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Geology: The Deeper the Rock the Colder? Lava Flow over Snow? Plus Other Conundrums
21 June 2013
Geological Society of America, The
New Geology articles posted online ahead of print 20 June 2013
These ten new Geology articles confront geologic conundrums and capture evidence toward answering even the most difficult questions on topics such as strain localization; atmospheric CO2; ultra-high pressure metamorphism; white chalk cliffs; lithospheric dripping; retreating trenches; microbial diversity beneath glaciers and ice-sheets; salt-marsh ecosystems; New Zealand glaciers -- biggest well before Europe's Little Ice Age; rock mechanics; tsunami hazards; and tracking the impact of the 2011 Tohoku-oki earthquake.
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Ductile strain rate measurements document long-term strain localization in the continental crust
Emmanuelle Boutonnet et al., Laboratoire de Géologie de Lyon, Université de Lyon, Université Lyon 1, ENS-Lyon, CNRS, 2 rue Raphaël Dubois, 69622 Villeurbanne, France; and Institute of Geosciences, Johannes Gutenberg University Mainz, J.-J.-Becher-Weg 21 D-55128 Mainz, Germany. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G33723.1.
Quantification of strain localization in the continental lithosphere is debated because of the lack of reliable deformation rate measurements in the deep crust. The Quartz-strain-rate-metry method, based on the combination of a flow law for dynamic recrystallization of quartz (by dislocation creep) and a piezometer calibrated for quartz, is a convenient tool for performing such measurements. Emmanuelle Boutonnet and colleagues tested and calibrated the method by identifying the best piezometer-rheological law pairs that yield a strain rate in agreement with that measured on the same outcrop by a more direct method taken as a reference. When applied to two major continental strike-slip shear zones, the Ailao Shan-Red River (southwest China) and the Karakorum (northwest India), they calculate across-strike strain rate variations, from less than 1 x 10-15 s-1 in zones where strain is weak, to greater than 1 x 10-13 s-1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates on the order of 1.1 cm yr-1 (Karakorum) and 4 cm yr-1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.
Intensified Southern Hemisphere Westerlies regulated atmospheric CO2 during the last deglaciation
C. Mayr et al., Institute of Geography, University of Erlangen-Nuremberg, Kochstrasse 4/4, 91054 Erlangen, Germany; and Earth and Environmental Sciences/GeoBio-Center, University of Munich, Richard-Wagner-Strasse 10, 80333 Munich, Germany. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34335.1.
The causes for the rise of the greenhouse-gas carbon dioxide in the atmosphere at the end of the last Glacial are a matter of controversial scientific debate. One explanation is the release of carbon dioxide from deep ocean reservoirs. C. Mayr and colleagues test the hypothesis of a stimulation of increased carbon dioxide release from the deeper ocean by intensification of the southern hemispheric Westerlies about 12,800 years ago. The authors investigated sediments from a Patagonian lake located in the present core region of the southern Westerlies. Sediment parameters demonstrate that drought conditions like today occurred at the same time when the greenhouse gas began to reach a level characteristic for the present warm period during pre-industrial times. This observation is interpreted as a southward shift and intensification of the Westerlies at the end of the last Glacial that evoked the end of the ice age by stimulating carbon dioxide release from the deep Southern Ocean. The study demonstrates that position and strength of the southern Westerlies are critical parameters for the global carbon cycle and thus for our climate.
Petrochronology of Himalayan ultrahigh-pressure eclogite
Dennis G. Donaldson et al., Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G33699.1.
This article addresses the timing of the India-Asia collision by investigating ultra-high pressure (UHP) rocks in the NW Himalaya. UHP rocks, if considered the leading edge of a subducting continental lithosphere, can be used to back calculate the timing of collision. The Himalaya records two UHP localities, separated upwards of 400 km along strike. These sites previously were interpreted to have experienced UHP metamorphism ~7 m.y. apart. However, new data presented in this article suggests that these UHP rocks experienced similar geochronology and metamorphism. This implies that the timing of the India-Asia collision took place synchronously in the NW Himalaya.
Shear heating not a cause of inverted metamorphism
Steven B. Kidder et al., Department of Geology, University of Otago, Dunedin 9054, New Zealand. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34289.1.
As miners are well aware, temperature increases with depth in the earth. A long-standing puzzle for geologists then has been understanding why, sometimes, we find rocks exposed at the surface that appear to have been colder at deeper levels. This feature is known as "inverted metamorphism." A long-standing possible explanation, based partly on a rock exposure in the San Gabriel Mountains near Los Angeles has been that rocks can be heated from above by heat generated on a fault. Steven B. Kidder and colleagues have reexamined this classic locality and created a computer model based on a modern understanding of the Cretaceous plate tectonic setting in which the rocks were formed. It is now known that the rocks were sequentially added from below along the fault in a process known as "accretion." Using a supercomputer, they tested thousands of models and were able to duplicate the temperature signature and ages of cooling of minerals from the area without any shear heating at all. They also found that heating on the fault actually reduces the magnitude of inverted metamorphism, and thus should no longer be considered a cause of this enigmatic metamorphic feature.
Seawater chemistry driven by supercontinent assembly, breakup, and dispersal
R.D. Müller et al., EarthByte Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34405.1.
After the Cambrian explosion of life, the mineralogy of marine cements, reef-builders, and organic sediment-producers has oscillated between aragonite and calcite seas. The Cretaceous calcite seas led to the formation of well-known chalk deposits spectacularly exposed as imposing white cliffs in many places, for instance along the south coast of England; however, the exact cause of these major changes in global ocean chemistry remains unknown. R.D. Müller and colleagues reconstruct a Jurassic to present-day history of oceanic heat flow and hydrothermal fluid flow of the global ocean basins based on reconstructions of now vanished ocean basins. They show that variations in hydrothermal flux calculated as a function of the changing age-area distribution of ocean floor after supercontinent break-up and dispersal correlate directly with Mg/Ca contents of marine carbonates over the last 200 million years. Cretaceous chalk formation coincided with rapid rates of ocean crust production and maximum area of young (less than 65 million year-old) ocean floor that drives hydrothermal flux. The model demonstrates that global tectonic processes impact not only on ocean chemistry and carbonate sedimentary cycles such as reef-building and chalk deposition, but may also contribute to the Earth's carbon cycle and ultimately to global climate.
Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes
M.N. Ducea et al., Universitatea Bucuresti, Facultatea de Geologie-Geofizica, Bucharest 010041, Romania; and University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34509.1.
Removal of lower parts of the lithosphere beneath mountain ranges is a fundamental mechanism responsible for the evolution of continents. However, the process has been difficult to fingerprint using geologic and geophysical tools. Little is known about the scales and speeds of detachment and dripping from direct observational evidence. In this paper, a group of geoscientists from the Universities of Bucharest, Arizona, and Toronto use a novel set of geochemical tools on young volcanic rocks from the Altiplano-Puna Plateau in central Andes, a classic area suspected of lithospheric dripping. Through a combination of geochemical parameters, thermometry, and geochronology, the authors demonstrate that lithospheric removal took place indeed under parts of the plateau, the size of drips is on the order of a few tens of kilometers and the time scales for dripping are short, on the order of one million years. The combination of techniques employed here on volcanic rocks can be used as a template for testing lower lithospheric dripping in other regions of the globe.
Slab rollback rate and trench curvature controlled by arc deformation
David Boutelier, School of Geosciences, Monash University, Clayton, VIC 3800, Australia; and Alexander Cruden. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34338.1.
In this study, David Boutelier and Alexander Cruden employ laboratory experiments to show that the shape of retreating trenches and rate of retreat are controlled by the rate of failure propagation in the magmatic arc. The retreating trench generates horizontal tension in adjacent segment of plate boundary which causes the failure propagation and eventually leads to an equilibrium between retreat and failure propagation rates which defines the shape of the created basin.
Insights on lava-ice/snow interactions from large-scale basaltic melt experiments
Benjamin R. Edwards et al., Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34305.1.
The 2010 eruption of Eyjafjallajokull in Iceland highlighted the importance of understanding what happens when volcanoes erupt through ice. In order to improve our understanding of these spectacular events, we have conducted large-scale experiments by pouring 100-300 kg (~220-660 pounds) of melted rock onto blocks of ice and snow (http://blogs.dickinson.edu/edwardsb/2012/08/29/lava-ice-snow-experiments-at-syracuse/). The preliminary results of this study are both expected (ice melts!) and somewhat unexpected (lava can travel on top of snow; melted water can bubble up through lava). Benjamin Edwards and colleagues have also tested the ability of layers of volcanic ash (sand in the experiments) to slow down the melting process. Results show that if ice or snow is covered by volcanic ash at the start of an eruption, this will significantly slow the rate of melting if lava flows later move over the ash-covered ice. These experiments show good agreement with observations from volcanic eruptions in Iceland and Sicily, including the ability of lava to find weaknesses in ice and a tunnel underneath ice. Once the lava has found its way to the base of the ice, it can continue to flow beneath the ice layers by exploiting pre-existing fractures.
Influence of bedrock mineral composition on microbial diversity in a subglacial environment
Andrew C. Mitchell et al., Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK; and Center for Biofilm Engineering, Montana State University, Bozeman, Montana 59717, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34194.1.
As a rule, life on Earth flourishes in the presence of sunlight and water. However, a growing body of evidence supports the presence of active ecosystems underneath glaciers and ice-sheets. Andrew C. Mitchell and colleagues investigate how bedrock minerals may strongly control microbial community structure under glaciers, likely through chemical energy derived from minerals in the bedrock. They demonstrate that iron and sulphar containing minerals play a key role in controlling the structure and abundance of subglacial microbial communities. Mineral-based energy may therefore serve a fundamental role in sustaining subglacial microbial populations and enabling their persistence during extended glacial-interglacial time scales, when ice masses covered between 30% and 100% of Earth's continental land surface.
How a marsh is built from the bottom up
John R. Gunnell et al., Department of Marine Sciences, University of North Carolina at Chapel Hill, 3202 Venable Hall, CB 3300, Chapel Hill, North Carolina 27599-3300, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34582.1.
Salt marshes are valuable landscapes that are rapidly disappearing. These ecosystems, which are often ancient, have been studied extensively, but their origins are only generally understood and not easily observed. At the Newport River in North Carolina, a marsh has been rapidly expanding for the past half-century due to land use changes. This presents a unique opportunity to examine the pattern of the marsh's growth in a more detailed way than was previously possible. The pattern of marsh growth was reconstructed using a combination of aerial photographs and 210Pb-based radioisotope dating of sediment cores taken from the marsh and adjacent bay. Each of the reconstructions from the sediment cores found a consistent trend: well before the colonization of marsh grasses, there was a rapid increase in sediment accumulation. This accelerated accumulation may be a positive feedback from the ongoing expansion of the marsh. As the marsh emerges, it shields adjacent waters from waves and currents, protecting recently deposited sediment from erosion and allowing it to rapidly accumulate. Ultimately, the marsh was built and maintained in a location that physical circumstances allowed. This insight about the proto-marsh landscape is unique and may affect how we consider marsh origins and wetland management.
The anatomy of long-term warming since 15 ka in New Zealand based on net glacier snowline rise
Michael R. Kaplan et al., Division of Geochemistry, Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34288.1.
An important goal for scientists who study glaciers is to learn whether past advances and retreats were synchronous across the globe. Michael R. Kaplan and colleagues used deposits left behind by glaciers in New Zealand, across the planet from the European Alps and North American Rockies, which are ideal for learning how climate has changed since the last Ice Age. In their study, Kaplan and colleagues show that the conventional wisdom on how climate has changed in the Northern Hemisphere is not entirely applicable in the Southern Hemisphere. They applied recent advances in dating to moraines left behind by glaciers in New Zealand. Since the Ice Age ended, in general, when New Zealand's glaciers were at their largest glaciers in Europe were at their smallest, and vice versa. Glaciers in New Zealand were at their largest well before Europe experienced its cold Little Ice Age of the 15th to 19th centuries. Kaplan and colleagues offer some possible explanations for opposite glacier behavior in New Zealand, including the far-field influence of tropical climates or how energy is received from the sun. Regardless of the specific causes, these results help provide a new foundation for understanding how glaciers and climate in New Zealand have evolved since the Ice Age.
Shear folding in low-grade metasedimentary rocks: Reverse shear along cleavage at a high angle to the maximum compressive stress
Patrick A. Meere et al., School of Biological, Earth & Environmental Science, University College Cork, Cork, Ireland. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34150.1.
Classic rock mechanics predict that faults develop at low angles (less 40 degrees) to the greatest principle stress which directly contradicts many field observations where faults are seen to form at very high angles (greater than 70 degrees) to the greatest principle stress. The field evidence of this "high angle" slip on faults requires them to be extremely weak. Faults are sites of repeated deformation and slip which can lead to significant mechanical weakening on a grain scale of the rocks in the immediate vicinity of the fault zone. This, coupled with the possibility of elevated fluid pressure within fault zones can lead to conditions amenable to "high angle" slip. This study presents an example of the systematic weakening of rocks, not along preexisting faults, but along tectonic cleavage foliation zones. Analysis of folded marine metasedimentary rocks from southern Ireland has provided unambiguous microstructural evidence for reverse shear on chemically weakened cleavage domains. This weakening, which is due to a tectonically induced diffusion anisotropy coupled with a reaction-diffusion chemical process, allows for systematic "high angle" slip across these foliation zones. This in turn facilitates the folding of the metasedimentary rocks containing the foliation by distributed shear along the sedimentary layers.
Tsunami recurrence revealed by Porites coral boulders in the southern Ryukyu Islands, Japan
Daisuke Araoka et al., Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan; and Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277-8564, Japan. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34415.1.
Information about past tsunami hazards, such as their recurrence interval and magnitude, is needed for future disaster prediction and mitigation. Daisuke Araoka and colleagues examine radiocarbon ages of the surfaces of massive coral boulders cast ashore by past tsunamis in the southern Ryukyu Islands, Japan, where few historical and geological records of past tsunamis are available. They selected only non-eroded Porites coral boulders along the shoreline, because their characteristics make it possible to determine the probable timing of their deposition by tsunamis, and applied a dating method that uses the cumulative probability distributions of large numbers of radiocarbon measurements of those boulders to determine the timing of past tsunamis. The results demonstrate that the southern Ryukyu Islands have repeatedly experienced tsunami events since at least 2400 years ago, with a recurrence interval of about 150-400 years. The largest Porites tsunami boulder that they studied (long axis, 9 m), which is probably the largest single-colony tsunami boulder in the world, was displaced by the A.D. 1771 Meiwa tsunami. Although the 1771 Meiwa tsunami was likely the largest event in at least the past 700 years, calculations of current velocity show that all identified tsunamis occurring before 1771 were probably large enough to cause considerable damage to human-built structures and loss of life. This study demonstrates that by reliably dating large numbers of selected coastal boulders it is possible to ascertain the timing, recurrence interval, and magnitude of past tsunamis in a location where few adequate survey sites of sandy tsunami deposits exist.
A slump in the trench: Tracking the impact of the 2011 Tohoku-Oki earthquake
M. Strasser et al., Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland. Posted online ahead of print on 20 June 2013; http://dx.doi.org/10.1130/G34477.1.
Large subduction zone earthquakes, such as the magnitude 9 earthquake that hit Japan in March 2011, trigger secondary effects, including tsunami and landslides. This impacts coastal communities and infrastructure, and affects the geological evolution of convergent plate boundaries, respectively. While such processes are, generically, generally well understood, the low reoccurrence of events so far hampered quantitative and holistic knowledge. The 2011 Tohoku-oki earthquake is the first big offshore earthquake, which was fully recorded by a series of measurement stations and for which data sets are available to characterizing the subduction system both before and after the earthquake. In this study, M. Strasser and colleagues present data from actual samples , collected in the deeper than 7 km deep trench, where the earthquake ruptured the seafloor. Their results ground-truth conceptual models derived from indirect geophysical and remote sensing campaigns and document a large earthquake-triggered slump in the trench. Strikingly, their data reveal that this slump resulted in a large (greater than 2 km) shift of the seafloor expression of the plate boundary. This documents that the evolution of the shallow plate boundary system is occurring, episodically, during a single-event during a period of seconds to minutes.