December GSA Bulletin media highlights

December 02, 2003

Boulder, Colo. - The December issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest is a new study of the seafloor morphology of the Arctic Ocean.

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Physiographic provinces of the Arctic Ocean seafloor
Martin Jakobsson, Center for Coastal and Ocean Mapping - Joint Hydrographic Center, University of New Hampshire, Durham, New Hampshire 03824, USA; et al. Pages 1443-1455.
Keywords: Arctic Ocean, physiography, bathymetry, ocean ridges, ocean basins.

The seafloor morphology of the Arctic Ocean has been analyzed and classified into physiographic provinces, abyssal plains, ridges, continental rises, continental shelves, and continental slopes. The study is based on the most recent version of the International Bathymetric Chart of the Arctic Ocean (IBCAO) digital depth compilation. The areas of the provinces classified are individually calculated, and their morphologies are subsequently discussed in the context of the geologic evolution of the Arctic Ocean Basin as described in the published literature.

Active and inactive groundwater flow systems: Evidence from a stratified, mountainous terrain
Alan L. Mayo, Brigham Young University, Department of Geology, Provo, Utah 84602, USA; et al. Pages 1456-1472.
Keywords: groundwater, conceptual model, isotope chemistry, groundwater age, solute chemistry, sedimentary rocks, mountains.

This article describes a new conceptual model of groundwater flow. The model envisions shallow, near surface groundwater systems that readily respond to seasonal and near term climatic cycles. Such systems are termed active. Inactive groundwater systems have limited or no communication with annual recharge events and have groundwater ages that do not progressively lengthen along their flow paths. Inactive flow systems may occur within a few hundred meters of land surface.

Orthogonal jointing during coeval igneous degassing and normal faulting, Yucca Mountain, Nevada
William M. Dunne, Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA; et al. Pages 1492-1509.
Keywords: orthogonal joints, welded tuffs, cooling joints, degassing tubes, perturbed stress fields, normal faults.

Joints characterized by networks of 5 to 10 mm diameter half-tubes, occurring in exact mirror image on opposing faces of the joints, are the oldest fractures in the welded volcanic tuffs at Yucca Mountain, Nevada. Tube-bearing joints are abundant at Yucca Mountain, but rare elsewhere in the world. The tubes provide the vital clue in demonstrating that the joints formed within a month of a massive caldera eruption about 12.7 Ma, and prior to welding of the tuff. During vertical expansion of the tuff, the joints and the tubes provided pathways for gases to escape from the erupted tuffs. The joints are unusual because they formed prior to welding, they exhibit no shear indicators, and have NW-SE and NE-SW trends that are not consistent with the E-W directed extension of the Basin and Range at 12.7 Ma. Analysis of new field observations and computer modeling, coupled with results of previous research, suggest that immediately after the eruption, slip on local faults perturbed the regional stress field, leading to joint formation. Formation of the NE-trending joints and differential compaction in a NW-trending basin caused a local 90° switch of horizontal extension direction, resulting in the formation of the NW-trending joints. Tube-bearing joints are large, persistent fractures that affect the present-day hydrologic characteristics of the site of the proposed high-level radioactive waste repository at Yucca Mountain.

Magmagenesis in the Ordovician backarc basins of the Northern Qilian Mountains, China
Lin-Qi Xia, Zu-Chun Xia, and Xue-Yi Xu, Xi'an Institute of Geology and Mineral Resources, Ministry of Land and Resources of China, Xi'an 710054, Shaanxi, China. Pages 1510-1522.
Keywords: backarc lava, arc lava, petrogeochemistry, magmagenesis, Northern Qilian Mountains.

This paper reports the results of a petrological and geochemical study of lavas from lower Palaeozoic back-arc basins in the Northern Qilian Mountains in the People's Republic of China. This study provides a good opportunity to understand the relationship between back-arc basin tectonic evolution and magmagenesis, from the time of rift initiation of island arc through development of back-arc seafloor spreading.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15'-33°45'S)
Jan Olov Nyström, Swedish Museum of Natural History, SE-10405, Stockholm, Sweden; et al. Pages 1522-1537.
Keywords: Chile, Andes, Tertiary, volcanism, extension, geochemistry.

This study deals with two Oligocene-Early Miocene volcanic formations situated in the Andes just east of Santiago, Chile, which were deposited during and after an inferred culmination of crustal attenuation. The >5200 m thick sequence erupted from volcanic arcs in continental basins. The geology, geochemistry, and isotope data suggest that the rocks formed during two episodes of extension terminated by short periods of contraction and uplift during continuous subduction. The two Tertiary formations belong to a belt of similar volcanic sequences along the Chilean Andes that become successively younger toward the south, probably due to oblique subduction of oceanic ridge.

Alleghanian deformation in the eastern Gaspé Peninsula of Quebec, Canada
Pierre Jutras, Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada; et al. Pages 1538-1551.
Keywords: Alleghanian orogeny, structural geology, Gaspé Peninsula, Ristigouche Basin, Cannes-de-Roches Basin, Carboniferous, strike-slip faults.

This paper provides evidence for Carboniferous transpressive deformation in a region that was thought to have only experienced mild extension during that time. It casts doubt on the assumed Devonian age of the youngest strike-slip displacements within the rest of the Quebec Appalachians, away from Carboniferous rock exposures.

Supra-subduction zone extensional magmatism in Vermont and adjacent Quebec: Implications for early Paleozoic Appalachian tectonics
Jonathan Kim, Vermont Geological Survey, Waterbury, Vermont 05671, USA; et al. Pages 1552-1569.
Keywords: metadiabases, supra-subduction zone, Rowe-Hawley belt, Dunnage zone, Bolton Igneous Group, Coburn Hill Volcanics, Mount Norris Intrusive Suite.

The paper interprets the origin of a group of ancient (ca. 470 Ma) volcanic rocks in northern Vermont near the border with Quebec. The chemistry of the rocks shows that they were magmas formed in a region where two plates collided in a great mountain building episode in the Ordovician time period. The specific region where the magmas erupted may have been a spreading basin behind a line of volcanoes that defined a volcanic arc.

Sheeted intrusion of the synkinematic McDoogle pluton, Sierra Nevada, California
Kevin H. Mahan, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111, USA; et al. Pages 1570-1582.
Keywords: emplacement, shear zones, plutons, sheeted dikes, Sierra Nevada.

This article addresses the emplacement style of a granitoid pluton in the Cretaceous Sierra Nevada magmatic arc in California. It also discusses the extent and significance of the deformed host rocks found within and adjacent to the pluton. These results may shed further light on important issues, such as the nature of temporal changes in pluton emplacement style during construction of a magmatic arc, processes of creation and preservation of host rock screens, and the tectonic significance of ductile shear zones in the Sierran arc.

Relationships among vegetation, climatic zonation, soil, and bedrock in the central White-Inyo Range, eastern California: A ground-based and remote-sensing study
W.G. Ernst, et al., Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA. Pages 1583-1597.
Keywords: hyperspectra microclimate, vegetation, White-Inyo Mountains, remote sensing, geobotany.

This study combines airborne remote sensing spectral data and hand-held radiometer measurements with detailed surface mapping of the geologic substrate, soil, and vegetative species and botanical communities in the Central White-Inyo range of easternmost California. Our work attempts to quantify environmental factors governing the nature and distribution of plant cover in deep valleys and on elevated mountain ridges. Microclimatic zonation is extreme in this rugged mountain range, lying in the rain shadow of the Sierra Nevada. Our five-year investigation demonstrates that vegetation assemblages in the Central White-Inyo range are sensitive indicators of temperature, precipitation-evaporation, and mineral nutrients. Rainfall (soil moisture) correlates directly with elevation and to a lesser extent with albedo of the geologic substrate - hence, these are the two main parameters controlling species and plant community distributions.
Geological Society of America

Geological Society of America

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