Shallow crustal structure beneath the Juan de Fuca Ridge from 2-D seismic refraction tomography

White, Don; Clowes, Ron M.

doi:10.1111/j.1365-246x.1990.tb00690.x

Cited by 52 publications

(50 citation statements)

References 47 publications

(50 reference statements)

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“…Both the MCS data and a seismic refraction tomography study indicate that layer 2A is thicker under abyssal hills than beneath inter-ridge valleys [White and Clowes, 1990;Rohr et al, 1992]. Unfortunately, there are no constraints on layer 2A thickness for zero-age crust on the Endeavor Segment from either MCS data or seismic refraction experiments.…”

Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

“…This consists primarily of a thin surficial lowvelocity (<2.5 km/s) layer, a transition zone where velocities increase by -2.5 km/s over a thickness of -100-150 m, and a layer with velocities of >5 km/s. A surficial low-velocity layer is not unique to young EPR crust, and has also been observed on the intermediatespreading Juan de Fuca Ridge [White and Clowes, 1990; and the slow-spreading Mid-Atlantic Ridge Purdy, 1987]. Because laboratory measured velocities for fresh oceanic basalts are typically -6 km/s, the low seismic velocities have been attributed to high porosity on scales larger than the laboratory samples [Hyndman and Drury, 1976;Schreiber and Fox, 1976;Wilkens et al, 1991].…”

Section: Physical Propertiesmentioning

confidence: 99%

“…Thus the magnetic data indicates a correlation between the magnetic source layer, seismic layer 2A, and the extrusive section and transition zone at the Juan de Fuca Ridge. If the layer 2A/2B boundary does represent the top of the sheeted dike complex at intermediatespreading ridges, then seismic refraction data argues that the average depth to the sheeted dikes is 400±200 m [White and Clowes, 1990;McDonald et al, 1993]. The similarity in layer 2A thicknesses between the intermediate spreading Juan…”

Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

“…The layer 2A/2B boundary at intermediate-spreading ridges has generally been interpreted as a porosity boundary within the extrusive section, primarily because of the discrepancy between the average thickness of layer 2A (-400-600 m) and the depth to the top of the sheeted dike complex at Hole 504B (-800 m) White and Clowes, 1990;. Deeptow magnetic data indicate that seismically defined layer 2A thicknesses correlate quite well with magnetic source layer thicknesses at the Endeavour, Cleft, and Vance segments of the Juan de Fuca Ridge [Tivey, 1993;Tivey and Johnson, 1993].…”

Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

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Seismic constraints on shallow crustal processes at the East Pacific Rise

Christeson¹

1994

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This thesis is concerned with understanding how oceanic crust is emplaced at mid-ocean ridges. The emphasis is upon fast-spreading ridges, and the use of seismic techniques to image the uppermost several hundred meters of the crust. We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise (EPR) crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high-resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC), over 'zero-age' crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low-velocity (<2.5 km/s) layer, a 100-150 m thick transition zone with velocities increasing by approximately 2.5 km/s, and a layer with velocities of -5 km/s at a depth below the seafloor of 130-190 m. The surficial low-velocity layer and transition zone are defined as seismic layer 2A, and the -5 km/s layer as layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within -1 km of the rise axis, with the depth to the 2A/2B boundary increasing from -150 m to 300-350 m over this range. The doubling of layer 2A thickness within 1-2 km of the rise axis is confirmed by multi-channel seismic (MCS) and wide-angle profile (WAP) data, which also indicate that there is no further systematic change in thickness with greater range from the rise axis.Inversions for attenuation structure demonstrate that the layer 2A/2B interface is not only a velocity boundary, but also an attenuation boundary, with Q increasing from 10-20 within layer 2A to >70 in layer 2B.The results of MCS and wide-angle experiments over plausible velocity structures are predicted quantitatively, based on velocity models constructed from on-bottom seismic refraction experiments and expanding spread profiles. We conclude that the accuracy of correlating the prominent shallow reflector observed in MCS and WAP data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may follow structure within layer 2A if the upper crust contains more than one high-gradient region (our step model). A shallow structure similar to the step model is consistent with onbottom refraction experiments and expanding spread profiles located over zero-age EPR crust. With distance from the rise axis, this step-like structure is apparently destroyed, and is converted into a single steep gradient similar in appearance to our gradient model. Layer 2A is interpreted to be composed of the extrusive section and transition zone, with layer 2B consisting of the sheeted dike complex. This implies that the top of the dikes subs...

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Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

Section: Physical Propertiesmentioning

confidence: 99%

Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

Section: Intermediate-spreading Ridgesmentioning

confidence: 99%

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Seismic constraints on shallow crustal processes at the East Pacific Rise

Christeson¹

1994

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“…A fine mesh is also important to obtain accurate graph theoretical solutions. The sheared mesh representation allows accurate travel time calculation in the presence of large topographic variations, with much smaller computational resources than a rectangular grid [e.g., White and Clowes, 1990;]. …”

Section: Joint Refraction and Reflection Tomography 131 Model Parammentioning

confidence: 99%

Magmatism and dynamics of continental breakup in the presence of a mantle plume

Korenaga¹

2000

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This thesis studies the dynamics of mantle melting during continental breakups by geophysical, geochemical, and numerical analyses. The first part focuses on the mantle melting and crustal accretion processes during the formation of the Southeast Greenland margin, on the basis of deep-crustal seismic data. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a compressional velocity structure, and a long-wavelength structure with strong lateral heterogeneity is successfully recovered, including 30-to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone. A nonlinear Monte Carlo analysis is also conducted to establish the absolute uncertainty of model parameters. The derived crustal structure is first used to resolve the origin of a margin gravity high, with new inversion schemes using both seismic and gravity constraints. Density anomalies producing the gravity high seem to be confined within the upper crust, not in the lower crust as suggested for other volcanic margins. A new robust framework is then developed for the petrological interpretation of the velocity structure of igneous crust, and the thick igneous crust formed at the continentocean transition zone is suggested to have resulted from vigorous active upwelling of mantle with only somewhat elevated potential temperature. In the second part, the nature of mantle melting during the formation of the North Atlantic igneous province is studied on the basis of the major element chemistry of erupted lavas. A new fractionation correction scheme based on the Ni concentrations of mantle olivine is used to estimate primary melt compositions, which suggest that this province is characterized by a large degree of major element source heterogeneity. In the third part, the nature of preexisting sublithospheric convection is investigated by a series of finite element analyses, because the strength of such convection is important to define the "normal" state of mantle, the understanding of which is essential to identify any anomalous behavior of mantle such as a mantle plume. The results suggest that small-scale convection is likely in normal asthenosphere, and that the upwelling velocity in such convection is on the order of 1-10 cm/yr. 271 Introductory NotesThis thesis is composed of three parts, all of which are related to the dynamics of mantle melting during continental breakups. In the first part, rifting magmatism associated with the opening of the North Atlantic (at -60 Ma) is investigated on the basis of deep-crustal seismic data collected across the Southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure (Chapter 1). Several key issues in travel time tomography such as velocity-depth ambiguity and absolute parameter uncertainty are identified, and a practical strategy is presented to resolve them. The derived crustal velocity structure is the...

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Mid‐Ocean Ridge Dynamics: Observations and Theory

Morgan

1991

Reviews of Geophysics

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Shallow crustal structure beneath the Juan de Fuca Ridge from 2-D seismic refraction tomography

Cited by 52 publications

References 47 publications

Seismic constraints on shallow crustal processes at the East Pacific Rise

Seismic constraints on shallow crustal processes at the East Pacific Rise

Magmatism and dynamics of continental breakup in the presence of a mantle plume

Mid‐Ocean Ridge Dynamics: Observations and Theory

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