Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

Swift, Stephen A.; Lizarralde, D.; Stephen, Ralph A.; Hoskins, Hartley

doi:10.1029/98jb00766

Cited by 32 publications

(41 citation statements)

References 62 publications

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“…All logging measurements display a sharp variation at the bottom of the lava pond at the contact with the basalt unit located immediately below. In the lava pond, Swift et al [2008] have also obtained lower values of the Poisson's ratio than those often measured in uppermost crust [e.g., Hyndman, 1979;Swift et al, 1998], consistent with a low extent of fracturing in the lava pond unit.…”

Section: Extent Of Rock Fracturingmentioning

confidence: 70%

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

Tartarotti

Fontana

Crispini

2009

Geochem Geophys Geosyst

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A complete “in situ” section of upper oceanic crust, from extrusive lavas, through dikes into gabbros has been recently drilled for the first time in 15 Ma old crust created at the superfast spreading East Pacific Rise (EPR). The upper igneous basement cored at Ocean Drilling Program (ODP)–Integrated Ocean Drilling Program (IODP) Holes 1256C and 1256D consists of sheet flows and massive flows, including a ponded lava flow unit (“lava pond”) interpreted as a thick lava flow delivered off‐axis and accumulated in a topographic depression. Fine‐scale structural analysis shows that the lava pond is much less fractured than the deeper lavas, probably as the effect of structure spacing that is larger than in the underlying lava flows. In Hole 1256D, structures are mainly subhorizontal in the lava pond and become steeper downhole, toward the sheeted dike complex. This reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth, inducing mostly subvertical eruptive fractures and fracture network, and the distance of the site from active rifting at the EPR. Contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns, as obtained by shipboard and downhole logging data. Electrical resistivity, compressional velocity, and bulk density are generally higher, whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole. The occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and, consequently, its permeability regime which, in turn, controls the geometry and distribution of hydrothermal circulation and basalt alteration.

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Section: Extent Of Rock Fracturingmentioning

confidence: 70%

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

Tartarotti

Fontana

Crispini

2009

Geochem Geophys Geosyst

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“…If such velocity anomalies existed, they would be expected to produce significant differences between the sonic log velocities and the vertical velocity obtained from VSP travel times. Figure 5b shows that sonic log velocities are up to 1 km/s lower in intervals with low Qi, but Swift et al [1998b] In Figure 7 the VSP amplitudes appear to parallel the amplitudes modeled by the geometrical spreading and scattering models. However, the downhole trends in the amplitudes difference reveal consistently different slopes (Figures 8a and 8b).…”

Section: Resultsmentioning

confidence: 87%

Seismic attenuation in upper ocean crust at Hole 504B

Swift¹,

Lizarralde²,

Stephen³

et al. 1998

J. Geophys. Res.

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Abstract. Seismic attenuation and its relationship to borehole stratigraphy in the upper 1.8 km of ocean basement at Hole 504B are determined from analysis of vertical seismic profile (VSP) data. VSP data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust, and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading, scattering, and intrinsic loss mechanisms to this decay. About 60% of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts. The remaining amplitude decrease is concentrated in two layers, at 500-650 and 800-900 meters below seafloor (mbsf) (225-375 and 525-625 rn below basement), across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified. Attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by Q of 10 and 8, respectively. It is likely that the Q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes. The shallow interval coincides with a change in alteration mineralogy deposited by late-stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off-axis flow through large lava tubes. Intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting. There is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf (-725 rn subbasement).

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“…This increase in P wave velocity with depth closely correlates with a large decrease in apparent bulk porosity with depth in Hole 504B [Becker et al, 1989[Becker et al, , 1992]. An increase in fracture density in the lowermost part of Hole 504B and a decrease in sonic velocities at -1600 m depth have been interpreted as evidence that a subhorizontal fault zone was penetrated near the base of the hole [Altet al, 1993] A comparison of the in situ physical properties measured in Hole 504B with available seismic data from in and around the drill site indicates that at this location the seismic layer 2/3 boundary is not associated with a lithostratigraphic transition from sheeted dikes to gabbro as is commonly assumed for oceanic crust [Detrick et al, 1994;Salisbury et al, 1996;Swift et al, 1998a]. Derrick et al [1994] place the seismic layer 2/3 boundary at -1200 + 200 m into the crust, within the sheeted dike section, where it is associated with a decrease in vertical velocity gradient estimated from coincident seismic refraction data.…”

Section: Introductionmentioning

confidence: 99%

“…If, instead of a change in velocity gradient, the layer 2/3 boundary is defined in terms of the classical layer 3 velocity of 6.7 km s -• [Raitt, 1963], then the boundary would fall in the lower part of the sheeted dike section -1725-1836-m subbasement [Salisbuo, et al, 1996]. Alternatively, Swift et al [1998a], using P wave velocities determined from a VSP profile in Hole 504B, place the seismic layer 2/3 boundary only -800-900 m into the crust, coincident with the top of the sheeted dike section. A comparison of core samples and logs from Hole 504B with these estimates of the depth to the seismic layer 2/3 boundary suggest that this boundary is caused by either a porosity transition marking the disappearance of interpillow voids and fractures or by a metamorphic front within the sheeted dikes controlled by the disappearance of chlorite [Salisbury eta[., 1996].…”

Section: Introductionmentioning

confidence: 99%

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

Detrick¹,

Toomey²,

Collins³

1998

J. Geophys. Res.

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Abstract. We have used over 16,000 compressional wave travel times recorded on 11 ocean bottom seismometers to determine the magnitude of P-wave heterogeneity and anisotropy in the upper 2 km of the oceanic crust in an -• 150 km 2 area around Hole 504B, the deepest hole drilled into the oceanic crust. Our best fitting one-dimensional, isotropic upper crustal velocity model for the 150 km 2 area around the drill site is similar to velocity models at Hole 504B determined using downhole sonic logs, vertical seismic profiles, oblique seismic experiments and conventional refraction profiling. There is no evidence seismic layer 2 is anomalously thin at Hole 504B, suggesting that the occurrence of the seismic layer 2/3 boundary within the upper sheeted dike section, which has been documented in Hole 504B, is a general feature of this 5.9 Ma crust. On the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in upper crustal P wave velocity around Hole 504B is comparatively small. Within the 571-m thick extrusive section maximum P wave velocity differences are -300-400 rn s '• (a 6%-8% velocity anomaly). These velocity differences decrease to <100-200 rn s -• (<2%-3% of the average velocity) within the sheeted dike section > 1 km into the crust. The highest upper crustal velocities are found beneath basement ridges west and south of Hole 504B, while the lowest upper crustal velocities occur beneath the flanking basement troughs. There is an azimuthal dependence to the travel time residuals (slow direction N-S) which is consistent with 2%-4% crack-induced anisotropy in the upper crust. The magnitude of upper crustal heterogeneity observed near Hole 504B is significantly less than that found in "zero-age" crust at the Mid-Atlantic Ridge but comparable to that observed off-axis in crust formed at faster spreading ridges.

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Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

Cited by 32 publications

References 62 publications

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

Seismic attenuation in upper ocean crust at Hole 504B

Three‐dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

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