Sound Velocity, Elastic Constants, and Related Properties of Marine Sediments in the Western Equatorial Pacific: Leg 7, Glomar Challenger

Gealy, E.L.

doi:10.2973/dsdp.proc.07.125.1971

Cited by 5 publications

(4 citation statements)

References 8 publications

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“…A refractor in the range of 34.5 km s-' cannot be identified as sedimentary on the basis of seismic velocity alone. Recent work, particularly that of JOIDES, has shown that this range can be common to both basalts and sediments (in particular, cherts, silicified limestones and evaporites) (for example, Fox, Schreiber & Peterson 1973;Christensen & Salisbury 1973;Talwani, Windisch & Langseth 1971;Gealy 1971). If a refractor can also be seen on a reflection record, then the character of the reflector may provide evidence in favour of one interpretation.…”

Section: Deep Crustal Structure: Seismic Refraction Resultsmentioning

confidence: 99%

The Eastern End of the Azores-Gibraltar Plate Boundary

Purdy¹

1975

Geophys J Int

109

103

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Summary The known relative motion of the African and Eurasian plates at the eastern end of the Azores‐Gibraltar plate boundary implies consumption of oceanic lithosphere at the low rate of 1–1.5 cm/yr. This section of the plate boundary, which crosses the area 34–38deg;N, 6–14° W, has none of the characteristic features of zones of oceanic lithosphere consumption (e. g. arcuate trench, island are volcanics, deep seismicity). A block of oceanic crustal and upper mantle material has been upthrust to form a 120 km long, 5 km high ridge, called Gorringe Ridge. Basalts, gabbros and ultramafics have been dredged from the northern face of this ridge. A mechanism for slow consumption is proposed which accounts for the anomalous features of this part of the plate boundary, and for the formation of Gorringe Ridge. Four deep seismic refraction lines show the major characteristics of the crustal structure to be 3–4 km thick sediment sequences, low upper mantle velocities of 7.3‐7.6 km s−1 and typical oceanic moho depths of about 11 km below sea level. Continuous seismic reflection profiles show 1–2 km of basement relief in the Western Horseshoe and Tagus Abyssal Plains and define a zone of acoustically opaque sediment in the Eastern Horseshoe Abyssal Plain. The free‐air gravity anomaly contour map shows 480 mgal. of relief. Closely‐controlled models for the structure of Gorringe Ridge devised to fit north‐south gravity profiles across the plate boundary require a large body with a density of at least 3.0 g cm−3 beneath the ridge to explain its associated 390 mgal. free‐air anomaly. The plate boundary is not well defined by the relocated epicentres but crustal structures suggest a 100 km southward offset immediately west of Gorringe Ridge. The calculation of finite difference poles of rotation for the African plate relative to Eurasia shows there have been two major periods of relative motion across the plate boundary in this region during the past 72 My. The slow compressive phase of the last 10 My was preceded 60–72 My before present by right lateral strike‐slip motion at a rate of 5.5 cm/yr. The direction of these relative motions was respectively perpendicular and parallel to 060°, a trend which is predominant in the bathymetry of this area and in the geology of southern Spain. This suggests that for the past 72 My a single line of weakness has existed in the lithosphere along which all motion between the African and Eurasian plates has been accommodated.

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Section: Deep Crustal Structure: Seismic Refraction Resultsmentioning

confidence: 99%

The Eastern End of the Azores-Gibraltar Plate Boundary

Purdy¹

1975

Geophys J Int

109

103

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“…This observation contradicts the expectation that velocity is directly related to density for a given lithology and that it follows a relationship such as Hamilton's [1978]. The resolution to this apparent conflict is that vertical variations in lithologies across the decollement (Figure 4) (tsqLu) qldea also exhibit low density and high velocity relative to normal clay lithologies at equivalent depths [Gealy, 1971]. We speculate that the large deviation from a linear trend for the samples within the decollement reflects microstructural details such as radiolarian abundance, whole versus fractured radiolarian tests, etc.…”

Section: Synthetic Seismogramsmentioning

confidence: 87%

Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust

Bangs¹,

Shipley²,

Moore³

et al. 1999

J. Geophys. Res.

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Abstract. A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 x 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecollement and decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the decollement. We construct a regional map of density, and inferred porosity, within the decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protodecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1-to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protodecollement. This zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the decollement.

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“…The high Vp/Vs ratio of marine sediments means that small differences in sediment thickness result in large differences in the relative arrival times of direct and converted phases. We explored the sensitivity of the constraints on G parameters to shallow structure by varying sediment thickness and Vp/Vs within ranges observed at nearby IODP drill sites (Gealy, 1971;Pälike et al, 2010aPälike et al, , 2010bShipboard Scientific Party, 2002). Our preferred sediment model based on prior NoMelt constraints has Vp/Vs of ∼8 and a thickness of 165 m. For the lowest Vp/Vs (2.4) and the minimum sediment thickness we tested (100 m), the predicted arrival time for the converted phase is ∼0.4 s closer to direct S compared to the sediment structure used in the synthetic calculations.…”

Section: Resultsmentioning

confidence: 99%

Constraints on the Depth, Thickness, and Strength of the G Discontinuity in the Central Pacific From S Receiver Functions

et al. 2021

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The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere‐asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.

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Sound Velocity, Elastic Constants, and Related Properties of Marine Sediments in the Western Equatorial Pacific: Leg 7, Glomar Challenger

Cited by 5 publications

References 8 publications

The Eastern End of the Azores-Gibraltar Plate Boundary

The Eastern End of the Azores-Gibraltar Plate Boundary

Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust

Constraints on the Depth, Thickness, and Strength of the G Discontinuity in the Central Pacific From S Receiver Functions

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