Detrick Robert S.

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Robert S.

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  • Article
    Constructing the crust along the Galapagos Spreading Center 91.3°–95.5°W : correlation of seismic layer 2A with axial magma lens and topographic characteristics
    (American Geophysical Union, 2004-10-21) Blacic, Tanya M. ; Ito, Garrett T. ; Canales, J. Pablo ; Detrick, Robert S. ; Sinton, John M.
    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 (1.0–2.5 km below the seafloor) and narrow (∼0.5–1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24–0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7°W, the magma lens is deeper (2.5–4.5 km) and wider (0.7–2.4 km), and layer 2A is thicker (0.36–0.66 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 ∼94°W the “transitional” 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.
  • Presentation
    Lessons learned from 104 years of mobile observatories [poster]
    ( 2007-12-10) Miller, Stephen P. ; Neiswender, Caryn ; Clark, Dru ; Raymond, Lisa ; Rioux, Margaret A. ; Norton, Cathy N. ; Detrick, Robert S. ; Helly, John ; Sutton, Don ; Weatherford, John
    As 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 ( 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.
  • Technical Report
    The crustal structure of the Kane fracture zone from seismic refraction studies
    (Woods Hole Oceanographic Institution, 1980-12) Detrick, Robert S. ; Purdy, G. Michael
    A detailed seismic refraction experiment was carried out across the Kane Fracture Zone near 24°N, 44°W using explosive and air gun sound sources and eight ocean bottom hydrophone receivers. The shooting lines and receive rs formed a 'T' configuration across the fracture zone, with two receivers located about SO km apart in the fracture zone trough and the remaining six receivers positioned 25-30 km apart on either side of the fracture zone. The crustal thicknesses and velocities observed at the receivers located north and south of the Kane Fracture Zone fall within the range of those typically observed for normal oceanic crust. There is no convincing evidence for signficantly different crustal thicknesses or upper mantle velocities on either side of the fracture zone despite a 10-m.y. age difference. Anomalously thin crust is present beneath the Kane Fracture Zone trough with total crustal thicknesses of only 2-3 km, about half the thickness of normal oceanic crust. This crust is also characterized seismically by low compressional wave velocities (~4.0 km/s) at shallow depths and the absence of a normal layer 3 refractor. This anomalous crust extends over a width of a t least 10 km. Dense, high-velocity mantle type material may also exist at shallow depths beneath the adjacent Kane Fracture Zone ridge. Results from other geological and geophysical studies of fracture zones suggest that this type of crustal structure may by typical of many Atlantic fracture zones. We propose that the anomalously thin crust found within these fracture zones is a primary feature caused by the accretion of a thinner volcanic and plutonic layer within the fracture zone. This anomalous crust, which probably is restricted to a zone no wider than a typical transform fault valley (~10 km) in most cases, is inferred to consist of a few hundred meters of extrusive basalts and dikes overlying about 2 km of gabbro and metagabbro, possibly interbedded with ultramafics. This anomalously thin crustal section may be extensively fractured and brecciated at shallow levels by faulting in the active transform domain. A relatively narrow zone of thin crust within fracture zones can ex plain a number of geological and geophysical characteristics of fracture zones including the depth of the transform fault valley and the exposure of deep crustal and upper mantle rocks in the walls of fracture zones.
  • Preprint
    Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge
    ( 2009-04-28) Canales, J. Pablo ; Nedimovic, Mladen R. ; Kent, Graham M. ; Carbotte, Suzanne M. ; Detrick, Robert S.
    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.
  • Preprint
    Frozen magma lenses below the oceanic crust
    ( 2005-06-15) Nedimovic, Mladen R. ; Carbotte, Suzanne M. ; Harding, Alistair J. ; Detrick, Robert S. ; Canales, J. Pablo ; Diebold, John B. ; Kent, Graham M. ; Tischer, Michael ; Babcock, Jeffrey M.
    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.
  • Article
    Upper crustal structure and axial topography at intermediate spreading ridges : seismic constraints from the southern Juan de Fuca Ridge
    (American Geophysical Union, 2005-12-14) Canales, J. Pablo ; Detrick, Robert S. ; Carbotte, Suzanne M. ; Kent, Graham M. ; Diebold, John B. ; Harding, Alistair J. ; Babcock, Jeffrey M. ; Nedimovic, Mladen R. ; Van Ark, Emily M.
    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.
  • Article
    Seismic structure of the Endeavour Segment, Juan de Fuca Ridge : correlations with seismicity and hydrothermal activity
    (American Geophysical Union, 2007-02-03) Van Ark, Emily M. ; Detrick, Robert S. ; Canales, J. Pablo ; Carbotte, Suzanne M. ; Harding, Alistair J. ; Kent, Graham M. ; Nedimovic, Mladen R. ; Wilcock, William S. D. ; Diebold, John B. ; Babcock, Jeffrey M.
    Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9–1.4 s (∼2.1–3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4–1.2 km wide and appears to dip 8–36° to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 ± 0.09 s (380 ± 120 m) below the seafloor on the along-axis line and 0.38 ± 0.09 s (500 ± 110 m) on the cross-axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9–2.4 s below the seafloor. By projecting hypocenters of well-located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal-related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.
  • Article
    Morphology and segmentation of the western Galápagos Spreading Center, 90.5°–98°W : plume-ridge interaction at an intermediate spreading ridge
    (American Geophysical Union, 2003-12-13) Sinton, John M. ; Detrick, Robert S. ; Canales, J. Pablo ; Ito, Garrett T. ; Behn, Mark D.
    Complete multibeam bathymetric coverage of the western Galápagos Spreading Center (GSC) between 90.5°W and 98°W reveals the fine-scale morphology, segmentation and influence of the Galápagos hot spot on this intermediate spreading ridge. The western GSC comprises three morphologically defined provinces: A Western Province, located farthest from the Galápagos hot spot west of 95°30′W, is characterized by an axial deep, rift valley morphology with individual, overlapping, E-W striking segments separated by non-transform offsets; A Middle Province, between the propagating rift tips at 93°15′W and 95°30′W, with transitional axial morphology strikes ∼276°; An Eastern Province, closest to the Galápagos hot spot between the ∼90°50′W Galápagos Transform and 93°15′W, with an axial high morphology generally less than 1800 m deep, strikes ∼280°. At a finer scale, the axial region consists of 32 individual segments defined on the basis of smaller, mainly <2 km, offsets. These offsets mainly step left in the Western and Middle Provinces, and right in the Eastern Province. Glass compositions indicate that the GSC is segmented magmatically into 8 broad regions, with Mg # generally decreasing to the west within each region. Striking differences in bathymetric and lava fractionation patterns between the propagating rifts with tips at 93°15′W and 95°30′W reflect lower overall magma supply and larger offset distance at the latter. The structure of the Eastern Province is complicated by the intersection of a series of volcanic lineaments that appear to radiate away from a point located on the northern edge of the Galápagos platform, close to the southern limit of the Galápagos Fracture Zone. Where these lineaments intersect the GSC, the ridge axis is displaced to the south through a series of overlapping spreading centers (OSCs); abandoned OSC limbs lie even farther south. We propose that southward displacement of the axis is promoted during intermittent times of increased plume activity, when lithospheric zones of weakness become volcanically active. Following cessation of the increased plume activity, the axis straightens by decapitating southernmost OSC limbs during short-lived propagation events. This process contributes to the number of right stepping offsets in the Eastern Province.
  • Article
    Asymmetric shallow mantle structure beneath the Hawaiian Swell—evidence from Rayleigh waves recorded by the PLUME network
    (John Wiley & Sons, 2011-10-31) Laske, Gabi ; Markee, Amanda ; Orcutt, John A. ; Wolfe, Cecily J. ; Collins, John A. ; Solomon, Sean C. ; Detrick, Robert S. ; Bercovici, David ; Hauri, Erik H.
    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.
  • Preprint
    Seismic structure of Iceland from Rayleigh wave inversions and geodynamic implications
    ( 2005-10-25) Li, Aibing ; Detrick, Robert S.
    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.
  • Article
    Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N) : evidence for focused melt supply and lower crustal dike injection
    (American Geophysical Union, 2005-09-09) Dunn, Robert A. ; Lekic, Vedran ; Detrick, Robert S. ; Toomey, Douglas R.
    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.
  • Technical Report
    A seismic refraction experiment in the central Banda Sea
    (Woods Hole Oceanographic Institution, 1979-02) Purdy, G. Michael ; Detrick, Robert S.
    A 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.
  • Thesis
    The crustal structure and subsidence history of aseismic ridges and mid-plate island chains
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1978-09) Detrick, Robert S.
    This 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.