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Das, Triparna, 1997

Crustal structure of western North America using surface waves and marine multichannel seismic reflection data

Bibliographic Reference

Das, Triparna, 1997, Crustal structure of western North America using surface waves and marine multichannel seismic reflection data: Princeton, New Jersey, Princeton University, Ph.D. dissertation, 115 p., illust., 3 folded sheets.

Abstract

This thesis concentrates on crustal structure of western North America using two seismic techniques--seismic surface waves (Rayleigh waves) to study crustal thickness in western United States and multichannel seismic reflection data to study crustal structure in northern British Columbia and southeastern Alaska. An extension of the Partitioned Waveform Inversion technique to include inversion for depth of Moho is presented. The feasibility of Moho-depth inversion from seismograms of regional earthquakes is investigated. Study on synthetic data followed by testing on real broadband data in the western United States showed that it is possible to fit high-frequency Rayleigh waves to invert for crustal thickness. This is a simple, inexpensive method for determining crustal thickness independently of the crustal shear wave velocity, with a resolution of 1-2 km. Results from 200 such individual waveform fits on broadband Rayleigh wave data in the western United States constitute a completely new data set on crustal thickness in the region. The constraints on average crustal thickness obtained from these individual waveform fits are combined in a complete linear inversion to obtain a crustal thickness map of the western United States. Previously published seismic reflection/refraction studies as well as some receiver function analyses were used as additional constraints in the inversion. The result is a detailed regional crustal thickness map of western United States. The ACCRETE project acquired high-quality, deep seismic reflection data across the major terrane boundaries of the Pacific Northwest. We concentrate on some profiles in the northern end of the Tertiary Queen Charlotte Basin, which is thought to overlie the boundary between the Wrangellia and Alexander terranes, as well as the more recent Alexander - North America suture. Under the Alexander terrane, several bands of pronounced southwest dipping seismic reflections obliquely cut a generally highly reflective crust. It is concluded that these southwest-dipping reflectors starting at the base of grabens and ending within 1 second two-way travel time from a shallowing Moho reflections, are extensional structures and are possibly related to the late Miocene extension previously reported in the region. Abrupt termination of lower crustal and Moho reflections observed on two lines (1250 and QC88-07) crossing the surface expression of the Kitkatla fault implies that the Kitkatla fault is a significant crustal scale feature. It is possible that it is the boundary between the Alexander terrane and the Wrangellia terrane. Such abrupt termination of reflections on another line (1261) makes it possible to extend the Kitkatla fault almost to the southern tip of the Prince of Wales Island.

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