From the Alps to the Deep Mantle

October 18, 2012

Boulder, Colo., USA - Geology articles posted online ahead of print this month survey topography, minerals, faults and tectonics, alluvium, modeling, snowball Earth, fossils and extinction, and pyrite-filled worm burrows. One notable study provides a new eruption date for the Salton Buttes (Calif., USA) of 30,000 years later than that determined by earlier studies, coinciding with the appearance of the earliest known obsidian tools there.

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Pre-glacial topography of the European Alps
Pietro Sternai et al., Swiss Federal Institute of Technology (ETH), Zürich, Switzerland. Posted online 2 October 2012;

This article by Pietro Sternai and colleagues presents an innovative technique for reconstructing the topography of a mountain range prior to Quaternary glaciations. An estimate of the pre-glacial topography of a mountain range is valuable for two purposes: first, the difference between pre-glacial and modern topography provides an estimate of the magnitude and distribution of glacial erosion; and second, removing the glacial overprint could reveal important differences in regional topography reflecting tectonic processes and precipitation patterns prior to glaciation. Sternai and colleagues apply this technique to the European Alps. The comparison between the reconstructed pre-glacial topography and the present-day Alpine landscape suggests that most glacial erosion has occurred in the lower and peripheral parts of the Alpine catchments. The inferred pattern of pre-glacial channel steepness is characterized by a region of high values in the western Alps, which cuts across lithologic and tectonic boundaries. The lack of evidence for young tectonic activity or lower precipitation rates restricted to this area suggests a potential change in regional isostatic support of the western Alps, for example by detachment of the European slab. The inferred pattern of pre-glacial channel steepness across the rest of the range can also be related to rock uplift, climate or erosion patterns prior to glaciation.

Phanerozoic sanukitoids from Caledonian Scotland: Implications for Archean subduction

Mike Fowler, Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK; and Hugh Rollinson. Posted online 2 October 2012;

Sanukitoids represent a volumetrically minor but important series of Neoarchean granitoids enriched in large-ion lithophile elements (e.g., Ba, Sr, and light rare earth elements), and with relatively high compatible elements (e.g., Mg, Ni, and Cr). Petrogenetic models have favored an origin in the suprasubduction mantle wedge, so their sudden appearance ca. 2.95-2.68 billion years ago has made them central to the ongoing debate about the beginnings of modern plate tectonics. Caledonian high Ba-Sr granites from the Northern Highlands of Scotland are petrological and compositional equivalents. The Caledonian examples have not undergone subsequent reworking, and their petrogenetic setting can be used to inform genetic models for sanukitoids. Prolonged subduction preceded emplacement of the high Ba-Sr granites in a short interval (about 10 million years ago) at the end of the Caledonian orogeny. Sediment subduction was responsible for elemental and isotopic enrichments and slab breakoff triggered melting in the subcontinental lithospheric mantle. This implies that during the Neoarchean the evolving tectonothermal regime moved the locus of melting from the basaltic slab to the sediment-infused mantle wedge, creating the first subcontinental lithospheric mantle. Sanukitoids may then have resulted from widespread, closely subsequent slab-breakoff events, producing evolved magmas by familiar fractionation mechanisms. Careful study of secular variations in high Ba-Sr magmas could constrain the evolution of the subcontinental lithosphere.

Onset of aseismic creep on major strike-slip faults
Ziyadin Çakir et al., Istanbul Technical University, Dept. of Geology, 34469, Maslak, Istanbul, Turkey. Posted online 2 October 2012;

Spaceborne geodetic data and field observations reveal that the central section of the MW = 7.4, 1999 Izmit (Turkey) earthquake segment of the North Anatolian Fault (NAF) began creeping aseismically following the earthquake. Global Positioning System measurements suggest that the aseismic surface creep may continue for decades and possibly until late in the earthquake cycle (250-350 years along the NAF), implying that stable surface creep observed along some major strike-slip faults may be triggered by large earthquakes. Geodetic measurements show that a significant amount of elastic strain has been released along the central section of the Izmit fault during the last decade, retarding stress accumulation on the fault as a whole and causing stress concentrations on locked portions of the fault, particularly at its tips. Accordingly, models of stress transfer need to include the additional changes in stress caused by aseismic fault creep that can promote or retard failure on adjacent faults. This is particularly important for the seismic gap segment of the North Anatolian fault in the Sea of Marmara south and west of Istanbul that is expected to be hit by a destructive earthquake within a couple of decades

Model shows that rivers transmit high-frequency climate cycles to the sedimentary record
Guy Simpson, University of Geneva, Section of Earth and Environmental Science, Rue des Maraîchers 13, 1205 Geneva, Switzerland; and Sébastien Castelltort. Posted online 2 October 2012;

Rivers are the main means by which terrigenous sediments are transported to sedimentary basins. Although it is widely accepted that rivers are perturbed by climatic variability, some studies have shown that the sediment supply at the mouth of major rivers remains remarkably constant through time, suggesting that rivers strongly buffer (or dampen) sediment flux variability. These results are difficult to reconcile with other evidence for abrupt changes in the sediment flux recorded in the shallow marine sediments near the mouth of major river systems. Simpson and Castelltort present results of a physically based numerical model that show convincingly how sediment flux signals are transferred and modulated by alluvial rivers. They show that depending on the frequency and the nature of sediment flux variability (e.g., sediment flux vs. water flux), rivers may dampen or actually even amplify the sediment flux signal at the outlet. These results suggest that it may be possible to distinguish climatic and tectonic forcing by inverting variability recorded in the shallow marine sedimentary record.

Submarine transitional flow deposits in the Paleogene Gulf of Mexico
Ian A. Kane, Statoil ASA, Research Centre Bergen, NO-5020 Bergen, Norway; and Anna S.M. Pontén. Posted online 2 Oct. 2012;

Gravity-driven flows on the seafloor are the largest, yet least well understood, sediment transport agents on Earth. Recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types, many hundreds of kilometers from paleocoastlines. These sedimentary deposits often defy conventional turbidite or debrite interpretations, having a character suggestive of deposition from flows with transient turbulent-laminar rheologies. Ian Kane and Anna Pontén present a process-based model that may be applicable to many deep-water settings and provides a framework for interpreting the stratigraphic and spatial distribution of these complex deposits.

Zn isotope evidence for immediate resumption of primary productivity after snowball Earth
Marcus Kunzmann et al., Dept. of Earth and Planetary Sciences/GEOTOP, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada. Posted online 2 October 2012;

The snowball Earth hypothesis explains unusual geological features that can be observed in the rock record with a global glaciation about 635 million years ago. It postulates an almost complete cessation of life on Earth due to uninhabitable conditions on a frozen planet. This raises the question when life recovered. A zinc isotope study of carbonate rocks from Australia, deposited in the immediate aftermath of the snowball Earth, indicates that primary producers recovered upon melting of the ice sheets. This means these microorganisms spread out in shallow marine environments as soon as the sea ice collapsed and, therefore, life immediately recovered from a global glaciation.

Diversity trends in the establishment of terrestrial vertebrate ecosystems: Interactions between spatial and temporal sampling biases
Roger B.J. Benson, Dept. of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK; and Paul Upchurch. Posted online 2 October 2012;

Vertebrates invaded land around 350 million years ago. Early diversification was unstable, apparently interrupted by several minor mass extinction events as terrestrial vertebrates got a foothold, diversifying until the present day to produce more than 25,000 species. This study by Roger Benson and Paul Upchurch shows that the early stages of this diversification were gradual. However, the fossil record, as currently known, provides only limited insights, because only tropical animals are known from the earliest intervals ("Late Carboniferous and Early Permian"), and most fossils of the "Middle and Late Permian" are from cooler, temperate latitudes, which contained fewer species, and especially fewer amphibians. This switch in how paleontologists have sampled fossil data has been interpreted as an extinction event called Olson's extinction, approximately 270 million years ago. However, it is more likely to be an artifact of uneven sampling.

Coseismic recrystallization during shallow earthquake slip
S.A.F. Smith et al., Istituto Nazionale di Geofi sica e Vulcanologia (INGV), Via di Vigna Murata 605, 00143 Rome, Italy. Posted online 15 October 2012;

When rocks slide against each other during earthquakes, the heat produced by friction is sometimes sufficient for rock melting. Once the rock melt cools down and hardens, it produces an earthquake glass that geologists call "pseudotachylyte." These glasses provide important evidence for fossil earthquakes in the geological record, but unfortunately they are quite rare and don't form in sedimentary rocks like sandstone and limestone. In this paper by S.A.F. Smith and colleagues, a new type of earthquake scar has been identified in limestone rocks. The authors used the world's most powerful rock friction machine (SHIVA) to recreate earthquake-like movements in the laboratory. They found that the frictional heat produced during earthquakes causes limestone to flow in a plastic way for only a few seconds, forming rock textures similar to those in deeper, much hotter parts of Earth's crust. Remarkably, the authors also identified very similar plastic flow of limestone in a natural fault in Korea. This newly identified type of earthquake scar will allow geologists to make significant progress understanding the complex physical and chemical processes that occur during earthquakes.

Weakening of the slab-mantle wedge interface induced by metasomatic growth of talc
Ken-ichi Hirauchi et al., High Pressure and Temperature Laboratory, Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA, Utrecht, Netherlands. Posted online 15 October 2012;

The rheology of the slab-mantle interface in subduction zones could potentially be controlled by weak hydrous minerals such as antigorite or talc, formed by reactions involving silica-rich aqueous fluids liberated during dehydration of the subducting slab. To determine the effect of Si-metasomatism on the rheological properties of antigorite, K. Hirauchi and colleagues performed a series of frictional sliding experiments on simulated, antigorite-bearing fault zones under silica-rich hydrothermal conditions. They found that the antigorite samples exhibit significant strain weakening toward the strength of pure talc. The weakening was mainly due to the development of thin, talc-rich shear zones. These results demonstrate that in the lowermost part of the forearc wedge, metasomatically produced talc slip surfaces or shear bands will form in the intensely sheared plate interface, causing a much larger weakening effect than expected for antigorite, even if the total amount of talc formed is minor (less than 10 vol%).

Solidus of alkaline carbonatite in the deep mantle
Konstantin D. Litasov et al., Dept. of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. Posted online 15 October 2012;

Minor amounts of alkalis (Na and K) can reduce drastically the solidus temperatures of carbonated silicate mantle, by as much as 400-500 degrees Celsius. Low-degree melting of carbonated peridotite and eclogite at pressures of 3-10 GPa produces Na- and K-bearing carbonatite melt. Mass-balance calculations of samples obtained below apparent solidi show clear deficits of alkalis, suggesting the presence of minor alkali-rich liquid or solid carbonate phases. In this paper, Konstantin Litasov and colleagues determine the true solidi in Na- and K-bearing carbonate systems and report the stability of alkaline carbonate phases. Melting of subducting alkaline carbonates would likely occur at transition zone depths to produce mobile carbonatite melt diapirs that migrate upward, modifying and oxidizing the upper mantle and initiating volcanism at the surface.

Syntectonic sedimentation effects on the growth of fold-and-thrust belts
Charlotte Fillon et al., Institut des Sciences de la Terre, Université Joseph Fourier, BP53, 38041 Grenoble, France. Posted online 15 October 2012;

To investigate the effects of syntectonic sedimentation on the external mountain ranges building, we use a numerical model that allows testing different settings of syntectonic sediment thicknesses and flexural parameters. Model results indicate a first-order control of syntectonic sedimentation on the thrust sheets geometry and kinematics. Thrust sheets are longer when syntectonic sediment thickness and/or flexural rigidity increase. Comparison with observations from several mountain belts confirms this first-order control of syntectonic sedimentation.

Petrological and Nd-Sr-Os isotopic constraints on the origin of high-Mg adakitic rocks from the North China Craton: Tectonic implications
Bin Chen et al., Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing 100871, China. Posted online 15 October 2012;

The Mesozoic high-Mg dioritic rocks in the North China Craton have been suggested to be part of adakitic rocks. The origin of the high-Mg diorites has been attributed to equilibration of partial melts from delaminated mafic crust (eclogite) with mantle peridotite. Bin Chen and colleagues present petrological and Os isotopic data against the delamination model and propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting for their origin. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively; and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Chen and colleagues note that the proposed model is probably applicable to the general mode of origin and tectonic settings of high-Mg adakitic magmas.

(U-Th)/He zircon and archaeological ages for a late prehistoric eruption in the Salton Trough (California, USA)
Axel K. Schmitt et al., Dept. of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA. Posted online 15 October 2012;

The latest eruption of the Salton Buttes, a chain of small volcanoes located in the center of Southern California's Imperial Valley, happened much more recently than previously thought. The Salton Buttes volcanoes were formed between 2,010 and 2,950 years ago by eruption of lava, which, in places, turned into obsidian during cooling. The decay of uranium in individual crystals taken from the Salton Buttes was used to date the eruption and to actually place it 30,000 years later than earlier studies had. Crystal ages also indicate that magma began to accumulate underneath the volcanoes for thousands of years prior to the latest eruption. This new eruption date also coincides with the appearance of the earliest known obsidian tools from Salton Buttes, whereas older prehistoric obsidian artifacts found in Southern California have been shown to come from other sources. Before uranium dating established this new date, it was theorized that the Salton Buttes and their obsidian deposits were submerged in ancient Lake Cahuilla, a precursor of the modern Salton Sea. But this new information shows that the exclusively late prehistoric obsidian use from Salton Buttes is due to a very recent eruption which was likely witnessed by humans in the area.

Iron isotope heterogeneity in pyrite fillings of Holocene worm burrows
Joonas J. Virtasalo et al., Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland. Posted online 15 October 2012;

Pyrite fills of worm burrows from Holocene lake sediments in the Baltic Sea basin demonstrate the strongest iron isotopic heterogeneity so far reported within individual iron sulfide mineral grains. Pyrite framboids (tiny spherical aggregates of microcrystals) that make up the cores of those pyrite fills were precipitated as a result of intense, localized microbial activity in the burrows. Iron isotopic composition of the framboids is extremely fractionated from the iron source materials (sedimentary iron hydroxides). With burial, the framboids were overgrown by poorly crystalline iron disulfide cement that displays less iron-isotopic fractionation, reflecting the 56Fe-enrichment of water remaining in the sediment pores and the preferential removal of 54Fe during the framboidal pyrite formation. Available data cannot discriminate between microbial and purely abiotic processes in the iron isotopic fractionation, yet microbial processes often result in faster dissolution of iron hydroxides. These results are supported by previous sulfur isotopic microanalyses of the same materials that demonstrate decreasing microbial activity but increasing sulfur isotopic fractionation in the framboids and cement formation sequence. Because iron and sulfur metabolisms are the earliest forms of microbial respiration, these results have value in the search for isotopic signatures of life in ancient Earth and extraterrestrial samples.

Legacy sediments and historic land use: Chemostratigraphic evidence for excess nutrient and heavy metal sources and remobilization
Jeffrey Niemitz et al., Dept. of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA. Posted online 15 October 2012;

The proliferation of late 18th to early 20th century mill dams in the Chesapeake Bay watershed lead to the detention of large volumes of runoff sediment. Early in this period, land use was predominantly agricultural, suggesting that the sequestered sediment may contain excess amounts of nutrients and heavy metals from fertilizers and other crop amendments. Over time, urban development has increased in the watershed and also contributed to the heavy metal inventory. This study compares two sub-watersheds of the Yellow Breeches Creek in Cumberland County, Pennsylvania, USA -- one that is dominated by forest and the other by agricultural land-use. Jeffrey Niemitz and colleagues determine from cores from legacy sediment deposits in each sub-watershed that nutrients and heavy metals in the agricultural land-use sub-watershed show significant increasing trends over background levels seen in the forested sub-watershed. The degradation and removal of many of these dams is remobilizing these sediments into downstream ecosystems including Chesapeake Bay with potential detrimental effects.

Geological Society of America

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