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Dynamic earth processes across time and space

GSA Bulletin articles posted online April 6-23

Geological Society of America

Boulder, Colo., USA - The dynamics of Earth are discussed in this new batch of GSA Bulletin papers posted online 6 April. Topics include the link between wildfire-flooding events and the supply of sand to beaches, with specific focus on a coastal California watershed; high-pressure metamorphism in the mountains of northwest China; generation of the Gold Hill shear zone and widespread tectonism in the Appalachian mountain belt; and the nature of magma-filled fractures (dikes) in the earth.

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The effects of wildfire on the sediment yield of a coastal California watershed

J.A. Warrick et al., U.S. Geological Survey, Santa Cruz, CA, USA. Posted online 6 April 2012; doi: 10.1130/B30451.1.

Wildfires are natural, recurring phenomena in the rural U.S. West. Intense wildfires will incinerate plant materials throughout a landscape, and this removal of vegetation often increases soil erosion during the first few years after a wildfire. New results presented by J.A. Warrick of the U.S. Geological Survey and colleagues suggest that the combination of a wildfire followed by an exceptionally wet winter can result in exceptional rates of soil erosion and sediment transport from rivers draining the burned landscape. Historically, these combinations of fire-and-flood occur with average recurrence intervals of about 1,000 years. However, Warrick and colleagues show that these rare fire-flood events are responsible for about half of the total sediment export from coastal watersheds, such as those studied along the central coast of California. These infrequent, yet exceptional, fire-flood events are likely important drivers of the coastal sediment budgets, including the supply of sand to beaches and the export of carbon to the sea.

Trace element transport during subduction-zone ultrahigh pressure metamorphism: Evidence from Western Tianshan, China

Yuanyuan Xiao et al., Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK. Posted online 6 April 2012; doi: 10.1130/B30523.1.

This study by Yuanyuan Xiao and colleagues presents new perspectives on the effects of the roles of subduction-zone metamorphism on arc magmatism and mantle compositional heterogeneity. A petrographic and bulk-rock geochemical study on blueschists and eclogites from the Western Tianshan ultrahigh pressure metamorphic belt, northwest China, has revealed the varying elemental mobility/immobility during subduction-zone metamorphism. The apparent immobility of elements such as uranium and light rare-earth elements (LREEs) during dehydration metamorphism suggests that the elevated abundances of these elements in arc magmas may have different origins from subduction-zone aqueous fluid enrichment as widely assumed. In addition, the authors note that the lack of rubidium/strontium (Rb/Sr)-Samarium/Neodynium (Sm/Nd) correlation in these and other similar metamorphosed rocks worldwide (i.e., rocks representing subducted residual ocean crust) is inconsistent with the observed first-order Sr-Nd isotope correlation in oceanic basalts. Hence, Xiao and colleagues conclude that the residual ocean crust that has undergone subduction-zone metamorphism cannot be the major source material for ocean island basalts as widely believed, although it can contribute to mantle compositional heterogeneity in general.

Kinematics, U-Pb geochronology, and 40Ar/39Ar thermochronology of the Gold Hill shear zone, North Carolina: The Cherokee orogeny in Carolinia, southern Appalachians

James P. Hibbard et al., Dept. Marine, Earth, and Atmospheric Sciences, Jordan Hall, North Carolina State University, Raleigh, North Carolina, 27695, USA. Posted online 6 April 2012; doi: 10.1130/B30579.1.

The Gold Hill shear zone is one of the most prominent bedrock structures in central North Carolina. On the basis of indirect evidence, it has been commonly perceived to be a Devonian, dextral strike-slip fault. Recent structural and geochronological studies focused on the shear zone by James P. Hibbard of North Carolina State University and colleagues indicate that it is a complex structure in both time and space. In particular, Hibbard and colleagues demonstrate that in central North Carolina it is mainly a thrust fault with a subordinate component of sinistral strike-slip; the timing of motion on the fault was mainly Late Ordovician, although it was reactivated in both the Devonian and Carboniferous. They conclude that the generation of the Gold Hill shear zone was coeval with widespread tectonism in the Appalachian mountain belt and it is considered to represent a manifestation of the southern Appalachian Cherokee orogeny, an event that marked the accretion of Gondwanan crustal blocks to North America.

The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

Katherine A. Daniels et al., School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK. Posted online 23 April 2012; doi: 10.1130/B30537.1.

Fractures in rock provide pathways for magma through the earth from great depths to the surface, leading to volcanic eruptions. However, not all of these magma-filled fractures, called dikes, reach the surface; most of them stall and solidify in Earth's crust. New findings presented by Katherine Daniels and colleagues help explain how Earth's crust deforms during volcanic eruptions. By measuring the shape and dimensions of dikes, estimates of the magma pressure during dike emplacement can be inferred from the theory of how solid rocks bend under pressure. Daniels and colleagues document the shapes of solidified dikes that are well exposed in eroded volcanoes from Scotland and South Africa and compare these field measurements with the dike shape predicted by theory. They find that the theory does not explain the dike shapes well and predicts pressures that are much larger than independent estimates based on the strength of rocks. Daniels and colleagues explain the disagreement with theory by taking into account (1) how rock fractures and (2) the solidification of liquid magma at the dike's edges. Solidified magma prevents dikes from closing as magma pressure declines. This effect can make it easier for magma to reach Earth's surface and erupt.

Provenance of the lower Ocoee Supergroup, eastern Great Smoky Mountains

Suvankar Chakraborty et al., University of Kentucky, Department of Earth and Environmental Sciences, Lexington KY 40506-0053, USA. Posted online 23 April 2012; doi: 10.1130.B30578.1.

The Great Smoky Mountains of western North Carolina and eastern Tennessee, which include some of the highest peaks in the eastern North America, are underlain by thick sequences of erosionally resistant sandstones and siltstones. Analysis by Suvankar Chakraborty and colleagues of the mineral and rock compositions and ages of detrital zircon grains deposited along with the quartz and feldspar framework grains indicate that the sediments were derived from local 1-1.3-billion-year-old basement sources at a time (0.65 billion years ago) when the eastern margin of the North American continent was undergoing rifting and breakup of the supercontinent Rodinia. The sediments were deposited in a restricted rift basin inboard of the continental margin.

Analogue modeling of lithospheric scale orocline buckling: Constraints on the evolution of the Ibero Armorican Arc

Daniel Pastor-Galán et al., Departamento de Geología. Universidad de Salamanca. Facultad de Ciencias, 37008, Salamanca, Spain. Posted online 23 April 2012; doi: 10.1130/B30640.1.

Daniel Pastor-Galán and colleagues report on a series of analogue modeling experiments that study the oroclinal buckling process as thick-skinned, involving the entire lithosphere. Results of the modeling suggest that during oroclinal buckling, extension in the outer arc and significant shortening in the inner arc are produced by tangential longitudinal strain as the main mechanism of deformation. The models also reveal that the mantle lithosphere thickens in different non-cylindrical ways depending on the initial lithospheric mantle thickness: from almost recumbent to folding with subvertical axial planes for the thinnest to the thickest mantle lithosphere, respectively. The results presented by Pastor-Galán and colleagues provide useful insights into thick-skinned orocline buckling as it is interpreted to have happened in the Iberian-Armorican Arc.


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