||30 April 2007
GSA Release No. 07-17
May / June Media Highlights:
The Geological Society of America Bulletin
Boulder, Colorado, USA - Geology topics of interest in the May-June issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN include: formation of volcanic seamounts offshore Newfoundland's Grand Banks; volcanic ash and jet engine problems; ice seeps in the McMurdo Dry Valleys of Antarctica; and impact of climate and sea-level change on the lower Mississippi River valley during the last 100,000 years.
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- Fragmentation of a foreland basin in response to out-of-sequence basement uplifts and structural reactivation: El Cajon-Campo del Arenal basin, NW Argentina
Estelle Mortimer et al., Universität Potsdam, Inst. f. Geowissenschaften, Potsdam, 14476, Germany. Pages 637-653.
- Keywords: foreland basin, basement uplift, stratigraphy, reactivation, thermochronology.
- The Sierras Pampeanas of NW Argentina is located along the eastern margin of the Puna plateau. Understanding basins preserved along the margin today provides insight into mechanisms involved in the incorporation of basins into the plateau. The thick-skinned setting is analogous to the Laramide province, western USA, and contrasts idealized thin-skinned foreland evolution models. The Sierras Pampeanas are characterized by Cenozoic basins separated by high-angle reverse-fault bounded basement ranges. In addition, the area is superimposed onto a Cretaceous extensional province, the Salta Rift, lending itself to structural reactivation. This study integrates apatite fission track, sedimentologic, and seismic reflection data documenting the evolution of the El Cajon-Campo del Arenal basin. It is bounded to the west by the plateau, the load from which controls basin architecture from the earliest sediments ~11 Ma. To the east, a previously extensional fault was reactivated in compression at ~6 Ma. This led to folding of basement rocks and the development of an intra-basin uplift within the El Cajon basin at 6-5 Ma. This intra-basin uplift developed progressively north to south, fragmenting the basin, while continuing deformation from the west led to further faulting and basin uplift. This pattern of deformation led to basin filling, possibly even closure, by 2.7 Ma, since which time the basin has been re-excavated. This paper highlights the importance of structural re-activation in the deformation, partitioning and uplift of these marginal basins, and their potential importance in controlling deformation in other thick-skinned tectonic settings.
- Early Cretaceous opening of the North Atlantic Ocean: Implications of the petrology and tectonic setting of the Fogo Seamounts off the SW Grand Banks, Newfoundland
Georgia Pe-Piper, David Piper (corresponding author), et al., Geological Survey of Canada (Atlantic), 1 Challenger Drive, Box 1006, Dartmouth, Nova-Scotia B2Y 4A2, Canada. Pages 712-724.
- Keywords: volcanism, Cretaceous, seamount, geochemistry, geochronology, continental margin, mantle convection.
- A 140-million-year-old chain of volcanic seamounts offshore from the Grand Banks of Newfoundland formed as a result of mantle convection across the abrupt strike-slip fault boundary between the Jurassic central Atlantic Ocean and the old continental block of the Grand Banks. The resulting high heat flux led to heating of the crust of the Grand Banks and the formation of a broad rift zone there, as eastern Canada and Europe separated to form the northern North Atlantic Ocean. Similar volcanic seamounts have formed along the strike-slip boundary between the Pacific Ocean and California in the past 10 million years.
- Quantitative scanning-electron microscope analysis of volcanic ash surfaces: Application to the 1982-1983 Galunggung eruption (Indonesia)
Orkun Ersoy et al., Hacettepe University, Geological Engineering, Ankara, Beytepe 06532, Turkey. Pages 743-752.
- Keywords: volcanic ash, roughness, gradient analysis, texture descriptors, Galunggung.
- During the 1982-1983 eruption of Galunggung volcano in western Java (Indonesia), the explosivity increased considerably with concomitant jet plane incidents. This drastic change in the style of the eruption is explained by an increase in the efficiency of groundwater-magma interaction. In these incidents, volcanic ash extensively damaged exterior surfaces, instruments, and engines, resulting in the loss of thrust. The eruptive columns were higher, being as much as 10 to 20 km, and caused engine trouble on jet airplanes forcing emergency landings in Jakarta. The textural analysis on volcanic ash surfaces may permit anticipation of the course of events in the eruption. The roughness and texture descriptors of pyroclastic material are important for distinguishing the products of magmatic eruptions from those of phreatomagmatic eruptions (water-magma interaction).The discrimination of products of different fragmentation mechanisms may permit forecasting of volcanic hazards and related jet plane incidents.
- A facies model for a submarine volcaniclastic apron: The Miocene Manukau Subgroup, New Zealand
Sharon Allen et al., University of Tasmania, CODES, Churchill Ave, Private Bag 79, Hobart, Tasmania 7001, Australia. Pages 725-742.
- Keywords: submarine volcaniclastic apron, basaltic-andesite volcanic complex, facies model, pumice, pillow lava, conglomerate, pyroclast.
- Island volcanoes are dynamic places built by eruptions and worn away continuously by the weather and periodically by catastrophic landslides. Aprons of sediment offshore contain a record of island growth and destruction. This paper describes a well-constrained pattern of spatial variations in grain size and texture in the offshore apron. The pattern can be used to reconstruct ancient successions and/or concealed island volcanoes.
- Late Cambrian (Steptoean) sedimentation and responses to sea-level change along the northeastern Laurentian margin: Insights from carbon isotope stratigraphy
Bosiljka Glumac and Laurel E. Mutti, Smith College, Department of Geology, Clark Science Center, Northampton, MA 01063, U.S.A. Pages 623-636.
- Keywords: Schodack Formation, Pine Plains Formation, Stockbridge Formation, Upper Cambrian, northern Appalachians, carbon isotopes.
- Stable isotopes of carbon were used in the study of poorly fossiliferous Upper Cambrian (about 500 million years old) strata in the northern U.S. Appalachians. The results provided unique insights into the patterns of sedimentation and the response to sea-level change along the northeastern margin of Laurentian (North American) continent.
- Quantitative and qualitative insights into bedrock landform erosion on the South Indian craton using cosmogenic nuclides and apatite fission tracks
Yanni Gunnell et al., University of Paris 7, Geography, 2 Place Jussieu, case 7001, Paris, France 75251 cedex 05, France. Pages 576-585.
- Keywords: inselberg, cosmogenic nuclide dating, erosion rate, backwearing, downwearing, etchplain, India.
- Inselbergs are sugar-loaf bedrock landforms that occur extensively in the Tropics, but the time scales at which they grow as erosion lowers the surrounding plain are poorly known. Our study attempts to quantify the growth of these landforms by balancing the long-term erosion rates of inselberg summits using in situ-produced cosmogenic beryllium, against the long-term erosion rate of the surrounding plain using the same technique as well as apatite fission-track dating. Whether the taller landforms exhibit greater relief because they are systematically older, or because landforms with lower relief have eroded faster during the same period of time, is open to discussion. However, results confirm the intuition that inselbergs are islands of low erosion (0 to 2 meters per million years) that grow persistently over time by differential erosion between contrasting lithologies. Taller landforms are older because they emerged on the land surface earlier, whereas lower adjacent bedrock landforms represent exposures of deeper crustal levels by a small margin. Their mean rates of denudation are a function of the rate of erosion on the surrounding plain, and may vary from region to region or continent to continent. This is broadly consistent with the geomorphic theory of etchplanation, but nevertheless raises issues of sampling design and of geocronological detection limitations in low-energy cratonic environments.
- Solute and isotope geochemistry of subsurface ice melt seeps in Taylor Valley, Antarctica
Katharine J. Harris, Anne Carey (corresponding author) et al., The Ohio State University, Dept. of Geological Sciences, 125 South Oval Mall, 275 Mendenhall Lab, Columbus, OH 43210-1398, U.S.A. Pages 548-555.
- Keywords: Antarctica, groundwater, stable isotopes, dry valleys, ice, hydrology.
- The McMurdo Dry Valleys of Antarctic are the largest ice-free area on the continent. Glacier melt water streams flow 6 to 12 weeks per year and are the dominant source of liquid water in this environment. We have observed a number of "seeps" that are not derived directly from glacial melt. This water is may be derived from permafrost, snow patches, buried ice, or refrozen precipitation that has accumulated in the subsurface. The geochemistry of the seep water is very distinctive when compared to nearby glacier melt waters. These seeps are important because they represent a previously overlooked component of this polar desert hydrological system.
- Fluvial evolution of the lower Mississippi River valley during the last 100-kyr glacial cycle: Response to glaciation and sea-level change
Tammy Rittenour et al., Utah State University, Dept of Geology, Logan, Utah 84322, U.S.A. Pages 586-608.
- Keywords: Mississippi River, lower Mississippi valley, braid belt, longitudinal profile, fluvial evolution, meltwater discharge, sea-level change.
- This paper presents the first detailed reconstruction of the evolution of the lower Mississippi valley during the last 100,000 years. New optically stimulated luminescence (OSL) ages are presented to provide insight into the response of the lower Mississippi River to glaciation of its headwaters and sea level change during the transition into and out of the last glaciation. Evidence from river deposits suggests that the Mississippi was flowing at an elevation 8-21 m below is modern floodplain during the last interglacial, then rapidly aggraded 15-19 m in response initial glaciation of its headwaters. Correlation to the southern margin of the Laurentide Ice Sheet in the Mississippi River headwaters suggests ice margin dynamics and resultant meltwater discharge influenced river dynamics 1000 km downstream, producing the large braided channel belts that fill the lower Mississippi valley. The geometry of the channel belt deposits indicates that the glacial-aged river gradient was steeper than today, with the deposits exposed as terraces in the northern lower Mississippi valley and dipping below the floodplain to the south to meet the lowered sea levels during glaciation. The new lower Mississippi River reconstructions presented here provide insight into the response of this continental-scale river system to climatic and sea-level forcing during the last glacial cycle.
- Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain
Peter G. DeCelles et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 654-680.
- Keywords: Tibet, arid paleoclimate, paleogeography, Cretaceous, Tertiary, sedimentary basins.
- The Tibetan Plateau is the largest region of high elevation on Earth. The time at which this high elevation was developed remains a critical component of models to explain the ongoing collision between the Asian and Indian continents. Our work documents the environmental conditions and the paleogeography in the central part of the Tibetan Plateau from about 115 million years ago to 10 million years ago at an unprecedented level of detail. We find evidence for large sandy deserts, large internally drained lakes, deltas, volcanoes, and rivers during this time span. By about 26 million years ago the region was at its present high elevation (4700 meters above sea level), and by about 80 million years ago arid climate was well established. Our data show that the Tibetan Plateau has a long complex history of development which runs counter to some explanations of the Plateau as a relatively recent (less than 10 million years ago) phenomenon.
- The Rangipo fault, Taupo rift, New Zealand: An example of temporal slip-rate and single-event displacement variability in a volcanic environment
Pilar Villamor et al., GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand. Pages 529-547.
- Keywords: Taupo volcanic zone, Taupo rift, active normal fault, active andesitic volcano, volcano-tectonic interactions, slip-rate variability, single-event displacement variability.
- ory of the Rangipo Fault, a major tectonic structure located at the southern end of the volcanic region, and compared it with the eruptive history of the nearby (~11 km) Ruapehu Volcano. We find that the fault was more active (i.e., the fault produced more earthquakes per year) in the past, ~20,000 years ago, at the same time that large volumes of magma erupted from Ruapehu Volcano, while currently both the fault and the volcano seem to be relatively quiet. This suggests that stretching of the crust in this region occurs in pulses where faults and volcanoes together contribute to the total extension. Our findings also have major impacts for hazard studies of this region. Although our results show a lower fault activity during the present time, which implies lower seismic hazard than previously reported, the possible interaction between earthquakes and volcanic eruptions raises the alert that a large earthquake on the Rangipo Fault could trigger a volcanic eruption, or vice versa.
- Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for central Andean foreland development
Federico M. Davila, Universidad Nacional de Córdoba, Facultad de Ciencias Exactas, Físicas y Naturales, Córdoba X 5016 GCA, Argentina,; and Ricardo A. Astini Ciudad Universitaria, Facultad de Ciencias Exactas, Físicas y Naturales, Córdoba X 5016 GCA, Argentina. Pages 609-622.
- Keywords: central Andes, Sierras Pampeanas, broken foreland, basin analysis, provenance, conglomerates.
- The manuscript focuses on how from conglomerate provenance analysis and complementary stratigraphic and structural knowledge, a synorogenic sedimentary wedge in the Sierras Pampeanas region (Argentina) can be understood. Interpretation suggests protracted intermontane basin formation within a broken foreland setting, which contrasts with previous analysis in the south Central Andes. We show that although the modern broken foreland can be linked to slab shallowing, as suggested by geophysical, volcanic evidence and basement thrusting, its protracted history, unconnected to the southward shift of the Juan Fernandez ridge, suggests that the early Neogene flat-slab subduction documented in this work might have not been linked to oceanic ridge collision.
- Late Neoproterozoic paleogeography of the Southeastern New England Avalon Zone: Insights from U-Pb geochronology and paleomagnetism
Margaret D Thompson et al., Wellseley College, Geosciences, Wellesley, MA 02481, U.S.A. Pages 681-696.
- Keywords: Avalon Zone, New England, Neoproterozoic, U-Pb geochronology, paleopole, paleogeography.
- Avalonian terranes consisting of latest Precambrian rocks of the Avalon Peninsula in Newfoundland and their counterparts stretching all the way to southeastern Massachusetts originated far from their current North American positions. They have been variously pictured as bordering either Africa or South America when these were joined to form the ancient continent called Gondwana. This paper documents the position of the magnetic pole in precisely dated volcanic rocks around Boston, Massachusetts that clarifies the geographic relationships at about 595 million years ago. These results are consistent with Avalonian terranes occupying a mid-latitude position off the North African portion of Gondwana which itself is inverted compared to other reconstructions in order to achieve reasonable velocities during drift to the south pole after 595 million years. Other types of evidence, especially Nd isotopic signatures and detrital zircon ages, that have previously been weighted over paleomagnetic data to argue for Avalonian terranes located along the South American margin of Gondwana are re-assessed in relation to the new reconstruction. The integrated geochronologic and paleomagnetic data will be valuable both to tectonics-minded students of the terminal Neoproterozoic and to those grappling with extreme climate events and early animal evolution special to that time.
- U-Pb geochronologic evidence for the evolution of the Gondwanan margin of the north-central Andes
David M. Chew et al., Trinity College Dublin, Geology, Dublin, Ireland. Pages 697-711.
- Keywords: Gondwana, Andes, Peru, geochronology, zircon, Paleozoic.
- The Andes are the classic example of a mountain belt that was not produced by continent-continent collision. Instead, the Andes have been an active margin for much of the past 500 million years up to the present day. However our knowledge of the early history of the Andes is very limited. U-Pb dating of zircon crystals from metamorphic and igneous rocks in the Eastern Cordillera of Peru demonstrates that the northern Andes experienced major orogenic events at 470 and 315 Ma. These ages are very similar to those recorded further south in Argentina and Patagonia, and suggest that major orogenic belts thousands of kilometres in length are buried under the recent volcanic cover of the Andes.
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