November-December 2008 GSA Bulletin media highlights

October 29, 2008

29 October 2008

Boulder, CO, USA - The latest issue of GSA BULLETIN spans the globe, examining ancient soils in Big Bend National Park, Texas; loess soils in Nebraska, including the greatest known thickness of the Peoria Loess in the world; folding, faulting, and metamorphism as seen in detailed geologic mapping across Pakistan; tectonic fractures in Southeast Viti Levu, Fiji; subsidence in Mexico City; groundwater arsenic in Araihazar, Bangladesh; the formation of the Andes; and earthquakes in the Seattle fault zone.

Highlights are provided below.

Paleogene paleosols and changes in pedogenesis during the initial Eocene thermal maximum: Big Bend National Park, Texas, USA
Paul D. White and Judith Schiebout, Physics Dept., Community College of Rhode Island. 400 East Avenue. Warwick, Rhode. Pages 1347-1361.
White and Schiebout test the hypothesis that continental chemical weathering increased during the initial Eocene thermal maximum (IETM) by comparing paleosols that formed before and during the event in western Texas. The chemical index of alteration (CIA) was used to investigate the weathering of silicate minerals. Paleosols generated before the IETM have CIA values ranging from 62 to 72; CIA values during the IETM range from 67 to 82. The CIA values for pre-IETM paleosols indicate moderate weathering conditions, and CIA values during the event indicate moderate to extreme weathering conditions. The clay mineralogy of the paleosols is dominated by smectite, and it is only within paleosols that formed during the IETM that there is a change. There is a notable increase in the amount of kaolinite in one paleosol horizon that is associated with the carbon excursion. In addition, there is an increase in the translocation of clays and iron and an increase in the leaching of calcite and plagioclase in IETM paleosols. The differences between soils that formed before and during the IETM indicate that chemical weathering did increase during this ancient global warming event. The mechanism responsible for increased weathering is interpreted to be an increase in hydrolysis reactions caused by an increase in humidity and an increase of carbonic acid in the soil due to elevated CO2 levels. Documentation of an increase in chemical weathering during the IETM is significant because it may have served as a negative feedback to reduce atmospheric CO2.

Isotopic evidence for the diversity of late Quaternary loess in Nebraska: Glaciogenic and nonglaciogenic sources
John N. Aleinikoff et al., U.S. Geological Survey. MS 963 Denver, Colorado 80225, USA, USA. Pages 1362-1377.
Lead (Pb) isotope compositions of detrital potassium-feldspars and U-Pb ages of detrital zircons were used as indicators for determining the sources of Peoria Loess deposited during the last glacial period (late Wisconsin, ca. 25-14 thousand years ago) in Nebraska and western Iowa. Aleinikoff et al.'s data indicate that only loess adjacent to the Platte River has Pb isotopic characteristics suggesting derivation from this river. The occurrence of 10-25 million-year-old detrital zircons suggests additional minor contributions of silt from the Oligocene-Miocene Arikaree Group and Miocene Ogallala Group. Isotopic data from detrital potassium-feldspar and zircon grains from Peoria Loess deposits in eastern Nebraska and western Iowa suggest that the immediate source of this loess was alluvium of the Missouri River. Alienikoff et al. conclude that this silt probably is of glaciogenic origin, primarily derived from outwash from the western margin of the Laurentide Ice Sheet. Identification of the White River Group as the main provenance of Peoria Loess of central Nebraska and the Missouri River valley as the immediate source of western Iowa Peoria Loess indicates that paleowind directions during the late Wisconsin were primarily from the northwest and west, in agreement with earlier studies of particle size and loess thickness variation. In addition, the results are in agreement with recent simulations of non-glaciogenic dust sources from linked climate-vegetation modeling, suggesting dry, windy, and minimally vegetated areas in parts of the Great Plains during the last glacial period.

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry
Daniel R. Muhs et al., U.S. Geological Survey. MS 980, Box 25046, Federal Center. Denver, Colorado 80225, USA. Pages 1378-1407.
Loess is one of the most extensive surficial geologic deposits in midcontinental North America, particularly in the central Great Plains region of Nebraska. Last-glacial-age loess (Peoria Loess) reaches its greatest known thickness in the world in this area. New stratigraphic, geochronologic, mineralogic, and geochemical data from Muhs et al. yield information about the age and provenance of Peoria Loess, as well as an evaluation of recent climate models. Sixteen new radiocarbon ages and recently acquired optically stimulated luminescence ages indicate that Peoria Loess deposition in Nebraska occurred between ca. 25,000 cal yr B.P. and ca. 13,000 cal yr B.P. After ca. 13,000 cal yr B.P., a period of pedogenesis began, represented by the dark, prominent Brady Soil. At some localities, further loess deposition was minimal. At other localities, sometime after ca. 11,000 cal yr B.P., there were additional episodes of loess deposition (Bignell Loess) intermittently throughout the Holocene. New mineralogical and geochemical data indicate that the most important sources of loess were likely Tertiary siltstones of the White River and Arikaree Groups, silt facies of Pliocene eolian sediments, and small contributions from Pierre Shale. It is likely that fine-grained silts were transported episodically through the Nebraska Sand Hills from Tertiary and Cretaceous bedrock sources to the north, in agreement with a model presented recently. The identification of Tertiary siltstones and silts as the primary sources of loess is consistent with isotopic data presented in a companion paper. Contributions of glaciogenic silt from the Platte and Missouri Rivers were limited to loess zones close to the valleys of those drainages. An earlier computer-based model of global dust generation during the last glacial period did not identify the Great Plains of North America as a significant source of non-glaciogenic eolian silt; however, a refined version of this model does simulate this region as a significant nonglacial dust source during the last glacial period, in good agreement with the results presented here.

Pedogenic carbonate isotopes as evidence for extreme climatic events preceding the Triassic-Jurassic boundary: Implications for the biotic crisis?
David M. Cleveland et al., Dept. of Geology. Baylor University. Waco, Texas 76798, USA. Pages 1408-1415.
Cleveland et al. have applied well-accepted methods of isotope geochemistry to pedogenic carbonates in order to reconstruct pCO2 and paleotemperature trends through the Late Triassic of equatorial Pangea. The data span the eight million years preceding the Triassic-Jurassic (T-J) boundary and supersede previous terrestrial records in temporal resolution. The results are interpreted to represent climatic fluctuations of aridity and elevated temperatures coupled with periods of increased pCO2 levels. Although the data in this study precede the T-J boundary, many studies conclude that the mass-extinction took place over a more prolonged period beginning in the Late Triassic. Thus, climate may have been a significant driving mechanism of the Late Triassic extinctions.

Mid-Cretaceous oceanic anoxic events in the Pacific Ocean revealed by carbon-isotope stratigraphy of the Calera Limestone, California, USA
Stuart A. Robinson et al., Dept. of Earth Sciences. University College London Gower Street. London, WC1E 6BT, UK. . Pages 1416-1427.
Oceanic anoxic events (OAEs) were geologically brief intervals of time in the Jurassic (200 to 145 million years ago) and the Cretaceous (145 to 65 million years ago) when vast quantities of organic carbon were deposited in marine sediments in many different ocean basins. OAEs represented major perturbations to the global carbon cycle and were likely associated with changes in elevated atmospheric CO2 levels and associated climatic and biotic changes. During the Cretaceous, the Pacific Ocean was by far the largest marine environment, yet our knowledge of OAEs in this basin is poor due to the subduction under North America of much of the ocean floor that existed during the Cretaceous. To date, most of our knowledge comes from deep-ocean drilling, but recovery of complete records has been difficult. To overcome these problems, Robinson et al. study limestones in central California that were originally deposited in deep water in the Pacific Ocean but were scraped off and stuck to the side of North America during subduction of the ocean crust on which they were deposited. This approach allows investigation of an area of the Cretaceous Pacific Ocean that cannot be accessed through ocean drilling. Robinson et al. find that the limestones in California contain records of two different OAEs, one of which was previously very poorly known from the Pacific Ocean. By comparing the sediment types and geochemistry of the Californian limestones with those from elsewhere in the Pacific Ocean, Robinson et al. are able to conclude that the OAEs were spatially variable, suggesting that the amount of organic carbon burial was not uniform everywhere in the Pacific Ocean.

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya
Joseph A. DiPietro et al., Dept. of Geology. University of Southern Indiana. Evansville, Indiana 47712, USA. Pages 1428-1440.
The Indus suture zone represents the collisional boundary between India and Asia across the Himalaya. In the northwest Himalaya of Pakistan this boundary separates rocks of the Indian plate from rocks of the Kohistan volcanic arc. It has long been thought that India was overthrust by the Kohistan arc in a north to south direction at or prior to 50 million years ago and that this collision is responsible for folding, faulting, and metamorphism on the Indian plate. DiPietro et al. present evidence for a different scenario. Detailed mapping across Pakistan from the Afghan border in the west to the Indian border in the east suggests that collision was ongoing as late as 25 million years ago. Folding, faulting, and metamorphism occurred during subduction of the Indian plate beneath oceanic lithosphere prior to collision with Kohistan. Additionally, it appears that the Kohistan arc was thrust in an east-to-southeast direction and not in a southward direction.

Overriding plate shortening and extension above subduction zones: A parametric study to explain formation of the Andes Mountains
W.P. Schellart, Research School of Earth Sciences. Australian National University. Canberra, ACT 0200, Australia. Pages 1441-1454.
Schellart's paper explains the formation of the Andes Mountains in South America, the longest and second highest mountain chain in the world. The Andes are located above a so-called subduction zone, a place where one tectonic plate (the subducting oceanic plate) sinks (subducts) into the Earth's interior below another plate (the overriding plate). Its location makes the Andes unusual, because most mountain belts are located at convergent zones, where two continents collide, like the Himalayas, which formed due to collision between India and Asia. Subduction zones are usually bordered by small ocean basins or low-lying continental margins, not by massive mountain ranges. The Andes span the entire west coast of South America, where the Nazca plate is subducting below the South American plate. It is the extreme size of the South American subduction zone that offers the first clue to the existence of the Andes. The subduction zone runs for more than 7000 km, from Colombia in the north to Patagonia in the south, and is thereby the largest on Earth. This large subduction zone is relatively immobile in the central part, near Bolivia, and is thereby capable of supporting large compressive stresses, stresses that smaller subduction zones would not be able to support. The second clue is the westward motion of South America. Such motion generates massive compressive stresses along the west coast of South America, as the South American plate effectively collides with the immobile, central part of the subduction zone. During collision, the South American continent wraps itself around the immobile central part near Bolivia, forming a major dent in the South American coastline that extends for more than a thousand miles. The shortening and crumpling of the South American crust eventually results in formation of the Andes. Schellart thus solves the paradox of how the Andes mountain range can exist at a subduction zone rather than a continental collision zone.

The Bikou basalts in the northwestern Yangtze block, South China: Remnants of 820-810 Ma continental flood basalts?
Xuan-Ce Wang et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics. Chinese Academy of Sciences. P.O. Box 9825 Beijing 100029, China. Pages 1478-1492.
The supercontinent of Rodinia, one of the oldest-known supercontinents and that once contained most of the Earth's landmass, was broken up during the middle Neoproterozoic time (about 830-720 million years ago). The mechanism of the supercontinent break up is not well understood. Mantle plume or superplume activities have often been invoked as a cause; however, associated Neoproterozoic continental flood basalts, a requisite product of mantle plume activities, have rarely been identified. Wang et al. carry out a systematic geological, geochronological, and geochemical investigation on the Bikou basalts from the largest Neoproterozoic volcanic units in the northwestern Yangtze Block in South China. These basalts were dated to be about 820-810 million years old when Rodinia was breaking up. Geochemical analyses indicate that the lower part of the Bikou basalts were mainly derived from partial melting of sub-continental lithospheric mantle, while the upper basalts were derived from a deeper and anomalously hot mantle plume about 160 °C hotter than the contemporary ambient mantle. Therefore, the Bikou basalts are most likely the remnants of Neoproterozoic continental flood basalts formed in response to a mantle plume that probably played a major role in breaking up the Rodinia.

Miocene magmatism and tectonics of the easternmost sector of the Calama-Olacapato-El Toro fault system in Central Andes at ~24°S: Insights into the evolution of the Eastern Cordillera
R. Mazzuoli et al., Dipartimento di Scienze della Terra. Universita di Pisa. Pisa, Italy. Pages 1493-1517.
The Andean Cordillera is a geological structure of large scientific interest because it constitutes the most spectacular example of a continental margin active from the Jurassic to present times. The Cordillera is the result of the complex geodynamic and tectonic processes linked to the convergence of the Pacific Plate under the South American continental plate, which yielded in time significant modifications of the lithospheric structures. Complex and extensive arc magmatic activity has accompanied the evolution of the Andean margin. In the Central Andes, recent (Miocene to Quaternary) calc-alkaline volcanism occurred as far as about 300 km back of the main volcanic arc, often focused along northwest-southeast-trending transverse fault systems, such as the Calama-Olacapato-El Toro at 24°S. The study of volcanism in such a backarc position raises questions of crucial importance for understanding the recent geodynamic evolution of Central Andes. Mazzuoli et al. present geological, petrological, and volcanological data on the Las Burras-Almagro-El Toro magmatic complex, located at the easternmost edge of the Calama-Olacapato-El Toro transverse fault structure, at the western border of the Eastern Cordillera. The magmatic complex consists of intrusive (monzogabbro to monzogranite) and volcanic (basalt to dacite) rocks, originated by mantle-derived magmas moderately modified by interaction with the crust. Our study has revealed that at 12 million years ago a change occurred in the geochemical characteristics of magmas. While a depleted lithospheric mantle was the source of the older phase magmas, different lithospheric and crustal domains originated the younger magmas. The distinct geochemical characteristics of the rocks emplaced in the older and the younger phases can be related to the Miocene geodynamic evolution of this Andean sector at 24°S. The extensional structures controlling the magma emplacement changed as well between the older and younger phase, suggesting a different role in time of the Calama-Olocapato-El Toro fault system. The results of Mazzuoli et al. permit the proposition of a geodynamical model concerning the evolution of the eastern Cordillera of the Central Andes at 24°S during the Miocene.

Analysis of lineaments and their relationship to Neogene fracturing, SE Viti Levu, Fiji
Tariq I.H. Rahiman and Jarg R. Pettinga, Dept. of Geological Sciences. University of Canterbury. Private Bag 4800. Christchurch 8140, New Zealand. Pages 1544-1555.
Rahiman and Pettinga have established a link between topographic lineaments identified from a range of remotely-sensed imagery and tectonic fractures mapped in the field in Southeast Viti Levu, Fiji. Broader zones of near-parallel surface lineaments are shown to represent fracture splay zones that merge into discrete through-going faults at depth in the crust and significantly offset the magnetic signature in bedrock. This lineament analysis for Southeast Viti Levu represents a new and viable approach to structural and tectonic analysis in regions where bedrock exposure is severely limited. In turn, such lineament data may be used to provide new insights into the tectonic evolution, seismicity patterns, and earthquake hazards of such regions.

Space geodetic imaging of rapid ground subsidence in Mexico City
Enrique Cabral-Cano et al., Departamento de Geomagnetismo y Exploracion. Instituto de Geofísica, Universidad Nacional Autonoma de Mexico. Ciudad Universitaria, Mexico D.F. 04510, Mexico. Pages 1556-1566.
Mexico City lies in a basin that once held Lake Texcoco, a large shallow body of fresh water that was urbanized since pre-Hispanic times and drained within the last two centuries. Much of the city's water is drawn from an aquifer within the lake's sediment deposits. It has been known for some time that groundwater overdraft (withdrawal of water in excess of recharge) results in compaction of these sediments and subsidence of the surface. Cabral-Cano et al., presenting new measurements by GPS and satellite Interferometric Synthetic Aperture Radar (InSAR), have quantified the rates and patterns of subsidence. Subsidence closely follows the shoreline of the old lake and reaches a maximum of 370 mm/yr southwest of the international airport, near the center of the old lake. Spatial gradients of subsidence, marking areas where areas with high subsidence rates are immediately adjacent to areas with lower subsidence rates, are also well-imaged and can be used to predict locations of infrastructure damage or risk damage from differential subsidence.

Contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in Araihazar, Bangladesh
Beth Weinman et al., Earth and Environmental Sciences. Vanderbilt University. Nashville, Tennessee 37235, USA. Pages 1567-1580.
Today, it is reported that more than 90% of Bangladesh's population use "tubewells" to tap into the country's groundwater. In doing so, they put themselves at risk of ingesting waters ridden with unsafe levels of arsenic. One of the major obstacles in combating this exposure is that groundwater arsenic can differ drastically over very short distances. Research often shows 10-100 part-per-billion change in dissolved arsenic over distances of 10-100 meters. In trying to unravel how this type of heterogeneity occurs, research in Araihazar, Bangladesh, by Weinman et al. demonstrates that much of the complexity in groundwater arsenic is linked with the evolution of the local landforms. A sedimentological investigation of the local deposits shows that the flat, featureless floodplain occupying Araihazar today developed after abandonment by a high-energy river. Aquifer units overlain by the sandy bar and levee deposits dating back to the high energy river are associated with low concentrations of arsenic, while areas that have been infilling with mud since abandonment have higher levels of groundwater arsenic. This provides a connection between the nature of floodplain sediments and the level of arsenic dissolved in the underlying groundwaters, giving a ground-truthed, evolutional context to much of the heterogeneity seen in arsenic distributions. For the people living in the delta, it means that knowledge of the local aquifer history can be used by villages to locate community wells and to differentiate between areas of supply and areas that require piped water. It also raises a question as to whether the groundwater is being affected by villagers modifying, digging, and relocating deposits within Bangladesh's floodplains.

Earthquakes generated from bedding-plane-parallel reverse faults above an active wedge thrust, Seattle fault zone
Harvey M. Kelsey et al., Dept. of Geology. Humboldt State University. Arcata, California 95521, USA, USA. Pages 1581-1597.
A key question in earthquake hazard analysis is whether individual faults within fault zones can trigger independent earthquakes. In the Seattle urban area, an earthquake on the Seattle fault zone about 1,000 years ago (A.D. 900-930) regionally uplifted the coastline around the Seattle area. This earthquake was caused both by a master fault at depth and by overlying shallower faults that moved at the same time. But evidence from uplifted beaches of limited extent show that an earthquake just centuries earlier than the A.D. 900-930 earthquake only uplifted areas within hundreds of meters north of individual faults of the Seattle fault zone. Such an earthquake, although smaller and shallower, nonetheless can be as large as magnitude 6.5 to 7. Kelsey et al. propose that faults in the Seattle fault zone produce two types of earthquakes: regional earthquakes that uplift widespread areas and localized earthquakes confined to one fault and affecting a smaller area. This latter type of earthquake has occurred at least twice and perhaps three times in the past 5,000 years, and all these earthquakes occurred before the biggest prehistoric earthquake about 1,000 years ago. In order to accommodate these two types of earthquakes, the Seattle fault zone is probably controlled by a wedge of crust that is pushing northward. The largest earthquakes are formed when the edges of the wedge break apart and the smaller earthquakes occur when the crust above the wedge breaks to accommodate the ever-present pushing of the wedge from below. The shallower earthquakes affect a smaller area of the Seattle urban area but even the smaller earthquakes will be accompanied by widespread shaking as well as localized earth surface rupture.
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