Dissertation/Thesis Abstract

Nature of the crust and upper mantle beneath Northwest Basin-and-Range and East Africa using passive-source seismic data
by Gashawbeza Mengesha, Ewenet, Ph.D., Stanford University, 2009, 140; 3364518
Abstract (Summary)

In this thesis, I present my contribution towards the understanding of rifting processes responsible for the shaping of two tectonic environments. The first region of interest is the Basin-and-Ranges province, a region actively deforming in a wide rifting mode, and the second is the Main Ethiopian Rift, a region actively deforming in a narrow rifting mode.

The Basin-and-Range Province of western North America is one of the best regions in the world for the study of processes related to continental rifting. It is a broad and elevated region of intracontinental stretching currently extending in a wide rift mode. I primarily focused on the northwest corner of the Basin-and-Range province, which is a transition zone between a highly extended region in the east to a region where there is less extension in the west. The composition and structure of the crust beneath this region and its relationship with the upper-crustal extension is not adequately understood.

The Main Ethiopian Rift is part of the East African Rift System, which provides a modern analog to help us understand how continents break apart. The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. Seismic characteristics of the upper-mantle and its relationship with the current rifting processes and volcanism need to be understood.

In this research project I demonstrated the use of receiver functions, wide-angle refraction, shear-wave splitting, and tomographic approaches to find answers for the above outstanding questions.

For the Basin-and-Range province study I used teleseismic data collected by a Stanford University seismic experiment supplemented by data from nearby USArray and Berkeley digital stations to develop insights into the characteristics of the crust in the northwest corner of the Basin-and-Range province. I combined teleseismic waveform data from a PASSCAL short-period seismic network and the USArray transportable broadband stations to study crustal thickness variations and rock properties beneath the northwest Basin-and-Range region.

For the Main Ethiopian Rift study I used data collected during the EAGLE (Ethiopia-Afar Geoscientific Lithospheric Experiment) project and the EBSE (Ethiopian Broadband Seismic Experiment). I used shear-wave splitting measurements to understand the relationship between splitting parameters and interaction of Cenozoic rifting and magmatism with the older Precambrian lithosphere fabrics in Ethiopia.

Upper-mantle anisotropy may be caused either by present asthenospheric mantle flow or by frozen signatures of former deformation and stress conditions in the lithosphere. We find that the mantle anisotropy beneath East Africa is more dominated by deformational events in the Proterozoic times occurring within the lithosphere than the present-day processes in the sublithospheric mantle. Consequently, the distribution and magnitude of anisotropy provided valuable data to constrain the role of the mantle in the tectonic evolution of this region. In addition to the shear-wave splitting technique I also used seismic attenuation tomography and S-receiver function techniques to obtain more constraints to understand the tectonics in this region. My seismic attenuation study in Ethiopia helped to obtain images of laterally varying anelastic structure of the lithosphere. Using the S-receiver function technique, I measured the depth to the lithosphere-asthenosphere boundary in Ethiopia and its variation across and along the Main Ethiopian Rift.

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Indexing (document details)
Advisor: Klemperer, Simon
School: Stanford University
School Location: United States -- California
Source: DAI-B 70/06, Dissertation Abstracts International
Subjects: Geophysics
Keywords: Basin and Range Province, East African Rift, Mantle, Upper mantle
Publication Number: 3364518
ISBN: 9781109238242
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