321 research outputs found
RIDGE and InterRidge: Cooperative interdisciplynary studies of mid-ocean ridges
RIDGE (gidge mer-Oisciplinary Boba1 bperiments) is a major research initiative in the United States designed to complement existing research on spreading centers. RIDGE has been designed to integrate exploration, expenment and theory into a major, decade-long effort to understand one of the pnmary processes that have shaped the evolution of our planet. Its objectives are twofold: (1) to provide a focus for a coordinated, interdisciplinary research program directed at geological, hydrothermal and biological processes associated with the formation of oceanic cmst, and (2) to provide a framework within which a rich diversity of investigator-initiated research can be undertaken. The RIDGE initiative has led to parallel efforts in other nations which have recently joined together to form InterRidge, an intemational collaboration of some sixteen countries focused on ridge crest studies
Seismic structure of Iceland from Rayleigh wave inversions and geodynamic implications
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 241 (2006): 901-912, doi:10.1016/j.epsl.2005.10.031.We have constrained the shear-wave structure of crust and upper mantle beneath
Iceland by analyzing fundamental mode Rayleigh waves recorded at the ICEMELT and
HOTSPOT seismic stations in Iceland. The crust varies in thickness from 20 to 28 km in
western and northern Iceland and from 26 to 34 km in eastern Iceland. The thickest crust
of 34-40 km lies in central Iceland, roughly 100 km west to the current location of the
Iceland hotspot. The crust at the hotspot is ~32 km thick and is underlain by low shearwave
velocities of 4.0-4.1 km/s in the uppermost mantle, indicating that the Moho at the
hotspot is probably a weak discontinuity. This low velocity anomaly beneath the hotspot
could be associated with partial melting and hot temperature. The lithosphere in Iceland
is confined above 60 km and a low velocity zone (LVZ) is imaged at depths of 60 to 120
km. Shear wave velocity in the LVZ is up to 10% lower than a global reference model,
indicating the influence of the Mid-Atlantic Ridge and the hotspot in Iceland. The lowest
velocities in the LVZ are found beneath the rift zones, suggesting that plume material is
channeled along the Mid-Atlantic Ridge. At depths of 100 to 200 km, low velocity
anomalies appear at the Tjornes fracture zone to the north of Iceland and beneath the
western volcanic zone in southwestern Iceland. Interestingly, a relatively fast anomaly is
imaged beneath the hotspot with its center at ~135 km depth, which could be due to
radial anisotropy associated with the strong upwelling within the plume stem or an Mgenriched
mantle residual caused by the extensive extraction of melts.This work is
supported by University of Houston, Woods Hole Oceanographic Institution, and NSF
grant OCE-0117938
A seismic refraction experiment in the central Banda Sea
Also published as: Journal of
Geophysical Research 83 (1978): 2247-2257A seismic refraction experiment in the central Banda Sea is interpreted by using both slope intercept
and delay time function methods. The crustal structure is shown to be oceanic, with velocities (4.97, 6.47,
7.18, and 7.97 km/s) typical of oceanic layers 2, 3A, and 3B and the mantle. Individual layer thicknesses
va ry systematica lly along the line, though the range of thicknesses observed for layers 2 ( 1.5-2.0 km) and
3A (2.0-3.5 km) falls well within the range observed for normal oceanic crust. Layer 3B is unusually thick
(2.5-4.6 km), the result being slightl y greater than normal depths-to Moho of9-IO km below the sea floor.
Shear head waves from layers 3A and 3B are identified on two receivers. In both cases, shear wave
conversion occurred at the sediment/layer 2 interface. The observed shear wave velocities and intercepts
indicate a Poisson's ratio of 0.25-0.28 in layer 3 and ~0.33 in layer 2. These and earlier results from the
southern Banda basin indicate that the entire Banda Sea is underlain by oceanic type crust.Prepared for the Office of Naval Research
under Contract N00014-74-C-0262; NR 083-004
and for the International Decade of Ocean
Exploration of the National Science Foundation
under Grant OCE 75-19150
The crustal structure and subsidence history of aseismic ridges and mid-plate island chains
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1978This thesis consists of three papers examining problems related to the crustal structure, isostasy and subsidence history of aseismic ridges and mid-plate island chains. Analysis of gravity and bathymetry data across the Ninetyeast and eastern Walvis Ridges indicates these features are locally compensated by an over thickening of the oceanic crust. Maximum crustal thicknesses are 15-30 km. The western Walvis Ridge is also compensated by crustal thickening; however, the isostasy of this part of the ridge is best explained by a plate model of compensation with elastic plate thicknesses of 5-8 km. These results are consistent with the formation of the Ninetyeast and Walvis Ridges near spreading centers on young lithosphere with flexural rigidities at least an order of magnitude less than those typically determined from flexural studies in older parts of the ocean basins. As the lithosphere cools and thickens, its rigidity increases, explaining the differences in isostasy between aseismic ridges and mid-plate island chains. The long-term subsidence of aseismic ridges and island/ seamount chains can also be explained entirely by lithospheric cooling. Aseismic ridges form near ridge crests and subside at nearly the same rate as normal oceanic crust Mid-plate island chains subside at slower rates because they are built on older crust. However, some island chains have subsided faster than expected based on the age of the surrounding sea floor, probably because of lithospheric thinning over midplate hot spots, like Hawaii. This lithospheric thinning model has major implications both for lithospheric and mantle convection studies as well as the origin of continental rift systems
Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N) : evidence for focused melt supply and lower crustal dike injection
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B09101, doi:10.1029/2004JB003473.We gathered seismic refraction and wide-angle reflection data from several active source experiments that occurred along the Mid-Atlantic Ridge near 35°N and constructed three-dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness. The tomographic images reveal anomalously thick crust (8â9 km) and a low-velocity âbull's-eyeâ, from 4 to 10 km depth, beneath the center of the ridge segment. The velocity anomaly is indicative of high temperatures and a small amount of melt (up to 5%) and likely represents the current magma plumbing system for melts ascending from the mantle. In addition, at the segment center, seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction. Our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust. Our results also indicate that the segment ends are colder, receive a reduced magma supply, and undergo significantly greater tectonic stretching than the segment center.This research was
supported by U.S. National Science Foundation grants OCE-0203228 and
OCE-0136793; support for V. Lekic was provided by the IRIS undergraduate
internship program
Lessons learned from 104 years of mobile observatories [poster]
Poster session IN13B-1211 presented 10 December 2007 at the AGU Fall Meeting, 10â14 December 2007, San Francisco, CA, USAAs the oceanographic community ventures into a new era of integrated observatories, it may be helpful to look back on the era of "mobile observatories" to see what Cyberinfrastructure lessons might be learned. For example, SIO has been operating research vessels for 104 years, supporting a wide range of disciplines: marine geology and geophysics, physical oceanography, geochemistry, biology, seismology, ecology, fisheries, and acoustics. In the last 6 years progress has been made with diverse data types, formats and media, resulting in a fully-searchable online SIOExplorer Digital Library of more than 800 cruises (http://SIOExplorer.ucsd.edu). Public access to SIOExplorer is considerable, with 795,351 files (206 GB) downloaded last year. During the last 3 years the efforts have been extended to WHOI, with a "Multi-Institution Testbed for Scalable Digital Archiving" funded by the Library of Congress and NSF (IIS 0455998). The project has created a prototype digital library of data from both institutions, including cruises, Alvin submersible dives, and ROVs. In the process, the team encountered technical and cultural issues that will be facing the observatory community in the near future. Technological Lessons Learned: Shipboard data from multiple institutions are extraordinarily diverse, and provide a good training ground for observatories. Data are gathered from a wide range of authorities, laboratories, servers and media, with little documentation. Conflicting versions exist, generated by alternative processes. Domain- and institution-specific issues were addressed during initial staging. Data files were categorized and metadata harvested with automated procedures. With our second-generation approach to staging, we achieve higher levels of automation with greater use of controlled vocabularies. Database and XML- based procedures deal with the diversity of raw metadata values and map them to agreed-upon standard values, in collaboration with the Marine Metadata Interoperability (MMI) community. All objects are tagged with an expert level, thus serving an educational audience, as well as research users. After staging, publication into the digital library is completely automated. The technical challenges have been largely overcome, thanks to a scalable, federated digital library architecture from the San Diego Supercomputer Center, implemented at SIO, WHOI and other sites. The metadata design is flexible, supporting modular blocks of metadata tailored to the needs of instruments, samples, documents, derived products, cruises or dives, as appropriate. Controlled metadata vocabularies, with content and definitions negotiated by all parties, are critical. Metadata may be mapped to required external standards and formats, as needed. Cultural Lessons Learned: The cultural challenges have been more formidable than expected. They became most apparent during attempts to categorize and stage digital data objects across two institutions, each with their own naming conventions and practices, generally undocumented, and evolving across decades. Whether the questions concerned data ownership, collection techniques, data diversity or institutional practices, the solution involved a joint discussion with scientists, data managers, technicians and archivists, working together. Because metadata discussions go on endlessly, significant benefit comes from dictionaries with definitions of all community-authorized metadata values.Funding provided by the Library of Congress and NSF (IIS 0455998
Upper crustal structure and axial topography at intermediate spreading ridges : seismic constraints from the southern Juan de Fuca Ridge
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B12104, doi:10.1029/2005JB003630.We use multichannel seismic reflection data to image the upper crustal structure of 0-620
ka crust along the southern Juan de Fuca Ridge (JdFR). The study area comprises two
segments spreading at intermediate rate with an axial high morphology with narrow
(Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both
segments we image the top of an axial magma chamber (AMC). The AMC along Cleft
deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and
hydrothermal vents near the southern end of the segment, to 2.3 km at the northern end
near the site of the 1980âs eruptive event. Along the Vance segment, the AMC also
deepens from south to north, from 2.4 km to 2.7 km. Seismic layer 2A, interpreted as the
basaltic extrusive layer, is 250-300 m thick at the ridge axis along the Cleft segment, and
300-350 m thick along the axis of the Vance segment. However off-axis layer 2A is
similar in both segments (500-600 m), indicating ~90% and ~60% off-axis thickening at
the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the
walls of the ASG, with the remaining thickening occurring within 3-4 km of the ASG.
Along the full length of both segments, layer 2A is thinner within the ASG, compared to
the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by
alternating periods of magmatism and tectonic extension. Our observations agree with
the evolving nature of the ASG. However, we suggest that its evolution is related to large
changes in axial morphology produced by small fluctuations in magma supply. Thus the
ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The
changes in axial morphology affect the distribution of lava flows along the ridge flanks,
as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may
occur at all spreading rates, but its effects on crustal structure and axial morphology are
most pronounced along intermediate spreading rate ridges.This study was supported by the National Science Foundation grants OCE-0002551 to
Woods Hole Oceanographic Institution, OCE-0002488 to Lamont-Doherty Earth
Observatory, and OCE-0002600 to Scripps Institution of Oceanography
Constructing the crust along the Galapagos Spreading Center 91.3°â95.5°W : correlation of seismic layer 2A with axial magma lens and topographic characteristics
Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B10310, doi:10.1029/2004JB003066.Multichannel seismic reflection data are used to infer crustal accretion processes along the intermediate spreading Galapagos Spreading Center. East of 92.5°W, we image a magma lens beneath the ridge axis that is relatively shallow (1.0â2.5 km below the seafloor) and narrow (âŒ0.5â1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24â0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7°W, the magma lens is deeper (2.5â4.5 km) and wider (0.7â2.4 km), and layer 2A is thicker (0.36â0.66 km) and thickens off axis by <40%. The positive correlation between layer 2A thickness and magma lens depth supports the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface. Our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis. Flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high, which is largest east of 92.5°W. West of âŒ94°W the âtransitionalâ axial morphology lacks a prominent bathymetric high and layer 2A no longer thickens off axis. We detect no magma lens west of 94.7°W where a small axial valley appears. The above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the Galapagos hot spot on the Galapagos Spreading Center.This project was funded by NSF-OCE-
0002189
Multiple expressions of plume-ridge interaction in the Galapagos : volcanic lineaments and ridge jumps
Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q05018, doi:10.1029/2012GC004093.Anomalous volcanism and tectonics between near-ridge mantle plumes and mid-ocean ridges provide important insights into the mechanics of plume-lithosphere interaction. We present new observations and analysis of multibeam, side scan sonar, sub-bottom chirp, and total magnetic field data collected during the R/V Melville FLAMINGO cruise (MV1007; MayâJune, 2010) to the Northern GalĂĄpagos Volcanic Province (NGVP), the region between the GalĂĄpagos Archipelago and the GalĂĄpagos Spreading Center (GSC) on the Nazca Plate, and to the region east of the GalĂĄpagos Transform Fault (GTF) on the Cocos Plate. The NGVP exhibits pervasive off-axis volcanism related to the nearby GalĂĄpagos hot spot, which has dominated the tectonic evolution of the region. Observations indicate that ~94% of the excess volcanism in our survey area occurs on the Nazca Plate in three volcanic lineaments. Identified faults in the NGVP are consistent with normal ridge spreading except for those within a ~60 km wide swath of transform-oblique faults centered on the GTF. These transform-oblique faults are sub-parallel to the elongation direction of larger lineament volcanoes, suggesting that lineament formation is influenced by the lithospheric stress field. We evaluate current models for lineament formation using existing and new observations as well as numerical models of mantle upwelling and melting. The data support a model where the lithospheric stress field controls the location of volcanism along the lineaments while several processes likely supply melt to these eruptions. Synthetic magnetic models and an inversion for crustal magnetization are used to determine the tectonic history of the study area. Results are consistent with creation of the GTF by two southward ridge jumps, part of a series of jumps that have maintained a plume-ridge separation distance of 145 km to 215 km since ~5 Ma.This work was
supported by NSF grant OCE-0926637 and OCE-1030904 to
DF and KH. DGâs work was supported by NSF grants EAR-
0838461 and EAR-1145271. Additional support was provided
to E.M. by the Deep Ocean Exploration Institute at the Woods
Hole Oceanographic Institution.2012-11-3
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