Earthquake swarms; marine Ediacaran fossil traces; Alca obsidian; Mammoth Mountain

(Press-News.org) Boulder, Colo., USA – Studies in this latest batch of GEOLOGY postings cover tiny Ediacara organisms, CO2 gas following seismic swarms, the growth of Mount Everest, methane seeps, the remarkably modern character of Cretaceous seawater composition, geodynamic models of the assembly of Rodinia and Gondwana, and whether subduction zones are invading the Atlantic. Other studies cover the Danube Basin, the Andes, the Central Range of Taiwan; and the seafloor near Costa Rica. All article abstracts are open access online.

Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above.

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The progressive evolution of a continental climate in southeast central European lowlands during the Middle Pleistocene recorded in loess paleosol sequences
Björn Buggle et al., ETH Zürich, Dept. of Earth Sciences, Biogeoscience Group, Sonneggstrasse 5, 8092 Zurich, Switzerland; and Chair of Geomorphology, University of Bayreuth, 95440 Bayreuth, Germany. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34198.1.

Little is currently known about the long-term climate evolution of central and southeastern Europe during the Quaternary, because suitable paleoenvironmental records from the terrestrial realm are scarce. However, loess plateaus in the middle and lower Danube Basin represent exceptional paleoenvironmental archives due to their thickness and the completeness of the loess paleosol successions, spanning more than 700 thousand years. Based on multiproxy investigations on these archives, new insights into the Middle and Late Pleistocene climate evolution of this region is presented. It can be shown that in these lowlands interglacial climate continentality progressively increased during the Middle Pleistocene. Corresponding trends are also expressed in other climate archives of these basins as well as in the lowlands of the northern Black Sea region but are not or are less clearly present in climate archives from outside these lowlands. Potential triggers and explanations are discussed, and Björn Buggle and colleagues conclude that a small-scale increase in paleo-elevation of Central European mountain ranges (Alps, Carpathians, Dinarides) during the Middle Pleistocene should be considered as a driving factor for the progressive increase in climate continentality of SE-European lowlands as well as for the westward expansion of the Eurasian steppe belt.

Multi-technique geochemical characterization of the Alca obsidian source, Peruvian Andes
Kurt Rademaker et al., Dept. of Anthropology, University of Maine, 5773 South Stevens Hall, Orono, Maine 04469, USA. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34313.1.

Kurt Rademaker and colleagues report results from comprehensive mapping and multi-technique geochemical characterization of obsidian from the Alca source in the Peruvian Andes, aimed at understanding the evolution of trade networks in one of the world's centers of complex civilization. Alca obsidian was a highly valued and widely distributed volcanic glass used for stone tool making in the central Andes from 13,000 years ago until recently. This team's mapping of obsidian over more than 330 square kilometers establishes Alca as among the largest sources of obsidian in the South American continent. Compositional analysis of Alca obsidian using traditional and emerging non-destructive techniques indicates that the source region contains six geochemically and geographically distinct subsources. When obsidian sources exhibit such variability, patterns of quarrying, exchange, and landscape use can be inferred by tracking changes in the distribution of artifacts deriving from different subsources through time. Now that the Alca source has been characterized via multiple methods, and the efficacy of non-destructive analysis to distinguish the subsources has been established, further non-destructive analysis of robust sets of obsidian artifacts within Peru will allow high-resolution study of the evolution of prehistoric Andean exchange systems.

No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits
Yunxing Xue et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34186.1.

About half of the world's total gold production comes from very old volcanic rocks from Western Australia, Canada, Africa, and India, and are called Archean greenstone belts. However, the source of gold in these deposits remains controversial. Was it carried by circulating ground water, by fluids released during heating of the crust, by fluids originating from the mantle below the crust, or by fluids derived from a crystallizing magma chamber? The gold that forms these deposits is carried in solution by gold-sulfur compounds, so it is likely that the gold and sulfur originate from the same source. Yunxing Xue, and co-workers from the Australian National University, report the first analyses of multiple isotopes of the sulfur from Archean gold deposits by SHRIMP SI, a new ion probe build at the Australian National University, which allows the in situ measurement of light isotopes on tiny spots as small as 20 microns. The results show unequivocally that the sulfur associated with the gold was not derived from the atmosphere, which allows the authors to rule out the first two hypotheses. The gold carrying fluids must therefore come from the mantle or, more likely, a hidden magma chamber below the gold deposits.

The Miocene elevation of Mount Everest
Aude Gébelin et al., Biodiversity and Climate Research Centre (BiK-F), Senckenberganlage 25, 60325 Frankfurt, Germany. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34331.1.

How high was Mount Everest in the past? Aude Gébelin and colleagues conducted stable-isotope paleo-altimetry to reconstruct the late Early Miocene elevation of Mount Everest, the highest peak on Earth. The method is based on a very simple technique: It recovers the isotopic composition of ancient rain water as oxygen and hydrogen isotopes in precipitation scale with elevation. In the absence of sedimentary deposits within the rapidly eroding Himalayan range, the "geologic archive" of such meteoric waters is the South Tibetan detachment. The South Tibetan detachment is an extensional fault structure in which meteoric water circulated during the time of detachment activity. Hydrous silicates formed during deformation and exchanged oxygen and hydrogen isotopes with meteoric water. Gébelin and colleagues circumvented the impact of long-term climate change on isotopes in precipitation by comparing their South Tibetan detachment data with the rainfall record in the Himalayan foreland basin. The difference in the oxygen isotopic composition of precipitation is consistent with mean elevations of more than 5,000 meters for the Mount Everest area. These similar-to-modern mean elevations for the Everest region suggest that an early Himalayan rain shadow influenced the late Early Miocene climatic and rainfall history along the Himalayan chain and the Tibetan plateau.

Rock magnetic record of the Triassic-Jurassic transition in pelagic bedded chert of the Inuyama section, Japan
Alexandra Abrajevitch et al., Research School of Earth Sciences, Australian National University, 142 Mills Road, Acton, ACT 0200, Australia. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34343.1.

The end-Triassic mass extinction event is regarded as one of the five largest extinction events of the Phanerozoic. The extinction is thought to be ultimately caused by volcanic activity at the Central Atlantic Magmatic Province (CAMP); however, the underlying environmental mechanisms that drove biotic turnover remain poorly understood. This rock magnetic study by Alexandra Abrajevitch and colleagues of a marine bedded chert sequence spanning the Triassic-Jurassic boundary transition provides a record of a two-stage environmental change. The disappearance of previously ubiquitous biogenic magnetite particles indicates that global environmental decline had started about a hundred thousand years prior to the formal Triassic-Jurassic boundary. Unusual optical and magnetic properties of hematite pigment in the key chert bed indicate an episode of a significant but short-lived change in bottom water chemistry at the end of the Triassic. The stepwise change in magnetic properties is suggestive of the protracted environmental deterioration, likely prompted by the early episodes of the CAMP volcanism, which was followed by a sudden ocean acidification event.

Sulfate availability and the geological record of cold-seep deposits
Thomas F. Bristow, NASA Ames Research Center, Moffett Field, California 94035, USA; and John P. Grotzinger, Division of Geological and Planetary Sciences, California Institute of Technology, MC170 25, 1200 East California Boulevard, Pasadena, California 91125, USA. Posted online ahead of print on 24 May 2013; http://dx.doi.org/10.1130/G34265.1.

Certain areas of the seafloor are subject to high rates of methane gas escape. These sites are called 'methane seeps' and host exotic biological communities, which make a living using the methane as an energy source. Relic seeps are preserved in rocks and found in the geological record, but are intriguingly restricted to a relatively recent period in Earth's history. In addition, some diagnostic geochemical signals found in recent seeps are absent in the older examples of seeps. This paper by Thomas Bristow and John Grotzinger explains these trends as a consequence of changes in abundance of one of the most important constituents of seawater -- seawater sulfate, the levels of which reflect how much free oxygen is in the atmosphere and oceans. Their work indicates low ocean sulfate levels in the first 200 million years following the Cambrian explosion of life. The implication is that early animals had to contend with low oxygen levels. This environmental stress is thought to have been an important influence on the tempo of evolution, extinction, and recovery of early animals.

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin
João C. Duarte et al., School of Geosciences, Monash University, Melbourne, Victoria 3800, Australia. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34100.1.

An outstanding question in plate tectonics is how do oceans start to close? It is known that the movement of Earth's tectonic plates tear apart supercontinents to form new oceans, as the Pangaea broke-up to form the Atlantic. These oceans will eventually close and continents recombine – the so-called Wilson Cycle. As oceanic lithosphere spreads and cools it becomes gravitationally unstable, and a few million years after it tends to collapse into the mantle, a process called subduction; and the pull at the subduction zones will drive the continents back together. However, oceanic plates also become stronger with aging making the process of spontaneous subduction unlikely. An alternative model considers that subduction zones can propagate from ocean to ocean, as the Scotia and Lesser Antilles arcs propagated from the Pacific into the Atlantic. Based on a new tectonic map, Duarte and colleagues provide evidence that a new subduction zone is just starting to propagate into the Atlantic Iberian margin, which could be a precursor of a large-scale Atlantic subduction system that can lead to its closing. The evidence suggests that subduction can be an invasive process that can have a crucial role in the Wilson Cycle.

Seismic investigation of magmatic unrest beneath Mammoth Mountain, California, USA
Guoqing Lin, Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34062.1.

In 1990, tree die-off was observed in the vicinity of Mammoth Mountain in eastern California (United States), which was concluded due to the emission of magmatic CO2 gas following a seismic swarm. In this study, Guoqing Lin uses high-resolution seismic data to investigate the magmatic unrest during this 11-month-long swarm between 1989 and 1990 beneath Mammoth Mountain. Lin inverts a three-dimensional compressional-wave velocity (Vp) model and estimate the ratio of compressional- to shear-wave velocity (Vp/Vs). The result shows an anomalous body below sea level, consistent with the seismic expression of the magma body that triggered the seismic swarm. Abrupt changes in Vp/Vs ratios near sea level indicate the source of the 1989 CO2 flux is a shallow CO2 reservoir.

Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C
Berend A. Verberne et al., Dept. of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Netherlands. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34279.1.

Despite the importance of aseismic-seismic transitions in natural faults, the microphysical mechanisms controlling such transitions remain largely unknown. In this paper, Berend Verberne and colleagues present new data on the slip stability and strength of calcite fault rocks, and on microstructural development down to the nanometer-scale. Their experiments consisted of direct shear tests performed dry at slip rates of 0.1-10 µm/s, at a constant normal stress of 50 MPa, at 18-150 degrees Celsius. The results show a transition from stable to unstable slip above ~80 degrees Celsius. All recovered samples revealed a microstructure characterized by narrow shear bands consisting of ultrafine grains with a crystallographic preferred orientation (CPO). Electron microscopy applied to shear bands showed angular grain fragments down to ~0.3 micrometers in size, plus dense aggregates of calcite crystals measuring ~5-20 nanometers. These nanocrystals are aligned with the r-slip system (sub-)parallel to the shear plane and direction, suggesting a role of crystal plasticity next to cataclasis. Since crystal plasticity is strongly thermally-activated, we infer that the transition to unstable seismic slip is due to enhanced crystal plastic flow at temperatures greater than 80 degrees Celsius. These results imply that tectonically active limestone terrains are prone to shallow-focus seismicity, consistent with recent observations from the Apennines (Italy) or the Corinth Rift Zone (Greece).

Inversion of a hyper-extended rifted margin in the southern Central Range of Taiwan
Kirk McIntosh et al., Institute for Geophysics, University of Texas at Austin (R2200), 10100 Burnet Road, Austin, Texas 78758-0000, USA. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34402.1.

The island of Taiwan exists because it is in an area where two tectonic plates are colliding. This results in a rapidly growing mountain belt emerging from the sea accompanied by frequent earthquakes. This paper by Kirk McIntosh and colleagues describes the results of a seismic profiling study across Taiwan and the areas southwest of Taiwan that provide new information about how this collision takes place. In general, plate collisions occur when an entire oceanic basin on one plate is thrust beneath the other plate and deep into the Earth in a process called subduction. When all the oceanic crust is underthrust then thicker, less dense continental crust collides with crust of an island arc on the over-riding plate. The data McIntosh and colleagues present show that the edge of the Eurasia tectonic plate (the Chinese continental margin) consists of a 200+ km-wide area of stretched continental crust rather than the thinner, denser oceanic crust. This crust starts to underthrust beneath the over-riding plate, but then gets transferred to the base of the over-riding plate, building up into a 30 km-thick mass over time. It is this mass that collides with thicker continental crust beneath the shallow waters of the continental shelf to create the Taiwan mountain belt.

Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds
Ekaterina S. Kiseeva et al., Dept. of Earth Sciences, University of Oxford, OX1 3AN Oxford, UK. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34311.1.

The transition zone (TZ) of the Earth's mantle (the depth interval between two major seismic discontinuities at 410 km and 660 km) is critical to understanding our planet's evolution. The only natural samples of Earth's TZ are inclusions in diamonds, which mostly originate from the upper mantle, but in some cases could be delivered from the deepest upper mantle, TZ or even the lower mantle. Majoritic garnets are one of the most common high-pressure inclusions in diamonds and they have been found in diamonds worldwide, including South Africa, Canada, Siberia, Brazil, and many other localities. In this study, Ekaterina Keseeva and colleagues compare natural majorite garnet inclusions in diamonds with the compositions of laboratory crystallized majorite garnets, and determine that a significant fraction of a hybrid rock (so-called metapyroxenite) is present within Earth's TZ. Metapyroxenite is intermediate in composition between the most common mantle lithologies: eclogite and peridotite. This result is consistent with the seismological data that suggests lithologically heterogeneous TZ.

Experimental evidence linking slip instability with seafloor lithology and topography at the Costa Rica convergent margin
Matt J. Ikari et al., MARUM, Center for Marine Environmental Sciences, University of Bremen, D-28359 Bremen, Germany. Posted online 6 June 2013; http://dx.doi.org/10.1130/G33956.1.

Subduction zones, where tectonic plates collide resulting in one plate sinking beneath the other, are where the world's biggest earthquakes occur. The cause of these earthquakes is still a mystery, but off the coast of Costa Rica they seem to occur in places where the seafloor is elevated, for example where there are large seamounts. A characteristic of the elevated seafloor near Costa Rica is that they are covered with hard, carbonate sediment (chalk). On the other hand, normal (not elevated) portions of the ocean floor are covered in weak clay. By testing these two types of sediment in laboratory friction experiments, Matt Ikari and colleagues show that the carbonate sediment is strong and reproduces the earthquake-like behavior, something not seen for the clay sediment. Therefore, it is not just the presence of elevated ocean floor that causes large earthquakes, but the type of sediment on top that may be the cause of large earthquakes near Costa Rica.

Evidence for Cnidaria-like behavior in ca. 560 Ma Ediacaran Aspidella
Latha R. Menon et al., Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34424.1.

When do the earliest animals appear in the fossil record? How deep were the roots of the Cambrian explosion? According to molecular clocks, the earliest animals arose nearly 700 million years ago, yet evidence for animal activity has been curiously lacking below about 555 million years ago (Ma). Meanwhile, the Ediacaran organisms that flourished ca. 579-541 Ma remain enigmatic. The discovery by Liu et al. in 2010 of horizontal trails in the 565 Ma Mistaken Formation, Newfoundland, opened the serious likelihood of metazoans at this time. In this paper, Latha Menon and colleagues report similar trails now directly associated with a key Ediacaran fossil, Aspidella terranovica Billings 1872, from ca. 560 Ma Fermeuse Formation , Newfoundland. Moreover, they present the earliest vertical traces found so far, also associated with Aspidella. Menon and colleagues interpret these as equilibrium traces, produced by an organism on the seafloor periodically adjusting its position vertically in response to small pulses of sediment. Such traces are well known from later rocks, and produced by marine animals today. This demonstrates that at least one Ediacaran organism was a true marine animal, probably of cnidarian grade, capable of behaviors requiring muscular action—a truly animal trait.

Locating South China in Rodinia and Gondwana: A fragment of greater India lithosphere?
Peter A. Cawood et al., Dept. of Earth Sciences, University of St Andrews, North Street, St Andrews KY16 9AL, UK; and Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34395.1.

The South China craton first formed and then occupied a position adjacent to Western Australia and northern India. Convergent plate margin magmatic arc-back arc assemblages in the craton range in age from about 1000 million years to 820 million years and display a sequential northwest decrease in age. These relations suggest formation and closure of arc systems through southeast-directed subduction and resulted in progressive northwestward accretion onto the periphery of an already assembled Rodinia. Siliciclastic units within an early Paleozoic (500-460 million year old) succession that transgresses across the craton were derived from the southeast and include detritus from beyond the current limits of the craton. Detrital zircon age spectra require an East Gondwana source and are very similar to the Tethyan Himalaya and younger Paleozoic successions from Western Australia, suggesting derivation from a common source and by inference accumulation in linked basins along the northern margin of Gondwana, a situation that continued until rifting and breakup of the craton in the late Paleozoic.

Dynamic polar climates in a greenhouse world: Evidence from clumped isotope thermometry of Early Cretaceous belemnites
Gregory D. Price, School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA, UK; and Benjamin H. Passey, Dept. of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34484.1.

Using samples from Siberia and employing new state-of-the-art isotope techniques, Gregory Price and Benjamin Passey show that the Arctic ocean was significantly warmer than at present during the Cretaceous. Although warm, Price and Passey also find evidence for substantially cooler times, cold enough to support polar ice. The presence of polar ice for brief intervals during the Cretaceous is controversial. Hence dynamic polar climates characterize this time. These analyses also show that the composition of seawater (in terms of oxygen isotopes) did have a remarkably modern character.

Not all supercontinents are created equal: Gondwana-Rodinia case study
Christopher J. Spencer et al., Dept. of Earth and Environmental Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34520.1.

Christopher Spencer and colleagues propose that the isotopic signature of seawater and detrital zircons through time associated with the formation of supercontinents is dependent upon the configuration of the subduction zones that assemble the various continents. In the case of Gondwana, single-sided subduction zones reworked a significant portion of the old, isotopically enriched continental margins controlling the signature of the resulting material eroded into the oceans and the zircons derived from the orogenic belt. Rodinia, on the other hand, was dominated by double sided subduction zones resulting in greater reworking of isotopically depleted volcanic arc material resulting in a more depleted isotopic signature. The proposed geodynamic models of the assembly of Rodinia and Gondwana provide a connection between the geodynamic configuration of supercontinent assembly and its resulting isotopic signature.

Numerical simulations of CO2 migration during charnockite genesis
Amlan Banerjee et al., Indian Statistical Institute, Geological Studies Unit, Kolkata 700108, India. Posted online 6 June 2013; http://dx.doi.org/10.1130/G34129.1.

Charnockite is considered to be generated either through the dehydration of granitic magma by CO2 purging or by solid-state dehydration through CO2 metasomatism during granulite facies metamorphism. To understand the extent of dehydration, CO2 migration is quantitatively modeled in silicate melt and metasomatic fluid as a function of temperature, H2O wt%, pressure, basal CO2 flux and dynamic viscosity. Results show that CO2 migration through porous and permeable high-grade metamorphic rocks can generate dehydrated patches close to the CO2 flow path, as illustrated by the occurrences of incipient charnockites. The reaction-front velocity constrained by field observations comes out to be 0.69 km/m.y., which matches well with other studies. On the other hand, temperature, rate of cooling, and basal CO2 flux are the critical parameters effecting CO2 diffusion through a silicate melt. CO2 diffusion through silicate melt can only occur at temperature greater than 840 degrees Celsius and during slow cooling (3.7 x 10-5 degrees Celsius per year), features that are typical of magma emplacement in the lower crust and explains why some deep-level plutons contain both hydrous and anhydrous mineral assemblages. CO2 diffusion through silicate melt is virtually insensitive to pressure.

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