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Along- and across-axis variations in crustal thickness and structure at the Mid-Atlantic Ridge at 5°S obtained from wide-angle seismic tomography : implications for ridge segmentation (2009)
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In: Journal of geophysical research. B, Solid earth, Hoboken, NJ : Wiley, 1978, 114(2009), 2169-9356
In: volume:114
In: year:2009
In: extent:20
Description / Table of Contents: Two end-member styles of crustal accretion are observed at two adjacent spreading segments at the Mid-Atlantic Ridge at 5°S: focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment. Our results were obtained by tomographic inversion of wide-angle seismic reflection and refraction data collected along three intersecting profiles. The segment north of the 5°S fracture zone is characterized by a well-developed median valley with a pronounced seafloor bulge in the segment center. A discrete portion of anomalously low velocities (-0.4 to -0.5 km/s relative to average off-axis structure) at depths of ~2.5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt. This suggests that this segment is currently in a magmatically active period, which is confirmed by the observation of fresh lava flows and ongoing high-temperature hydrothermal activity at the seafloor. Close to the current spreading axis, the crust thins rapidly from 8.5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center. The reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3. The transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7.2-7.3 km/s indicating that serpentinization could be as much as 25% at 3.5 km depth. In contrast, ~4.1 Ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end. Here, the transform is characterized by a normal seismic layer 2/3 transition suggesting robust melt supply to the segment end at those times. In the adjacent southern segment, the crust thickens from ~2.5 km beneath the flank of an oceanic core complex to ~5.0 km at the segment boundary. The observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers.
Type of Medium: Online Resource
Pages: 20 , graph. Darst
ISSN: 2169-9356
DOI: 10.1029/2008JB006103
Language: English
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  • 2
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    Joint inversion of receiver functions, surface wave dispersion, and magnetotelluric data (2010)
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    In: Journal of geophysical research. B, Solid earth, Hoboken, NJ : Wiley, 1978, 115(2010), 2169-9356
    In: volume:115
    In: year:2010
    In: extent:23
    Description / Table of Contents: We present joint inversion of magnetotelluric, receiver function, and Raleigh wave dispersion data for a one-dimensional Earth using a multiobjective genetic algorithm (GA). The chosen GA produces not only a family of models that fit the data sets but also the trade-off between fitting the different data sets. The analysis of this trade-off gives insight into the compatibility between the seismic data sets and the magnetotelluric data and also the appropriate noise level to assume for the seismic data. This additional information helps to assess the validity of the joint model, and we demonstrate the use of our approach with synthetic data under realistic conditions. We apply our method to one site from the Slave Craton and one site from the Kaapvaal Craton. For the Slave Craton we obtain similar results to our previously published models from joint inversion of receiver functions and magnetotelluric data but with improved resolution and control on absolute velocities. We find a conductive layer at the bottom of the crust, just above the Moho; a low-velocity, low-resistivity zone in the lithospheric mantle, previously termed the Central Slave Mantle Conductor; and indications of the lithosphere-asthenosphere boundary in terms of a decrease in seismic velocity and resistivity. For the Kaapvaal Craton both the seismic and the MT data are of lesser quality, which prevents as detailed and robust an interpretation; nevertheless, we find an indication of a low-velocity low-resistivity zone in the mantle lithosphere. These two examples demonstrate the potential of joint inversion, particularly in combination with nonlinear optimization methods.
    Type of Medium: Online Resource
    Pages: 23 , graph. Darst
    ISSN: 2169-9356
    DOI: 10.1029/2009JB006369
    Language: English
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  • 3
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    Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico : implications for the generation of volcanism and subduction dynamics (2006)
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    In: Journal of geophysical research. B, Solid earth, Hoboken, NJ : Wiley, 1978, 111(2006), 2169-9356
    In: volume:111
    In: year:2006
    In: extent:22
    Description / Table of Contents: In order to study electrical conductivity phenomena that are associated with subduction related fluid release and melt production, magnetotelluric (MT) measurements were carried out in southern Mexico along two coast to coast profiles. The conductivity-depth distribution was obtained by simultaneous two-dimensional inversion of the transverse magnetic and transverse electric modes of the magnetotelluric transfer functions. The MT models demonstrate that the plate southern profile shows enhanced conductivity in the deep crust. The northern profile is dominated by an elongated conductive zone extending 〉250 km below the Trans-Mexican Volcanic Belt (TMVB). The isolated conductivity anomalies in the southern profile are interpreted as slab fluids stored in the overlying deep continental crust. These fluids were released by progressive metamorphic dehydration of the basaltic oceanic crust. The conductivity anomalies may be related to the main dehydration reactions at the zeolite ? blueschist ? eclogite facies transitions and the breakdown of chlorite. This relation allows the estimation of a geothermal gradient of 8̃.5°C/km for the top of the subducting plate. The same dehydration reactions may be recognized along the northern profile at the same position relative to the depth of the plate, but more inland due to a shallower dip, and merge near the volcanic front due to steep downbending of the plate. When the oceanic crust reaches a depth of 80-90 km, ascending fluids produce basaltic melts in the intervening hot subcontinental mantle wedge that give rise to the volcanic belt. Water-rich basalts may intrude into the lower continental crust leading to partial melting. The elongated highly conductive zone below the TMVB may therefore be caused by partial melts and fluids of various origins, ongoing migmatization, ascending basaltic and granitic melts, growing plutons as well as residual metamorphic fluids. Zones of extremely high conductance (〉8000 S) in the continental crust on either MT profile might indicate extinct magmatism.
    Type of Medium: Online Resource
    Pages: 22 , graph. Darst
    ISSN: 2169-9356
    DOI: 10.1029/2005JB003739
    Language: English
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  • 4
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    Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua (2008)
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    In: Journal of geophysical research. B, Solid earth, Hoboken, NJ : Wiley, 1978, 113(2008), 2169-9356
    In: volume:113
    In: year:2008
    In: extent:13
    Type of Medium: Online Resource
    Pages: 13
    ISSN: 2169-9356
    DOI: 10.1029/2007JB005291
    Language: English
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  • 5
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    Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at 3̃8°S (2008)
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    In: Journal of geophysical research. B, Solid earth, Hoboken, NJ : Wiley, 1978, 113(2008), 2169-9356
    In: volume:113
    In: year:2008
    In: extent:19
    Description / Table of Contents: A joint interpretation of swath bathymetric, seismic refraction, wide-angle reflection, and multichannel seismic data was used to derive a detailed tomographic image of the NazcaSouth America subduction zone system offshore southern Arauco peninsula, Chile at 3̃8ʿS. Here, the trench basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ) is obliquely subducting underneath the South American plate. The velocity model derived from the tomographic inversion consists of a 7̃-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities 〉8.4 km s-1. In the trenchouter rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst-and-graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending-related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10-15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 68 km into the upper mantle, where mantle temperatures are estimated to be 400-430ʿC. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a 2̃0 km wide frontal prism below the continental slope with Vp ≥3.5 km s-1, (2) a 5̃0 km area with Vp = 4.5-5.5 km s-1, interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp ̃6.0 km s-1. Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.
    Type of Medium: Online Resource
    Pages: 19
    ISSN: 2169-9356
    DOI: 10.1029/2007JB005569
    Language: English
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