Boulder, Colo., USA - Twenty-five new articles have been posted online ahead of print on Geology's website since 28 June. The science covers a gamut of topics, from microbial mats to super-eruptions; sand to monsoons; glaciers to sinkholes. All Geology articles go through a rigorous peer-review process prior to print. Geology is the highest rated journal for geoscience for the seventh year in a row, according to a 2012 Thomson Reuters survey.
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28 JUNE 2013
Tracking the evolution of large-volume silicic magma reservoirs from assembly to supereruption
Jörn-Frederik Wotzlaw et al., Earth and Environmental Sciences Department, University of Geneva, 1205 Geneva, Switzerland. First published on 28 June 2013, http://dx.
The most voluminous silicic volcanic eruptions in the geological past were associated with caldera collapses above giant silicic magma reservoirs. The thermal evolution of these sub-caldera magma reservoirs controls the volume of eruptible magma and eruptive style. Here we combine high-precision zircon U-Pb geochronology, trace element analyses of the same mineral grains, and mass balance modeling of zircon trace element compositions allowing us to track the thermal and chemical evolution of the Oligocene Fish Canyon Tuff magma reservoir (Colorado, United States) as a function of absolute time. Systematic compositional variations in U-Pb dated zircons record ~440 thousand years of magma evolution. An early phase of volumetric growth was followed by a period of cooling and crystallization, during which the Fish Canyon magma approached complete solidification. Subsequent remelting, due to underplated andesitic recharge magmas, began 171 plus or minus 58 thousand years prior to eruption, and led to the generation of ~5000 cubic kilometers of eruptible crystal-rich (~45 vol%) dacite. Age-equivalent, but compositionally different, zircons in an andesite enclave from late-erupted Fish Canyon Tuff tie the growth and thermal evolution of the upper-crustal reservoir to a lower-crustal magma processing zone. Our results demonstrate that the combination of high-precision dating and trace element analyses of accessory zircons can reveal invaluable information about the chemical and thermal histories of silicic magmatic systems and provides critical input parameters for fluid dynamic modeling.
Thermal decomposition along natural carbonate faults during earthquakes
Cristiano Collettini et al., Dipartimento di Scienze della Terra, Università di Perugia, Piazza dell'Università 1, Perugia 06123, Italy; and Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy. First published on 28 June 2013, http://dx.
Theory and laboratory experiments propose that fault lubrication during earthquakes is generated by physico-chemical processes triggered by a temperature rise due to frictional heating. This frictional heating is generated in a way similar to rubbing both hands together. However, solidified friction melts (pseudotachylyte) are the only unambiguous signature of seismic slip along natural faults but are rare in the geologic record. This has led the scientific community to question whether thermally activated mechanisms, observed during laboratory experiments and modeled in theoretical studies, are also relevant for the mechanics of natural, seismic faults. This has created a paradox that is arguably one of the most debated topics in the Geosciences. In our Geology manuscript we show a gamut of new natural fault rocks that are the unambiguous signature of thermal processes active during earthquake slip.
Isotopic shifts in the Cenozoic Andean arc of central Chile: Records of an evolving basement throughout cordilleran arc mountain building
Marcia Muñoz et al., Departamento de Geología, FCFM, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. First published on 28 June 2013, http://dx.
Late Cenozoic arc magmas from central Chile have been formed during the construction of the modern Andes. This archetypical example of a cordilleran arc orogenic system has been built over an heterogeneous basement and a significant volume of continental lithosphere has ben underthrusted under the arc area due to shortening processes. Based on Hf and Nd isotopic data along with the latest orogenic models for the region, this contributions shows that major magmatic isotopic shifts are related to different basement domains and also to the evolution of such basement throughout the assemblage of the modern Andes. In particular, such isotopic shifts signal the arrival of older basement to the crustal magmatic source resulting from the deep western underthrusting responsible for the main Andean uplifting event. These results provide an example of the dynamic interplay between magmatism and tectonism in an evolving orogen, and draw back the attention to the major role of continental lithospheric processes in conditioning the composition of arc magmas.
Concordant monsoon-driven postglacial hydrological changes in peat and stalagmite records and their impacts on prehistoric cultures in central China
Shucheng Xie et al., State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China. First published on 28 June 2013, http://dx.
Drought and flooding exert important impacts on anthropogenic activities in modern days or in ancient times. However, in comparison with the paleo-temperature, we know much less about the variation of hydrological conditions in ancient times. This is primarily due to the difficulty in the reconstruction of rainfall amount, resulting from the restricted proxies. Two new independent proxy records from peatland and stalagmite archives were proposed to trace monsoon-driven hydrological changes occurring since the last deglaciation in a broad region of middle Yangtze area of China. The wet periods are recorded by reduced mass accumulation rates of hopanoids, biomarkers for aerobic microbes in Dajiuhu peatland. The hopanoid-based reconstruction is supported by the first report of the environmental magnetism parameter, the ratio of fine magnetic particles to total ferromagnetic particles in a stalagmite from Heshang cave in central China. Spatiotemporal distributions of over 1600 prehistoric settlement sites of six distinctive cultures of the Neolithic to Iron Ages were observed to correlate with the fluctuating hydrological conditions referred by the two proxies, with enhanced flooding risk forcing major relocations of human settlements away from riparian zones. Flooding may also lead to the collapse of the ancient cultures in this region.
Size variation of conodonts during the Smithian-Spathian (Early Triassic) global warming event
Yanlong Chen et al., Institute of Earth Sciences, University of Graz, Heinrichstrasse 26, 8010 Graz, Austria; and State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Hubei 430074, China. First published on 28 June 2013, http://dx.
The end-Permian mass extinction is believed to be the most severe biodiversity crisis in the history of life. Immediately after the extinction, many marine taxa suffered a temporary but dramatic size reduction (i.e., the 'Lilliput Effect'). Compiled data show that recovery from the end-Permian mass extinction was not a smooth process but included several episodes of environmental disturbance and three further episodes of extinction. The third one occurs near the Smithian/Spathian boundary (SSB), around 1.8 million years after the end-Permian extinction and was recently associated to an episode of extreme warmth. Ammonoids and Conodonts suffered especially. Conodonts are an extinct group of marine chordates with a feeding apparatus composed of microscopic 'tooth-like' elements. They are one of the fastest evolving fossil groups, sensitive to environment stress, and are thus an ideal organism to test ecological responses to past episodes of climatic change. Detailed size measurements show for the first time that this clade suffered a temporary, but significant, size reduction during the SSB crisis, followed by gradual and steady size increase. Size reduction of conodonts was caused by an episode of global warming, further strengthening the link between morphological and climatic changes recorded in the fossil record.
Magnetostratigraphic determination of the age of ancient Lake Qinghai, and record of the East Asian monsoon since 4.63 Ma
Chaofeng Fu et al., State Key Laboratory of Loess and Quaternary Geology, Institute of Earth and Environment, CAS, Xi'an, Shaanxi, 710075, China; and Key Laboratory of Western Mineral Resources and Geological Engineering, Ministry of Education of China & Chang'an university, Xi'an, Shaanxi, 710054, China. First published on 28 June 2013, http://dx.
Lake Qinghai is the largest interior plateau lake in North China, and is sensitive to climate change and the environmental effects of Tibetan Plateau uplift. An almost continuous 626 m long sediment core have been obtained from an in-filled part of the southern lake basin, which documents both the age of the origin of the lake and the evolution of the East Asian monsoon during the Late Cenozoic. The article presents a high-resolution magnetostratigraphy work which provides a chronology back to about 5.1 million years ago. Analysis of lithofacies and depositional environments reveal that the change from eolian to lacustrine facies occurred ~4.63 million years ago, corresponding to a shift from an arid/semi-arid to a more humid climate, which resulted in the origin of Lake Qinghai. Changes in sediment lithology and mean grain-size indicate that the lake level fluctuated considerably, superimposed on a long-term trend from higher to lower levels in response to variations in the East Asian Monsoon. This archive is a significant additional source of information on regional and global environmental change, complementing the existing records from North China which are mainly based on analysis of loess deposits. This study is extremely important, because long, continuous, terrestrial lacustrine sedimentary records are extremely rare.
Restraining bend tectonics in the Santa Cruz Mountains, California, imaged using 10Be concentrations in river sands
Maria H. Gudmundsdottir et al., Department of Geological and Environmental Sciences, Braun Hall, Building 320, 450 Serra Mall, Stanford University, Stanford, California 94305-2115, USA. First published on 28 June 2013, http://dx.
Certain sections of strike-slip faults, known as restraining bends, are sites of unusual tectonic activity because they undergo more contractional motion than surrounding areas. These bends can be the site of major contraction-dominated earthquakes that are difficult to characterize using traditional means. Over millions of years, these earthquakes build substantial mountain ranges, such as the site of the present study, the Santa Cruz Mountains, California. This study tests a method for estimating where and how fast these tectonic motions build mountains based on rates of river erosion at various points in the mountain range. The faster a mountain is uplifted, the steeper it will become, causing rivers to erode it more rapidly -- thus high erosion rates should occur where uplift rates are high. We calculate erosion rates by measuring chemical changes that occur in rock as it reaches the Earth's surface and is exposed to radiation from outer space. Using erosion rates calculated from these chemical changes, we are able to replicate known variations in the rate at which the Santa Cruz Mountains are being uplifted. This technique has the potential to reveal areas of rapid mountain-building, which might be particularly prone to hazardous earthquakes.
3 JULY 2013
Carbon cycle feedbacks during the Oligocene-Miocene transient glaciation
Elaine M. Mawbey and Caroline H. Lear (corresponding), School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK. First published on 3 July 2013, http://dx.
New evidence published in Geology helps solve the mystery surrounding a collapse of the Antarctic ice sheet 23 million years ago. The surface of the East Antarctic ice sheet is so cold that models can only simulate its collapse by applying a significant climatic warming. Yet numerous lines of evidence suggest that 23 million years ago the Antarctic ice sheet decayed in size as changes in Earth's orbit around the Sun drove more subtle changes in Earth's seasons. Scientists from Cardiff University's School of Earth and Ocean Sciences analyzed fossil "foraminifera". These microscopic animals live in the ocean - on death their shells collect on the seafloor making a geological record of the past. The team found that while the ice sheet was expanding, more carbon was buried on the seafloor. Later, while the ice sheet decayed, carbon was introduced back into the oceans. It seems the subtle changes in Earth's orbit around the Sun were amplified by a positive feedback effect, most likely involving the greenhouse gas carbon dioxide. Dr. Lear said: "Understanding what caused dramatic collapses of the Antarctic ice sheet in the past is necessary to test the accuracy of current ice sheet models."
Coastline retreat via progressive failure of rocky coastal cliffs
Nick J. Rosser et al., Department of Geography, Durham University, South Road, Durham DH1 3LE, UK. First published on 3 July 2013, http://dx.
Despite much research on the many processes that erode rocky coastal cliffs, accurately predicting the nature, location and timing of coastline retreat remains challenging. This is also confounded by the apparently episodic nature of cliff failure. The dominant drivers of coastal erosion, marine and sub-aerial processes, are anticipated in future to increase, so understanding their present and combined efficacy is fundamental to improving predictions of coastline retreat. We capture change using repeat laser scanning across a series of near-vertical rock cliffs on the UK North Sea coast over 7 years to determine the controls on the rates, patterns and mechanisms of erosion. For the first time we document that progressive upward propagation of failure dictates the mode and defines the rate at which marine erosion of the toe can accrue retreat of coastline above; notably a failure mechanism not conventionally considered in cliff stability models. Propagation of instability and failure operates over decadal timescales, and is moderated by local rock mass strength and the time-dependence of rock fracture. Once initiated, failure propagation can operate ostensibly independently to external environmental forcing, and so may not be tightly coupled to prevailing conditions, which holds implications for modeling future coastal change.
Rheological controls on the emplacement of extremely high-grade ignimbrites
Geneviève Robert et al., Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. First published on 3 July 2013, http://dx.
Geneviève Robert and colleagues used a combination of field observations, rheology experiments, and thermal modeling to constrain the conditions that can lead ash deposits from pyroclastic flows to weld together and flow like lava, forming "lava-like ignimbrites". Their results show that welding and ductile flow are possible during emplacement of ignimbrites if emplacement temperatures are sufficiently high or if viscous heating is efficient in a shear zone at the interface between the deposit and the pyroclastic flow. Furthermore, viscous heating in rapidly-emplaced ignimbrites removes the requirement for high eruption temperatures or for high dissolved water contents at fragmentation, which are frequently invoked in order to explain lava-like features. Viscous heating can heat the underlying deposits up so that it is potentially hotter than the pyroclastic flows themselves. The history of these materials begins at high temperature in a magma chamber, goes through fragmentation and transport as ash particles in a pyroclastic flow, and ends with welding and heating back to a liquid that may be hotter even than the magma chamber from whence it came.
Sinkhole precursors along the Dead Sea, Israel, revealed by SAR interferometry
Ran N. Nof et al., Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Mailbox 653, Beer-Sheva 84105, Israel. First published on 3 July 2013, http://dx.
Toward operational sinkhole early warning system along the Dead Sea, Israel: The water level in the Dead Sea has been dropping at an increasing rate since the 1960s, exceeding a meter per year during the past decade. This drop has triggered the formation of sinkholes and widespread land subsidence along the Dead Sea shoreline, resulting in severe economic loss and infrastructural damage. We examined the spatiotemporal evolution of sinkhole-related subsidence using Satellite based Interferometric synthetic aperture radar (InSAR) measurements and field surveys, and resolved millimeteric-scale precursory subsidence in all sinkhole sites that we examined. It seems that filling of newly formed sinkholes with gravel and mud injections into drill-holes enhance land subsidence, enlarge existing sinkholes and form new sinkholes. Apart from shedding light on the mechanical process, the results of our study may pave the way for the implementation of an operational sinkhole early warning system.
The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone
Christie D. Rowe et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK. First published on 3 July 2013, http://dx.
How thick is a plate boundary fault in a subduction zone, and does it grow with depth? A compilation of 18 measurements from active and ancient subduction thrust faults around the world shows that subduction thrust faults form one or more high-strain strands about 5-35 m thick. Earthquakes and interseismic creep take place on these strands. If there is more than one strand, they may be braided through a section ~100-350 m thick. Thicker faults may have more strands but the thickness of individual fault strands is the same. This means subduction thrust faults don't grow gradually by incorporating more wallrock, as has been previously suggested to explain thick mélange units. Instead, kilometers-thick mélanges must form by accumulation through time, and by off-fault deformation. The faults are too thin to allow buoyant "backflow" from depth as has been suggested to explain mélanges containing high-pressure metamorphic rocks.
Rapid coastal subsidence in the central Ganges-Brahmaputra Delta (Bangladesh) since the 17th century deduced from submerged salt-producing kilns
Till J.J. Hanebuth et al., MARUM-Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany. First published on 3 July 2013, http://dx.
The densely populated low-lying Ganges-Brahmaputra Delta is highly vulnerable to the global sea-level rise. In order to estimate the subsidence of the delta over historical time scales, as a complement to existing entire-Holocene or modern data, we examined submerged salt-producing kiln sites in the coastal Sundarbans. These kilns were built just above the previous winterly spring high-tide level, but are currently located ~155 (plus or minus 15) cm below the corresponding modern level. The kilns were ultimately fired ~300 years ago (1705 plus or minus 35 AD), as shown by optically stimulated luminescence (OSL) dating. Salt production was terminated abruptly by a catastrophic event (major cyclone). Radiocarbon ages additionally measured on the charcoal layers at the kiln's bases and on associated mangrove stump horizons support the OSL ages. Consequently, the outer delta sank at 5.2 plus or minus 1.2 mm/a, including a 0.8 mm/a global sea-level rise over this period. Expecting an accelerating eustatic sea-level rise of up to 7 mm/a, a coastal sinking of up to 8.9 plus or minus 3.3 mm/a is calculated for the next few decades, which will dramatically aggravate the already present problematic situation. Only a prudently-managed control of sediment accretion will keep southern Bangladesh above the sea level.
Massive Cretaceous-Paleogene boundary deposit, deep-water Gulf of Mexico: New evidence for widespread Chicxulub-induced slope failure
Richard A. Denne et al., Marathon Oil Corporation, 5555 San Felipe, Houston, Texas 77056, USA. First published on 3 July 2013, http://dx.
The largest known submarine landslide has been identified in the Gulf of Mexico, generated by the Chicxulub extraterrestrial impact which also caused the mass-extinction at the end of the Cretaceous. The landslide was triggered when seismic shock waves and tsunamis caused sediments on the Gulf of Mexico seafloor deposited during the prior 10 million years to be eroded and lifted up into the water column. These sediments then settled back down onto the seafloor over the following weeks, blanketing the entire deep-water Gulf of Mexico with a deposit up to 200 meters thick and a volume of 100,000 cubic kilometers, 10 times larger than any other known submarine landslide deposit. The deposit was recognized in 31 industry-drilled wells from the deep-water Gulf of Mexico and on seismic images. These new findings provide crucial evidence of the Chicxulub impact's destructive effects on the Gulf of Mexico region and place doubt on claims of the existence of a separate extraterrestrial impact larger than Chicxulub at the end of the Cretaceous.
11 JULY 2013
A potential barrier to deep Antarctic circumpolar flow until the late Miocene?
I.W.D. Dalziel et al., University of Texas, Institute for Geophysics, 10100 Burnet Road (R2200), Austin, Texas 78758-4445, USA. First published on 11 July 2013, http://dx.
Scientists have found geologic evidence which casts doubt on one of the conventional explanations for how Antarctica's ice sheet formed. The Antarctic circumpolar current (ACC), an ocean current flowing clockwise around the entire continent, isolates Antarctica from warmer ocean water to the north, thereby helping to maintain the ice sheet. For several decades it has been surmised that the onset of a complete ACC played a critical role in the initial glaciation of the continent about 34 million years ago. Now, dredging in the central Scotia Sea near Antarctica reveals the remnants of a now-submerged volcanic arc that formed from 28 to 12 million years ago and may have formed a physical barrier to the formation of the ACC until after 12 million years ago. This work bolsters an alternate hypothesis which claims that dropping levels of atmospheric carbon dioxide around 34 million years ago was the primary trigger for the formation of the Antarctic ice sheet. Indeed, ocean temperatures globally fell at that time. Hence the onset of the ACC may be related to the well documented descent of the planet into a much colder 'icehouse' glacial state rather than to the initial glaciation of Antarctica.
Paleozoic-aged brine and authigenic helium preserved in an Ordovician shale aquiclude
I.D. Clark et al., University of Ottawa, Department of Earth Sciences, Ottawa, Ontario K1N 6N5, Canada. First published on 11 July 2013, http://dx.
The subsurface isolation of nuclear waste, as well as CO2 and hazardous fluids from fracking relies on the integrity of geological barriers to protect the surface environment from contamination. This study has examined the origin and age of water and gas in sedimentary rocks which are being considered for construction of a nuclear waste repository in Ontario, Canada. New analytical methods were developed to extract fluids and gases from the pores in these highly impermeable shale and limestone rocks. Geochemical and isotope measurements show that these fluids are ancient, originating as hypersaline seawater during Silurian time some 400 million years ago. Helium has accumulated in the pore fluids from the slow decay of uranium in these rocks, retained in-situ for more than 260 million years, despite its being the most mobile component found in the subsurface. The results provide a glimpse of the geochemical makeup of these ancient fluids. Their great age and immobility provide confidence that we can find deep geological settings for the safe isolation of nuclear waste.
Elevated pCO2 leading to Late Triassic extinction, persistent photic zone euxinia, and rising sea levels
Caroline M.B. Jaraula et al. (Kliti Grice, corresponding), Curtin University, Western Australia Organic and Isotope Geochemistry Centre, Department of Chemistry, Curtin University, Perth, WA 6845, Australia. First published on 11 July 2013, http://dx.
Elevated pCO2 leading to Late Triassic extinction, photic zone euxinia and rising sea levels are comparable to conditions that occurred during the end-Permian event reported in Science in 2005. Such conditions will have occurred during all past episodes of rapid global warming and biotic crisis that are associated with similar rises in pCO2.
Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction
Bernadette C. Proemse et al., Department of Geoscience, University of Calgary, 2500 University Drive, Calgary, Alberta T2N 1N4, Canada. First published on 11 July 2013, http://dx.
Ocean Anoxia did not cause the Latest Permian Extinction. With loss of over 90% of marine life, the Latest Permian Extinction, ~252 million years ago represents the most severe extinction event in Earth's history. Previous studies have suggested that strong oxygen depletion in the oceans was a principal cause of the extinction, and that marine life survived only in shallowest waters where wave action maintained oxygen levels. In this new study, novel techniques using molybdenum stable isotopes were applied to examine marine oxygen levels from shallow to deep waters during the extinction. Ancient sediments were collected that represent a gradient in the marine water depth, from sites now located in the Canadian High Arctic. Results show that the deepest part of the ocean turned anoxic (oxygen depleted), explaining a near complete loss of marine life. However, shallower shelf environments well below the base of storm waves, also lacking signs of life, showed oxygen rich conditions across the mass extinction horizon. This suggests that ocean anoxia was not a global phenomenon and cannot have been the main cause of the extinction. Other killing mechanisms responsible for the extinction remain uncertain, but may be a combination of impacts associated with dramatic climate change occurring at that time.
CO2 degassing from hydrothermal vents at Kolumbo submarine volcano, Greece, and the accumulation of acidic crater water
Steven Carey et al., Graduate School of Oceanography, University of Rhode Island, 215 S. Ferry Road, Narragansett, Rhode Island 02882, USA. First published on 11 July 2013, http://dx.
Hydrothermal vents in the crater of Kolumbo submarine volcano (Aegean Sea) are discharging carbon dioxide together with fluids at temperatures up to 220 degrees C. The gas is dissolving into seawater close to the bottom, leading to significant acidification of the water inside the crater with pH levels down to 5.0. This water is trapped within the bowl-shaped crater because of its increased density associated with high carbon dioxide contents. Lack of abundant macrofauna at the Kolumbo hydrothermal vents and the occurrence of carbonate-poor sediment in the crater appear to be consequences of this local anomaly in ocean acidification. These sites provide important opportunities to study the composition of gases released to the ocean and their contribution to the global carbon flux. Build-up of CO2-rich water in the bowl-shaped crater of Kolumbo may be producing hazardous conditions analogous to some African volcanic lakes (Monoun and Nyos) where overturn of gas-rich bottom waters led to abrupt releases of CO2 at the surface. Water samples collected near the floor of Kolumbo volcano released gas when they were brought to the surface and exposed to atmospheric pressure. A minimum estimate of 1.4 by 105 cubic meters of excess CO2 may exist in the bottom 10 meters of the Kolumbo crater.
Holocene sea-level change derived from microbial mats
Daniel Livsey and Alexander R. Simms, Department of Earth Science, University of California-Santa Barbara, 1006 Webb Hall, MC 9630, Santa Barbara, California 93106-9630, USA. First published on 11 July 2013, http://dx.
This study presents an innovative approach for reconstructing past sea level using intertidal microbial mats from Baffin Bay, Texas. Intertidal microbial mats are commonly found in arid regions wherein most sea-level proxies only have meter-scale precision. Studying past sea level is necessary for understanding how the coastline may respond to future change in sea level expected over the next century. 95% of all 1,297 microbial mats surveyed in Baffin Bay where found to live within 29 cm of mean sea level. Therefore buried microbial mats were used to determine past sea level within a 29 cm range for the last 5,000 years. The 29 cm precision of intertidal microbial mats to past sea level is much better than the typical precision of sea level proxies used within arid regions.
Rapid pulses of uplift, subsidence, and subduction erosion offshore Central America: Implications for building the rock record of convergent margins
Paola Vannucchi et al., Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK. First published on 11 July 2013, http://dx.
Paola Vannucchi and colleagues describe the impact of the subduction of a high "mountain chain" on the margin of the overriding plate. A detailed record of the last 2.5 million years has been preserved in the sedimentary sequence drilled and recovered in 2011 by IODP Exp. 334. Reading this record and its correlation with the architecture of the margin as it appears today has allowed us to reconstruct the phases of disruption and how the margin is trying to heal through building deep, fast filling sedimentary basin on the forearc.
Lithospheric convective instability could induce creep along part of the San Andreas fault
Laetitia Le Pourhiet, Université Pierre-et-Marie-Curie-Paris 06, Institut des Sciences de la Terre Paris F-75005, France; and Jason Saleeby. First published on 11 July 2013, http://dx.
The 150 km long portion of the San Andreas located north of Parkfield (California, USA) is exempt of large destructive earthquakes. Until now, scientists have focused their research on characterizing the properties of rocks located inside that portion of the fault to explain this anomaly. Le Pourhiet and Saleeby alternatively propose that the lack of large earthquakes is caused by an anomaly in external loading. The heavy root of the Sierra Nevada Mountains was indeed ripped off into the earth mantle 3 to 4 million years ago and still remained attached like an anchor to the Great Valley plate today. With the help of numerical models, they show that the load exerted by this anchor causes the plate to bulge at a location that matches the location and extent of the silent portion of the fault. They proposed this bulged area favors geometrically the formation of large patches of over-pressured water into the rocks. In a similar manner as a car undergoing aquaplaning, the fault portion located within this bulge therefore lacks adherence allowing the Pacific and American plates to slide past each other continuously with no earthquakes.
Slab flattening trigger for isotopic disturbance and magmatic flare-up in the southernmost Sierra Nevada batholith, California
Alan D. Chapman et al., Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA. First published on 11 July 2013, http://dx.
Deep exposures into the crust of Southern California permit examination of an ancient (Late Cretaceous) shallow subduction zone. Zircon U-Pb ages from rocks belonging to the lower plate of the subduction zone indicate that these rocks were deposited at ca. 100 Ma. The same zircon grains reveal a significant shift in oxygen isotope values, which resulted from subduction and metamorphism. These zircon age and isotopic results provide evidence for upward percolation of volatile phases and/or partial melt from the lower plate into the upper plate of the shallow subduction zone. This event coincided with voluminous magmatism in Southern California. We suggest that as the downgoing plate of the subduction system flattened, rocks of the lower plate were plastered to the base of the upper plate, which triggered an episode of copious magmatism. Shallow subduction of the Cocos plate beneath central Mexico represents a close modern analog to this model.
Upwelling, rifting, and age-progressive magmatism from the Oki Daito mantle plume
Osamu Ishizuka et al., Institute of Geology and Geoinformation, Geological Survey of Japan/AIST, Central 7, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan. First published on 11 July 2013, http://dx.
Until now, one of the world's major volcanic provinces has remained undetected beneath the Philippine Sea. The extent and size of the volcanism has been made clear by a study of material recovered from this region by a team of Japanese and UK scientists. A stream of hot rock from deep in the Earth, known as a mantle plume, first made contact with the Philippine Sea bed 51 million years ago. This produced volcanoes covering an area of 250,000 cubic kilometers, and resulted in a major "crack" in the Earth's crust that generated further volcanic activity. Analyzing the age and chemical composition of the lava from these submarine volcanoes has revealed that this mantle plume, called the Oki-Daito plume, continued its activity for 15 million years. During this period of activity, the plume spread out beneath the crust and "infected" other volcanoes within 800 km of the center. Subsequently, around half of the original volcanoes within this province have been consumed back into the deep Earth by plate movements. However the remainders have provided new information on how this important volcanic region developed.
Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing
Huayu Lu et al., School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, China. First published on 11 July 2013, http://dx.
East Asian monsoon precipitation influences the lives of 1.6 billion people today, its swings have attracted many attentions but the forcing mechanism has not been well understood. To investigate the monsoon variation in the past can provide critical insight on causes of the changes. The natural archives such as lake sediments and loess deposits preserved information of the past monsoon precipitation changes, because the monsoon precipitation modulated vegetation density and composition which left vestiges in these sediments. Dr. Lu and his colleagues use Chinese loess deposits, which are independently dated, to reconstruct the monsoon precipitation since 21 ka (ka = thousand years ago). Their results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8-3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation at long time scale. They use TraCE-21000 modeling experiment to test their interpretation and the results support their conclusion.