Boulder, Colo., USA – Geosphere articles posted online 16 November 2012 cover a variety of topics, such as the geophysics of the Hogri fault zone, 5 km offshore of the Diablo Canyon nuclear power plant; using web-based GIS technologies and readily available global remote sensing datasets for investigations of arid land; the structure and evolution of the U.S. Sierra Nevada; the ANDRILL McMurdo Ice Shelf and Southern McMurdo Sound Drilling Projects; and climate-tectonic interactions in the southern Alaskan orogen.
Abstracts for these and other Geosphere papers are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of Geosphere articles by contacting Kea Giles at the address above.
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Influence of fault trend, bends, and convergence on shallow structure and geomorphology of the Hosgri strike-slip fault, offshore central California
Samuel Y. Johnson and Janet T. Watt, U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00830.1.
Comprehensive geophysical data are used to document and map the active, continuous, strike-slip Hosgri fault zone for more than 95 km in offshore central California. Slight bends and changes in fault trend control the development of fault-bounded seafloor uplifts and sedimentary basins. Fault slices develop in complex areas where the Hosgri fault obliquely converges with the Los Osos and Shoreline faults, and the regional uplift at Piedras Blancas formed above diverging Hosgri fault strands. Results presented by Samuel Y. Johnson and Janet T. Watt provide a model that can be applied more broadly for understanding the shallow structure and evolution of strike-slip fault zones, as well as new information to better constrain regional earthquake hazard assessment. Enhanced hazard assessment is especially timely because the Hosgri fault zone lies just 5 km offshore of the Diablo Canyon nuclear power plant.
Eruptive history of Mount Katmai, Alaska
Wes Hildreth and Judy Fierstein, Volcano Science Center, U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00817.1.
Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, Wes Hildreth and Judy Fierstein reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling, so they provide instead a photographic and interpretive record of the wall sequences exposed.
Deposition and deformation in the deepwater sediments of the offshore Barreirinhas Basin, Brazil
Ana Krueger et al., Dept. of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00805.1.
The Barreirinhas Basin is an ideal location to study shale-dominated gravity-driven thrusting systems because of the limited areal extent of the deformed areas compared to other areas in the world. Regional seismic reflection profiles across the Barreirinhas Basin on the Brazilian Equatorial margin show two major deepwater fold and thrust belts linked landward to extensional fault systems. Thrust faults are interpreted to be products of shortening caused by gravity-driven extension on the continental margin that involve rocks of both the shelf and the slope. Results show two main deformation events during the Cretaceous (between 84 and 89 million years ago [89 to 84 Ma]) and several episodes during the Later Cenozoic (ca. 55 Ma).
Timing of intense magmatic episodes in the northern and central Sierra Madre Occidental, western Mexico
Fred W. McDowell and William C. McIntosh, Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00792.1
The Sierra Madre Occidental volcanic field is the largest component of the Tertiary ignimbrite flare-up of western North America. Laser-fusion 40Ar/39Ar geochronology of sanidine and anorthoclase from ignimbrites sampled within three areas of concentrated geologic mapping in the Mexican states of Chihuahua and Durango provides the first examination of fine-scale temporal fluctuations within this significant area of the ignimbrite flare-up.
Integrated solutions for hydrologic investigations in arid lands
Doris Becker et al., Dept. of Geosciences, Western Michigan University, Kalamazoo, Michigan 49008, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00777.1.
Hydrological assessment studies across vast regions of the arid world are often hindered by the inaccessibility of these areas and the paucity of datasets, as well as the high expenses and difficulties entailed in acquiring these datasets, their unpublished nature, and their varying scales, projections, and datum. Using the Eastern Desert of Egypt (225,000 square kilometers) and the Sinai Peninsula (61,000 square kilometers) as test sites, Doris Becker and colleagues demonstrate practical and cost-effective integrated (geochemistry, geophysics, modeling) solutions that utilize web-based GIS (http://www.esrs.wmich.edu/webmap) technologies and take advantage of readily available global remote sensing datasets.
Shallow laccolithic emplacement of the Land's End and Tregonning Granites, Cornwall, UK: Evidence from aureole field relations and P-T modeling of cordierite-anthophyllite hornfels
Jonathan M. Pownall et al., Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00802.1.
The Land's End and Tregonning-Godolphin granites of the more than 250-km-long Permian Cornubian Batholith are heterogeneous medium- to coarse-grained peraluminous biotite-, tourmaline-, and lithium-mica granites traditionally thought to be emplaced as massive magmatic diapirs. Jonathan Pownall and colleagues present an investigation focusing on the contact metamorphism and deformation of the aureole rocks adjacent to the Land's End and Tregonning granites as an approach to better determine the method of granite emplacement and the depth at which it occurred. New 1:5000-scale geological maps for ~15 km of coastal exposure of the granites and their aureoles are included.
Is chemical zonation in plutonic rocks driven by changes in source magma composition or shallow-crustal differentiation?
Drew S. Coleman et al., Dept. of Geological Sciences, CB#3315 Mitchell Hall, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00798.1.
New mapping in the Half Dome Granodiorite of the Tuolumne Intrusive Suite of Yosemite National Park reveals previously unrecognized heterogeneities. The composition of the rocks varies predictably, and is cyclic, with repeated occurrence of dark-colored, low-silica rocks in the east grading to light-colored, high-silica rocks in the west. At the western margin of a cycle, there is an abrupt change back to the low-silica rocks and another gradational change to higher silica concentrations. The cycles are interpreted to result from in situ differentiation of the Half Dome Granodiorite in the shallow crust. Although the cycles span the compositional range of all map units in the Tuolumne Intrusive Suite, trace-element trends preserved in the cycles differ dramatically from those of both the intrusive suite and other plutons of the eastern Sierra Nevada batholith. Drew S. Coleman and colleagues suggest that the distinction between chemical variation in the cycles and that of the batholith as a whole indicates that upper crustal differentiation is relatively insignificant in driving the overall chemistry of the batholith and associated volcanic rocks.
Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, California, part I: Implications of thermomechanical modeling
J. Saleeby et al., Tectonics Observatory, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00746.1.
Jason Saleeby and colleagues apply thermomechanical numerical modeling to the physical processes that govern the removal of high-density upper mantle over the past five-million years of geologic time from the base of the crust beneath the southern Sierra Nevada and adjacent eastern San Joaquin Basin of California. The models are tested against observational data on crust and upper mantle structure and composition, uplift and volcanism of the Sierra Nevada, and subsidence and sedimentation in the San Joaquin Basin. This paper is part of Geosphere's "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California," series.
Imaging lithospheric foundering in the structure of the Sierra Nevada
H. Gilbert et al., Dept. of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47901, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00790.1.
The foundering, or removal, of dense lithospheric material has been proposed as a contributor to uplifting areas of high topography. Because a large portion of the material removed during foundering would be expected to be mafic, this process would also provide a mechanism by which continental crust reaches its intermediate composition from mantle melts. Earlier tomographic investigations identified a body with high seismic wave speeds in the mantle beneath the Great Valley, to the southwest of the southern Sierra Nevada, and suggested that it marks lithospheric material that has been removed from the southern Sierra. Signs of young uplift in the Sierra Nevada make it an ideal location to study foundering because the process may have occurred recently enough for its signature to not be overprinted and it may be ongoing. The tomographic model presented by H. Gilbert and colleagues demonstrates that high-speed lithospheric material remains intact to the base of the crust beneath the western foothills of the central Sierra Nevada and extends to depths greater than 100 km. In contrast, no high-speed material underlies the southern Sierra, or along the eastern portion of the central Sierra, where low-speed asthenosphere reaches the base of the crust. These observations suggest complete removal of the lithosphere from beneath the southern Sierra and ongoing removal within the central Sierra. The removal of this dense material appears to have contributed to the uplift of the southern Sierra and the high seismic velocities in the mantle beneath the Great Valley. This paper is part of Geosphere's "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California," series.
Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation
Katherine R. Barnhart et al., Dept. of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Ave., Boulder, Colorado 80302, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00765.1.
Katherine Barnhart and colleagues present data from crustal xenoliths -- fragments of the crust transported to the surface in volcanic eruptions -- that allow documentation of the physical and chemical characteristics of the lower portion of the continental crust in central Montana. Studies of this sort allow geologists to understand how continental crust formed and how it has evolved through time. This region is geologically notable because a thick layer of high seismic velocity material makes up nearly half of the crust under much of Montana and Wyoming. It is difficult to determine how this high-velocity layer formed because it is not exposed at the Earth's surface. Barnhart and colleagues analyze a suite of 11 xenoliths sourced from within and above this high-velocity layer to determine the mineralogy, petrology, geochronology, and bulk seismic velocity of the samples. Their observations indicate that there is considerable variation in how this high velocity layer formed. This paper is part of Geosphere's "Investigations of North America as EarthScope Reaches Its Maturity" series.
Multistage Cenozoic extension of the Albion-Raft River-Grouse Creek metamorphic core complex: Geochronologic and stratigraphic constraints
Alexandros Konstantinou et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00778.1.
Metamorphic core complexes represent a suite of rock outcrops that once used to be deep (10-20 kilometers) within Earth's crust, but plate tectonic extensional processes are responsible for exposing them to the surface, where they can be studied. Many years of research of various examples of metamorphic core complexes has produced a wealth of data, and a large number of different models have been developed to explain their formation. Alexandros Konstantinou and colleagues utilize geochronology and geologic mapping to study the basin that formed during the uplift of the Albion-Raft River-Grouse Creek metamorphic core complex in southern Idaho and northwestern Utah. They show that the basin formed between seven and 14 million years ago and outline the evolution of sources of detritus through this time period. Konstantinou and colleagues also summarize their previous work regarding the timing of the high temperature deformation of the rocks within the complex, which happened deep in the crust of the earth, between 32 and 25 million years ago. Their conclusion is that the high temperature deformation of the rocks, happening deep in the earth, is at least 10 million years older than the final uplift of the complex and the formation of the sedimentary basin, and hence these two events are probably not related. These results bear important implications on how to interpret metamorphic core complexes and give insight into all of the different models that have been proposed to explain them. This paper is part of Geosphere's "Investigations of North America as EarthScope Reaches Its Maturity" series.
Evolution of marine sedimentation in the Bering Sea since the Pliocene
Ivano W. Aiello and A. Christina Ravelo, Moss Landing Marine Laboratories, Moss Landing, California 95039, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00710.1.
Aiello and Ravelo's paper is the first to describe and document regional patterns of sedimentation in the Bering Sea, the third largest marginal sea and one of the most productive regions of the world. The analysis of several thousand meters of sediment cores representing the last five million years of the geological record allowed them to link vertical and horizontal changes in sedimentary sequences to global climatic and oceanographic changes through the entire Northern Hemisphere glaciation. The authors found that productivity in the Bering Sea was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period decreasing as Northern Hemisphere glaciation intensified some three million years ago. This paper is a hallmark for information on the style and evolution of late Cenozoic sedimentation in the high north Pacific. This paper is part of Geosphere's "Exploring the Deep Sea and Beyond" series.
Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone
H.F. Ryan et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00815.1.
Seismic reflection data are used to describe forearc basin evolution and relate this to areas of coupling on the Aleutian megathrust that can generate great earthquakes. Both the subduction of the Amlia fracture zone and a mid-Pleistocene increase in glaciation have an influence on plate coupling. This study by H.F. Ryan and colleagues has implications for the generation of massive tsunamis from both earthquakes and earthquake-triggered large-scale failures of the Aleutian Ridge. This paper is part of Geosphere's "Exploring the Deep Sea and Beyond" series.
Neogene benthic foraminiferal assemblages and paleoenvironmental record for McMurdo Sound, Antarctica
Molly O. Patterson and Scott E. Ishman, Dept. of Geology, Southern Illinois University Carbondale, Mailcode 4324, Carbondale, Illinois 62901, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00771.1.
In the austral summer of 2007-2008, the international Antarctic Drilling Program's (ANDRILL) Southern McMurdo Sound (SMS) project successfully recovered an approx. 1000-meter-deep drill core beneath the Ross Ice Shelf in the McMurdo Sound, Antarctica. The aim of the SMS project was to establish a robust history of past Antarctic ice sheet variation and climate evolution that can integrated into continental and global records that will help further understanding of the East Antarctic Ice Sheet's role in the past, present and future global climate system. The SMS project recovered a sediment record inferred to be Miocene (~17 million years ago) and younger in age. The sediment record highlights major fluctuations in ice sheet extent during periods warmer than present and during a potential transition in ice sheet stability. The results presented here by Molly Patterson and Scott Ishman, through the use of benthic foraminifera, give further insight into ice sheet extent and glacial proximity as well as paleobathymetric estimates that will be useful concerning efforts to extract eustatic sea-level trends during the Miocene. This paper is part of Geosphere's "The ANDRILL McMurdo Ice Shelf (MIS) and Southern McMurdo Sound (SMS) Drilling Projects" series.
Early Miocene volcanic activity and paleoenvironment conditions recorded in tephra layers of the AND-2A core (southern McMurdo Sound, Antarctica)
A. Di Roberto et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, via della Faggiola 32, I 56126 Pisa, Italy. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00754.1.
A. Di Roberto and colleagues present results of sedimentological, morphoscopic, petrographic, and geochemical studies on volcanic deposits recovered from a 1138.54 m-long core (AND-2A) sampled in the Ross Sea (Antarctica). They studied these deposits in order to draw information about style of volcanic eruption from which they have been produced as well as to provide insight on the provenance and the environments of deposition. On the basis of volcanological, geochemical, and age constraints, the long-lived Mount Morning eruptive center, located ~80 km south of the drill site, was recognized as the probable source from which the deposits were emitted. The analysis of volcanic deposits in the studied core also provides important information on the paleoenvironment of deposition by revealing that they were deposed under generally open-water condition when ice only partially covered sea surface. This paper is part of Geosphere's "The ANDRILL McMurdo Ice Shelf (MIS) and Southern McMurdo Sound (SMS) Drilling Projects" series.
Plate margin deformation and active tectonics along the northern edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska, and Yukon, Canada
Ronald L. Bruhn et al., Dept. of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00807.1.
This study by Ronald Bruhn and colleagues discusses the complex history of deformation and the earthquake potential of the Saint Elias Mountains in southern Alaska. The Saint Elias Mountains are formed by the subduction and collision of the Yakutat Terrane, a group of rocks that has moved northward over time along the Fairweather fault. These rocks are now colliding into southern Alaska, driving uplift to form mountains and creating many earthquakes, some of which generate tsunamis that then propagate southward through the Pacific Basin. The rapid uplift and high elevations of the Saint Elias Mountains result in extensive glaciation, both in the interior of the mountains and on the broad coastal plains. The glaciers bury the largest faults, making them impossible to map and study using standard techniques of structural geology. Bruhn and colleagues use remote sensing techniques to map the surface topography and the movement of ice on the glaciers, which provide evidence for the locations of large faults and folds that lie beneath the ice. The approach is based on the concept that undulations in the topography at the base of a glacier can be detected by subtle changes in the topographic relief and directions and rates of ice flow on the surface of the glacier. The major results of this study are (1) a new and comprehensive model of the structure of the mountains where the tectonic plate motion changes from mostly strike-slip to strong convergent plate motion and uplift by faulting and folding of the crust; (2) new information concerning the style and history of faulting beneath the spectacular Bagley Ice Valley which spans the spine of the Saint Elias Mountains; and (3) a regional model for how the major faults form a seismically active fault network that accommodates deformation to form the mountains, and also generates earthquakes, both large and small. This paper is part of Geosphere's "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen" series.
Reconnaissance geochronology and geochemistry of the Mont-Tremblant gneiss of the Morin terrane, Grenville Province, Québec
William H. Peck, Dept. of Geology, Colgate University, Hamilton, New York 13346, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00828.1.
The Mont-Tremblant gneiss is a granulite facies quartzofeldspathic orthogneiss unit in Morin terrane of the southwestern Grenville Province. These gneisses are the oldest rocks in the Morin terrane and country rocks to intrusions of the 1.15-billion-year-old anorthosite-mangerite-charnockite-granite (AMCG) suite. The Mont-Tremblant gneiss may also be basement to the Grenville Supergroup, but until now it has not been directly dated. Sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) geochronology of zircon from two penetratively deformed samples of Mont-Tremblant gneiss yields igneous ages of 1324 plus or minus 38 million years and 1333 plus or minus 32 million years, and one sample shows metamorphic zircon growth during the Shawinigan orogeny 1159.4 plus or minus 15.6 million years ago. Geochemically, Mont-Tremblant gneiss samples are calc-alkaline, granitic, and show hallmarks of an evolved arc environment. These characteristics are consistent with the Mont-Tremblant gneiss being correlative with the Geon 14 Lacoste and Bondy suites in the Central Metasedimentary Belt of Québec to the west, but contrast with the more juvenile tonalitic Geon 14 calc-alkaline suites in the Adirondack Highlands and other Grenville outliers in the Appalachian Mountains. This transition may reflect differences in the lithosphere along strike of the Laurentian margin during Geon 14 subduction. This paper is part of Geosphere's "New Developments in Grenville Geology" series.
New Series
Introduction: CRevolution 2: Origin and Evolution of the Colorado River System II
Karl Karlstrom et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 16 November 2012; http://dx.doi.org/10.1130/GES00716.1.
This volume contains papers that highlight the continued scientific debate about the origin and evolution of the Grand Canyon and the Colorado River System. This iconic region was uplifted from sea level starting in the late Cretaceous to present elevations that exceed 4 km in the Rocky Mountains and 1.5 km over large regions of the Colorado Plateau. The new contributions come at a time of intense debate about the driving forces for uplift of plateau regions and connections between the mantle dynamics and surface topographic systems. This introduction by Karl Karlstrom and colleagues reflects results from a 2010 Colorado River symposium held in Flagstaff, Arizona, USA, in May 2010 that had 70 participants. This meeting built on two previous decadal scientific meetings and focused on forging scientific consensus where possible while also articulating continued controversies. New developments involved hypotheses that Neogene mantle flow is driving plateau tilting and differential uplift, with consensus that multidisciplinary studies are needed to test the relative importance of tectonic, climatic, and geomorphic forcings in shaping the spectacular landscapes of the Colorado Plateau region. Building on a century-long tradition, this region still provides a globally important natural laboratory for studies of the interactions of erosional and tectonic processes in shaping landscape of elevated plateaus.
Journal
Geosphere