Boulder, CO, USA - Topics include: new evidence supporting the existence of mantle plumes; evidence from northwest Scotland of the largest meteorite ever to hit the British Isles; insights into California's still-active Long Valley caldera volcano; discovery of a microtektite field (microscopic impact glass particles) from Victoria Land Transantarctic Mountains; and long-term effects of the Chesapeake Bay impact structure on coastal Virginia. The GSA TODAY science article presents surprising evidence regarding depth and strength of Earth's tectonic plates.
Seismic imaging of subduction zone metamorphism
Stéphane Rondenay et al., Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue 54-512, Cambridge, Massachusetts 02139, USA. Pages 195-198.
Rondenay et al. seek to determine the depths at which fluids are released in subduction zones. This is an important objective because fluid release in such areas may trigger arc volcanism and intraslab earthquakes. By analyzing seismic images of the Alaska and Cascadia subduction zones in conjunction with thermal models and petrological constraints, they found that (1) seismic images provide a direct estimate of the depth range over which fluids are released in the system; (2) this depth range corresponds to that where the subducted crust transforms into eclogite, a high-pressure/temperature mineral assemblage; and (3) the location of this reaction is dependent on the thermal structure of the subducted plate.
Did magma ascent rate control the explosive-effusive transition at the Inyo Volcanic Chain, California?
Jonathan M. Castro, Smithsonian Institution, Mineral Sciences, National Museum of Natural History, 10th and Constitution Ave. NW, Washington, DC 20013, USA, and James E. Gardner, University of Texas at Austin, Department of Geological Sciences, 1 University Station C1100, Austin, Texas 78712-0254, USA. Pages 275-278.
Castro and Gardner’s paper describes natural and experimental evidence for the rage of magma rise beneath the Inyo Volcanic Chain, California. The authors found that magma may rise very slowly, yet despite these conditions, the magma may still explode violently.
Excess temperatures at ocean islands: Implications for mantle layering and convection
Keith Putirka, California State University–Fresno, Department of Earth and Environmental Sciences, 2576 E. San Ramon Avenue, MS/ST24, Fresno, California 93710-8039, USA. Pages 279-282.
Many volcanic ocean islands, like Hawaii, occur at locations that are far from plate boundaries, and are therefore not easily explained by plate tectonics. Morgan suggested that such volcanoes are the result of high-temperature thermal upwellings, called mantle plumes; these plumes presumably derive their heat from the core-mantle boundary. A consequence of this idea is that ocean islands, like Hawaii, tap a deep mantle source, bringing up material that lies at the base of Earth’s mantle. A common assumption is that a high 3He/4He ratio at ocean islands is a geochemical signal of this deep mantle source. These ideas have been controversial, but find support in new estimates of mantle temperatures at 28 ocean islands by Putirka. These temperature estimates show that the mantle beneath 27 of 28 volcanic ocean islands is hotter than ambient mantle by 115–290 °C. Mantle plumes are thus common, not rare. In addition, though, hotter plumes tend to have higher 3He/4He, which suggests that high 3He/4He is indeed a signal from the deep mantle; this means that Earth’s mantle, despite active convection, is compositionally layered. Finally, a comparison of mantle temperatures to Pb isotope ratios indicates that even the deepest mantle is not primitive or undifferentiated, but rather, it has been depleted by prior partial melting events, probably as a result of formation of the continental crust. Putirka’s paper thus indicates that plumes are common, they tap a deep mantle source, and the deep mantle is depleted.
Controls on sinuosity evolution within submarine channels
Ian A. Kane et al., Earth Sciences, University of Leeds, Leeds LS29JT, UK. Pages 283-286.
River channels have been the focus of study for centuries; however, processes within river channels familiar to schoolchildren, such as meandering and ox-bow lakes, are still very poorly understood when it comes to submarine channels. Submarine channels extend from the continental slope into deep-ocean basins and are built by the passage of turbidity currents carrying large volumes of terriginous sediment. The study of submarine channels has advanced greatly over the last 50 years or so with new techniques allowing us to visualize the sea floor. One thing that has struck scientists is the relative stability of submarine channels, in comparison to fluvial channels. To investigate this phenomenon, Kane et al. used experimental modeling to attempt to create a process-similarity with submarine channels. The results of the modeling suggest that submarine channels may be subject to deposition on either the inner or the outer bend of meander, which is very different from river channels where deposition occurs primarily at the inner bend, inducing meander-loop expansion. This mechanism may explain why submarine channels generally lack ox-bow lake–type features and may remain stable over large time periods.
Microtektites from Victoria Land Transantarctic Mountains
L. Folco et al., Museo Nazionale dell'Antartide, Universita' di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 287-290.
Folco et al. report on the discovery of a microtektite (microscopic impact glass particles) field from Victoria Land Transantarctic Mountains. Microtektites were found trapped in the local detritus accumulated in weathering pits and joints of several glacially eroded summits (approximately 2600 meters above sea level) distributed latitudinally for 520 kilometers. It is suggested that these microtektites define the southern extension of the Australasian tektite/microtektite strewn field. The margin of the Australasian tektite/microtektite strewn field is thus shifted southward by approximately 3000 kilometers, and the maximum distance from the putative parent impact site in Indochina increased by approximately 2000 kilometers. This emphasizes the paradox of the missing parent crater of the largest (more than 10 percent of the Earth’s surface) and youngest tektite-strewn field discovered on Earth.
Toroidal mantle flow through the western U.S. slab window
G. Zandt, University of Arizona, Department of Geosciences, Gould-Simpson Building, Building 77, Tucson, Arizona 85721, USA; and E. Humphreys, University of Oregon, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 291-294.
The circular pattern of anisotropic fast-axis orientations of split SKS arrivals observed in the western U.S. cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global-scale plate motion. A plume origin for this pattern accounts more successfully for the anisotropy field, but little evidence exists for an active plume beneath central Nevada. Zandt and Humphreys suggest that mantle flow around the edge of the sinking Gorda–Juan de Fuca slab is responsible for creating the observed anisotropy. Seismic images and kinematic reconstructions of Gorda–Juan de Fuca plate subduction have the southern edge of this plate extending from the Mendocino triple junction to beneath central Nevada, and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab, consistent with the observed pattern of anisotropy. This flow may enhance uplift, extension, and magmatism of the northern Basin and Range while inhibiting extension of the southern Basin and Range.
The roof of an axial magma chamber: A hornfelsic heat exchanger
Kathryn M. Gillis, School of Earth and Ocean Sciences, P.O. Box 3055 STN CSC, University of Victoria, Victoria, British Columbia V8W 3P6, Canada. Pages 295-298.
Mid-ocean ridges are dynamic features where magma generated within the mantle accumulates in axial magma chambers and builds the ocean crust. The heat released from these axial magma chambers drives the circulation of seawater-derived fluids through the porous crust. These fluids are ultimately released back into the ocean at hydrothermal vent sites. Gillis studied hornfelsic rocks that are the remnants of a critical boundary that separates the circulating hydrothermal fluids and axial magma chambers—the so-called conductive boundary layer. The characteristics of these rocks allow us to calculate the amount of heat transferred across the conductive boundary layer, and thus test theoretical models for how heat and mass are transported from Earth's interior to its surface.
A Precambrian proximal ejecta blanket from Scotland
Kenneth Amor et al., Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. Pages 299-302.
Scientists have found evidence of the biggest meteorite ever to hit the British Isles, in rock formations in northwest Scotland. Amor et al. believe a meteorite at least 500 meters in diameter hit the region about 1.2 billion years ago. Ejected material from the huge meteorite strike is scattered over an area about 50 kilometers across, roughly centered on the northern Scottish town of Ullapool. The evidence lies in a layer of rock that experts believe is the ejected material thrown out during the formation of a meteorite crater. Chemical testing of the rock found the characteristic signature of meteoritic material, which has high levels of the key element iridium, normally only found in low concentrations in surface rocks on Earth. The target rocks, analyzed under a microscope, also imply a meteorite strike, revealing tell-tale microscopic parallel fractures. These are formed during the very high pressures of a meteorite impact. This is the most spectacular evidence for a meteorite impact within the British Isles found to date, and the actual meteorite crater is thought to lie within the immediate vicinity, buried under younger rocks.
Porphyroblast rotation versus nonrotation: Conflict resolution!
T.H. Bell et al., School of Earth and Environmental Sciences James Cook University, Townsville, Queensland 4811, Australia. Pages 303-306.
Spiral shaped trails of inclusions in large garnet crystals, as well as other mineral phases with a similar large growth habit relative to the matrix (porphyroblasts), have always been thought of as a product of rotation of the crystal as it grew in a deforming rock. Two decades ago it was proposed that they formed without rotation of the porphyroblast and that the inclusion trails represented successively orthogonally overprinted planar structures (foliations). It was argued that these were overgrown by the garnet and could be used to access the full deformation history of the rock in the orientation in which it formed relative to the surrounding rock mass. Most geologists are highly skeptical of this, even though the data gathered on these structures in natural rocks supports this proposition. This skepticism is reinforced by the fact that over 99% of all theoretical, experimental, and computer modeling of this phenomenon resulted in rotation of the porphyroblast. The computer modeling presented by Bell et al. reveals, for the first time, that this nonrotation phenomenon does occur in the same geometric circumstances of deformation in which large porphyroblasts grow. This is a major scientific breakthrough and will eventually allow general acceptance of inclusion trails as a tool to enable understanding of many unresolved geologic phenomena.
Lithosphere erosion and crustal growth in subduction zones: Insights from initiation of the nascent East Philippine Arc
Colin G. Macpherson, Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK. Pages 307-310.
Subduction, where one lithospheric plate is pushed beneath another, provides a key driving force for plate tectonics and is believed to have played an important role in generating the continents. However, the way that subduction begins is poorly understood due to the lack of examples. Macpherson examines magmatism in a rare, active example of a nascent subduction zone in the eastern Philippines. The depth at which magma stalls and crystallizes depends on the thickness of the upper plate. In turn, this provides insights into production of the continental crust. Compositions of melts that reach the upper crust are sensitive to variations in crystallization depth, and the eastern Philippines demonstrate that deeper crystallization produces magmas of the type required to help build continental crust.
Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison
K. Panchuk et al., 134 Edmund Park, Saskatoon, Saskatchewan S7H 0Z4, Canada. Pages 311-314.
A sudden and extreme increase in global temperatures 55 million years ago has been attributed to the release of methane, a strong greenhouse gas, due to the destabilization of seafloor methane hydrate deposits by long-term global warming. This interpretation has dire implications for the consequences of modern day climate change. Panchuk et al.’s study suggests, however, that the distribution of deep-sea calcium carbonate sediments cannot be explained by methane from hydrate deposits alone. Instead, the release of carbon related to volcanism in Greenland, and the opening of the northern Atlantic Ocean, might come closer to explaining the observation. This study represents the first time the observed spatio-temporal record of carbonate sediments has been incorporated into an Earth-system model to understand ancient climate change.
Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation
Stephen R. Meyers, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA. Pages 315-318.
A long-standing controversy in the field of geochronology pertains to the use of Milankovitch orbital cycles for the construction of deep-time astrochronologies. When vestiges of these cyclic perturbations to Earth's orbit are preserved in the geologic record, they provide an important high-resolution method for measuring the passage of time. Yet a major challenge to the development of accurate deep-time astrochronologies is the lack of sufficient independent time control (e.g., radiometric age data) that would reliably calibrate the stratigraphic rhythms to temporal periods, and thus directly confirm their orbital tempo. Meyers investigates two of the most controversial stratigraphic units in the field of astrochronology, using a new statistical approach that overcomes the independent time control problem. In both the Triassic Latemar Limestone and the Eocene Green River Formation, Meyers demonstrates that the null hypothesis (no orbital signal) can be rejected with an extremely high degree of confidence. These results underscore the high fidelity of the orbital chronometer, and have far-reaching consequences for the field of geochronology. In the case of the Latemar Limestone, the orbital chronometer disagrees with the existing uranium/lead zircon geochronology by a factor of 4, indicating that sources of error in the zircon-based time scale must be reconsidered. More generally, the new astrochronologic method provides an objective standard for refinement of Cenozoic-Mesozoic geochronologies, and will permit extension of orbital time scale construction into the Paleozoic.
Unzipping Long Valley: An explanation for vent migration patterns during an elliptical ring fracture eruption
Eoghan P. Holohan et al., Department of Geology, School of Natural Sciences, Trinity College, Dublin D2, Ireland. Pages 319-322.
California is home to the still-active Long Valley caldera volcano, which formed during the third largest eruption in Pleistocene North America. Stratigraphic and petrologic studies of this eruption had deciphered an intriguing pattern of vent migration during the caldera's collapse. This vent migration was thought to mirror the lateral propagation (“unzipping”) of the magma-conducting ring fractures, along which the Long Valley magma chamber roof subsided by some 2–3 kilometers. Reasons for this migration pattern, however, were ill-understood. From simple, scaled analog models, easily re-run in the classroom, Holohan et al. show that this unzipping pattern inferred at Long Valley was intrinsically related to the highly elliptical plan-view shape of the pre-collapse magma chamber roof. They also explain how the elliptical roof shape systematically influences the initial location and subsequent lateral propagation of a caldera's ring fractures. Such a systematic development of ring fracturing above an elliptical magma chamber may help explain the distribution of major vents at other elliptical calderas, and it enhances our ability to predict how such hazardous volcanoes may behave in the future.
Impact effects and regional tectonic insights: Backstripping the Chesapeake Bay impact structure
Travis Hayden et al., Western Michigan University, Department of Geosciences, 1187 Rood Hall, 1903 W. Michigan Avenue, Kalamazoo, Michigan 49008, USA. Pages 323-326.
Studies of impact structures provide a powerful window into a number of aspects of geology and tectonics. Hayden et al. use a modeling technique known as backstripping to model the long-term effects of the Chesapeake Bay Impact Structure on the tectonics and sedimentary history of coastal Virginia. This impact occurred 35 million years ago, and this study represents the first time this backstripping technique has been applied to an impact structure. The results of this study indicate that the crustal-scale effects of the impact persisted for 7 million years, longer than previously thought. In addition, these results increase our understanding of sedimentary processes along the passive margin of eastern North America, as well as increasing our understanding of the evolution of passive margins around the world.
Toasting the jelly sandwich: The effect of shear heating on lithospheric geotherms and strength
Ebbe H. Hartz, Oslo University, Physics of Geological Processes, P.O. Box 1048 Blindern, Oslo 0316, Norway, and Aker Exploration, Oslo, Norway; and Yuri Y. Podladchikov, Aker Exploration, Oslo, Norway. Pages 331-334.
Hartz and Podladchikov show that frictional heating, the heat produced when rocks deform, is a major component of Earth’s heat budget, which in turn controls many Earth processes. It has long been accepted that the temperature of Earth’s stiff outer shell (the lithosphere, also called “the jelly sandwich”) controls its strength, and thus how it deforms. Hartz and Podladchikov suggest that the opposite is also. Frictional heating is not a new concept in Earth science, but it has not previously been recognized that deformation locally can compete with radiogenic processes in being the main heat producer on a lithospheric scale. Furthermore, the calculations applied here are simple and easy to reproduce: Earth’s strength (“forces”) is multiplied by its rate of deformation. Both factors are given in lithospheric-scale strength diagrams. The simplicity of the new method allows the results to be checked against the strength of Earth and its surface heat flow (both are model-independent parameters that can be directly measured), and thus we can predict weakening due to shear heating without any additional assumptions. This new quantification of shear heating provides simple explanations for many grand-scale Earth processes, including the remarkable weakness of active mountain belts, their high heat flow, abundant magmatism, and deep earthquakes.
GSA TODAY Science Article
Temporal evolution of continental lithospheric strength in actively deforming regions
Wayne Thatcher and Fred F. Pollitz, U.S. Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, California 94025-3591, USA
Earth has a strong, brittle outer shell that is broken into a series of plates. It is the interaction of these brittle plates that gives us mountain ranges, earthquakes, tsunamis, and volcanoes. But what drives plate tectonics, and how did the plates form" To answer these questions, we need to first determine the depth to which the plates extend and to understand what makes the plates brittle and strong. In an article published in the April-May 2008 GSA Today, Wayne Thatcher and Fred Pollitz, geophysicists with the U.S. Geological Survey, report on their studies of how plates deform in various settings, ranging from the shaking resulting from major earthquakes to the slow bending of plates beneath the weight of glaciers. Their results may require a major re-think of plate tectonics, for it appears that on geological time scales, only the upper 10 to 15 km of plates have any strength at all. This unexpected result contrasts markedly with the prevailing view of plates consisting of 100-km-thick beams, and may indicate that our understanding of the processes that give rise to mountains and that control plate movements is in need of reevaluation.
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