March/April GSA Bulletin media highlights

02/24/04

Boulder, Colo. – The March/April issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics include: controversy over whether the Northern and Southern hemispheres experience cold ice-age climates at the same time; large, rapid changes in sea level during the Late Cretaceous greenhouse world that challenge current understanding of sea level dynamics; and giant ice avalanches in Tibet.

Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for additional information or assistance.

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Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia
Benyamin Sapiie and Mark Cloos, Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA. Pages 277–293.
Keywords: New Guinea, Central Range, strike-slip, porphyry copper.

Evidence for recent strike-slip faulting has been discovered in the core of the highlands of west New Guinea. This faulting created pathways for the intrusion of magma from deep in the earth and for the flow of hydrothermal fluids that formed important copper and gold deposits about 3 million years ago. The Grasberg porphyry copper-gold deposit was formed and contains the world's third largest copper and largest gold reserves, which are extracted by open pit mining at the nearly 14,000 ft elevation, very top of the mountain belt.

New conceptual fold-fracture model includes prefolding joints, based on field data from Emigrant Gap anticline, Wyoming
Stephan Bergbauer and David D. Pollard, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Pages 294–307.
Keywords: folding, fracturing, structural geology, models, Laramide orogeny, petroleum geology.

In structural geology, various conceptual models assume that fractures or faults that form in sedimentary layers during folding are oriented symmetrically with respect to the geometry of the fold. The underlying assumption of these conceptual models is that, prior to folding, the sedimentary layers were free of heterogeneities on the scale of the fractures/faults to form. This would allow fractures/faults to form symmetrically with respect to the fold geometry during folding. However, our field observations in a well-exposed Laramide fold near Casper, Wyoming, suggest that the folded sedimentary layers contained two sets of systematic joints prior to folding, and that the existence of these joints significantly influenced the geometry and spatial distribution of fractures/faults that formed during folding. We found that fractures and faults on the anticline did not form parallel and perpendicular to the fold axis, but in the direction of the pre-folding joints, which strike obliquely to the fold axis. Moreover, we found that one of the two pre-folding joint sets is aligned closer with the fold axis, and that fractures/faults oriented parallel to this joint set accommodate the majority of shear deformation during folding. Therefore, we propose a new conceptual model that includes the influence of the pre-folding joints on the fracture/fault geometry that form during folding. The proposed model of fracture-fold relationships differs from existing conceptual models because it postulates that folding-related fracture orientations are controlled by the orientations of pre-folding fractures.

Cosmogenic nuclide chronology of millennial-scale glacial advances during O-isotope stage 2 in Patagonia
Michael R. Kaplan, Department of Geology and Geophysics, University of Wisconsin-Madison, Wisconsin 53706, USA et al. Pages 308–321.
Keywords: South America, ice ages, glacial maximum, last, exposure age, paleoclimatology, and geochronology.

We address a longstanding problem plaguing scientists who research past climates: whether the Northern and Southern hemispheres experience cold ice-age climates at the same time. The answer to this problem is important because it may help us to better understand how the climate system works. The amount of solar energy received varies spatially between the hemispheres (for example, the seasons are out of phase); thus, if it turns out that ice ages occur at the same time, then some factor must override solar changes over (at least most of) one half of Earth. We attacked this problem by utilizing a relatively new method to determine when the glaciers last grew in the middle latitudes of the Andes. The new data indicate that glaciers underwent major expansion out of the Andes onto the Patagonian plains at roughly the same time as the great Northern Hemisphere ice sheets over North America and Europe existed, from about 27,000 to 16,000 years ago. Glaciers grew in South America despite a high in solar energy. However, this was at a time that the Northern Hemisphere had a low in solar energy. The similar ice age history in both hemispheres, despite spatial differences in solar energy received, implies that a global climate forcing, such as atmospheric cooling, must synchronize ice age conditions over most of Earth on time scales of a few thousand years.

Characterization and evolution of fractures in low-volume pahoehoe lava flows, eastern Snake River Plain, Idaho Conrad J. Schaefer and Simon A. Kattenhorn, Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA. Pages 322–336.
Keywords: basalt, fracture, lava flow, pahoehoe, inflation entablature.

The paper describes the cooling history of low-volume basalt lava flows of the Eastern Snake River Plain, which are good analogs for flows in Hawaii, Iceland, and elsewhere. The cooling fracture patterns differ from sheet-style flows, such as those of the Columbia River flood basalts. Four distinct fracture types produce fracture patterns that are controlled by cross-sectional flow shape as well as the inflationary history during cooling. Unlike sheet flows, the final stage of cooling results from rapid cooling when an inflation fracture pierces the molten core of the lava flow.

Hawaiian submarine manganese-iron oxide crusts-A dating tool?
James G. Moore, U.S. Geological Survey, Menlo Park, California 94025, USA, and David A. Clague, Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA. Pages 337–347.
Keywords: Hawaii, manganese, ferro-manganese.

Black oxide crusts that are rich in manganese and iron form on most rocky outcrops on the ocean floor. These crusts are well developed on steep lava slopes of the Hawaiian Ridge and more than a thousand samples from the ocean floor have been collected from 140 general places. Early samples were taken by dredging, but more recently research submarines and remotely operated vehicles have taken over the sampling. The black crusts are thicker on older rocks. The thickness increases toward the northwest from Hawaii (the youngest island), toward Kauai (the island with the oldest volcanoes in the study area). Comparison of the maximum crust thickness with the dated volcanic features indicates that the crusts thicken at the rate of about 2.5 mm, or about a tenth of an inch in a million years. On the very youngest volcano, Loihi, off the south coast of the Big Island, the crusts are microscopic, whereas the crusts off Oahu are thickest, almost one-half inch. The thickness information not only allows a comparison of the relative exposure ages of two or more features offshore from different volcanoes, but also provides specific age estimates of volcanic and landslike deposits.

A depositional record of deglaciation in a paleofjord (Late Carboniferous [Pennsylvanian] of San Juan Province, Argentina): The role of catastrophic sedimentation
Ben Kneller, School of Earth Sciences, University of Leeds, UK, et al. Pages 348–367.
Keywords: deglaciation, fjord, mass transport, catastrophic, Carboniferous, Pennsylvanian, Argentina.

This article describes processes and the resulting deposits that relate to a time of major climate change in the geologic past. These deposits, in present-day western Argentina, formed in one of a series of ancient fjords that existed on the mountainous margin of the supercontinent of Gondwana approximately 300 million years ago. The valley was originally carved by ice flowing out from the glaciated interior of the continent. During global warming at the end of a major glaciation, the valley was flooded as the main valley glacier began to melt and sea level rose. The resulting flooded valley (fjord) was rapidly filled with sediment supplied first by local glaciers, and later by large rivers flowing from the glaciers of the continental interior. Occasional rock avalanches into the fjord caused major submarine landslides and tsunami-like waves that periodically stripped away the forest that had grown along the shoreline.

Upper Cretaceous sequences and sea-level history, New Jersey Coastal Plain
Kenneth G. Miller, Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08854, USA, et al. Pages 368–393.
Keywords: eustasy, sequence stratigraphy, sea-level history, New Jersey Coastal Plain, Late Cretaceous, backstripping.

Geologists have long thought that sea level changes were large (10s of m) and rapid (occurring on scales ranging from 1000s to less than a million years). Such large, rapid changes can only be explained by the waxing and waning of large continental ice sheets. What is perplexing is that large and rapid sea level changes have been reported for the Greenhouse world of the Late Cretaceous, an interval supposed to be ice free. However, until now, the evidence for such large changes has been equivocal because the effects of tectonics and sediment supply also affect interpretation of sea level change. In this paper, Miller and colleagues present recent drilling results from the New Jersey Coastal Plain (Ocean Drilling Program Leg 174AX) that provide a Late Cretaceous record of sea level. This record requires that either ice sheets grew and decayed in this Greenhouse world or that our understanding of sea level mechanisms is fundamentally flawed.

Giant, ~M8 earthquake-triggered ice avalanches in the eastern Kunlun Shan, northern Tibet: Characteristics, nature and dynamics
Jerome van der Woerd, Tectonique Active, Institut de Physique du Globe de Strasbourg, France; et al. Pages 394–406.
Keywords: ice avalanches, earthquakes, glaciers, Kunlun Mountains, Tibet, remote sensing.

Several giant ice avalanches were initiated by failure of ice caps due to strong ground motion during the November 14, 2001, MW = 7.9 Kokoxili earthquake along the Kunlun Fault in northern Tibet (China). Four ice avalanches were identified on the north slope of the Burhan Budai Shan, near the Kunlun Pass along the Golmud to Lhassa highway. Two additional avalanches occurred 50 km to the west on the south slope of the eastern Yuxi Feng. The ice avalanches, mostly composed of ice and snow, reached 2–3 km down valley beyond the snouts of the present glaciers. Detailed high resolution image analysis and field campaigns helped to better describe and understand the occurrence of such avalanches. Velocities of the avalanches were estimated to reach over 120 km/h and then flowed at less than about 75 km/h before coming to rest on the valley floors. The frequency of such events is unknown, but such phenomena may become more common as a consequence of human-induced climate change and may constitute a significant geologic hazard in the near future.

Structural framework of the Edwards Aquifer recharge zone in south-central Texas
David A. Ferrill, CNWRA, Southwest Research Institute®, San Antonio, Texas, 78238-5166, USA, et al. Pages 407–418.
Keywords: Edwards Aquifer, Edwards Group, Balcones fault zone, faults and faulting, normal faults.

The Edwards Aquifer of south-central Texas is composed of Cretaceous limestones, and is the major source of water for many communities, including the city of San Antonio. Population growth and development over the aquifer recharge zone threatens the quality and quantity of water in the aquifer. Both recharge and subsurface flow of groundwater are controlled by faults and fractures that developed along the Balcones fault zone within the last 20 million years. Rapid aquifer response to rainfall events and variable flow from wells indicate that groundwater flow is strongly "channeled" by the fault and fracture network existing in the Edwards Aquifer limestones. Our investigation focuses on the geologic structure of the Edwards Aquifer; analysis of fault offset and stratigraphic juxtaposition relationships; evaluation of fault zone deformation, dissolution, and fault system architecture; and investigation of fault block deformation and scaling of small scale (intra block) normal faults. Characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault block communication and flow paths. We show that although the dominant trend of large faults is ENE, there is an interconnected network of smaller faults and fractures that have significantly different orientations that are causally linked to the larger faults. Small faults and fractures are also conduits for groundwater recharge and flow and contribute to the complexity of flow path shapes in the aquifer system. In addition, the larger faults may, in some areas, have caused the rocks of the Edwards Aquifer to come into contact with the older rocks of the underlying Trinity Aquifer, which is a primary source of water for many Texas Hill Country communities. This opens the possibility of underground recharge of the Edwards from the Trinity Aquifer, which may contribute to both the quantity of water available and the potential for contamination of the Edwards Aquifer from the Trinity Aquifer.

Petrogenesis of Carboniferous rift-related volcanic rocks in the Tianshan, northwestern China
Lin-Qi Xia, et al., Xi'an Institute of Geology and Mineral Resources, Ministry of Land and Resources, Xi'an, Shaanxi 710054, China. Pages 419–433.
Keywords: Tianshan Carboniferous rift-related volcanic rocks, geochemistry, petrogenesis, asthenosphere, lithosphere, northwest China.

This paper reports the results of a petrological and geochemical study of dominant Carboniferous basaltic lavas in the Tianshan Carboniferous–Permian Rift System of northwestern China. This study allows us to address important aspects of the petrogenesis of Tianshan Carboniferous rift volcanic rocks, such as the nature of volcanic rocks, mantle melting conditions, relative contribution of the asthenospheric and lithospheric components in magma generation, as well as their geodynamic implication

Oxygen isotope trends of granitic magmatism in the Great Basin: Location of the Precambrian craton boundary as reflected in zircons
Elizabeth M. King, Department of Geography-Geology, Illinois State University, Normal, Illinois 61790, USA, et al. Pages 451–462.
Keywords: Great Basin, oxygen isotopes, zircon, terrane boundaries.

This article examines the oxygen isotope geochemistry of granitic rocks of the northern Great Basin of Utah and Nevada to determine terrane boundaries in the continental crust. Zircons preserve magmatic oxygen isotope ratios through alteration events better than other minerals, such as quartz and feldspar, and therefore are the best indicator of magmatic source. Previous oxygen isotope studies have suggested that the location of terrane boundaries differ from those determined by radiogenic isotope studies. Zircon oxygen isotope ratios indicate that the western edge of the buried Precambrian craton and continental sedimentary rocks is located approximately where radiogenic isotope studies indicate. The difference between previous studies using whole-rock powders and this study using zircons suggests that plutons in the Great Basin have experienced variable assimilation and cooling rates, making regional studies using whole-rock powders or quartz difficult.

High latitude meteoric d18O compositions: Paleosol siderite in the Middle Cretaceous Nanushuk Formation, North Slope, Alaska David F. Ufnar, Department of Geology, University of Southern Mississippi, Hattiesburg, Mississippi 39406, USA, et al. Pages 463–473.
Keywords: Siderite, oxygen isotopes, Cretaceous, paleosols, carbon isotopes, paleoclimatology.

Millimeter-scale nodules of siderite (iron carbonate) found in ancient wetland soils of the Nanushuk Formation of North Slope, Alaska, have proven to be a critical link in analyses of the mid-Cretaceous "greenhouse-world" hydrologic cycle of North America. The oxygen isotopic values of the siderite nodules are a proxy record of ancient meteoric fluid oxygen isotopic values (soil groundwater), and hence, precipitation. The Nanushuk Formation paleosol siderite oxygen isotopic values are the high-latitude end-point values for a mass balance model that has been used to quantify precipitation rates throughout the Cretaceous Western Interior Basin and the North Slope, Alaska. Calculated precipitation rates for the coal-bearing paleosols of the Nanushuk Formation agree very well (approximately 480 mm/yr of precipitation) with precipitation rates necessary to maintain modern peat-forming environments.

K-bentonites, volcanic ash preservation, and implications for Early to Middle Devonian volcanism in the Acadian orogen, eastern North America Charles A. Ver Straeten, Center for Stratigraphy and Paleontology, New York State Museum, State Education Department, Albany, New York 12230, USA. Pages 474–489.
Keywords: K-bentonites, Devonian, preservation, marine processes, Acadian orogeny.

Detailed examination of altered volcanic ash layers (K-bentonites) from the Devonian Period indicate that many beds do not represent single volcanic eruptions, as they are generally interpreted to represent. Multi-layered beds, fossil layers within beds, authigenic minerals (e.g., glauconite, phosphate), subjacent hardgrounds, and an irregular distribution of beds through space and time raise questions about the depositional history and preservation potential of volcanic ash in marine environments, and the degree to which the beds represent a primary record of volcanism. In this paper, the author proposes a model for volcanic ash layer preservation, modification, or destruction, recognizing the effects of a broad range of physical, biological, and chemical processes in shallow marine epicontinental sea, foreland basin, or continental shelf settings, and applies the model to the Devonian record of volcanism and mountain building in eastern North America.

Neogene-Holocene rift propagation in central Tanzania: Morphostructural and aeromagnetic evidence from the Kilombero area Bernard Le Gall, UMR-CNRS 6538 'Domaines Océaniques,' Institut Universitaire Européen de la Mer, 29280 Plouzané, France, et al. Pages 490–510.
Keywords: East African Rift, central Tanzania, Karoo/Cenozoic extensional faulting, satellite imagery, drainage pattern analysis, aeromagnetic data.

The best known worldwide example of extended continental crust occurs along the Kenya Rift (Eastern Africa) where prominent NS-trending depressions, bounded by morphological fault scarps and occupied in places by shallow lakes and active volcanoes, are the clearest expressions of the crustal rifting process at the surface. Typical rift morphology is evidenced as far south as the Kenya-Tanzania border, which is generally considered as the southern extremity of the Kenya active rift system. A revised model for active rift propagation is proposed in our study on the basis of three-dimensional morphological modeling (elaborated from analysis of topographic sheets), complemented by (1) river drainage analysis (extracted from SPOT and Landsat satellite imagery), and (2) field structural observations. Evidence for active-recent extensional tectonics is documented in the Kilombero area (Central Tanzania) (i.e., 300 km south of the previously accepted rift deformation tip zone). Geophysical records analysis (aeromagnetic dataset) helps to further image the deep structure of the Kilombero fault-bounded trough, where 3 km of young sediments are assumed to have been deposited during the rift-related subsidence that occurred since the last million years. At a greater scale, our model postulates that the two active rift branches (Kenya to the east and Tanganyika-Malawi to the west) of the East African Rift System connect southward in the vicinity of Lake Malawi, so that the Tanzanian rigid crustal block (mainly composed of old cratonic rocks) appears to be completely surrounded by two active extensional faulted belts.

Source: Eurekalert & others

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