52 research outputs found
Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan
Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles
Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60â100âkm depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120â160âkm depth suggests that the slabâs mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics
Paving (through) Amazonia: Neoliberal Urbanism and the Reperipheralization of Roraima
This paper examines the neoliberal reshaping of infrastructure provision in Brazil's extreme north since the mid-1990s, when roadway investments resulted in unprecedented regional connectivity. The BR-174 upgrade, the era's most important project, marked a transition from resource-based developmentalism to free-market transnationalism. Primarily concerned with urban competitiveness, the federal government funded the trunk roadway's paving to facilitate manufacturing exports from Manaus. While an effort was made to minimize deforestation, planners sidelined development implications in adjacent Roraima. The state's urban system has thus experienced reperipheralization and intensified primacy. Market-led growth now compounds the inheritance of hierarchical centralism and ongoing governmental neglect. Our study shows a vast territory dependent on primate cities for basic goods and services. Travelling with Roraimans from bypassed towns, we detected long-distance passenger transportation and surface logistics with selective routes. Heterogeneous Roraiman (im)mobilities comprise middle-class tourism and heightened consumerism as well as informal mobility tactics and transnational circulations of precarious labor. The paper exhorts neoliberal urbanism research to look beyond both Euro America's metropoles and their Global South counterparts. Urbanization dynamics in Brazil's extreme north demonstrate that market-disciplined investments to globalize cities produce far-reaching spatial effects. These are felt even by functionally-articulated-yet-marginalized peripheries in ostensibly remote locations
Arc magmas sourced from melange diapirs in subduction zones
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 862-867, doi:10.1038/ngeo1634.At subduction zones, crustal material is recycled back into the mantle. A certain proportion, however, is returned to the overriding
plate via magmatism. The magmas show a characteristic range of compositions that have been explained by three-component
mixing in their source regions: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin
sedimentary veneer are added to the depleted peridotite in the mantle beneath the volcanoes. However, currently no uniformly
accepted model exists for the physical mechanism that mixes the three components and transports them from the slab to the
magma source.
Here we present an integrated physico-chemical model of subduction zones that emerges from a review of the combined findings
of petrology, modelling, geophysics, and geochemistry: Intensely mixed metamorphic rock formations, so-called mélanges, form
along the slab-mantle interface and comprise the characteristic trace-element patterns of subduction-zone magmatic rocks. We
consider mélange formation the physical mixing process that is responsible for the geochemical three-component pattern of the
magmas. Blobs of low-density mélange material, so-called diapirs, rise buoyantly from the surface of the subducting slab and
provide a means of transport for well-mixed materials into the mantle beneath the volcanoes, where they produce melt. Our model
provides a consistent framework for the interpretation of geophysical, petrological and geochemical data of subduction zones.H.M. was funded
by the J. LamarWorzel Assistant Scientist Fund and the
Penzance Endowed Fund in Support of Assistant Scientists.
Funding from NSF grant #1119403 (G. Harlow)
is acknowledged.2013-05-1
Coupled onshore erosion and offshore sediment loading as causes of lower crust flow on the margins of South China Sea
Non-volcanic seismic swarm and fluid transportation driven by subduction of the Philippine Sea slab beneath the Kii Peninsula, Japan
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Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone
Abstract:
Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and shortâterm arrays. Parallel teleseismic travelâtime measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to localâearthquake attenuation studies but at 10Ă lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subductionâdominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subductionârelated melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in highâfrequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and localâearthquake body waves. Differences between P and S attenuation suggest primarily shearâmodulus relaxation. Overall the attenuation measurements show consistent, coherent subductionârelated structure, complementary to travel times
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