A SEISMIC INVESTIGATION OF CRUST AND UPPER MANTLE STRUCTURE BENEATH THE ZAGROS MOUNTAINS AND THE SOUTHERN AND EAST AFRICAN PLATEAUS
ABSTRACT
This thesis addresses crustal and upper mantle structure in three regions in and around the African Superswell: the Zagros Mountains, southern Africa, and the East African Plateau, each representing a different tectonic regime. In the Zagros Mountains of southwestern Iran, source mechanisms for six moderate-sized earthquakes are investigated using a combination of moment tensor inversion and depth phase analysis. The six earthquakes that are studied were reported in global earthquake catalogs as having lower crustal or upper mantle source depths, but upon further study, it was found that all six nucleated within the upper crust. This finding contributes to a growing number of studies indicating that seismicity in the Zagros Mountains is limited to the upper crust.
In both southern and eastern Africa, upper mantle structure was investigated to evaluate the thermal state of the upper mantle and implications for the source of uplift in each area. Rayleigh wave phase velocities were measured for these regions using a two plane wave approximation method and were then inverted for a quasi-three dimensional shear wave velocity model. In southern Africa, it was found that the lithospheric lid structure and the sublithospheric velocity reduction for the Kaapvaal Craton is comparable to the upper mantle structure beneath other Archean Cratons. Thus, little seismic evidence was found of an upper mantle thermal anomaly sufficient to support high elevations in that area. In East Africa, evidence was found for a broad thermal anomaly across the study region that extends from the base of the lithosphere into the iv transition zone. This anomaly is larger than previous studies have indicated and its presence and size imply a possible connection to the African Superplume.
TABLE OF CONTENTS
LIST OF FIGURES ………………………………………………………………………………………..viii
LIST OF TABLES………………………………………………………………………………………….xiv
ACKNOWLEDGEMENTS……………………………………………………………………………..xv
Chapter 1 Introduction ……………………………………………………………………………………1
Chapter 2 Source Parameters for Moderate Earthquakes in the Zagros Mountains
with Implications for the Depth Extent of Seismicity …………………………………..8
2.1 Abstract …………………………………………………………………………………………….8
2.2 Introduction ……………………………………………………………………………………….8
2.2.1 Geologic Setting ………………………………………………………………………….11
2.3 Datasets & Methodology …………………………………………………………………….15
2.3.1 Datasets ……………………………………………………………………………………..15
2.3.2 Methodology ………………………………………………………………………………15
2.4 Discussion and Conclusions ………………………………………………………………..18
2.5 References …………………………………………………………………………………………21
Chapter 3 Phase Velocities and Shear Wave Velocity Structure of the Southern
African Upper Mantle ………………………………………………………………………………25
3.1 Abstract …………………………………………………………………………………………….25
3.2 Introduction ……………………………………………………………………………………….26
3.3 Background ……………………………………………………………………………………….27
3.3.1 The Geology of Southern Africa ……………………………………………………27
3.3.2 Previous Studies of Southern Africa ………………………………………………30
3.4 Phase Velocity Inversion …………………………………………………………………….37
3.4.1 Data Processing …………………………………………………………………………..37
3.4.2 Inversion Methodology ………………………………………………………………..42
3.4.3 Phase Velocity Results …………………………………………………………………45
3.5 Inversion for Shear Velocity Structure ………………………………………………….53
3.5.1 Method of Inversion for Shear Wave Velocities ……………………………..54
3.5.2 Shear Wave Velocity Results ………………………………………………………..56
3.6 Discussion …………………………………………………………………………………………61
3.6.1 Review of Shear Wave Velocities and Models ………………………………..61
3.6.2 Comparison to Previous Studies ……………………………………………………62
3.6.3 Comparison with Other Cratons ……………………………………………………66
3.7 Summary and Conclusions ………………………………………………………………….69
3.8 References …………………………………………………………………………………………71
Chapter 4 Upper Mantle Shear Wave Velocity Structure Across East Africa from
the Inversion of Rayleigh Wave Phase Velocities ………………………………………..77
4.1 Abstract …………………………………………………………………………………………….77
4.2 Introduction ……………………………………………………………………………………….78
4.3 Background ……………………………………………………………………………………….79
4.3.1 The Geology of East Africa ………………………………………………………….79
4.3.2 Previous Studies of East Africa …………………………………………………….83
4.4 Phase Velocity Inversion …………………………………………………………………….91
4.4.1 Data Processing …………………………………………………………………………..91
4.4.2 Model Parameterization and Inversion Methodology ……………………….96
4.4.3 Phase Velocity Results …………………………………………………………………101
4.5 Shear Velocity Inversion …………………………………………………………………….109
4.5.1 Inversion for Shear Wave Velocity ………………………………………………..109
4.5.2 Shear Wave Velocity Results ………………………………………………………..111
4.6 Discussion …………………………………………………………………………………………118
4.7 Summary and Conclusions ………………………………………………………………….128
4.8 References …………………………………………………………………………………………130
Chapter 5 Conclusion ……………………………………………………………………………………..138
Appendix A Supplementary Figures for Chapter 2 ……………………………………………..141
Appendix B List of Events Used for Surface Wave Tomography in Southern Africa …………………………………………………………………………………………………….147
Appendix C Supplementary Figures for Chapter 3 ……………………………………………..151
Appendix D List of Events Used for Surface Wave Tomography in East Africa ……186
Appendix E Supplementary Figures for Chapter 4 ……………………………………………..192
Chapter 1
Introduction
In this thesis, I investigate crustal and upper mantle structure in three separate tectonic regimes within the Afro-Arabian region to advance our understanding of processes that lead to rifting, uplift, and seismicity within and surrounding the African Superswell (Figure 1-1). The depth extent of seismicity and implications for lithospheric rheology in the convergent margin of the Zagros Mountains are presented in Chapter 2. In Chapter 3, upper mantle shear wave velocity structure beneath the Kaapvaal Craton and surrounding mobile belts in southern Africa is examined to determine if there is seismic evidence for an upper mantle thermal anomaly supporting the Southern African Plateau, and in Chapter 4, upper mantle structure beneath East Africa is imaged to evaluate candidate geodynamic models for the uplift of the East African Plateau.
Figure 1-1: Locations of study areas throughout the Afro-Arabian region discussed in this thesis.
In Chapter 2, I use a combination of moment tensor inversion and depth phase modeling to determine source parameters of six moderate-sized earthquakes in the Zagros Mountains. These events were reported as having lower crustal or upper mantle source depths by global earthquake catalogs. Previous studies, however, have debated whether the lower crust and upper mantle beneath the Zagros Mountains might be aseismic, deforming ductiley. I find that upon detailed study, all six events appear to have nucleated in the upper 11 km of the crust, lending support to the assertion that seismicity in the area is limited to the upper crust, and suggesting that global catalogs may overestimate source depths in this region.
Chapter 3 is an investigation of the upper mantle beneath southern Africa, focusing on the upper mantle beneath the Kaapvaal Craton. Some studies have suggested that the high elevations found throughout southern Africa may be supported by buoyancy due to a thermal anomaly in the upper mantle that appears seismically as a low velocity zone (e.g., Priestley, 1999; Li and Burke, 2006; Priestley et al., 2006; Wang et al., 2008). Other studies, however, have not found evidence for a low velocity zone in the upper mantle indicative of a thermal anomaly (e.g., Zhao et al., 1999; Freybourger et al., 2001; James et al. 2001; Saltzer, 2002; Fouch et al., 2004; Larson et al., 2006; Chevrot and
Zhao, 2007; Hansen et al., 2009). With a combined dataset from the Southern African Seismic Experiment, AfricaArray and Global Seismic Network stations, I build a quasithree dimensional shear wave model by inverting phase velocities measured using a two plane wave approximation method (Forsyth and Li, 2005). I compare the shear wave velocity structure beneath the Kaapvaal Craton to velocity structure beneath other Archean Cratons, and find that the velocity reduction beneath the Kaapvaal Craton lithosphere is comparable to that beneath other Archean cratons, indicating that there is little seismic evidence for an upper mantle thermal anomaly beneath the Kaapvaal Craton supporting the Southern African Plateau.
In Chapter 4, I apply the same methodology from Chapter 3 to eastern Africa to address the origin of the Cenozoic rifting and uplift of the East African Plateau by imaging the lateral and depth extent of low velocities beneath the plateau. In this area, the East African Rift System bifurcates around the Tanzania Craton. The entire region is characterized by anomalously high topography, which is likely supported by buoyancy from a thermal anomaly in the upper mantle. Evidence for the upper mantle thermal anomaly comes from seismic, petrological, and gravity studies (e.g., Ritsema et al., 1998; Simiyu and Keller, 1997; Chesley et al., 1999; Owens et al., 2000; Nyblade et al., 2000; Nyblade and Brazier, 2002; Weeraratne et al, 2003, Park et al., 2006). However, the depth extent, lateral extent, and source of the thermal anomaly is uncertain. I use data from the Tanzania Broadband Seismic Experiment together with new data from the AfricaArray East African Seismic Experiment to build a quasi-three dimensional shear wave model from our measured phase velocities. Because of the inclusion of new data, I am able to image structure across the entire southern East African Plateau, which has not been possible in previous studies. I find evidence of a broad low velocity zone underlying the entire region, which extends to and possibly into the mantle transition zone. This indicates that the low velocity anomaly beneath eastern Africa is broader than has been proposed in previous studies.
Chapters 2, 3, and 4 are each written in the format of independent journal articles. For this reason, there is some repetition of content in the description of methodologies and broad scale African geology in chapters 3 and 4. Chapter 2 was published in the Bulletin of the Seismological Society of America (Adams, A., R. Brazier, A. Nyblade, A.
Rodgers, and A. Al-Amri, 2009, Source Parameters for Moderate Earthquakes in the
Zagros Mountains with Implications for the Depth Extent of Seismicity, Bulletin of the Seismological Society of America,99, 2044-2049). Included in this thesis are five appendices, designed to provide additional information on each chapter that is not suited to the journal article format. Appendix A contains supplemental figures for Chapter 2, which were included as online supplements to Adams et al. (2009). Appendix B includes a list of events that were used for the surface wave tomography in southern Africa presented in Chapter 3. Appendix C includes additional figures and resolution tests for southern Africa that are not shown in Chapter 3. Appendix D is a list of the earthquakes used for the surface wave tomography in eastern Africa (Chapter 4), and Appendix E contains additional figures not included in that chapter.
References
Adams, A., R. Brazier, A. Nyblade, A. Rodgers, and A. Al-Amri (2009), Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity, Bulletin of the Seismological Society of America,99, 2044-2049
Chesley, J., R. Rudnick, and C. Lee (1999), Re-Os systematics of mantle xenoliths from the East African rift: Age, structure and history of the Tanzania craton: Geochimica et Cosmochimica Acta, 63, 1203-1217
Chevrot, S. and L. Zhao (2007), Multiscale finite-frequency Rayleigh wave tomography of the Kaapvaal craton, Geophysical Journal International, 169, 201-215
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A SEISMIC INVESTIGATION OF CRUST AND UPPER MANTLE STRUCTURE BENEATH THE ZAGROS MOUNTAINS AND THE SOUTHERN AND EAST AFRICAN PLATEAUS