A low velocity zone underlying a fast-spreading rise crest

Orcutt, John A.; Kennett, B. L. N.; Dorman, LeRoy M.; Prothero, William A.

doi:10.1038/256475a0

Cited by 79 publications

(44 citation statements)

References 9 publications

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“…Velocities which are rather low compared to normal mantle velocities (8.0-8.1 km s-'), either at the base of the crust or within the upper mantle, could be a consequence of water injection in some circumstances (Bowin 1973;Orcutt et al 1975;Lewis & Snydsman 1977;Lewis 1983;Whitmarsh et al 1986). Hence, serpentinized peridotite is suspected.…”

Section: The Kerguelen-heard Plateaumentioning

confidence: 96%

Deep structure of the northern Kerguelen Plateau and hotspot-related activity

Charvis

Recq

Operto

et al. 1995

Geophysical Journal International

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SUMMARY Seismic refraction profiles were carried out in 1983 and 1987 throughout the Kerguelen Isles (southern Indian Ocean, Terres Australes & Antarctiques Franqaises, TAAF) and thereafter at sea on the Kerguelen‐Heard Plateau during the MD66/KeOBS cruise in 1991. These profiles substantiate the existence of oceanic‐type crust beneath the Kerguelen‐Heard Plateau stretching from 46°s to 55°S, including the archipelago. Seismic velocities within both structures are in the range of those encountered in ‘standard’ oceanic crust. However, the Kerguelen Isles and the Kerguelen‐Heard Plateau differ strikingly in their velocity‐depth structure. Unlike the Kerguelen Isles, the thickening of the crust below the Kerguelen‐Heard Plateau is caused by a 17 km thick layer 3. Velocities of 7.4 km s−1 or so within the transition to mantle zone below the Kerguelen Isles are ascribed to the lower crust intruded and/or underplated by upper mantle material. The crust‐mantle boundary below the Kerguelen‐Heard Plateau is abrupt and devoid of any underplated material. The difference in structure between the northern edge of the Kerguelen Plateau (including the archipelago) and the Kerguelen‐Heard Plateau may be related to variability of the time‐dependent hotspot activity. The Kerguelen‐Heard Plateau was emplaced during the Cretaceous time (110 Ma) when the volcanic output rate of the Kerguelen Plateau and the Ninetyeast Ridge was high (as well as high potential temperature). The northernmost Kerguelen Plateau and the archipelago were emplaced during Tertiary time (40–45 Ma), as the volcanic output rate reduced. Furthermore, intraplate volcanism continued in the Kerguelen archipelago for at least 40 Ma. The isostatic compensation of the Kerguelen Isles and the Kerguelen‐Heard Plateau is achieved by low‐density mantle material, as shown by refraction and geoid studies. The velocity‐depth structure below the Kerguelen Isles is similar to that found below intraplate oceanic islands such as Hawaii. Despite the differences in age, the crust below Iceland (0Ma) and the Kerguelen Plateau (100–120 Ma) are strikingly akin. The similarity between the Kerguelen‐Heard Plateau and Iceland, then, strongly supports a similar origin for both structures, the Kerguelen‐Heard Plateau being a fossil equivalent of present‐day Iceland. Crustal thickening beneath the Kerguelen‐Heard Plateau, which may result from an Iceland‐type setting (i.e. an active spreading centre over a hotspot), is mostly produced by thickening of layer 3, layer 2 representing 25 per cent only of the thickness of the igneous crust. The Kerguelen Isles, despite the initial volcanism near the active Southeast Indian Ridge, behave as a midplate volcanic island and are definitely not representative of the whole Kerguelen Plateau structure.

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Section: The Kerguelen-heard Plateaumentioning

confidence: 96%

Deep structure of the northern Kerguelen Plateau and hotspot-related activity

Charvis

Recq

Operto

et al. 1995

Geophysical Journal International

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“…Inferences about the nature of sub-axial magma reservoirs are based on a limited amount of data from scattered ridge segments, which need not necessarily be typical of their respective ridge systems. For example, geophysical studies of segments of the East Pacific Rise have revealed the existence of a low-velocity zone beneath the ridge axis (Orcutt et al 1975, McClain et al 1985, which could represent a magma reservoir. In contrast, similar studies of the much slower spreading FAMOUS area of the Mid-Atlantic Ridge have failed to reveal any substantial axial magma chamber (Fowler 1976).…”

Section: Magma Storage and Releasementioning

confidence: 99%

Igneous Petrogenesis

Wilson

2000

330

383

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“…However, even on the East Pacific Rise, which has a high magmatic budget such models have been known for some while to be inconsistent with seismological observations. Early refraction experiments on the EPR [Orcutt et al, 1975[Orcutt et al, , 1976Rosendahl et al, 1976] detected a low-velocity zone whose top coincided with an upper crustal reflector [Herron et al, 1978[Herron et al, , 1980Hale et al, 1982] which was interpreted to be the roof of a magma chamber. However, the vertical extent and velocities of the low-velocity anomaly detected by the refraction experiments were not consistent with a very magma body.…”

Section: The Axial Low-q Regionmentioning

confidence: 99%

The seismic attenuation structure of the East Pacific Rise

Wilcock¹

1992

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Studies of seismic propagation through oceanic crust have contributed enormously to our understanding of the generation and evolution of oceanic crust. However, such work has largely been confined to the seismic velocity structure. In this thesis we present results from a study of seismic attenuation using a data set collected for three-dimensional tomographic imaging of a fast-spreading ridge. The experiment location at 9 0 30'N on the East Pacific Rise is the site of a strong mid-crustal seismic reflector which has been inferred to be the roof of a small axial magma chamber at about 1.6 km depth.A spectral method is used to estimate t*, a measure of the integrated attenuation along a wave path. Such a method assumes that the dominant frequency-dependent component of propagation is intrinsic attenuation. A logarithmic parameterization is then used to invert t* measurements for Q-I structure assuming that the velocity structure is given from earlier studies. To evaluate the method of Q tomography a full-waveform finitedifference technique which does not include attenuation is used to calculate solutions for seismic propagation through a two-dimensional velocity model. The results show a complex pattern of seismic propagation in the vicinity of the axial magma chamber. The first arrival always passes above the magma chamber. However, for paths of significant length that cross the rise axis the amplitude of this arrival is very small, and the first phase with significant amplitude is a diffraction below the magma chamber. High-amplitude Moho turning and PP arrivals may also be important secondary arrivals. Synthetic inversions show the importance of selecting time windows for power spectral estimation which are dominated by a single phase and of using wave paths which closely corresponds to that of the selected phase.A comparison of the finite difference solutions and the predictions of the a twodimensional, exact ray-tracing algorithm with record sections obtained during the tomography experiment significantly improves our understanding of seismic propagation across the East Pacific Rise. The results enable an objective choice of the position and length of the time window for t* estimation. Moreover, additional constraints are incorporated into an approximate three-dimensional ray-tracing algorithm used in the inversion so that the wave paths more closely correspond to those of the desired phase. The full data set to be inverted comprises about 3500 t* estimates and includes crustal paths which do not cross the rise axis, diffractions above and below the axial magma chamber, and Moho-turning phases. Wave paths for the Moho-turning phases cross the rise axis at a wide range of lower crustal depths.The Q-1 models resulting from two-dimensional and three-dimensional tomographic inversions show that the attenuation of seismic waves on the East Pacific Rise is dominated by two regions of low Q; one in the upper 1 km of crust, and one at depths greater than about 2 km below the rise axis. While the data do not ...

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A low velocity zone underlying a fast-spreading rise crest

Cited by 79 publications

References 9 publications

Deep structure of the northern Kerguelen Plateau and hotspot-related activity

Deep structure of the northern Kerguelen Plateau and hotspot-related activity

Igneous Petrogenesis

The seismic attenuation structure of the East Pacific Rise

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