|02 March 2010
GSA Release No. 10-07
Director - GSA Communications & Marketing
MARCH 2010 MEDIA HIGHLIGHTS
Boulder, CO, USA – In GEOLOGY, examination of Chicxulub impact crater sediments supports existing K-T mass extinction theories; Cretaceous nannofossils help clarify climate change; a debris flows study breaks up the sieve-lobe paradigm; Walter Alvarez and coauthor take a look back at the Copernican Revolution; microbial mats display evolutionary creativity through wax esters; and study of thermophilic microbes in the Canadian High Arctic impacts the search for life on Mars. GSA TODAY examines "nature's sandbox" in assessing oil reserves.
Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. View abstracts for this issue of GEOLOGY at http://geology.gsapubs.org/.
GSA TODAY articles are open access. Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service,
Geometric difference between non-feeder and feeder dikes
Nobuo Geshi et al., Geological Survey of Japan, AIST (National Institute of Advanced Industrial Science and Technology), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan. Pages 195-198.
A dike is a path of magma in Earth's interior; most volcanic eruptions are fed by dikes. Structural development of dikes controls the behavior of magma and therefore also controls the beginning and end of the eruption. To reveal the controls on the development of dikes, Geshi et al. investigated many dikes exposed on the 400-m-high cliff of the Miyakejima Caldera (Japan). Their results show that the magmatic overpressure and mechanical properties of the host rock controls the entire structure of dikes. Dike thicknesses increase with altitude, reflecting changes in the mechanical properties of the rocks surrounding the dikes and the gradient of the magmatic overpressure inside the dikes. When a dike reaches the ground surface and erupts, dike thickness decreases with the drop of magmatic overpressure inside the dike. Geshi et al.'s results suggest that careful and detailed observation of ground deformation by dike intrusion can reveal the change of magmatic overpressure within a dike and, therefore, could contribute to the mitigation of eruption hazards.
* * * * * * * * * * *
Grain size of Cretaceous-Paleogene boundary sediments from Chicxulub to the open ocean: Implications for interpretation of the mass extinction event
Timothy Bralower et al., Dept. of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Pages 199-202.
The causes of the mass extinction of 75% of marine and 50% of terrestrial species (including the dinosaurs) at the Cretaceous-Tertiary boundary, 65 million years ago, has been the subject of raging debate. The majority of scientists support the theory that the impact of an asteroid on the Yucatan Peninsula (Mexico) was the trigger of the extinctions. The most potent evidence for this theory is a layer of rock containing telltale signs of impact, including melt droplets and shocked mineral grains that can be traced from the Yucatan around the world. Basing their research on fossils in rocks around the Gulf of Mexico, a group of scientists have recently proposed that the Yucatan impact preceded the mass extinction by 300 thousand years, and that the extinction was caused by a massive volcanic event in India. The current study by Bralower et al., however, leads to a different conclusion through the analysis of sediment particle size in Cretaceous-Tertiary boundary samples to determine the origin of fossil shells. The data demonstrate that fossils in Cretaceous-Tertiary boundary rocks around the Gulf of Mexico region are eroded from underlying layers by a tsunami at the impact event. Thus they rule out the correlation of the mass extinction event with Indian volcanism and conclusively support the connection with the Yucatan impact.
Latitudinal migration of calcareous nannofossil Micula murus in the Maastrichtian: Implications for global climate change
Nicolas Thibault et al., Dept. of Geography and Geology, University of Copenhagen, Oster Voldgade 10, 1350 Copenhagen C., Denmark. Pages 203-206.
The first appearance of the calcareous nannofossil Micula murus is one of the main time markers in the microplankton of tropical and intermediate latitudes in the upper Maastrichtian stage (uppermost Cretaceous). A review of its first occurrence at 14 deep-sea sites and sections by Thibault et al. shows that it is actually a diachronous event. Micula murus first appeared about 68.5 million years ago in the Tropical Realm, where it remained confined for about 1.2 million years, thus supporting high-latitudinal thermal gradients. This species subsequently spread out about 67.3 million years ago to temperate latitudes and to the Tethys, in coincidence with the onset of a major change in surface-water circulation and interocean communications. The demonstrated migration patterns of this species should thus draw attention to the use of its first appearance in biochronological schemes.
The sieve lobe paradigm: Observations of active deposition
Juan Pablo Milana, InGeo-CONICET, Universidad Nacional de San Juan, 5401 San Juan, Argentina. Pages 207-210.
Understanding the way sediment is transported across alluvial fans is fundamental in many aspects, including natural hazard management. Milana demonstrates that an almost abandoned sediment depositional mechanism concept - the sieve-lobe - due to many criticisms raised afterward, it is actually a valid assumption and serves to explain many deposits interpreted as modified debris flows. The process is the simple rapid infiltration of entire mud-depleted floods over permeable alluvial gravel, and as a result, a lobate matrix-free, well-sorted, highly permeable alluvial gravel deposit is formed. The work shown here suggests therefore that we need more research on the basics of transport and depositional mechanisms of sediments if we expect to fully understand arid and semiarid alluvial fan and alluvial stream processes.
Shocked quartz and other mineral inclusions in Australasian microtektites
L. Folco et al., Museo Nazionale dell'Antartide, Universita di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 211-214.
In a study by Folco et al., the application of high-resolution analytical techniques including syncrothron X-ray diffraction, field emission scanning electron microscopy, and microraman spectroscopy led to the discovery of microscopic mineral inclusions in Australasian microtektites recovered from deep-sea sediment cores within 2000 km of Indochina. Inclusions consist of frequent shocked quartz plus a Zr-phase and trace of Fe-oxide crystallites. The shocked quartz and the Zr-phase are interpreted as relicts of the target rock. The occurrence of partially melted quartz relicts and fluidal structures (schlieren) confirms that microtektites are quenched molten droplets and not condensates from a hot plume of vaporized crustal rocks. Furthermore, the internal homogeneity of Australasian microtektites in terms of abundance of relict mineral inclusions, vesicles, and schlieren increases with distance from Indochina. This finding strengthens the current hypothesis that the source crater of the largest and youngest tektite-strewn field on Earth is located in the Indochina region, as internal heterogeneity characterizes normal impact glass found in or near the source crater. This finding also indicates that the Australasian microtektites with the longest trajectories experienced the highest temperature-time regimes. Lastly, the definition of microtektites should include the possible occurrence of microscopic relict inclusions as an indication of proximity to the source crater.
Syncollisional rapid granitic magma formation in an arc-arc collision zone: Evidence from the Tanzawa plutonic complex, Japan
Kenichiro Tani et al., Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan. Pages 215-218.
Reaching the deeper parts of intra-oceanic arc crust is crucial in understanding how continents grow, as this is where continental crust is believed to be currently forming. However, it is difficult to directly sample this crust, as ocean and sediments mask it. Consequently current studies rely on crustal sections uplifted to the surface as a result of collision or accretion. The Tanzawa plutonic complex (TPC), located in central Japan, has been widely accepted to be such a section of the Izu-Bonin-Mariana (IBM) arc. It has been exposed as a result of the ongoing collision between the intra-oceanic IBM and mature Honshu arcs. However, new thermogeochronology data for TPC zircons collected in this study, by Tani et al., show that the main pulse of TPC magmatism was simultaneous with the IBM-Honshu arc collision and rapidly emplaced. Furthermore, geochemical signatures of TPC zircons indicate assimilation of continental sediments from the Honshu arc during the TPC magma genesis. This argues against the TPC being representative of juvenile IBM arc crust. To understand how continental crust forms in the intra-oceanic arc, therefore, it is essential to directly sample the deep crust via deep drilling within the Integrated Ocean Drilling Program.
Volcano destabilization by magma injections in a detachment
Vincent Famin and Laurent Michon, Universite de la Reunion, Laboratoire Geosciences Reunion, Institut de Physique du Globe de Paris, CNRS, UMR 7154, 15 avenue Rene Cassin, BP 7151, 97715 Saint Denis messag Cedex 9, La Reunion, France. Pages 219-222.
Famin and Michon report the discovery of a detachment (i.e., a large low-angle fault) exposed by erosion at Piton des Neiges, the extinct volcano of La Reunion hotspot. Such faults are thought to play an important role in the destabilization of ocean island volcanoes but are usually too deep to be observed. The movement of the fault uncovered an extinct magma chamber of the volcano. Their study also reveals that detachment slip was caused by repeated injections of magmas into the fault plane. This new mechanism of destabilization may occur elsewhere (Canary, Hawaiian, and Galapagos Islands), and could in particular explain some peculiar eruptions of Piton de la Fournaise, the active twin of Piton des Neiges.
Dissipation of fast strike-slip faulting within and beyond northeastern Tibet
Alison R. Duvall and Marin K. Clark, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Pages 223-226.
Duvall and Clark challenge the long-standing view that major strike-slip faults accommodate lateral extrusion of Tibetan crustal blocks toward a "free" boundary to the east, and instead propose a new model where strike-slip motion is dissipated northward in the direction of India-Asia plate convergence. Using geodetic velocities, Quaternary slip rates, and map faulting patterns, they show how left-lateral slip at the eastern end of the Kunlun fault is accommodated within the Tibetan Plateau and into the surrounding region. Their results do not support significant lateral motion beyond the plateau margin to the east. Rather, they demonstrate that slip is transferred northward, partly to the Haiyuan fault and partly into a region of distributed faulting or rotation. They also suggest that the transfer of Kunlun fault motion across a wide deforming zone over 500 km northward develops due to the underlying weak lower crust.
The paradox of tropical karst morphology in the coral reefs of the arid Middle East
S.J. Purkis et al., National Coral Reef Institute, Nova Southeastern University Oceanographic Center, Dania Beach, Florida 33004, USA. Pages 227-230.
Purkis et al., demonstrate that patterning in the geomorphology of coral reefs in the Middle East can be used to determine climate for the region during the last glacial sea-level lowstand. The reefs of both the Arabian Gulf and Red Sea, despite their geographic separation and vastly different physical and ecological conditions, display the same curious honeycomb motif. By developing a Pleistocene/Holocene chronology of sea level and climate for the areas, and through the use of pattern analysis and computer simulation, they reveal the mechanism of formation for this patterning to be dissolution of the then-exposed carbonate bedrock by rainwater. This finding confirms that although this broad geographic region is now hyper-arid, subjected to very little rainfall, the climate in the mid-Holocene was, as has previously been postured, much wetter than today and with considerable monsoonal influence. They further demonstrate that aspects of reef morphology in the region are controlled by antecedent topography formed as recently as the mid-Holocene epoch.
The neglected early history of geology: The Copernican Revolution as a major advance in understanding the Earth
Walter Alvarez and Henrique Leitão, Dept. of Earth and Planetary Science, University of California-Berkeley, Berkeley, California 94720-4767 USA. Pages 231-234.
Historians of science have long considered the Copernican Revolution to mark the beginning of modern astronomy and modern physics. Copernicus placed the Sun at the center of the universe, rather than Earth, and that insight eventually led to correct astronomy and to Newton's fundamental laws of physics. But this part of Copernicus's work was wildly incorrect, because the Sun is not at the center of the Universe — it is just one star in our galaxy out of hundreds of billions of galaxies. Copernicus also stressed that Earth is a planet. This was an astonishing idea for the time, but it has turned out to be entirely correct. Geologists and the other scientists who study Earth usually trace the origin of geology to a time much later than Copernicus. Alvarez and Leitão suggest that since the work of Copernicus fundamentally concerned the Earth, historians of science should trace the origin of modern geology back to the Copernican Revolution. Because of the environmental threats to our planet, geology may be the most important science for the 21st century, and our suggestion that geology has a much longer history than it is usually accorded adds a new dignity to this essential science.
Effect of late Cenozoic aridification on sedimentation in the Eastern Cordillera of northwest Argentina (Angastaco Basin)
Sharon Bywater-Reyes et al., Dept. of Geology and Geophysics, University of Wyoming, 1000 E University Avenue, Laramie, Wyoming 82071, USA. Pages 235-238.
This study uses stable isotope geochemistry and radiometric dating to establish a relationship between change in depositional environment and a change in climate. Bywater-Reyes et al. establish that within the Angastaco Basin of the Argentinean foreland region grain size increases and the environment changes from a meandering-stream to braided fluvial as aridity is established by the Pliocene.
Cirques, peaks, and precipitation patterns in the Swiss Alps: Connections among climate, glacial erosion, and topography
Alison M. Anders et al., Dept. of Geology, University of Illinois, 1301 W. Green Street, Urbana, Illinois 61801, USA. Pages 239-242.
Anders et al. studied the impact of climatic variability on glacial erosion rates and mountain elevations. In the Swiss Alps, spatial variability in precipitation is associated with spatial variability in the elevation of glacially carved cirques. In wetter regions, cirques occur at lower elevations and the heights of surrounding mountain peaks are lower than in drier regions. The relief of cirque basins is constant across the regions. These relationships support the hypothesis that mountain peak elevations are controlled by erosion of glacial cirques and subsequent slope failure of the cirque headwalls. These findings highlight the interrelationship between climate, erosion, and topography and point to the primary importance of glaciers in controlling mountainous topography.
Large variations of oxygen isotopes in precipitation over south-central Tibet during Marine Isotope Stage 5
Yanjun Cai et al., State Key Lab of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China. Pages 243-246.
In the study by Cai et al., oxygen isotope (δ18O) records of two stalagmites from Tianmen Cave, on the south-central Tibetan Plateau, which grew during much of Marine Isotope Stage 5, were presented. The Tianmen record, as the first cave record from the Tibetan Plateau, characterizes a precipitation δ18O history larger in amplitude, but similar in structure to Asian Monsoon records from the adjacent regions, providing essential evidence that the Asian Monsoon system, including the East Asian and Indian Monsoon subsystems, responds largely to changes in Northern Hemisphere summer insolation. Extremely low δ18O values during Marine Isotope Stages 5a, 5c, and 5e suggest that precipitation, rather than temperature, was a major factor controlling δ18O in precipitation on orbital time scales in the south-central Tibetan Plateau. The Tianmen record may help in the interpretation of regional ice core δ18O records. The large range of orbital-scale shifts in meteoric δ18O (>9 ‰) raises important considerations related to reconstructing the uplift history of the plateau.
Microbial biosynthesis of wax esters during desiccation: An adaptation for colonization of the earliest terrestrial environments?
David B. Finkelstein et al., Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA. Pages 247-250.
Seasonal changes in the amount of rain or snow controls the amount of water delivered to desert lakes in eastern Oregon. When coupled with evaporation during the summer, the resulting changes in water chemistry drive the microbial mats to produce compounds that aid their survival during dry periods. These compounds, termed wax esters, have been previously described in marine and hot spring environments. The study by Finkelstein et al. documents these compounds and their structure from mats composed of similar microbial organisms that live at the water’s surface and from dried lake beds. The production of wax esters by microbial colonies is advantageous because it results in a gain of cellular water during evaporation and delays desiccation. Finkelstein et al. observed both an increase in the abundance and size of the wax esters, which may represent an evolutionary strategy to survive dry periods. Wind transport of dried mat materials assists microbial community migration between lake flats and basins during arid times. Re-wetting of the microbial mats breaks down these waxes so that the components may be used again by the next generation. Wax ester production likely influenced microbial migration to lake environments in the earliest terrestrial ecosystems as well as their survival through dry periods.
Valanginian isotope variation in glendonites and belemnites from Arctic Svalbard: Transient glacial temperatures during the Cretaceous greenhouse
Gregory D. Price and Elizabeth V. Nunn, School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK. Pages 251-254.
In a study by Price and Nunn, past temperatures calculated using oxygen isotopes of fossil belemnites from the Arctic yield cool temperatures consistent with transient glacial polar conditions paradoxically during the Cretaceous greenhouse. Such cool polar temperatures during the Cretaceous help reconcile some geologic data with the predictions of general circulation models. Nevertheless, beyond this postulated and transient cool event within the early Cretaceous (Valanginian), the remainder of the data are explained in terms of warm polar conditions (considerably above freezing) during the Cretaceous greenhouse.
Oxidation of petrogenic organic carbon in the Amazon floodplain as a source of atmospheric CO2
Julien Bouchez et al., GeoForschung Zentrum (Sektion 3.4), Telegrafenberg, 14473 Potsdam, Germany. Pages 255-258.
In a study by Bouchez et al., burial of organic matter in marine sediments built up a large pool of organic carbon in sedimentary rocks. The oxidation of this "petrogenic" organic carbon is a source of CO2 to the atmosphere, which has not yet been well quantified. The Amazon River transports huge amounts of solid particles resulting from the erosion of Andean sedimentary rocks. During riverine transport through the Madeira floodplain (one of the largest tributaries of the Amazon basin), the petrogenic organic carbon contained in these particles is largely oxidized, resulting in a net input of 0.25 MTC/yr to the atmosphere. This CO2 flux is comparable to the atmospheric CO2 sink by silicate minerals weathering over the same area. Graphite is the most stable carbon phase with respect to this oxidation process, whereas more structurally "disordered" petrogenic organic carbon is relatively prone to oxidation. Finally, the petrogenic organic carbon is better preserved during riverine transport in the Andes-Amazon system than in the Himalaya-Ganga Brahmaputra system. The extent of oxidation of petrogenic organic carbon during riverine transport is likely to depend both on climatic and geodynamic settings. Altogether, petrogenic organic carbon oxidation in large river floodplains appears as a significant process in the global carbon cycle.
Relationships among climate, erosion, topography, and delamination in the Andes: A numerical modeling investigation
Jon D. Pelletier et al., Dept. of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA. Pages 259-262.
The Andes mountain range has grown in height and width over the past 60 million years. Recent evidence has suggested that much of this growth occurred rapidly in one or more pulses lasting just a few million years. Pelletier et al. combine the processes leading to growth of the Andes together with the processes that lead to decay (erosion) and show how they interact along the full length of the Andes over time to create the mountain range we see today.
Rapid, high-temperature formation of large-scale rheomorphic structures in the 2.06 Ma Huckleberry Ridge Tuff, Idaho, USA
John W. Geissman et al., Dept. of Earth and Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA. Pages 263-266.
The ~2.06-million-year-old Huckleberry Ridge Tuff is one of the large-volume, explosive eruptive products associated with the Yellowstone-Snake River plain hotspot, a major volcanic and physiographic feature in the Western United States. In eastern Idaho, south of its caldera source, the Huckleberry Ridge Tuff is some 130 m thick and exhibits exceptionally large-magnitude folds defined by tuff compaction fabrics that are parallel to both the primary internal zonation and the basal contact with older sedimentary deposits. Paleomagnetic data by Geissman et al., from a large-amplitude (over 150 m) overturned fold near the failed Teton Dam show that the fold developed at elevated (>580 °C) temperatures. The in situ magnetization throughout the fold is indistinguishable in direction from results from areas where the tuff is flat-lying (undeformed). Because deformed and undeformed Huckleberry Ridge Tuff exposures preserve the same anomalous magnetization directions, the unusually large-scale structures (referred to as rheomorphic) in the tuff must have formed rapidly (less than a few hundred years) at high temperatures shortly after development of compaction fabrics. Post-welding, high-temperature deformation is consistent with field evidence indicating rapid, plastic secondary deformation of much of the tuff prior to cooling and vapor-induced alteration (devitrification).
Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton
E. Tohver et al., School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Pages 267-270.
The middle of South America is marked by a large suture zone that separates the Amazon craton on the west from the smaller Sao Francisco and Rio de Plata cratons to the east. The central portion of this suture zone, the Paraguai belt, is marked by a prominent, greater than 90-degree curve. New paleomagnetic data by Tohver et al., demonstrate that this curvature formed as a secondary feature. Together with a novel geochronological technique for determining the age of secondary magnetizations, our observations demonstrate that the Gondwana supercontinent was not fully formed until the mid- to late-Cambrian.
Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes
John Parnell et al., Dept. of Geology, University of Aberdeen, Aberdeen AB24 3UE, UK. Pages 271-274.
Meteorite impact craters have been proposed as possible sites to find microbial life on Mars, as they are a focus for heat and water circulation. However the limited lifetime of crater hydrothermal systems raises the question of whether colonization by microbes could take place fast enough before both heat and liquid water are lost. Using the composition of sulfur isotopes in sulfide minerals in the Haughton Impact Structure, Canadian High Arctic, Parnell et al., have shown for the first time that a crater was pervasively colonized by microbes, and that colonization of over 20 cubic kilometers of impact breccia rock was rapid, within 10,000 years after impact, while the rock was still warm. This emphasizes the potential of impact craters in the evolution of early life on Earth, and the search for life on other planets.
Timing and magnitude of the sea-level jump preluding the 8200-yr event
Marc P. Hijma and Kim M. Cohen, Faculty of Archaeology, Leiden University, Reuvensplaats 3, NL-2311 BE, Leiden, Netherlands. Pages 275-278.
It was 8200 years ago that the most pronounced climate excursion of the last 12,000 years occurred. There is general consensus that this was caused by the shutdown of a warm ocean current due to a multiple drainage event in the Hudson Bay area. An ice dam that held two large lakes collapsed and huge amounts of water and ice entered the North Atlantic, resulting in a sea-level jump. However, there has been no direct measurement of this jump using precise sea-level data, and also the exact timing for the beginning of the event was unknown. By dating numerous peat layers in the western Netherlands, Hijma and Cohen show that the period of rapid sea-level rise commenced 8450 plus or minus 44 yr ago. The jump in sea level reached a local magnitude of 2.11 plus or minus 0.89 m within 200 yr. This magnitude is almost double the size of previous estimates. The discrepancy suggests either a coeval Antarctic contribution or, more likely, a previous underestimate of the total American lake drainage. The new data can be used as important input in climate models that try to hindcast the 8200 yr event, but also try to predict future climate developments.
Did the Late Ordovician African ice sheet reach Europe?
Juan C. Gutiérrez-Marco et al., Instituto de Geologia Economica (CSIC-UCM), Facultad de Ciencias Geologicas, Jose Antonio Novais 2, E-28040 Madrid, Spain. Pages 279-282.
Several paleovalleys of Hirnantian age are described from the Cantabrian Zone in northern Spain, and interpreted as subglacial tunnel valleys based on their geometry and infilling succession. They were most likely related to the North Gondwana Ice Sheet, so far believed to be restricted to Africa, the Arabian Peninsula, and South America. This discovery, by Gutiérrez-Marco et al. constitutes the first mention of glacial valleys outside the Gondwana landmass and suggests that glacial lobes reached Europe. It has noticeable implications for a variety of studies dealing with Paleozoic paleogeography. The two most important implications are related to (1) Ordovician paleoglacial reconstructions, and (2) pre-Variscan distribution of the present-day, peri-Mediterranean, Gondwana-derived blocks. Any studies dealing with tunnel valley structures (petroleum geology in North Africa or Arabia, Quaternary successions in the North Sea region) should also be interested in the original description of Ordovician paleovalleys from Spain.
A low-velocity zone with weak reflectivity along the Nankai subduction zone
Jin-Oh Park et al., Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan. Pages 283-286.
Park et al. investigate a low-seismic velocity zone with weak reflectivity along the Nankai subduction zone, which is the best-studied subduction zone around the globe. They propose a straightforward analysis for a low velocity zone within the Nankai accretionary wedge, based on an extremely valuable set of three-dimensional seismic reflection data. The observation of the low-velocity zone beneath the thrust-faulted, highly reflective interval is a stunning result. This is the first such investigation to show such a low-velocity zone in detail. This work may be of significant public interest as it appears to be related to tsunami generation. The authors propose that a fluid-rich sediment unit is underplated beneath the Nankai Trough margin, resulting in a rigidity-lowered outer wedge that may favor a tsunami by being more easily uplifted during a coseismic rupture. They put an emphasis on using seismic wave velocity for extracting geological information in addition to using the reflection images.
Rock to regolith conversion: producing hospitable substrates for terrestrial ecosystems
R.W.H. Butler, Geology and Petroleum Geology, School of Geosciences, University of Aberdeen AB24 3UE, UK; and D.A. Paton. Pages 4-9.
The thick sedimentary sequences that characterize many of the world’s continental margins hold an unstable secret of gravitational collapse, the scale of which has only become apparent through exploration for hydrocarbons in the past 20 years or so. Although inaccessible, submerged beneath many kilometers of water, these structures are revolutionizing the understanding of the geometry of contractional deformation in sedimentary successions. According to authors Butler and Payton, they are "nature's sandbox" — large-scale versions of the laboratory physical models currently in vogue in some parts of the structural geology community. In the latest issue of GSA Today, Butler and Payton examine one well-imaged system, the continental margin of southwest Africa, offshore Namibia, to better understand the large-scale deformation of poorly lithified sedimentary rocks. Through independent estimates of extension and contraction, the authors show that a considerable amount of deformation is not accounted for in three-dimensional modeling, and call for further, detailed studies of other areas that take into consideration their regional context.