Boulder, Colo., USA Geosphere has posted additions to several themed issues: History and Impact of Sea-Level Change Offshore New Jersey; Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California; Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain-Yellowstone Region and Adjacent Areas; Origin and Evolution of the Sierra Nevada and Walker Lane. Two other articles cover terrestrial laser scanning and the earthquake hazard of the Hat Creek fault.
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Testing sequence stratigraphic models by drilling Miocene foresets on the New Jersey shallow shelf
Kenneth G. Miller et al., Dept. of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00884.1. Themed issue: Results of IODP Exp313: The History and Impact of Sea-Level Change Offshore New Jersey.
Kenneth Miller and colleagues present seismic, core, log, and chronologic data on three early to middle Miocene sequences sampled across a transect of seismic clinothems (prograding sigmoidal sequences) in topset, foreset, and bottomset locations beneath the New Jersey shallow continental shelf (Integrated Ocean Drilling Program Expedition 313, Sites M27-M29). They recognize stratal surfaces and systems tracts by integrating seismic stratigraphy, lithofacies successions, gamma logs, and foraminiferal paleodepth trends. Their interpretations of systems tracts, particularly in the foresets where the sequences are thickest, allow them to test sequence stratigraphic models. Miller and colleagues find little evidence for correlative conformities; even in the foresets, where sequences are thickest, there is evidence of erosion and hiatuses associated with sequence boundaries. They do not resolve the issue of fractal versus hierarchical order, but the data are consistent with arrangement into orders based on Milankovitch forcing on eccentricity.
Mantle lithosphere as a source of postsubduction magmatism, northern Sierra Nevada, California
G. Lang Farmer et al., Dept. of Geological Sciences and CIRES, University of Colorado, Boulder, Colorado 80309, USA Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00885.1. Themed issue: Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California.
Cenozoic volcanism in the southern Sierra Nevada, California, has been attributed to the loss of the deep Sierran continental lithosphere to the deep mantle, associated with an episode of "bottom-down" mountain building. But can the same process be responsible for late Cenozoic volcanism in the northern half of the mountain range, including volcanism in the Lake Tahoe region? G. Lang Farmer and colleagues show that detailed chemical and isotopic studies of the northern Sierran volcanic rocks do not share the same time-composition patterns as their southern Sierran counterparts. For example, no high potassium volcanic rocks have been identified in the Lake Tahoe region. A chemical transition through time does exist in the northern Sierran volcanic rocks, from normal continental arc compositions to high niobium post-orogenic volcanism, but this transition seems to mark the opening of a "slab less" window beneath the region related to the formation of the San Andreas transform fault. The opening of this slab less window in the Pliocene exposed intact deep continental lithosphere to upwelling hot mantle, resulting in the melting of deep continental mantle that had been metasomatized from below by arc-related magmas generated during the Miocene. None of the Late Cenozoic volcanic activity in the northern Sierra Nevada requires downwelling and removal of deep mantle lithosphere in this region.
Carbon-isotope stratigraphy from terrestrial organic matter through the Monterey event, Miocene, New Jersey margin (IODP Expedition 313)
Linhao Fang et al. (Stephen P. Hesselbo, corresponding author), Dept. of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00851.1. Themed issue: Results of IODP Exp313: The history and Impact of Sea-level Change Offshore New Jersey.
The stratigraphic utility of carbon-isotope values from terrestrial organic matter is explored for Miocene siliciclastic sediments of the shallow shelf, New Jersey margin, USA (Integrated Ocean Drilling Program [IODP] Expedition 313). These shallow marine strata, rich in terrestrial organic matter, provide a record of deposition equivalent to the Monterey event, a prolonged interval of time characterized by relatively positive carbon-isotope values recorded from foraminiferal carbonate in numerous oceanic settings. Coherent stratigraphic trends and short-term isotopic excursions are observed consistently in palynological preparation residues, concentrated woody phytoclasts, and individually picked woody phytoclasts obtained from the New Jersey sediments. A bulk organic matter curve shows somewhat different stratigraphic trends but, when corrected for mixing of marine-terrestrial components on the basis of measured C/N ratios, a high degree of conformity with the woody phytoclast record is observed. However, assuming that the correlations based on strontium-isotope values and biostratigraphy are correct, the carbon-isotope record from the New Jersey margin contrasts with that previously documented from oceanic settings (i.e., lack of positive excursion of carbon isotope values in terrestrial organic matter through the Langhian Stage). Factors that may potentially bias local terrestrial carbonisotope records include reworking from older deposits, degradation and diagenesis, as well as environmental factors affecting vegetation in the sediment source areas. These possible factors are assessed on the basis of pyrolysis data, scanning electron microscope observations, and comparison to palynological indices of environmental change. Some evidence is found for localized degradation and/or reworking of older woody phytoclasts, but where such processes have occurred they do not readily explain the observed carbon-isotope values. It is concluded that the overall carbon-isotope signature for the exchangeable carbon reservoir is distorted, to the extent that the Monterey event excursion is not easily identifiable. The most likely explanation is that phytoclast reworking has indeed occurred in clinoform toe-of-slope facies, but the reason for the resulting relatively heavy carbon-isotope values in the Burdigalian remains obscure.
Geochemical and paleomagnetic variations in basalts from the Wendell Regional Aquifer Systems Analysis (RASA) drill core: Evidence for magma recharge and assimilationfractional crystallization from the central Snake River Plain, Idaho
Marlon M. Jean et al., Dept. of Geology, Northern Illinois University, Davis Hall 312, DeKalb, Illinois 60115, USA. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00914.1. Themed issue: Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain-Yellowstone Region and Adjacent Areas.
This article highlights the stratigraphy, paleomagnetism, petrography, and geochemistry from lava flows recovered from a hydrologic test well near Wendell, Idaho, USA. Marlon M. Jean and colleagues first present core observations (i.e., stratigraphy of the hole, paleomagnetism, and petrography) and then detail the major element and trace element chemistry of the lavas. By integrating these results with depth, they demonstrate that crystal fractionation and magma recharge cycles occur within the magma chambers responsible for these lava flows. This research enhances our look into the volcanism of the Snake River Plain and is likely to be cited beyond the strict confines of research in the Snake River Plain with the growing interest in examining flood basalt flows in high-resolution elsewhere.
Synvolcanic crustal extension during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental, Mexico: Evidence from the Guazapares Mining District region, western Chihuahua
Bryan P. Murray et al., Dept. of Earth Science, University of California, Santa Barbara, Webb Hall, Santa Barbara, California 93106-9630, USA. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00862.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
This paper by Bryan P. Murray and colleagues presents new geologic mapping and geochronology from the previously unstudied Guazapares Mining District region of western Chihuahua, Mexico, located along the western edge of the northern Sierra Madre Occidental. The Sierra Madre Occidental of western Mexico is the largest and best-preserved silicic large igneous province of the Cenozoic and was active during the extensive mid-Cenozoic ignimbrite flare-up that affected much of the southwestern North American Cordillera from the Middle Eocene to Late Miocene. Silicic large igneous provinces may be characteristic of continental regions undergoing broad lithospheric extension; however, a large part of the Sierra Madre Occidental remains unmapped and undated, therefore the age relationships between ignimbrite flare-up volcanism and crustal extension remain unclear. Murray and colleagues show that extension preceded the onset of magmatism in the study area and demonstrate that extension was active in the study area during deposition of approx. 27.5-million-year-old outflow ignimbrites, presumably erupted from calderas of similar ages identified to the north and east by other workers. Extension continued during growth of an approx. 27 to 24.5 million-year-old andesitic volcanic center in the study area, followed by continued extension during 24.5 to 23 million-year-old silicic flare-up magmatism in the study area. This study shows how extensional structures controlled the location of andesitic and silicic volcanic vents and shallow-level intrusions. This study also shows that the onset of extension in the study area overlaps with the end of peak Oligocene silicic magmatism to the east, and that extension in the study area preceded and coincided with a second peak of silicic magmatism in the Miocene, which is represented in the study area. Finally, Murray and colleagues show that their data supports the interpretation that regional silicic flare-up magmatism and extension migrated southwestward with time.
Birth of a plate boundary at ca. 12 Ma in the Ancestral Cascades arc, Walker Lane belt of California and Nevada
Cathy J. Busby, Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00928.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
The Walker Lane belt of eastern California and western Nevada is the northernmost extension of the Gulf of California transtensional rift, where the process of continental rupture has not yet been completed, and rift initiation can be studied on land. GPS and earthquake focal mechanism studies demonstrate that the Walker Lane belt currently accommodates NW-SEdirected movement between the Sierra Nevada microplate and the North American plate, but the timing and nature of rift initiation remains unclear. In this paper, Cathy J. Busby presents a model for plate-margin-scale initiation of the Gulf of California and Walker Lane transtensional rifts approx. 12 million years ago. Busby notes that localization of rifting in both was initiated by thermal weakening in the axis of a subduction- related arc undergoing extension due to slab rollback, and thermal weakening in the arc was enhanced by stalling of the trenchward- migrating precursor arc against a thick Cretaceous batholithic lithospheric profile on its western margin. Rifting succeeded very quickly in the Gulf of California, due to stalling of Farallon slabs, but the Walker Lane transtensional rift has been unzipping northward along the axis of the Cascades arc, following the Mendocino triple junction.
Late Oligocene to middle Miocene rifting and synextensional magmatism in the southwestern Sierra Madre Occidental, Mexico: The beginning of the Gulf of California rift
Luca Ferrari et al., Centro de Geociencias, Universidad Nacional Autnoma de Mxico, Campus Juriquilla, 76230, Quertaro, Qro., Mexico. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00925.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
Although Basin and Range-style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended about 14 to 12.5 million years ago. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comond Group as subduction related, whereas volcanism after approx. 12.5 million years ago is extension related. Here Luca Ferrari and colleagues present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension.
The Hat Creek fault is one of the largest active normal faults in northeastern California; however, the seismic hazard of this fault has not been determined with great confidence in the past. Using a combination of field mapping of Late Pleistocene through Holocene earthquake ruptures and theoretical analysis, the active portion of the fault is determined to have a rupture length of at least 30 km and is capable of magnitude 6.7 earthquakes approximately every 667 years (with an uncertainty of plus or minus 167 years). There have been no historical surface-rupturing earthquake events in the past 200 years, implying a moderate near-term seismic hazard. In past events, the fault ruptured through three ages of Late Pleistocene basaltic lavas, producing a surface morphology characteristic of normal faults in analogous environments. Therefore, the seismic hazard analysis method described has general applicability to normal faults that rupture through young lava flows elsewhere.
On the estimation of geological surface roughness from terrestrial laser scanner point clouds
Graham Mills and Georgia Fotopoulos, Dept. of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada. Posted online 13 Sept. 2013; http://dx.doi.org/10.1130/GES00918.1.
In recent years, terrestrial laser scanning (TLS) has become an increasingly important tool in the geosciences, allowing for the development of accurate virtual models of natural surfaces from a distance of up to hundreds of meters. In particular, the quantitative study of surface roughness, useful in modeling natural processes at work on the surface of the Earth, has been greatly expanded by the convenience and accuracy of TLS measurements. In this paper, the effect of errors in TLS range measurements and the measurement procedure is investigated with respect to roughness calculations through physical and numerical simulations. Range errors were found to correlate with scan range, instrument parameters and real surface morphology, causing roughness to be underestimated by up to 20%.
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