Boulder, Colo., USA - New Geosphere articles posted online 11 Jan. and 5 Feb. 2013 include additions to the "Origin and Evolution of the Sierra Nevada and Walker Lane" series, the "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen" series, and the "Crevolution 2: Origin and Evolution of the Colorado River System II" series. A new series is also introduced: "Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey."
1. Fresh water and the New Jersey shelf
2. Adobe Hills, California-Nevada, USA
3. Centimeter-scale analog models of continental rifts
4. The Organ magma complex
5. Textures of volcanic glass
6. The Hexi corridor basin, Tibetan Plateau
7. A comparison of the Seward-Malaspina and the Bagley-Bering-Tana glacier systems in Alaska
8. Two-dimensional models for the Border Ranges fault system, Alaska
9. New analytical techniques for understanding Grand Canyon
10. The value of different modalities in studying the Sierra Nevada
Abstracts for these and other Geosphere papers are available at http://geosphere.
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Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin
Robert van Geldern et al., GeoZentrum Nordbayern, University of Erlangen-Nuremberg, Schlossgarten 5, 91054 Erlangen, Germany. Posted online 11 Jan. 2013; now part of the Feb. 2013 issue; http://geosphere.
The presence of a large fresh water lens below the sea floor in the sediments of the New Jersey Shelf is known from early scientific drillings in the 1970s. However, the origin of this large freshwater body is still under debate. Recent groundwater flow models suggest that the water originates mainly from molten ice that entered the ground below large continental ice sheets during the last ice age. In this case, the potential resource of fresh water a few miles off the coast of New Jersey would be a non-renewable resource. In contrast to that, analyses of the pore waters that were sampled during the 2009 drilling expedition of the International Ocean Drilling Program (IODP Exp. 313) revealed the following sources of fluids beneath the shelf: (1) modern rainwater from onshore New Jersey, (2) modern seawater, and (3) a brine that ascends from deep sediments. This rules out the ice age origin theory. To investigate, Robert van Geldern and colleagues used the chemical composition of pore water and a naturally occurring label within the water molecule (H2O) -- the so-called "stable isotope composition" of the elements oxygen (O) and hydrogen (H). This ratio can be used to identify the origin and mixing of water masses. Further geochemical investigations of the sediment also revealed the existence of active methane formation in depths below 350 meters below the seafloor. Under today's sea level, this major greenhouse gas is not released from the sediment into the ocean or the atmosphere. The methane is oxidized in the sediment column to carbon dioxide that in turn is mostly trapped in the pore waters as dissolved inorganic carbon. This situation, however, could have been completely different during ice ages, where the sea level was much lower than today. Here, the methane could have vented out and contributed a significant portion of greenhouse gas into the atmosphere. This paper is part of a new "Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey" series.
Pliocene sinistral slip across the Adobe Hills, eastern California-western Nevada: Kinematics of fault slip transfer across the Mina Deflection
Sarah Nagorsen-Rinke et al., Dept. of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA. Posted online 11 Jan. 2013; now part of the February 2013 issue; http://geosphere.
The Adobe Hills, California-Nevada, USA, is a region of faulted volcanic rock located within what is called the Mina deflection, a zone of faults that connects the right-lateral fault slip dominated northern Eastern California shear zone (ECSZ) to the south with the right-lateral fault slip dominated Walker Lane belt (WLB) to the north. New geologic mapping, fault studies, and geochronology in the Adobe Hills allow researchers Sarah Nagorsen-Rinke and colleagues to calculate fault slip rates and test predictions for how fault slip is transferred from one fault system to another. Rocks exposed in the Adobe Hills include 11-million-year-old explosive volcanic rock overlain by three- to four-million-year-old (Pliocene age) sandstones, basalt lava flows, and basalt cinder cones, and younger (less than one-million-year-old) sands, alluvium, and mud. The authors propose that a set of faults located west of the White Mountains fault zone and east of Long Valley Caldera transfer a portion of right lateral Owens Valley fault slip northwestward onto the left lateral faults in the Adobe Hills. Fault slip in the Adobe Hills is part of a regional pattern of initiation and renewal of fault slip during the Pliocene that extends from latitude ~40 degrees N to ~36 degrees N within the ECSZ-WLB and along the western margin of the Basin and Range Province. This regional deformation episode may be related to changes in gravitational potential energy. This paper is part of the "Origin and Evolution of the Sierra Nevada and Walker Lane" series.
Experimental modeling of rifting at craton margins
Giacomo Corti et al., Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, U.O. Firenze, Via G. La Pira, 4, 50121 Florence, Italy. Posted online 11 Jan. 2013; now part of the Feb. 2013 issue, http://geosphere.
Continental rifting is one of the most important geodynamic processes that shape our planet: During its evolution, lithospheric plates are torn apart and broken, and -- eventually -- a new oceanic basin is formed in between. Yet, the dynamics by which continental extension is progressively focused to form mid oceanic ridges is not well understood, especially when the process occurs at the margins of the strongest portions of the continents, the old cratonic areas. Continental rifts, such as the system of rift valleys in East Africa, Lake Baikal, or the basins that characterize large portion of the Antarctic plate, represent typical examples of these conditions, where the juxtaposition between the old, cold, and resistant lithosphere and an adjacent weaker domain is likely to influence the architecture and evolution of the extension-related deformation. In this study by Giacomo Corti and colleagues, centimeter-scale analog models are used to reproduce and analyze in the laboratory this large-scale geological process, providing results of important relevance for improving our knowledge of development of continental rifts at the margins of old, strong cratonic areas.
Geochronologic evidence of upper-crustal in situ differentiation: Silicic magmatism at the Organ caldera complex, New Mexico
Matthew J. Zimmerer and William C. McIntosh, Dept. of Earth and Environmental Science and New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. First posted online 11 Jan. 2013; now part of the Feb. 2013 issue; http://geosphere.
Determining the processes that generate caldera-related silicic magmas and the origin of compositional zonation patterns of ignimbrites is central to our understanding of caldera-forming eruptions. Though the hazards associated with calderas are well known, caldera eruptions are infrequent and have not been directly observed. Because of this, most caldera magmatism models are developed using extinct caldera systems. In this study, Matthew Zimmerer and William McIntosh, conducted 40Ar/39Ar and laser ablation-inductively coupled plasma-mass spectrometry U/Pb zircon dating of the volcanic and plutonic rocks from the Organ caldera complex in order to accomplish two primary goals: (1) to investigate the time scales of caldera magmatism, from inception to cessation; and (2) to determine whether the caldera-forming silicic ignimbrites were generated by upper-crustal in situ differentiation or were generated at deeper crustal levels. This was accomplished by comparing the timing of caldera ignimbrite eruptions to the emplacement history of the Organ Needle pluton, the proposed residual crystal mush of the caldera magma chamber.
Atlas of Alteration Textures in Volcanic Glass from the Ocean Basins
Martin Fisk, College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97330, USA; and Nicola McLoughlin. First posted online 5 Feb. 2013; http://dx.
Martin Fisk and Nicola McLoughlin provide a comprehensive photographic atlas of the intricate alteration features found in glass in igneous rocks from the ocean basins. These textures have previously been termed "bioalteration textures" or "etch pits." Fisk and McLoughlin use transmitted-light color photomicrographs to illustrate the range of granular and tubular textures as well as their relationship to fractures, minerals, vesicles, and multiple episodes of alteration in the same sample. They describe the tubular forms using seven morphological characteristics: (1) length and width; (2) density; (3) curvature; (4) roughness; (5) variations in width; (6) branching; and (7) tunnel contents. The photomicrographs are a starting point for understanding the factors that control the formation of the alteration textures, for evaluating the biogenicity of the various forms, for inferring subsurface conditions during alteration, and for making comparisons to similar textures in ancient ophiolites, some of which have been attributed to the earliest life on Earth.
Late Quaternary slip rates of the thrust faults in western Hexi Corridor (Northern Qilian Shan, China) and their implications for northeastward growth of the Tibetan Plateau
Zheng Wen-Jun et al., State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China. Posted online 5 Feb. 2013; http://dx.
Based on 10Be exposure dating and topographic profiling, Zheng Wen-Jun and colleagues determined vertical components of slip rates for the Jiayuguan Fault and the Jintanan Shan Fault in the NE Tibetan Plateau. They found that the rates are consistent with previous geological and GPS constraints, and conclude that the Tibetan Plateau continues to grow northeastward by thrust faulting at low rates and by folding on the northeastern edge of the Hexi Corridor basin.
Examination of the interplay between glacial processes and exhumation in the Saint Elias Mountains, Alaska
Rachel M. Headley et al., Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. Posted online 5 Feb. 2013; http://dx.
The Saint Elias Range in southeast Alaska is a tectonically active and heavily glaciated coastal mountain system. In this study, Rachel Headley and colleagues review and combine glaciology and thermochronology data from the largest glacier systems in the region, the Seward-Malaspina and the Bagley-Bering-Tana. These datasets record the glacial flow, the hydrological system, and the exhumation of the bedrock under the ice. The combined datasets reveal that the two glacier systems, despite close proximity and similar size, show very different patterns of erosion and sediment transport. To fully understand this difference, says Headley, it is necessary to take into account glacier-specific properties and processes, such as subglacial water flow, surging of the glaciers, and bedrock topography and structural setting. This paper is part of the "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen" series.
Interpretation of gravity and magnetic data and development of 2D cross-sectional models for the Border Ranges fault system, south-central Alaska
Niti Mankhemthong et al., Dept. of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA. Posted online 5 Feb. 2013; http://dx.
Extensive glacial cover and lack of dense geophysical data within the Cook Inlet basin (CIB) of south-central Alaska make locating and determining the geometry of the Border Ranges fault system (BRFS), a major feature of the region, difficult. Niti Mankhemthong and colleagues use recently collected gravity data, available aeromagnetic data, and other geophysical information as constraints to develop plausible 2D cross-section models that better image the BRFS and related geologic structures of the CIB. Their models suggest the BRFS dips 50 to 70 degrees toward the west-northwest and extends to at least 15 km. Their integrated models also show a thick sequence of sedimentary rocks and volcanic rocks (6 to 20 km depth) overlying a high metamorphosed, hydrous rock body (serpentinite) at a depth of 16 to 34 km. The volcanic rocks and serpentinite are interpreted as possible sources of the south Alaska magnetic high over the CIB. The CIB's eastern boundaries are characterized by gravity and magnetic highs of the Border Range ultramafic and mafic assemblages (BRUMA), rocks high in iron and magnesium that have been derived from the lower crust and upper mantle. Formation of the BRUMA may be related to the presences of the deeper serpentinite body. A model that includes underplated sediments at the base of the crust beneath the northwestern Chugach Mountains (12 to 40 km) is consistent with an observed regional gravity low. The underplating may be associated with the process of subducting and shortening Yakutat microplate in south-central Alaska. This paper is part of the "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen" series.
New thermochronometric constraints on the Tertiary landscape evolution of central and eastern Grand Canyon, Arizona
J.P. Lee et al., U.S. Geological Survey, Denver Federal Center MS974, Denver, Colorado 80225, USA. Posted online 5 Feb. 2013; http://dx.
The Grand Canyon of Arizona, USA, is both a geologic wonder and enigma. Since its first geologic description over one hundred years ago, it has been the source of vigorous debate regarding the timing and processes of its formation. Ironically, the enormous magnitude of erosion that characterizes Grand Canyon is what obscures a description of its geologic history due to the almost complete removal of the Tertiary rock record. However, new analytical techniques have revived the century-old debate. In this study, J.P. Lee and colleagues present new data that allow insight into the thermal evolution of the rocks collected from the area of Grand Canyon. These thermal histories allow them to infer patterns of erosion through time, and therefore constrain the age of Grand Canyon. Results indicate that the region of Grand Canyon has undergone a spatially variable erosion history with segments of Grand Canyon forming at 28-20 million years ago while the easternmost segments of Marble Canyon formed no earlier than 10 million years ago. These observations indicate that the coherent landscape feature that we know today as Grand Canyon was actually assembled in a segmented fashion from inherited landscape features. The results here bring the geologic community one step further in constructing a unified theory that describes the complicated origin of Grand Canyon while honoring many previously described lines of geologic evidence. This paper is part of the "Crevolution 2: Origin and Evolution of the Colorado River System II" series.
Paleochannels, stream incision, erosion, topographic evolution, and alternative explanations of paleoaltimetry, Sierra Nevada, California
John Wakabayashi, Dept. of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA. Posted online 5 Feb. 2013; http://dx.
This paper by John Wakabayashi presents data bearing on the topographic evolution of the Sierra Nevada, California, USA. The systematic relationship between the azimuth and gradient of Eocene river channel deposits (paleochannels) strongly support late Cenozoic tilting and uplift of the range. Paleochannel reaches trending perpendicular to the range axis display the steepest gradients. In contrast, modern Sierran rivers do not show a systematic relationship between gradient and azimuth. Late Cenozoic initiation of significant stream incision (canyon cutting) also suggests late Cenozoic uplift, because this downcutting began in spite of apparently decreasing stream discharge and increasing sediment load. An increase in stream gradient (by tilting and uplift) best explains the onset of incision at that time. Late Cenozoic uplift and stream incision began earlier in the southern than in the northern part of the range. The southern part of the range has experienced two episodes of late Cenozoic uplift compared to a single one for the north. Stable isotope data, that have been interpreted to indicate a lack of late Cenozoic uplift, may reflect relatively recent reequilibration and/or the progressive advance of an erosional/weathering front downward into fresh rock. This study by Wakabayashi is potentially important for comparing and evaluating interpretations of the topographic evolution of mountainous regions derived from different methodologies, such as geomorphic-stratigraphic, thermochronologic, and stable isotope based paleoaltimetry. This paper is part of the "Origin and Evolution of the Sierra Nevada and Walker Lane" series.