GSA Bulletin posts new studies from China, Egypt and Israel, Argentina, Mexico, California, Appalachia

November 25, 2013

Boulder, Colo., USA - GSA Bulletin articles posted online ahead of print in November cover sedimentology in the Sinai-Negev erg of Egypt and Israel; petrology in the Tongling area of Anhui Province in eastern China; paleotopography in the Central Andes of Argentina; sedimentology of the Monterey Submarine Canyon, offshore California, USA; geochronology of Volcán Tepetiltic, western Mexico; and thermochronology of the Appalachian Mountains.

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Particle-size fractionation of eolian sand along the Sinai-Negev erg of Egypt and Israel
Joel Roskin et al., Department of Geography and Environmental Development, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel, Published online ahead of print on 22 Nov. 2013;

This study analyzes changes in the particle-size of (eolian) sand along the west - east transport system of the Nile Delta -- northern Sinai Peninsula -- northwestern Negev erg (sand sea) of Egypt and Israel during the late Pleistocene. We infer that the sand grain size gradually fines down the transport path ("fractionation"), and this has to do with a past and current climate gradient that is also expressed in the different geomorphologies of linear dunes.

Petrogenesis of high-K, calc-alkaline and shoshonitic intrusive rocks in the Tongling area, Anhui Province (eastern China), and their tectonic implications
Cailai Wu et al., State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China, Published online ahead of print on 22 Nov. 2013;

This paper describes the petrological features of the Mesozoic intermediate-acid intrusive rocks and enclaves in famous Tongling metallogenic district of eastern China, which has been the copper capital of the country for many years. The paper discusses the ages of the intrusive rocks, their compositions, and their relationship to the metallogenesis. The relationship between the magmatic and tectonic processes is also discussed. We suggest a connection between the magmatic activity and transtensional deformation on the famous Tan-Lu fault of eastern China.

The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina
Barbara Carrapa et al., Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA, Published online ahead of print on 22 Nov. 2013;

High elevation mountain belts such as the Andes in South America are the result of a combination of tectonic and erosional processes. With this study we show that the history of deformation and exhumation preceding Andean contractional tectonics in the Cenozoic strongly controls the magnitude and location of exhumation. Areas that were already elevated during the Cretaceous, as a result of rift flank uplift, were not as exhumed as areas that were instead depositional centers in the Cretaceous and later in the Cenozoic. Low temperature thermochronology shows old Cretaceous ages indicative of limited exhumation of paleo-rift flanks and younger Cenozoic ages indicative of higher exhumation for areas in a sedimentary basin position. This study shows that exhumation is strongly affected by the tectonic history of an area and low temperature thermochronology is a good fingerprint of paleotopography.

The timing of sediment transport down Monterey Submarine Canyon, offshore California
T. Stevens et al., Centre for Quaternary Research, Department of Geography, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK, Published online ahead of print on 22 Nov. 2013;

The transport of eroded sediment from land to sea is a fundamental component of the rock cycle. Submarine canyons act as conduits for sand transport to the deep sea in mass movement events known as turbidity currents, trigged by storms, earthquakes or sediment loading. However, the timing and range of these events is poorly understood. We use Remotely Operated Vehicles equipped with coring equipment to sample and date sediment to 4-km depths in the Monterey Canyon, offshore California; a system comparable in size to the Grand Canyon. We date the timing of the entry of sand into the canyon through the first application of optically stimulated luminescence dating to canyon sediments. The technique measures when sediments were last exposed to light at the canyon head and tells us how fast sediment moves through the canyon. Our results demonstrate that sand moves in multiple flows before emerging out of the canyon. The canyon temporarily stores sediment with the deeper canyon experiencing less frequent events. In water depths of less than 1 km mass transport events occur over years and decades. Further down the canyon events occur with a 150- to 250-year recurrence frequency. Sediment transport down the canyon is therefore remarkably active.

40Ar/39Ar geochronology of Volcán Tepetiltic, western Mexico: Implications for the origin of zoned rhyodacite-rhyolite liquid erupted explosively from an andesite stratovolcano after a prolonged hiatus
Holli M. Frey et al., Department of Geology, Union College, Schenectady, New York 12308, USA, Published online ahead of print on 6 Nov. 2013;

An outstanding question in the study of intermediate (andesite-dacite) stratovolcanoes at subduction zones is why some evolve to erupt rhyolite and/or rhyodacite, whereas others do not. If rhyolitic melt is derived from crystal-rich andesite-dacite magma in a sub-volcanic chamber, why do segregation and eruption of the evolved liquid occur in some cases but not others? In the western Mexican Volcanic Belt, Volcán Tepetiltic, underwent caldera collapse during explosive eruption of zoned rhyodacite-rhyolite. 40Ar/39Ar geochronology was used to determine the eruptive history of Volcán Tepetiltic. The main edifice was largely constructed between 560 and 450 thousand years ago (ka), during which time ~42 cubic kilometers of phenocryst-rich andesite-dacite lavas were erupted. After a hiatus of ~180,000 years, there was a climactic Plinian eruption of ~4-8 cubic kilometers of zoned magma rhyodacite-rhyolite with crystal-poor pumice. The age of the climactic eruption was bracketed to be ~236 plus or minus 52 ka by peripherally erupted basaltic andesite flows. Given the long hiatus (~180,000 years) between the cone-building episode and the explosive eruption of rhyodacite-rhyolite, it is proposed that the influx of voluminous basaltic andesite into the upper crust drove partial melting of the sub-solidus magma chamber beneath Volcán Tepetiltic.

Decay of an old orogen: Inferences about Appalachian landscape evolution from low-temperature thermochronology
Ryan E. McKeon et al., Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA, Published online ahead of print on 6 Nov. 2013;

The modern Appalachian Mountains of eastern North America have long been described as the slowly eroding roots of a once great mountain range, built hundreds of millions of years ago during the formation and breakup of the supercontinent Pangaea. Models predict that mountain ranges should decay away quickly when the tectonic driving forces that built them cease, which makes the persistence of rugged topography and comparatively lofty summits of the Blue Ridge Mountains of western North Carolina difficult to explain in the context of an old and long-decaying mountain range. By analyzing the low-temperature thermal history of rocks in the central and southern Appalachians, we corroborate pervious work that suggests the long-term rate of erosion and landscape evolution has been slow. However, detailed investigation and modeling of two samples from the Blue Ridge Mountains indicate that river valleys eroded more quickly than ridge tops for tens of millions of years during the Cretaceous, generating more rugged topography long after tectonically driven mountain building ended. While the cause of renewed mountain building remains to be determined, it appears that parts of the Appalachians are not as old as we thought.

Geological Society of America

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