68 research outputs found
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 (8–9 km) and a low-velocity “bull's-eye”, 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, 10–14 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 1980’s 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
Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge
Author Posting. © The Author(s), 2009. 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 460 (2009): 89-93, doi:10.1038/nature08095.The oceanic crust extends over two thirds of the Earth’s solid surface and is
generated along mid-ocean ridges from melts derived from the upwelling mantle.
The upper and mid crust are constructed by dyking and seafloor eruptions
originating from magma accumulated in mid-crustal lenses at the spreading axis,
but the style of accretion of the lower oceanic crust is actively debated. Models
based on geological and petrological data from ophiolites propose that the lower
oceanic crust is accreted from melt sills intruded at multiple levels between the
Moho transition zone (MTZ) and the mid-crustal lens, consistent with
geophysical studies that suggest the presence of melt within the lower crust.
However, seismic images of molten sills within the lower crust have been elusive.
To date only seismic reflections from mid-crustal melt lenses and sills within
the MTZ have been described, suggesting that melt is efficiently transported
through the lower crust. Here we report deep crustal seismic reflections off the
southern Juan de Fuca Ridge that we interpret as originating from a molten sill
presently accreting the lower oceanic crust. The sill sits 5-6 km beneath the
seafloor and 850-900 m above the MTZ, and it is located 1.4-3.2 km off thespreading axis. Our results provide evidence for the existence of low permeability
barriers to melt migration within the lower section of modern oceanic crust
forming at intermediate-to-fast spreading rates, as inferred from ophiolite
studies.This research was supported by grants form the US NSF
Asymmetric shallow mantle structure beneath the Hawaiian Swell—evidence from Rayleigh waves recorded by the PLUME network
Author Posting. © The Author(s), 2011. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 187 (2011): 1725–1742, doi:10.1111/j.1365-246X.2011.05238.x.We present models of the 3-D shear velocity structure of the lithosphere and asthenosphere beneath the Hawaiian hotspot and surrounding region. The models are derived from long-period Rayleigh-wave phase velocities that were obtained from the analysis of seismic recordings collected during two year-long deployments for the Hawaiian Plume-Lithosphere Undersea Mantle Experiment. For this experiment, broad-band seismic sensors were deployed at nearly 70 seafloor sites as well as 10 sites on the Hawaiian Islands. Our seismic images result from a two-step inversion of path-averaged dispersion curves using the two-station method. The images reveal an asymmetry in shear velocity structure with respect to the island chain, most notably in the lower lithosphere at depths of 60 km and greater, and in the asthenosphere. An elongated, 100-km-wide and 300-km-long low-velocity anomaly reaches to depths of at least 140 km. At depths of 60 km and shallower, the lowest velocities are found near the northern end of the island of Hawaii. No major velocity anomalies are found to the south or southeast of Hawaii, at any depth. The low-velocity anomaly in the asthenosphere is consistent with an excess temperature of 200–250 °C and partial melt at the level of a few percent by volume, if we assume that compositional variations as a result of melt extraction play a minor role. We also image small-scale low-velocity anomalies within the lithosphere that may be associated with the volcanic fields surrounding the Hawaiian Islands.This research was financed by the National Science Foundation under
grants OCE-00-02470 and OCE-00-02819. Markee was partly
sponsored by a SIO graduate student fellowship
Frozen magma lenses below the oceanic crust
Author Posting. © The Authors, 2005. 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 436 (2005): 1149-1152, doi:10.1038/nature03944.The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust. Thermal modelling, tomography, compliance and wide-angle seismic studies, supported by geological evidence, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of approximately 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies
Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges
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