Boulder, CO, USA - GEOLOGY topics include "the best submarine record of displacement," geophysical data from the Black Sea, hazardous volcanic ice-slurry flows, the controversy over riverbank erosion rates, surface cracks in the Atacama Desert, CO2 sequestration, ultradeep Australian diamonds, Earth's magnetic field and the cosmic-ray-climate theory, fresh-water megafloods into the Pacific, early marine fossils preserved in French amber, tiny fossil fish teeth recovered by the Ocean Drilling Program, and alkaline groundwater at the dawn of land plant radiation.
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Post-glacial (after 20 ka) dextral slip rate of the offshore Alpine fault, New Zealand
Philip M. Barnes, National Institute of Water and Atmospheric Research, Private Bag 14 901, Kilbirnie, Wellington, New Zealand. Pages 3-6.
Barnes presents what is arguably the best submarine record of displacement rate yet documented on a major submarine plate boundary strike-slip fault. It will be of wide interest to a multidisciplinary global community of workers undertaking research on active plate boundaries, continental margins, and also zones of intracontinental deformation in which strike-slip faulting is significant. It is particularly relevant to current tectonic studies of western North America, where a substantial part of the Pacific-North America plate boundary zone is similarly offshore. The high-precision slip rates are derived for the southern, submarine part of the Alpine fault, New Zealand. The derivation of slip rate is innovative, utilizing dextral offsets of relict and dated last-glacial geomorphology imaged in high-quality multibeam bathymetric data. Such technology now provides an opportunity to image at high resolution the tectonic geomorphology associated with major submarine faults, at a precision now comparable to routinely available topographic data sets. Analysis of the displacements rates involves robust statistical treatment of the data. The post-glacial slip rate is determined to vary along strike from 27 mm/year in northern Fiordland to 31 mm/year off central Fiordland. These rates represent some of the highest strike-slip rates on Earth, and have implications for kinematic models of plate boundary deformation.
Abrupt transition from magma-starved to magma-rich rifting in the eastern Black Sea
Donna J. Shillington et al., Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, New York 10964, USA. Pages 7-10.
Continental breakup is a fundamental part of the plate tectonic cycle whereby continents rift apart and new oceans are formed. When a continent ruptures, magma can be generated at depth and released at the surface, but the amount of magma released can vary substantially, even along individual rifts. There are extensive observations of the most and least magmatic rifts, and well-developed theories to explain them, but until now no direct observations of the transition between them. Shillington et al. present new geophysical data from the eastern Black Sea that show for the first time the transition from a rift where there was almost no magma to a rift where there was voluminous magma. Surprisingly, this transition is very sharp, which indicates that magmas must be focused from a broad area at depth to create an abrupt shallow expression. Focusing of melts, together with gradual changes in the properties of the mantle, might also produce sharp changes in magmatism at other rifts around the world.
Explaining the extreme mobility of volcanic ice-slurry flows, Ruapehu volcano, New Zealand
Gert Lube et al., Volcanic Risk Solutions, Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand. Pages 15-18.
The near-invisibility of ice-slurry flows in the geological record belies their significant hazard at snow-capped volcanoes. These four-phase flows exhibit extreme rates of volumetric bulking and unusually high mobility. Lube et al. clarifies the mechanisms of their motion through two examples generated on 25 September 2007 at Mt. Ruapehu, New Zealand. Brief explosions through Crater Lake ejected 5700 cubic meters of acidic water that entrained 60 times this volume of snow as it traveled over a snow-covered glacier. The resulting ice slurry traveled up to 7.7 km. A co-generated second flow took a more tortuous initial path, before riding over the already frozen deposits of the first unit and beyond. For the first time, downstream evolution of the kinematic properties of propagating ice-slurry fronts could be characterized, as well as the longitudinal variation of the physical properties of their resulting deposits. The chemistry and composition of the deposits show that during flow, vertical percolation of water through the porous ice-particle-water-air mixture generated a basal zone of high internal pore pressure. This effect is particularly strong when a thick, high-density flow front forms, which races ahead of the tail, to control runout and consequent hazard.
Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia
Jim Pizzuto and Michael O'Neal, Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA. Pages 19-22.
Research published in Science early in 2008 suggested that the rivers in most of the eastern United States owe their morphology and ongoing processes to the construction of mill dams following European settlement, and their recent demise around the middle of the twentieth century. This idea was very controversial because huge expenditures for stream restoration and watershed management are based on the idea that streams of the region are equilibrium-meandering channels adjusted to a constant bankfull discharge. Pizzuto and Neal test one aspect of the controversial hypothesis at a field site in Virginia: that current bank erosion rates have increased in recent decades, due to the demise of mill dams. Using historical aerial photographs, they document an increase in river bank erosion rates after 1957, and find that the demise of mill dams along the river around the same time provides the best explanation for their observations.
Surface cracks record long-term seismic segmentation of the Andean margin
John P. Loveless et al., Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, New York 14853, USA. Pages 23-26.
Knowledge of the location and size of past earthquakes along the world's subduction zones can aid in assessing future seismic hazard. However, geologic evidence documenting the effects of historical earthquakes often exists only for the past several events, limiting our ability to recognize truly long-lived patterns of seismicity. Here Loveless et al. demonstrate that in the arid Atacama Desert, Chile, meter-scale surface cracks that open in response to large earthquakes preserve a record of the locations and extent of earthquakes over the past several hundred thousand years. They used high-resolution satellite imagery from Google Earth to map over 50,000 of these features in coastal regions of northern Chile and southern Peru. The distribution of these cracks indicates that large earthquakes have occurred in approximately the same regions for long periods of time, and they infer that future events will follow these large-scale patterns.
Long-term performance of a mudrock seal in natural CO2 storage
Jiemin Lu et al., School of GeoSciences, The University of Edinburgh, Edinburgh EH9 3JW, Scotland. Pages 35-38.
Geological storage of anthropogenic CO2 (carbon sequestration) is cited as a critical technology to battle against global warming, driven by increasing amounts of greenhouse gases in the atmosphere. Safe storage requires a robust cap rock to hold CO2 in the injection zone deep underground for tens of thousand years. Lu et al. studied a mudrock unit from an oilfield with naturally high CO2 under the North Sea. For the first time, they measured, using analysis of minerals in the rock, that CO2 only moved 10 meters (30 feet) in 60 million years. This proves that natural rock units can perform very well as safe seals to store injected CO2 and prevent upward leakage for time scales much longer than required.
Deep mantle diamonds from South Australia: A record of Pacific subduction at the Gondwanan margin
Ralf Tappert et al., Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, 5005, South Australia. Pages 43-46.
Tappert et al. have discovered that ultradeep diamonds from Australia formed in remnants of the Pacific plate, which was subducted deep beneath the former supercontinent Gondwana. Ultradeep diamonds from Eurelia, South Australia, grew at depths of around 670 km, which is much deeper than most other diamonds in the world. The composition of the diamonds provides evidence that they grew deep beneath the Gondwanan supercontinent in subducted oceanic rocks. The kimberlites that host the ultradeep diamonds in South Australia, and similar-aged ultradeep diamond deposits in southern Africa and South America, were emplaced along the margin of the former supercontinent Gondwana. This suggests that diamond formation and the kimberlite magmatism that brought the diamonds to the surface are related to the same subduction process that transferred the Pacific plate deep into the Earth's mantle. This process may have also caused the widespread basaltic volcanism in Gondwana during the Mesozoic.
Taking mylonites' temperatures
Matthew J. Kohn and C.J. Northrup, Dept. of Geosciences, Boise State University, Boise, Idaho 83725, USA. Pages 47-50.
Ductile shear zones are the mid- to deep-crustal expressions of faults, yet understanding their deformation behavior has proved elusive. One key problem is determining the temperatures at which shear zones deform. Kohn and Northrup show that a novel new thermometer, the titanium content of quartz, recovers temperatures in ductile shear zones with unprecedented precision. This result, in turn, improves estimates of the mechanical behavior of the crust, particularly the viscosities and strain rates during deformation. Applications indicate that some commonly observed textures result not simply from temperature, but also from huge differences in strain rate (four orders of magnitude).
Australian desert dune fields initiated with Pliocene-Pleistocene global climatic shift
Toshiyuki Fujioka et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Pages 51-54.
Using an innovative approach, burial dating using cosmogenic beryllium-10 and aluminium-26, Fujioka et al. reveal that Australian desert dune fields began to form one million years ago, when global ice age cycles changed their pace from 40 thousand years to 100 thousand years. This is the first paper to use cosmogenic nuclides for dating dune deposits, combined with the conventional luminescence method.
Wind-driven reorganization of coarse clasts on the surface of Mars
Jon D. Pelletier et al., Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA. Pages 55-58.
Individual rocks in some areas of Mars are regularly spaced. Here, Pelletier et al. show that rocks can move around on the surface of Mars and organize into a regular pattern due to the action of the wind, without being picked up. This occurs because of the way that air flows around the rocks, and the resulting pattern of erosion of the underlying sandy surface. Repeated cycles of erosion around the rocks and subsequent rolling cause them to move and organize spatially.
Evidence for focused magmatic accretion at segment centers from lateral dike injections captured beneath the Red Sea rift in Afar
Derek Keir et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Pages 59-62.
Keir et al. have captured the emplacement of two new vertical sheets of volcanic rock in Ethiopia, where Arabia and Africa are separating to create a new ocean. Observations of earthquakes and the motion of Earth's surface show that each new sheet of volcanic rock is about 10 km long, about 2 m wide, and created rapidly over approximately hour-long time scales. The new sheets of rock are initially molten, and travel horizontally in the upper 10 km of the Earth from a magma reservoir beneath a major volcano. The results of Keir et al. suggest that repeated emplacement of similar volcanic structures allows continents to split apart and new oceans to form.
Uplift and magma intrusion at Long Valley caldera from InSAR and gravity measurements
Pietro Tizzani et al., CNR-Istituto per il Rilevamento Elettromagnetico dell'Ambiente, Via Diocleziano 324, 80124 Naples, Italy. Pages 63-66.
A study by Tizzani et al. provides compelling evidence that magma is forcing Long Valley caldera, located in the east side of the Sierra Nevada in Central California, upward. The caldera has been the site of major geologic unrest in the past 33 years, characterized by 75 cm of uplift of the resurgent dome in the central section of the caldera, and earthquake activity followed by periods of relative quiescence. Tizzani et al. investigate the cause of unrest by a joint inversion of surface deformation measurements, based on differential synthetic aperture radar interferometry (InSAR), and microgravity data. The critical convergence of geodetic and microgravity data require a local mass increase beneath the resurgent dome consistent with magma intrusion, contributing to the two decades of Long Valley caldera unrest from 1980 through early 2000.
Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography
Claudio Chiarabba et al., Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, 00143 Rome, Italy. Pages 67-70.
In this study, Chiarabba et al. outline the space and time variations of body wave velocities which occurred during one of the best-documented series of normal faulting earthquakes, the 1997 Umbria-Marche sequence in central Italy. They show the first ever observations of rock fracturing and fluid overpressure propagation along a fault system by using space-time resolved variations of Vp/Vs anomalies (4D variations), which accompany earthquake migration and precede large aftershocks. The Vp/Vs increase observed before the mid-October earthquakes is related to a pore pressure increase on fluid-filled cracks in the volume around the fault. Chiarabba et al. also document that such variations are measurable by only using S- and P-wave arrival times.
Is there a link between Earth's magnetic field and low-latitude precipitation?
Mads Faurschou Knudsen and Peter Riisager, Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. Pages 71-74.
Based on an observed correlation between the Earth's magnetic dipole moment and oxygen isotope paleo-precipitation records from caves in Oman and southern China, Knudsen and Riisager suggest that the Earth's magnetic field may have influenced the amount of rainfall in low-latitude regions during the past 5000 years. The physical mechanism that underpins the geomagnetic-climate link is provided by the cosmic-ray-climate theory, which suggests that galactic cosmic ray (GCR) particles entering the atmosphere influence the formation of low-altitude clouds and, in turn, climate. The geomagnetic field shields the Earth from GCR particles and, according to the cosmic-ray-climate theory, it therefore has the potential to influence cloud formation, rainfall, and climate. Since the amount of GCR particles entering the atmosphere is also modulated by the Sun, the cosmic-ray-climate theory is central to the ongoing scientific debate regarding the role of the Sun in climate change. Knudsen and Riisager lend support to the notion that variations in the Earth's magnetic field may influence the climate of our planet. They also deliver independent support for certain aspects of the cosmic-ray-climate theory.
Testing the Mojave-Sonora megashear hypothesis: Evidence from Paleoproterozoic igneous rocks and deformed Mesozoic strata in Sonora, Mexico
Jeffrey M. Amato et al., Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Pages 75-78.
In the 1970s, an 800-km-long strike-slip Late Jurassic fault called the "Mojave-Sonora megashear" was postulated to explain the geologic similarities between rocks in the southwest United States and those in northwest Sonora, Mexico. To test this idea, Amato et al. determined the age and isotopic composition of granites in Sonora that would have been affected by this faulting. They determined that granites in the field area had uranium-lead zircon ages of 1763-1737 million years old and 1076 million years old, with isotopic compositions typical of rocks near Caborca, Mexico, on the southwest side of the fault, even though they were collected from the northeast side of the fault. This relationship was previously explained by 40 km of northward thrusting over the megashear, but Amato et al. used structural and stratigraphic relationships to show that the granites were not far-traveled, and that they must have been near their present location by 190 million years ago. These relationships are inconsistent with Late Jurassic megashear displacement and require either that no major strike-slip fault is present in Sonora or that strike-slip displacement occurred prior to Early Jurassic time.
Pleistocene megafloods in the northeast Pacific
C. Lopes and A.C. Mix, Departamento de Geologia Marinha, Laboratorio de Energia e Geologia, 2721-866 Amadora, Portugal. Pages 79-82.
Lopes and Mix show that anomalous amounts of fresh water entered the northeast Pacific Ocean from about 15,000 to 30,000 years ago. Some freshwater inputs can be linked to mega-floods from glacial lakes in land; other inputs may reflect deglacial water from massive ice sheets. These inputs (whatever the source of fresh water) were recorded in deep ocean sediments through microfossils (such as diatoms and foraminifera), and their study allows researchers to estimate past salinity variations for this particular area.
Exceptional preservation of marine diatoms in upper Albian amber
Vincent Girard et al., Universite Rennes 1, UMR CNRS 6118, Rennes, France. Pages 83-86.
Late Albian amber from Charente-Maritime (southwestern France) contains the first known marine diatoms preserved in a fossil resin. About 70 inclusions were assignable to the genera Basilicostephanus, Coscinodiscus, Hemiaulus, Melosira, Paralia, Rhizosolenia, Skeletonema, Stephanopyxis, Trochosira, and ?Aulacoseira (some of these likely represented by several species). The diatom assemblage is mainly composed of colonial planktonic genera, which are typical for coastal shallow waters. Girard et al. find that these amber inclusions extend the fossil record of six genera from the Late Cretaceous period and support certain molecular phylogenetic assumptions regarding the diversification of marine diatoms in the Early Cretaceous period. French amber probably recorded a transition from an Early Cretaceous ancestral microflora to a more modern one. Indeed, the fossils represent a typical Late Mesozoic diatom assemblage; however, several genera are new for the Early Cretaceous period. The unusual introduction of diatom shells from beach or sea by wind, spray, or high tide onto the resin flows was possible because the amber forest grew close to the seashore.
Circulation through the Central American Seaway during the Miocene carbonate crash
Derrick R. Newkirk and Ellen E. Martin, Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. Pages 87-90.
Tiny fossil fish teeth found in deep sea sediments record past variations in the neodymium (Nd) isotopic composition of bottom waters, which can be used to reconstruct past ocean circulation patterns. Nd data from teeth recovered from Ocean Drilling Program sites in the eastern equatorial Pacific and Caribbean Basin indicate that during the mid to late Miocene (about 16-6 million years ago), prior to closure of the Isthmus of Panama, the predominant direction of intermediate-to-deep water flow through the Central American Seaway was from the Pacific to the Atlantic. Although surface wind patterns suggest shallow flow from east to west, several General Circulation Models predict west-to-east flow at deeper levels, and Newkirk and Martin's Nd isotopic data present a unique method to corroborate this prediction. In particular, the Nd data indicate that the Caribbean was essentially filled with waters sourced with corrosive Pacific waters during intervals of low carbonate accumulation, referred to as the Caribbean Carbonate Crash. In addition, times of greater Pacific throughflow identified using Nd isotopes correlate to intervals of enhanced formation of deep water in the North Atlantic. This result contradicts most General Circulation Model results that predict North Atlantic waters were too fresh to sink when there was strong equatorial exchange through the Central American Seaway.
Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation
Pierre Jutras et al., Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada. Pages 91-94.
Jutras et al. present evidence from ancient petrified soils for surface conditions of high alkalinity, close to that of household bleach, at the dawn of land plant development, 450 to 475 million years ago. They argue that the ultra-alkaline conditions at that time are the result of a gradual decrease in atmospheric carbon through geologic times. Atmospheric carbon normally combines with water to produce acid rain, and decreasing atmospheric concentrations therefore result in less acidic rains. However, the strong acids produced by land plants prevent most modern soils from developing the alkalinity that would otherwise naturally develop through groundwater-mineral interactions.
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