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  • 1
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    Online Resource
    Coincident seismic reflection/refraction studies of the continental lithosphere: A global review
    American Geophysical Union (AGU) ; 1987
    In:  Reviews of Geophysics Vol. 25, No. 4 ( 1987-05), p. 723-742
    Details
    In: Reviews of Geophysics, American Geophysical Union (AGU), Vol. 25, No. 4 ( 1987-05), p. 723-742
    Abstract: Nearly 50 coincident seismic reflection/refraction studies to depths of at least the Moho provide an improved understanding of the continental lithosphere. Some conclusions include the following: (1) A transparent upper crust, a common observation on vertical reflection profiles, cannot generally be correlated with velocity gradients or low‐velocity zones. Rather, a commonly transparent upper crust may be explained by short‐wavelength, steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source‐generated noise. (2) The reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity (6.6–7.3 km/s) and to consist of laminated high‐ and low‐velocity layers with typical thicknesses of 100–200 m. (3) Landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high‐ and low‐velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin. (4) The crust‐mantle boundary may differ sufficiently when imaged with vertical incidence and wide‐angle data to justify the retention, for the present, of the concept of separate reflection and refraction Mohos. While there is good evidence that these features are coincident within measurement uncertainties in most regions, recently recorded data from the Basin and Range admit the possibility for noncoincidence in that area. (5) Upper mantle reflections which cannot be migrated into the lower crust remain rare, despite isolated unequivocal examples. Thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust. The wide‐angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come. (6) There appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments; these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes. (7) No truly three‐dimensional coincident experiment (i.e., including three‐dimensional migration) has been conducted, but some three‐dimensional data have been collected using both methods. Measurements of attenuation, Poisson's ratio, and anisotropy within the crust using coincident data sets remain frontiers.
    Type of Medium: Online Resource
    ISSN: 8755-1209 , 1944-9208
    URL: Article
    DOI: 10.1029/RG025i004p00723
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1987
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  • 2
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    Multichannel seismic evidence for a subcrustal intrusive complex under Oahu and a model for Hawaiian volcanism
    American Geophysical Union (AGU) ; 1987
    In:  Journal of Geophysical Research: Solid Earth Vol. 92, No. B13 ( 1987-12-10), p. 13687-13707
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 92, No. B13 ( 1987-12-10), p. 13687-13707
    Abstract: Coincident multichannel seismic reflection and refraction data acquired during a wide‐aperture two‐ship experiment provide evidence for a complex crust‐mantle (C‐M) transition under Oahu, Hawaii. Several large‐aperture common depth point lines and three expanding spread profiles suggest the existence of an anomalously thick (3–6 km) C‐M transition zone underneath the volcanic ridge which extends for distances of 100 km to the north and south from the center of Oahu. The anomalous C‐M transition may represent a plutonic complex which intruded into the upper mantle and the lower crust in a 200‐km‐wide area centered at Oahu. The existence of such a large volume of intrusions near the base of the crust implies that the surficial expression of volcanism constitutes only a small fraction of the amount of melt generated at depth under the Hawaiian Islands. This interpretation is in accord with previous petrological models which predict trapping and accumulation of upwelling magma at and below the Moho. We have constructed a model which suggests that the interaction between the upwelling magma and the lithospheric flexural stress field may modulate the characteristic eruption history of Hawaiian volcanoes. In particular, the model for the plane stress field which accompanies the flexure of the oceanic crust around island chains indicates that the stress field under individual volcanoes varies considerably with its position relative to the tip of the chain. As a Hawaiian‐sized volcano develops, the magnitude of deviatoric compressive stresses under it is probably sufficient to block the conduits of the upwelling magma within the oceanic crust and to terminate eruptions. Further upwelling magma is predicted by the models to be ponded at the base of the crust. Resumption of posterosional volcanism seems to occur at a constant distance behind the center of active shield volcanism, as the horizontal compressive stresses along the axis of the chain are released. Observed orientations of dikes of this volcanic phase agree with the directions of the maximum calculated stresses. Our model implies that magma upwells over a 300‐km‐wide zone and that the oceanic plate may not be fractured under the islands.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB092iB13p13687
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1987
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  • 3
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    Online Resource
    Coincident seismic reflection/refraction studies of the continental lithosphere: a global review
    Oxford University Press (OUP) ; 1987
    In:  Geophysical Journal International Vol. 89, No. 1 ( 1987-04), p. 1-6
    Details
    In: Geophysical Journal International, Oxford University Press (OUP), Vol. 89, No. 1 ( 1987-04), p. 1-6
    Type of Medium: Online Resource
    ISSN: 0956-540X , 1365-246X
    URL: Issue
    URL: Article
    DOI: 10.1111/gji.1987.89.issue-1
    DOI: 10.1111/j.1365-246X.1987.tb04379.x
    Language: English
    Publisher: Oxford University Press (OUP)
    Publication Date: 1987
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  • 4
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    Seismic velocity structure at Deep Sea Drilling Project site 504B, Panama Basin: Evidence for thin oceanic crust
    American Geophysical Union (AGU) ; 1989
    In:  Journal of Geophysical Research: Solid Earth Vol. 94, No. B7 ( 1989-07-10), p. 9283-9302
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 94, No. B7 ( 1989-07-10), p. 9283-9302
    Abstract: We present an analysis of wide‐angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch 3 air gun array and fixed‐gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near‐normal‐incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity‐depth profile that better explains the observed data is characterized by high‐velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8‐km‐thick low‐velocity zone (V p = 7.1–6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high‐velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16–19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide‐angle reflection event observed at a range of 16–28 km and an MCS reflection event with a crustal travel time of 1.4–1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wide‐angle event demonstrates that these events are generated at the Moho.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB094iB07p09283
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1989
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  • 5
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    Geometry and subsurface lithology of southern Death Valley basin, California, based on refraction analysis of multichannel seismic data
    Geological Society of America ; 1987
    In:  Geology Vol. 15, No. 12 ( 1987), p. 1159-
    Details
    In: Geology, Geological Society of America, Vol. 15, No. 12 ( 1987), p. 1159-
    Type of Medium: Online Resource
    ISSN: 0091-7613
    URL: Article
    DOI: 10.1130/0091-7613(1987)15〈1159:GASLOS〉2.0.CO;2
    Language: English
    Publisher: Geological Society of America
    Publication Date: 1987
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  • 6
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    Seismic stratigraphy of the oceanic Moho based on ophiolite models
    Geological Society of America ; 1985
    In:  Geology Vol. 13, No. 1 ( 1985), p. 62-
    Details
    In: Geology, Geological Society of America, Vol. 13, No. 1 ( 1985), p. 62-
    Type of Medium: Online Resource
    ISSN: 0091-7613
    URL: Article
    DOI: 10.1130/0091-7613(1985)13〈62:SSOTOM〉2.0.CO;2
    Language: English
    Publisher: Geological Society of America
    Publication Date: 1985
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  • 7
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    A high‐resolution seismic reflection/refraction study of the Chugach‐Peninsular Terrane Boundary, southern Alaska
    American Geophysical Union (AGU) ; 1989
    In:  Journal of Geophysical Research: Solid Earth Vol. 94, No. B4 ( 1989-04-10), p. 4441-4455
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 94, No. B4 ( 1989-04-10), p. 4441-4455
    Abstract: We present results from a high‐resolution seismic refraction analysis of the shallow (approximately 2 km) crustal structure along the 107‐km‐long Trans‐Alaska Crustal Transect Chugach reflection line in southern Alaska and a comparison with laboratory measurements of field samples. The refraction analysis includes the two‐dimensional interpretation of several thousand first‐ and secondary‐arrival travel times digitized from 1024‐channel split‐spread common shot gathers. The velocity model derived from this analysis better defines the location and geometry of terrane boundaries than does the normal incidence reflection section and agrees well with surface mapping of lithologies. Furthermore, the model predicts travel times within 100 ms of the reflection times recorded from the base of the Quaternary on the Chugach reflection section. Thicknesses of Quaternary deposits, with velocities between 1.1 and 2.0 km/s, correlate inversely with the quantity of observed lower crustal reflections on the Chugach section, suggesting that the presence or absence of these sediments in sufficient thickness exerted primary control on the quality of the deeper portion of the section. There is a significant velocity contrast between crystalline rocks across the Border Ranges fault (5.0 versus 5.6 km/s), the major contact between the Chugach and Peninsular terranes, in agreement with laboratory measurements of field specimens. In the Peninsular terrane the modeling indicates that an unnamed fault delimiting the southern flank of the Copper River Basin dips steeply northward at 50° and has about 1300 m of vertical offset. Laboratory measurements document a maximum velocity anisotropy of 20% for phyllitic schists of the Valdez Group in the Chugach terrane. In agreement with the observed E‐W strike and near‐vertical dip of the Valdez Group, we determined a significant (14%) velocity anisotropy for ray paths oriented N‐S versus NE‐SW.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB094iB04p04441
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1989
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  • 8
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    A comparison of two sets of synthetic reflection seismograms for a complex two-dimensional model of the oceanic crust-mantle transition
    American Geophysical Union (AGU) ; 1987
    In:  Geophysical Research Letters Vol. 14, No. 8 ( 1987-08), p. 801-803
    Details
    In: Geophysical Research Letters, American Geophysical Union (AGU), Vol. 14, No. 8 ( 1987-08), p. 801-803
    Type of Medium: Online Resource
    ISSN: 0094-8276
    URL: Article
    DOI: 10.1029/GL014i008p00801
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1987
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  • 9
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    Multichannel seismic evidence for variations in crustal thickness across the Molokai Fracture Zone in the Mid‐Pacific
    American Geophysical Union (AGU) ; 1988
    In:  Journal of Geophysical Research: Solid Earth Vol. 93, No. B2 ( 1988-02-10), p. 1119-1130
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 93, No. B2 ( 1988-02-10), p. 1119-1130
    Abstract: Coincident multichannel seismic reflection and refraction data from a N–S transect near Oahu, Hawaii, provide evidence for thickening of the Pacific crust by 1–2 ± 1 km south of the large‐offset (16 m.y.) Molokai Fracture Zone (FZ). Tau‐p stacks, tau‐sum inversions, and forward modeling of the refraction data indicate that the crustal thickening occurs primarily within the lower portion of seismic layer 2. Assuming isostatic balance, the differences in crustal thickness predict that seafloor having the same age will have different elevations across the FZ. Observations of sea‐floor depths across the FZ east of the Hawaiian Islands are consistent with this prediction implying that the processes which have generated the crustal differences have been stable for over 50 m.y. Previous correlations between the chemical composition of ridge crest basalts, crustal thickness, and ridge crest elevation have been attributed to variations in the thermal regime of the upper mantle under mid‐ocean spreading centers. In accord with this hypothesis, we propose that the observed differences in crustal structure across the Molokai FZ may have been produced by small (25°C) differences in the thermal regime of the upper mantle beneath the ancestral East Pacific Rise. Discontinuous intracrustal reflections located about 1.6 s below the sediment/basement interface are observed in migrated reflection data south of Oahu. These reflections are similar in character to the lower crustal “Horizon R” event observed in the western North Atlantic. Shallower intracrustal reflections, possibly from within seismic layer 2, are also observed. The observation of these intracrustal reflections in both the Atlantic and Pacific oceans suggests that they are a fundamental signature of the crustal accretion process at a variety of spreading rates and that they are mappable using modern seismic reflection/refraction methods.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB093iB02p01119
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1988
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  • 10
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    Two‐dimensional seismic reflection modeling of the inferred fossil oceanic crust/mantle transition in the Bay of Islands Ophiolite
    American Geophysical Union (AGU) ; 1986
    In:  Journal of Geophysical Research: Solid Earth Vol. 91, No. B12 ( 1986-11-10), p. 12520-12538
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 91, No. B12 ( 1986-11-10), p. 12520-12538
    Abstract: We investigate the origin and character of oceanic Moho reflections by computing two‐dimensional synthetic seismogram profiles of the inferred fossil oceanic crust/mantle transition observed in the Bay of Islands Ophiolite. To simulate a seismic reflection experiment, we calculated near‐vertical‐incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length. In the synthetic profiles the Moho reflection event varies from a single phase to two or more phases of up to 1‐s (two‐way travel time) total duration. Individual phases show lateral variation in amplitude, and their two‐way travel times vary by as much as 0.25 s over horizontal distances as short as 10 km. Lateral discontinuity of phases results in abrupt variations in the travel times of first‐arriving, high‐amplitude Moho phases. The geological structures generating the highest‐amplitude Moho reflections vary from high‐ and low‐velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the Moho Transition Zone. Reflection amplitudes from the residual upper mantle are insignificant, and our modeling suggests that using the first‐arriving, high‐amplitude Moho phase to estimate thickness of magmatic material might result in errors of up to 1‐s two‐way travel time (∼3–4 km). Multichannel seismic data from both the western Pacific and western North Atlantic show Moho travel time variations similar to those observed in the synthetic profiles. The western North Atlantic data also show multiphase Moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data, suggesting that the structures observed in the inferred fossil crust/mantle transition of the ophiolite are characteristic of oceanic lithosphere.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB091iB12p12520
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1986
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