Basement Logs from the Mouth of the Gulf of California, Deep Sea Drilling Project Leg 65

Salisbury, Matthew H.

doi:10.2973/dsdp.proc.65.108.1983

Cited by 4 publications

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References 4 publications

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“…With the advent of the Deep Sea Drilling Project, the potential arose for testing these models against the actual velocity structure of the crust as determined by downhole logging. Although several attempts have been made in the past to drill and log deep into the crust (e.g., Kirkpatrick, 1979;Salisbury etal., 1979;Salisbury, 1983;Cann and Von Herzen, 1983), this potential was first fully realized on Leg 83, when Hole 504B on the Costa Rica Rift (Fig. 1) was deepened to a sub-bottom depth of 1350 m and logged to a sub-basement depth of 1013 m through a spectacular sequence of pillow basalts and minor flows underlain by sheeted dikes-a sequence reminiscent of those observed in the upper levels of ophiolites (e.g., Moores and Vine, 1971;Williams, 1971).…”

Section: Introductionmentioning

confidence: 99%

The Velocity Structure of Layer 2 at Deep Sea Drilling Project Site 504 from Logging and Laboratory Experiments

Salsibury¹,

Christensen²,

Becker³

et al. 1985

Initial Reports of the Deep Sea Drilling Project

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Compressional-and shear-wave velocity logs (V p and V % , respectively) that were run to a sub-basement depth of 1013 m (1287.5 m sub-bottom) in Hole 504B suggest the presence of Layer 2A and document the presence of layers 2B and 2C on the Costa Rica Rift. Layer 2A extends from the mudline to 225 m sub-basement and is characterized by compressional-wave velocities of 4.0 km/s or less. Layer 2B extends from 225 to 900 m and may be divided into two intervals: an upper level from 225 to 600 m in which V p decreases slowly from 5.0 to 4.8 km/s and a lower level from 600 to about 900 m in which V p increases slowly to 6.0 km/s. In Layer 2C, which was logged for about 100 m to a depth of 1 km, F p and F s appear to be constant at 6.0 and 3.2 km/s, respectively. This velocity structure is consistent with, but more detailed than the structure determined by the oblique seismic experiment in the same hole. Since laboratory measurements of the compressional-and shear-wave velocity of samples from Hole 504B at P con fj n. ing = ^differential average 6.0 and 3.2 km/s respectively, and show only slight increases with depth, we conclude that the velocity structure of Layer 2 is controlled almost entirely by variations in porosity and that the crack porosity of Layer 2C approaches zero. A comparison between the compressional-wave velocities determined by logging and the formation porosities calculated from the results of the large-scale resistivity experiment using Archie's Law suggest that the velocity-porosity relation derived by Hyndman et al. (1984) for laboratory samples serves as an upper bound for V p , and the noninteractive relation derived by Toksöz et al. (1976) for cracks with an aspect ratio a = 1/32 serves as a lower bound.

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Section: Introductionmentioning

confidence: 99%

The Velocity Structure of Layer 2 at Deep Sea Drilling Project Site 504 from Logging and Laboratory Experiments

Salsibury¹,

Christensen²,

Becker³

et al. 1985

Initial Reports of the Deep Sea Drilling Project

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“…Since the advent of the Deep Sea Drilling Project in 1968, many of the logging techniques developed by industry have been applied in boreholes drilled in the ocean basins for scientific purposes. Although many DSDP sediment boreholes have been logged (e.g., Montadert, Roberts, et al, 1979;Boyce, 1980;Yeats, Haq, et al, 1981;Curray, Moore, et al, 1982), before Leg 78B only five serious attempts were made to log holes drilled in oceanic basement: Hole 396B on the Mid-Atlantic Ridge (Kirkpatrick, 1979); Hole 417D on the Bermuda Rise Salisbury, Stephen, et al, 1980); Hole 462A in the Nauru Basin (Boyce, 1981); Holes 482C, 483, and 485A in the mouth of the Gulf of California (Salisbury, 1983); and Hole 504B on the Costa Rica Rift (Cann and Von Herzen, 1983). Of these, several were in anomalous crust.…”

Section: Introductionmentioning

confidence: 99%

Basement Logging on the Mid-Atlantic Ridge, Deep Sea Drilling Project Hole 395A

Mathews¹,

Salisbury²,

Hyndman³

1984

Initial Reports of the Deep Sea Drilling Project

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On Leg 78B, the Glomar Challenger returned to Hole 395A on the Mid-Atlantic Ridge and logged the upper 0.5 km of the crust, obtaining excellent resistivity, natural-gamma, and caliper logs throughout the section and reasonable density and velocity logs near the base. The logs show that the crust may be divided into two distinct geophysical units. The upper 400 m of the crust displays high porosities and low velocities, densities and resistivities. Below this, between 400 and 500 m sub-basement, the crust displays a low crack porosity (1-2%), high resistivities (up to 1000 öm), and high velocities (up to 6.0 km/s). For the lower unit, a value of 2.2 was obtained for the exponent m in Archie's Law, which is consistent with low permeabilities measured in the same interval with a packer (2 to 9 µdarcies). Since the boundary between these two units is marked by a sharp increase in alteration products and the lower unit behaves as if it were sealed, we conclude that Hole 395A penetrated a major geophysical discontinuity.

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“…The results from Hole 735B provide the opportunity to test these hypotheses and models using downhole and physical properties measurements. Although several attempts to drill and log deep into the oceanic crust have been previously made (Kirkpatrick, 1979;Salisbury et al, 1980;Salisbury, 1983;Cann and Von Herzen, 1983;Salisbury et al, 1985), this potential was more fully realized in results from Hole 735B, DSDP Hole 504B in the Costa Rica Rift, and the findings from the Hess Deep area during Leg 147.…”

Section: Nature Of Seismic Layermentioning

confidence: 99%

Velocity structure of the lower oceanic crust: results from Hole 735B, Atlantis II Fracture Zone

Iturrino¹,

Ildefonse²,

Boitnott³

2002

Proceedings of the Ocean Drilling Program

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Laboratory compressional wave (V P) and shear wave (V S) velocities were measured as a function of confining pressure for the gabbros from Hole 735B and compared to results from Leg 118. The upper 500 m of the hole has a V P mean value of 6895 m/s measured at 200 MPa, and at 500 meters below seafloor (mbsf), V P measurements show a mean value of 7036 m/s. V S mean values in the same intervals are 3840 m/s and 3857 m/s, respectively. The mean V P and V S values obtained from log data in the upper 600 m are 6520 and 3518 m/s, respectively. These results show a general increase in velocity with depth and the velocity gradients estimate an upper mantle depth of 3.32 km. This value agrees with previous work based on dredged samples and inversion of rare element concentrations in basalts dredged from the conjugate site to the north of the Atlantis Bank. Laboratory measurements show V P anisotropy ranging between 0.4% and 8.8%, with the majority of the samples having values less than 3.8%. Measurements of velocity anisotropy seem to be associated with zones of high crystal-plastic deformation with predominant preferred mineral orientations of plagioclase, amphiboles, and pyroxenes. These findings are consistent with results on gabbros from the Hess Deep area and suggest that plastic deformation may play an important role in the seismic properties of the lower oceanic crust. In contrast to ophiolite studies, many of the olivine gabbros show a small degree of anisotropy. Log derived V S anisotropy shows an average of 5.8% for the upper 600 m of Hole 735B and tends to decrease with depth where the overburden pressure and the age of the crustal section suggests closure of cracks and infilling of fractures by alteration

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Basement Logs from the Mouth of the Gulf of California, Deep Sea Drilling Project Leg 65

Cited by 4 publications

References 4 publications

The Velocity Structure of Layer 2 at Deep Sea Drilling Project Site 504 from Logging and Laboratory Experiments

The Velocity Structure of Layer 2 at Deep Sea Drilling Project Site 504 from Logging and Laboratory Experiments

Basement Logging on the Mid-Atlantic Ridge, Deep Sea Drilling Project Hole 395A

Velocity structure of the lower oceanic crust: results from Hole 735B, Atlantis II Fracture Zone

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