November 2011 Geology highlights: New research posted Oct. 5

(Press-News.org) Boulder, CO, USA - Another packed issue of GEOLOGY, The Geological Society of America's premier journal and the top-most cited geoscience journal in the world (http://www.scimagojr.com/journalrank.php?category=1907) is online in pre-issue publication. Topics include the San Andreas Fault and SAFOD; the 1944 Tonankai earthquake, Japan; Bonin Island beach sands; the fluid properties of ice; fracturing of the Panamanian Isthmus; diatoms from Ocean Drilling Program Site U1304; China's Loess Plateau; deglaciation and climate change in Turkey; an ancient Death Valley landslide; and cephalopod hatchling habits.

Keywords: Nankai Trough, Tonankai earthquake, large igneous province, Olympic Dam, Gawler Range Volcanics, Mesoproterozoic, Panamanian Isthmus, thermochronology, Boston Blue Clay, Ocean Drilling Program Site U1304, Northern Atlantic, Red Sea Rift, Alpine fault, New Zealand, San Andreas, glaciers, Chinese Loess Plateau, Tibet, Qaidam basin, ocean carbonate chemistry, Atlantic, inoceramids, OAE 2, Vallecillo, Mexico, San Andreas fault, SAFOD, Younger Dryas, Turkey, end-Permian mass extinction, Guadalupian extinction, iron cycling, cephalopods, Phanerozoic, South America monsoon, Brazil, North AMOC, Bond events, Red Sea, Levantine corridor, Aqaba, Jordan, paleoflood, OSL dating, storm surge, Netherlands, Tecopa paleolake, Death Valley

Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.



Progressive illitization in fault gouge caused by seismic slip propagation along a megasplay fault in the Nankai Trough Asuka Yamaguchi et al., Dept. of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; doi: 10.1130/G32038.1.

The question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafloor is critical for assessing the risk of tsunami disaster. Asuka Yamaguchi of the University of Tokyo and colleagues present geochemical and mineralogical data of drilled core samples obtained from the shallow portion of the megasplay fault that coincides with the rupture area of the A.D. 1944 Tonankai earthquake in the Nankai Trough. Chemical and mineralogical features of the localized slip zone may reflect a transformation in clay mineralogy caused by frictional heating, and suggest that the seismic slip can propagate to very shallow levels along megasplay fault systems.



Osmium behavior in a subduction system elucidated from chromian spinel in Bonin Island beach sands Katsuhiko Suzuki et al., Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka 237-0061, Japan; doi: 10.1130/G32044.1.

Osmium (Os) isotopes are a potential tracer of a recycled crust in mantle or volcanic rocks because of the significant contrast between Os isotope ratios of crust and mantle. However, possible contamination of crust with the high Os isotope ratio to magma during magma ascent may overprint this original signal of the Os isotopic ratios. Katsuhiko Suzuki of JAMSTEC and colleagues explored the use of Os isotopes in chromian spinel (Cr-spinel) as a discriminator of primitive magma Os isotopic compositions in the Izu-Bonin arc, using beach sands as composite samples of the boninite and tholeiite magmas. Cr-spinel is an early-stage crystal that preserves its isotopic composition even during later crustal contamination. Suzuki et al. found very low Os isotope ratios in Cr-spinels from boninites, suggesting that they represent primitive magmas with no Os recycled from the subducting slab during the boninite generation in the infant arc stage. Conversely, the high Os isotopic ratios in Cr-spinels from tholeiites likely reflect the contribution from a slab-derived component, because more oxidative conditions in the subarc mantle probably allowed Os to mobilize from the subducting slab during the transitional arc stage. Although contamination of crustal components may overprint the original signature of magma during its ascent, Cr-spinel allows us to compensate such possibilities.



The fluorine link between a supergiant ore deposit and a silicic large igneous province Jocelyn McPhie et al., ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia; doi: 10.1130/G32205.1.

Olympic Dam is a supergiant ore deposit that is mined for copper, uranium, gold, and silver. These metals occur in small grains of sulfide and oxide minerals within granite and volcanic rocks that have been intensely broken up. The metal-rich minerals were precipitated from hot, metal-rich water that permeated through the broken-up rocks. Jocelyn McPhie of the University of Tasmania and colleagues have found that originally, the granite and volcanic rocks contained a lot of fluorine, and fluorine-rich minerals are also abundant in the ore deposit. They propose that the hot, metal-rich water was able to concentrate and transport metals because it inherited high fluorine content from the rocks through which it circulated. Further, McPhie and colleagues infer that hydrofluoric acid, the most corrosive acid known, was partly responsible for breaking up the granite and volcanic rocks by dissolving them along fractures and faults. It is no accident that the world's largest hydrothermal ore deposit occurs in granite and volcanic rocks that are fluorine-rich.



Fracturing of the Panamanian Isthmus during initial collision with South America David W. Farris et al., Florida State University, Dept. of Earth, Ocean, and Atmospheric Sciences, Tallahassee, Florida 32306, USA; doi: 10.1130/G32237.1.

David W. Farris of Florida State University and colleagues report evidence that tectonic collision between South America and Panama began approximately 23-25 million years ago. This is much earlier than previously thought, and is significant because it ultimately led to development of the Panamanian Isthmus, which in turn had wide-ranging oceanic, climatic, biologic, and tectonic implications. The interpretation of earlier collision is based on distinct changes in the chemistry of Panama volcanic arc rocks and synchronous exhumation of both Panama and the adjacent northern Andes in Colombia. Exhumation on both the Panama and South American sides of the collision is what would be expected once the collision began, and this is what is observed at 23-25 million years ago. Farris and colleagues also present a new tectonic model in which Panama fractured and rotated in several pieces to produce the modern curved shape of the Panamanian Isthmus that we observe today.



Insights into pore-scale controls on mudstone permeability through resedimentation experiments Julia Schneider et al., Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713, USA; doi: 10.1130/G32475.1.

Fine-grained marine sediments have limited ability to flow water. This ability is strongly dependent on the fraction of coarser-grained sediments that are present. These properties are difficult to interpret from sediments taken in natural settings because of natural variability and sample disturbance. Instead, Julia Schneider of the Jackson School of Geosciences and colleagues follow a systematic approach, adding coarse material to fine-grained sediment in different proportions to test the effect of composition on flow. They prepared the samples using a new technique termed resedimentation that avoids sample variation and disturbance. Schneider and colleagues show that by replacing 50 percent of the fine-grained sediment with coarse material, the ability of fluid to flow through the sample (permeability) is increased by an order of magnitude. Coarser samples have larger pores and fewer aligned particles than finer-grained samples providing easily accessible pathways for fluids. This behavior is described by a dual-porosity model where one rock fraction is dominated by silt, large pore throats are present, and the majority of flow occurs, and another fraction is dominated by fine-grained particles, where limited flow occurs. These results are important because fine-grained sediments act as seals for both carbon dioxide and hydrocarbons in the subsurface, behave as gas reservoirs, and can potentially cause harm in offshore drilling.



A highly productive Subarctic Atlantic during the Last Interglacial and the role of diatoms O.E. Romero et al., Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, 18002 Granada, Spain; doi: 10.1130/G32454.1.

Most paleoclimatic and paleoceanographic studies have focused on coarse time scales that have a more academic and less immediate appeal. A new development is the increasing recognition of the occurrence of laminated sediments composed of diatom mats in open-ocean, deep-sea environments. Because of the laminated nature of these deposits, origins have been ascribed previously to implausible occurrences of reduced oxygen conditions. Using an expanded section of the Last Interglacial (LIG), gained at Integrated Ocean Drilling Program Site U1304 in the Subarctic Atlantic, O.E. Romero of the University of Granada and colleagues demonstrate that the early interglacial stage 5e was marked by oceanographic conditions conducive for high diatom production and accumulation. Though the Subarctic Front provided the physical conditions for high diatom production and deposition, these processes alone are insufficient to explain the high rates of siliceous productivity and the formation of diatomaceous sediments. Instead, the additional presence of an increased nutrient pool provided by Subantarctic Mode Waters played the decisive role in initiating and sustaining diatom production. These observations by Romero and colleagues provide new insight into the workings of the marine biological silica and carbon pumps during early LIG and have important implications for understanding variability in diatom productivity during other glacial/interglacial transitions.



Initial burst of oceanic crust accretion in the Red Sea due to edge-driven mantle convection Marco Ligi et al., Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy; doi: 10.1130/G32243.1.

Throughout Earth's history, the continents periodically assemble in a single supercontinent, then fragment and disperse again in cycles each lasting about 500 million years. Rupturing of a continent and the birth of a new ocean are taking place now in the Red Sea, where Arabia is separating from Africa. Geophysical surveys and bottom rock samples were obtained in a recent expedition in the central Red Sea, where continental breakup and accretion of new oceanic crust are just starting. Data collected by Marco Ligi of Italy's Consiglio Nazionale delle Ricerche suggest that as soon as the lid of continental crust breaks, an initial strong burst of oceanic crust generation takes place, with intense basaltic volcanism and fast opening velocity. This initial active pulse fades then into steady, more passive, slower accretion. The initial pulse may be triggered by secondary convective movements in Earth's mantle below the rift, caused by a strong horizontal thermal gradient between the cold walls of the continental plates and the hot, axially upwelling subrift mantle. This mechanism of initial continental breakup may have acted also during the earliest stages of the opening of the Atlantic Ocean more than 100 million years ago.



Inverted metamorphic sequences in Alpine fault mylonites produced by oblique shear within a plate boundary fault zone, New Zealand Alan F. Cooper and Richard J. Norris, Geology Dept., University of Otago, Dunedin 9016, New Zealand; doi: 10.1130/G32273.1.

The east-dipping Australia-Pacific plate boundary in southern New Zealand, the Alpine fault, is bounded on its eastern side by a sequence of mylonites whose metamorphic grade is lower than the hanging-wall sequence that overlies them. Rather than being caused by an inverted thermal gradient, this inverted metamorphic sequence is attributed to lateral and vertical shear within the mylonite zone translating low-grade rock from its source area, 110 km to the south west. As documented by Alan F. Cooper and Richard J. Norris of the University of Otago, New Zealand, this transpressional shear occurred subsequent to the post-late Miocene inception of oblique convergence on the plate boundary.



Stress modulation on the San Andreas fault by interseismic fault system interactions John P. Loveless and Brendan J. Meade, Dept. of Geosciences, Smith College, Northampton, Massachusetts 01063, USA; doi: 10.1130/G32215.1.

Earthquake hazard is directly related to the level of stress on major faults. In the branching southern California fault system, the rate at which stress increases on one fault is affected by activity throughout the entire fault system. By calculating the interseismic (between earthquakes) stressing rate on all faults in southern California, John P. Loveless and Brendan J. Meade of Smith College find that neighboring faults amplify the stress accumulation rate on the Mojave and San Bernardino sections of the San Andreas fault. Since the 1857 Mw = 7.9 Fort Tejon earthquake, the last large earthquake on the San Andreas fault, stress accumulated interseismically on these sections due to interactions with nearby faults is about three times larger than the coseismic stress changes induced by the Landers and Hector Mine earthquakes. The amplification of stress accumulation rates on the San Andreas fault serves as a type example of the way in which fault system interactions may affect physics-based seismic hazard estimates.



How nonlinear is the creep deformation of polar ice? A new field assessment K.M. Cuffey, Dept. of Geography, University of California, Berkeley, California 94720-4740, USA; and J.L. Kavanaugh, Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada; doi: 10.1130/G32259.1.

Glacier ice flows across Earth's surface much in the same way that pancake batter spreads when poured onto a skillet, or syrup spreads when poured on a pancake. The fluid properties of ice are very different from those of syrup or pancake batter, however. The purpose of this study by K.M. Cuffey of the University of California at Berkeley and J.L. Kavanaugh of the University of Alberta, Canada, is to determine the fluid properties of ice. To accomplish this task, they analyzed measurements of flow and thickness of a large Antarctic glacier together. Information derived from this study will help geoscientists to understand how glaciers in polar regions, including the vast continental ice sheets, are responding to climate change.



Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications Alex Pullen et al., Dept. of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA; doi: 10.1130/G32296.1.

The Chinese Loess Plateau (central Asia) is one of Earth's most complete terrestrial records of wind-derived sediment for the past 2.5 million years. Alex Pullen of the University of Rochester and colleagues analyze detrital zircon crystals from glacial-period loess strata of the Loess Plateau to better understand dust storm sources and general wind patterns over central Asia during Quaternary glacial periods. Using the data collected during this project and observations of modem-interglacial dust storm tracks, Pullen et al. note a substantial southward shift in the source areas and tracks of dust-generating storms between modem-interglacial and glacial periods. The ages of zircon crystals included in the loess suggest the Qaidam Basin and northern Tibetan Plateau were significant source areas for the loess. This is at odds with modem-interglacial observations that suggest source areas to the north. Pullen and colleagues interpret this shift to represent an equatorward shift in the mean annual position of the subtropical jet stream during glacial periods. This work suggests that the southerly jet position inhibited the northward movement of precipitation associated with the East Asian monsoon into central East Asia during glacial periods.



Modern and late Pleistocene B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi determined with laser ablation ICP-MS M. Raitzsch et al., MARUM, Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany; doi: 10.1130/G32009.1.

The ocean, the largest carbon reservoir besides the solid earth, plays a major role in governing the carbon dioxide concentration in the atmosphere. The carbon dioxide concentration of the deep sea is part of the oceanic carbonate chemistry regulating the carbonate saturation state, a term for the degree of calcium carbonate preservation and dissolution. Benthic foraminifers, microorganisms dwelling on the sea floor, may record this saturation state of deep waters in the composition of their calcite shells, specifically, the ratio of the elements boron to calcium (B/Ca). Hence, B/Ca of fossil shells of foraminifers can be used to reconstruct past variations of the deep oceanic carbonate chemistry, which is essential for understanding past atmospheric carbon dioxide changes. M. Raitzsch of the University of Bremen, Germany, and colleagues measure samples from sediments accumulated in the past 135 thousand years in the equatorial Atlantic. It was already known that this region was alternately influenced by different deep water masses with different carbonate chemistries, which is clearly reflected in their new B/Ca data. However, the exact mechanism of boron incorporation into biogenic calcite is still largely unknown. Their new data demonstrates that boron is heterogeneously distributed within single shells, suggesting that the boron uptake is biologically influenced by the foraminifer metabolism.



Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 reflected by benthic and planktic fossils Christina Ifrim et al., Institute of Geosciences, Ruprecht Karls University, 69120 Heidelberg, Germany; doi: 10.1130/G32161.1.

Oceanic Anoxic Events (OAEs), oceanic crises resulting from ocean acidification and a drastic reduction of oxygen dissolved in the water, repeatedly occurred in Earth history. They appear to be related to extreme Greenhouse climate. The study of OAEs' effects on marine organisms is thus of increased interest due to comparison with the recent climatic changes in climate and the increase of carbon dioxide in the atmosphere. The fossil record shows that shelled organisms suffered from these rapid chemical oceanic perturbations. One specific OAE occurred in the early Late Cretaceous about 94 million years ago, and caused massive faunal extinction on a global scale. One group of Cretaceous bivalves, the inoceramids, benefitted. They conquered hostile bottom environments at the seafloor. However, quantitative analysis of data from a section in northeastern Mexico by Christina Ifrim of Ruprecht Karls University, Germany, and colleagues suggests that inoceramid larvae remained vulnerable to the lack of oxygen. The team suggests that the abundance of these bivalves is a paleoproxy for oxygen levels in the seawater. Their data indicate that a stepwise increase of oxygen levels toward the end of the OAE led to a recovery of the marine ecosystem about 93 million years ago, including a fast evolution of the inoceramidae into numerous new species. This data thus provides insight into the paleobiology of these exceptional bivalves.



High pore pressure, or its absence, in the San Andreas Fault Chi-yuen Wang, Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720, USA; doi: 10.1130/G32294.1.

The weakness of the San Andreas Fault Zone, as revealed by a number of indirect observations, has major implications on the mechanics of earthquakes and the forces that move the lithospheric plates. Yet the mechanism for this weakness is not well understood. Several hypotheses have been proposed, each implying different faulting mechanisms, making it important to determine which is correct. Drilling projects into active faults have been conducted around the world to obtain direct information on the constitution and pore pressure in the fault. The SAFOD (San Andreas Fault Observatory at Depth) drilling project near Parkfield, California, is the best documented and reported. Observations made during drilling led some scientists to conclude that there was no evidence of high pore pressure in the fault cores. If the conclusion holds, it would have important implications on our understanding of earthquake mechanisms. Chi-yuen Wang of the University of California at Berkeley shows, however, that these observations may not be sufficient to reject the hypothesis of high pore pressure in the San Andreas fault; definite knowledge of pore pressure in the fault zone may require long-term monitoring at the SAFOD site. The result may also be useful for interpreting results from other active fault-drilling projects around the world.



Remarkably extensive glaciation and fast deglaciation and climate change in Turkey near the Pleistocene-Holocene boundary Marek Zreda et al., Dept. of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA; doi: 10.1130/G32097.1.

Looking at how climate changed in the geological past can provide a useful perspective for studying modern climate change and for predicting climate changes in the next century. Marek Zreda of the University of Arizona and colleagues use mountain moraines to reconstruct the former glaciers and to determine climatic changes in Turkey at the beginning of the Holocene, the current interglacial epoch. They found that the glaciers were unusually large for that time, with snow lines lower than present by more than 1400 meters, implying a temperature 9 degrees Celsius lower than modern long-term average temperature. The main glacier melting phase lasted 500 years during which the ice margin retreated at the average rate of 1700 meters per century, which is higher than modern glacier retreat rates computed over comparable time. This corresponds to the temperature increase at the rate of 1.4 degrees Celsius per century, which exceeds the global warming trend of the past century, 0.6 degrees Celsius, showing that natural causes can lead to fast and large climate changes, and that the magnitude and the rate of climate change observed in the past century are not unprecedented.



Seismically induced slump on an extremely gentle slope ( END