Caterpillar-walk exhumation, the downfall of the Moche, and trilobites in camouflage

(Press-News.org) Boulder, Colo., USA – New Geology articles posted online ahead of print cover everything from cratering on Mars to leopard-like camouflage in trilobites. Locations studied include the Ries Impact Crater; Hydrate Ridge, Oregon; Stromboli volcano; northern Peru; the Bushveld Complex, South Africa; western and central New York state; the Sahara Desert; and the French Alps. Brief highlights follow:

1. Analogous cratering at the Ries Impact Crater, Germany, and on Mars;
2. A presentation of the first secure, high-resolution land-sea PSV-based sediment-core synchronization;
3. Helium emissions as a sign of volcanic unrest;
4. Study of a methane seep site on a submarine ridge called Hydrate Ridge, offshore Oregon;
5. Caterpillar-walk exhumation;
6. Understanding the patterns in a Strombolian bomb field;
7. Unfolding rocks;
8. A testable thermo-mechanical shear zone model;
9. Mineral-rich volcanic glass from the seafloor;
10. Halite and extreme paleoweather conditions;
11. The first systematic documentation of decadal trends in streambed elevation on continental scales;
12. Analysis of clam shells used in ancient funeral ceremonies points to climate change as a factor in the downfall of the Moche;
13. The Bushveld Complex, South Africa, and the trapped liquid shift effect;
14. Leopard-like patterns in trilobites used as light disrupting camouflage;
15. The unique nature of shale;
16. The first apatite (U-Th)/He dating of the Saharan Tuareg Shield; and
17. 3,036 tree-ring series extracted from 759 conifers provide evidence for 61 landslide reactivations since A.D. 1898 in the French Alps.

Detailed 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 Christa Stratton at the address above.

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The Ries impact, a double-layer rampart crater on Earth
Sebastian Sturm et al., Albert-Ludwigs-Universität Freiburg, Institut für Geowissenschaften, Geologie, Albertstrasse 23B, 79104 Freiburg, Germany. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33934.1.

The complex Ries impact crater in Germany, with its observed ejecta distribution, thickness, and fabric characteristics, is comparable to double-layer rampart craters on Mars. Interpolation results of the ejecta blanket outside the crater show that the distribution deviates from a steady decrease with radial range as seen for lunar impact craters. Sebastian Sturm and colleagues found a depression (called moat) followed by a ridge (called rampart) of thick ejected Bunte Breccia material at about two crater radii from the crater center. The Ries crater, with its rampart position and width, perfectly fits to equivalent rampart dimensions of Ries-scaled Martian double-layer craters (e.g., Steinheim crater). The presence of a massive continuous rampart structure, as well as the ejecta fabric, indicates the presence of fluids during the emplacement process of the ejecta blanket outside the crater. This strong analogy means that the formation conditions for rampart structures can be studied on Earth.

Synchronizing Holocene lacustrine and marine sediment records using paleomagnetic secular variation
Sædís Ólafsdóttir et al., Institute of Earth Sciences, University of Iceland, IS-101 Reykjavík, Iceland. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33946.1.

In this manuscript, the first secure, high-resolution land-sea PSV-based sediment-core synchronization is presented, supported by several diagnostic tephra layers. Two high-sediment-accumulation-rate lake sediment cores are synchronized with a well-dated nearby marine core, allowing the derivation of unusually precise age models for the lake sediment records. These PSV-derived age models capture a high level of non-linear, centennial-scale variability in sedimentation rates that were not resolved by conventional dating methods, and the records now reveal important abrupt climate transitions. Our results show the great amplitude of PSV around Iceland, demonstrate the potential of PSV as a stratigraphic land-sea correlation tool and illustrate how PSV-synchronization can greatly improve geochronologies for hard-to-date lacustrine sediment records. Both land-sea synchronization and improved geochronologies provide valuable information about forcing and response of the climate system, indicating its global potential. The conclusions of this manuscript are therefore expected to be of great interest to the paleomagnetic, geochronological and paleoclimate communities.

Diffusive helium emissions as a precursory sign of volcanic unrest
Eleazar Padrón et al., Environmental Research Division, Instituto Tecnológico y de Energías Renovables, 38611 Granadilla de Abona, Tenerife, Canary Islands, Spain. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34027.1.

This research represents the first intensive and extensive study on diffusive helium emission from an island active volcano during the pre-eruptive, eruptive, and post-eruptive phase of a submarine eruption off the island coast. Eleazar Padrón and colleagues show the geochemical behavior of the diffusive helium emission at El Hierro (Canary Islands, Spain), during the recent magmatic reactivation and related submarine eruption off the southern coast of El Hierro Island. Prior to the eruptive phase, both diffusive helium emission through the soil environment from the entire island and its estimated magmatic fraction showed a steady increase, consistent with pressure buildup of magmatic gas and its subsequent leakage to the surface environment. The properties of helium minimize the interaction with the surrounding rocks/aquifers on its movement toward Earth's surface. The reported results provides a significant insight on the buildup of magmatic gas pressure in active volcanoes and its posterior release toward the surface environment, highlighting the usefulness of noble gases such as helium as potential geochemical precursors of volcanic eruptions, particularly regarding the timing of gas release with respect to the onset of seismic activity. This research suggests that diffusive helium emission studies on these volcanoes are tremendously useful for the geochemical programs on volcano surveillance.

Drivers of focused fluid flow and methane seepage at south Hydrate Ridge, offshore Oregon, USA
Gareth J. Crutchley et al., Helmholtz Centre for Ocean Research Kiel, GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34057.1.

The seepage (or leakage) of methane gas out of submarine sediments into the ocean is important for sustaining biological communities on the seafloor. Additionally, there is concern about the potential for climate change to cause large-scale release of methane into the ocean, which could make it to the atmosphere and thereby contribute to global warming. Therefore, it is important to understand the geological conditions that allow methane to migrate upward through sub-seafloor sediments to be released into the ocean. Gareth J. Crutchley and colleagues use 3-D seismic data to image geological structures beneath the seafloor at a well-known methane seep site on a submarine ridge called Hydrate Ridge, offshore Oregon. The data reveal key geological structures in detail that channel methane-rich fluids toward the seep site; structures include dense networks of faults and a sedimentary horizon known as "Horizon A." Through numerical modeling, Crutchley and colleagues show how fluid pressure is expected to build up beneath the seep site in response to focused fluid flow through Horizon A. This fluid pressure drives gas-charged fluids upward toward the seafloor. These results reveal the nature of fluid flow beneath the seafloor and show how and why methane seepage at this site has persisted for many thousands of years.

A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust
C. Tirel et al., Dublin Institute for Advanced Studies, Geophysics Section, 5 Merrion Square, Dublin 2, Ireland. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33862.1.

The scars of continental collisions that shaped today's continents are marked by very peculiar rocks. These rocks were once brought to depths of tens to hundreds of kilometers by sinking tectonic plates, metamorphosed under high pressure there, and then exhumed to the surface, often rapidly and without being strongly deformed. These "high-pressure" (HP) rocks are important markers of continental assembly. Yet, the mechanism of their exhumation has been a puzzle for decades. Of the solutions proposed, erosion was much too slow, and viscous circulation of the rocks in a "subduction channel" was difficult to reconcile with the lack of strong deformation in the HP rock units. Numerical simulations by C. Tirel and colleagues reveal a surprisingly simple solution to the problem: When the buoyant continental crust of a subducted micro-continent detaches from the lithospheric slab that dragged it down, the dense slab starts sinking faster, the subduction zone retreats, and the metamorphosed rocks ascend into the space that opens. In this process, the crustal rocks follow a path reminiscent of the walk of a caterpillar. The newly identified caterpillar-walk exhumation and accretion of HP crustal rocks is likely to be one of the fundamental mechanisms behind the evolution of Earth's continents.

Classification, landing distribution, and associated flight parameters for a bomb field emplaced during a single major explosion at Stromboli, Italy
L. Gurioli et al., LMV, OPGC, CNRS, Université Blaise Pascal, 63038 Clermont-Ferrand, France. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33967.1.

This paper presents a unique, multidisciplinary database built for a single volcanic major explosion at Stromboli volcano (Italy). Strombolian eruptions are probably the most common volcanic events on Earth, with Stromboli lending its name to this common eruption style. Because of the reliability of activity, Stromboli has also become a laboratory for execution of experiments to better understand volcanic activity. During two field seasons in 2010 and 2011, L. Gurioli and colleagues were, for the first time, able to make a rare map for landing positions of all fragments within a bomb field and combine these data with coincident geophysical metrics to fully characterize the emission. They argue that their unique dataset of deposit and geophysical data is invaluable if geoscientists are to better (i) place such volcanically common events within modern classification schemes, (ii) define the dynamics of the event that emplaced the bomb field, (iii) understand the relation between the explosive event and the subsequent distribution of bombs in terms of size and areal extent, and (iv) map the hazard posed by such common, but well-visited, events.

When do folds unfold during progressive shear?
Maria-Gema Llorens et al., Department of Geosciences, Eberhard Karls University Tübingen, Wilhelmstrasse 56, 72074 Tübingen, Germany. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33973.1.

Folds form when one pushes on a table cloth. When one pulls again, the folds disappear, without leaving a trace that the cloth was once folded. In a similar way, folds form in strong rock layers when contracted by tectonic forces. Would such folds also disappear without a trace if these forces were to be reversed? Could there be straight rock layers that were once folded? Maria-Gema Llorens and colleagues addressed this question with numerical finite-element simulations of folding and unfolding. They found that a single folded layer can unfold completely if its mechanical properties do not change between the shortening and stretching phase. Characteristic tight folds remain, however, if the layer becomes softer during the process. When a stack of layers with a variety of mechanical properties is folded, some layers may unfold, while others do not. The simulations show that folds in rocks may indeed unfold, but unlike an ironed table cloth, traces of the unfolding usually remain in rocks. The study provides indicators for recognizing unfolding in rocks, which is of importance to unravel their deformation history, for example, during mountain building.

Thermo-mechanical model for the finite strain gradient in kilometer-scale shear zones
Arthur Bauville (corresponding) and Stefan M. Schmalholz, Institute of Earth Sciences, University of Lausanne, CH-1015 Lausanne, Switzerland. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33953.1.

Ductile shear zones ranging from millimeter to kilometer scale are common structures in orogenic belts. Many of the kilometer-scale crustal shear zones with a thrust-type shear sense exhibit a similar nonlinear decrease of the finite shear strain with increasing distance from the shear zone's base. To explain this shear strain variation, Arthur Bauville and Stefan Schmalholz present an analytical one-dimensional thermo-mechanical model that describes the shearing of a fluid with temperature-dependent viscosity. The model is controlled by a single dimensionless parameter, β, which depends on the activation energy of the fluid and the temperature distribution in the shear zone. Least squares fits of measured natural strain data from several shear zones worldwide provide β-estimates for each shear zone and show that the model can describe the strain variation in the shear zones. Independent β-estimates based on temperature from field data and typical activation energies for rock agree with β-estimates from data-fitting showing that the model is also physically feasible. The study presents a testable thermo-mechanical shear zone model and shows that the temperature increase toward the shear zone's base and the related decrease in viscosity can be the dominant cause for the observed strain variation in kilometer-scale shear zones.

Noble metals potential of sulfide-saturated melts from the subcontinental lithosphere
Vadim S. Kamenetsky et al., ARC Centre of Excellence in Ore Deposits, and School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia; and Steinmann Institut, Universität Bonn, Bonn 53115, Germany. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34066.1.

The origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic. Vadim Kamenetsky and colleagues describe, for the first time, the occurrence of nickel-platinum-palladium-gold–rich sulfide droplets in a modern volcanic glass, dredged from the Atlantic seafloor. The parental high-Mg andesitic melt is derived from an ancient lithospheric source within the ambient convecting mantle. The melt is exceptionally enriched in Ni and saturated in Fe-Ni-PGE-Au sulfide at high magmatic temperature and crustal pressure. Sulfide saturation of this particular silicate melt coupled with silicate-sulfide liquid unmixing and strong partitioning of noble metals into the sulfide are helpful to our understanding of magmatic Ni-PGE sulfide deposits within the continents, especially a mantle source of silicate and associated sulfide melts parental to the ore.

Extremely high temperatures and paleoclimate trends recorded in Permian ephemeral lake halite
James J. Zambito IV (corresponding) and Kathleen C. Benison, West Virginia University, Department of Geology and Geography, P.O. Box 6300, Morgantown, West Virginia 26506-6300, USA. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34078.1.

Pangaea was hotter than anywhere on Earth today. This study by James J. Zambito and Kathleen C. Benison reconstructs extreme paleoweather conditions and paleoclimate changes recorded in Permian (~275 million year ago) bedded halite from the subsurface of Kansas by using fluid inclusion homogenization temperatures to directly measure the water temperature when the halite precipitated. This Permian halite has characteristics similar to halite that precipitates today in shallow ephemeral lakes, like in Death Valley, where the lake water temperature is an excellent air temperature proxy. In the Permian, maximum air temperatures were extremely hot, up to 73 °C (~165 °F), and daily temperature ranges were as large as 32 °C (~90 °F). Furthermore, paleoclimate trends recorded in this halite suggests that climate was variable, with relatively rapid transitions between these extremely hot and dry conditions and somewhat cooler conditions with maximum temperatures around 40 °C (~104 °F). The late Paleozoic deglaciation, from ~305 to 265 million years ago, when Earth was transitioning from an icehouse to a greenhouse climate state, is one of the best deep-time analogs for current and predicted climate change. The extremely high temperatures, wide temperature ranges, and relatively rapid climate transitions recorded in this halite provides new insights into the late Paleozoic deglaciation.

Imprint of climate and climate change in alluvial riverbeds: Continental United States, 1950–2011
Louise J. Slater (corresponding) and Michael Bliss Singer, Department of Earth and Environmental Sciences, University of St Andrews, Irvine Building, North Street, St Andrews KY16 9AL, UK. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34070.1.

It is commonly believed that the elevation of riverbeds is more or less constant, so any change in flood risk is due to changes in hydrology. However, Louise J. Slater and Michael Bliss Singer have found significant trends in the elevation of riverbeds at ~70% of sites studied across the continental USA, indicating that river channels are filling in with sediment or that sediment is being eroded through time. If the riverbed elevation goes up, this could increase flood risk, while if it bed elevation goes down, it would be reduced. Slater and Singer find that the magnitude of these trends is related to the variability in streamflow, which is higher in drier regions of the USA. Flow variability affects the quantity of sediment eroded from the drainage basin, and with climatic shifts expressed as changes to streamflow, there is potential for the evolution of streambed elevation to change. This is the first systematic documentation of decadal trends in streambed elevation on continental scales. The findings have important implications for the stability of riverine infrastructure, the navigability of rivers, the distribution of aquatic habitat, flood management, and the flood insurance industry.

Marine radiocarbon reservoir age variation in Donax obesulus shells from northern Peru: Late Holocene evidence for extended El Niño
Miguel F. Etayo-Cadavid et al., BP America Inc., Houston, Texas 77079, USA. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34065.1.

Analyses of clam shells used in ancient funeral ceremonies offer additional evidence as to how climate change may have contributed to the gradual collapse of an early South-American civilization. This research by Miguel F. Etayo-Cadavid and colleagues indicates that El Niño, a temporary, cyclical change in the Pacific's circulation, and an intertwined ocean phenomenon, known as upwelling, likely contributed to the 6th-century downfall of an advanced civilization called the Moche. The Moche once flourished along the northern coast of Peru. The research illustrates some climate-related challenges faced by the Moche around 540 A.D., prior to their collapse. It indicates that upwelling (a wind-driven act that brings deep water and nutrients back to the ocean's surface) diminished during this time-frame because of changes in El Niño patterns. This would have resulted in less seafood for the Moche and may have changed the types of fish species common in the area. Through radiocarbon dating of recovered shells and other materials left in excavated tombs and by studying the archaeological contexts of the burial tombs, the scientists can estimate the age of the shells and the relative amount of upwelling that occurred when the shells grew.

Rare earth element abundances in apatite in the Bushveld Complex -- A consequence of the trapped liquid shift effect
R.G. Cawthorn, School of Geosciences, University of the Witwatersrand, PO Wits, 2050, South Africa. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34026.1.

The Bushveld Complex, South Africa, formed by the slow cooling of the largest body of intruded magma in the world. During the cooling and crystallization, many complex interactions between the liquid and the formed crystals occurred. Predicting the behavior of elements with low concentrations in the liquid and their variable concentrations in the final solid rocks is complicated. One process that may occur during the cooling of this liquid is called liquid immiscibility (think of the cooling of a bowl of dirty washing-up water, which produces oily droplets on the surface). It has been proposed to have happened in the Bushveld Complex, and the evidence presented was based on the rare earth element (elements in the periodic table from Lanthanum to Lutetium in the periodic table, called REE) content of the mineral apatite. In this paper, R.G. Cawthorn shows that the liquid immiscibility model fails to explain many features. An alternative process is described here whereby the early formed apatite with a minor concentration of REE reacted with the surrounding liquid as it cooled, and that a high concentration of REE resulted by the apatite extracting further REE from the liquid. This model explains the reason for the anomalous low concentration of Europium in apatite that cannot be explained by the liquid immiscibility process.

Original spotted patterns on Middle Devonian phacopid trilobites from western and central New York
Christopher A. McRoberts et al., Geology Department, State University of New York at Cortland, P.O. Box 2000, Cortland, New York 13045, USA. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34158.1.

Although color (and colored patterning) is ubiquitous in the epidermis and exoskeletons of modern animals, preservation of color in the fossil record is exceptionally rare, with very few examples preserved within fossil skeletons. Christopher McRoberts and colleagues and students from SUNY Cortland and Western Illinois University document some exceptionally preserved fossil trilobites belonging to Eldredgeops rana that exhibit original spotted markings embedded in their exoskeleton. These exquisitely preserved trilobites collected from the Middle Devonian of western and central New York exhibit distinctive biologic patterning and are preserved as low-Mg calcite spheres that likely served as windows through a pigmented exoskeleton to the underlying epidermis of the trilobites. Spot patterning, composition, and structure rule out previous hypotheses of muscle attachment sites or diagenetic artifacts. The leopard-like patterns would have served a light disrupting camouflage.

Geochemical controls on shale microstructure
John J. Valenza II et al., Schlumberger-Doll Research, 1 Hampshire Street, Cambridge, Massachusetts 02139, USA. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33639.1.

John Valenza and colleagues utilize conventional characterization and processing techniques to quantify the effects of composition and natural processing on the structure of shale. As a result, they find that the shale is a unique natural material in that its physical structure is highly dependent on the generation and expulsion of mobile hydrocarbons. By intuitive extension, Valenza and colleagues demonstrate that this process also governs the behavior (e.g., mass transport or interaction with fluids of varying nature) of these systems.

Eocene exhumation of the Tuareg Shield (Sahara Desert, Africa)
Sylvain Rougier et al., UMR 8148 IDES, bât. 504, Université Paris Sud XI, 91405 Orsay, France. Posted online 18 March 2013; http://dx.doi.org/10.1130/G33731.1.

Sylvain Rougier and colleagues have published the first apatite (U-Th)/He datings on the Tuareg Shield, a substratum swell of up to 1000 km in diameter, associated to Cenozoic volcanism, in the Saharan desert. These results show that the currently outcropping basement underwent an exhumation of 1 to 3 km during the Eocene, before any volcanic occurrence. Moreover, given the presence of Mesozoic sediments near some of the analyzed samples, the authors propose that the samples were previously buried beneath a plurikilometric sedimentary cover, which would have been deposited between the Upper Cretaceous and the Eocene.

Climate change increases frequency of shallow spring landslides in the French Alps
Jérôme Lopez Saez et al., Institut National de Recherche en Sciences et Technologies pour l'Environnement et l'Agriculture, UR EMGR, 38402 St-Martin-d'Hèrescedex, France. Posted online 18 March 2013; http://dx.doi.org/10.1130/G34098.1.

In this contribution, past process activity is reconstructed on seven landslide bodies of the Riou Bourdoux catchment (southeastern French Alps). Based on an unusually dense data set of 3,036 tree-ring series extracted from 759 conifers, Lopez Saez and colleagues provide evidence for 61 landslide reactivations since A.D. 1898. Based on logistic regressions and threshold analyses of monthly rainfall data and temperature anomalies, they determine that the combination of snow-rich winters and positive temperature anomalies in spring (enhanced snowmelt) seems to have driven landslide reactivations in the past. Since the early 1990s, however, landslide reactivations clearly have been on the rise and thereby exhibit excessive and unprecedented rates of activity at the scale of the Riou Bourdoux catchment. From the data, evidence exists for a shift from snowmelt-induced landslides (controlled by winter precipitation) to reactivations controlled by spring temperatures. Therefore, this contribution also adds evidence to the hypothesis that climate change (and related warmer springs) could further enhance landslide activity in the course of the 21st century.

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