The continental Mohorovicic discontinuity is most often interpreted as a step‐function velocity boundary. However, on deep crustal seismic reflection profiles, reflections, at depths where refraction data places the Moho, have laminated character and are laterally discontinuous. These observations on continental reflection data point to a model of the Moho that has thin layered rather than block structure. Previous workers, who have synthetically generated seismic responses from various crust‐mantle boundary models, have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed Moho arrivals. Geologic interpretations of the thin layering include relatively undeformed metasediments, cumulate layering, tectonic banding, and lenses of partial melt. To obtain more direct geologic evidence, a synthetic seismogram is generated from geologic cross‐sections of the Ivrea‐Verbano Zone, an exposed section through the lower crust and upper mantle. The synthetic shows laminations similar to those observed on reflection data. Without more direct evidence, no single geologic interpretation of the Moho is reasonable. Indeed, the Moho may be a laterally variable boundary, its composition and structure dependent upon the geologic history of the overlying crust.
Multichannel seismic reflection data over the axial region of the East Pacific Rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples. The data are from the Lamont Doherty Geological Observatory's line 17 shot across the rise at 9°N. The migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof. The polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge. The asymmetry is probably real and not an artifact of data collection. Modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally. A reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis. The shape of the roof reflection is convex upward with an approximate slope of 10° and a width of 4 km. If extrapolated symmetrically to the other side of the ridge axis, the magma chamber roof event is consistent with the funnel‐shaped chamber proposed by Pallister and Hopson (1981) for the Samail ophiolite. If the chamber roof steepens rapidly beyond the extent of the reflection, it would be consistent with the mush‐filled model of Sleep (1975, 1978) and Dewey and Kidd (1977).
Although seismic reflection data in batholithic terranes are scarce, datasets available to us from three different regions show surprising similarities. All three profiles, one across Precambrian rocks in Texas, and two across Mesozoic batholiths in California and Nevada, show strong subhorizontal reflections at depths of 6–10 km. These reflections are interpreted to originate from the base of the batholiths, and indicate that these batholiths are tabular in shape. The large amplitudes of the reflections require a marked contrast in acoustic impedance (product of velocity and density) between the batholith and the underlying rocks. Two of the profiles show reflective zones having appreciable thicknesses (0.5–2.5 km), while the third profile shows a single continuous reflection. The four most favored candidates for sub‐batholithic reflections are (1) underplated gabbro, (2) cumulate layers, (3) migmatites, and (4) thrust faults. Cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance.
Although seismic reflection data in batholithic terranes are scarce, datasets available to us from three different regions show surprising similarities. All three profiles, one across Precambrian rocks in Texas, and two across Mesozoic batholiths in California and Nevada, show strong subhorizontal reflections at depths of 6-10 km. These reflections are interpreted to originate from the base of the batholiths, and indicate that these batholiths are tabular in shape. The large amplitudes of the reflections require a marked contrast in acoustic impedance (product of velocity and density) between the batholith and the underlying rocks. Two of the profiles show reflective zones having appreciable thicknesses (0.5-2.5 km), while the third profile shows a single continuous reflection. The four most favored candidates for sub-batholithic reflections are (1) underplated gabbro, (2) cumulate layers, (3) migmatites, and (4) thrust faults. Cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance.
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