Seismic Properties of Sheeted Dikes from Hole 504B, ODP Leg 111

Christensen, Nikolas I.; Wepfer, William W.; Baud, Richie D.

doi:10.2973/odp.proc.sr.111.153.1989

Cited by 21 publications

(24 citation statements)

References 14 publications

(12 reference statements)

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“…Laboratory measured velocities on eight dike samples from Hole 504B are slightly higher, 6.2-6.7 km/s [Christensen et al, 1989], but are consistent with the results of fullwaveform sonic logging [Moos et al, 1990]. The porosity of the dike core samples from Hole 504B is very low, about 0.3% [Christensen et al, 1989], but in complete agreement with electrical logs once corrections have been made to Archie's law [Pezard, 1990]. If we accept the Poisson's ratio of 0.3 (Vp/Vs = 1.9) inferred from the full waveform logs at Hole 504B [Moos et al, 1990], the seismic measurements at Site 504 would require a large-scale porosity of about 2% [Ludwig et al, in prep].…”

Section: Physical Propertiesmentioning

confidence: 99%

“…Hole 504B is the only piece of oceanic crust where dike velocities at the scale of hundreds of meters are unequivocally known, and at this location the estimated velocity at the top of the sheeted dikes is -5.7-5.8 km/s (Figure 3.16) [Little and Stephen, 1985;Becker et al, 1988]. Laboratory measured velocities on eight dike samples from Hole 504B are slightly higher, 6.2-6.7 km/s [Christensen et al, 1989], but are consistent with the results of fullwaveform sonic logging [Moos et al, 1990]. The porosity of the dike core samples from Hole 504B is very low, about 0.3% [Christensen et al, 1989], but in complete agreement with electrical logs once corrections have been made to Archie's law [Pezard, 1990].…”

Section: Physical Propertiesmentioning

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: Physical Propertiesmentioning

confidence: 99%

Section: Physical Propertiesmentioning

confidence: 99%

Seismic constraints on shallow crustal processes at the East Pacific Rise

Christeson¹

1994

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“…Compressional-wave velocities measured at room temperature and 100 MPa, the in situ pressure at the base of the hole, are shown in Figure 5 along with the Leg 148 velocity log and laboratory velocities from previous legs (Wilkens et al, 1983;Christensen and Salisbury,1985;Christensen et al, 1989;Iturrino et al, 1995). Also shown (solid line) is the best fit to the laboratory V p data at estimated in situ pressures after the velocities have been corrected to the in situ temperatures determined from geophysical logging (Shipboard Scientific Party, 1993) using a correction factor, δV /; = -0.5 × 10~3 km/s°C (Christensen, 1979).…”

Section: Comparison Of Laboratory Logging and Seismic Velocitiesmentioning

confidence: 99%

Nature of the Layer 2/3 Transition from a Comparison of Laboratory and Logging Velocities and Petrology at the Base of Hole 504B

Salisbury¹,

Christensen²,

Wilkens³

1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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Compressional-and shear-wave velocities obtained by logging and laboratory measurements on samples from the base of Hole 504B between 1725 and 1836 m sub-basement are indistinguishable from classic Layer 3 velocities obtained by refraction (V p = 6.7 km/s, V s = 3.75 km/s). Point counts of thin sections from this interval show that the rocks consist not of gabbro, but of greenschist facies metadiabase dikes dominated by plagioclase and actinolite. Comparison with thin sections and logs from higher levels suggests that the Layer 2/3 boundary is a metamorphic front within the sheeted dikes controlled by the disappearance of chlorite, or a porosity transition marking the disappearance of interpiUow voids and large-scale fractures, depending on whether the boundary is defined in terms of velocities or velocity gradients.

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“…Added to the figure are sample velocities measured in the laboratory at 100 MPa confining pressure (Wilkens et al, 1983;Christensen and Salisbury, 1985;Christensen et al, 1989; Iturrino et al, in press). With the exception of several local excursions, velocities in basement generally fall between 4.5 and 5.5 km/s above 1100 mbsf.…”

Section: Discussionmentioning

confidence: 99%

“…The bulk density and porosity data compiled from previous Hole 504B studies are plotted with Leg 148 data in Figure 66 (Karato, 1983;Wilkens et al, 1983;Christensen and Salisbury, 1985;Christensen et al, 1989; Shipboard Scientific Party, 1983, 1985, 1992a Iturrino et al, in press). Densities measured on Leg 148 are between 2.88 and 3.05 g/cm 3 , with most greater than 2.90 g/cm 3 .…”

Section: Physical Propertiesmentioning

confidence: 99%

Site 504

1992

Proceedings of the Ocean Drilling Program

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Principal results: The primary objective of Leg 148 was to revisit Hole 504B in the eastern equatorial Pacific in order to deepen the hole into oceanic Layer 3 and to determine the nature of the transition from Layer 2 to Layer 3. Site 504 is located on 5.9-m.y.-old crust 201 km south of the Costa Rica Rift, the easternmost segment of the Galapagos Spreading Center (at 1°13.61 l'N, 83°43.818'W, at a water depth of 3460 m). Prior to Leg 148, Hole 504B was the deepest scientific drill hole in the oceans, extending 2000.4 meters below seafloor (mbsf; 1725.9 m sub-basement). It is the only hole to penetrate through the volcanic section into the underlying sheeted dike complex, and the site has become an important in-situ reference section for the physical and chemical structure of the upper oceanic crust. Results from recent drilling in Hole 504B during Leg 140 in late 1991, as well as seismic evidence, suggested that the bottom of the hole lay within the lower portion of the sheeted dike complex, close to the seismic Layer 2/Layer 3 boundary. Many scientists believe that the Layer 2/Layer 3 transition coincides with the change downward from sheeted dikes to underlying gabbros as is observed in ophiolites, but the seismic boundary may be a metamorphic transition within gabbros or within the lower sheeted dikes. Although the transition from sheeted dikes to gabbros has been observed by submersible (Auzende et al., 1989;Francheteau et al., 1992) in tectonic exposures of both Atlantic and Pacific ocean crust, this transition has never been observed in situ in undisturbed ocean crust, and its relation to the Layer 2/Layer 3 boundary remains unproven.Upon re-entering Hole 504B on 28 January, 1993, temperatures were logged from seafloor to just above the total depth of the hole (2000.4 mbsf) with somewhat ambiguous results caused by a calibration error or instru-1 Alt, J.C., Kinoshita, H., Stokking, L.B., et al., 1993. Proc. ODP, Init. Repts., 148: College Station, TX (Ocean Drilling Program).2 Shipboard Scientific Party is as given in the list of participants preceding the contents. mentation problem. The measured gradient through the cased section was close to the background value in the sediments, indicating that presently there is no downhole flow like that observed in the past. A slight dip in temperatures immediately below the bottom of the casing is similar to that recorded during Leg 140, indicating some hydrologic activity in the uppermost basement. Deep in the hole the gradient is generally linear and consistent with logs taken during Legs 137 and 140, indicating a maximum recorded temperature of about 180°C at the bottom of the hole. Following temperature measurements, borehole waters were sampled on 28-29 January. Water was collected in 6 of 8 runs, at 560, 796, 1031, 1266, 1501, and 1970 mbsf. One sample (1501 mbsf) clearly displays chemical characteristics of seawater that has reacted with basalt at elevated temperatures, whereas the compositions of other samples are close to seawater. Following drilling ...

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Seismic Properties of Sheeted Dikes from Hole 504B, ODP Leg 111

Cited by 21 publications

References 14 publications

Seismic constraints on shallow crustal processes at the East Pacific Rise

Seismic constraints on shallow crustal processes at the East Pacific Rise

Nature of the Layer 2/3 Transition from a Comparison of Laboratory and Logging Velocities and Petrology at the Base of Hole 504B

Site 504

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