion occurred. This study investigates the late Cenozoic (~2.7 million years ago) evolution of the landscape affected by the northwestern sector of the Fennoscandian Ice Sheet through detailed seismo-stratigraphic studies of the continental margin deposits. In total, the sediments deposited along the margin correspond to the removal of about 100 m of sediments by glacial erosion, implying one order of magnitude variation in sediment yield along the Fennoscandian/Barents Sea ice sheet margins. This is related to the size and bedrock composition of the catchment areas and timing of ice growth and ice sheet dynamics.

Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada

R.J. D'Souza et al., School of Earth and Ocean Science, University of Victoria, Victoria, British Colombia V8W 3P6, Canada. This article is online at http://dx.doi.org/10.1130/B31289.1.

Vancouver Island, Canada, is the site of the Bonanza island arc, which was emplaced in the Jurassic period, between 203 and 164 Ma. Previous studies have tentatively correlated the Bonanza arc with the well-studied Talkeetna arc, Alaska, on the basis of their similarities in rock types, emplacement age, and field setting. Lower-crustal rocks exposed in the Talkeetna arc section contain garnet; however, no similar rocks are known from the Bonanza arc. Rameses D'Souza and colleagues have found geochemical signatures in the Bonanza arc rocks consistent with the presence of unexposed garnet-bearing rocks in the lower crust of the Bonanza arc. This finding is supported by the geochemical modelling they present and its implication of an at least 23 km thick, the minimum depth at which garnet can form in arcs, is consistent with field mapping constraints. Thus, this study presents a previously unknown correlation between the Talkeetna and Bonanza arcs.

An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater

N.J. Tosca et al., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. This article is online at http://dx.doi.org/10.1130/B31339.1.

Banded iron formation preserves critical information about early life and the evolution of Earth's surface before, during, and after the rise of atmospheric oxygen. However, exactly how these major iron ore deposits formed from ancient seawater is vigorously debated. To shed light on the genesis of iron formation, Nicholas J. Tosca and colleagues conducted experiments designed to mimic the chemistry of ancient seawater in the absence of oxygen. These experiments show that an enigmatic mineral long known to occur in iron formation (the mineral greenalite) crystallized rapidly from seawater when pH increased in response to the upwelling of deep, iron- and silica-rich water. Their experiments also show that this reaction would have played a major role in determining the iron content of ancient oceans. This helps unlock relationships between the iron, carbon, and silica cycles on the early Earth that set the environmental backdrop for early evolution of life.

Late Oligocene-Early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of southern Chile: Implications on the tectonic development of the North Patagonian Andes

Alfonso Encinas et al., Departamento de Ciencias de la Tierra, Universidad de Concepción, Casilla 160-C, Concepción, Chile. This paper is online at http://dx.doi.org/10.1130/B31303.1.

The Chilean margin has been used as the model of an ocean-continent convergent system dominated by compression and active mountain building. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata of the Traiguén Formation in southern Chile were deposited in a deep-marine extensional basin during the Late Oligocene-earliest Miocene. Geochemistry and petrography of its pillow basalts suggest that the unit accumulated on a thinned crust in a convergent margin rather than at an oceanic spreading center. These deposits constitute the only reliable records of submarine supra-subduction volcanism during the Cenozoic in southern South America.

A 2000-yr rupture history for the Alpine fault derived from Lake Ellery, South Island, New Zealand

Jamie D. Howarth et al., GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand. This paper is online at http://dx.doi.org/10.1130/B31300.1.

Scientists don't yet understand the full range of rupture behavior produced by transform plate boundary faults, even though we know they are capable of producing large to great earthquakes and are significant sources of seismic hazard for society and infrastructure. A major challenge is identifying the role that fault segmentation plays in controlling the size and location of earthquake ruptures. In this study we used lake sedimentary records to determine that segments of the Alpine Fault in New Zealand have ruptured independently of each other in the past, but have also ruptured in concert at other times. Significant advances in understanding the behavior of transform plate boundary faults may be gained by applying the same technique in similar settings world-wide.

Quantifying lava flow hazards in response to effusive eruption

Ciro Del Negro et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Italy. This paper is online at http://dx.doi.org/10.1130/B31364.1.

We present an emergent strategy able to provide an efficient response in case of volcanic unrest where satellite images and volcanological data are coupled with advanced flow simulation models for scenario forecasting and hazard assessment. The combined exploitation of various multispectral satellite data acquired in day and night time modes offers great promise to provide a high rate of information refresh and to permit a complete comprehension of volcanic feature evolution and chronology. Physics-based models driven, or validated, by satellite-derived parameters are now capable of fast and accurate forecast of lava flow inundation scenarios (hazard). Our results demonstrate that the integrated application of satellite observations and modelling may provide more accurate and reliable eruptive scenarios, and at last give a global perspective allowing timely predictions of lava flow hazards during effusive crises possible at the great majority of volcanoes that lack any kind of monitoring.

Late Cenozoic evolution of high-gradient trough mouth fans and canyons on the glaciated continental margin offshore Troms, northern Norway -- Paleoclimatic implications and sediment yield

T.A. Rydningen et al., Department of Geology, University of Tromsø, The Arctic University of Norway, N-9037 Tromsø, Norway. This paper is online at http://dx.doi.org/10.1130/B31302.1.

The spectacular scenery of Norway is to a large degree a result of the action of glaciers under climate conditions very different from today. Little is however known on the paleoenvironment of this period and the rate at which the glacial erosion occurred. This study investigates the late Cenozoic (~2.7 million years ago) evolution of the landscape affected by the northwestern sector of the Fennoscandian Ice Sheet through detailed seismo-stratigraphic studies of the continental margin deposits. In total, the sediments deposited along the margin correspond to the removal of about 100 m of sediments by glacial erosion, implying one order of magnitude variation in sediment yield along the Fennoscandian/Barents Sea ice sheet margins. This is related to the size and bedrock composition of the catchment areas and timing of ice growth and ice sheet dynamics.

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