Petrologic nature of the oceanic Moho: Comment and reply

Luyendyk, Bruce P.; Nichols, Jerry

doi:10.1130/0091-7613(1977)5<578:pnotom>2.0.co;2

Cited by 3 publications

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“…The problem of thinness is relieved if a lower portion of the oceanic crust is serpentinized. The negative velocity gradient brought about by the serpentinization in zones 3b, c and d would produce a low velocity layer beneath the clinopyroxenites which would most likely go undetected in marine refraction surveys (Vine & Moores 1972;Luyendyk & Nichols 1977). In this model based on the Point Sal ophiolite, the seismic M-discontinuity would correspond to a phase boundary between fresh and serpentinized peridotite occurring perhaps 5 km below the sediments.…”

Section: Resultsmentioning

confidence: 97%

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Seismic velocity structure of the ophiolite at Point Sal, southern California, determined from laboratory measurements

Nichols

Warren

Luyendyk

et al. 1980

Geophysical Journal International

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The Jurassic ophiolite at Point Sal, California, contains all the rock types typically found in other ophiolites. The total thickness is just over 3 km, much thinner than seismically determined ocean crust thicknesses of 5-6 km. We measured the seismic velocities of 34 representative watersaturated samples in the laboratory under pressure and at room temperature. The velocity profile is a function of both metamorphic grade and the concentration of mafic minerals. Oceanic layer 2A compressional velocities are not found in the ophiolite. Layer 2B velocities of 4.9-5.4 km s-l are found for greenschist facies metabasalts. A velocity of 6.0 km s-l represents high grade greenschist facies mineral assemblages and amphibole in a dyke and sill complex. Late stage differentiates in the upper plutonic section have layer 3A velocities over 6.0 km s-'. Velocities increase with depth to abov 7.1 km s-l due to the introduction of cumulus olivine and other mafic p k ;es. Olivineclinopyroxene gabbros and troctolites have layer 3B velocities of about 7.4 km s-'. Beneath this, clinopyroxenites and cumulate ultramafics have slightly higher velocities. Serpentinized peridotite (dunite and harzburgite) occurs at the base of the ophiolite and has compressional velocities around 5.5 km s-'. The velocity profile with depth shows prominent gradients in the basaltic and gabbroic layers, but marked discontinuities are seen between the dyke and sill complex and the plutonic section and also between the upper plutonic section and the lower cumulate gabbros and ultramafic rocks. A pronounced velocity inversion, produced by serpentinized dunite underlying 166 J. Nichok et al.the gabbros, has been seismically modelled as a possible in situ characteristic of ocean crust and mantle structure. If a basal low velocity serpentinized layer underlies the ocean crust in some areas, then the Moho is within the mantle and marks the top of unaltered peridotite. Model travel-time curves and seismograms for such a structure show that it would be erroneously interpreted as a thicker ocean crust and that it would yield prominent late reverberations on seismic records.

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Section: Resultsmentioning

confidence: 97%

“…head wave. Second, the presence of the LVL above the mantle can delay seismic arrivals from the mantle, making the crust appear thicker than it really is (Rosendahl et al 1976;Luyendyk & Nichols 1977;Lewis & Snydsman 1977).…”

Section: Synthetic Travel-time Curvesmentioning

confidence: 99%

Seismic velocity structure of the ophiolite at Point Sal, southern California, determined from laboratory measurements

Nichols

Warren

Luyendyk

et al. 1980

Geophysical Journal International

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“…Water can now slowly pormeate the lower crust, serpentinizing the olivine in the basal crustal 'layer,' thus decreasing the seismic velocity in that region. The extent of alteration to the lower crust and uppermost mantle is questionable Woollard [1975],Clague and Straley [1977],Luyendyk and Nichols [1977],Lewis [1978],. andNichols et al [1980] propose various degrees of serpentinization of upper mantle material based on petrologic, seismic refraction, and synthetic modeling data.…”

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confidence: 99%

Ophiolites, synthetic seismograms, and oceanic crustal structure: 2. A comparison of synthetic seismograms of the Samail Ophiolite, Oman, and the ROSE refraction data from the East Pacific Rise

Kempner¹,

Gettrust²

1982

J. Geophys. Res.

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Synthetic seismograms for the Samail ophiolite, computed using the full reflectivity technique, are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust. If so, then this ophiolite complex can be compared with the older (45 m.y.) Bay of Islands (BOI) ophiolite to develop an aging model for oceanic crust. Synthetics for this young emplacement age (5-8 m.y.) ophiolite are compared with ocean bottom hydrophone data from 0.5-and 4.5-m.y.-age crust obtained during the Rivera Ocean Seismic Experiment. Second-arrival phases from reflected refractions from one ophiolite velocity-depth model and data from 4.5-m.y.-age crust are in excellent agreement; this agreement suggests that the Samail ophiolite is a good working model for young oceanic crust. Earlier work has shown that the BOI ophiolite is a good prototype for mature oceanic crust. The BOI ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited. We use the petrologic and geochemical properties of the Samail and BOI ophiolites to develop a two-stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials. refractions.' The crustal models used are based on velocitydepth functions from both Manghnani et al. [1981] and Christensen and Sinewing [1981, and personal communication, 1982] for the Ibra and northern section of the ophiolite complex, respectively. The seismograms are compared with ocean bottom seismometer (OBS) refraction data from phase I Of the Rivera Ocean Seismic Experiment (ROSE) that samples oceanic crust of similar (4.5 m.y.) and younger (0.5 m.y.) age [see Ewing, 1979].Our results sugget that within the limits of the seismic velocity and density samples for the Samail ophiolite, synthetic seismograms can be produced which offer good approximations to seismic refraction observations from young oceanic crust. Some variation between the synthetics and the observations occurs in the second arrivals. Our results suggest that this problem arises because reflected refractions are particularly sensitive to the P velocity gradient in the uppermost crust. Unfortunately, this portion of the crust and the ophiolite suites is poorly constrained. However, the agreement between synthetics for the Samail ophiolite and seismic refraction observations from young crust is impressive; the differences noted above are well within the expected variability of oceanic crustal structure.We have investigated the BOI ophiolite complex of Newfoundland in a similar manner [Kempnet and Gettrust, this issue] and show that synthetic seismograms for the BOI ophiolite fit marine crustal refraction data from older (60 m.y.) ocean basin crust. That investigation is based on a comparison of seismic refraction observations from the northeast Pacific with the BOI ophiolite complex. These similarities between seismic observations sampling young and mature oceanic crust with synthetic seismograms for ophiolite suites of similar ages suggest that it is rea...

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A comparison of synthetic P‐Tau sections for proposed models of two ophiolites

Brocher¹,

Kempner²,

Gettrust³

1982

J. Geophys. Res.

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Analyses of slant stacks of synthetic record sections generated from velocity models for ophiolites by the reflectivity method show that even 90‐km‐long refraction lines having a 10‐Hz source and a 1‐km intertrace spacing barely distinguish between fine‐scale crustal structures which are of current interest. Such data are sufficient, however, to distinguish between young and old oceanic crust. Tripling the spatial sampling period degrades the quality of the synthetic slant stacks for this temporal frequency content. A major implication of the study is that most currently available field data, being sparser and poorer in quality (due to both noise and instrument responses), offer little hope of resolving thin low‐velocity layers or small changes in velocity gradients by using newly proposed inversion techniques based on slant stacking. Last, a simple, first‐order correction to a downward continuation algorithm, namely, the inclusion of the phase shift caused by postcritical reflection at each ray parameter, is proposed.

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Petrologic nature of the oceanic Moho: Comment and reply

Cited by 3 publications

References 0 publications

Seismic velocity structure of the ophiolite at Point Sal, southern California, determined from laboratory measurements

Seismic velocity structure of the ophiolite at Point Sal, southern California, determined from laboratory measurements

Ophiolites, synthetic seismograms, and oceanic crustal structure: 2. A comparison of synthetic seismograms of the Samail Ophiolite, Oman, and the ROSE refraction data from the East Pacific Rise

A comparison of synthetic P‐Tau sections for proposed models of two ophiolites

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