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Widespread hillslope gullying on the southeastern Tibetan Plateau: Human or climate-change induced?
Jon D. Pelletier et al., Dept. of Geosciences, The University of Arizona, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA; doi: 10.1130/B30266.1.
Landscapes on the southern Tibetan Plateau have been intensively gullied in the recent geologic past. Jon D. Pelletier of the University of Arizona and colleagues date the onset of this gullying and explore alternative explanations for it, including climate change and overgrazing. They conclude that increases in the population of the plateau several thousand years ago led to widespread devegatation, which pushed the landscape past a hydrologic and geomorphic tipping point that led to the dramatic gullying they observed.
Paleovalley morphology and fluvial system evolution of Eocene-Oligocene sediments ("auriferous gravels"), northern Sierra Nevada, California: Implications for climate, tectonics, and topography
Elizabeth J. Cassel, Dept. of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA; and Stephan A. Graham; doi: 10.1130/B30356.1.
The Sierra Nevada (California, United States) has been studied since Eocene (55-33 million years ago) river deposits were hydraulically mined for gold in the 19th century, but questions remain regarding the development and elevation history of the range. Elizabeth J. Cassel of Franklin and Marshall College and Stephen A. Graham of Stanford University present detailed descriptions and interpretations of paleovalley surfaces and Eocene river deposits to reconstruct the evolution of the river system and propose possible climatic and tectonic controls on that evolution. River deposits range from coarse gravel conglomerate to clay and soil horizons, and are interpreted as braided river deposits influenced by both internal and external drivers, including migration of the main area of deposition over time; the warm Eocene climate, which contributed to increased chemical weathering and high rates of sediment supply; and changes in base level due to sea-level fluctuations and Laramide plate tectonics. River terrace mapping and grain size analysis provide estimates of moderate local slopes, consistent with little to no recent surface uplift. The diachronous, localized nature of paleovalley incision and deposition invalidates some previous estimates of the timing and amount of range uplift based on reconstructed regional channel gradients.
Late Ordovician (Chatfieldian) catastrophic volcanism and abrupt carbonate platform-interior subsidence: A tectonic link across a Taconian foreland basin (Quebec Embayment) due to inherited crustal weakness
George R. Dix and M.J. Al-Dulami, Ottawa-Carleton Geoscience Centre and Dept. of Earth Sciences, 2160 Herzberg Laboratories, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada; doi: 10.1130/B30371.1.
Dix and Al-Dulami document an example of far-reaching tectonic influence of plate-boundary collision on distant continental interior sedimentation in eastern North America during the Late Ordovician, ~440 million years ago. Using regional changes in thickness of sedimentary units, the distribution of shale-limestone interbeds, and a regional volcanic ash bed as a time marker, Dix and Al-Dulami show how abrupt and differential subsidence along the interior margin of a Late Ordovician foreland basin in eastern North America was contemporaneous with catastrophic volcanism that appears to have reworked the continental margin of ancient North America. The history of abrupt subsidence is unexpected this far removed from the continental margin. It may represent the inherited predisposition of an underlying, shallowly buried fault system of Late Precambrian age to short-term changes in regional stress driven by plate-boundary collision along ancient eastern North America.
Rock avalanches and the pace of late Quaternary development of river valleys in the Karakoram Himalaya
Kenneth Hewitt et al. (John J. Clague, corresponding), Dept. of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada; doi: 10.1130/B30341.1.
New terrestrial cosmogenic nuclide (TCN) ages on deposits of large landslides that blocked upper Indus streams in the Karakoram Himalaya range from 3000 to 8000 years. The dated deposits are key to understanding late Quaternary events that have shaped the Himalayan landscape. Each landslide dammed the Indus or a major tributary and controlled base level and sedimentation for millennia. Until the 1990s, most of the landslides were interpreted as moraines; related lacustrine and other sediments continue to be attributed to glacial damming, and stream terraces to tectonic processes. The sediments and landforms that Kenneth Hewitt of Simon Fraser University and colleagues describe here are thought to have originated tens of thousands to hundreds of thousands of years earlier than their TCN ages require. Instead, they argue that TCN ages record an interplays during the Holocene of geomorphic processes in landslide-fragmented valleys. The landslides have buffered climatic and tectonic events, rather than being indicators of such. Millennia-long episodes of zero net bedrock incision at each site are surprising, but rates of sedimentation above landslide barriers and erosion controlled by their breaching are close to today's measured rates. Hewitt and colleagues propose that landslide-fragmented rivers may, in fact, characterize future upper Indus landscape evolution on long timescales.
Kinematic evolution of the Patagonian retroarc fold-and-thrust belt and Magallanes foreland basin, Chile and Argentina, 51°30'S
Julie C. Fosdick et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA; doi: 10.1130/B30242.1.
Julie C. Fosdick of Stanford University and colleagues present a regional kinematic history of foreland crustal shortening across the Patagonian Andes in southernmost South America, based on geologic mapping, subsurface stratigraphic interpretations, and zircon uranium-lead geochronology. Fosdick et al.'s work builds upon the existing retroarc foreland basin stratigraphic record and integrates the Patagonian fold-thrust belt deformational history with concurrent Magallanes Basin evolution. The Patagonian-Magallanes retroforeland system is perhaps the best type-example of an end-member foreland basin developed atop quasi-oceanic and transitional crust. They speculate that Neogene foreland shortening was accommodated by reactivation of Mesozoic normal faults zones and accounts for broad uplift of the Patagonian fold-thrust belt. Fosdick et al.'s results indicate approximately 32-40 km (approximately 19-23%) of crustal shortening. Orogenic shortening began 100 million years ago, progressed during six stages of deformation, and transitioned from thin-skinned deformation to basement-involved uplift in middle Miocene time. This work is of particular significance to a diverse group of geologists studying thrust belts, topographic development, and foreland sedimentation in retroforeland thrust belt settings.
Relating magma composition to eruption variability at andesitic volcanoes: A case study from Mount Taranaki, New Zealand
Michael B. Turner et al., Australian Research Council, National Key Centre for Geochemical Evolution and Metallogeny of Continents, Dept. of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia; doi: 10.1130/B30367.1.
Acquiring accurate eruption records and understanding the volcanic processes behind eruption periodicity are important to the development of realistic hazard assessments and volcanic emergency planning. Michael B. Turner of Macquarie University and colleagues use a detailed study of the Holocene (less than 10,000 years B.P.) eruption record of Mount Taranaki volcano to explore the link between magmatic processes and eruption frequency, helping to demystify these outwardly complex magma systems. Throughout the studied period, six compositionally distinct magma batches have been identified at this volcano. The batches erupted on 1500-2000-year timescales, which were synchronous with variations in eruptive frequency. The larger volume eruptions appear to be statistically predictable because they tend to occur just prior to a period of repose before the onset of a new magma batch. The fundamental properties of magma-volcano systems identified by Turner and colleagues provide a paradigm for constraining the timescales and nature of magmatic processes as well as a foundation for more robust probabilistic time-varying hazard forecasts.
Denudational response to surface uplift in east Tibet: Evidence from apatite fission-track thermochronology
Christopher J.L. Wilson and Andrew P. Fowler, School of Geosciences, Monash University, Clayton, Victoria 3800, Australia; doi: 10.1130/B30331.1.
Wilson and Fowler's apatite fission-track results from the eastern Tibetan Plateau indicate Cretaceous cooling (~90 million years ago) and low denudation rates where elevations exceed 3000 m. Some localized areas have experienced the effects of structurally enhanced rock uplift that was accompanied by rapid denudation from the Miocene to Recent. Significant Cenozoic denudation occurs in river valleys where the incision was initiated in the Oligocene to Miocene (28-12 million years ago). Estimated mean denudation in east Tibet during the Cenozoic is ~1-2 km from the low relief/high elevation plateau interior and at least 5 km at the high relief plateau margin in the Longmen Shan.
Metamorphic history of the central Himalaya, Annapurna region, Nepal, and implications for tectonic models
S.L. Corrie and M.J. Kohn, Dept. of Geosciences, Boise State University, Boise, Idaho 83725, USA; doi: 10.1130/B30376.1.
The timing and pressure-temperature conditions of metamorphism have been determined in the Annapurna region of central Nepal to place new constraints on the evolution of the core of the Himalayan mountain belt. Calculated rates of movement along the major fault surfaces during the past ~25 million years are compatible with estimates from elsewhere in the central Himalaya and with modern measurements of total Himalayan convergence (2 cm per year). S.L. Corrie and M.J. Kohn of Boise State University show that although the timing and duration of movement on discrete thrust surfaces may differ, the broad geodynamics in one part of a mountain belt can be realistically extrapolated to another part within a few hundred kilometers. Corrie and Kuhn explore the implications of this result in the context of tectonic models of thrust belt development.
Journal
Geological Society of America Bulletin