|17 July 2013
GSA Release No. 13-45
Aerial view of Lake Tahoe, USA. See related article by Jillian M. Maloney et al., http://dx.doi.org/10.1130/GES00877.1.
GEOSPHERE Presents New Studies on the Nature and Structure of North America and Taiwan
Boulder, Colorado, USA – Eight new studies posted 26 June and 16 July add to Geosphere's cache of solid research on the nature and structure of North America. Locations studied: Black Canyon of the Gunnison, Colorado; the northern Cascade Mountains, Washington; the Sierra Nevada batholith; the New Jersey shelf; the Appalachian Basin of northwestern Alabama; the Sierra Nevada microplate (Walker Lane rift); and the West Tahoe-Dollar Point fault. A ninth study covers erosion and crustal deformation in central Taiwan.
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.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geosphere in articles published. Contact Kea Giles for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Incision history of the Black Canyon of Gunnison, Colorado, over the past ~1 Ma inferred from dating of fluvial gravel deposits
Magdalena S. Donahue et al., Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00847.1
Magdalena S. Donahue and colleagues write that spatio-temporal variability in fluvial incision rates in bedrock channels provides data regarding uplift and denudation histories of landscapes. They note, "The longitudinal profile of the Gunnison River (Colorado), tributary to the Colorado River, contains a prominent knickzone with 800 m of relief across it within the Black Canyon of the Gunnison," and go one to discuss average bedrock incision rates and paleosol ages. In the article abstract, Donahue et al. conclude, "The Black Canyon knickpoint overlies a strong gradient between low-velocity mantle under the Colorado Rockies and higher-velocity mantle of the Colorado Plateau. We interpret recent reorganization and transient incision of both the Gunnison River and upper Colorado River systems to be a response to mantle-driven epeirogenic uplift of the southern Rockies in the last 10 Ma [million years]."
Holocene tectonics and fault reactivation in the foothills of the north Cascade Mountains, Washington
Brian L. Sherrod et al., U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00880.1.
Brian L. Sherrod and colleagues use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). They write that "mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland." For example, "The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults." According to their paper's abstract, Sherrod and colleagues "infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake." Further data analysis, they note, demonstrates that "reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada)."
Intrusive history and petrogenesis of the Ash Mountain Complex, Sierra Nevada batholith, California (USA)
Julia E. Holland et al. (Diane R. Smith, corresponding author), Dept. of Geosciences, Trinity University, One Trinity Place, San Antonio, Texas 78212-7200, USA. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00890.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
Julie E. Holland and colleagues study the Ash Mountain Complex (AMC) in the western Sierra Nevada batholith (SNB; California, USA), which is an exposure of six compositionally diverse intrusive lithologies with clear crosscutting relationships that permit a focused investigation of magma source characteristics and the relative roles of petrogenetic processes on the evolution of the system. They use new field observations, zircon U-Pb dates, major and trace element data, and Sr-Nd-Pb isotopic data to develop a model that can be applied to similar SNB intrusive suites. Data analysis suggests that "stage 1 gabbroids were derived by partial melting of lithospheric mantle, whereas coeval felsic magmas were derived by partial melting of a mafic, juvenile crustal source. Stage 2 and stage 3 granitoids were derived from similar sources that generated stage 1 granitoids, but there was greater input from evolved crust." For further discussion, see their paper, which is part of the "Origin and Evolution of the Sierra Nevada and Walker Lane" series," online at http://dx.doi.org/10.1130/GES00890.1.
Fresh-water and salt-water distribution in passive margin sediments: Insights from Integrated Ocean Drilling Program Expedition 313 on the New Jersey Margin
Johanna Lofi et al., Geosciences Montpellier, UMR5243, Bâtiment 22, Université Montpellier 2, 34095 Montpellier cedex 5, France. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00855.1. Themed issue: Results of IODP Exp. 313: The History and Impact of Sea-level Change Offshore New Jersey.
Johanna Lofi and colleagues study the New Jersey shelf (offshore North America) and note that "the presence of pore water fresher than seawater is known from a series of boreholes completed during the 1970s and 1980s." To account for this fresh water, they write, "a first hypothesis involves possible present-day active dynamic connections with onshore aquifers, while a second involves meteoritic and/or sub–ice-sheet waters during periods of lowered sea level." Integrated Ocean Drilling Program (IODP) Expedition 313 drilled three boreholes on the middle shelf, offering the team a unique opportunity to study the internal structure of the siliciclastic system at scales ranging from the depositional matrix to the continental margin. This, Lofi and colleagues write, "enables the stratigraphic architecture to be correlated with the spatial distribution and salinity of saturating fluids." Expedition 313 revealed both very low salinities at depths exceeding 400 m below the seafloor and evidence for a multilayered reservoir organization, with fresh- and/ or brackish-water intervals alternating vertically with salty intervals. Lofi and colleagues go on to present a revised distribution of the salinity beneath the middle shelf. Their observations suggest that the processes controlling salinity are strongly influenced by lithology, porosity, and permeability. "Saltier pore waters generally occur in coarse-grained intervals and fresher pore waters occur in fine-grained intervals. The transition from fresher to saltier intervals is often marked by cemented horizons that probably act as permeability barriers. In the lowermost parts of two holes, the salinity varies independently of lithology, suggesting different mechanisms and/or sources of salinity." Their interpretation of the sedimentary facies distribution, derived from core, logs, and seismic profile analyses, "is used to discuss the margin-scale two-dimensional reservoir geometry and permeability distribution. These proposed geometries are of primary importance when considering the possible pathways and emplacement mechanisms for the fresh and salty water below the New Jersey shelf."
Statistical classification of log response as an indicator of facies variation during changes in sea level: Integrated Ocean Drilling Program Expedition 313
Jennifer Inwood et al., Dept. of Geology, University Road, University of Leicester, Leicester LE17RH, UK. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00913.1. Themed issue: Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey.
Jennifer Inwood and colleagues utilize a novel application of a statistical approach for analysis of downhole logging data from Miocene-aged siliciclastic shelf sediments on the New Jersey Margin (eastern USA). A multivariate iterative nonhierarchical cluster analysis (INCA) of spectral gamma-ray logs from Integrated Ocean Drilling Program (IODP) Expedition 313 enables them to infer the lithology within this siliciclastic succession as well as, through comparison with the 1311 m of recovered core, a continuous assessment of depositional sequences is constructed. Inwood and colleagues find that "significant changes in INCA clusters corroborate most key stratigraphic surfaces interpreted from the core, and this result has particular value for surface recognition in intervals of poor core recovery". Their analysis contributes to the evaluation of sequence stratigraphic models of large-scale clinoform complexes that predict depositional environments, sediment composition, and stratal geometries in response to sea-level changes. This novel approach of combining statistical analysis with detailed lithostratigraphic and seismic reflection data sets will be of interest to any scientists working with downhole logs, especially spectral gamma-ray data, and also provides a reference for the strengths and weaknesses of multicomponent analysis applied to continental margin lithofacies. The method presented by Inwood and colleagues "is appropriate for evaluating successions elsewhere and also has value for hydrocarbon exploration where sequence stratigraphy is a fundamental tool."
Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa
Mark G. Steltenpohl et al., Dept. of Geology and Geography, Auburn University, Petrie Hall, Auburn, Alabama 36849, USA. Posted online 16 July 2013, http://dx.doi.org/10.1130/GES00703.1
Mark G. Steltenpohl digitally merge and filter aeromagnetic and gravity data sets obtained for Alabama (United States) to enhance upper-crustal anomalies. They describe their research area: "Beneath the Appalachian Basin in northwestern Alabama, broad deep-crustal anomalies of the continental interior include the Grenville front and New York-Alabama lineament (dextral fault). Toward the east and south, high-angle discordance between the northeast-trending Appalachians and the east-west-trending wedge of overlapping Mesozoic and Cenozoic Gulf Coastal Plain sediments reveals how bedrock geophysical signatures progressively change with deeper burial. High-frequency magnetic anomalies in the Appalachian deformed domain (ADD) correspond to amphibolites and mylonites outlining terranes, while broader, lower-amplitude domains include Paleozoic intrusive bodies and Grenville basement gneiss. Fundamental ADD structures (e.g., the Alexander City, Towaliga, and Goat Rock–Bartletts Ferry faults) can be traced southward beneath the Gulf Coastal Plain to the suture with Gondwanan crust of the Suwannee terrane. Within the ADD, there is clear magnetic distinction between Laurentian crust and the strongly linear, high-frequency magnetic highs of peri-Gondwanan (Carolina-Uchee) arc terranes. The contact (Central Piedmont suture) corresponds to surface exposures of the Bartletts Ferry fault. ADD magnetic and gravity signatures are truncated by the east-west–trending Altamaha magnetic low associated with the Suwannee suture. Arcuate northeast-trending magnetic linears of the Suwannee terrane reflect internal structure and Mesozoic failed-rift trends." Using such geophysical data, Steltenpohl and colleagues make inferences on surface and subsurface geology and vice versa, which, they note, "has applicability anywhere that bedrock is exposed or concealed beneath essentially non-magnetic sedimentary cover."
Sierra Crest graben-vent system: A Walker Lane pull apart within the ancestral Cascades arc
Cathy J. Busby et al., Dept. of Earth Science, University of California, Santa Barbara, Santa Barbara, California 93106, USA. Posted online 26 June 2013; http://dx.doi.org/10.1130/GES00670.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
Cathy J. Busby and colleagues show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began about 12 million years ago in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fissures within a series of grabens, which Busby and colleagues refer to as "the Sierra Crest graben-vent system." They describe the graben-vent system in detail and present evidence for a dextral component of slip on the north-northwest-south-southeast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. In the abstract for their paper, Busby and colleagues conclude that "the Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fills under a transtensional strain regime in an arc volcanic field." This article is part of the "Origin and Evolution of the Sierra Nevada and Walker Lane" themed issue.
Structural inheritance and erosional controls on thrust kinematics in western Taiwan
T. Wilcox et al., Dept. of Geological Sciences, University of Colorado at Boulder, 2200 Colorado Avenue, Boulder, Colorado 80309, USA. Posted online 26 June 2013, http://dx.doi.org/10.1130/GES00819.1
Recognition that erosion of actively growing mountain ranges plays a major role in controlling not only topography but also crustal scale deformation patterns has driven researchers to learn more about the various impacts of erosion on tectonically active regions. Currently, the impacts of locally enhanced erosion resulting from the presence of powerful rivers or glaciers on patterns of deformation within mountain ranges are better understood and documented than the impacts of how relatively erodible and/or deformable different types of rocks within a single erosional setting are. This work by T. Wilcox and colleagues compares different types of physical observations and numerical model results to explore how different rock types which erode and/or deform at different rates within a single environmental setting may affect the long-term patterns of crustal deformation in central Taiwan.
Paleoseismic history of the Fallen Leaf segment of the West Tahoe–Dollar Point fault reconstructed from slide deposits in the Lake Tahoe Basin, California-Nevada
Jillian M. Maloney et al., Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Driver, La Jolla, California 92093, USA. Posted online 26 June 2013; http://dx.doi.org/10.1130/GES00877.1.
Jillian M. Mahoney and colleagues study the West Tahoe-Dollar Point fault (WTDPF), which extends along the western margin of the Lake Tahoe Basin (northern Sierra Nevada, western United States) and is characterized as its most hazardous fault. They write that "Fallen Leaf Lake, Cascade Lake, and Emerald Bay are three sub-basins of the Lake Tahoe Basin, located south of Lake Tahoe, and provide an opportunity to image primary earthquake deformation along the WTDPF and associated landslide deposits." In this paper, Mahoney and colleagues present results from high-resolution seismic CHIRP (compressed high intensity radar pulse) surveys in Fallen Leaf Lake and Cascade Lake, multibeam bathymetry coverage of Fallen Leaf Lake, onshore LiDAR (light detection and ranging) data for the southern Lake Tahoe Basin, and radiocarbon dates from piston cores in Fallen Leaf Lake and Emerald Bay. This improved mapping of the WTDPF reveals the fault geometry and architecture south of Lake Tahoe and improves the geohazard assessment of the region.