Boulder, Colo., USA - Terrestrial Laser Scanner (TLS) is a powerful mapping tool that helps us to image natural surfaces at centimeter scale, which is established in this research study by evaluating the performance of long-range TLS, Riegl Z620i and Riegl LPM-800HA, on characterizing natural surfaces. Prabin Shilpakar and colleagues have developed a procedure to test componential uncertainty budgets of the TLS system, such as instrumental error, georeferencing error, and surface modeling error.
Shilpakar and colleagues also have established the relationship between reference network uncertainty and the repeatability and resolution of imaged natural surfaces. To achieve their goals, they used a combination of the two instruments to image fault scarps and erosional ravines in Panamint Valley and the San Gabriel Mountains of California, respectively. In both experiments, a Total Station (TS) was used to establish a control network geometry of reflectors and georeferenced with the Global Navigation Satellite System (GNSS) in Real Time Kinematic (RTK) and Static (S) modes in the first and second experiment, respectively. Shilpakar and colleagues document that the combined uncertainty for the reference network and surface interpolation represents the repeatability of an imaged natural surface.
Assessment of the uncertainty budget and image resolution of terrestrial laser scans of geomorphic surfaces
Prabin Shilpakar et al., Dept. of Geosciences, University of Texas at Dallas, ROC 21 800 West Campbell Road, Richardson, Texas 75080, USA. This article is online at http://geosphere.
Other recently published Geosphere articles are highlighted below:
The intrusion breccia in the valley of Roaring Brook, Giant Mountain, Adirondack Highlands, New York: A modern interpretation
James McClelland et al., Dept. of Geology, Colgate University, 13 Oak Drive, Hamilton, New York 13346, USA. This article is online at http://geosphere.
The breccia consists of rounded and angular inclusions of rocks that were originally interpreted as derived from a much older, and no longer exposed, basement that was invaded by molten material now forming the matrix for the fragments. However, application of modern geochemical techniques reveals that the inclusions consist of material that is identical to, and of the same age, as the host rocks. The distinctive features of the inclusions are due to the interaction between molten, partially crystallized, and solidified intrusive material.
Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa Clara basin, California
Edward A. Mankinen and Carl M. Wentworth, U.S. Geological Survey, 345 Middlefield Road, MS 937, Menlo Park, California 94025, USA. This article is online at http://geosphere.
The Quaternary alluvial sedimentary section beneath the Santa Clara Valley, at the southern end of San Francisco Bay, California, contains a rich record of past variation in Earth's magnetic field over the million years or so, during which the section accumulated. Study of cores of these sediments, collected during the drilling of five 265-to 407-meter deep wells in the valley, identifies the two most recent global magnetic reversals: the 780-thousand-year-old boundary between the Brunhes normal and Matuyama reversed paleomagnetic intervals, and the 990-thousand-year-old top of the 60-thousand-year-long Jaramillo normal interval, which occurs within the upper Matuyama. At least 100 m of normal Jaramillo section are recognized in the two most deeply penetrating wells, but no reversed bottom to that interval was found. In addition, several paleomagnetic excursions -- relatively short regional departures from predominant magnetic behavior -- are recognized: the 32-thousand-year-old Mono Lake, 210 thousand year old Pringle Falls, 565 thousand year old Big lost, possibly the 40-thousand-year-old Laschamp and 110-year-old Blake, as well as three other unidentified excursions. These paleomagnetic events provide independent control on the age of the sedimentary section and are fully consistent with previous subdivision of the upper 300 meters of section into eight sedimentary cycles and with the ages assigned to those cycles through correlation with the marine oxygen isotope record.
Quantifying comparison of large detrital geochronology data sets
Joel E. Saylor and Kurt E. Sundell, Dept. of Earth and Atmospheric Sciences, University of Houston, Science & Research 1 Building, 3507 Cullen Boulevard, Room 312, Houston, Texas 77204, USA. This paper is online at http://geosphere.
The increase in detrital geochronological data presents challenges to existing approaches to data visualization and comparison, and highlights the need for quantitative techniques able to evaluate and compare multiple large data sets. We test five metrics commonly used as quantitative descriptors of sample similarity in detrital geochronology: the Kolmogorov-Smirnov (K-S) and Kuiper tests, as well as Cross-correlation, Likeness, and Similarity coefficients of probability density plots (PDPs), kernel density estimates (KDEs), and locally adaptive, variable-bandwidth KDEs (LA-KDEs). We assess these metrics by applying them to 20 large synthetic data sets and one large empirical data set, and evaluate their utility in terms of sample similarity.
Petrogenesis and provenance of distal volcanic tuffs from the Permian-Triassic Karoo Basin, South Africa: A window into a dissected magmatic province
Matthew P. McKay et al., Dept. of Geology and Geography, West Virginia University, 98 Beechurst Ave., Morgantown, West Virginia 26505, USA. This article is online at http://geosphere.
The age and geochemistry of zircon grains that were found in volcanic ash within the sedimentary strata in South Africa record volcanic activity in the Permian (300 to 251 million years ago) and earliest Triassic (251 to 240 million years ago). The chemical composition of the zircon grains suggests that the grains formed in an extensional backarc volcanic system, which would have been located within the supercontinent Gondwana, but was lost during opening of the Atlantic and Southern Oceans. This volcanic rift system may have played a role in the eventual breakup of the supercontinent Gondwana millions of years later, beginning around 170 million years ago.
Sedimentary provenance of the Taza-Guercif Basin, South Rifean Corridor, Morocco: Implications for basin emergence
Jonathan R. Pratt et al., Dept. of Earth and Ocean Sciences, University of South Carolina, 710 Sumter Street, Columbia, South Carolina 29208, USA. This article is online at http://geosphere.
This research investigates the cause of the emergence of the Taza-Guercif Basin in Morocco. This basin was part of the Rifean Corridor, a marine connection between the Atlantic Ocean and the Mediterranean Sea open from approximately 8 to 6 million years ago. The closure of the Rifean Corridor was essential in initiating the Messinian Salinity Crisis, an event in which the Mediterranean Sea was nearly dried out. The Rifean Corridor opened and closed between two actively uplifting mountain belts, the Rif and Middle Atlas mountains. To determine the role of this mountain uplift in causing the emergence of the Taza-Guercif Basin, Jonathan Pratt and colleagues have compared the U-Pb and fission-track ages of detrital zircons in sediments deposited prior to, during, and after the Messinian Salinity Crisis within the basin with zircons found in the Rif and Middle Atlas mountains. Their evaluation finds the uplift of the Middle Atlas Mountains was an important cause of the emergence of the Taza-Guercif Basin and likely, the entire Rifean Corridor.
Cenozoic paleogeographic evolution of the Elko Basin and surrounding region, northeast Nevada
J.-E. Lund Snee et al., Dept. of Geological Sciences, 450 Serra Mall Building 320, Room 118, Stanford University, Stanford, California 94305, USA. This article is online at http://geosphere.
The geologic history of the western U.S. between the end of Cretaceous crustal thickening and the beginning of Miocene Basin and Range extension is controversial. We studied a strategic group of rocks in northeastern Nevada, adjacent to the Ruby Mountains, to gain insight into the tectonic history of Earth's surface across this time span. Geologic mapping and radiometric ages constrain the chronostratigraphy of volcanic and sedimentary strata that were deposited between Late Cretaceous and Miocene time. These data support a growing consensus that extensional faulting did not occur as a consequence of, or in the immediate aftermath of, crustal thickening, as has been proposed by many workers. Instead, extension began in middle Miocene time -- about 17 million years ago. Our work showing this geologically young age for the onset of surface extension must be reconciled with decades of previous work that suggest that the once deeply buried metamorphic rocks exposed in the Ruby Mountains and East Humboldt Range were brought closer to the surface by extensional faulting at an earlier time, between 85-20 million years ago. This inconsistency might be explained by a two-part history where dome-like upwelling of hot middle crust represented by the future Ruby Mountains took place until about 29 million years ago without producing faults with significant offset at the surface. Final uplift was accomplished by younger Miocene normal faults, as documented by the sedimentary record of the Humboldt Formation between about 17 and 8 million years ago. These results also have implications for understanding the evolution of topography and climate in the western United States over the same controversial time span. Previous workers suggested that high elevations and rugged topography developed from north to south across the western U.S. between about 50-23 million years ago. Our enhanced chronostratigraphy shows that the suggested elevation gain could have occurred across this time span or later, but that rugged topography began to develop only about 17 million years ago.
A calibration-free approach for measuring fracture aperture distributions using X-ray computed tomography
Da Huo et al., Dept. of Energy Resources Engineering, Stanford University, Green Building, Room 065, 367 Panama Street, Stanford, California 94305, USA. This article is online at http://geosphere.
In this paper, Huo et al. present and validate a Calibration-Free Missing CT Attenuation (CFMA) imaging method for measuring fracture apertures. This model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. They also analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning. The aperture measurement error is significantly reduced by replicate scans.
Testing models of Tibetan Plateau formation with Cenozoic shortening estimates across the Qilian Shan-Nan Shan thrust belt
Andrew V. Zuza et al., Dept. of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095-1567, USA. This article is online at http://geosphere.
The Tibetan Plateau, often called "the Roof of the World," is the highest, largest, and youngest plateau on Earth. The collision of India with Asia since 65-55 million years ago has caused the modern plateau to achieve its > 4 km average elevation and 55-70 km crustal thickness (i.e., double that of the average continental crust). The mechanism by which the plateau was constructed is still debated. To test existing models of plateau development, we examined deformation across the northern plateau boundary by analyzing seismic reflection profiles to observe the subsurface geology. By integrating our work with existing structural studies, we demonstrate that a combination of processes is responsible for the growth of northern Tibet: (1) southward underthrusting of Asia beneath Tibet and (2) distributed folding and faulting leading to the wholesale thickening of Tibetan crust. In addition, this margin is accommodating significantly more convergence between India and Asia than previously thought. We show that deformation is concentrated along the northern frontal thrust system, which is similar to deformation along the southern margin of the plateau, where deformation is highly concentrated along the Main Frontal Thrust of the Himalaya. This is significant for geologic hazard assessment, as concentrated deformation in northern Tibet may lead to large devastating earthquakes.
Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene-Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada
C.J. Busby et al., Dept. of Earth and Space Sciences, University of California Davis, One Shields Ave., Davis, California 95616, USA. This article is online at http://geosphere.
From the abstract: Eocene to Pliocene paleochannels of the Sierra Nevada (California, USA) were first exploited for gold placer deposits during the California gold rush (1848), and then mapped in surveys more than century ago. The surveys showed that the paleochannels flowed westward, like the modern rivers of the range; it then was assumed that the heads of the paleochannels were at the modern range crest. A first paradigm shift occurred ~50 yr ago, when it was recognized that at least some of the paleochannel fill was sourced from the region of the current state of Nevada, and it was proposed that the Sierra Nevada range was younger than the paleochannels (younger than 6 million years). More recent work has demonstrated that Sierran paleochannels are ancient features that formed on the shoulder of a broad high uplift (the Nevadaplano) formed during Cretaceous crustal shortening; the headwaters were in central Nevada prior to disruption of the plateau by Basin and Range extension. A second paradigm shift occurred in the past decade: the Sierra Nevada range front is formed of north-northwest transtensional structures of the younger than 12 Ma Walker Lane belt, not north-south to north-northeast-south-southwest extensional structures of the Basin and Range. In this paper, C.J. Busby and colleagues use detailed geologic mapping to reconstruct the paleogeographic evolution of three Oligocene to Pliocene east-west paleochannels in the central Sierra Nevada, and their progressive south to north derangement by Walker Lane structures: the Stanislaus in the south, the Cataract in the middle, and the Mokelumne in the north.
The stratigraphic expression of decreasing confinement along a deep-water sediment routing system: Outcrop example from southern Chile
Erin A.L. Pemberton et al., Dept. of Geoscience, University of Calgary, 2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada. This article is online at http://geosphere.
From the abstract: The products of sediment-laden turbidity currents that traverse areas of decreasing confinement on submarine slopes include erosional and depositional features that record the inception and propagation of deep-sea channels. The cumulative stratigraphic expression and deposits of such transitions, however, are poorly constrained relative to depositional settings dominated by end-member confined (i.e., submarine channel fill) and unconfined (i.e., lobe) deposits. Upper Cretaceous strata of the Magallanes foreland basin in southern Chile are characterized by a variety of stratigraphic architectural elements in close juxtaposition both laterally and vertically. This includes (1) low-aspect-ratio channelform bodies attributed to slope channel fills; (2) high-aspect-ratio channelform bodies interpreted as the deposits of weakly confined submarine channels; (3) lenticular sedimentary bodies considered to represent the infill of laterally coalesced scours; (4) discontinuous channelform bodies representing isolated scour fills; and (5) a cross-stratified, positive-relief sedimentary body, which is interpreted to record an upslope-migrating depositional bedform. These elements are interpreted to have formed at a submarine sediment routing system segment characterized by a break in slope, and an accompanying decrease in confinement. The various architectural elements examined are interpreted to record a unique stratigraphic perspective of turbidite channels at various stages of development, from early-stage discontinuous and isolated scour fills to low-aspect-ratio channel units.
Thermal evolution of the Sierra Nevada batholith, California, and implications for strain localization
Elisabeth Sophia Nadia et al., Dept. of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska 99775-5780, USA. This article is online at http://geosphere.
From the abstract: The Sierra Nevada batholith (California, USA) hosts multiple shear zones of different ages and different styles of deformation. In this study we present new data syntheses and maps of U-Pb zircon and hornblende and biotite Ar age distributions through the batholith in order to examine the temporal and thermal settings under which contractional and transpressional shear zones arose. These maps highlight the localization of intrabatholithic shear zones at the boundaries between swaths of some of the oldest and youngest plutons, and help to distinguish deformation styles in the southern and central Sierran arc. We also present new 40Ar/39Ar ages and crystallization and deformation temperatures from along the Kern Canyon fault system in the southern part of the batholith, and contrast these new constraints with previously published thermochronological conditions for shear zones to the north.
All GEOSPHERE articles are available at http://geosphere.