Lithospheric Structure of North Africa and Western Eurasia

Open Access
Author:
Kosarian, Minoo
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 13, 2006
Committee Members:
  • Charles James Ammon, Committee Chair
  • Sridhar Anandakrishnan, Committee Member
  • Kevin Patrick Furlong, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • Crustal thickness
  • Shear velocity structure
  • Surface wave dispersion
  • Receiver functions
  • Poisson`s ratio
Abstract:
Although much progress has been made over the last few decades towards understanding the structure of the Earth, many questions regarding the details of Earth's lithospheric structure remain unanswered. The primary goal of this study is to gain a better understanding of upper and lower continental crustal composition and structure to improve our knowledge of the tectonic evolution of the Earth. To contribute this goal, the present study focuses on the estimation of first order seismic structure using receiver functions, and the construction of a library of shear-velocity structures in the vicinity of seismic stations across western Eurasia and north Africa using receiver functions and tomography based surface wave dispersion estimates. One hundred and seventy one stations recording a total of about 6,000 teleseismic events producing more than 100,000 seismograms have been investigated. The distribution includes 78 stations in the Middle East and Asia, 57 stations in Europe, and 36 stations in central and north Africa. I have examined receiver functions for 119 stations with the best data for the period of 1990-2004 and applied the receiver function stacking procedure of Zhu and Kanamori (2000) to estimate Poisson's ratio and crustal thickness. The research area is divided to five tectonics environments, explicitly Shields, Platform, Paleozoic orogenic belts, Mesozoic-Cenozoic orogenic belts, and rift zones based on Condie's (1989) simplified classifications. The results from this study show a slightly lower value of Poisson's ratio s = 0.25 for Shield compared to the Orogenic-belts with s = 0.26 Crustal thickness H ranges from 32-47 km with an average of 38 km and standard deviation of 3 km for the shields. The less well sampled platforms show a wider distribution of crustal thickness, ranging from 30-58 km with an average 42 km and a standard deviation of 9 km. Orogenic regions also show a wide variation of crustal thickness, with thickness values from 20 to 55 km and standard deviations in the range of 8-10 km. Since the ultimate goal of this study is to provide an improved imaged of global continental structure and composition, I combine observations obtained in this study with receiver functions results from other published analysis. In total, I have integrated observations from 606 stations located in different geologic settings. The compiled results show a value of s = 0.26 for Poisson's ratio and H = 39 km for crustal thickness in Shields and Platforms, and s = 0.26-0.27 with H = 35-37 km for the Orogenic belts. The results show that previously suspected correlations between Poisson's ratio and tectonic age or crustal thickness are more complex - regional variations are more important. Comparison with the standard reference seismic model Crust 2.0 shows that it systematically underestimates crustal shear velocities. A detailed analysis of data from southern Algeria, near the central Hoggar uplift, indicates a significant crustal root beneath the mount Tahat, consistent with underplating of the lower crust. Converted phases from the transition zone suggest relatively low upper mantle velocities to a depth near 410 km, but a strong, azimuthal consistent arrival from 660 km suggests that boundary is relatively unperturbed.