North-South Variations in Structure, Topography, and Melting Regime along the Ultra-Slow Spreading Red Sea Ridge by Emilie Elisabeth Bowman B.S. Geological Sciences (2017) University of Texas at Austin Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in partial fulfillment of the requirements for the degree of Master of Science in Geology at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2019 C 2019 Massachusetts Institute of Technology. All rights reserved. Author............................................Signature redacted Department of Earth, Atmospheric, and Planetary Sciences May 22, 2019 Signature redacted C ertified b y . ..................................................................... ..... .. .... ,. .. .- Oliver Jagoutz Associate o ssor of Geology Signature redacted hesis Supervisor Accepted by.................... ................................. Robert D. van der Hilst MASSACHUSES INSTITUTE Schlumberger Professor of Earth and Planetary Sciences Of T.ECHNOLOGY Department Head JUN 17 2019 LIBRARIES ARCHIVES 2 North-South Variations in Structure, Topography, and Melting Regime along the Ultra-Slow Spreading Red Sea Ridge by Emilie Elisabeth Bowman Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on May 22, 2019 in partial fulfillment of the requirements for the degree of Master of Science in Geology Abstract The Red Sea rift is a nascent ultra-slow spreading ridge superimposed on the Afar plume. Based on high-resolution seismic data, the southernmost (south of the Danakil rift at 17.05'N), southern (17.05-19.75 0N), and central (19.75-23.8 0N) segments display seafloor spreading that is anomalously magma-rich compared to other ultra-slow spreading centers. In contrast, the northern segment (23.8-280N) exhibits magma-poor extension along large-offset east- and west-dipping detachments. Sediment-corrected basement depths along the northern Red Sea reveal an axial valley as deep as the Gakkel Ridge (4200-5100 m). South of 19.75 0N, plume-supported axial shoaling matches that of adjacent parts of Arabia, Africa, and the Gulf of Aden. Geochemically, the southernmost Red Sea is the locus of plume-ridge interaction. Here, E-MORBs are enriched in alkali, incompatible, and light rare-earth elements. High mantle potential temperatures (Tp; 1326 5'C), melting pressures (12 0 kbars) and temperatures (1306 6'C), and fractionation pressures (5.3 1.6 kbars) calculated using the reverse fractional crystallization model of Brown (2019) suggest thickened oceanic crust created by high-degree partial melting of a plume-like source. North of the Danakil rift, Tp (1307+ 1"C) spans a narrow range and is within the range of ambient mantle. The southern Red Sea contains N- to E-MORB depleted in alkali, incompatible, and light rare-earth elements indicating limited mixing with Afar plume material, while the central segment is host to the most depleted magmas along the ridge (La/SmN 0.8). Within the southern and central regions, fractionation pressures (2.0 1.2 and 4.8 2.1 kbars, respectively) indicate lithosphere (5-15 km) thinner than that of normal ultra-slow spreading ridges (15-35 km). In the northern Red Sea, high Na8 and deep pressures of melting (10.4 1.4 kbars) suggest thickened lithosphere, undulations in which induce melt focusing into volcanic deeps. Based on these results, we propose that the Red Sea south of at least 26.5 0N is an oceanic spreading center. We find that anomalously magma-rich spreading in the central and southern segments cannot be related to the Afar plume. Instead, the Danakil rift diverts plume-related mantle flow northeast beneath Arabia. Thus, the southern and central Red Sea must be characterized by vigorous mantle upwelling that causes heightened melt production and lithospheric thinning. Thesis Supervisor: Oliver Jagoutz Title: Associate Professor of Geology 3 4 Table of Contents A b strac t ........................................................................................................................................... 3 Table of Contents ............................................................................................................................ 5 List of Figures ................................................................................................................................. 6 L st of Tables .................................................................................................................................. 6 1. Introduction................................................................................................................................. 7 2. Background and Previous W ork .............................................................................................. 8 2.1. Geologic Setting................................................................................................................... 8 2.2. The Red Sea ....................................................................................................................... 12 3. Structure of the Red Sea Rift .................................................................................................. 15 3.1. M aterials and M ethods....................................................................................................... 15 3.2. Southern Red Sea............................................................................................................... 15 3.3. Central Red Sea.................................................................................................................. 18 3.4. N orthern Red Sea............................................................................................................... 20 3.4.1. N ature of the Basem ent in the N orthern Red Sea ................................................... 26 4. Com position of A xial Basalts from the Red Sea Ridge....................................................... 31 4.1. Previous W ork ................................................................................................................... 31 4.2. Red Sea G eochem istry ....................................................................................................... 34 4.2.1. M ajor Elem ents...........................................................................................................34 4.2.2. M ajor Elem ent D ifferentiation................................................................................. 42 4.2.3. V ariations in M antle Source along the Red Sea Ridge.......................................... 43 5. Changes in M elting Regim e along the Red Sea Ridge ......................................................... 50 5.1. Results of the Maximum Likelihood Reverse Fractional Crystallization Model ....... 52 5.1.1. Calculated Prim ary M elt Com positions.................................................................. 61 5.1.2. Major Element Oxide Compositions of Basalts from the Red Sea Compared to Other U ltra-Slow Spreading Ridges ............................................................................................... 63 6. The Effect of Plume-Ridge Interaction on the Topography of the Red Sea and Surrounding R e g io n s ......................................................................................................................................... 6 4 7. D iscussion................................................................................................................................. 70 7.1. The N orthern Red Sea as an U ltra-Slow Spreading Ridge ............................................ 71 7.2. Central and Southern Red Sea ....................................................................................... 79 7.3. Plume Influence Beneath Ramad Seamount and the Southern Islands .......................... 84 7.4. Role of the D anakil Rift in D iverting Plum e Flow ......................................................... 85 8. Conclusions............................................................................................................................... 86 References ..................................................................................................................................... 88 5 List of Figures Figure 1. M ap of the R ed Sea region ........................................................................................... 9 Figure 2. Map of the southern and central Red Sea................................................................. 17 Figure 3. M ap of the northern Red Sea..................................................................................... 21 Figure 4. Interpretations of representative seismic lines from the Red Sea............................... 22 Figure 5. Topographic profile of the Red Sea axial ridge......................................................... 29 Figure 6. Red Sea axial basalt sample location map.................................................................. 35 Figure 7. Total alkali (Na20 + K 20) versus SiO 2 diagram...................................................... 36 Figure 8. K/Ti versus M gO diagram ......................................................................................... 38 Figure 9. M ajor elem ent oxides versus M gO............................................................................ 39 Figure 10. Variations in Na8 , K/Ti, Nb/Zr, and La/SmN of axial basalts along the Red Sea........ 44 Figure 11. Variations in isotopic signatures of axial basalts along the Red Sea .....................