A search for layering in the oceanic crust
Collins, John A.
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LocationWestern North Atlantic
The results of numerous seismic refraction and reflection experiments have shown that the seismic structure of the oceanic crust can be usefully parameterized by a small number of locally horizontal layers within which the rates of change of velocity and impedance as a function of depth are approximately constant. Layer boundaries are defined by changes in velocity and/or impedance gradient. This dissertation discusses the structure of seismic layer boundaries within the oceanic crust, and investigates the relationships between the seismic characteristics of these boundaries and the geological structure of the crust. The seismic signature of the crust/mantle boundary (Moho) is a prominent event on multichannel seismic (MCS) reflection data. In the Western North Atlantic, the character of the Moho reflection event varies from a single well-defined phase to a more complex event consisting of two or more overlapping phases of up to 1.0 s total duration. In Chapter 1 of this dissertation, the geological structures generating Moho reflections are investigated by calculating synthetic reflection profiles for three laterally varying velocity models totaling 64 km in length. These velocity models were derived from the observed distribution of lithologies that comprise the inferred fossil crust/mantle transition found in the Bay of Islands Ophiolite. Along the synthetic profiles, the Moho reflection is characterized by both single-phase and multi-phase events, the geometry and durations of the latter being similar to those observed on MCS data from the Western North Atlantic. In addition, the lateral variation in Moho travel time, up to 0.25 s over distances of less than 10 km, is similar to that observed on MCS data. The similarities between the observed and synthetic data suggest that the complicated interlayered sequences of mafic and ultramafic rocks that comprise the inferred crust/mantle transition in ophiolites might also be characteristic of the oceanic crust. Although ophiolites provide a useful model of the lithological structure of the oceanic crust, the unambiguous correlation of geologic and seismic structures can only be achieved by conducting seismic experiments in the vicinity of deep crustal drillholes. Chapters 2 and 3 of this dissertation present analyses of the velocity and reflectivity structure of the crust in the immediate vicinity of Deep Sea Drilling Project Hole 504B in the Panama Basin, currently the deepest drillhole (1.288 km) into oceanic igneous crust. Reflectivity synthetic seismogram modeling of amplitude features common to four sonobuoy profiles collected in the immediate vicinity of Site 504B shows that crustal thickness at the drillsite is only 5 km. A critical constraint on this interpretation is the observation, on four MCS profiles passing through the drillsite, of a near-normal-incidence reflection event with a crustal travel time of 1.4-1.5 s. This event is assumed to correlate with a wide-angle reflection/refraction event observed at ranges of 16-28 km on the sonobuoy profiles. Seismic modeling demonstrates that both of these events are generated at the Moho. The crustal velocity-depth profile at Site 504B is unusual in comparison to typical oceanic profiles in having high velocity gradients (up to 0.6 km s- 1 km- 1 ) in the middle crust and a 1.8 km thick low-velocity zone (Vp=7.1-6.7 km s- 1 ) immediately above Moho. A simple explanation for this unusual profile is that the velocity of the middle crust has been increased by the addition of a high-velocity mineral component such as olivine. The olivine concentration of the middle crust need be no greater than 34-37%. Hole 504B is the only site where the volcanics/sheeted-dike boundary, predicted by the ophiolite model to Qe a fundamental feature of oceanic crust, has been drilled. The downward change in rock type coincides with changes in a variety of logged physical properties. The normal-incidence travel time to this boundary is similar to the travel times of shallow reflection events observed in other areas. Accordingly, Site 504B is an ideal location to test the hypothesis that shallow reflection events correlate with the extrusives/dike boundary. Despite extensive processing, MCS data collected 'in the immediate vicinity of Hole 504B show no conclusive evidence for a laterally coherent reflection event generated within the upper crust. The lack of a detectable reflection event from the upper crust is consistent with the results of synthetic seismogram modeling of velocity-depth profiles constructed from the logged downhole variation in physical properties. On these normal-incidence synthetic seismograms, low-amplitude reflections from the volcanic/dike contact are obscured by the high-amplitude basement reflection and by sediment-column multiples. In contrast to the synthetic reflection data, the seismic signature of the volcanics/dike boundary is readily recognizable on a synthetic wide-angle reflection/refraction profile. The change in velocity across this boundary causes focusing of refracted arrivals in the range window 6-7 km. High-amplitude arrivals are observed at similar ranges on the sonobuoy profiles collected near the drillsite, suggesting that at Site 504B, variations in depth to this layer boundary are more easily mapped with the wide-angle reflection/refraction method.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October 1988
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