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Goodwin, E.B., 1989

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Bibliographic Reference

Goodwin, E.B., 1989, Studies of extension and crustal accretion in the Basin and Range Province and Alaska from vertical-incidence and wide-angle seismic reflection profiling: Palo Alto, California, Stanford University, Ph.D. dissertation, 205 p., illust., maps.

Abstract

This thesis presents the results of four seismic reflection studies with the purpose of improving our knowledge of the structure and composition of the crust and hence out understanding of the geologic processes that have modified it. In Chapter 1 we present synthetic seismogram modeling recently collected P- and S-wave seismic reflection data which suggests that the intrabasement Bagdad reflectors in west-central Arizona are intrusive mafic sheets. The identification is made based on the seismic polarity of the reflections and the absence of S-wave reflections on coincident three-component data. If the Bagdad reflectors are Cenozoic-age mafic sheets, they may be the product of rapid intrusion into an area of weak extension where the principal stresses are approximately equal. In Chapter 2, coincident P- and S-wave data are used to determine the composition of the lower crust and assess the role of crustal underplating in the Basin and Range - Colorado Plateau transition zone. The main P and S observations for the lower crust include high-amplitude P-wave reflections, moderate-amplitude S-wave reflections, an average P-wave velocity of 6.6 km s-1, and a Poisson's ratio of [sigma] = 0.27. We argue for an active mantle in the Transition Zone, but emphasize that most of the extension-related melts have remained in the upper mantle or are confined to a thin zone at the base of the crust. Chapter 3 presents the results of synthetic seismogram modeling of the subhorizontal and laterally discontinuous reflections that appear to characterize the crust of highly extended terranes. This synthetic modeling study based on data from the Picacho Mountains metamorphic core complex in Arizona suggests that the reflectivity results from alternating layers of high and low velocity that extend laterally for about 500 m. Sources for the reflectivity include fracture zones, compositional layering of mafic and granitoid rocks, and zones of velocity anisotropy. Chapter 4 presents seismic data from a 160-km-long refraction profile across the Wrangellia terrane in southeastern Alaska. The data show that the Wrangellia terrane extends to at least 10 km depth, that Wrangellia and adjacent terranes are cross cut at about 23 km depth by a low-velocity zone, that the crust may be as thick as 55 km, and that the P-wave velocities in the upper 23 km of the crust are appropriate for intermediate-composition plutonic rocks. This last finding, together with new geologic mapping in the Wrangellia terrane, suggests that Wrangellia was constructed on pre-existing transitional/continental crust rather than oceanic crust as was previously thought.

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