GSA Bulletin highlights: New research posted June 24, 2011

June 29th, 2011
Locations of study highlighted in this new research include the central and Patagonian Andes, the Tibetan Plateau, Croatia, the Quebec Appalachians, the Blue Mountain Province of Oregon, the Rio Grande, Papua New Guinea, and Australia. Highlights are provided below. Abstracts for issues of GSA Bulletin are available at .

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Multiple slope failures associated with neotectonic activity in the Southern Central Andes (37 degrees-37 degrees 30 minutes S), Patagonia, Argentina

Ivanna M. Penna et al., Laboratorio de Tectonica Andina, Departamento de Geologia, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, pabellon II, 1427 Buenos Aires, Argentina; doi: 10.1130/B30399.1.

The transition area between the Central and Patagonian Andes presents an anomalous cluster of rockslides, developed in basaltic rocks with horizontal attitude. Ivanna M. Penna of the University of Buenos Aires and colleagues explore the relationship between these rockslides and Quaternary tectonic activity. They differentiated three groups of settings: rockslide failures lining up along folds are the largest rockslides with volumes up to 4 cubic kilometers; rockslides along active faults are one order smaller (up to 0.5 cubic kilometers); and rockslides not linked with tectonic structures are smallest with the largest 0.17 cubic kilometers in volume. This last group counts for only 2.5% of the entire rockslide volume in the area. This characteristic and the absence of large-scale sliding planes inclined toward the valleys led Penna and colleagues to propose that rock fracturing due to neotectonic activity is a major conditioning factor for failures, which is most effective along folds. In addition, although most rockslides postdate the Last Glacial Maximum by several thousand years, 75% of the rockslide volume occurs in sector of the valleys carved by glacial erosion. Only 25% occur in sectors of the valleys carved by fluvial erosion, indicating the additional contribution of glacial oversteepening in conditioning large rock-slope failures.

Late Quaternary right-lateral slip rates of faults adjacent to the Lake Qinghai, northeastern margin of the Tibetan Plateau

Dao-Yang Yuan et al., Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China; doi: 10.1130/B30315.1.

Dao-Yang Yuan of the China Earthquake Administration and colleagues determined the slip rates of two right-lateral strike-slip faults, the Elashan and Riyueshan faults, by combining the terrace riser offsets with terrace ages dated by 14C, optically stimulated luminescence, and 10Be techniques in the northeastern margin of the Tibetan Plateau. The slip rates of these two faults are about ~1 mm per year. Yuan and colleagues discuss active tectonics and tectonic deformation in this and an adjacent area. They provide new evidences about tectonic deformation in this area beginning near ~8-10 million years ago. The results are useful for understanding the process and dynamics of continental deformation, especially in this area.

Microbial laminite versus rooted or burrowed caps on peritidal cycles: Salinity control on parasequence development, Early Cretaceous isolated carbonate platform, Croatia

Antun Husinec and J. Fred Read, Dept. of Geology, St. Lawrence University, 23 Romoda Drive, Canton, New York 13617, USA; doi: 10.1130/B30305.1.

Meter-scale, carbonate cycles (parasequences) capped by microbial laminites are common on many ancient peritidal carbonate platforms that formed during greenhouse times. These cycles are less common on platforms that developed during icehouse and transitional times. Antun Husinec of St. Lawrence University and J. Fred Read, emeritus professor at Virginia Tech, provide data from the Early Cretaceous, paleotropical greenhouse isolated platform, Croatia, that show some parasequences lack the typical regressive tidal flat laminites. Instead, theses bioturbated emergent parasequences have burrowed and rooted upper parts, capped with thin greenish paleosols, which result from colonization of prograding coastal zones by plants under non-hypersaline conditions. Husinec and Read suggest that the presence or absence of laminites from the parasequences may be used to track gross salinity changes of coastal waters on the platform through time. They also suggest means to evaluate whether these bioturbated versus laminate caps relate to salinity increase into broad embayments, or to Milankovitch climate changes at the parasequence scale.

Hinterland-directed transtensional faulting at an orogen structural front: The example of the Cap-Chat melange, Quebec Appalachians

Nicolas Pinet et al., Geological Survey of Canada, 490 rue de la Couronne, Quebec, Quebec G1K 9A9, Canada; doi: 10.1130/B30309.1.

Melanges are complex geological assemblages that are usually mapped as chaotic units, without further study. Nicolas Pinet of the Geological Survey of Canada and colleagues present evidence that beyond the general complexity of melanges, geological observations may still contribute to unraveling their history. In the case of the Cap-Chat melange, located close to the western boundary of the Quebec Appalachians, most of its complexity results from the superimposition of several tectonic events.

Late Jurassic magmatism, metamorphism, and deformation in the Blue Mountains Province, northeast Oregon

Joshua J. Schwartz et al., Dept. of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487, USA; doi: 10.1130/B30327.1.

The Blue Mountains Province of northeast Oregon contains a remarkably well-preserved record of early to mid-Mesozoic tectonic activity including sedimentation, magmatism, and metamorphism. Joshua J. Schwartz of the University of Alabama and colleagues investigated Late Jurassic deformation in the province, which involved folding and faulting primarily along terrane boundaries (e.g., Wallowa-Baker and Baker-Izee-Olds Ferry boundaries) and within the composite Baker oceanic melange terrane (e.g., Bourne-Greenhorn subterrane boundary). They determined that the timing of contraction in the province occurred between 159 and 154 million years ago. Schwartz and colleagues suggest that the widespread, approximately N-S-directed contractional features in the province record a short-lived, intense deformational event, and preserve an example of upper-crustal strain localization associated with terminal arc-arc collision between the Olds Ferry and Wallowa island arc terranes. These brittle-to-semi-brittle deformation zones and contractional features may be analogous to the modern-day, Molucca Sea-type orogenic arc-arc collision model.

Stratigraphic, sedimentologic, and dendrogeomorphic analyses of rapid floodplain formation along the Rio Grande in Big Bend National Park, Texas

D.J. Dean et al., Dept. of Watershed Sciences, Utah State University, 5210 Old Main Hill, Logan, Utah 84322-5210, USA; doi: 10.1130/B30379.1.

The channel of the Rio Grande in the Big Bend region rapidly narrowed between 1991 and 2008 through the development of new floodplains on top of channel bars. D.J. Dean of Utah State University and colleagues excavated floodplain trenches, mapped and analyzed these deposits, and used a relatively new tree-ring dating method to reconstruct the timing and style of floodplain formation during this period. This dating method identifies changes in tree-ring anatomy that occurs following the burial of riparian plant stems to accurately date deposition to within one year. They show that narrowing began by the lateral and vertical deposition of sand, mud, and sometimes gravel on top of and against channel bars. As the channel contracted due to the lateral growth of these bars, narrowing slowed. However, deposition during moderate floods continued to cause vertical growth of these developing surfaces. Vertical growth due to suspended-sediment deposition eventually resulted in the formation of distinctive levee and trough floodplain topography. Vegetation establishment within the channel played an integral role in the conversion of bare, channel bars to floodplains, because it helped stabilize fine sediment deposits and trap sediment during floods.

Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea

Nathan R. Daczko et al., Geochemical Evolution and Metallogeny of Continents Australian Research Council (GEMOC ARC) National Key Centre, Dept. of Earth and Planetary Sciences, Macquarie University, NSW 2109, Sydney, Australia; doi: 10.1130/B30326.1.

Nathan R. Daczkoof Macquarie University, Australia, and colleagues gained an understanding of the processes that control continental rifting by examining an active plate tectonic setting in eastern Papua New Guinea, which is presently undergoing extension. They investigated this fundamental plate tectonic process that has played an important role in the development of many continental margins and is critical to understanding Earth evolution. They incorporate their examination of field relationships and laboratory analyses, and determine the structural evolution of a greater-than-2-km tall domed landform that took place in the past few million years.

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

P.A. Cawood et al., Dept. of Earth Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK; doi: 10.1130/B30415.1.

In classic models of mountain building, deformation and uplift are related to continental collision (e.g., Alpine-Himalayan chain). Such mountain belts form at the termination of oceanic subduction and occupy an internal location within an assembled continent (supercontinent). However, a significant number of mountain belts also develop at the edge of continents (e.g., Cordillera) and form within an environment of continuing subduction and accretion. The east Australian segment of the Terra Australis accretionary orogen developed along the eastern margin of Gondwana from the Neoproterozoic until the Mesozoic (830-230 million years ago). Geochronological and regional relations indicate mountain building involved alternating pulses of compression and extension that can be linked to magmatism, crustal thickening and basin subsidence from 300-230 million years ago and took place in an evolving plate boundary framework. This switching of regimes was accompanied by the termination of a Devonian-Carboniferous magmatic arc system and the establishment of a new (Permian-Triassic) outboard arc. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of supercontinent Pangea.

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