|11 March 2011
GSA Release No. 11-17
Director of Education, Communication, & Outreach
NASA satellite image of the U.S. coast along North
and South Carolina.
See Thieler and Ashton, "'Cape capture': Geologic data and modeling results suggest the Holocene loss of a Carolina Cape," doi: 10.1130/G31641.1.
GEOLOGY HIGHLIGHTS: NEW SCIENCE POSTED ONLINE 8 MARCH 2011
Boulder, CO, USA – Highlights of articles scheduled for GEOLOGY (posted online 8 March 2011) are provided below. Topics include the April 2009 L'Aquila earthquake; the Taupo Volcanic Zone on New Zealandís North Island; high-resolution autonomous underwater vehicle (AUV) seafloor images of the "Lucia Chica" area offshore central California; the legacy of land-use by aboriginal peoples and Europeans arriving in North America; and another entry in the long-standing debate over the Carolina Capes.
GEOLOGY is now regularly posting pre-issue publication content — finalized papers that are ready to go to press and not under embargo. GSA invites you to sign up for e-alerts and/or RSS feeds to have access to new journal content the minute it is posted online. Go to http://www.gsapubs.org/cgi/alerts and enter your e-mail address to manage your subscriptions for pre-issue postings, full tables of contents alerts, and more.
Keywords: L'Aquila earthquake, rare earth elements, Jiangnan fold belt, South China, Millbrig K-bentonite, Kinnekulle K-bentonite, Hamersley Basin, Paraburdoo spherule layer, Taupo Volcanic Zone, salt diapirs, snowball Earth, early Aptian oceanic anoxic event, Lucia Chica, ocean chemistry, ocean volcanoes, Panama, cape capture, Carolina, Global Ocean Conveyor, Deep Western Boundary Currents, serpentinite, KORDI scientific team, Levant Basin, land-use, aboriginal people, North America, Pasig-Potrero River, Mount Pinatubo, Mount St. Helens, Pennsylvanian fossil forests, coal forests, New Mexico, coniferopsid woodland, Pliocence free-living corals, Paleocene Sierra Nevada, Kerguelen Plateau, fossil fish teeth, double domes, Oman ophiolite, Integrated Ocean Drilling Program, NanTro SEIZE, zircon dating, subvolcanic plumbing systems, Frankland River, continental crust, Izu-Bonin Trench
Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. GEOLOGY abstracts are available at http://geology.gsapubs.org/.
Non-media requests for articles may be directed to GSA Sales and Service, .
Evidence for localized active extension in the central Apennines (Italy) from global positioning system observations
Nicola D'Agostino et al., Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, Via di Vigna Murata 605, 00143 Rome, Italy. First published online 8 March 2011; doi: 10.1130/G31796.
D'Agostino et al. describe the distribution of present-day tectonic deformation in the central Apennines area (central Italy) using global positioning system measurements. This region has been affected by several destructive earthquakes in the past, and suffered from the April 2009 Mw 6.3 L'Aquila earthquake. Understanding the present-day distribution of deformation, which is likely to be released in future earthquakes, contributes to the improvement of seismic hazard assessment. D'Agostino et al.'s observations support the hypothesis that elastic deformation is accumulating on a 50-km-wide area (perpendicular to the Apennines) that produced the largest historical earthquakes. A striking correlation between active deformation and distribution of regional topographic elevations also supports the hypothesis that the localization of deformation is controlled by differences in horizontal variations in gravitational potential energy.
Unusual rare earth element fractionation in a tin-bearing magmatic hydrothermal system
Thomas Monecke et al., Dept. of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA. First published online 8 March 2011; doi: 10.1130/G31659.1.
Normalized rare earth element patterns of rock and mineral samples from evolved granitic systems and associated hydrothermal tin deposits frequently show an unusual split into four consecutive curved segments, referred to as tetrads. Thomas Monecke of the Colorado School of Mines and colleagues describe for the first time the simultaneous occurrence of complementary convex and concave tetrads for vein fluorite, suggesting that this unusual trace element signature may develop during a single evolutionary stage within the hydrothermal environment. Based on geochemical analysis of vein fluorite, fluid inclusion studies, and thermodynamic modeling, Monecke and colleagues show that fractionation of rare earth elements can be linked to fluid immiscibility and preferential partitioning of these elements between vapor and coexisting liquid. The findings provide the first direct evidence constraining the geological conditions responsible for the occurrence of the tetrad effect in tin-bearing magmatic-hydrothermal systems, and require a reassessment of the current understanding of the origin of tin deposits.
Reappraisal of the ages of Neoproterozoic strata in South China: No connection with the Grenvillian orogeny
Jun-Hong Zhao et al., State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, People’s Republic of China First published online 8 March 2011; doi: 10.1130/G31701.1.
The Jiangnan fold belt separates the Yangtze and Cathaysia blocks in South China and has long been considered Grenvillian in age in order to accommodate South China in central Rodinia. It consists of deformed Early Neoproterozoic strata that are unconformably overlain by undeformed Late Neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons. Zircons from the Early Neoproterozoic strata yield U-Pb ages as young as 830 million years old, and one granitic pluton has a zircon U-Pb age of about 827 million years old. The Early Neoproterozoic strata are proposed to have been deposited on an active continental margin, prior to amalgamation of the Yangtze and Cathaysia blocks around 830 million years ago. The overlying Late Neoproterozoic strata were deposited in the intra-continental rifted Nanhua Basin 820 to 730 million years ago, resulted from back-arc spreading above the oceanic subduction zone along the northern and western margins of the Yangtze block. The Jiangnan fold belt is therefore not a Grenvillian feature as previously suggested, and there is no evidence to place South China in central Rodinia. Instead, Jun-Hong Zhaoof China University of Geosciences and colleagues believe that South China was located in a marginal position of this supercontinent.
Apatite phenocryst compositions demonstrate a miscorrelation between the Millbrig and Kinnekulle K-bentonites of North America and Scandinavia
Bryan K. Sell and Scott D. Samson, Dept. of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA. First published online 8 March 2011; doi: 10.1130/G31425.1.
Bryan K. Sell and Scott D. Samson of Syracuse University contribute to an ongoing debate concerning the relationship between two ash-fall beds that have been proposed to be the result of one of the largest eruptions of the Phanerozoic. Both ash-fall beds are approximately 453 million years old, with one located in North America (Millbrig K-bentonite) and the other in Scandinavia (Kinnekulle K-bentonite). Conventional geologic methods cannot be used to assess whether these two beds are from one mega-eruption because ash (mostly volcanic glass) is not entirely preserved after millions of years. However, many small crystals of volcano-derived apatite are usually preserved despite the harsh sedimentary environments that decomposed much of the original volcanic ash. To test whether these two beds arc from the same gigantic eruption, Sell and Samson examine the composition of the very small and well-preserved crystals of volcanic apatite. The apatite trace element composition indicates that these two beds do not comprise one gigantic eruption. Also, an examination of the chemical variation among apatite crystals at different levels within these ash-fall beds suggests that the ash may have been derived from multiple eruptions. The ash is likely from large-magnitude eruptions; however, they may not have been as voluminous as once thought.
Paraburdoo spherule layer (Hamersley Basin, Western Australia): Distal ejecta from a fourth large impact near the Archean-Proterozoic boundary
Scott W. Hassler et al., Geology Dept., Oberlin College, Oberlin, Ohio 44074, USA. First published online 8 March 2011; doi: 10.1130/G31526.1.
Scott W. Hassler of Oberlin College and colleagues report the discovery of a thin layer of formerly molten sand-size spherules among sedimentary strata in the Hamersley Basin, Western Australia. The layer is similar to the one formed 65 million years ago by the impact that created Chicxulub crater, but the new layer is about 40 times older; it formed when a large asteroid struck Earth about 2.57 billion years ago. At least four discrete asteroid impacts have produced spherule layers in the Hamersley Basin, all within a span of about 140 million years. Having this many large impacts in such a short time raises questions about whether the rate of impacts fluctuated during Earth's history. The original components of the spherules were replaced by other minerals over the eons, but textures preserved inside the spherules indicate the impact probably happened somewhere in the ocean rather than on dry land. It is a close match for a spherule layer of similar age in the Griqualand West Basin (South Africa). If confirmed by subsequent work, this correlation demonstrates that under favorable circumstances, scientists can use impact ejecta to pinpoint strata deposited on separate continents within the same few hours billions of years ago.
Degassing of the H2O-rich rhyolites of the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand
Emily R. Johnson et al., ARC (Australian Research Council) Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia. First published online 8 March 2011; doi: 10.1130/G31543.1.
The Taupo Volcanic Zone (TVZ), on the north island of New Zealand, is one of the most volcanically active regions on Earth. Here Emily R. Johnson of the Australia Research Council, University of Tasmania, and colleagues use the analysis of melt inclusions -- tiny blebs of melt trapped in a crystal as it grows in a magma -- to obtain the geochemical compositions and the concentration of volatile elements (water, chlorine, and carbon dioxide) dissolved in melts from eight recent (more than 60,000 years ago) eruptions. Johnson and colleagues measure high concentrations of dissolved volatiles in the TVZ melts, indicating crystallization occurred at depths of about 4 to 8 km in the crust. As the magmas ascended from these depths, the dissolved volatiles began to move into a vapor phase forming bubbles (much like a bottle of soda when the lid is removed). Comparison of the volatile contents of volcanic glasses erupted at the surface and the melt inclusions trapped at depth provides an estimate of the extent of gas loss during ascent and eruption. Their estimates suggest that the total emissions of water and chlorine into the atmosphere during the TVZ eruptions were high, contributing roughly 10 to 100% (depending on the eruption) of the average yearly global volcanic arc output of these gases.
Southern Louisiana salt dome xenoliths: First glimpse of Jurassic (ca. 160 Ma) Gulf of Mexico crust
Robert J. Stern et al., Geosciences Dept., The University of Texas at Dallas, Richardson, Texas 75083-0688, USA. First published online 8 March 2011; doi: 10.1130/G31635.1.
In spite of its importance to the economy and environment of the United States, Mexico, and Cuba, there is much still to learn about the Gulf of Mexico. It opened by seafloor spreading about 160 million years ago when the supercontinent Pangea broke up, but scientists have until now not been able to study samples of the seafloor lavas formed at that time. These are deeply buried beneath 6 to 10 miles of sediment. Salt diapirs -- which rise like blobs in a lava lamp from the bottom of the Gulf of Mexico sediment pile -- often pluck fragments of surrounding rocks and carry them toward the surface, where they are found in salt mines. Robert J. Stern of The University of Texas at Dallas and colleagues analyze samples of lava from southern Louisiana salt bodies that were brought to the surface in this way. Their results, including radiometric age, mineral and whole rock chemical composition, and isotopic composition show that these are unusual lavas, made up of a mixture of minerals from the mantle that were overprinted by magmas generated by low degrees of mantle melting. In conjunction with magnetic data for the region, these results support earlier suggestions that the Louisiana margin formed as a magma-poor rift, in contrast to the magma-rich Texas margin. These results also encourage the search for similar material in other salt mines around the region.
Condensation origin for Neoproterozoic cap carbonates during deglaciation
Martin J. Kennedy and Nicholas Christie-Blick, Dept. of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia. First published online 8 March 2011; doi: 10.1130/G31348.1.
The great "snowball Earth" ice ages at the end of the Precambrian ended in what has been proposed by some to be the greatest global warming event in Earth history. Much of the evidence for that warming event comes from a blanket of carbonate (limestone and dolomite) that drapes glacial deposits throughout the world. This enigmatic deposit has been suggested to record a widespread chemical precipitation event as carbon dioxide from the atmosphere, generated from oxidation of methane clathrates or built up during the snowball Earth glaciation, came to equilibrium with the oceans. Martin J. Kennedy of the University of Adelaide and Nicholas Christie-Blick of Columbia University suggest that the cap carbonates record more typical processes of background sedimentation of calcium carbonate, concentrated on the shelves by a major rise in sea level that pushed other types of sediment landward. Background carbonate precipitation throughout the oceans would likely have been more widespread in the Precambrian, prior to the advent of carbonate-secreting organisms, and the magnitude of the postglacial rise likely concentrated these sediments in the global deposit evident today. The longer duration of deposition of these deposits contrasts with earlier ideas of catastrophic deposition following the snowball Earth deglaciations, to a more typical pattern of carbon burial.
Transient global cooling at the onset of early Aptian oceanic anoxic event (OAE) 1a
Wolfgang Kuhnt et al., Institut fur Geowissenschaften, Christian-Albrechts-Universitat zu Kiel, Olshausenstrasse 40, 24118 Kiel, Germany. First published online 8 March 2011; doi: 10.1130/G31554.1.
Cretaceous anoxic events mark striking episodes in Earth's climate history, when marine productivity became substantially enhanced, oxygen-deficient conditions prevailed in the ocean, and organic matter became preserved across environments ranging from the deep ocean to shelf seas. To date, the trigger and climatic repercussions of such events remain enigmatic, and continue to be the source of vigorous debate. Wolfgang Kuhnt of Christian-Albrechts-Universitat zu Kiel and colleagues provide new records from a subtropical, open marine carbonate shelf succession (La Bedoule, southeastern France) with minimal diagenetic overprint track the onset of the early Aptian anoxic event (about 120 million years ago) in unprecedented resolution. Kuhnt et al.'s records reveal that the onset of this major perturbation of the carbon cycle was accompanied by protracted climate swings, reflecting an intricate interplay between atmospheric carbon dioxide release and drawdown. Unraveling forcing mechanisms and climate feedbacks during ancient greenhouse climates and oceanic anoxic events is especially relevant as we move into a new era of high carbon dioxide and human-induced climate change.
The elusive character of discontinuous deep-water channels: New insights from Lucia Chica channel system, offshore California
Katherine L. Maier et al, Dept. of Geological and Environmental Sciences, Braun Hall Building 320, Stanford University, Stanford, California 94305, USA. First published online 8 March 2011; doi: 10.1130/G31589.1.
Self-propelled devices "flying" at low altitudes above the seafloor are revealing unprecedented details of submarine topography unattainable from ship-towed instruments. Katherine L. Maier of Stanford University and colleagues present new high-resolution autonomous underwater vehicle (AUV) seafloor images that reveal unexpected rugosity and low-relief (greater than 10 m), discontinuous conduits over ~70 square kilometers of the seafloor offshore central California, in an area informally termed the "Lucia Chica." The high-resolution images capture multiple, disconnected, low-relief channels and other seafloor features, including scours, buried channels, megaflutes, channel floor textures, and other unidentified depressions, which were not known to exist in the Lucia Chica. Continuous channels were interpreted from lower-resolution images, but newly acquired AUV data indicate that a single sinuous channel fed a series of discontinuous lower-relief channels. These discontinuous features formed when channels shifted position, were cut off from feeders, and were subsequently reactivated. The diverse features now imaged in the Lucia Chica channel system (joining the Lucia Canyon, offshore California) are likely common in modern and ancient systems with similar overall morphologies, but have not been previously mapped with lower-resolution detection methods.
The role of SO4 in the switch from calcite to aragonite seas
P. Bots et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. First published online 8 March 2011; doi: 10.1130/G31619.1.
Several times during Earth's history, the chemistry of the oceans has changed dramatically. The last major change was simultaneous with the extinction event that wiped out the dinosaurs. So far, scientists know that these changes in chemistry forced evolution of marine organisms and their biominerals (e.g., shells) causing a change from calcite- to aragonite-forming organisms (the main components of chalk and limestone). However, it is not well understood what these changes were, and how ocean chemistry influences biomineral formation. P. Bots of the University of Leeds and colleagues developed a unique laboratory method to recreate conditions in the ancient oceans, and used computer models to look at how ocean water evaporates to produce salt deposits, to study prehistoric ocean chemistry. Their work shows that in addition to magnesium (which scientists believed mainly determines biomineral formation), the sulfate content of seawater also played a key role in controlling which biomineral was favored by ancient marine organisms. Bots et al. also found that the magnesium and sulfate content in the oceans must have been much less than previously thought. These findings may have important implication for future projections of changing ocean chemistry (e.g., acidification).
Oceanic intraplate volcanoes exposed: Example from seamounts accreted in Panama
David M. Buchs et al., Institut de Geologie et Paleontologie, Universite de Lausanne, CH-1015 Lausanne, Switzerland. First published online 8 March 2011; doi: 10.1130/G31703.1.
The study of volcanoes in the ocean, which form islands such as those observed in Hawaii, French Polynesia, or the Galapagos Archipelago, has increased the understanding of the dynamics of Earth's interior. However, poor access to the interior of these volcanoes has limited the extent of this knowledge. Volcanoes formed in the ocean travel long distances because of plate tectonics and related motion of the ocean floor. Ultimately, the plate driving the volcanoes will plunge under another plate and be recycled into the mantle. During this process, ancient oceanic islands can be scraped off the ocean floor and be exhumed and eroded at Earth's surface. David M. Buchs of Universite de Lausanne, Switzerland, and colleagues show that such ancient ocean volcanoes occur in Panama, and their internal anatomy remained remarkably well preserved during their exhumation. Buchs et al. propose that similar occurrences of ancient ocean volcanoes may occur elsewhere in the world, which will help scientists to better characterize the dynamics of Earth's interior.
'Cape capture': Geologic data and modeling results suggest the Holocene loss of a Carolina Cape
E. Robert Thieler and Andrew D. Ashton, U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. First published online 8 March 2011; doi: 10.1130/G31641.1.
For more than a century, the consensus view of the origin and evolution of the Carolina Capes -- Hatteras, Lookout, Fear, and Romain -- along the U.S. East Coast has been that these capes have existed for much or all of the past 15,000 years, and that the number of capes has not changed. But is it possible that there were more than four Carolina Capes in the past? E. Robert Thieler of the U.S. Geological Survey and Andrew D. Ashton of Woods Hole Oceanographic Institution revisit the long-standing debate over the origins of the Carolina Capes, bringing together geologic information with new numerical modeling techniques. Thieler and Ashton suggest that a cape existed southwest of Cape Hatteras, North Carolina, between 9000 and 4000 years ago, but was abandoned as the coast changed over time. Their study provides new evidence for what may be a widespread process in coastal evolution. The process, termed "cape capture," suggests that the number and spacing of capes can be dynamic, and that a coast can self-organize in response to sand transport by waves. In their shoreline evolution model simulations, smaller capes are subsumed or “captured” by larger neighbors over several-thousand-year time scales. Thieler and Ashton suggest this capture event may be the first documented field evidence of a theoretically predicted large-scale dynamic coastal change.
Coupled deep-water flow and climate variability in the middle Pleistocene North Atlantic
H.F. Kleiven et al., Bjerknes Centre for Climate Research, Uni Research, Allegaten 55, N-5007 Bergen, Norway. First published online 8 March 2011; doi: 10.1130/G31651.1.
Rapid changes in the strength of the Global Ocean Conveyor are widely believed to have caused abrupt climate changes in the past. A sensitive place to assess this circulation change is in the cold return flow of Deep Western Boundary Currents. Detailed paleoceangraphic reconstructions by H.F. Kleiven of Norway’s Bjerknes Centre for Climate Research and colleagues show a strong coupling between deep ocean flow speeds, and that climate conditions occurred at a range of time scales ranging from 1000 to 100,000 years some 0.75 million years ago. This suggests that, given the right boundary conditions, rapid climate shifts can also occur during relatively warm climate conditions, in comparison to more recent times.
Hyperextension, serpentinization, and weakening: A new paradigm for rifted margin compressional deformation
Erik R. Lundin and Anthony G. Doré, Statoil ASA, Trondheim 7005, Norway. First published online 8 March 2011; doi: 10.1130/G31499.1.
There is mounting evidence that Earth's crust (in large sections of its ocean margins) became highly stretched, or hyperextended, before continental break-up. In these areas, water percolates through the thin crust, reacting with the mantle underneath to form a rock called serpentinite. Because serpentinite is weak and easily deformed, and the crust is thin, Erik R. Lundin of Statoil ASA and colleague Anthony G. Doré believe that these areas react more readily to compressive forces, resulting in folding. They use evidence from the northeast Atlantic Ocean to show a convincing connection between these hyperextended areas and previously unexplained folding episodes. It may also be that folding on ocean margins, not normally expected to deform, indicates that weak, hydrated mantle lies beneath.
Metal-bearing molten sulfur collected from a submarine volcano: Implications for vapor transport of metals in seafloor hydrothermal systems
Jonguk Kim et al., Deep-sea and Marine Georesources Research Dept., Korea Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, Korea. First published online 8 March 2011; doi: 10.1130/G31665.1.
A metal-bearing molten sulfur pool was found at an active, off-axis caldera in the northeastern Lau Basin, southwestern Pacific Ocean. The KORDI scientific team deployed a dredge haul to the central volcanic cone where vigorous hydrothermal activity was detected. The dredge haul was returned with unexpected, solidified molten sulfur adhering to its frame and chain bag. More interesting is the fact that the molten sulfur shows high metal content, especially copper. The metals occur as small, irregular-shaped inclusions of copper sulfide (covellite), possibly precipitated directly from magmatic vapor. The enriched metals in the molten sulfur were found to be commonly associated with magmatic fluid. The role of magmatic fluids is poorly understood in the formation of submarine massive sulfide deposits, especially in terms of the source of metals. This work by Jonguk Kim of Korea Ocean Research and Development Institute and colleagues provides field-scale evidence for metal transport via sulfur-rich volcanic degassing in the submarine massive sulfide deposits. The occurrence of a molten sulfur pool would play an important role in enhancing the efficiency of transferring magmatic fluid (and metals) to the submarine hydrothermal system.
Origin and nature of the rapid late Tertiary filling of the Levant Basin
J. Steinberg et al., Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel. First published online 8 March 2011; doi: 10.1130/G31615.1.
The recent huge gas discovery in Early Miocene siliciclastics offshore Israel highlighted a new world-class gas province, and has subsequently sparked great interest in the deep-water sediments of the Levant Basin. J. Steinberg of the Geological Survey of Israel and colleagues present two fundamental observations. First, that more than half of its sedimentary column accumulated within a mere 15% of the basin's life span, that is, a 6-km-thick section within 37 million years. Second, that this young section first accumulated in the deep basin and only then did large amounts of sediment amass along the eastern Levant continental margin. These fundamental observations allude to two feasible source-to-sink scenarios. One, that the thick late Tertiary section is composed of large amounts of terrigenous material that originated in Arabia and was transported via the Israeli continental margin, and two, that the offshore material originated in Africa and was transported northward via the region that eventually evolved into the Nile River. Steinberg et al.'s data emphasize the difficulties of the first scenario and suggest that Africa was the dominant sediment source, though turbidity currents probably entered the basin both from south and east. This suggestion raises new questions regarding ocean circulation at a time in which the neo-Tethys was closing and the marine connection to the Mesopotamian Basin gradually ceased.
Middle Miocene reorganization of deformation along the northeastern Tibetan Plateau
Richard O. Lease et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA. First published online 8 March 2011; doi: 10.1130/G31356.1.
Large expanses of high terrain on the Tibetan Plateau represent the foremost expression of the collision of India with Asia. When and how mountain ranges of diverse orientations on the Tibetan Plateau began growing remains a key parameter in assessing how plateau deformation evolves over tens of millions of years. Richard O. Lease of the University of California at Santa Barbara and colleagues determine the low-temperature cooling histories of rocks that have been brought up to the surface of the earth by tectonic compression. The onset of rapid cooling and erosion of the rocks indicates the initiation of mountain range growth. By documenting the timing of growth of different mountain ranges on the northeastern margin of the Tibetan Plateau, Lease et al. interpret a reorganization of plateau deformation in the Middle Miocene (about 13 million years ago). At that time, long-standing NNE-SSW compression that mimicked the direction of the India-Asia collision gave way to the inclusion of new structures accommodating the east-west motion of Tibet.
Pre-colonial (A.D. 1100-1600) sedimentation related to prehistoric maize agriculture and climate change in eastern North America
G.E. Stinchcomb et al., Terrestrial Paleoclimatology Research Group, Dept. of Geology, Baylor University, Waco, Texas 76798-7354, USA. First published online 8 March 2011; doi: 10.1130/G31596.1.
Hunting, gathering, and farming practices of the aboriginal peoples of eastern North America affected landscapes and indirectly impacted drainage systems several hundred years prior to the arrival of Europeans, according to Gary E. Stinchcomb of Baylor University and colleagues. Using a suite of geoarchaeological techniques combined with an extensive literature review, Stinchcomb et al. show that intensified American Indian land use and maize-based farming from A.D. 1100 to 1600 likely fostered erosion during the Medieval Warm Period and Little Ice Age climate intervals. Together, these processes are reflected in a marked increase in sedimentation resulting from the periodic flooding of streams. "There is growing interest in the environmental impact Europeans had upon their arrival in the New World, largely due to modern-day stream restoration efforts. Our study shows that we must also concern ourselves with the legacy of much earlier land use and its unintended effects on streams," say Gary Stinchcomb.
Long-term elevated post-eruption sedimentation at Mount Pinatubo, Philippines
Karen B. Gran et al., Dept. of Geological Sciences, University of Minnesota, 1114 Kirby Drive, Duluth, Minnesota 55812, USA. First published online 8 March 2011; doi: 10.1130/G31682.1.
The 1991 eruption of Mount Pinatubo (Philippines) was the second largest eruption of the 20th century, resulting in record high sediment yields in rivers draining the volcanic flanks. Sediment yields declined exponentially for the first decade following the eruption. Research by Karen B. Gran of the University of Minnesota-Duluth and colleagues measures the rate of deposition on the upper alluvial fan of the Pasig-Potrero River from 1997 to 2009. The results show that sediment yields remain higher than predicted two decades after the eruption, with depositional rates of 0.7 m/yr from 2002 to 2009. This finding compares well with results from the North Fork Toutle River at Mount St. Helens (United States) following the 1980 eruption. Post-eruptive sediment yields at both volcanoes went through two phases of erosion and sediment export. In the first, erosion of hillslope tephra and carving of the initial valley network lead to high sediment yields that declined exponentially over 5 to 10 years. In the second phase, continued valley widening and river instability have maintained high sediment yields for at least two decades. A comparison of post-eruption deposition with pre-eruption sediment yields indicates that the depositional record in century to millennial time scales is dominated by sediment from eruptions.
Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome
Howard J. Falcon-Lang et al., Dept. of Earth Sciences, Royal Holloway, University of London, Surrey TW20 0EX, UK. First published online 8 March 2011; doi: 10.1130/G31764.1.
Pennsylvanian fossil forests are known from hundreds of sites across tropical Pangea, but nearly all comprise remains of humid coal forests. Here, Howard J. Falcon-Lang of the University of London and colleagues report a unique occurrence of seasonally dry vegetation, preserved in growth position along less than 5 km of strike, in the Pennsylvanian (early Kasimovian, Missourian) of New Mexico (United States). Analyses of stump anatomy, diameter, and spatial density, coupled with observations of vascular traces and associated megaflora, show that this was a deciduous, mixed-age, coniferopsid woodland (about 100 trees per hectare) with an open canopy. The coniferopsids colonized coastal sabkha facies and show tree rings, confirming growth under seasonally dry conditions. Such woodlands probably served as the source of coniferopsids that replaced coal forests farther east in central Pangea during drier climate phases. Thus, the newly discovered woodland helps unravel biome-scale vegetation dynamics and allows calibration of climate models.
Rise and fall of Pliocene free-living corals in the Caribbean
James S. Klaus et al., Dept. of Geological Sciences, University of Miami, Coral Gables, Florida 33124, USA. First published online 8 March 2011; doi: 10.1130/G31704.1.
Climate change is already impacting coral reef ecosystems around the globe, and models of future climate change predict levels of greenhouse gases and temperature will rise within the next century above anything seen for at least the past 650,000 years. The Pliocene epoch (5.3 to 2.6 million years ago) may provide a geologic analog for such conditions in the Caribbean. James S. Klaus of the University of Miami and colleagues highlight a striking difference between modern and Pliocene coral communities. The Pliocene epoch was characterized by a greater diversity of free-living corals. Unlike most reef corals, these corals lived unattached to the sea floor. Free-living corals were well suited to the increased productivity, increased sedimentation, and warmer temperatures of the Pliocene. Between 8 and 4 million years ago, the origination of new free-living coral species was roughly double that of other corals. However, free-living corals experienced abrupt extinction between 2 and 1 million years ago as environmental conditions changed and suitable habitat was eliminated. The free-living corals of the Pliocene would have been well suited to predicted ocean conditions for this century. However, the evolutionary bottleneck of Pliocene-Pleistocene extinctions may leave modern coral faunas vulnerable with the return to Pliocene-like conditions.
Sedimentologic and isotopic constraints on the Paleogene paleogeography and paleotopography of the southern Sierra Nevada, California
Alex R. Lechler and Nathan A. Niemi, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. First published online 8 March 2011; doi: 10.1130/G31535.1.
The age and elevation history of the Sierra Nevada mountain range (California, USA) is a contentiously debated topic. Alex R. Lechler and Nathan A. Niemi of the University of Michigan report detrital zircon U-Pb ages from Paleocene to Eocene sandstones that span the southernmost Sierra Nevada, accompanied by delta-18O values of interfingering lacustrine carbonates. The detrital zircon age "fingerprint" found in sandstones located within and to the east of the present-day Sierra Nevada demonstrate an integrated river system with a source region located in the modern Owens Valley, and which traversed westward across the Paleocene Sierra Nevada to the Pacific Ocean. Previously described marine fossils in the eastern section place these strata at or near sea level in Paleocene time. Eocene sandstones to the west of the Sierra Nevada reveal only western Sierra Nevada detrital zircons, suggesting that the Paleocene river system was redirected by this time. These data, combined with the elevation of present-day exposures of the sections within and to the east of the Sierra Nevada, requires the southern Sierra Nevada to have undergone about 1.5 km of surface uplift since the Paleocene.
Two-stepping into the icehouse: East Antarctic weathering during progressive ice-sheet expansion at the Eocene-Oligocene transition
Howie D. Scher et al., Ocean Sciences Dept., and Institute of Marine Sciences, University of California, Santa Cruz, California 95064, USA. First published online 8 March 2011; doi: 10.1130/G31726.1.
In conjunction with increasing benthic foraminiferal delta-18O values at the Eocene-Oligocene transition (EOT; ~34 million years ago), coarse-grained, ice-rafted debris (IRD) appears abruptly alongside fossil fish teeth with continentally derived neodymium (Nd) isotope ratios in Kerguelen Plateau (Southern Ocean) sediments. Increased Antarctic weathering flux, as inferred from two steps to less radiogenic Nd isotope ratio values, coincides with two steps in benthic foraminiferal delta-18O values. These results, presented by Howie D. Scher of the University of California at Santa Cruz, indicate that two distinct surges of weathering were generated by East Antarctic ice growth during the EOT. Weathering by ice sheets during a precursor glaciation at 33.9 Ma did not produce significant IRD accumulation during the first Nd isotope ratio shift. Glacial weathering was sustained during a terrace interval between the two steps, probably by small high-elevation ice sheets. A large increase in weathering signals the rapid coalescence of small ice sheets into an ice sheet of continental proportions circa 33.7 million years ago. Rapid ice-sheet expansion resulted in a suppression of weathering due to less exposed area and colder conditions. Parallel changes in Antarctic weathering flux and deep-sea carbonate accumulation suggest that ice-sheet expansion during the EOT had a direct impact on the global carbon cycle; possible mechanisms include associated changes in silicate weathering on the East Antarctic craton and enhanced fertilization of Southern Ocean waters, both of which warrant further investigation.
Viscous collision in channel explains double domes in metamorphic core complexes
Patrice F. Rey et al., Earthbyte Group, School of Geosciences, The University of Sydney, Sydney NSW 2006, Australia. First published online 8 March 2011; doi: 10.1130/G31587.1.
Hot rocks, formed deep in the crust through partial melting, are often exposed at Earth's surface as domes surrounded by much cooler sedimentary rocks. This strange association demands an explanation. Erosion alone, the most obvious exhumation process, cannot be invoked because this process would affect the sedimentary rocks as well. Patrice F. Rey the University of Sydney and colleagues present a computer model that shows that the deep and hot continental crust can be sucked up into gaps created by the extending upper crust. In this process, the hot lower crust flows into the opening gap, forming a double domes structure that mimics that observed in natural gneiss domes.
Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite
Benedicte Abily et al., Toulouse University, OMP (Observatoire Midi-Pyrenees), CNRS-UMR 5562, 14 avenue E. Belin, 31400 Toulouse, France. First published online 8 March 2011; doi: 10.1130/G31652.1.
This structural and petrological study of the Oman ophiolite by Benedicte Abily of Toulouse University and colleagues shows that normal faults at oceanic spreading centers can root in incompletely crystallized cumulates and form at the same time as melt is being supplied to the magma chamber. In the vicinity (greater than or equal to 200 m) of a ridge-facing normal fault located a couple of kilometers off a former axial mantle diapir, blocks of layered cumulates were tilted at high angles relative to the Moho. Block rotation was accommodated by a swarm of anastomosed fault planes connected to the main fault. These planes are underlain by screens of gabbroic micropegmatites (former melt layers) and by ptygmatic folds, pointing to viscous deformation of a compacting crystal mush. Flat-lying, undeformed cumulate layers from the same parent melt as the deformed ones settled directly over the tilted blocks. Water introduction into the melt is evidenced by the crystallization of anomalously high anorthite cumulus plagioclase, regardless of the degree of evolution of the magma. Shearing continued at subsolidus temperatures along some, but not all, fault planes, attested to by the local development of plastic deformation structures. The subsolidus development of amphibolespinel coronas around olivine grains points to pervasive percolation of high-temperature hydrothermal fluids through the cumulates. As the crust cooled further, water penetration progressively focused within the main fault zone, where the crust was fractured and eventually altered in greenschist facies conditions.
Seismic slip propagation to the updip end of plate boundary subduction interface faults: Vitrinite reflectance geothermometry on Integrated Ocean Drilling Program NanTro SEIZE cores
Arito Sakaguchi et al., Institute for Frontier Research on Earth Evolution (FREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 173-25 Showa, Kanazawa ward, Yokohama 236-0001, Kanagawa, Japan. First published online 8 March 2011; doi: 10.1130/G31642.1.
Seismic faulting along subduction-type plate boundaries plays a fundamental role in tsunami genesis. During the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) Stage 1, shallow portions of plate subduction faults were drilled and cored in the Nankai Trough (offshore Japan), where repeated large earthquakes and tsunamis have occurred, including the 1944 Tonankai (Mw = 8.1) earthquake. Arito Sakaguch of the Japan Agency for Marine-Earth Science and Technology used the largest deep-sea scientific drilling vessel, Chikyu, for the project detailed here. Samples were obtained from the frontal thrust, which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge, and from a megasplay fault that branches from the plate boundary décollement. The toe of the accretionary wedge has classically been considered aseismic, but organic alteration geothermometry in sediment reveals that the two examined fault zones underwent localized temperatures of more than 380 degrees Celsius. This suggests that frictional heating occurred along these two fault zones, and implies that coseismic slip must have propagated at least one time to the up-dip end of the megasplay fault and to the toe of the accretionary wedge. This is the first evidence of tsunamigenic faulting of abyssal plate subduction fault.
Continental-scale detrital zircon provenance signatures in Lower Cretaceous strata, western North America
A.L. Leier, Dept. of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada; and G.E. Gehrels. First published online 8 March 2011; doi: 10.1130/G31762.1.
The modem political boundary between the United States and Canada originated roughly 200 years ago. However, it appears that even as much as 120 million years ago, rivers and sediments in the two regions developed their own distinct characteristics. A.L. Leier of the University of Calgary and G.E. Gehrels of the University of Arizona use recently developed laser-based techniques to reconstruct the origin of individual sand grains that were deposited during the Cretaceous Era (120 million years ago) in western North America. Sand grains made of the mineral zircon show that Cretaceous sediment in the United States has a clear "American" signature, whereas that in the Canadian Rockies has a different and definable "Canadian" signature. These signatures are related to the original crystallization ages of individual zircon grains, which differ between Canada and the United States.
Subvolcanic plumbing systems imaged through crystal size distributions
Oleg E. Melnik et al., Institute of Mechanics, Moscow State University, 1-Michurinskii prospect, Moscow 119192, Russia. First published online 8 March 2011; doi: 10.1130/G31691.1.
The texture of volcanic rocks holds cryptic clues to the geometry of the underground magma system from which the sample derives and the path it took to the surface. Deciphering this information in terms of the critical magma system variables required to interpret and model volcano behavior has long been a goal of volcanology. Oleg E. Melnik of Moscow State University and colleagues present a novel method to extract conduit diameter and magma chamber depth from textural analysis of a single volcanic sample. This quick, low-cost method could be employed by volcano monitoring teams anywhere in the world, using readily available equipment and observational constraints. Melnik et al.'s method requires quantification of the rock texture using crystal-size distribution data and observational constraints on magma discharge rate during eruption, and petrological estimates of eruption temperature. The latter are readily available for most active volcanoes. This method provides robust estimates of magma chamber depth, with greater vertical resolution than is possible with more costly and time-consuming seismic or potential-field methods. As for conduit dimension, Melnik et al.' s technique has a lateral resolution that far exceeds that possible by any other method. This is a major breakthrough in linking rock texture and process beneath volcanoes.
From sediments to their source rocks: Hf and Nd isotopes in recent river sediments
Bruno Dhuime et al., Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK. First published online 8 March 2011; doi: 10.1130/G31785.1.
Bruno Dhuime of the University of Bristol and colleagues have developed a new way to unravel continental evolution from the sedimentary record. Radiogenic isotopes in sediments are widely used to investigate the evolution of the continental crust; however, sediments often provide biased records because some units in their source regions are more susceptible to erosion than others. Unraveling continental evolution from the sedimentary record therefore requires an understanding of erosion laws that link sediments to their source rocks, and the extent to which erosion laws vary in different erosion systems. Using integrated hafnium and uranium and lead isotopes in detrital zircons, and neodymium isotope ratios of bulk sediments along the Frankland River in southwestern Australia, the relative contributions of different source terrains in the sedimentary record have been determined for the first time. The results have been integrated in a model to calculate the continental growth for the Gondwana supercontinent. These indicate that at least 50% of the present-day volume of the continental crust was generated by the end of the Archean (i.e., 2.5 billion years ago).
Diversity of melt conduits in the Izu-Bonin-Mariana forearc mantle: Implications for the earliest stage of arc magmatism
Tomoaki Morishita et al., Frontier Science Organization, Kanazawa University, Kanazawa 920-1192, Japan. First published online 8 March 2011; doi: 10.1130/G31706.1.
Tomoaki Morishita of Kanazawa University, Japan, and colleagues examine peridotites recovered from an exhumed crust/mantle section exposed along the landward slopes of the northern Izu-Bonin Trench (offshore Japan) to address magmatic processes during the earliest stage of subduction initiation.