Boulder, Colo., USA – Locations studied for this month's posting of Geology articles include New Zealand's Taupo Volcanic Zone; Llaima volcano, Chile; the Mississippi Fan (Gulf of Mexico); the eastern equatorial Pacific Ocean; Wax Lake Delta, Louisiana; the Atlas Mountains of Morocco; the East Antarctic Ice Sheet; southern Tibet; the Longmenshan fault, Wenchuan, China; the Regab pockmark, offshore Africa; the Siberian Traps; the eastern California shear zone; Cabo de Gata, southern Spain; and the northwest Borborema province, Brazil.
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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Conditions of melt generation beneath the Taupo Volcanic Zone: The influence of heterogeneous mantle inputs on large-volume silicic systems
Tyrone O. Rooney, Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA, rooneyt@msu.edu; and Chad D. Deering, Department of Geology, University of Wisconsin-Oshkosh, Oshkosh, Wisconsin 54901-8649, USA. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34868.1.
Establishing the origin of the magmas that create the massive eruptions common in New Zealand's Taupo Volcanic Zone requires a knowledge of processes active in the Earth's interior. Previous studies have noted a spatial variation in eruptive compositions in the Taupo Volcanic Zone. In this contribution we link these spatial variations in eruptive compositions to processes that are active deep beneath New Zealand. These processes, which are related to the continual pushing of the Pacific Plate deep into the Earth beneath New Zealand, reveal differences in the fluids contributing to magmas being created. Differences in magma composition are retained as they rise through the Earth's crust, evolve, and eventually erupt to the surface as lavas and large, pyroclastic flows. The results of our research provide the first direct linkage between deep processes and the production of the voluminous eruptive products characteristic of the Taupo Volcanic Zone.
Plate tephra: Preserved bubble walls from large slug bursts during violent Strombolian eruptions
Dawn C.S. Ruth (corresponding) and Eliza S. Calder, (Ruth and Calder) Department of Geology, 411 Cooke Hall, State University of New York University at Buffalo, Buffalo, New York 14260, USA, dcs34@buffalo.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34859.1.
Volcanologists use the characteristics of eruptive products to glean information about volcanic processes. When new types of products are identified, it is possible to further refine the current understanding of the physical volcanic processes, either in the conduit or in the eruption plume. Ruth and Calder characterize an uncommon, erupted particle, called plate tephra, produced during the 2008 eruption of Llaima volcano, Chile. These particles are plate-shaped scoria, which sometimes have other fragments adhered to their outer surfaces. Internally, large crystals are commonly aligned. The plate tephra have been found as far as 9 km away from the summit. Similar particles have been found at other volcanoes that experience the same type of eruption. The authors propose that these particles are the remnants of large bubble walls that burst during the violent, initial phase of the eruption. Their presence in deposits from Llaima and at other volcanoes indicates that the plate tephra may be diagnostic of these types of violent eruptions.
Continuous deep-seated slope failure recycles sediments and limits levee height in submarine channels
Derek E. Sawyer et al., Department of Earth and Environmental Sciences, University of Kentucky, 101 Slone Building, Lexington, Kentucky 40506-0053, USA, derek.sawyer@uky.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34870.1.
Oceanic channel-levee systems meander across the ocean floor for hundreds of kilometers and are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. A key characteristic of these channels is the great thickness of the channel-bounding levees, which in most cases reaches several hundred 100 meters taller than the surrounding seafloor. The stability of the levee is critical to maintain and open conduit. Existing models generally assume that levees aggrade in response to the cumulative overspill of in-channel turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee height in channels on the Mississippi Fan (Gulf of Mexico) is limited by the mechanical strength of the levee, not the flow behavior. Our model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee growth and high fluid pressure in the underlying strata. We further show that levee failures reintroduced previously deposited levee material back into the channel system, which is a new and important finding for understanding channel and fan evolution.
Middle Miocene climate cooling linked to intensification of eastern equatorial Pacific upwelling
Ann Holbourn et al., Institute of Geosciences, Christian-Albrechts-University, D-24118 Kiel, Germany,
ah@gpi.uni-kiel.de. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34890.1.
During the middle Miocene, Earth's climate transitioned from a relatively warm phase (Miocene climatic optimum) into a colder mode with re-establishment of permanent ice sheets on Antarctica, approximately 14 million years ago. Understanding this fundamental step in Earth's climate evolution provides a long-term perspective to explore the range of natural variability and to evaluate the main processes driving climate development in the absence of large northern-hemisphere ice sheets. We present high-resolution climate records from the eastern equatorial Pacific Ocean, one of the most climate sensitive regions, which track changes in primary productivity and ice volume with unprecedented resolution over this remarkable transition. Our results show that the Miocene climatic optimum was characterized by high-amplitude climate fluctuations, marked by major perturbations of the carbon cycle. Our findings also support the view that increased primary productivity due to enhancement of eastern equatorial Pacific upwelling played a role in fostering atmospheric CO2 drawdown and climate cooling. Thus, our study highlights the tight coupling of high and low latitude climate systems and the dynamic role of the eastern equatorial Pacific Ocean as climate regulator during warmer phases of Earth's history.
The importance of erosion in distributary channel network growth, Wax Lake Delta, Louisiana, USA
John B. Shaw (corresponding) and David Mohrig, Jackson School of Geosciences, University of Texas at Austin, C9000, Austin, TX 78712, USA, jshaw@mail.utexas.edu; john.burnham.shaw@gmail.com. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34751.1.
The development of distributary channel networks on river deltas has profound implications for human geography, ecology, hydrocarbon reservoirs, and interpreting ancient environments, yet direct measurements of their formation remain scarce. In this paper, Shaw and Mohrig collected four 3-D bathymetric surveys at the tips of distributary channels on the Wax Lake Delta over 20 months, to directly measure channel growth. Distributary channels extend 2 to 6 km beyond their sub-aerially exposed levees, creating an extensive region for flow to leave channels laterally. Distributary channel extension patterns were found to vary with river discharge. During low river discharge, three measured distributary channels extended basinward by eroding at each channel tip. During high river discharge, only one channel extended basinward, while two retreated as their previous courses were filled with sediment. The single extending channel became dominant in terms of cross-sectional area at this time. The authors identify the distributed channel extension through erosion at tips and the single channel dominating extension as a two key morphological patterns that occur in the distributary channel networks of many modern river deltas.
Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains
Meghan S. Miller (corresponding) and Thorsten W. Becker, Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, MC704, Los Angeles, California 90089, USA, msmiller@usc.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34959.1.
Huge mountains, such as the Himalayas, form by thickening of the outermost, crustal layer of the Earth when tectonic plates collide. However, lateral variations in strength within the plates, which may be the consequence of ancient geological processes, appear to guide where mountains form, even within the continents themselves. The Atlas Mountains of Morocco are a prime example of such anomalous mountains in continental Africa. We present new evidence from a seismological experiment across the Moroccan Atlas Mountains for localized, deformation at the flanks of the mountain range. We find recent, near-vertical deformation that extends through the entire African plate. This deformation coincides with ancient faults on the surface, that originally formed when Africa rifted away from North America nearly 200 My ago, and have recently been reactivated. The mechanism for making the Atlas appears to be weak and thinned plate, and the cause is an anomalously hot layer beneath the plate, which also explains recent volcanism in the region.
Glaciovolcanic evidence for a polythermal Neogene East Antarctic Ice Sheet
John L. Smellie et al., Department of Geology, University of Leicester, Leicester LE1 7RH, UK, jls55@le.ac.uk. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34787.1.
A debate about the relative stability of the East Antarctic ice sheet (EAIS) has raged for more than 30 years. Much of the discussion has focused on when the ice sheet made an irreversible step change from wet-based and dynamic ice in the warmer past, to the comparatively stable ice and hyper-arid polar desert conditions that occur today. New information has been obtained from volcanic centers scattered along 800 km of the Ross Sea margin of the Transantarctic Mountains in Antarctica. The centers interacted with ice during eruptions and the evidence they preserve demonstrates that a single unique step change never took place. Instead, the EAIS was in a "transitional" state during the past 12 million years. It maintained a spatially and temporally variable regime either with a basal water layer or without any water at its base, similar to the modern EAIS. An important shift is thus required in the prevailing paradigm describing the thermal state of the EAIS. The results of studies like this can help in forecasting the future behavior of ice sheets under changing climatic conditions.
Zircon xenocrysts in Tibetan ultrapotassic magmas: Imaging the deep crust through time
Dong Liu et al. (Zhidan Zhao, corresponding), State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34902.1.
Mantle-derived post-collisional ultrapotassic volcanic (UPV) rocks in southern Tibet carry not only information on mantle sources but also crustal signals within fragments entrained during ascent through the crust. These UPV rocks contain abundant zircon xenocrysts of pre-eruption age, offering a prime opportunity to reveal signatures of cryptic magmatic pulses, and also to image lithospheric thickening and crustal evolution through time. We use a combined in situ method of zircon chronology, trace element, and Hf isotope geochemistry, and present a systematic study of these xenocrystic zircons. Zircon data set shows a more detailed pattern of mantle input and crustal thickening in the Central Lhasa sub-terrane than those revealed by surface rocks, and, as a case study. Similar age (Proterozoic-Paleozoic) distributions between these zircon xenocrysts and those in the Lhasa Terrane detrital spectra demonstrate the continental origin of xenocrystic zircons with high U/Yb. Zircon Epsilon Hf(t) display three major magmatic pulses at ~90 million years ago (Ma), ~50 Ma, and ~20 Ma, suggesting the significant crustal growth events in the Lhasa Terrane at those times. The increasing DyN/YbN and U/Yb ratios since ~55 Ma are interpreted to reflect significant and progressive crustal thickening in response to India-Asia convergence.
Gouge graphitization and dynamic fault weakening during the 2008 Mw 7.9 Wenchuan earthquake
Li-Wei Kuo et al., National Taiwan University, Taipei City, Taiwan, and State Key Laboratory of Continental Tectonic and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China, liweikuo@ntu.edu.tw, liweikuo@gmail.com. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34862.1.
The Longmenshan fault that ruptured during the 2008 Mw 7.9 Wenchuan (China) earthquake was drilled to a depth of 1200 m, and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575� m. We report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths. Results indicate that the Longmenshan fault at this locality is extremely weak at seismic slip rates. In situ synchrotron X-ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products, similar to the occurrence of graphite in the natural principal slip zone of the 2008 Wenchuan rupture. We surmise that graphitization occurred due to frictional heating of carbonaceous minerals. Because graphitization was associated with strong dynamic weakening in the experiments, we further infer that the Longmenshan fault was extremely weak at borehole depths during the 2008 Wenchuan earthquake, and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust.
Stylolites in limestones: Barriers to fluid flow?
Michael J. Heap et al., École et Observatoire des Sciences de la Terre, Université de Strasbourg (UMR 7516 Centre National de la Recherche Scientifique), F-67081 Strasbourg cedex, France, heap@unistra.fr. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34900.1.
Stylolites -- products of intergranular pressure-solution -- are laterally extensive, planar features. They are a common strain localization feature in sedimentary rocks. Their potential impact on regional fluid flow has interested geoscientists for almost a century. Prevalent views are that they act as permeability barriers, although laboratory studies are extremely rare. Here we report on a systematic laboratory study of the influence of stylolites on permeability in limestone. Our data demonstrate that, contrary to conventional wisdom, the studied stylolites do not act as barriers to fluid flow. In detail, when a stylolite occurs perpendicular to the direction of flow, the permeability simply follows the same power law permeability-porosity trend as the stylolite-free material. We show, using a combination of high-resolution (4 µm) X-ray computed tomography, optical microscopy, and chemical analyses, that the stylolites of this study are not only perforated layers constructed from numerous discontinuous pressure solution seams, but comprise minerals of similar or lower density to the host rock. The stylolites are not continuous high-density layers. Our data affirm that stylolites may not impact regional fluid flow as much as previously anticipated.
Fluid flow regimes and growth of a giant pockmark
Yann Marcon et al., MARUM, University of Bremen, Leobener Str., 28359 Bremen, Germany, ymarcon@marum.de, yann.marcon@gmail.com. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34801.1.
Fluid escape and release of methane from the seabed is a widespread phenomenon in the world's oceans, which is believed to contribute noticeably to the global carbon budget. Methane release has also more direct and localized impacts at the seafloor. However, the extent to which such impacts may influence the seepage intensity is still unclear, and the contribution of seabed fluid flow to the global carbon budget remains poorly constrained. In order to better understand the dynamics of seabed fluid seepage, researchers from the University of Bremen (Germany) and IFREMER (French Oceanographic Institute) visited the Regab pockmark, a large seafloor depression associated with fluid escape, located at 3160 m water depth offshore Africa. Using state-of-the-art imaging techniques they mapped the entire site and provided an unprecedented detailed view of the pockmark. They found that the pockmark is composed of two different zones that reflect the occurrence of two distinct fluid flow regimes within the sub-seafloor. Based on their observations, they explain how the seepage of methane shapes the environment and controls the faunal distribution. In particular, they suggest that methane seepage may trigger self-sealing processes that cause rising fluids to laterally spread and the pockmark to expand.
Acid rain and ozone depletion from pulsed Siberian Traps magmatism
Benjamin A. Black et al., Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, bablack@mit.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34875.1.
The eruption of the Siberian Traps was one of the largest volcanic cataclysms of the past 500 million years, and it has been linked with the end-Permian mass extinction. The eruption probably occurred in episodic pulses over the course of a million years. This paper describes results from a global climate model simulations of the Earth's climate and atmospheric chemistry 252 million years ago. The study presents global maps of the acid rain and ozone depletion that could have resulted from pulses of magmatism. During volcanic episodes, rain over much of the northern hemisphere was so acidic it resembled fresh-squeezed lemon juice. Repeated ozone collapses may have exposed land plants and animals to much higher levels of ultraviolet radiation. The authors argue that in conjunction with global warming, acid rain and ozone depletion could have linked magmatism to the end-Permian mass extinction on land.
How much can off-fault deformation contribute to the slip rate discrepancy within the eastern California shear zone?
Justin W. Herbert et al., Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA, jherbert@geo.umass.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34738.1.
Movement between the Pacific and North American plates is primarily accommodated by localized slip along the San Andreas fault and faults across the eastern California shear zone (ECSZ) within the Mojave Desert. While we can measure slip on faults, it is very difficult to directly measure the deformation going on between the faults. We use three-dimensional mechanical models to simulate the active deformation in the region and assess how much strain occurs between the faults. First, we modified existing fault models to better represent the disconnected nature of active faults in the ECSZ. The models with revised fault geometry produce fault slip rates that better match measured slip rates from geologic investigations. Within the revised model 40% ± 23% of the total strain across the ECSZ is accommodated in the rock between the faults. Because GPS stations in the region move in response to both fault slip and off-fault deformation, models that use GPS to determine fault slip could over-estimate slip rates. Within the ECSZ geologic slip rates are less than half the slip rates estimated from models that invert slip from GPS velocities. Consequently, a significant portion of the discrepancy between the geologic and geodetically modeled slip rates in the ECSZ could be due to permanent off-fault deformation.
Amino acid ratios in reworked marine bivalve shells constrain Greenland Ice Sheet history during the Holocene
Jason P. Briner et al., Department of Geology, University at Buffalo, State University of New York, Buffalo, New York 14260, USA, jbriner@buffalo.edu. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34843.1.
Reconstructing past changes of the Greenland Ice Sheet is important for constraining its sensitivity to warm intervals longer than those contained within the brief historic interval. In particular, constraining the size of the Greenland Ice Sheet during the warm middle Holocene is key for quantifying ice sheet sensitivity to climate change. However, because the ice sheet was smaller than it is today during the middle Holocene, most evidence of its past fluctuations is buried beneath its present footprint. We developed a procedure that mines the bulldozed piles of debris that the ice sheet scooped out of the ocean during recent fluctuations. We search these piles of debris for marine shells, which provide evidence that the ice sheet was smaller than it is today. Our method quantifies the configuration of amino acids contained in the shells; the configuration changes as shells age. Thus we are able to develop histories of past ice sheet fluctuations, even during times when the ice sheet was smaller than it is today. We found that the ice sheet was smallest between 3000 and 5000 years ago, which is lagged behind the time of warmest atmospheric temperatures, but similar to the time of warmest oceanic temperatures.
Hyaloclastite fragmentation below the glass transition: An example from El Barronal submarine volcanic complex (Spain)
Massimiliano Porreca et al., Istituto Nazionale di Geofisica e Vulcanologia, 67100 L'Aquila, Italy, massimiliano.porreca@ingv.it. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34744.1.
We investigated volcanic products of an ancient submarine volcanic eruption occurred in Cabo de Gata, in the western Mediterranean sea (southern Spain). The eruption occurred during Miocene (around 12 million years ago). The deposits of this eruption are now visible along excellent and continuous marine cliff exposures, which allowed investigations on emplacement mechanisms of lavas in a subaqueous environment. These processes are in fact poorly understood because of scarce direct observations of this kind of eruptions in submarine conditions. We applied a paleomagnetic study to understand how the magma interact with water producing huge amount of pyroclastic products, called hyaloclastite. The main target of this study was to determine the thermal history of lava fragmentation and hyaloclastite production. The paleomagnetic results, integrated with estimation of glass transition (liquid-solid behavior transition) temperatures, indicate that the fragmentation progressed down to very low and unexpected temperatures (210 to 390 degrees Celsius). We conclude that hyaloclastite fragmentation in thick lavas may occur over most of the cooling history, as a result of the progressive access of sea water toward the lava interior by development of a complex network of contraction fractures.
Electromagnetic constraints for subduction zones beneath the northwest Borborema province: Evidence for Neoproterozoic island arc–continent collision in northeast Brazil
Antonio L. Padilha et al., Instituto Nacional de Pesquisas Espaciais—INPE, C.P. 515, 12201-970 São José dos Campos, Brazil, padilha@dge.inpe.br. Published online ahead of print on 22 Nov. 2013; http://dx.doi.org/10.1130/G34747.1.
Correlation attempts between northeast Brazil and West Africa in the complex Neoproterozoic West Gondwana reconstruction have been hampered because key links in the internal structure of northeast Brazil have yet to be identified. A magnetotelluric study undertaken in northeast Brazil provides models of the crustal–upper mantle electrical conductivity beneath this region and allows mapping, for the first time, of dipping resistive features corresponding to remains of Neoproterozoic collision and subduction activities. On the basis of geological and geochemical information, a model of collision of an intraoceanic magmatic arc coalesced into an earlier passive margin is proposed for the Neoproterozoic tectonic evolution of this area, involving processes of reversal of subduction polarity and oceanic slab breakoff. These results support the presence of an ancient seaway in northeast Brazil that was closed by the assembly of West Gondwana in the Neoproterozoic.
Journal
Geology