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Dynamics of Mantle Flow Around the Azores Triple Junction: Constraints from Bathymetry and Gravity Data Ravi Darwin Sankar

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Dynamics of Mantle Flow Around the Azores Triple Junction: Constraints from Bathymetry and Gravity Data Ravi Darwin Sankar Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2009 Dynamics of Mantle Flow Around the Azores Triple Junction: Constraints from Bathymetry and Gravity Data Ravi Darwin Sankar Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCES DYNAMICS OF MANTLE FLOW AROUND THE AZORES TRIPLE JUNCTION: CONSTRAINTS FROM BATHYMETRY AND GRAVITY DATA by RAVI DARWIN SANKAR A Thesis submitted to the Department of Geological Sciences in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Spring Semester, 2009 Copyright © 2009 Ravi Darwin Sankar All Rights Reserved The members of the Committee approve the Thesis of Ravi Darwin Sankar defended on March 06th, 2009. Jennifer Georgen Professor Co-Directing Thesis Jim Tull Professor Co-Directing Thesis Vincent Salters Committee member William Parker Committee member Approved: __________________________________________________ Leroy Odom, Chair, Department of Geological Sciences The Graduate School has verified and approved the above named committee members. ii To the four pillars of my life: God, my wife and my parents. I am not skilled to understand what God has willed or planned but walking with Him and you through this journey has given me strength. Danika, we were joined in marriage during the course of this study. I love you. You mean everything to me, without your love, encouragement and understanding I would not be able to make it. My parents, you have given me so much. This is your degree. Mom, thanks for continuous faith in me, and for teaching me that I should never relent. Dad, you always told me to “never be satisfied with anything ordinary.” Thanks for inspiring my love for earth sciences. We made it…. For I know the plans I have for you, declares the LORD, plans to prosper you and not to harm you, plans to give you hope and a future Jeremiah 29:11-13 iii ACKNOWLEDGMENTS I would like to express my sincere appreciation to my advisor, Dr. Jennifer Georgen for her patience, understanding, professional and personal support throughout the duration of this project, as well as funding for the latter part of the research. Thanks for giving me the opportunity to be part of this work. It has been a tremendous honor working with you. You made excellence a habit. Thank you! My advisory committee: Dr. Jim Tull, Dr. Vincent Salters and Dr. William Parker. I admire your intelligence and leadership. I am grateful for your comments and suggestions. Dr Stephen Kish, thanks for your presence in the lab and your encouragement. I am eternally grateful to the United States Department of State for the award of the FULBRIGHT Faculty Development Scholarship. You allowed me to integrate with a nation, university, people and culture that I have come to love and appreciate. Jacqui Gregoire, Amy Whitish, Renee-Hahn Burke, thanks for the second chance. There are not enough words to describe your excellence. Pastor Matt and Laura Cates, thanks for your friendship and support. It was a tremendous pleasure serving in ministry with you. Great is your faithfulness oh God! Your grace is enough for me. Thanks for never forsaking me. Thank you Father, Son and Holy Spirit. I owe you everything! iv TABLE OF CONTENTS LIST OF FIGURES……………………………………..……………………………...vi ABSTRACT…………………………………………………………………………….vii 1. INTRODUCTION and MOTIVATION……………………………………………1 2. GEOLOGICAL SETTING OF STUDY AREA Mid-Atlantic Ridge……………………………………………………….3 The Azores Triple Junction……………………………………………….4 The Terceira Rift………………………………………………………….6 Mantle Plumes and the Azores Hotspot…………………………………..8 Alternatives to the Thermal Plume Hypothesis………………………….11 3. METHODOLOGY Bathymetry and Free-Air Gravity Data………………..………………...13 Mantle Bouguer Anomaly……………………………………………….14 4. RESULTS MBA Patterns along the Mid-Atlantic Ridge……………………………16 MBA Patterns for the Azores Archipelago and along the TR…………...18 5. DISCUSSION Azores/Terceira Rift Waist Width…………………………………….....20 Influence of Triple Junction Geometry…………………………………. 22 6. CONCLUSIONS……………………………………………………………………..24 7. APPENDIX A………………………………………………………………………...25 8. REFERENCES……………………………………………………………………….37 9. BIOGRAPHICAL SKETCH………………………………………………………..42 v LIST OF FIGURES Figure 1: Location of primary study area in reference to the Mid-Atlantic Ocean. Base- map produced from data obtained by Smith and Sandwell (1997)………………...…….25 Figure 2: Map of the ridge-ridge-ridge (RRR) Azores Triple Junction indicating the branch locations of the Mid-Atlantic Ridge and the Terceira Rift, in reference to the Azores hotspot…………………………………………………………………………...26 Figure 3: Simplified Azores Triple Junction geometry and ridge configuration, indicating variations in ridge spreading rate and plate motion with respect to the triple junction.....27 Figure 4: Regional location map of the Azores plateau indicating the existing position of the triple junction, the islands comprising the Azores volcanic archipelago as well as the major fracture zones in the region……………….............................................................28 Figure 5: Isotope systematics of Terceira Rift lavas…………………………………….29 Figure 6: Averages of crustal thickness and Na8 at various mid-ocean ridges, as well as for the Azores region confirming the negative correlation of Na8 with Fe8………………..............................................................................................................30 Figure 7: Regional scale bathymetric map for the Azores Triple Junction, indicating shiptrack coverage, V-shaped bathymetric features as well as the location of the Azores hotspot, in reference to the triple junction………………………………….....................31 Figure 8: Free-air anomaly (FAA) map for the Azores region, reflecting the density contrast of the seafloor in the area……………………………………………………….32 Figure 9: Map of Mantle Bouguer anomaly correction, generated from bathymetry data……………………………………………………………………………………….33 Figure 10: Mantle Bouguer anomaly (MBA) map, calculated by subtracting the mantle Bouguer correction from FAA, while assuming a constant 5 km thick reference crust………………………………………………………………………………………34 Figure 11: Axial bathymetry profile (11a) and filtered MBA profile (11b) along the Terceira Rift, with increasing distance from the Azores Triple Junction……………..........................................................................................................35 Figure 12: Along-isochron widths of residual bathymetric anomalies W, versus half- spreading rates U (12a). Figure 12b is a plot of along-isochron amplitudes of MBA plotted against ridge-hotspot distances…………………………………………………..36 vi ABSTRACT Mid-ocean ridge interactions with hotspots strongly affect mantle flow processes. This study analyzes the geophysical anomalies produced as a result of the interaction between a hotspot and an oceanic ridge-ridge-ridge triple junction, in close proximity to one another. The complex three dimensional (3D) nature of the Azores Triple Junction (ATJ), in which two near-collinear faster-spreading ridges are joined orthogonally with a slower-spreading ridge, provides an excellent opportunity to quantify the effect of triple junction geometry on along-axis magmatic accretion and mantle dynamic processes as a result of the interaction with a hotspot. For the ATJ, the faster-spreading ridges are two branches of the Mid-Atlantic Ridge (MAR), and the slower-spreading ridge is the Terceira Rift (TR). Using shipboard bathymetry and satellite free-air gravity data, we obtain mantle Bouguer anomaly (MBA) by eliminating from free-air gravity the attractions of seafloor topography and a reference crust. Along the TR, the Azores hotspot has a maximum MBA axial gravity low of –100 mGal, suggesting localized crustal thickening, elevated mantle temperatures and/or low density mantle. The entire Azores plateau along the TR is associated with a large (~ -80 mGal) broad MBA low. Dispersion of plume material along the TR, a distance in the range of 550 km, is likely minimized by the rift system’s obliqueness, immature nature and ultra-slow spreading rate, as well as the presence of the Gloria FZ. Further, along-axis profiles along the TR suggest that MBA shows a strong dependence on the tectonic segmentation of the ridge axis. vii CHAPTER 1 INTRODUCTION AND MOTIVATION This investigation focuses on the dynamics and generalized characteristics of mantle flow processes in the vicinity of the Azores Triple Junction (ATJ). The ATJ is an oceanic ridge-ridge-ridge (RRR) triple junction formed at the intersection of the North American (NA), African (AF) and Eurasian (EA) plates. The ATJ is made up of northern and southern branches of the Mid-Atlantic Ridge (MAR) and a third branch to the east of the MAR referred to as the Terceira Rift (TR). The TR represents a unique oceanic rift system situated within thickened, relatively old oceanic lithosphere and it exhibits both oceanic and continental features (Beier et al., 2008). The Azores archipelago is the surface manifestation of a hotspot that has come into contact with a mid-ocean ridge. Studies have shown that where a ridge and a plume coincide, a large quantity of melt production in the mantle, amplified crustal thickening, and unusually robust seafloor volcanism take place. The stable configuration of the ATJ offers an exceptional opportunity to study the interaction of three spreading centers with varying divergence rates in close proximity to
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