Boulder, Colo., USA - New Geology studies include a mid-Cretaceous greenhouse world; the Vredefort meteoric impact event and the Vredefort dome, South Africa; shallow creeping faults in Italy; a global sink for immense amounts of water on Mars; the Funeral Mountains, USA; insect-mediated skeletonization of fern leaves in China; first-ever tectonic geomorphology study in Bhutan; the Ethiopian Large Igneous Province; the Central Andean Plateau; the Scandinavian Ice Sheet; the India-Asia collision zone; the Snake River Plain; and northeast Brazil.
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High sea-surface temperatures during the early Aptian Oceanic Anoxic Event 1a in the Boreal Realm
Jörg Mutterlose et al., Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universitätsstrasse 150, 44801 Bochum, Germany. Published online 21 Mar. 2014; http://dx.
Paleotemperature changes, based on molecular and calcareous fossils, are reconstructed for a paleolatitude of about 39 degrees north for the onset of the mid-Cretaceous greenhouse world (about 122 million years ago). This period coincides with the Oceanic Anoxic Event 1a. So far reliable paleotemperature data are missing for the interval under discussion. (1) The integrated dataset indicates that "super greenhouse" conditions prevailed during this period at northern latitudes. It is for the first time that a distinctive warming peak is documented for about 39 degrees north. (2) Sea-surface temperatures of about 31 to 34 degrees Celsius are only slightly lower than those estimated for coeval low latitudinal sites. The data suggest that an equable warm climate, with reduced latitudinal gradients, characterized this period. (3) The high temperatures detected are explained by a major oceanic perturbation in the Pacific causing the huge Ontong Java volcanic province. The increased production of oceanic crust caused an increase of the CO2 emissions, giving way to the onset of the mid-Cretaceous greenhouse phase. At the same time an increase in humidity resulted in a high nutrient supply causing relative high productivity conditions as suggested by our calcareous fossil findings.
Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event
Matthew S. Huber et al., Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria. Published online 21 Mar. 2014; http://dx.
Meteorite impact craters are an important part of the geological record. However, prior to 2020 million years ago, when the Vredefort impact structure formed, the terrestrial record of impact craters is nonexistent. For the older impact cratering record, it is necessary to find materials that were ejected from the craters, such as impact spherules, which are mm-scale droplets of melted material. Many estimates of the size of impact craters have been made based on the size of impact spherules, but there has been little evidence to test the models. This paper by Matthew S. Huber and colleagues describes a previously unknown occurrence of impact spherules that have the same age as the Vredefort impact event, and describes the likelihood that the ejecta is derived from the event. This extends our understanding of the cratering record on Earth and provides an additional constraint on models of the relationship between impact craters and the impact spherules, which can be applied in the future to resolve outstanding questions in the field of impact crater research.
The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments
Fabrizio Balsamo et al., NEXT -- Natural and Experimental Tectonics Research Group, Department of Physics and Earth Sciences, Parma University, Parco Area delle Scienze 157A, 43124 Parma, Italy. Published online 21 Mar. 2014; http://dx.
Earthquakes occur episodically along shallow, creeping faults in poorly lithified sediments. This represents an unsolved paradox, largely due to (1) our poor understanding of the mechanics governing creeping faults and (2) the lack of documented geological evidence showing how coseismic rupturing overprints creeping, stable faults in near-surface conditions. In this paper, Fabrizio Balsamo and colleagues describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated sediments in the forearc Crotone Basin (south Italy). Field observations of deformation band-dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin black gouge layers. Compared to foliated cataclasites, black gouges possess much lower grain size/porosity/permeability and are characterized by distinct mineralogical assemblages, compatible with high temperatures (180 to 200 degrees Celsius) due to frictional heating during seismic slip. Foliated cataclasites and black gouges were also produced by laboratory friction experiments performed on host sediments at sub-seismic and seismic slip rates, respectively. The results of the study show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults, thus helping to understand the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels.
Evidence for widespread aqueous sedimentation in the northern plains of Mars
Mark R. Salvatore and Philip R. Christensen, School of Earth and Space Exploration, Arizona State University, P.O. Box 876305, Moeur Building, Room 131, 201 E. Orange Mall, Tempe, Arizona 85287-6305, USA. Published online 21 Mar. 2014; http://dx.
The massive networks of outflow channels are some of the most prominent fluvial features on Mars. However, the sedimentary deposits associated with these features have never been definitively identified due overprinting by subsequent geologic processes. This study by Mark R. Salvatore and Philip R. Christensen identifies a widespread sedimentary unit through a large portion of the martian northern lowlands by interrogating exposures in the walls of impact craters, which expose the subsurface stratigraphy. This unit, whose observed extent covers an area greater than that of Greenland, exhibits unique morphologies, spectral signatures, and associations with regional topography that all support an origin from the martian outflow channels. This is the first identification of such sedimentary deposits and the first to characterize their outcrop-scale properties. These properties all suggest that the northern lowlands of Mars served as a global sink for immense amounts of water and sediment resulting from outflow channel activity.
Jurassic Barrovian metamorphism in a western U.S. Cordilleran metamorphic core complex, Funeral Mountains, California
Thomas D. Hoisch et al., School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA. Published online 21 Mar. 2014; http://dx.
This study by Thomas D. Hoisch and colleagues establishes that major crustal shortening in the Sevier orogenic belt began earlier than previously thought. Large-magnitude shortening of Cretaceous age is well documented, but evidence for major Late Jurassic shortening has been lacking. This study combines two different dating methods with pressure-temperature paths from metamorphic rocks to document substantial burial of Late Jurassic age in the Funeral Mountains in eastern California. The Funeral Mountains are located in the hinterland of the Sevier orogenic belt and constitute a classic Barrovian metamorphic terrain. Pressure-temperature paths derived from chemical zoning in garnet indicate steep increases in pressure at temperatures of about 525 to 535 degrees Celsius, and are consistent with thrust loading. The age of the thrust loading was determined to be 158.2 plus or minus 2.6 million years old by dating the garnet using the Lu-Hf method. Cooling related to uplift and exhumation 146 to 153 million years ago, determined by dating muscovite using the 40Ar/39Ar method, shortly followed garnet growth. This study finds that the period of large-magnitude crustal shortening extends into the Late Jurassic, and may have closely followed the onset of east-dipping Franciscan subduction along the western margin of North America.
Proterozoic onset of crustal reworking and collisional tectonics: Reappraisal of the zircon oxygen isotope record
Christopher J. Spencer et al., NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. Published online 21 Mar. 2014; http://dx.
A newly compiled global database of uranium, lead, and oxygen isotopic analyses of zircon shows temporal changes in the magmatic record related to changes in the degree of crustal reworking. The delta-18O composition of bulk sediment remains relatively constant through geologic time. In contrast, the delta-18O values in magmatic zircons go from relatively low values in the Archean to increasingly higher and scattered values defining a series of peaks and troughs in post-Archean data wherein peaks of crustal reworking increases is associated with supercontinent assembly. Hence we attribute the pattern of post-Archean delta-18O values recorded by magmatic zircons to the onset of collisional tectonics, especially during formation of supercontinents.
Complex mineral zoning patterns caused by ultra-local equilibrium at reaction interfaces
Stacey Borg et al., CSIRO Earth Science and Resource Engineering, P.O. Box 1130, Bentley, 6102 Western Australia, Australia. Published online 21 Mar. 2014; http://dx.
Chemically zoned minerals are useful records of ambient conditions and bulk chemical composition of the fluid from which the minerals precipitate. In fluid-buffered systems, zoning of mineral compositions is expected to reflect directly the evolution of fluid composition. This study shows that during rapid fluid-rock reactions, ultra-local equilibrium can form complex mineral zoning patterns, even when the overall system is highly fluid buffered. Stacey Borg and colleagues reacted cleaved calcite single crystals with aqueous arsenate-phosphate solutions at 250 degrees Celsius and water-saturated pressure. They found that complex zoning patterns and solid solution between apatite and arsenate-bearing apatite that replace calcite formed within hours, and these zoning patterns were destroyed within days during secondary reactions. Borg and colleagues propose a two stage reaction process in the formation of the final reaction product. This paper highlights the complexity that can arise from ultra-local fluid composition variations due to rapid fluid-rock interaction in a short-lived event, for example during a seismic cycle. Interpretation of the complex zoning patterns to reflect the evolution of bulk fluid would not only be extremely complex but also erroneous.
Discovery of mafic impact melt in the center of the Vredefort dome: Archetype for continental residua of early Earth cratering?
C.L. Cupelli et al., Earth Science, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7, Canada. Published online 21 Mar. 2014; http://dx.
A Canadian, U.S., and British geoscience team has discovered remnants of a giant magma lake created by flash melting of the crust by the largest and oldest known meteorite impact crater, the ~250 km wide Vredefort structure, south of Johannesburg, South Africa. The melt bodies produced just over two billion years ago were long presumed erased by erosion and have gone unrecognized over generations of geological research in the famous area. However through a combination of careful field mapping and analysis of the highly shock resistant mineral zircon, doctoral candidate Lisa Cupelli and her team have shown that bodies of impact melt did indeed survive in the ancient rocks of the crater floor. The unit is enveloped in gneiss and recrystallized by crater adjustments so that it no longer looks like the conventional "Sudbury-style" melt rocks well known to geologists. The discovery opens a new door on Vredefort research, and aids the search for residua of older giant impacts believed, based on lunar records, to have melted and altered much of the Earth's first continental crust.
Evidence for insect-mediated skeletonization on an extant fern family from the Upper Triassic of China
Zhuo Feng et al., Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, China; and Key Laboratory of Tropical Forest Ecology Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla 666303, China. Published online 21 Mar. 2014; http://dx.
Leaf skeletonization represents a distinctive form of insect feeding behavior. It commonly occurs in angiosperm leaves after their initial appearance at about 145-million-year ago, the beginning of a geologic period called the Cretaceous. This type of feeding behavior rarely has been documented in older fossils. A new study by Yunnan University paleobotanist Zhuo Feng and colleagues present the earliest evidence of insect-skeletonized leaves which belong to the extant umbrella ferns in the 210-million-year old (a geologic period called the Triassic) Yipinglang flora from Yunnan Province, southwestern China. Very similar to the modern insect feeding habit, in the skeletonized area of the fossil umbrella fern leaves, the interveinal tissue is completely removed, exposing the leave veins forming polygonal meshes of varying size. This study of insect-mediated skeletonization of fern leaves from southwestern China not only fills a spatiotemporal gap in the currently known data on the paleo-geographical distribution and stratigraphic occurrence of plant-arthropod associations, but also sheds light on the antagonistic relationship between a fern host and its insect consumer during their early evolutionary stage.
Active tectonics of the eastern Himalaya: New constraints from the first tectonic geomorphology study in southern Bhutan
Théo Berthet et al., Géosciences Montpellier, UMR5243, Université Montpellier 2, Place E. Bataillon, 34095 Montpellier, France. Published online 31 Mar. 2014; http://dx.
The Himalayan arc, which extends more than 2500 km along the plate boundary between Indian and Eurasian plates, has experienced three great earthquakes during the last century. However the seismic behavior of the Bhutan region is unknown, because it corresponds to the only portion of the arc where no evidence of major earthquakes has been reported. In this study, the first of its kind in Bhutan, the analysis of offset fluvial terraces shows that two major earthquakes ruptured the Himalayan frontal thrust during the last millennium, and that a comparable rate of Holocene deformation (~20 mm/yr) is accommodated across the Himalaya in Bhutan as in central Nepal. This suggests that the propensity for great earthquakes in Bhutan is similar to what is observed in neighboring portions of the Himalaya arc.
The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates
Tyrone O. Rooney et al., Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA. Published online 31 Mar. 2014; http://dx.
This paper by Tyrone O. Rooney and colleagues presents geochemical results on the first highly silica under-saturated lavas ever discovered in the Ethiopian Large Igneous Province. These well-characterized lavas represent a heretofore hypothesized but unproven mantle end-member located within the regional lithospheric mantle. Their results demonstrate that melting of enriched domains within the African-Arabian lithospheric mantle is a viable mechanism for producing the HIMU-like magmas observed in the region. Rooney and colleagues show that these enriched HIMU-like lithospheric domains are isotopically distinct from the Afar plume and are instead the result of long-term evolution of amphibole-bearing metasomes associated with the Neo-Proterozoic construction of the regional lithospheric mantle. Their data shows, for the first time, that lithosphere-derived HIMU-like magmas may be distinguished from "C"-like Afar plume magmas, constraining the source of reservoirs that contribute to rifting. This is an important new result, not only because it gives new insight into the processes of melt generation within the lithosphere regionally, but also because it explains the geographically and temporally widespread distribution of HIMU-like magmatism found on the African-Arabian plates by melting of an easily-fusible component associated with the initial formation of the lithosphere.
Early Cenozoic uplift of the Puna Plateau, Central Andes, based on stable isotope paleoaltimetry of hydrated volcanic glass
Robin R. Canavan et al., Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. Published online 31 Mar. 2014; http://dx.
The Central Andean Plateau is one of the largest mountain plateaus on Earth, second only to the Tibetan Plateau. Unlike Tibet, the Central Andean Plateau is not the result of two continental plates colliding and thus the processes by which the Central Andean Plateau achieved its high elevation remains an area of debate. To date, most study has focused on the northern portion of the Central Andean Plateau, the Altiplano of Bolivia. Isotope ratios that reflect the temperature and water composition during the formation of carbonate minerals collected from the northern Altiplano suggest that the Central Andes were rapidly uplifted during the last 10 million years. Authors Robin R. Canavan and colleagues present new evidence from the Puna Plateau, the southern-most portion of the Central Andean Plateau, using hydrogen isotopes from volcanic ash deposited over the last 36 million years. Interpreting this geochemical data with regards to an atmospheric model that describes how isotopes vary with elevation, their results indicate that the Puna Plateau was already about 4 km high by 36 million years ago. Integrating the Puna Plateau and Altiplano data suggests a much more complex uplift history for the Central Andean Plateau than previously thought.
Asynchronous response of marine-terminating outlet glaciers during deglaciation of the Fennoscandian Ice Sheet
Chris R. Stokes et al., Department of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK. Published online 31 Mar. 2014; http://dx.
Recent studies show that marine-terminating outlet glaciers (e.g., in Greenland) are thinning, retreating and contributing to sea-level rise. This behavior has been linked to atmospheric and oceanic warming, but non-climatic factors such as the depth of their fjords, can also influence the rate of retreat, and there is uncertainty about which of these factors is most important. Moreover, observations are restricted to the last few decades, making it difficult to separate short-term variability from longer-term trends. This new study by Chris R. Stokes and colleagues uses radiocarbon dating and moraines to reconstruct the retreat of outlet glaciers during deglaciation of the Scandinavian Ice Sheet between 18,000 and 10,000 years ago. Interestingly, retreat rates averaged over several millennia (~30 m/yr) are less than half those observed on modern-day outlet glaciers (more than 100 m/yr), but there were episodes of rapid retreat (up to ~150 m/yr). More significantly, phases of rapid retreat were not synchronous between neighboring glaciers and most occurred irrespective of any atmospheric warming. Rather, the highest retreat rates occurred in deep, wide fjords. This implies that if scientists are to accurately predict the response of modern-day outlet glaciers over the next few centuries, there is an urgent need to measure the landscapes currently under the ice.
Miocene burial and exhumation of the India-Asia collision zone in southern Tibet: Response to slab dynamics and erosion
B. Carrapa et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 31 Mar. 2014; http://dx.
This manuscript presents the first multi-dating study of the Indus-Yarlung collision zone in southern Tibet. This study includes data from the Oligo-Miocene Kailas Formation and underlying Gangdese batholith rocks preserved along a ~600 km stretch of the collision zone. The IYS collision zone forms the boundary between the Indian and Asian plates, and marks the location of the greatest intercontinental collision during the past 300 million years of Earth history. Major mountain ranges flanking this collision zone have high relief and elevation, intense deformation, active seismicity, and thick crust. Although the region has been subjected to intense deformation leading to high elevation and great crustal thickness, the Kailas Formation contains evidence for crustal extension and deposition at relatively low elevation under a humid-tropical climate. The data presented here by B. Carrapa and colleagues document deep burial and subsequent high-magnitude exhumation of the Indus-Yarlung collision zone between 23 and 17 million years ago, which are best explained by southward rollback of India under Asia and subsequent renewed northward underthrusting. The documented amount of exhumation requires removal of a minimum of 180,000 cubic kilometers of rock from the collision zone, which is best explained by deep incision of the Yarlung River.
Cumulate fragments in silicic ignimbrites: The case of the Snake River Plain
Ben S. Ellis et al., Institute for Geochemistry and Petrology, ETH Zurich, NW Clausiusstrasse 25, 8092 Zurich, Switzerland. Published online 31 Mar. 2014; http://dx.
The Snake River Plain of southern Idaho contains a record of the early volcanism from the Yellowstone province in the form of voluminous rhyolitic ignimbrites and lavas. Within these deposits, crystals occur both as single, discrete grains ("phenocrysts") surrounded by melt and as aggregates of many individual crystals. Mineral chemistry indicates the presence of several populations, found both in phenocrysts and in the crystal aggregates. Hence, it appears that these large volcanic eruptions are the results of the amalgamation of multiple melt lenses during eruption. In order to effectively segregate melt batches prior to eruption and insulate them, rheologically strong and warm barriers are necessary. Ben S. Ellis and colleagues suggest that these "hot walls" are generated by melt removal from crystal-rich regions of the reservoirs. The melt then pools in individual, but nearby lenses, which co-erupt. The crystal aggregates represent fragments torn from the walls during eruption.
Neoproterozoic oceanic crust remnants in northeast Brazil
Fabrício Caxito et al., Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil; and Geotop, Université du Québec à Montréal, Montréal H3C 3P8, Canada. Published online 31 Mar. 2014; http://dx.
This paper reports the discovery of an ancient ocean within the northeast Brazilian highlands, which existed around 820 million years ago. Fragments of this ancient ocean were entrapped between two colliding tectonic plates during a mountain building process around 630 million years ago, in a similar way to modern-day collision of India and Tibet that generated the Himalayan mountains. This finding helps geoscientists to better understand how Earth's continent-oceanic distribution looked like in this very special time known as the Neoproterozoic era (1000 to 542 million years ago), when major climatic, geochemical and biological changes took place, with extreme global glaciations and culminating with the explosive diversification of complex life.