Boulder, Colo., USA - Most plant fossils are isolated organs, making it difficult to reconstruct the type of plant life or its ecosystem structure. In their study for GEOLOGY, published online on 28 Aug. 2014, researchers Camilla Crifò and colleagues used leaf vein density, a trait visible on leaf compression fossils, to document the occurrence of stratified forests with a canopy dominated by flowering plants.
Using a 40-meter-tall canopy crane equipped with a gondola, they were able to collect leaves from the very top of trees in Panama and the United States. They measured leaf vein density in 132 species from two Panamanian tropical forests and one temperate forest in Maryland (USA). The team also compared the leaf vein values of canopy-top and forest-bottom leaves (i.e., leaf litter on the forest floor).
The authors show that venation density, like plant metabolism (i.e., transpiration and photosynthesis), is higher in the leaves located in the forest canopy and decreases in leaves at lower levels. Furthermore, they found that leaves from the forest floor, which are the closest analog to fossil floras, preserve this pattern.
The team also reanalyzed vein density data from the literature from the Early Cretaceous (132.5 million years ago) to the Paleocene (58 million years ago) to determine when flowering plants became part of the upper forest canopy. Vein density values similar to present ones appeared about 58 million years ago, indicating that the emergence of flowering plants in the canopy occurred by the Paleocene.
Variations in angiosperm leaf vein density have implications for interpreting life form in the fossil record
Camilla Crifò et al., Dept. of Geology and Environmental Earth Science, Miami University, 114 Shideler Hall, Oxford, Ohio 45056, USA, and Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Ancón, Republic of Panamá. Current address: Dept. of Biology, University of Washington, Box 351800, Seattle, Washington 98195, USA. Published online on 28 Aug. 2014; http://dx.
Other recently posted GEOLOGY articles (see below) cover such topics as
- 1. A difference of 5 mm per year between estimates and direct observation of motion between the Pacific and North America plates;
2. Seismic hazard in the Sichuan Basin, China; and
3. Wildfire chars versus coal combustion and the Permian-Triassic boundary extinction event.
GEOLOGY articles published online ahead of print can be accessed online at http://geology.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.
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Pacific plate deformation from horizontal thermal contraction
Corné Kreemer, Nevada Bureau of Mines and Geology, and Seismological Laboratory, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557-0178, USA; and Richard G. Gordon, Dept. of Earth Science, Rice University, MS 126, 6100 Main Street, Houston, Texas 77005, USA. Published online 15 Aug. 2014; http://dx.
Plate tectonics is one of the few universal theories most people have learned about in school. In this study, Corné Kreemer and Richard Gordon challenge here a fundamental part of this theory: that the plates are rigid. Plate rigidity is plate tectonics' central approximation and gives the theory its rigor and predictive power. This study predicts and quantifies significant deformation within the young parts of the Pacific plate by assuming that the plate must contract horizontally due to the cooling of the plate as it moves away from its adjacent spreading centers. Thus, one cannot assume that parts of the plate near the Pacific-Antarctic spreading center move rigidly with parts off the coast of California. Estimates of the relative motion between the Pacific and North America plates from seafloor data in the south Pacific and Atlantic oceans differs by 5 mm per year from direct observations, and this study can explain about half of this difference (the remainder may come from deformation of the oceanic parts of other plates). Kreemer and Gordon find that the implied deformation in the Pacific plate has the same spatial distribution as the locations of earthquakes, which suggests that thermal contraction is (at least, partly) released seismically and improves our understanding of the cause of those enigmatic earthquakes.
The 2013 Lushan earthquake: Implications for seismic hazards posed by the range front blind thrust in the Sichuan Basin, China
Maomao Wang et al., Dept. of Earth Sciences, Nanjing University, Nanjing 210093, China; and Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA; and Dept. of Geophysics, Kyoto University, Kyoto 606-8502, Japan. Published online on 28 Aug. 2014; http://dx.
A team of scientists from Nanjing University (China), Harvard University (USA), Kyoto University (Japan), Nanyang Technological University (Singapore), and China Earthquake Administration (China) report they have discovered an active blind-thrust fault beneath the populated western Sichuan basin in China. They identified this fault, called the Range Front blind thrust, from seismic reflection profiles, petroleum wells, and relocated earthquakes. Their study shows that the 2013 Mw 6.6 Lushan, China, earthquake ruptured a small segment of the fault on this fault. Their study suggests that unruptured parts of the fault may pose significant seismic hazards to the population centers in the Chengdu Plain. They found the 2008 Wenchuan and 2013 Lushan earthquakes, along with results from this new study, demonstrate that the Longmen Shan and western Sichuan basin are underlain by an active, imbricate thrust system. This suggests that there are several potential sources of large earthquakes in the region, and presents the possibility of multi-segment earthquakes that may involve two or more of these faults. Because of these fault are close to large cities, and the prospect for basin amplification of seismic waves, these newly discovered faults represent significant hazards.
Latest Permian chars may derive from wildfires, not coal combustion
Victoria A. Hudspith et al., Earth System Science Group, Dept. of Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QE, UK. Published online on 28 Aug. 2014; http://dx.
The Permian-Triassic boundary extinction event was one of the greatest mass extinctions on Earth. A substantial body of evidence suggests that massive volcanic eruptions in Siberia (Russia) played a significant role in the extinction. One of the greenhouse contributors is thought to have been extensive CH4 release from the combustion of coals and organic-rich shales, during the emplacement of shallow intrusions, as part of the Siberian Trap eruptions. Observations of organic matter interpreted to be coal combustion products (fly ash) in latest Permian marine sediments have been used to support this hypothesis. However, this interpretation is dependent upon this fly ash forming from coal combustion and not by alternative mechanisms. Hudspith et al. studied the appearance of wildfire-derived products (char) from late Permian Russian coals as well as char from modern tundra, peatland, and boreal forest fires, using reflected-light microscopy, to demonstrate that wildfires can produce charred organic matter that is morphologically comparable to end-Permian fly ash. These observations, coupled with extensive global evidence of wildfires during this time interval, call into question the contribution of coal combustion to the end-Permian extinction event.
Microanalyses link sulfur from large igneous provinces and Mesozoic mass extinctions
Sara Callegaro et al., Dept. of Geosciences, University of Padua, via Gradenigo 6, 35131 Padua, Italy. Published online on 28 Aug. 2014; http://dx.
Climate change is nowadays considered to be chiefly a responsibility of mankind driven pollution, but also natural phenomena such as volcanic eruptions can affect the environment through emission of poisonous or polluting gases. The history of our Earth has seen episodes called mass extinctions -- such as the famous extinction of dinosaurs -- in which life has been threatened to be totally or partially wiped out from our planet. Some scientists seek for the cause of these events in ancient volcanic eruptions, others in meteorite impacts. Sulfur -- what makes volcanic fumes smell of rotten eggs -- has been measured in crystals from ancient volcanic rocks with a new method. Volcanic rocks from three areas of the world (India, South America, and the coasts of the Atlantic Ocean) dating back to 65, 135, and 200 millions of years, were analyzed. It turns out that magmas coeval with mass extinctions (65 and 200 millions of years) contained twice the sulfur as the other, harmless magmas. This could pollute the atmosphere, causing climate change and extinctions. More investigations and measurements will be needed to confirm this hypothesis, but this is another important brick in the wall of the understanding of Earth's history and climate.
Periplatform drift: The combined result of contour current and off-bank transport along carbonate platforms
Christian Betzler et al., Institut für Geologie, Universität Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. Published online 15 Aug. 2014; http://dx.
Hydroacoustic and sedimentological data of the western leeward flank of Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulating a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward thinning wedge of the periplatform drift at the foot of the bank escarpment displays along and down-slope variations in sedimentary architecture. Sediments are muddy carbonate sands, coarsening basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth related facies belts develop, whereas strike-discontinuous sediment lobes, scarps and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration.
Global continental weathering trends across the Early Permian glacial to postglacial transition: Correlating high- and low paleolatitude sedimentary records
Jianghai Yang et al., State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China. Published online 15 Aug. 2014; http://dx.
Fine-grained sediments/sedimentary rocks provide informative paleoclimatic archive. These climatic information can be read upon weathering geochemistry studies. A well-dated Lower Permian sequence from low latitude North China is correlated with the high latitude Gondwana sedimentary successions. Correlation suggests that during the Early Permian glacial to postglacial transition continental weathering was enhanced globally. This enhancement in continental weathering coincides with significant elevation in atmospheric CO2 contents. In the early Permian glacial interval, latitudinal distribution of chemical weathering degrees of muddy sediments/sedimentary rocks reflects the land surface temperature gradient. This study throws new light on using sedimentary geochemistry to reconstruct paleoclimate evolution.
Holocene Indian Ocean tsunami history in Sri Lanka
Kelly L. Jackson et al., Dept. of Marine Geosciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA. Published online on 28 Aug. 2014; http://dx.
Karagan Lagoon, southeast Sri Lanka, contains a 7,000-year record of Indian Ocean tsunamis. Sediment cores retrieved tsunami deposits from the 2004 Indian Ocean tsunami and seven older paleo-tsunami deposits. Assuming that these tsunamis were generated by giant earthquakes along the Sumatra-Andaman subduction zone, this record extends the giant-earthquake history for the Indian Ocean region. Radiocarbon dating indicates that one paleotsunami occurred between 2417 plus or minus 152 calendar years before present (cal. yr B.P.) to 2925 plus or minus 98 cal. yr B.P., and six between 4064 plus or minus 128 cal. yr B.P. and 6665 plus or minus 110 cal. yr B.P. The recurrence interval is variable, ranging from 181-517 yr to 1045 plus or minus 334 yr, with an average recurrence interval of 434 plus or minus 40 yr during the approx. 4000 to 7000 cal. yr B.P. continuous interval. The sediment record documents that Sri Lanka and much of the Indian Ocean basin is affected by large tsunamis at non-uniform intervals, from a few hundred years to over a thousand years, which could be as large or even larger than the 2004 tsunami.
An Indian Ocean tsunami triggered remotely by an onshore earthquake in Balochistan, Pakistan
Gösta Hoffmann et al., Dept. of Applied Geosciences, German University of Technology, PO Box 1816, PC 130 Muscat, Sultanate of Oman, and Institute of Neotectonics and Natural Hazards, RWTH Aachen University, Lochnerstrasse 4-20, 52056 Aachen, Germany. Published online 15 Aug. 2014; http://dx.
Hoffmann and colleagues present measurements of tsunami waves observed on 24 September 2013 within the Arabian Sea (Northern Indian Ocean). Their results indicate that the waves were in the range of 1 m or less in height. They were first recorded along the eastern coast of Oman and propagated toward the west. The authors suggest that the waves must have been triggered by a submarine slide as a secondary effect of a 7.7 magnitude earthquake that struck southern Pakistan. Cascading effects resulting in large submarine slope failures within the Arabian Sea are more frequent than postulated. The authors write that hazard potential along the shorelines is underestimated, because such secondary effects of distant ruptures onshore were previously not taken into account.
Sub-decadal turbidite frequency during the early Holocene: Eel Fan, offshore northern California
Charles K. Paull et al., Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA. Published online 15 Aug. 2014; http://dx.
From the abstract: Remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an ~100-m-thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on Eel submarine fan, offshore northern California (USA). These outcrops provide a rare opportunity to connect young deep-sea turbidites with their feeder system. Carbon-14 measurements reveal that from 12.8 to 7.9 thousand years ago, one turbidite was being emplaced on average every seven years. This emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the Pacific margin of North America. The turbidites contain abundant wood and shallow-dwelling foraminifera, demonstrating an efficient connection between the Eel River source and the Eel Fan sink. Turbidite recurrence intervals diminish fivefold to about 36 years from 7.9 thousand years ago onward, reflecting sea-level rise and re-routing of Eel River sediments.
Transient explosions at open-vent volcanoes: The case of Stromboli (Italy)
L. Pioli et al., Section des Sciences de la Terre et de l'Environnement, Université de Genève, Rue des Maîrachers 13, 1205 Geneva, Switzerland. Published online 15 Aug. 2014; http://dx.
From the abstract: Stromboli is a persistently active, open-vent basaltic volcano whose activity is controlled by the balance between magma supply, outgassing, and eruptive rates, and is characterized by low-intensity, regular Strombolian explosions. However, two types of large, transient, violent explosive eruptions suddenly occur with no clear precursory activity. These explosions, called "major" and "paroxysmal" depending on size, cover a large variability in intensity and magnitude, but are all marked by short duration. Paroxysms have significantly larger intensities (greater than 106 kg/s) than major explosions (104 kg/s) and fundamental differences in the characteristics (composition, crystallinity, vesicularity) of the erupted tephra, suggesting that different sources feed these two eruption types. Paroxysms are generated by the explosive fragmentation of low porphyricity (LP) magma mingled with high-porphyricity resident magma in the shallow reservoir, whereas major eruptions are likely associated with destabilization of the lower portion of the shallow magmatic system, continuously hybridized by the arrival of LP magma. In general, the intensity of these explosions is related to the amount of the LP magma erupted (greater 107 kg in paroxysms and 104 to 105 kg in major explosions), suggesting that the magma plays a major role in the fragmentation mechanism. Despite its primary importance in the hazards of Stromboli, the total amount of magma erupted in these events in the past 10 years is less than 1% of the total mass erupted by the volcano.
Permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity
Satoshi Okumura and Osamu Sasaki, Division of Earth and Planetary Materials Science, Dept. of Earth Science, Graduate School of Science, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan, and Division of GeoEnvironmental Science, Dept. of Earth Science, Graduate School of Science, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan. Published online 15 Aug. 2014; http://dx.
Repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit. To trap the volcanic gases in the conduit, the lava dome or plug must be impermeable. However, silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization. Since fractured magma has high permeability, the reduction of permeability is a key process that allows gases to accumulate, leading to an explosion. Satoshi Okumura and Osamu Sasaki performed uniaxial compression experiments on rhyolite fragments at temperatures of 700 to 900 degrees Celsius to investigate the permeability reduction of fractured rhyolite. Their experimental results show that permeability is controlled by porosity, and permeability reduction can be caused by compaction of fractured magma. Based on the observed relationship between permeability and porosity, Okumura and Sasaki calculate the time scale of permeability reduction, i.e., the transition from permeable to impermeable magma. They conclude that their modeling and comparison with eruption cyclicity indicate that permeability reduction and gas accumulation may explain eruption cyclicity at some silicic volcanoes, but other mechanisms can also contribute to control cyclicity.
How waves shape salt marshes
Nicoletta Leonardi and Sergio Fagherazzi, Dept. of Earth and Environment, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA. Published online on 28 Aug. 2014; http://dx.
Salt marshes are eroding at an alarming rate and we risk losing their valuable ecosystem services (for example, they mitigate the impact of hurricanes and provide the habitat for a variety of animal species). Several years of high-resolution field measurements at five sites along the U.S. Atlantic Coast and numerical simulations have been used to investigate marshes erosion by wind waves. When wind waves are small, the prediction of failure events could be impossible because the erosion rate is independent from wave energy, and very big erosion events can happen despite of the reduced wave exposure. This happens because local marsh properties affect the global erosion rate. This can, for example, be connected to the fact that there are areas characterized by crab burrowing, other areas characterized by denser vegetation, different soil properties, and so on. On the contrary, when waves are very high, the erosion rate has a well-defined mean intensity value that depends on the wave energy magnitude. Finally, when salt marshes are exposed to high wave energy conditions, their boundaries erode uniformly, yielding a relatively smooth shoreline. On the other hand, when wind waves are weak, jagged marsh boundaries form.
Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface
Tahar Hammouda et al., Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France, and Laboratoire Magmas et Volcans, CNRS, UMR 6524, F-63038 Clermont-Ferrand, France; and Institut de Recherche pour le Développement, R 163, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France. Published online on 28 Aug. 2014; http://dx.
Carbonatites are very unusual magmatic rocks that were once molten carbonates. The oldest ones are more than 2.5 billion years old. Today, there is only one active carbonatitic volcano, Oldoinyo Lengai, in the East African Rift region of Tanzania. Carbonatites carry information about the deep carbon cycle, and studying them helps us understand carbon fluxes between Earth's surface (oceans, biosphere, sedimentary rocks) and its deep interior. The eruption of calcium-rich carbonatites at Earth's surface is at odds with them being equilibrated with the mantle at depth because previous studies found significant magnesium contents. By combining high-pressure technology and synchrotron radiation, we have monitored the decompression of molten carbonates mixed with mantle material. We obtained calcic melts with compositions similar to those of most carbonatites. We demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, therefore without catastrophic CO2 release. We show that high temperature tends to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, but here, we show that emplacement of carbonatites at or near the surface necessitates a hot environment, such as rift zones.
Time-scale uncertainty of abrupt events in the geologic record arising from unsteady sedimentation
David B. Kemp and Philip F. Sexton, Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Published online on 28 Aug. 2014; http://dx.
Defining the timescales of abrupt climate change events in the geologic record is a vital prerequisite for accurately assessing the severity and rates of ancient climatic processes, and for appropriate comparison with recent environmental change. Sedimentary rocks typically provide the only archive of these ancient events, with sediments laid down in ancient oceans preserving chemical and physical fingerprints of climate change. Dating the rock layers to determine the duration of short-lived (less than or equal to 1000 years) events is problematic, however, because rocks can usually not be dated at such high resolution. Instead, paleoclimatologists can make estimates of the duration of abrupt events by estimating rates of sedimentation using lower resolution dating on longer, multi-millennial timespans. David Kemp and Philip Sexton of the Open University (UK) argue that this approach is flawed because sedimentation rates are not constant. By modeling sedimentation rate variability typical of ancient marine environments, they show that the actual duration of millennial-scale events can be longer than estimated by well over 100%, and shorter than estimated by up to 60%. These findings thus have implications for accurately quantifying the timescales and rates of ancient climate change and other Earth system processes.
Rapid shifts in subarctic Pacific climate between 138 and 70 ka
Lars Max et al., Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Am Alten Hafen 26, D-27568 Bremerhaven, Germany. Published online on 28 Aug. 2014; http://dx.
During the past decades, instrumental observations document the sensitivity of the Arctic and subarctic regions to changes in global climate. However, without adequate knowledge about long-term natural climate variability far beyond instrumental observations it is quite delicate to understand forcing and climate feedback mechanisms in present and future climate. In this study, we present proxy reconstructions based on marine organic biomarkers to study past variability in sea surface temperature and sea-ice from a sediment record recovered in the subarctic Pacific realm. Here, we focused on the time interval between 138,000 to 70,000 years ago. This geological interval comprises the last interglacial (Eemian), which is our nearest analogue to modern climate conditions. We found that subarctic Pacific sea surface temperatures were about two degrees Celsius warmer during the early Eemian (128,000 to 126,000 years ago) compared to today. However, subarctic Pacific climate evolution was far from stable and marked by rapid oscillations in sea surface temperatures and sea-ice on millennial timescales. These rapid climate transitions are nearly synchronous to short-term temperature oscillations well known from Greenland and the North Atlantic. Our results point to a rapid circumpolar climate coupling in the Northern Hemisphere and highlight the role of oceanic-atmospheric feedback mechanisms in past climate.
Fingerprints of late Neoproterozoic ridge subduction in the Pan-African Damara belt, Namibia
Francesca Meneghini et al., Dept. of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, Stellenbosch, South Africa; and Departamento de Geologia, Campus do Morro do Cruzeiro, Universidade Federal de Ouro Preto, Ouro Preto MG 35400-000, Brazil. Published online on 28 Aug. 2014; http://dx.
In plate tectonics the production of new lithosphere between two diverging plates (i.e., along a ridge), is counteracted by its consumption through subduction (the under-thrusting of one plate below another). An inevitable consequence of the coexistence of these processes is the encounter of a ridge with a trench, i.e. the area where consumption starts. During ridge subduction, the diverging plates continue moving apart at depth, causing the opening of the so-called slab window. Hallmark of this process is the formation of granitic bodies whose features are unusual in subduction settings. The ridge subduction process is seen in actual convergent margin and described in Meso-Cenozoic complexes (i.e., back to 250 million years ago), but rarely documented in older margins. Our paper report exceptionally well-preserved field evidence of ridge subduction in the Damara Belt of Namibia (Africa, 580 to 500 million years ago), in the form of primary intrusive interaction between rocks produced along ridges (basalts and gabbros) with sediments deposited along trench. Based on this physical evidence and on new dating of granitic rocks emplaced during subduction, we propose a new evolution model for the Damara Belt based on ridge subduction that allows explaining some peculiar characteristics that have always considered at odds with subduction."
Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India
Joydip Mukhopadhyay et al., Dept. of Geology, Presidency University, Kolkata 700073, India, Corresponding Author: Quentin G. Crowley, Dept. of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland. Published online on 28 Aug. 2014; http://dx.
The widely accepted model for evolution of the atmosphere states that oxygen levels did not appreciably rise until about 2.4 billion years ago. This Great Oxidation Event (GOE) enriched the atmosphere and oceans with oxygen and heralded one of the biggest shifts in evolutionary history. Up until very recently however, it has been unclear if any oxygenation events occurred prior to the GOE and the argument for an evolutionary capability of photosynthesis has largely been based on the first signs of an oxygen build-up. This paper presents chemical evidence for the oldest known example of oxidative weathering from a terrestrial environment. This is remarkable in that the studied ancient soil horizon (termed a paleosol) is at least 3.02 billion years old and predates the GOE by some 600 million years. Clastic sediments immediately overlying the paleosol were deposited in a shallow water environment and contain minerals which are not stable in the presence of elevated oxygen. It is likely therefore that the oxygenation event recorded by the paleosol was short-lived. Importantly, such oxygen levels could only have been produced by micro-organisms capable of oxygenic photosynthesis, which is some 60 million years earlier than previously recognized from the Archean rock record.
A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States -- A relic of craton formation
Erin A. Wirth and Maureen D. Long, Dept. of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. Published online 15 Aug. 2014; http://dx.
Earth's continents have played a crucial role in controlling the style of plate tectonics and the evolution of life; however, our understanding of how continents form and why they remain stable over billions of years remains incomplete. A recent discovery is the presence of the mid-lithospheric discontinuity, a seemingly sharp decrease in seismic velocity at depths internal to the lithosphere that appears to be a pervasive feature beneath continents. The origin of this unexpected feature is puzzling, but its presence within cratons (the oldest part of continents that have remained stable for long periods of geologic time) suggests that the mid-lithospheric discontinuity may result from processes associated with continent formation. Erin A. Wirth and Maureen D. Long use seismic techniques to study the properties and origin of the mid-lithospheric discontinuity beneath the central United States to gain insight into the past tectonic processes that formed the easternmost North American craton. They find multiple mid-lithospheric discontinuities that correspond to sharp changes in anisotropy, the directional dependence of seismic wavespeeds, with depth. Wirth and Long note that their observations of multiple sharp gradients in anisotropy internal to the lithosphere supports models for craton formation via stacked subducted slabs or a series of underthrusting events.
Dynamic ups and downs of the Himalaya
Laurent Husson et al., Université de Grenoble Alpes, Institut des Sciences de la Terre (ISTerre), F-38041 Grenoble, France, and CNRS, Institut des Sciences de la Terre, F-38041 Grenoble, France. Published online on 28 Aug. 2014; http://dx.
The subduction of the Indian plate underneath the Himalaya and Tibet is not a steady process. During the northern route of the Indian continent, the Indian slab deforms and moves southward relative to the Himalaya. This motion stirs the mantle, which in turns deforms the surface. This transient dynamic topography may explain the fast Miocene uplift of the Himalaya and the recent and the rapid subsidence of the
Phosphorus zoning reveals dendritic architecture of olivine
Benoit Welsch et al., Dept. of Geology and Geophysics, University of Hawaii-Manoa, 1680 East-West Road, Honolulu, Hawaii 96822, USA. Published online 15 Aug. 2014; http://dx.
This article by Benoit Welsch and colleagues presents a micro-analytical study of the spatial distribution patterns of slow diffusing impurities (phosphorus, aluminum) in volcanic, plutonic and experimental olivine. The analysis reveals that all the crystals have a dendritic P-rich architecture, suggesting that the mineral always experience a first event of rapid growth in the melts. These results also show the crystallization of the mineral is not a straightforward time progression from crystal core to rim, and that dendritic growth and subsequent infilling unify diverse but ubiquitous features of igneous olivine.