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North-South Variations in Structure, Topography, and Melting Regime along the Ultra-Slow Spreading Red Sea Ridge

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

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

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 Red 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 ...... 47 Figure 12. Variations in K/Ti and Nb/Zr ratios of axial basalts along the Gulf of Aden ...... 48 Figure 13. Diopside-olivine-quartz ternary diagram illustrating correction of erupted basalts to prim ary m elts using the Brown (2019) m odel ...... 53 Figure 14. Results from the reverse fractional crystallization model plotted against latitude along th e R ed S ea ridg e ...... 5 6 Figure 15. La/SmN versus Sm/YbN and Sm/NdN versus Lu/HfN of Red Sea ridge basalts...... 58 Figure 16. Histograms of calculated mantle potential temperatures in the Red Sea ...... 60 Figure 17. A1203 versus CaO and Na20 versus FeO of calculated primary magmas ...... 62 Figure 18. Map showing the location of transects along swells and extent of plume topography in th e Red S ea reg io n ...... 6 7 Figure 19. Topographic profiles of the northern African, southern African, Arabian, Gulf of A den, and R ed Sea sw ells...... 68 Figure 20. Schematic along-axis cross section of the Red Sea...... 76 Figure 21. Na8 vs. melting pressure of axial basalts along the Red Sea ...... 78

List of Tables

Table 1. Depth to basement measurements and sediment-corrected bathymetry in the northern R ed S ea ...... 2 8 Table 2. Thermobarometry results for axial basalts along the Red Sea...... 55

6 1. Introduction

At mid-ocean ridges, plate separation causes mantle upwelling and volcanism that results in the formation of new oceanic lithosphere and prolonged oceanic spreading, but the processes that control the formation of new oceanic spreading systems remain unconstrained. Previous studies in the Red Sea and other immature ocean basins have produced a range of interpretations, from wholesale nucleation of individual ridge segments (e.g. Taylor et al., 1995) to spreading initiation within individual cells centered along the ridge axis and separated by attenuated continental crust (e.g. Bonatti 1985).

In addition, more than 20 hot spots are spatially associated with mid-ocean ridges, and approximately one-fifth of the global ridge system displays chemical (e.g. alkali, light rare-earth element, and/or incompatible element enrichments; Campbell, 2001) and physical properties (e.g. anomalously shallow bathymetry, enhanced volcanism, thickened crust, few transform faults; e.g.

Schilling, 1973; Verma et al., 1983; Ito and Lin, 1995; Miller et al., 1998; Lithgow-Bertelloni and

Silver, 1998; Dick et al., 2003) clearly influenced by proximity to a hot spot (Ito et al., 2003).

The Red Sea, a nascent ocean basin formed by the separation of Arabia from Africa, provides an excellent opportunity to study the development and onset of seafloor spreading above an active mantle plume. With full spreading rates less than 20 mm/year, the Red Sea is an ultra- slow spreading ridge (Dick et al., 2003). According to Augustin et al. (2016), the Red Sea displays features characteristic of its ultra-slow spreading rates (deep axial valley, focused magmatism in dome-shaped volcanoes, few transform faults, prevalence of nontransform discontinuities/accommodation zones), and all anomalous features can be attributed to the Red

Sea's proximity to the Afar plume.

7 In this paper, we investigate the onset and development of seafloor spreading in the Red

Sea, including the relative influence of spreading rate and plume-ridge interaction along the length of the rift axis. This study is based on analysis of geochemical data in addition to new structural analyses of high-resolution bathymetry and seismic data that cover the eastern (Saudi Arabian) half of the Red Sea. These latter data are proprietary. We thus present interpretations or schematic depictions as representative of these data, although we use publicly available bathymetry to corroborate our interpretations. We find that the northern Red Sea is undergoing magma-poor seafloor spreading, while the central and northern half of the southern Red Sea (17.05 to -23.3"N) exhibit magma-rich spreading that is not associated with the Afar plume. In order to determine the source of magma-rich spreading within these regions, we use the maximum likelihood reverse fractional crystallization model of Brown (2019) to investigate north-south variations in the melting regime along the ridge and to better constrain the thermal structure of the mantle beneath the Red Sea. We conclude that the entirety of the Red Sea has geochemical and topographic characteristics and melting regimes that indicate enhanced extents of partial melting and anomalously buoyant upwelling compared to other ultra-slow spreading ridges.

2. Background and Previous Work

2.1. Geologic Setting

The Red Sea is part of a rift-rift-rift triple junction and converges with the Main Ethiopian

Rift (MER) and the western Gulf of Aden in the region of Afar (Fig. 1; McKenzie and Morgan,

1969; Dewey and Bird, 1970). This triple junction lies above the Afar plume and sits within the triangle-shaped Afar depression. The northern vertex of the Afar depression is defined by the

Danakil rift zone, which is most likely the current locus of extension rather than along the axis of

8 300N- Gulf of Aqaba- Legend Boundary between southern and central Red Sea Midyan Zabargad Fracture Zone (ZFZ) Inter-trough zone - Plate boundary Hijaz Neoproterozoic shear zones and prominent foliation 25oN- * q V % strikes from Johnson et al. - (2011) Arabian-Nubian Jiddah Boundary between Neoproterozoic terranes Shield from Johnson et al. (2011) Ad Damm fault zone - Barka shear zone, boundary between the southern and ebeit northern Arabian-Nubian 20oN- Shield (Johnson et al., 2011) Harrat HaaAsir Haya

4300 m 15oN-Nu Nu i I Tokar/Barka

tiob lOoN A 100N- XiisA Kharshirukalla . Al I rqah f.

Main 5oN -5400m ft

300E 35oE 400E 45oE 500E

Figure 1. Map of the Red Sea region showing the different rift systems involved in the Afar Triple Junction. The Arabian-Nubian Shield (ANS) is composed of Neoproterozoic structures (Johnson et al., 2011) that have been shown to influence the evolution of the Red Sea rift. Suture zones that formed during the amalgamation of the ANS define boundaries between tectonic terranes. Tectonic terranes (such as Haya) display different structural grains. The variety of orientations of Neoproterozoic structures surrounding the Red Sea region and affecting the underling basement contribute to the diverse rifting evolution of this area.

9 the southernmost Red Sea (Prodehl and Mechie, 1991; Bosworth et al., 2005). A NNE-trending line of seismicity indicates that the active Danakil rift intersects the southern Red Sea rift axis at

~-17.05 0 N (Chu and Gordon, 1998).

Volcanism related to the Afar plume began as early as 45 Ma (Davidson and Rex, 1980;

Ebinger and Sleep, 1998; Vetel and Le Gall, 2006), but arrival of the Afar plume head at 31 Ma resulted in an acceleration of volcanic activity in the form of voluminous flood basalts in Ethiopia,

Yemen, and surrounding regions that lasted until 25 Ma (Hofmann et al., 1997). Possibly influenced by the plume's impingement, rifting of the Gulf of Aden began by the Early Oligocene

(-30 Ma; Hughes et al., 1991; Storey, 1995; Bosworth et al., 2005). The MER was not active until later, with the northern MER developing at 11 Ma (Wolfenden et al., 2004).

Formation of the Red Sea via breakup of the Arabian-Nubian Shield (ANS), which formed during the East African Orogen (715-530 Ma) as an amalgamation of sutured Neoproterozoic island arc terranes, may have begun as early as 34 Mya (Omar and Steckler, 1995). However, apatite fission track and (U-Th)/He dating of rift shoulder uplift indicate that significant continental extension in the Red Sea began in the early Miocene between 25-20 Ma (Bohannon, 1989; Omar et al., 1989; Omar and Steckler, 1995; Ghebreab et al., 2002; Szymanski et al., 2016). Tectonism during this time was characterized by rift-normal extension and rift-margin uplift (Bosworth,

2015). In the central to northern Red Sea, diffuse continental rifting began in extensional half- grabens, as opposed to the initial rift system in the southern Red Sea, which was significantly narrower with extension only occurring within the present-day coastal rift escarpments (Szymanski et al., 2016; Stockli and Bosworth, 2019). This discrepancy may have been caused by N-S variations in thermal structure along the Red Sea. In the south, the Afar plume may have facilitated thermal weakening of the lithosphere and induced focused extension (Stockli and Bosworth,

10 2019). Rifting in the northern Red Sea may have been controlled by pre-existing structures such as the NW-trending Neoproterozoic Najd fault system (Makris and Rihm, 1991; Cochran, 2005;

Stockli and Bosworth, 2019). Either way, the N-S transition from distributed to localized extension

is apparent in the present-day morphology of the Red Sea margins: a high-topography escarpment runs parallel to the southern Red Sea coastline, but diminishes north of Jeddah.

Early continental rifting was accompanied by magmatism along the Arabian margin in the form of Red Sea-parallel dike intrusions (-24-21 Ma; Feraud et al., 1991; Sebai et al., 1991;

Bosworth and Stockli, 2016) and NW-trending volcanic complexes, known as Harrats (30-20 Ma;

Camp and Roobol, 1992). During this time, rift-related volcanism on the Arabian margin was predominantly tholeiitic (Camp and Roobol, 1992). The lack of syn-rift magmatic activity along the African margin of the Red Sea is often cited as evidence for asymmetric rifting and simple shear accommodated by detachment faulting (Wernicke, 1985; Voggenreiter et al., 1988).

Systematic seafloor spreading in the Gulf of Aden developed by -17 Ma (Leroy et al.,

2004). Soon after, the sinistral NNE-trending Gulf of Aqaba-Levant transform developed as

Arabia collided into Eurasia at 14 Ma (although some estimates place transform initiation at -10

Ma; Reilinger and McClusky, 2011; Bosworth et al., 2005). Displacement along this new plate boundary dramatically reduced extension along the Gulf of Suez, originally the northernmost extent of the Red Sea (Moretti and Colletta, 1987; Steckler et al., 1988). Slip along the Gulf of

Aqaba-Levant transform resulted in a transition from orthogonal to oblique rifting and promoted the formation of NNE-trending transform zones, such as the Zabargad Fracture Zone (ZFZ) in the northern Red Sea (Szymanski et al., 2016; Stockli and Bosworth, 2019). Between 12-10 Ma, a second period of major uplift resulted in the formation of the Red Sea escarpment (Girdler and

Southren, 1987; Bayer et al, 1988; McGuire and Bohannon, 1989; Bohannon et al., 1989). Also

11 during this time, the locus of extension in the south shifted from the southernmost Red Sea to the

Danakil rift (Bosworth et al., 2005). Harrat volcanism began again and continued to the present, with a peak after 5 Ma (Camp and Roobol, 1992). This late Miocene to Present volcanism can be linked to an Afar plume-related thermal mantle anomaly imaged by seismic tomography that is migrating N-S across the Arabian plate (Hansen et al., 2007; Chang and Van der Lee, 2011). In fact, the younger Harrats trend N-S and have plume-like alkaline basaltic compositions (Camp and

Roobol, 1992).

2.2. The Red Sea

The Red Sea is floored by Miocene-age (25-20 Ma) evaporite sequences deposited during continental rifting as a result of multiple seawater incursions and subsequent evaporation (Girdler and Whitmarsh, 1974). These deposits consist of massive halite +/- anhydrite up to 7 km thick

(Lowell and Genik, 1972; Whitmarsh et al., 1974). This sequence is often unconformably overlain by a thin layer of Plio-Pleistocene carbonate and siliciclastic deposits, although rifting-induced salt diapirism and gravity-driven halokinesis in the form of allochthonous salt sheets have disturbed the stratigraphy (Mart and Hall, 1984; Mitchell et al., 2010; Tubbs et al., 2014).

The Arabia-Nubia Euler pole is located in northwest Egypt (Chu and Gordon, 1 998;

McClusky et al., 2003; ArRajehi et al. 2010) and results in counter-clockwise rotation of Arabia relative to Nubia and much faster rates of opening in the southern Red Sea (16 mm/yr full spreading rate at 180 N; Chu and Gordon, 1998) relative to the north (-10 mm/yr at 25.5' N). In addition, the off-axis Afar plume, which impinges the base of the lithosphere of the Ethiopian

Plateau, has been shown to produce geochemical (e.g. Schilling et al., 1992; Volker et al., 1997), topographic (e.g. Daradich et al., 2003), and morphological anomalies (e.g. Augustin et al., 2016) in the southern Red Sea. The Red Sea rift is thus divided into three regions from south to north

12 based on its stage of development (Cochran et al., 1986). The southern Red Sea (south of 19.75'

N) is characterized by continuous seafloor spreading along a well-developed axial ridge (Roeser,

1975). Oceanic spreading began ~5 Ma (Cochran 1983; Gurvich, 2006) at 170 N, although some authors argue for 10-12 Ma as the first occurrence of true oceanic basalts (Izzeldin, 1987; Augustin et al., 2014; Augustin et al., 2016). Despite its ultra-slow spreading rates, the southern Red Sea ridge is a magma-rich spreading segment, leading to the suggestion that topography and volcanic activity along the ridge is enhanced by proximity to the Afar plume (e.g. White and McKenzie,

1989; Ebinger and Sleep, 1998; Daradich et al., 2003; Mohriak, 2014). Basalts of the axial valley south of the Danakil rift show geochemical signatures similar to those of the Afar plume, and are enriched in incompatible elements and range in composition from E-MORB to alkaline basalts

(Altherr et al., 1988; Altherr et al., 1990; Volker et al., 1997; Haase et al., 2000). In addition, the

Red Sea escarpment runs along the Arabian coast of the southern Red Sea.

The central Red Sea (19.75 to -23.3"N) is thought to be in an intermediate stage characterized by discontinuous seafloor spreading (Searle and Ross, 1975; Bonatti, 1985). In this region, axial accretion of oceanic crust initiated between 3 and I Ma in discrete bathymetric deeps greater than 2000 m in depth (Searle and Ross, 1975; Bonatti, 1985; Ligi et al., 2012). These oceanized cells are floored by basalts of N-MORB affinity, indicating hot asthenosphere at depth capable of high degrees of partial melting (Altherr et al., 1988; Haase et al., 2000). Nodes of seafloor spreading are separated by comparatively shallower intertrough zones (ITZs) covered by downslope evaporite flows, known locally as submarine namakiers (Searle and Ross 1975;

Augustin et al., 2014). ITZs display gravity and magnetic signatures clearly different from those of the ridge (e.g. Searle and Ross 1975; Ligi et al., 2012). Thus, many authors propose that the basement underlying ITZs is composed of thinned continental lithosphere that rifts apart as

13 neighboring cells expand and merge into a continuous oceanic ridge (Bonatti, 1985; Cochran,

2005; Ligi et al., 2012). Other authors contend that the large-scale salt glaciers that make up ITZs in the central Red Sea actually cover an otherwise tectonically continuous spreading segment

(Searle and Ross, 1975; Girdler, 1985; Izzeldin, 1987; Sultan et al., 1992; Augustin et al., 2014), and favor the interpretation that Red Sea deeps south of the ZFZ represent second-order segments offset by nontransform discontinuities (Augustin et al., 2016).

Finally, the northern Red Sea rift zone (-23.8 to 280N) appears to have only limited syn- rift volcanism and is comparatively magma-poor (Cochran, 2005). However, the rift zone is entirely obscured by the thick evaporite layer and only a shallow axial depression -1200 m deep and 10-25 km wide is discernible (Girdler and Styles, 1974; Searle and Ross, 1975; Cochran and

Martinez, 1988; Ehrhardt and HUbscher, 2015). It thus remains unclear whether extended continental crust (e.g. Cochran, 1983; Bonatti, 1985; Cochran and Martinez, 1988; Cochran, 2005;

Mitchell and Park, 2014) or an axial trough of oceanic crust (e.g. Girdler, 1985; Gaulier et al.,

1988; Sultan et al., 1992) lies beneath the evaporite layer. Furthermore, the northern Red Sea is composed of two sections. From -23.8 to -25.25"N, the trough trends N-S along the ZFZ, a magnetically quiet and amagmatic incipient transform fault that offsets the ridge axis by -95 km

(Bonatti et al., 1984; Ligi et al., 2012; Stockli and Bosworth, 2019). The ZFZ may have formed from the influence on rifting of the Neoproterozoic Oko-Hamisana shortening zone (Dixon et al.,

1987; Crane and Bonatti, 1987; Johnson et al., 2011). Further north, from -25.25 to 280 N, the axial depression trends NW-SE and contains scattered deeps that are often shallow relative to those in the central Red Sea (Cochran, 2005). The three largest deeps in this region (Mabahiss, Shaban, and Conrad deeps) are volcanically active and have been geophysically imaged to confirm the presence of magmatism (e.g. Guennoc et al., 1988; Ehrhardt et al., 2005). Although Mabahiss deep

14 may be floored by oceanic basement (Guennoc et al., 1990), Shaban and Conrad deeps probably do not have any surficial expression of basement structure (Ehrhardt et al., 2005; Ehrhardt and

Htibscher, 2015). Samples obtained from Mabahiss and Shaban deeps are low degree, E-MORB- like melts that may indicate a melting regime influenced by lower spreading rates, the presence of cool, shallow mantle, and a potentially enriched source (Altherr et al., 1988; Altherr et al., 1990;

Haase et al., 2000). The presence of mid-ocean ridge basalts and hydrothermal activity within the northern Red Sea deeps is evidence for sub-salt magmatism, possibly indicating incipient oceanic spreading (Stockli and Bosworth, 2019).

3. Structure of the Red Sea Rift

3.1. Materials and Methods

This section presents interpretations based on proprietary high-resolution bathymetry (- m cell size) of the eastern (Saudi Arabian) half of the Red Sea from 28 to 21.3 0N. These data were used to map the structural, volcanic, and sedimentary characteristics of the eastern Red Sea. We use publicly available bathymetry data from GEBCO_2014 (30 arc-second interval grid; https://www.gebco.net) and from POSEIDON cruise POS408 and PELAGIA cruises 64PE350 and

64PE351 (1 arc-second; Augustin et al., 2014) to illustrate our observations and to allow interpretation of areas outside our high-resolution data coverage. Structural mapping of the Red

Sea was aided by examination of proprietary dip- and strike-oriented 2D seismic data of the eastern

Red Sea from 28 to 160N. Due to confidentiality reasons, only interpretations of representative seismic lines are shown and discussed in this report.

3.2. Southern Red Sea

15 The Red Sea is unique in that the northern, central, and southern segments display very different segmentation and structural styles (for a more in-depth analysis of segmentation and morphology of the Red Sea ridge, see Cochran (2005) and Augustin et al. (2014)).

The southern Red Sea (south of 19.750 N) is relatively continuous, with only small offsets occurring along transform faults as a result of hot, low viscosity mantle and accompanying thin lithosphere (Fig. 2a; Schilling et al., 1992). This area is also marked by a prominent long- wavelength axial high that rises from -2,000 m below sea level (BSL) in the north to 300 m BSL in the south (Fig. 5). This axial rise is related to the Afar plume swell and is discussed further in

Section 6. Associated with this topographic rise is the nearly constant high positive free air gravity anomaly that characterizes the active spreading center of the southern Red Sea ridge (e.g. Almalki et al., 2015). The southern Red Sea is most comparable to the Reykjanes ridge, which is also continuous, magma-rich, and slow-spreading as it shallows toward the Icelandic hotspot (Talwani et al., 1971; Vogt and Avery, 1974; Searle et al., 1998).

The southernmost Red Sea rift is currently undergoing a ridge jump toward the Afar plume, with extension occurring predominantly along the Danakil rift (Eagles et al., 2002). North of this intersection, the ridge assumes its characteristic behavior of active spreading along right-stepping en-echelon ridge segments. We thus divide the southern Red Sea into two sections: north and south of the Danakil rift (17.050 N). The latter consists of Ramad seamount at ~ 16.90 N and numerous axial volcanic islands (from north to south: Jebel at Tair, the Zubair group, and the Hanish-Zukur group). However, the ridge axis in this region has been accommodating only limited extension since 10 Ma, with modern spreading and rifting occurring predominantly on the Danakil rift to the west. The volcanism in this domain can instead be attributed to proximity to the Afar plume.

16 24eN- Legend Legend Boundary between southern Ier Volcano ~pand central Red Sea he- Zabargad Fracture Zone Hummocky volcanics dee Inter-trough zone (ITZ) --- Ridgeaxis Tranform faults - 220N- Non-transform discontinuities Seismic lines shown in Figure 4 _Boundary between At Neoproterozoic terranes from Johnson etal. (2011) Hat a AdDamm fault zone -Barka shear zone, boundary - - 0 20 N between the southern and northern Arabian-Nubian Shield (Johnson et al., 2011

Central RedI Harrat - Sea T

0 18 N- -Z

AtlantiA

-2850 m e-

0 360E 38-E 400E 42 E

Figure 2. a) Map of the southern and central Red Sea. The southern Red Sea is offset by small transform faults, including where the Danakil rift is inferred to intersect the Red Sea ridge axis. The southern and central segments are separated by the fossil Neoproterozoic Barka-Ad Damm suture zone. b) Zoom-in of the central Red Sea. Offsets in this region occur predominantly along nontransform discontinuities. Intertrough zones (ITZ) mark locations of incipient transform faults with valleys obscured by salt flowage. High-resolution bathymetry data from Augustin et al. (2014) was used to map the ridge axis and nontransform discontinuities in this region.

17 The structure of the southern segment has been robustly shown to involve organized seafloor spreading. An interpretation of a representative across-axis seismic image of the southern

Red Sea (Fig. 2a) is shown in Figure 4a. Subsidence of oceanic crust in accordance with half-space cooling is enhanced by loading of the newly formed basement by migrating salt sheets to depths as deep as 6750 m. The basement is offset by numerous east- and west-trending normal faults, resulting in rotated blocks of variable width (~0.5-2 km) that give the oceanic basement a high- frequency trough-and-ridge topography. Furthermore, while discussion of the nature of the continent-ocean transition (COT) in the Red Sea is beyond the scope of this paper, we note that a distinct continent-ocean boundary (COB) is not apparent in the seismic line.

3.3. Central Red Sea

Segmentation and morphology of the Red Sea changes at around 19.75" N. North of this latitude, the central Red Sea (Fig. 2a-b), with spreading rates of 10.8-13.8 mm/yr (Chu and Gordon,

1998), is composed of a series of axial bathymetric deeps floored by oceanic crust at depths of

-2000 m. These deeps are separated by shallow ITZs covered by namakiers. Augustin et al. (2014) argues that these areas are floored by oceanic crust and are loci of large-scale sediment and evaporite slumping.

Across-axis seismic lines show that the structure of the crust in the central Red Sea greatly resembles that of the south (Fig. 4b). Basement continuously subsides with distance from the axial ridge, a process that occurs within oceanic crust as it moves laterally away from the axis and gradually cools. Further east, oceanic crust subsides to depths of up to 6500 m until proximal west- dipping normal faults disrupt subsidence and uplift the basement to levels of thinned continental crust. We interpret the uplifted basement -80 km east of the ridge as the landward limit of "pure" oceanic crust. Izzeldin (1987) also observed the COB around 80 km on either side of the axial

18 ridge at 19"N. The presence of a transition from oceanic to continental crust along seismic lines in the central Red Sea is consistent with the later initiation of organized seafloor spreading in this domain (<0.78 to ~-1-3 Ma; Roeser, 1975; Bicknell et al., 1986; Ligi et al., 2012).

Thus, we conclude that the entire central Red Sea is undergoing oceanic spreading.

Namakier flows in ITZs blanket volcanic ridges and fault scarps, and covered volcanoes can be inferred beneath these deposits (Augustin et al., 2014). Areas that are not obscured by salt have continuous rift valleys. Because the resolution of bathymetry data in this region is significantly higher, the ridge axis was drawn to follow the trend of axial volcanic ridges (AVRs), which are offset in en-echelon, partially overlapping patterns (Fig. 2b; Augustin et al., 2016). This second- order segmentation is common in other ultra-slow spreading ridges (Carbotte et al., 2015). Because they deepen at their ends, second-order segments produce oblique-trending lineaments of older basins over time (MacDonald, 1988) that are most likely covered by salt in the Red Sea. Due to the apparent lack of spreading-perpendicular valleys with considerable offset, Augustin et al.

(2014) argue that the central Red Sea is devoid of transform faults and is instead a continuous oceanic super-segment offset only by overlapping non-transform discontinuities. However, across

ITZs, the axial ridge is offset by 5-30 km. Along-axis seismic lines that intersect ITZs show that they are grabens ~4000 m deep bounded by north- and south-dipping normal faults. As a result,

ITZs may represent 20-30 km-wide incipient transform valleys that induce gravity-driven halokinesis into the deepest parts of the basin. For comparison, transform valleys along the ultra- slow spreading Southwest Indian Ridge can be as wide as 30 km and as deep as -6000 km.

The transition from the southern to central Red Sea is marked by a nontransform discontinuity, which correlates with the position of the boundary between the northern and southern ANS (formed by the Barka suture on the Nubian plate and the Ad Damm fault on the

19 Arabian plate) if projected across the basin (El-Isa and Shanti, 1989; Johnson et al., 2011).

Likewise, the northern tip of Nereus deep, which is the northern extent of unequivocal oceanic spreading (Pautot, 1983; Bicknell et al., 1986), correlates with the location of the Yanbu-Sol

Hamed-Allaqi-Heiani suture zone on land (Fig. 1). These patterns indicate that preserved

Neoproterozoic basement structures may be influencing the evolution of rifting and defining major boundaries within the rift system (Manatschal, 2004). Pre-existing structures on the Arabian-

Nubian shield further impact segmentation along the central Red Sea. For example, ITZs and nontransform discontinuities are oriented parallel (NE-SW) to the structural trends of the Hijaz,

Jiddah, and Gebeit terranes.

Lastly, as mentioned above, the axial ridge in this region is anomalously shallow (~2000 m; Fig. 5). Because the central Red Sea is spreading at ultra-slow rates, the axial valley should resemble that of the Gakkel Ridge and the Southwest Indian Ridge (SWIR), and should be around

4200-5100 m deep. Therefore, the question remains as to what processes could be causing these anomalously shallow depths.

3.4. Northern Red Sea

Due to the prevalence of thick, high-velocity evaporites, geophysical imaging of the northern Red Sea (Fig. 3) has remained of poor quality. Here, we present two interpretations (Fig.

4c-d) of high resolution across-axis seismic images that illustrate two main types of crustal architecture in the NW-trending half of the northern Red Sea. Interestingly, crustal architecture continuously alternates between these two types. As such, based on basement structure, the northernmost Red Sea can be divided into five zones (indicated by arrows in Fig. 3). In three domains along the axis, crustal structure is dominated by tilted fault blocks offset along a system

20 Legend O Axial salt-walled mini-basins 0 Volcanically active deep - -ZabargadFractureZone (ZFZ) Dip of major detachment

---- Inferred rift axis ....- Inferred transform zones rqCe rpd Ma' TZ1I-TZ4 Location of depth to basement measurement N-N9 volcano 27 0N ) A Seismic lines shown in V Figure 4 N3 Boundary between Neoproterozoic terranes 4f from Johnson et al. (2011) -4W Harrat TZ1 4 4# N 5

260N - TZ2

T5 3 N F

4400 mMs

250Nf nR e n Ns ad Fracture Zone

-2850 m

340E 350E 360E 370E

Figure 3. Zoom-in of the northern Red Sea. Points Nl-N9 represent locations of depth to basement measurements used to calculate the depth of the axial ridge if it were exposed on the seafloor. Dashed lines TZl-TZ4 are transfer zones that separate regions of east- and west-dipping detachment faults. Also highlighted is the location of the marginal basin, which to the west borders a high-standing terrace.

21 FV

WA Red Sea E 00 -Axial ridge Southern Salt body sag basin- 3000: s000- Layered evaporites 0 7000 9000- Oceanic crust 11000 km

Central Red Sea 100 Axial ridg

3000 s 000- Layered evaporites--

7000

tA Oceanic crust 90, i 9000Transitional Ga b E5k crust

Northern Red Sea: zone of west-dipping detachment faulting depression _000-- Axial IV -I,"----'I / Salt body 3000- Marginal fault- s000 Layered evaporites .Uplifted basement 0 beneath terraces bounded graben 7000: -, 'COT? 9000 Newly accreted 211000- C crust 5 km

Northern Red Sea: zone of east-dipping detachment faulting Axial depression

00 -Layered evaporites Marginal fault- 0 bounded graben

Uplifted basement Uplifted basement 9000 Uplifted basement beneat terac 9000 beneath terrace Newly accreted beneath terrace 1000.d crust |sk Legend Fault Transitional crust - Base of sag basin Seafloor spreading- Salt body 'S'reflector related crust

Figure 4. Interpretations of representative seismic lines from the southern (a), central (b), and northern (c-d) Red Sea super-segments. Seismic images are proprietary. The southern and central segments (a-b) are characterized by abundant small-offset normal faults while the northern Red Sea is composed of a series of large-displacement west (c) and east-dipping (d) detachment systems and associated horsts and grabens. COT = continent-ocean transition.

22 of basement-involved, east-dipping large-displacement normal faults (Fig. 4d). For example,

basement in Fig. 4d is offset by two major east-dipping normal faults. These faults are listric

(downward-flattening and concave upward), and most likely rotate to low angles as they sole into

a detachment fault within the lithosphere (Manatschal, 2004). Along the eastern flank of the axial

rift, the westernmost major detachment fault produces a subhorizontal domal structure that

resembles an extinct oceanic core complex truncated by a west-dipping normal fault (Smith et al.,

2006). This detachment system is superimposed by a number of small-offset normal faults. In

addition, in between the three east-dipping domains, crustal structure along the rift is characterized

by moderately to highly tilted fault blocks offset by major west-dipping, high-angle listric normal

faults that probably sole into detachment surfaces at lithospheric depths (Fig. 4c). Overall, we note that, in both lines, deep grabens define the basement beneath mini-basins within the axial depression. To the east of the axial depression, a series of large-scale horsts and grabens is produced by movement along major detachment faults, which produce high- and low-standing basement cut by in- and out-of-sequence faults. Based on these observations, the evolution of the

Red Sea is predominantly driven by tectonic processes, and specifically by multiple detachment faulting. Our interpretations are consistent with those of Wernicke (1985) and Voggenreiter et al.

(1988), who argue that the asymmetry in uplift and volcanism along the Red Sea rift margins could result from simple shear accommodated along a single low-angle detachment surface within the lithosphere.

We interpret transitions in basement architecture as incipient transfer zones that trend parallel to the Gulf of Aqaba-Levant transform. The locations of these zones are consistent with other previously defined boundaries in the northern Red Sea, such as those determined by Cochran

(2005). For example, Gaulier et al. (1988) placed the boundary between northern continental

23 rifting and southern oceanic spreading through Brothers Islands, near the location of Transfer Zone

2 (TZ2). In addition, Ehrhardt et al. (2005) observed a NE-SW line of seismic events along an offset in the axial depression at ~26.9' N. They related this seismicity to a transform fault in the northern Red Sea, which we have now identified as Transfer Zone 1 (TZ I). TZI lies just south of

Conrad deep and trends parallel to the basin. We thus attribute the opening of Conrad deep to an extensional component along the transform. Furthermore, interpreted locations of transfer zones are also consistent with the morphology of basin fill and salt geometry throughout the northern

Red Sea (Fig. 3).

Although the northern Red Sea rift axis is buried beneath a package of evaporites, a series of deeps that extend from the ZFZ to the entrance of the Gulf of Aqaba provide insight into the trend of the rift axis (Fig. 3). Deeps in this area are much shallower (1300-1500 m) and are only a few km across in size (Cochran, 2005). One exception is Mabahiss deep, a -15 km wide and -2300 m deep fault-bounded bathymetric low located at the top of the N-S trending ZFZ. Mabahiss deep has been interpreted to be a volcanically active pull-apart basin (Pautot et al., 1986; Guennoc et al., 1988; Guennoc et al., 1990). The majority of deeps in the northern Red Sea, such as Kebrit deep in the ZFZ, are sediment-floored salt-walled mini-basins formed either from differential sediment loading and subsequent salt withdrawal, evaporite dissolution due to high heat flow and water penetrating along faults, or basement faulting-triggered halokinesis (Heaton et al., 1995;

Mitchell et al., 2010; Banham and Mountney, 2013). An argument for the latter is illustrated in

Figure 4c-d, where large-displacement normal faults sometimes define the distal limits of individual allochthonous salt sheets. In the NW-SE-trending half of the northern Red Sea, mini- basins within the axial depression are elongate perpendicular to spreading direction and are interpreted to represent the location of the buried rift axis (Fig. 3). Beginning at the intersection of

24 the Red Sea with the Gulf of Aqaba, a series of deeps define an arcuate ridge axis. This curved trace of mini-basins in the northernmost Red Sea most likely represents a synthetic horsetail structure developed at the southern termination of the Gulf of Aqaba-Levant transform. Because this strike-slip fault is left-lateral, the northernmost rift segment must be accommodating extension along an east-dipping detachment fault. This is consistent with the crustal structure in this area

(Fig. 3), which we interpreted to be dominated by east-dipping normal faults.

Furthermore, to the east of the axial depression, two sets of large-displacement detachment faults seen in both seismic lines uplift off-axis basement to shallow depths, while in the hanging wall down-dropped basement often spatially corresponds to basins on the seafloor. Likewise, uplifted crust forms prominent horsts that manifest on the surface as terraces within the basin fill.

Off-axis volcanoes (Fig. 3) are located on the edges of fault-uplifted terraces because magmas ascend along these fault surfaces (Cochran, 2005; Standish and Sims, 2010). These off-axis volcanoes are either located on the edges of the first terraces within the axial depression (Cochran,

2005) or are located on the east and west edges of the highest-standing terrace in the east (~500-

600 m deep; Fig 3). Perhaps these regions of shallow basement along E-W transects of the Red

Sea represent portions of exhumed mantle (Dick et al., 2003). In this case, highly reflective top- basement throughout the seismic lines may represent an overlying basaltic carapace. In addition, to the east of the highest terrace on the Saudi Arabian side of the Red Sea, there is a basin that spans, although discontinuously, the NW-trending half of the northern Red Sea and is deepest between 27.1 and 26.40N (Fig. 3). This basin is subdued to nonexistent on the Egyptian side of the

Red Sea. This marginal basin corresponds to the large near-symmetrical landward basement graben bounded by normal faults that face the basin center and progressively down-drop tilted fault blocks. Interestingly, this proximal graben observed along both seismic lines is situated at the

25 oblique intersection between the seismic lines and our inferred transfer zones. Thus, deep basement in these regions may be related to transform valley development.

3.4.1. Nature of the Basement in the Northern Red Sea

The top of crystalline basement consistently extends to great depths (~6000-10000 m) beneath the deepest part of the axial depression encountered along each seismic line (Fig. 4c-d), although we also observe comparably deep basement off-axis. We note that the northern Red Sea is opening at full spreading rates (5.6 1 mm/yr at 27 N to 10 1.6 mm/yr at 25.5 N; McClusky et al., 2003; Chu and Gordon, 1998) comparable to those of the Gakkel ridge (14.6 mm/yr in the west to 6.3 mm/yr in the Laptev Sea; DeMets et al., 1994). The Gakkel ridge is characterized by an axial valley around 4200-5100 m deep, although shallows to as low as 3,500 m at volcanic centers (Coakley and Cochran, 1998; Michael et al., 2003). Thus, if the northern Red Sea is a true oceanic spreading center, and if the axial depression was not obscured by basin fill, we would expect a deep rift valley at depths similar to those of the Gakkel Ridge. In order to approximate the depth of the Red Sea rift valley if basin fill were removed, we took a series of dip lines (Nl-

N8) that intersect the axial depression north of Mabahiss deep and recorded depth to basement with respect to the seafloor beneath the axial depression along each line. We also recorded depth to basement at one point (N9) in the eastern Zabargad Fracture Zone. Locations of measurements are shown in Figure 3. At some points (N4, N6, and N9), we also documented the thickness of sediment above the 'S' reflector, a distinct seismic reflector that marks the unconformity between the evaporite formation and overlying sediments (Phillips and Ross, 1970).

To calculate the depth of the ridge valley if exposed at the seafloor, we assume that the basement is entirely overlain by evaporites (except for those points with sediment thickness data), and thus depth to basement represents the thickness of the evaporite formation. In order to remove

26 the basin fill overlying the crust and replace it with water, we must assume Airy isostatic compensation. Along the Red Sea axis, isostatic disequilibrium can be accomplished by 1) the creation of dynamic topography in response to viscous stresses caused by upwelling of mantle plume material (Daradich, 2003) and 2) the presence of rift valleys, which have been shown to be isostatically non-compensated (Cochran, 1979). In the northern Red Sea, the former is negligible, as the axial rise produced by the Afar plume diminishes north of 19.75 N (discussed in Section 6;

Fig 19). In addition, any inaccuracy introduced via assuming isostatic compensation of rift valleys will most likely be within error of our measurements ( 1 000 m; Canales et al., 2002). We can thus assume local Airy isostasy to calculate the depth of the sediment-free axial ridge (corrected bathymetry) at each point. Assigning a compensation depth within the asthenosphere, we using the equation

corrected bathymetry = bathymetry + ts * (P Ps) + te * (P -P Pa - Pw Pa - Pw where ts is the thickness of the sediment layer when recorded, te is thickness of the evaporite formation, pa is the density of the asthenosphere (pa = 3300 kg/m3), ps is the density of the Plio-

Pleistocene sediment (ps = 2400 kg/m 3), Pe is the density of the Miocene evaporite formation (Pe=

2200 kg/M 3 assuming halite is the dominant phase; Ellis and Singer, 2008), and pw is the density of water (p,= 1000 kg/m 3). The results are summarized in Table 1.

We compute depths of the axial ridge that range from ~3350 to 5750 m below sea level.

These depths are consistent with that of other ultra-slow spreading ridges, such as the Gakkel ridge

(~4200-5100 m deep; Fig. 5). In our calculation of the axial ridge depth, varying the density of the evaporite sequence to account for layered anhydrite and pre-evaporite sediments consistently yields average axial ridge depths that exceed 4000 m. We hypothesize that in the north, where the

27 Table 1. Locations of depth to basement measurements along the axis of the northern Red Sea, thicknesses of evaporite sediment packages at each location, and sediment-corrected bathymetry calculated assuming isostatic equilibrium.

LaiueLniue Actual Seiet Eaoie Total Corrected Measurement Latitde Longitude Tdimentm)SeBahmtry ThEvorite Basin Fill Bathymetry (m) Thickness (m) (m)

NI 27 34.93 -1181 9500 9500 -5724

N2 26.91 35.14 -1197 7600 7600 -4832

N3 26.69 35.10 -1132 7600 7600 -4767

N4 26.46 35.24 -1250 250 5700 5950 -4074

N5 26.18 35.45 -1115 8000 8000 -4941

N6 26.08 35.53 -1000 375 9625 10000 -5750

N7 25.95 35.61 -1151 4600 4600 -3351

N8 25.71 35.83 -1241 6000 6000 -4111

N9 24.41 36.53 -1224 910 7620 8530 -5224

28 Ramad &Southern Islands Southern Red Sea Central Red Sea Northern Red Sea

Ramad Radial dista nce from Afar plume (km) 200 400 seamou nt600 800 1000 1200 1400 1600 1800 2000 0' I I I -100+ v~v~ I I I I E) -2000- 00"* E -3000 e Zaborgad I I I I f.Z. 4-0 -4000 'U | I QI I Depth range of the Gakkel Ridge axial valley i l. -5000 0 I I -600 14 16 18 20 22 24 26 28 Latitude (ON) Figure 5. Topographic profile of the Red Sea axial ridge plotted against latitude and radial distance from the Afar plume. Small black points in the central and southern Red Sea represent the topography of the oceanic crust-floored axial valley, while points in gray mark the topography of basin fill covering the rift axis in the northern and central Red Sea. Larger black dots in the northern Red Sea are evaporite- and sediment-corrected basement depths indicating the depth of the ridge axis if it was exposed on the seafloor. We interpret these depths as representing the true topography of the rift axis in the northern Red Sea. These depths correlate very well with the depth range of the Gakkel ridge axial valley. Thus, ultra-slow seafloor spreading along the northern Red Sea most likely occurs within a deep axial ridge valley. Dashed lines mark the locations of transfer zones (TZ 1 - TZ4) that separate regions of east-dipping and west-dipping detachment faulting.

29 measured heat flow is very high (200-300 W/m 2 ), continental crust cannot exist at these deep depths. In order to test this, we employ a simple isostatic balance to quantify how deep the incipient ridge must be once continental crust thins to 0 km. For example, if continental crust is thinned by

50%, its elevation should also decrease by 50%. Based on this principle, we assume local isostatic equilibrium and use the equation

do -dR d, d-dR

10 - 'R 11 ~ 'R where do and l are the elevation and thickness, respectively, of the initial continental crust, d1 and

1i are the elevation and thickness, respectively, of the thinned continental crust, and dR is the depth of the ridge once continental crust has thinned to 0 km. The Egyptian coast of the northern Red

Sea has a crustal thickness (lo) of -26 km at an elevation of -300 m (do) above sea level (Hosny and Nyblade, 2016). Further east, according to Gaulier et al. (1988), 7-8 km thick (1i) crystalline continental crust sits at a depth of 8 km (di) beneath sea level. In order to compare the depth of continental crust on land to that covered by basin fill within the Red Sea, we water-load the coast of Egypt and isostatically remove sediment from the continental crust within the western Red Sea.

Using the above equation, we calculate that at a continental crustal thickness of 0 km, the axial ridge should form a valley 4600-4800 m below sea level. This range of depths for the axial valley floor agrees with those that we record from seismic analysis.

In Figure 5, the calculated depths of the ridge valley if exposed at the seafloor are compared to a topographic profile of the Red Sea. Areas covered by basin fill are marked in gray. We interpret points Nl-N9 as representing the true topography of the ocean- and/or mantle-floored axial northern Red Sea. We note that our inferred transform zones are always accompanied by a large- offset uplift or down-drop of top-basement. In some areas, the northern Red Sea basement extends to depths as great as 5750 m. These deep areas may be part of immature transform valleys. For

30 example, point N1, which is at a depth of -5720 m, is located just south of Transform Zone 1.

Similarly, deep basement within the ZFZ is further evidence that this N-S striking domain most

likely represents an incipient transform fault with a developing transform valley. Point N6 is also

unusually deep, and may indicate an area where anomalous lithospheric stresses produced by a

change in the thermal structure of the lithosphere result in deep valleys (Phipps Morgan et al.,

1987; Eberle and Forsyth, 1998). Shallower areas, such as N7, most likely reveal volcanic centers

(Michael et al., 2003). In fact, just east of the rift axis at this latitude, a small volcano crops out of

the basin fill within the axial depression. Interestingly, Mabahiss deep and associated seamount,

when compared to surrounding basement topography, rise -2000-3000 m above average basement

depths. This is consistent with previous interpretations that Mabahiss deep is a large faulted-

bounded basin, which provides a locus for mantle upwelling and volcanism. If Mabahiss deep

represents a shallow crustal exposure and is a region of melt focusing, then its shallow depths may

in part be due to thicker crust (Cannat et al., 2003). Like Mabahiss seamount, large localized

volcanic constructions along the Gakkel ridge can be as wide as 35 km and rise up to 2500 m above

the average depth of the axial valley (Michael et al., 2003). Shaban and Conrad deeps may be

younger centers of melt focusing, with volcanic activity just barely cropping out of the salt.

4. Composition of Axial Basalts from the Red Sea Ridge

4.1. Previous Work

Basalts from the Red Sea ridge show N-S variations in composition from incompatible

element-enriched MORB (E-MORB) in the north and south to comparatively depleted normal-

MORB (N-MORB) in the central Red Sea (Altherr et al., 1988; Volker et al., 1993). Because fractionation-corrected Na20 (Na8) increases toward the north, this compositional variability has been attributed to changes in degree of partial melting caused by the increase in spreading rate

31 toward the south (Altherr et al., 1988; Haase et al., 2000; Augustin et al., 2014). Basalts from

Shaban and Mabahiss deeps also have MORB-like Nb/U values, precluding contamination by crustal assimilation (Haase et al., 2000). Compared to other Red Sea MORBs, basalts from the

Ramad seamount located along the southernmost Red Sea axis have the most plume-like isotopic signatures (Volker et al., 1993). In Pb-Pb, Sr-Pb, and Nd-Pb spaces, Ramad seamount and Red Sea ridge MORB form a linear array suggestive of mixing between a depleted (central Red Sea N-

MORB) and an enriched HIMU (Ramad seamount E-MORB) source (Volker et al., 1997). Basalts from the islands south of the Ramad seamount have similarly enriched isotopic signatures with values skewed toward those of Yemen volcanics, illustrating the existence of a third mantle source.

Volker et al. (1997) also analyzed trace element ratios from the southern islands of the Red Sea and found no indication of crustal assimilation. These geochemical signatures indicate that not only changes in spreading rate but also the existence of regionally heterogeneous mantle control

N-S variations in basalt composition along the Red Sea ridge.

Systematic changes in the geochemistry of axial basalts from the Red Sea are similar to those along the Gulf of Aden. For example, Schilling et al. (1992) examined the isotopic signature of basalts along this ridge from the Afar triple junction to the Owen Fracture Zone. East of 480 E, axial basalts have depleted MORB compositions corresponding with an asthenospheric source.

Between 440 and 480E, incompatible element enrichment and HIMU-type isotopic signatures reflect the active influence of a mantle plume. In the Gulf of Tadjoura, west of 44'E, the Gulf of

Aden continues westward onto the African continent where considerable melting of a hybrid EMI -

EM2 source component, most likely sub-continental lithospheric mantle, results in incompatible element enrichment and radiogenic Pb and Sr isotopic ratios.

32 In addition, Volker et al. (1997) recognized that HIMU-type compositions are most evident in basalts from Ramad and Jizan at 17'N and decrease to the north. South of Ramad and Jizan, a transition to hybrid EMI-EM2 compositions is apparent in volcanic rocks from Zubair and Hanish islands. These patterns are consistent with Schilling et al. (1 992)'s torus plume model, where the plume influence is concentrated at 17'N and decreases to the north and south. In this model, ambient mantle is assimilated into the core of an ascending plume head, and as a result original plume material is forced into a donut-shaped torus around the plume axis.

Estimates of mantle potential temperature (Tp) are useful in quantifying plume-related thermal anomalies. Tp is the hypothetical temperature of the solid adiabatically convecting mantle if it were to reach the surface of the Earth unmelted (McKenzie and Bickle, 1988). Mantle plumes create large mantle thermal anomalies (AT) between 160-3000C (e.g. Sleep, 1990; Schilling, 1991;

Herzberg and O'Hara, 2002; Putirka, 2005) compared to ambient mantle (1350 50 0C; Courtier et al., 2007; Herzberg et al., 2007; Katsura et al., 2010). At mid-ocean ridges, Tp has been calculated by many authors to range from 1260-1295'C (McKenzie and Bickle, 1988; Robinson et al., 2001; Presnall et al., 2002) to 1410-1475'C (Kinzler and Grove, 1992; Asimow et al., 2001;

Putirka, 2005). Using PRIMELTI, Hertzberg et al. (2007) employ a reverse fractional crystallization model based on olivine addition to estimate TP at mid-ocean ridges to range between

1280-14000 C. At ultra-slow spreading ridges, thin oceanic crust indicates limited mantle melting either due to cooler mantle potential temperatures or the depression of the top of the melting column by conductive cooling from above (Reid and Jackson, 1981; Jackson and Reid, 1982; Dick et al., 2003). Robinson et al. (2001) uses a decompression melting model with conductive cooling to predict a Tp of 1280 50C for the SWIR. They argue that this near-normal Tp strongly suggests slow spreading-induced conductive cooling as the cause of reduced crustal thicknesses at the

33 SWIR. Also for the SWIR and consistent with this interpretation, Brown (2019) derive a Tp of

1298 1 00C.

4.2. Red Sea Geochemistry

The Red Sea provides a unique opportunity to investigate the melting processes along a nascent ultra-slow spreading ridge interacting with an active mantle plume. We first present major and trace element compositions of basalts along the Red Sea rift axis to illustrate the variable influence of ultra-slow spreading rates, plume activity, and mantle source. To do this, we collected a geochemical data set of glass and whole rock analyses of all axial basalts from the Red Sea and the Gulf of Aden and all volcanic rocks from the southern Red Sea islands using the EarthChem

Portal (www.earthchemportal.org). Sample locations are shown in Figure 6. Additional location and geochemical data from original sources were inputted where necessary. All major elements were normalized to a sum of 100.

In the discussion below we divide our geochemical data set into four groups based on location along the Red Sea axis: I) Ramad and the Red Sea southern islands (south of 17.05 N),

2) southern Red Sea (17.05 - 19.75'N), 3) central Red Sea (19.75 - 23.3'N), and 4) northern Red

Sea (north of 23.80 N).

4.2.1. Major Elements

Major element compositions of basalts from the Red Sea axis cover a range of compositions from alkaline basalt to hawaiite in the southernmost Red Sea to dominantly tholeiitic basalt along the fully developed spreading axis (Fig. 7). Basalts from the axial ridge of the southern and central spreading segments have compositions that range from ~6-11.5 wt.% MgO (Fig. 9), with basalts from the central Red Sea extending to the highest MgO contents. In the northern Red

34 28oN -

260N N N ern, Re

240N-

R a.

220N

200N 200N -Southe ed Se

180N

Sample legend for Red Sea axial basalts 160N - 0 Shaban Deep Northernh

* Mabahiss Deep Red Sea * Central Red Sea 14aN -A Southern Red Sea * Ramad seamount Southern- most Red * Southern islands oSea

34oE 360E 380E 400E 420E 44oE Figure 6. Map showing the locations of all axial basalts used in this study. Geochemical data was downloaded from the EarthChem Portal. Basalts can be divided into four geochemically distinct domains: the northern Red Sea, central Red Sea, southern Red Sea, and the southernmost Red Sea composed of Ramad seamount and the southern islands.

35 Red Sea Global MORB 0 Shaban deep Northern dataset from Mabahiss deep I Red Sea Gale et al. (2013) Central Red Sea All MORB A Southern Red Sea Ramad seamount Southern- * Ultra-slow Southern islands I most Red Sea spreading ridges

I I I I I I II I 'It 7 Hawaiite

6 -$ mMM

Alkali basalt + 0 0 5 0 40

04 z -0 3 10go 21- Tholefite U- 1 i I - I I I _ II I- L1 46 47 48 49 50 51 52 53 54 55 SiO 2 (wt.%)

Figure 7. Total alkalis (Na2 O + K20) versus SiO 2 classification (Cox et al., 1979) for erupted basalts from the Red Sea with tholeitte, alkali basalt, and hawaiite fields denoted (Macdonald and Katsura, 1964). Symbols for basalts are based on location, with northern (circles in shades of pink), central (red squares), and southern (blue triangles) Red Sea lavas plotting as tholeii- ties and those from Ramad seamount (light green diamonds) and the southern islands (dark green diamonds) predominantly ranging from alkaline basalts to hawaiites. Also plotted is the global database of MORB (grey) from Gale et al. (2013) with points from ultra-slow spreading ridges (Gakkel and Southwest Indian Ridges) highlighted in dark gray.

36 Sea, which is traditionally thought of as a zone of late stage continental rifting, basalts from Shaban and Mabahiss deeps have significantly lower MgO contents (2.5-8 wt.% MgO). Basalts from

Ramad and the southern islands have MgO contents that span the entire range defined by the northern, central, and southern Red Sea.

With K/Ti ratios greater than 1.5, basalts from Ramad and the southern islands, as well as those from the northern Red Sea, are E-MORB in composition (Fig. 8; Cushman et al., 2004).

Southern Red Sea basalts have variable K/Ti ratios (0.03-0.25) that range from N- to E-MORB.

Predominantly N-MORB and T-MORB are present within the central segment. Within this segment, Atlantis 11 and Nereus deeps are host to the most depleted basalts with Mg#s (molar

MgO/(MgO + FeO*)) closest to equilibrium with the mantle (not shown). Thus, this suggests that these rocks solidified from melt that has undergone only minimal amounts of fractional crystallization. Secondly, basalts from the central Red Sea have K/Ti ratios (0.03-0.18) that fall on the low end of the range defined by ultra-slow spreading ridges (~0.04-0.3), suggesting a relatively depleted source. Based on their major elements, the four regional groups also define geochemically distinct domains.

4.2.1.1. Northern Red Sea

Compared to other ultra-slow spreading ridges, the dominant group of basalts from the northern Red Sea has relatively lower MgO (4.5-7.9 wt.%), TiO 2 (1.1-3.1 wt.%), and Na20 (-2.8-

3.9 wt.%) contents and higher K 2 0 (0.21-0.59 wt.%), Si0 2 (50-53 wt.%), and CaO (~9.5-12

wt.%) contents (Fig. 9). Within the Red Sea, higher Na 2 0, K20, and A1 2 0 3 (~-14-20 wt.%) as well slightly elevated TiO 2 contents of basalts from the northern segment at least partly reflect lower degrees of melting caused by ultra-slow spreading rates, where conductive cooling lowers the top

37 1 I I I. 0.9-

0.8-

0.7-

0.6- 4 4*

0.5-

0.4-

0.3 - 0

0.2 E-$AORR

- T-iOR 0.1 ?B AL N-MO 0 .~mE.- 2 3 4 5 6 7 8 9 10 11 12 MgO (wt.%)

Figure 8. Classification of basalts from the Red Sea based on K/Ti versus MgO (wt.%). N-MORB has K/Ti < 0.09, T-MORB has K/Ti from 0.09 to 0.15, and E-MORB has K/Ti > 1.5 (e.g. Cushman et al., 2004). Legend as in Figure 7.

38 6.5

55 .a 6

54 - 5.5

53- 5 52 4.5

51 4 5-1 0 3.5 50 z 3 e. . 2.5

1.5

Ah. - 1 -. .WV......

A C d 3. 5 -

ee 3 -

2. ~1= 10 0 21V

9 , * 1. 5

8 5

7 0, 0.

1.6 20 0 0 1.4 19 +I* * 1.2 18

17 o 0.8 C: 16

0.6 - M~ 15

0.4 14 0.2 13 0 2 3 4 5 6 7 8 9 10 11 12 15 MgO (wt.%) 14 Figure 9. Major element oxides including 13 -. . a) SiO2 , b) Na 20, c) CaO, d) TiO2 , e) K2 0, 12 f) Al 0 , and 2 3 g) FeO* (=FeO +0.89Fe 2 0 3) i~11 versus MgO content. Basalts from the b D 10 Red Sea ridge do not plot along a single 9 liquid line of descent, and instead display

8 0 different trends related to N-S variations in melting conditions and in parental magma 7 composition. Legend as in Figure 7. 6 2 3 4 5 6 7 8 9 10 11 12 MgO (wt.%)

39 of the melting column to greater depths (Reid and Jackson, 1981; Niu and Hekinian, 1997; White et al, 2001; Dick et al., 2003; Montdsi and Behn, 2007; Standish et al., 2008). Greater concentrations of incompatible elements in the northern Red Sea compared to the rest of the region may also indicate a more enriched mantle source (Haase et al., 2000). In addition, FeO* (8-11.5 wt.%) content increases with decreasing MgO in a trend that is lower in FeO* but parallel to that of the central and southern segments (Fig. 4g). Lower FeO* contents in the northern Red Sea may reflect shallower or water-assisted melting. Overall, although the northern segment is comparatively more differentiated and scattered, this group has characteristics (low FeO* and high

A1 2 03, Na20, and K20) that most resemble the ultra-slow spreading Gakkel and Southwest Indian

Ridges, although they do not completely overlap like we would expect.

4.2.1.2. Central Red Sea

The central Red Sea forms well-defined linear trends on major element oxide versus MgO plots (Fig. 9). Compared to the northern Red Sea, this group has high MgO (7.0-11.5 wt.%), CaO

(-11-13.3 wt.%). and FeO* (8.7-13.7 wt.%) contents and low K20 (<0.16), Al 20 3 (-14-16.7

wt.%), Si0 2 (48-51 wt.%), and TiO 2 (0.5-1.5) contents. Low Si0 2 contents in this region most likely correspond to anomalously deep melting along this segment. Due to comparatively higher degrees of partial melting in the central Red Sea as a response to faster spreading rates, basalts extend to lower Na20 (2.0-2.7 wt.%) contents in this area (Fig. 9b). In addition, using the Brown

(2019) model (implemented in Section 5), we calculate the average Mg# of clinopyroxene saturation in the most primitive central Red Sea lavas to be -0.68. This is consistent with the CaO content in these basalts, which begins decreasing around 10 wt.% MgO (Fig. 9c), which

corresponds to a Mg# of -0.67. At greater MgO contents, CaO and A1 2 0 3 define a linear relationship with Mg# that is commonly interpreted to suggest olivine-only fractionation.

40 However, according to the Brown (2019) model, plagioclase in these primitive samples saturates at average Mg#s of 0.71, and it is thus likely that all lavas in the Red Sea have fractionated at least both olivine and plagioclase during their early evolution. Unfortunately, because there is a lack of primitive basalts in our database, inflection points associated with the point of plagioclase saturation are not resolvable. In fact, many mid-ocean ridge basalts have olivine-plagioclase co- crystallization that begins at Mg#= 0.68-0.70 and thus undergo little olivine-only crystallization during cooling (Brown, 2019). Furthermore, the central Red Sea is alkaline-poor compared to other ultra-slow spreading ridges (Fig. 7).

4.2.1.3. Southern Red Sea

Compared to the central Red Sea, the southern Red Sea has similarly low K 20 (<0.21 wt.%) and high CaO (10.5-14 wt.%) and FeO* (8.5-15 wt.%) contents (Fig. 9). The southern Red Sea

spans a slightly lower range of MgO contents (6-10 wt.%) and a higher range of SiO 2 (49-53 wt.%) contents that may reflect increased differentiation or low-pressure melting (Gale et al.,

2014). Low Na20 (1.5-2.4 wt.%), TiO 2 (0.5-1.7 wt.%), and A1 2 03 (-13-16.5 wt.%) contents indicate higher degrees of partial melting as a result of the relatively faster spreading rates in this area. Furthermore, a striking feature of the southern Red Sea is that, compared to the other tectonomagmatic segments, basalts from this section are significantly alkaline depleted (Fig. 7).

This is especially surprising because, as the geographically closest region to the Afar plume, we would expect alkaline contents of the southern Red Sea basalts to be similar to those of Ramad seamount and the southern islands.

4.2.1.4. Ramad Seamount and the Southern Islands

Basalts from Ramad seamount and the southern islands are alkali-rich compared to the rest of the Red Sea (Fig. 7). The Ramad seamount, a volcano located at the approximate latitude of

41 seafloor spreading initiation (-17'N), has relatively high MgO (8.5-8.8 wt.%), Na20 (2.6-3.4 wt.%), CaO (12.5-13 wt.%), and TiO 2 (1.4-1.6 wt.%) contents and low FeO* (8.4-8.6 wt.%) contents (Fig. 9). Basalts from this seamount also show anomalously high K20 (0.65-0.70 wt.%)

0 and low Si0 2 (48-49.5 wt.%) contents. South of -16 N, along the attenuated continental crust- floored rift axis, the southern islands have major element concentrations that exhibit a much greater amount of compositional diversity. Basalts from the southern islands have highly variable MgO

(1.5-11.5 wt.%), K20 (0.25-1.7 wt.%), Si0 2 (46-54 wt.%), Na20 (-2.4-2.7 wt.%), CaO (~6.8-

13 wt.%), A12 0 3 (- 14.7-19 wt.%), TiO 2 (-1.5-4 wt.%), and FeO* contents (-8-14 wt.%) that do not form well-defined trends.

4.2.2. Major Element Differentiation

Excluding the southern islands, the northern Red Sea is generally the most fractionated domain in the Red Sea, while the central domain is the least differentiated with the largest population of primitive MORB. The southern Red Sea is transitional between the two. With the exception of Ramad seamount and the southern islands, FeO*, Na20, K20, and TiO2 increase and

A12 0 3 and CaO decrease with decreasing MgO contents. These trends can be attributed to olivine, plagioclase, and clinopyroxene fractionation. Although fractional crystallization has contributed to modifying lava compositions, the compositional diversity of the four groups cannot be produced by polybaric differentiation of a single parental melt. Compared to the other three tectonomagmatic groups, Ramad and the southern islands are highly enriched in alkalis, A12 0 3, TiO 2 and depleted

in CaO, Si0 2 , and FeO at a constant MgO. This region, particularly the Ramad seamount, has the most pronounced HIMU component interpreted to reflect a significant influence of the Afar plume on the asthenospheric mantle source (Volker et al., 1997).

42 Likewise, in total-alkali versus silica space, Ramad and the southern islands are enriched in alkalis as a result of the Afar plume (Fig. 7). In contrast, the southern Red Sea plots at anomalously alkali-poor values. This strongly suggests against large degrees of mixing between the Afar plume and ambient mantle beneath the southern Red Sea north of 17.05'N. Compared to

0 basalts south of 17.05 N, the northern Red Sea has the highest Na20, A1 203, K20, and K/Ti, the lowest FeO*, and relatively high SiO 2 for a given MgO content. This geochemical signature possibly reflects shallow and low degree partial melting of an enriched source. Furthermore, the southern Red Sea shows trends that overlap with those of the central segment (similarly high FeO*

and low K 2 0 and K/Ti at a constant Mg#), but displays elevated SiO 2 and MgO and lower TiO2 ,

Na20, and total alkali contents. These geochemical signatures predict shallower and higher extents of melting in the southern Red Sea (e.g. Langmuir et al., 1992; Gale et al., 2014).

4.2.3. Variations in Mantle Source along the Red Sea Ridge

Basalt chemistry varies systematically as the Afar hotspot is approached from the north along the Red Sea ridge. As Augustin et al. (2014) show, fractionation corrected Na 2O (Na8; Plank and Langmuir, 1992) decreases along axis from the northern to the southern Red Sea (Fig. I Oa).

This trend reflects systematic increases in spreading rate from north to south along the Red Sea

(Haase et al., 2000; Augustin et al., 2014), although changes in mantle source composition can also cause these trends. For example, a dramatic increase in Na8 is observed at Ramad and the southern islands due to derivation of basaltic melts from an enriched mantle plume source (e.g.

Schilling et al., 1992; Volker et al., 1997). This can be further illustrated by analyzing variations in ratios of incompatible elements such as K/Ti and Nb/Zr, which reveal the relative proportion of plume-derived versus ambient mantle undergoing melting along the Red Sea (Fig. I Ob-c). As

43 Radial distance from Afar plume (km) 200 400 600 800 1000 1200 1400 1600 1800 A 1400 1600 1800 3 ~, ~ abarg~__I-2000 Z 2 3 a I -11ft. .5 ______- 13000 i

1 . 5 1 - -.

0. 5 J Zabargad -

0.4 0.3 ~ZabargadI .d0.2 1 z 0.1 0

E 2 Zabargad

IIz. 0[ A f 14 16 18 20 22 24 26 Latitude (ON)

Figure 10. a) Na2 0 corrected for fractional crystallization (Na8 ), b) K/Ti, c) Nb/Zr, and d) La/Sm normalized to primitive upper mantle (PUM; McDonough and Sun, 1995) versus latitude (ON) on the bottom x-axis and

distance from the Afar plume (km) on the top x-axis. In a, Na8 was calculated using the equation from Plank and Langmuir (1992) and the topographic profile of the Red Sea is plotted in gray for reference. Legend as in Figure 7.

44 expected, K/Ti and Nb/Zr ratios spike to on average of 0.5 and 0.15, respectively, at Ramad seamount and the southern islands, which previous authors have suggested is the locus of plume activity along the Red Sea (Schilling et al., 1992; Volker et al., 1997). Unlike other plume-affected ridges, such as the Galapagos ridge, K/Ti and Nb/Zr ratios do not increase smoothly toward the plume (Detrick et al., 2002). For example, the southern Red Sea is the geographically closest segment to the Ramad seamount and is the region where axial topography begins to shoal toward the south with proximity to the plume. However, basalts from this segment have significantly lower K/Ti (<0.21) and Nb/Zr (0.06-0.09) ratios than those from Ramad and the southern islands that are not easily explained by variations in partial melting. These trace element differences thus suggest very limited interaction between plume material and ambient mantle north of the Danakil rift at I 7.050 N (-600 km from the plume center). Instead, trace element signatures within the southern Red Sea are most like those of the central segment, suggesting that these two domains are underlain by mantle sources of similar fertility. The geochemical signature of the Afar plume completely diminishes north of-20ON (-950 km from the plume center), which corresponds with the boundary between the southern and central Red Sea. In the northern Red Sea, K/Ti (0.17-0.66) and Nb/Zr (0.04-0.1) ratios rise to significantly higher values than those of the central and southern domains and overlap with the range defined by other ultra-slow spreading ridges (0.05-0.35 and

0.01-0.1, respectively). Basalts along the northern segment are derived from a fertile mantle source most likely sequestered by the Zabargad Fracture Zone (Haase et al., 2000).

These patterns are also evident in La/SmN ratios (Fig. 1 Od). La/SmN is cOmmonly used as a proxy for the degree of partial melting, but is also affected by differences in mantle source. For example, the Afar plume component within the mantle source of basalts from Ramad seamount and the southern islands produces incompatible element enrichments with elevated La/SmN ratios

45 (avg. 2.6). Like Nb/Zr and K/Ti along the southern segment, only slightly elevated La/SmN (0.4-

1.1) ratios in this domain compared to the central Red Sea indicate very small amounts of mixing between plume-derived and ambient mantle.

Volker et al. (1997) observed similar trends in Sr, Nd, and Pb isotopic variations along the

Red Sea (Fig. 11). Samples from Ramad seamount show a positive anomaly in Pb and Sr isotopes and a negative anomaly in Nd isotopes related to the Afar plume. In the central Red Sea, isotopic

2 06 2 04 signatures (low Pb/ Pb and 17Sr/ 6Sr ratios and high 143Nd/ 144Nd ratios), although considerably scattered, are the most depleted along the Red Sea and indicate an asthenospheric source (Volker et al., 1997). Towards the north and south, Pb and Sr isotopic ratios increase while 14 3Nd/ 144Nd decreases (Volker et al., 1993; Haase et al., 2000). In the southern Red Sea, isotopic signatures

(higher 206Pb/204Pb and 8 Sr/86Sr and lower 143Nd/ 44Nd compared to the central Red Sea) indicate interaction between an Afar plume-like component beneath Ramad seamount and the asthenospheric central Red Sea mantle (Volker et al., 1997). In addition, the northern Red Sea is underlain by mantle characterized by distinct major, trace, and isotopic compositions (high

8 7 r/ 86 Sr for a similar range of 14 3Nd/1 44Nd) produced from interaction between ambient mantle and a more enriched source (Volker et al., 1997; Haase et al., 2000). Variation in mantle sources should be further constrained by conducting a principal component analysis, which is beyond the scope of this study.

Furthermore, axial basalts in the Gulf of Aden have geochemical signatures that show similar patterns as they approach the Afar plume from the east (Fig. 12). For an in-depth analysis of plume-ridge interaction in the Gulf of Aden, the reader is referred to Schilling et al. (1992).

West of around 46.5-46.8'E (500 km from the plume center), significantly elevated K/Ti and

46 *

Radial distance from Afar plume (km) 200 -400 600 800 1000 1200 1400 1600 1800 0.7036 -a 03 0.7034 7j -1000 - 0.7032 3 0.7030 abrgA -2000 3D 0.7028 - - 0.7026 jiii~~~~~~ ______F__ abargadf-3000 1_____

0.51 32 I 00.51315 -b I M z : 0.5131 - 22 A E - Zabargad e -0.51305- AA r-4..z0.513 - $ * A k I I

19.5 CI ~ Zabargad CL 19 ~ A* a.~ A 0- 18.5

18 14 16 18 20 22 24 26 Latitude (ON)

Figure 11. a) 87Sr/86Sr versus latitude (ON) on the bottom x-axis and distance from the Afar plume (km) on the top x-axis. The topographic profile of the Red Sea rift axis is plotted in gray for reference. Also plotted along these x-axes are isotopic compositions b) '43Nd/1 44Nd and c) 2 06Pb/204Pb. Legend as in Figure 7.

47 Radial distance from Afar plume (km)

1.5 100 200 300 400 500 600 700 800 900 I I I I sq Es i i I -1000 1.0 a - 3 ~ -2000 3 0i - ~ V -3000 0.5 : : *1 \.A. . - 0 -4000 3 0 0 10 008. . 0 *P 00 a I . ,9.0.. .* 0 -5000

0.4

Z 0.3 .a 0.2

. ______-__-t ~ 0.1 - 0 - 0 43 44 45 46 47 48 49 50 51 Longitude (OE)

Figure 12. a) K/Ti and b) Nb/Zr of axial basalts along the Gulf of Aden versus longitude ( 0E) on the bottom x-axis and distance from the Afar plume (km) on the top x-axis. The topographic profile along the Gulf of Aden ridge axis is plotted in gray in a. Also marked are the locations of prominent fracture zones (f.z.) that serve to obstruct plume-related mantle flow eastward along the ridge axis.

48 Nb/Zr ratios of axial basalts reflect a major contribution of plume-affected mantle to the parental melt, although this effect diminishes west of ~44'E as a result of the torus-shaped plume (Schilling et al., 1992). Between 46.5-46.8 0E, K/Ti and Nb/Zr ratios are relatively lower, indicating only limited interaction between the Afar plume and ambient mantle. Thus, like the Danakil rift, the

Khanshir Al lrqah fracture zone at -46.75 0E (-475 km from the plume center) appears to interfere with the passage of plume material along the ridge. The presence of MORB-like K/Ti and Nb/Zr ratios east of the Xiis Al Mukalla fracture zone at 48'E (~600 km from the plume center) marks the eastern periphery of plume-influenced mantle in the Gulf of Aden. The ability of transform faults and fossil structures to obstruct mantle flow has been well documented along other mid- ocean ridges (e.g. Vogt and Johnson, 1975; Georgen and Lin, 2003). It thus appears that the Afar plume is asymmetrical and channelized flow of plume material extends further up the Red Sea ridge axis than it does the Gulf of Aden. This is most likely a consequence of the relative continuity of the Red Sea ridge, which lacks the segmentation by large transform faults characteristic of the

Gulf of Aden.

Overall, the observed compositional heterogeneity within the Red Sea may reflect differences in mantle composition (e.g. Le Roex et al., 1983; Schilling et al., 1992; Volker et al.,

1997; Shen and Forsyth, 1995; Haase et al., 2000), axial thermal structure and mantle potential temperature (Gale et al., 2014; Dalton et al., 2014), or a combination of the two (Dick et al., 1984;

Standish et al., 2008). Either way, geochemical diversity along the Red Sea is influenced by proximity to the Afar plume (e.g. Schilling et al., 1992; Volker et al., 1997), the stage of development of the nascent ridge axis (e.g. Ligi et al., 2012), and the reduction in full spreading rates from south to north along the ridge (e.g. Augustin et al., 2014; Haase et al., 2000). Therefore, despite Augustin et al. (201 9)'s argument that the Red Sea is an ordinary ultra-slow spreading

49 ridge with all abnormalities due to the proximity of plume activity, we find that erupted basalts clearly isolated from the Afar plume do not have geochemical signatures that resemble those of the SWIR or the Gakkel ridge.

5. Changes in Melting Regime along the Red Sea Ridge

According to Gale et al. (2014), Na20 and A12 0 3 enrichment and SiO 2 and CaO depletion characterize the geochemistry of basalts from ultra-slow spreading ridges as a result of deep and low degree melting of a cold, wet, and garnet-bearing peridotite or pyroxenite mantle source

(Holness and Richter, 1989; Asimow and Langmuir, 2003; Brown, 2019). They also hold that the vertical dimension of the melting regime beneath ultra-slow ridges is shortened by thicker lithosphere. In her coupled reverse and forward fractional crystallization model (discussed below),

Brown (2019) investigate basalts from the 9-25 0 E SWIR and conclude that the geochemistry of ultra-slow spreading ridges does not require deep or cold melting and is actually best modelled by low degree melting of a plagioclase lherzolite assemblage.

Surprisingly, erupted basalts from the Red Sea north of 17.05'N have major element signatures that do not completely conform to those of the global array of basaltic compositions from similarly slow spreading mid-ocean ridges. As a whole, Red Sea lavas north of the Danakil

rift have elevated CaO and lower A1 2 03, Na20, and TiO 2 contents compared to other ultra-slow ridges, although basalts from the northern Red Sea have A12 03, Na20, and TiO 2 contents that overlap with the field defined by the Gakkel and SWIR. Furthermore, in the southern Red Sea, basalts are enriched in Si0 2 compared to the Gakkel and Southwest Indian Ridges. This leads us to the fundamental question at the basis of all geochemical studies on the Red Sea: what is the origin (mantle source and/or thermal structure) of basaltic compositional diversity along the Red

50 Sea? We further ask: why does the composition of each segment individually differ from that of the global array of ultra-slow spreading ridge basalts?

To address these questions for the Red Sea, we need to understand the dominant causes of variability. In this section, we investigate N-S variations in melting processes in order to provide insight into the role of axial thermal structure in producing the compositional heterogeneity observed along the ridge. To do this, we need to correct each erupted basalt for fractional crystallization to achieve the composition of the multiply saturated primary melt. However, the majority of available basaltic glass samples from the Red Sea ridge are plagioclase saturated, and we thus cannot use the PRIMELT model of Herzberg et al. (2007) to calculate representative primary melts. Instead, we employ a recently developed maximum likelihood reverse fractional crystallization model presented by Brown (2019) that uses major and trace element compositions of basaltic glasses to elucidate the 1) pressure and temperature conditions of melting and 2) the temperature of the mantle source from which all basalts melted.

We input into this model only those data described above that have major and trace elements obtained exclusively from basaltic glass analyses. This method is applied to 138 basaltic glasses located along the Red Sea rift axis from 16.90 N at Ramad seamount to 26.3'N at Shaban deep. Although we use the same compositional groups as above, we exclude data from the southern islands in our analyses such that the southernmost group is now named Ramad.

A brief review of the model is given below, and we refer the reader to Brown (2019) for more information. This model assumes that the melt, or aggregate of melts, was in equilibrium with lherzolitic peridotite (Mg#mman, = 0.895-0.905; Chatterjee and Sheth, 2015, Asimow and

Longhi, 2004; Brown, 2019). Primary mantle melts have multiple saturation points that have been experimentally determined for melting within the plagioclase, spinel, and garnet peridotite stability

51 fields (Kinzler and Grove, 1992; Kinzler, 1997; Till et al., 2012; Grove et al., 2013). For all inputted samples, Brown (2019) then corrects the liquid to the Mg# of the primary melt

(Mg#Mar ) by calculating the Mg# of the plagioclase-in (Mg#MIt, where OP stands for olivine- plagioclase) and clinopyroxene-in (Mg# 't, where OPA stands for olivine-plagioclase-augite)

Mg#Melt transition points and the endpoint Primary in equilibrium with the mantle. In this way, this model eliminates errors associated with using an incorrect phase assemblage to restore a differentiated basalt back to Mg#Mray (Shen and Forsyth, 1995; Asimow and Longhi, 2004; Hertzberg et al.,

2007; Till, 2017). In addition, because primary mantle melts are multiply saturated, their compositions have been empirically constrained by Kinzler and Grove (1992). Thus, this model is able to compare in Tormey composition space the corrected primary liquids of erupted basalts to the actual compositions of primary mantle melts (Fig. 13). Outputs of this method include the maximum pressure of fractional crystallization (P x) that minimizes the compositional difference between the basaltic sample and an augite-plagioclase-olivine saturated melt (Yang et al., 1996).

Using different, experimentally constrained values for Mg#0 ," and Mg#,"' such that

Mg# 0 ,* > Mg# ,Me,the model computes every fractional crystallization path possibly experienced by the melt. To estimate the phase proportions of olivine, plagioclase, and augite, a KDFe-Mg of 0.30 and KDCa-Na of I between olivine and melt and plagioclase and melt, respectively, is used. Lastly, the pressure and temperature of melting, mantle potential temperature (using a 1.5 0C/kbar mantle adiabat with no correction for latent heat of melting), and the best-fit peridotite stability field of the corrected primary melt composition with the lowest normalized root mean square difference

(NRMSD) compositional distance from the true primary melt are derived.

5.1. Results of the Maximum Likelihood Reverse Fractional Crystallization Model

52 Diopside

Global MORB dataset from 0 Gale et al., (2013)* Olivine Quartz

004F0

Plagiociase- Spinel Transition 11-12 kbarg Primary mantle

log. *kbars 0s j eog ars

Spinel-Garnet 40 kbars Transition Red Sea Reverse Crystallization 0 Shaban deep Northern erupted basalt * Mabahiss deep Red Sea corrected basalt * Central Red Sea * Southern Red Sea A closest primary * Ramad seamount melt

Figure 13. Reverse fractional crystallization computation and accompanying figure from Brown (2019) applied to axial basalts along the Red Sea. Green points define the array of primary mantle melts that are generated from compositionally variable sources within the plagioclase, spinel, and garnet stability fields. Most Red Sea basalts are corrected to primary melts that indicate origination from the plagioclase-spinel transition.

53 Results of the maximum likelihood reverse fractional crystallization model are shown in

Table 2 and Figure 14. On average, corrected primary melt compositions display small (0.01)

NRMSD compositional distances from true primary melts. One exception to this is the Ramad group, which have calculated primary melts with compositions further away from true primary melts (average NRMSD = 0.05). Increasing the KDFe-Mg to 0.33 decreases this compositional difference (average NRMSD = 0.03) but does not significantly change the results of the model.

Compositions of basaltic glasses from the Red Sea record melting within the plagioclase (62%) and spinel (38%) fields at temperatures of 1294 23'C (average I-sigma standard deviation) and pressures of 10.4 1.7 kbars. These melting parameters coincide with a mantle potential temperature of 1307 1 IC, which is slightly elevated compared to the range of mantle temperatures calculated for other ultra-slow spreading centers (Robinson et al., 2001; Brown,

2019). Contrary to the general consensus that magmatism at ultra-slow spreading centers is a product of cold and deep melting of garnet-bearing peridotite, the results of this model confirm the absence of deep, garnet field melting and mantle significantly colder than average ambient temperatures (-1300-1400'C) recognized by Brown (2019). Instead, these results, coupled with our major element analysis, agree well with those presented in Brown (2019) for the 9-25 0E SWIR.

In both the SWIR and the Red Sea, low-degree melting within the plagioclase field dominates. As with the SWIR, the fractional crystallization-corrected primary melts of erupted basalts away from the Afar plume are within 1% of equilibrium with lherzolitic peridotite, indicating a lack of evidence for garnet pyroxenite melting (Brown, 2019).

We find that the model-predicted melting fields are consistent with variations in rare-earth element abundances along the Red Sea, as shown by the correlations between La/SmN and Sm/YbN

54 Table 2. Thermobarometry results for 138 axial basalts along the Red Sea using the maximum likelihood reverse fractional crystallization model and accompanying table from Brown (2019). P = pressure in kbars, T = temperature in 0C, Tmp mantle potential temperature.

Ramad Southern Central Northern Mabahiss Shaban Seamount Red Sea Red Sea Red Sea Deep Deep All

#points 3 48 51 36 20 17 138 Mg# glass 0.64 0.55 0.57 0.56 0.52 0.60 0.56 la 0.00 0.04 0.06 0.07 0.06 0.04 0.06 NaK# glass 0.22 0.16 0.17 0.25 0.25 0.24 0.19 lo 0.02 0.02 0.03 0.05 0.06 0.03 0.05 FC P 5.3 2.0 4.8 1.7 1.4 2.0 3.1 J o 1.6 1.2 2.1 1.3 1.2 1.3 2.2 NRMSD FC P 0.01 0.01 0.02 0.12 0.03 0.23 0.04 l 0.00 0.01 0.02 0.26 0.02 0.34 0.14 NRMSD PM 0.05 0.01 0.01 0.01 0.01 0.02 0.01 lo 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Mg# Cpx 0.64 0.65 0.65 0.63 0.63 0.63 0.65 Jo 0.00 0.02 0.02 0.02 0.02 0.03 0.02 Mg# Plag 0.73 0.72 0.72 0.71 0.71 0.70 0.72 lT 0.00 0.01 0.01 0.02 0.02 0.02 0.02 %OPC 0 31 27 23 34 12 27 lT 0 16 16 19 14 16 17 %OP 34 14 13 22 20 25 16 Jo 0 10 9 11 9 12 11 %0 1 12 10 2 1 3 8 1 0 20 18 2 1 2 17 FC extent 35 51 46 48 55 40 48 ]a 1 7 13 12 7 11 11 Mg# Prim Melt 0.74 0.73 0.73 0.73 0.73 0.73 0.73 Jo 0.00 0.01 0.01 0.01 0.01 0.01 0.01 Mg# Olivine 0.904 0.899 0.901 0.902 0.901 0.902 0.901 lo 0.001 0.003 0.003 0.003 0.003 0.002 0.003 NaK# Prim Melt 0.21 0.13 0.15 0.21 0.20 0.22 0.16 lo 0.02 0.01 0.01 0.02 0.03 0.02 0.04 # pts garnet 0 0 0 0 0 0 0 # pts spinel 0 10 31 11 4 7 52 # pts plag 3 38 20 25 16 10 86 % plag pts 100 79 39 69 80 59 62 % spinel pts 0 21 61 31 20 41 38 Best Fit P (kbars) 12.0 9.2 11.5 10.4 10.1 10.7 10.4 Jo 0.0 1.0 1.6 1.4 1.3 1.4 1.7 Best Fit T 1306 1281 1311 1288 1284 1292 1294 la 6 14 22 21 21 20 23 Best Fit Tmp 1326 1304 1310 1303 1304 1302 1307 Jo 5 10 11 9 10 8 11

55 Radial distance from Afar plume (km) 600 800 1000 1200 1400 1600 1800 a 0 I *I-- " - - 2 N a 76 .0 4 - e Zabargad - 6 W 4 l f.z. - U 8 . -____- 10

1F 113014 1 1 1 F 1320 b b A

Cn 1280 -A-

9AnI 6 I I I I a 1' ~ 8 -C 10 - A - .Z 12 A 14

1340 d 1320

~)1300 - A A 1280 __ AEII - 146 17 18 19 20 21 22 23 24 25 26 27 Latitude (ON)

Figure 14. a) Pressure of fractional crystallization, b) melting temperature, c) melting pressure, and d) mantle potential temperature plotted against latitude along the Red Sea ridge and radial distance from the Afar plume. Melting conditions were calculated from compositions of erupted basalts using the reverse fractional crystallization model of Brown (2019). Legend as in Figure 7. In most cases, the central Red Sea displays the hottest and deepest melting conditions and the deepest pressures of fractional crystallization. Calculated temperatures of melting and pressures of melting and fractionation in the southern and northern Red Sea are comparatively lower. Ramad seamount has the highest average mantle potential temperature due to its proximity to the Afar plume.

56 ratios and between Sm/NdN and Lu/HfN ratios (Fig. 15). Garnet field melts have kimberlite-like heavy rare-earth element depletion and light rare-earth element enrichment, and thus have higher

Sm/YbN and La/SmN and lower Sm/NdN and Lu/HfN ratios than all MORB (e.g. Brown, 2019). As can be seen, lower La/SmN and Sm/YbN and higher Sm/NdN and Lu/lfN ratios in the Red Sea compared to those produced during garnet field melting indicate that melting of a garnet-bearing peridotite is not required to produce our sample suite. We note that basalts from the northern Red

Sea have ratios like those of the Gakkel and SWIR, reflecting enrichment that supports the role of low degree partial melting of an enriched source at ultra-slow spreading rates. Basalts from the central Red Sea on the other hand have ratios within the range of depleted-MORB (La/SmN 0.8 according to the terminology of Gale et al. (2013)).

As predicted, melting conditions vary systematically among the four tectonomagmatic groups of the Red Sea (Fig. 14). Calculated melting conditions within individual segments vary greatly, so in the following discussion we continue to report averages with I-sigma standard deviations. Along the northern segment (n=36), far from the Afar plume, 69% of primary melts were generated in the plagioclase peridotite stability field, with 80% and 59% of melting occurring in the plagioclase field at Mabahiss and Shaban deeps, respectively. In this region, melting initiates at 1288 21 C and 10.4 1.4 kbars. In the central Red Sea (n=54), only 39% of melting takes place in the plagioclase field, with the additional 61% occurring within the spinel field. Melting in this region occurs at higher temperatures (131 1 220C) and pressures (11.5 1.6 kbars) compared to the northern Red Sea. In addition, plagioclase-field melting (79%) along the southern segment (n=45) occurs at comparatively shallower depths (9.2 1.0 kbars) and lower temperatures (1281 140C). In addition, despite variations in temperature, pressure, and field of melting, the potential temperature

57 101 a

0- Garnet field E0 a melts

" 100 0 Er oo

_J Depleted MORB f ield

100 Sm/Yb normalized to PUM 101 2 1.8 1.6 1.4 1.2 0- 0 1 Garnet field M 0.8 $ melts

0 0.6

E S 0 0 Lt) 0.4 %0 S. e

p * - 10-1 100 Lu/Hf normalized to PUM

Figure 15. a) La/Sm N vs. Sm/Yb N and b) Sm/NdN vs. Lu/HfN all normalized to primitive upper mantle (PUM; McDonough and Sun, 1995). Fields of garnet melting are from Brown (2019). Small red circles are south African kimberlites (compiled by Brown (2019)) that have clear garnet signatures. Erupted basalts from the Red Sea axis and the southern islands do not have trace element signatures consistent with melting in the garnet stability field. The majority of basalts from the central Red Sea have depleted MORB-like La/SmN ratios (La/Sm N<0.8; Gale et al., 2013), while those from the northern Red Sea have compositions that plot within the range of ultra-slow spreading ridges. Legend as in Figure 7.

58 of the mantle beneath the northern (1303 9"C), central (1310 1 IC), and southern Red Sea

(1 304 1 00C) is fairly consistent, although mantle temperatures extend to higher values in the northern Red Sea (Fig. 16). This narrow range of mantle potential temperatures supports compositional heterogeneity resulting mainly from variations in spreading rate and lithospheric thickness (Brown, 2019). Lastly, variations in melting pressure (Fig. 14c) agree with those qualitatively constrained from Fe8 (Haase et al., 2000; Augustin et al., 2014), which correlates with the pressure of melting (Klein and Langmuir, 1987).

Within the vicinity of the Afar plume, compositions of basalts from Ramad seamount (n=3) indicate melting temperatures of 1306 60C at melting pressures of 12.0 0 kbars, which correlate with mantle potential temperatures of 1326 50C. These parameters reflect hotter and deeper melting in the southernmost Red Sea consistent with the presence of the strong Afar plume-derived thermal anomaly (e.g. Rooney et al., 2012). However, these mantle potential temperatures are much lower than those calculated from other plume-influenced ridges (e.g. Galapagos, Iceland; e.g. Herzberg and Gazel, 2009; Rooney et al., 2012), although higher potential temperatures at these locations may be a consequence of the method by which they were computed (Brown, 2019).

In the Red Sea, all erupted magmas become plagioclase saturated at Mg#Me = 0.72 0.02. which is within error of the Mg# of the primary melt in equilibrium with the mantle (0.70-0.72).

This is further evidence that all Red Sea magmas are saturated in plagioclase and experienced only limited olivine-only fractionation during their early evolution (Brown, 2019). In addition, clinopyroxene in all Red Sea basalts began crystallizing at Mg# = 0.65 0.02, a transition point not only controlled by bulk composition but also by pressure (Yang et al., 1996). Furthermore, along the Red Sea, fractional crystallization occurs at pressures that span a large range from 0.001

59 a 14 .I Mabahiss 12 - Shaban

0- --c8

26- U-

Mantle Potential Temperature (00)

Central Red Sea 12

Cr) 8

Mantle Potential Temperature (C) c 20 .. r Southern Red Sea E- mRamad Seamount

15 -

C 0-10- (.-

Mantle Potential Temperature (C) Figure 16. Histograms of calculated potential temperatures of the mantle beneath the northern (a), central (b), and southern (c) Red Sea. The majority of calculated mantle potential temperatures in the Red Sea north of 17.050 N define a narrow range (~] OoC). South of the Danakil rift and within proximity of the Afar plume, basalts from the Ramad seamount record on average higher mantle potential temperatures (-1 326 50C).

60 to 10 kbars with an average of 3.1 2.2 kbars (Fig. 14a). This is within the range of those calculated for the SWIR (Brown, 2019). In the southern and northern Red Sea, fractional crystallization occurs at relatively shallow pressures of 2.0+1.2 kbars and 1.7 1.3 kbars, respectively, although the southern segment undergoes slightly larger extents of fractionation (51 7%) compared to the north (48 12%). Basalts from the central Red Sea experience 46 13% fractional crystallization at much deeper pressures (4.8 2.1 kbars). Deep pressures of fractional crystallization in this domain indicate thick oceanic lithosphere compared to the southern and northern segments. Similarly,

Ramad basalts fractionally crystallize at comparatively higher pressures (5.3 1.6 kbars), consistent with enhanced magma supply and crustal thickening within proximity to the Afar plume. These magmas have experienced much lower extents of fractional crystallization (35%) as also evidenced by their low Mg#s, FeO, and SiO2 contents.

5.1.1. Calculated Primary Melt Compositions

The maximum likelihood reverse fractional crystallization model by Brown (2019) also generates corrected primary melt compositions based on calculated best-fit paths of fractional crystallization (Fig. 17). By analyzing the compositions of the primary melts throughout the Red

Sea, we can better discriminate between mantle source and melting conditions as the dominant cause of compositional heterogeneity within our sample suite of fractionated basalts. For example, a similar range in A1 20 3 content of all primary basalts indicates that the variability of A1203 content in fractionated samples is due to decreasing spreading rate and hence degree of partial melting from the southern to northern Red Sea. Similar conclusions can be drawn from the Na20 contents of primary melts, although enhanced Na 20 contents in the north are most likely derived from the enriched mantle source beneath this segment. In addition, according to the model, the southern

61 -1

a 20 I I 19

18 -0

- 0 0- 17 -- -0 16 0 N\ 15

14 //

13 I I 12 8 10 12 1 4 CaO (wt%) 5 All b MORB 4.5 - Frac. cryst. path 4 Erupted oom 0 - o basalt 3.5 -b Primary 0 - basalt N 3 2.5

1i:. I5 4 6 8 10 1 2 14 16 FeO (wt%)

Figure 17. a) Al 2 0 3 vs. CaO and b) Na 2 0 vs. FeO of erupted basalts along the Red Sea and their corrected primary magmas connected by best-fit fractional crystallization paths calculated from the model by Brown (2019). Framework for figure generated by MATLAB script provided by Brown (2019). All MORB from Gale et al. (2013). Color scheme as in Figure 13.

62 Red Sea begins melting at shallower pressure relative to the central domain. Although elevated

SiO 2 contents of basalts from the south are consistent with this calculation, similar FeO contents, which are sensitive to initial melting pressure (Langmuir et al., 1992; Gale et al., 2014), of basalts from the southern and central Red Sea are not. According to Figure I7b, primary melts generated in the southern Red Sea are comparatively FeO-rich such that effects of shallow melting on FeO content are muted. Likewise, variation in CaO content within fractionated basalts mirror that within primary melts.

5.1.2. Major Element Oxide Compositions of Basalts from the Red Sea Compared to Other

Ultra-Slow Spreading Ridges

CaO contents of erupted basalts from the Red Sea are significantly higher than those of lavas from other ultra-slow spreading ridges and in CaO-MgO space plot within the field of slow to fast spreading ridges (Fig. 9c). High CaO contents of slow to fast spreading ridges result from shallower pressures of fractionation, which is due to a thin or absent lithosphere lid (e.g. Dick et al., 2003; Gale et al., 2014). Results from the reverse fractional crystallization model also predict significantly shallower pressures of fractionation along the Red Sea ridge indicative of thin lithosphere and hence magma-rich seafloor spreading. Normal ultra-slow spreading ridges, on the other hand, have very thick lithosphere (15-35 km) as a result of passive upwelling and cold mantle potential temperatures. In addition, the reverse fractional crystallization model computes relatively constant mantle potential temperatures throughout the Red Sea north of I 7.050N that are only slightly elevated compared to other ultra-slow spreading ridges (Robinson, 2001; Brown, 2019) and are within the range of ambient mantle (3000-40000 C). Thus, heightened melt production, as

indicated by lower Na2O, TiO 2, K20, and A1 2 0 3 contents in the central and southern Red Sea compared to other ultra-slow spreading ridges, and thinned lithosphere cannot be due to thermal

63 anomalies within the Red Sea, and in particular those associated with the Afar plume. Instead, enhanced degrees of partial melting must be associated with anomalously buoyant mantle flow that experiences fast upwelling velocities (Dick et al., 2003). Buoyant flow serves to mitigate the effects of convective heat loss and, coupled with the production of more melt, generates a thinned and weakened lithosphere.

6. The Effect of Plume-Ridge Interaction on the Topography of the Red Sea and

Surrounding Regions

So far, we have seen that plume-ridge interaction in the Red Sea and the Gulf of Aden is characterized by the flow of hot fertile plume material into the subaxial asthenospheric channel of the two ridges, where it interacts with MORB-derived mantle as it is delivered to the seafloor

(Morgan, 1971; Ribe et al., 1995). This interaction produces heightened mantle temperatures, thickened oceanic crust, and uplifted topography (Crough, 1983; Weir et al., 2001; Hooft et al.,

2006). Characterizing the processes responsible for creating and maintaining resulting hotspot swells can help further constrain the dynamics of plume-ridge interaction. In order to explain the formation and longevity of these bathymetric swells, previous authors invoke a combination of uplift caused by viscous forces induced by convection of mantle plume material (e.g. Courtney and White, 1986; Von Herzen et al., 1989; Daradich et al., 2003), reductions in lithospheric density produced by lithospheric reheating (e.g. Detrick et al., 1986), compositional buoyancy of the depleted root of a mantle plume (e.g. McNutt and Bonneville, 2000; Zhou and Dick, 2013), and crustal thickening by enhanced volcanism (Burke and Whiteman, 1973; Weir et al., 2001).

Buoyancy may also be enhanced by the presence of melt retained within the mantle residue (Phipps

Morgan et al., 1995).

64 Beginning south of -19.750 N along the southern Red Sea, the axial depth shoals from

north to south by -2000 m (Fig. 5). This regional, long-wavelength shallowing of the axial ridge

towards the Afar plume defines the swell topography in the Red Sea. North of -19.750 N, the

topographic signature of the plume disappears as indicated by the break in slope of the swell.

Where exposed, the central Red Sea ridge consistently extends to depths of only -2000 m. In

Sections 4-5, we showed that the melting regime of central Red Sea volcanics predicts normal

mantle potential temperatures and that the geochemical anomaly of the plume does not persist into

the central Red Sea. Therefore, we can say with confidence that the geochemical, thermal, and

topographic signatures of the plume do not persist north of-19.75' N.

Furthermore, seismic tomography has identified low-velocity anomalies that concentrate

in southwest Arabia and extend to mid-mantle depths (Chang and Van der Lee, 2011; Hansen et

al., 2012). Channels of low velocity material extend southwestward beneath Ethiopia and eastward

beneath the Gulf of Aden. Plume-derived mantle flow bypasses the central Red Sea, and instead

migrates north-northeastward beneath Arabia (Chang and Van der Lee, 2011). At the Afar triple junction, anomalously high topography has been attributed to the presence of this megaplume,

although local rift shoulder uplift contributes to this effect. Dynamic topography has been a

favored mechanism for the African superswell (Lithgow-Bertelloni and Silver, 1998; Daradich et

al., 2003; Sembroni et al., 2016; Gvirtzman et al., 2016). However, these studies calculate residual

topography using models that rely on accurate assumptions for crustal thickness (Daradich et al.,

2003) and mantle density heterogeneities (Lithgow-Bertelloni and Silver, 1998). For example,

Daradich et al. (2003) observes wholescale tilting of the Arabian plate in response to seismically

imaged plume-derived mantle flow beneath Arabia. They use viscous-flow simulations and

characterize the long-wavelength increase in African-Arabian flank uplift as dynamic topography

65 produced by viscous flow within the mantle. The results of this study, however, rely on accurate crustal thickness estimates, which are scarce along the Red Sea and Gulf of Aden.

The Afar hotspot swell presents a unique opportunity to study the importance of different processes in producing topographic uplift. This is because the Afar plume is centered over the continental crust of both the African and Arabian plates as well as young oceanic crust in the Gulf of Aden and southern Red Sea. If the topographic response to plume-induced uplift is the same in all four domains, we can clearly state that the African-Arabian swell is indeed a reaction primarily to dynamic uplift caused by material upwelling within the mantle plume.

We thus generate five topographic profiles extending from the Afar plume: two transects across the oceanic crust of the Gulf of Aden ridge (projected onto a 770 line) and the Red Sea ridge

(projected onto a 1500 line) and three profiles extending across the continental crust of the northern

African swell, southern African swell, and Arabian swell (Fig. 18, 19a). In order to compare the profiles to each other, on-shore topography is water-loaded using the equation

water - loaded topography = topography x P a - Pair) (P a -P water/ where Pa is the density of the asthenosphere (3300 kg/m 3). In order to analyze swell topography, we plot elevation with respect to the non-swell-influenced terrane for each profile (Fig. 19b), which is estimated to be the average elevation of terrane found outward of the break in slope of the swell along each profile (Fig. 19a). We translate the Gulf of Aden profile up by 3000 m (depth of the normal oceanic crust along the ridge). As discussed above, although the central Red Sea ridge axis is much shallower (-2000 m) than other ultra-slow axial valleys (-4000-5000 m), it is clearly not influenced by the Afar plume. Therefore, we translate the Red Sea up by 2000 m. Because the Afar plume is asymmetric, some areas display more extensive swells. In order to compare the

66 30oN-

25 0N

20O N-

150N- Legend Northern African swell # Red Sea swell Arabian swell Gulf of Aden swell 100N- Southern African swell Khansh Geochemical extent I Al lrqah f . of plume e Plume center o Outward limit of plume topography 50% along swell N 60% along swell

0 0 35oE 400E 45 E 50 E circles reflect Figure 18. Location of transects along swells in the Red Sea region. Yellow Orange the outward limit of dynamically supported topography (0% plume topography). and the dots mark the midpoint between the plume center (100% plume topography) of the Afar outward limit of each swell. Lines join these markers to illustrate the path in the plume northeast beneath the Arabian plate. Plume-related mantle flow is obstructed Red Sea and within northern Africa by the Barka-Ad Damm suture zone. The topographic of Aden influence of the plume extends further along both the Red Sea and the Gulf by the compared to plume-related geochemical thermal anomalies, which are constrained of Aden. Danakil rift in the Red Sea and the Khanshir Al Irqah fracture zone in the Gulf

67 3000 - non-swell- 3000 a ~ 'E00 influenced terrane- >% 1000 - - c. 0~

o2 -2000 V

~ 3000 W -4000 II i i I I I I I 200 400 600 800 1000 1200 1400 1600 1800 Radial distance from plume (km) 5000

E 4000-b 000 To Afar plume 3000

C)2000-

~0 1000. 4-L

-100C-

-200J 75% 50% 25% 0% Percentage along swell Legend Red Sea swell S Exposed oceanic crust * North African swell * Gulf of Aden swell Basin fill I * South African swell 0 Arabian swell

Figure 19. a) Topographic profiles of the northern African, southern African, Arabian, Gulf of Aden, and Red Sea swells. In the Red Sea, points plotted in black mark the topography of the oceanic ridge, while points plotted in gray are those areas along the Red Sea ridge covered by evaporite deposits and thus do not represent true axial ridge depths. b) Water-loaded on-land profiles are plotted against the topography of oceanic swells. 3000 m (average depth of the axial ridge) has been added to the topography of the Gulf of Aden, while 2000 m has been added to the Red Sea as the central Red Sea has been shown to be isolated from interaction with the plume (this study). We plot the corrected topography (see text for correction) against the percentage along each swell, where 0% indicates the absence of plume topography and 100% represents the plume center. Ignoring rift flank uplift, similar amounts of shoaling with decreasing distance to the Afar plume imply dynamically supported topography.

68 topographic responses of different profiles at the same point along their respective uplifts, we plot in Figure 19b the corrected topography against percentage along each swell, where 100% represents the plume center and 0% represents the break in slope of the swell toward non-swell-

influenced terrane.

All transects show an increase in elevation as they approach the plume center (Fig. 19b).

Ignoring the effect of rift shoulder uplift on on-shore topography and the contribution of changes in axial morphology along ridges (Canales et al., 2002), the increase in relative elevation along all

swells occurs at similar rates. Because the oceanic and continental regions surrounding the Afar

plume show similar swell topography, crustal thickness variations are probably not a main factor controlling uplift. In addition, calculated mantle potential temperatures in the southernmost Red

Sea are significantly lower than those predicted for other hot spots, indicating that reduction in

lithospheric density via enhanced temperature probably does not cause large-scale uplift in this

region. We therefore conclude that the long-wavelength signal of the African-Arabian swell is

dynamic in origin. Our results are consistent with all previous studies that invoke dynamically

sustained topography to explain uplift of the Afar superswell. Of course, localized regions of melt

retention and compositional buoyancy may contribute to small-scale changes in uplift (Daradich,

2003).

The break in slope of the swell along each profile marks the limit of dynamically created topography and the Afar plume's influence on uplift. We mark the location of this inflection point

along each profile in map view (yellow dot in Fig. 18). As previously suggested, the Barka suture-

Ad Damm fault zone, which forms the boundary between the northern and southern ANS, defines the northern extent of dynamic topography in both the Red Sea and on the African plate. The high- elevation African-Arabian escarpment diminishes rapidly north of this boundary, indicating that

69 the steep topography in these regions is not only a product of rift shoulder uplift but is also a dynamic response to plume convection (Daradich et al., 2003). Therefore, this fossil suture zone, most likely preserved during interaction with rifting, serves to mitigate the viscous stresses produced by upwelling of Afar plume material.

7. Discussion

We have presented in this report evidence that the northern, central, and southern tectonomagmatic sections, along with Ramad seamount and the southern islands, define supersegments of the Red Sea characterized by different structural, topographic, geochemical, and thermal systems. In the central and southern segments, where the spreading rate is the fastest (12-

16 mm/yr), an absence of magmatic segmentation, coupled with increased melt supply, are reminiscent of faster spreading ridges. North of the ZFZ, the character of the spreading ridge changes such that magmatic and topographic characteristics of this area are more typical of other ultra-slow spreading centers. Similar transitions between magmatic and amagmatic spreading have been observed along the Gakkel ridge (Jokat et al., 2003).

In addition, geochemical variability along the Red Sea results from different extents, temperatures, and pressures of melting of variably enriched/depleted plagioclase- spinel-bearing peridotite and different degrees of interaction with the Afar plume. Variations in pressure and extent of crystallization also contribute to the observed major element diversity along the Red Sea.

Surprisingly, mantle potential temperature does not vary strongly throughout the Red Sea until within the vicinity of the Afar plume. We summarize our findings in the context of the geochemistry, thermal regime, topography, morphology, and structural evolution of the four tectonomagmatic segments and construct a model to explain the observed variation in all aspects along the Red Sea ridge.

70 7.1. The Northern Red Sea as an Ultra-Slow Spreading Ridge

The topography, morphology, and structure of the northern Red Sea is mostly obscured by thick accumulations of sediment and evaporite deposits. As such, gravity signatures that exclude the effects of salt are difficult to resolve, while magnetic lineations, when present, are subdued, episodic, and most likely represent axial diking and local volcanism within bathymetric deeps

(LaBrecque and Zitellini, 1985; Girdler, 1985; Dyment et al., 2013). It thus remains enigmatic as to whether the basement along the rift axis north of the Zabargad Fracture Zone is composed of attenuated continental crust (Cochran, 1983; Bonatti, 1985; Cochran and Martinez, 1988; Guennoc et al., 1988; Bosworth, 1993; Ghebreab, 1998; Cochran, 2005; Bosworth et al., 2005; Mitchell and

Park, 2014; Almalki et al., 2015; Almalki et al., 2016) or young oceanic lithosphere (LaBrecque and Zitellini, 1985; Girdler, 1985; Gaulier et al., 1988; Sultan et al., 1992; Dyment et al., 2013;

Tapponnier et al., 2013; Stern and Johnson, 2019). Based on the analysis provided in this paper and supplemented by previous work, we favor the latter interpretation that the northern Red Sea is a magma-poor ultra-slow spreading ridge that exhibits melt focusing and has an axial ridge characterized by accreted oceanic crust at least up to 26.5-270 N. First-order observations include the 40-60 km wide, evaporite-floored axial depression. Here, flowage of salt probably exceeds the ultra-slow spreading rates such that the salt-floored oceanic basin widens via intrusion of dikes and sills (Girdler, 1985). High temperatures, unable to diffuse through the insulating evaporitic blanket, weaken magnetization and cause slow cooling of basalts (Cochran and Martinez, 1988).

The presence of off-axis volcanoes suggests that crustal accretion is probably not restricted to the axial ridge valley (Standish and Sims, 2010). Based on poor sample coverage, we cannot rule out the possibility that the northern Red Sea is composed of magmatic segments separated by amagmatic accretionary zones of exhumed mantle peridotite (Dick et al., 2003). These zones are

71 characterized by weak magnetization, a wide axial trough, and a thin basaltic carapace. We also cannot comment on the extent of mantle exhumation in the eastern half of the northern Red Sea, as this area lacks exploration (such as seismic refraction data).

Within the axial depression, a trail of elongated salt-walled mini-basins that formed from a combination of faulting and salt dissolution (Banham and Mountney, 2013) demarcate the trace of the axial valley. Relatively deep basement is observed beneath the diffuse rift axis. In fact, the presence of an ultra-slow spreading ridge-like axial valley that is on average -4700 m deep is confirmed from the sediment-corrected topography derived from depth to basement measurements taken along 2-D reflection seismic images. Seismic refraction studies conducted in the western

Red Sea at a latitude of 260N beneath the axial depression reveal 6.7 km/s crustal velocities that extend from -7 to 11 km below sea level (Gaulier et al., 1988). These velocities support the presence of oceanic crust at this latitude, which leads Gaulier et al. (1988) to conclude that the Red

Sea is undergoing oceanic crustal accretion south of Brothers Island. At this latitude, continental crust is present -50 km west of the rift axis. A COB at this distance from the ridge is consistent with the recent initiation of seafloor spreading in the northern Red Sea compared to the rest of the ridge, although Rihm et al. (1991) observe oceanic crust only 20 km off the western Red Sea coast in this same area.

However, a matter of controversy is that north of -26.5"N, Gaulier et al. (1988) find -7-8 km thick continental crust just west of the rift axis at depths of 8 km. Within the axial depression,

Gaulier et al. observe continental crust-like seismic velocities of 5.7-6.2 km/s between depths of

-7-8 and 13 km, although poor quality of arrivals along the axis limits interpretation. In addition, in the northeastern Red Sea east of the rift axis (line PIV parallel to the coast), Rihm et al. (1991) record 15 km thick continental crust with a velocity of-6 km/s. We thus find that from 26.50N to

72 27'N, the presence of continental crust in the Red Sea proposed by Gaulier et al. (1988) and Rihm et al. (1991) is at odds with the deep basement along the rift axis. We argue that continental crust cannot isostatically exist at these depths (4000-6000 kin). Instead, basement beneath the axial depression in the north sits at depths reminiscent of ultra-slow spreading ridge axial valleys and cannot be characterized by hot continental crust. There is thus a discrepancy between isostatic predictions and seismic refraction results within the axial region between 26.5 and 270N. This region, as argued by Gaulier et al. (1988), most likely represents the transition from oceanic spreading in the south to continental rifting in the north. Here, proto-oceanic crust could form deep ridges immediately adjacent to attenuated continental crust. Furthermore, this potential 'transition zone' is bounded by Transfer Zones I and 2. TZ2 may extend to the east to truncate the northern extent of the marginal basin, which is underlain by deep basement most likely composed of oceanic crust or exhumed mantle. The latitude of this proposed transition zone also corresponds with the transition from predominantly aseismic seafloor spreading in the region between Mabahiss and

Shaban deeps to seismically active tectonics in the north (Bosworth et al., 2019).

South of this transition zone, seismic data reveal that the northern Red Sea basement is dissected by simple-shear listric and low-angle east- and west-dipping detachment faults. In an east-dipping domain of the northern Red Sea, we observe a slightly concave downward detachment fault and corresponding basement high, features which are commonly related to large-scale mantle exhumation (Whitmarsh et al., 2001; Dick et al., 2003; Manatschal, 2004). To the west of this potential oceanic core complex, where the detachment fault is cut by a high-angle out-of-sequence normal fault, the ridge deepens to axial valley topography. Unexpectedly, although multiple domains of uplifted basement are observed in E-W transects along the Red Sea, prominent mantle- exhuming, concave downward detachment faults common in magma-poor slow to ultra-slow

73 spreading ridges are not often observed along these transects (e.g. Whitmarsh et al., 2001; Cannat et al., 2006; Smith et al., 2006; Escartin et al., 2008; Gillard et al., 2016). Likewise, within and to the west of the axial depression south of the transition zone, anomalously shallow Moho reminiscent of mantle exhumation is not identified (Gaulier et al. 1988; Rihm et al., 1991).

However, because topography, morphology, volcanism, and structure within and around the Red

Sea display striking asymmetries (Wernicke, 1985; Voggenreiter et al, 1988), an across-axis asymmetrical occurrence of mantle exhumation would also not be surprising (Oyarzun et al, 1997).

Based on seismic data, within the eastern half of the northern Red Sea, major high-angle listric normal faults that dominate extension in this area may represent early stage detachment faulting and mantle exhumation, suggesting that in time these faults may rotate to low angles as the footwall is flexurally unloaded (Buck, 1988; Reston and McDermott, 2011; Reston and Ranero, 2011).

Detachment systems overlain by thin basaltic carapaces may also be obscured by a series of fault blocks offset by secondary steep, small-displacement, in- and out-of-sequence antithetic and synthetic normal faults, which also accommodate extension within the northern Red Sea (Reston and Ranero, 2011; Lutz et al., 2018). In fact, similar basement highs and graben-types structures observed in the Red Sea are present within the exhumed serpentinized mantle of the magma-poor

Newfoundland margin (Gillard et al., 2016).

Lastly, along a N-S transect of the northern Red Sea, basement-involved listric detachment faults flip polarity across Gulf of Aqaba-trending transfer zones. Alternating zones of west- and east-dipping detachment systems may indicate a temporal difference in development between each structural domain (Geoffroy et al., 2014; Reston, 2018) or may highlight the influence on rifting of the orientations of pre-existing lithospheric weaknesses (e.g. Schilling et al., 1992). Simple shear extension in this domain may indicate that the Red Sea is rifting subparallel to pre-existing

74 structures preserved in the crystalline basement, such as the Najd fault system (Ziegler and

Cloetingh, 2004).

Overall, the structural and topographic characteristics of the northern Red Sea suggests seafloor spreading accommodated by detachment faulting. Yet seismic refraction studies are not consistent with this interpretation in areas north of 270N, and there is not enough data to comment on the extent of mantle exhumation. We hold that, without borehole data, complete across-axis seismic lines, mantle Bouguer anomalies, and high-resolution velocity structure, it is difficult to resolve with certainty the nature of the northern Red Sea basement. Yet the geochemistry of axial basalts and the thermal structure predicted from their compositions are consistent with our structural interpretation.

The presence of discrete volcanic centers within axial deeps along the northern Red Sea probably reflects along-axis magmatic focusing (Fig. 20; Jokat et al., 2003), where melt is funneled into Conrad and Shaban deeps and Mabahiss seamount (Standish et al., 2008; Schlindwein et al.,

2013) most likely as a result of undulations in the lithosphere-asthenosphere boundary (Cannat et al., 2003; Standish et al., 2008; Montesi et al., 2011). That recent magmatic emplacement is localized within these deeps is consistent with the large, normally magnetized, dipolar magnetic anomalies displayed in these areas (Cochran et al., 1986, 2005). In fact, a seismic gap exists along the segment between Mabahiss and Shaban deeps (Bosworth et al., 2019). Focused magmatism, and perhaps related axial magma chambers, establish high temperatures that suppress seismic activity by rendering brittle faulting impossible (Schlindwein et al., 2013; Yu et al., 2018).

Reductions in seismicity may also be caused by the presence of amagmatic lithosphere in this region (Schlindwein and Schmid, 2016).

75 Radial distance from Afar plume (km) 200 400 600 800 1000 1200 1400 1600 1800 SI I I I I I I

Ramad seamount Oceanic Spreading Hatiba Mons Mabahiss Shaban

W Basin fill Crust Raad seamount u I sea and the southern t Sea e islands Central Red Sea san \Northern Red Sea 10

Zabargad

- . . Fracture I.Zone E 20 -G

4-J G t 0~ Focused melt flow Plagioclase-bearing I Litho- w hrtsphere . 30 -0I E 0

- Plaioclase sinel- Plaqioclase-beina9Iocase-earng ------I lherzolite Active beri upwelling lherzolite 40 -. tMore Active mantle passive Active plume- Asthenospheric upwelling from mantle km deep related upwelling mantle T0-400 = 32eC uT=we13in11& 50C T = 1307 1 10C T= 1326 I

14 16 18 20 22 24 26 Latitude ON

Legend Peridotite Base of oceanic Trapped melt solidus lithosphere

Figure 20. Schematic along-axis cross section of the Red Sea showing north-south variations in oceanic lithospheric thickness and depth of melting. Significant tectonic boundaries (Danakil rift, Barka-Ad Damm suture zone, Zabargad Fracture Zone) are highlighted. Melting is initiated when the temperature of the mantle exceeds the peridotite solidus, as there is no evidence for pyroxenite melting within the Red Sea. Basalts from the southern Red Sea record only very slight mixing with Afar plume material and instead display a significant central Red Sea mantle component. See text for further explanation.

76 Decompression melting of ascending and narrowing asthenosphere has led to magmatism of E-MORB-like composition along the axial valley, which has generated deep and thin oceanic crust (Fig. 20). Magmas are derived from shallow (-31 km), low degree melting of relatively fertile mantle (alkali and incompatible element-rich with high Nb/Zr, K/Ti, La/SmN, and Sm/YbN). At

Mabahiss deep, melting originates predominantly in the plagioclase field, whereas both plagioclase- and spinel-field melting are predicted beneath Shaban deep. Results of the reverse fractional crystallization model suggest that magma then ascends through the mantle until it reaches the base of the conductive lithospheric lid, where fractional crystallization begins to reduce the mass of the melt parcel. However, for a similar range in melting pressures as the central Red

Sea, Na8 contents of basalts from the northern segment suggest lower degrees of partial melting

(Fig. 21). Because the northern Red Sea is a magma-poor ultra-slow spreading segment, it is likely that lower extents of melting reflect a melting column truncated at its top by a thicker conductive lithospheric lid. Basalts that erupt at the surface are those high-flux magmas that remain molten during their ascent through the extensive lithosphere to the surface (Herzberg et al., 2007). Because they experience fractionation at shallower depths within the lithosphere, these erupted magmas record pressures of fractionation shallower than the base of the lithosphere. Most melts, however, lose too much heat to their surroundings and fractionally crystallize to an extent that they become trapped in the thick lithosphere.

Furthermore, although they are more differentiated (lower Mg#s), basalts erupted in the

northern Red Sea have FeO*, A12 03, K20, and Na20 contents that overlap with the compositional array of ultra-slow spreading ridges. Varying proportions of melting within the plagioclase and spinel stability fields most likely contribute to the compositional heterogeneity between Mabahiss

77 - 3.6 I -1 I IV I I 3.4 - 0

3.2 - o o 0 0 0 Oc 3 U F 0 0 em 2.8 1- 0 U CO 0 zm 2.6 U 2.4

2.2 irn 2 -

1.8 V

1.6 I I I I I I I 7 8 9 10 11 12 13 14 15 Melting Pressure (kbar)

Figure 21. Na8 vs. melting pressure (kbar) for basalts along the Red

Sea rift axis. Na8 , or Na2 0 calculated at 8% MgO, is an indicator of the degree of partial melting (Plank and Langmuir, 1992). The existence of a positive correlation between melting pressure and Na8 indicates that greater extents of melting occur at shallower depths. Legend as in Figure 7.

78 and Shaban deeps (Brown, 2019). For example, higher Na20 and A1 20 3 contents of basalts from

Shaban deep reflect greater proportions of spinel-field melting compared to Mabahiss deep. In addition, K/Ti, Nb/Zr, La/SmN, and Sm/YbN ratios of northern Red Sea basalts fall within the range of ultra-slow spreading centers. Melt focusing and potentially more buoyant mantle upwelling beneath Mabahiss and Shaban deeps suggest thinner lithosphere, shallower pressures of differentiation, and longer mantle melting columns beneath these regions compared to normal ultra-slow spreading ridges. As such, magmas erupted within these deeps have higher CaO and incompatible major element contents compared to the majority of basalts from the Gakkel and

SWIR.

Altogether, compared to the rest of the Red Sea, the northern segment, spreading at 5.6-10 mm/yr (McClusky et al., 2003; Chu and Gordon, 1998), is characterized by detachment fault- dominated terrain, low extents of melting, melt focusing, and eruption of enriched basalts. South of the Zabargad Fracture Zone, faster spreading rates (10.8-16 mm/yr) and heightened and continuous volcanism of normal to depleted MORB-like basalts may indicate a transition to comparatively more buoyant flow in the central and southern segments (Fig. 20; Dick et al., 2003).

7.2. Central and Southern Red Sea

The Zabargad Fracture Zone isolates the northern Red Sea from the central and southern segments. South of this zone, the southern Red Sea has been undergoing organized seafloor spreading at least since 5 Ma (Cochran 1983; Gurvich, 2006). As shown by Augustin et al. (2014), continuous oceanic accretion extends into the central Red Sea. Here, seafloor spreading initiated around 3-1 Ma (Ligi et al., 2012) but may have begun as late as 0.78 Ma (Roeser, 1975; Bicknell et al., 1986). Previously interpreted as zones of attenuated continental crust (Bonatti, 1985), intertrough zones between prominent axial deeps are actually incipient transform valleys into

79 which gravity-driven namakiers have flown (Augustin et al., 2014). Spreading segments in between these incipient transform faults are defined by a series of axial deeps that potentially tap mantle of only slightly varying temperature and/or enrichment (Ligi et al., 2012).

Interestingly, in seismic dip lines along the eastern halves of the central and southern Red

Sea, large detachment faults that offset top-basement to very deep or shallow depths are not identified. Instead, the uppermost 2-3 km of the oceanic crust is cut by small-offset, closely spaced outward- and inward-facing normal faults, resulting in a series of horsts and grabens overlain by greater amounts of basin fill as basement subsides away from the axial ridge. In these regions, fault patterns most closely resemble those along the fast spreading (90-120 mm/yr) magma-rich northern East Pacific Rise (NEPR), where a horst and graben geometry results from movement along inward- and outward-facing faults of similar size and quantity (Edwards et al., 1991;

Alexander and Macdonald, 1996). In addition, faults appear to accommodate only a small part of tectonic extension. This tectonic fabric reveals that, unlike other ultra-slow spreading centers, rifting in the central and southern Red Sea is comparatively active and magma-dominated, where decompression melting and subsequent accretion of oceanic crust contributes significantly to plate separation. As a result, oceanic lithospheric mantle may be much thinner in this region than in normal ultra-slow spreading ridges.

Likewise, although the central and southern Red Sea spread at rates similar to the SWIR, the compositions of basalts erupted along these segments diverge from those of the northern Red

Sea and other ultra-slow spreading ridges. First of all, basalts are N- to E-MORB and are alkali and incompatible element-depleted. Due to its proximity to the Afar plume, the southern Red Sea exhibits slight enrichment in incompatible trace elements and LREEs and Afar plume-like isotopic

ratios as a result of minimal mixing between ambient asthenospheric mantle and plume material.

80 In addition, compared to the global array of ultra-slow ridges, axial volcanics have major element compositions depleted in TiO 2, Na 20, K2 0, and A12 0 3 and enriched in FeO* and CaO contents.

Because mantle potential temperatures remain relatively constant throughout the Red Sea north of the Danakil rift, these geochemical discrepancies suggest that magmas from the central and southern Red Sea are associated with anomalously fast and buoyant mantle upwelling and hence originated from high degrees of partial melting of a normal to depleted asthenospheric source.

Heightened magmatism related to larger extents of melting thus contribute to the magma-rich nature of these ultra-slow spreading segments. Magma-rich spreading in these regions probably weakens and thermally thins the lithosphere to normal oceanic crustal thicknesses (Dick et al.,

2003). As such, we predict thicker oceanic crust and much thinner lithospheric mantle along these

Red Sea segments compared to the SWIR (crustal thickness > 2.0; Muller et al., 1999) and Gakkel

(crustal thickness between 1.9-3.3 km; Jokat et al., 2003).

Melts from the central and southern Red Sea have trace element ratios indicating derivation from ambient mantle of similar fertility. However, in major element oxide versus MgO space, the

central Red Sea has higher A12 0 3, Na20, and TiO 2 and lower SiO2 contents relative to the southern segment, differences that most likely reflect variations in melting regime. For example, lower

A12 0 3, Na20, and TiO 2 contents in the southern domain reflect faster spreading rates and larger degrees of partial melting (Altherr et al., 1988; Haase et al., 2000; Augustin et al., 2014).

In addition, according to the results from the maximum likelihood reverse crystallization model (Brown, 2019), melting of the mantle beneath the central segment occurs at -34.5 km and

-1311 C (Fig. 20). Melting in the southern domain occurs at shallower depths (-27.6 km) and lower temperatures (~1281C). Plagioclase field melting dominates both regions, although 41% of central Red Sea glasses are better fit by melting in the spinel stability field. As such, the larger

81 proportion of spinel field melting in the central segment contributes to higher A1 2 0 3 and Na20 contents of these basalts (Brown, 2019). In the southern Red Sea, lower pressures of crystallization

(-2 kbar) relative to the central Red Sea (-4.8 kbar) indicate thinner oceanic lithosphere. In this region, shallow mean pressures of melting enhanced by thin lithosphere produce erupted basalts with elevated SiO 2 contents (Gale et al., 2014). In the southern Red Sea, the average pressure of fractional crystallization most likely corresponds to the base of the oceanic crust. This is because enhanced melt production along this ridge segment (as shown by low Na8 values of erupted basalts) caused by vigorous plume-related mantle upwelling (as inferred from plume-induced dynamic topography in this region), most likely erodes the base of the oceanic lithosphere. Crustal thicknesses in the southern Red Sea are thus on average 2-4 km thicker than normal ultra-slow spreading ridges.

In contrast, our results predict much thicker lithosphere in the central Red Sea (-15 km), and that the thickness of oceanic lithosphere increases by -10 km across the boundary from the southern to central Red Sea (Fig. 20). Because basalts from the central Red Sea display lower Na8 values and a shorter mantle melting column relative to the southern segment, central Red Sea magmas must originate from lower degrees of partial melting. This indicates that, compared to the southern Red Sea, the central segment is probably characterized by thinner oceanic crust (less than

-6 km). Thus, the presence of thicker lithosphere (-15 km) in the central Red Sea must be related to mantle upwelling rates that are slower than those in the southern segment but are still much more rapid than those at normal ultra-slow spreading ridges. In fact, active asthenospheric upwelling and mantle convection has been shown to occur in the central Red Sea by the global mantle convection model of Petrunin et al. (2017). In this section, seismic tomography has revealed that, around mantle transition zone depths (400 km within the mantle), a strong low-velocity

82 anomaly exists beneath the central Red Sea. Here, buoyant mantle unrelated to the Afar plume rises from the mantle transition zone and contributes to more efficient delivery of magma to the ridge axis of the central Red Sea compared to normal ultra-slow spreading ridges. The central segment also defines a seismic gap, where M > 4 earthquakes are rare (Bosworth et al., 2019). We therefore suggest that the central Red Sea taps deep asthenosphere that is hot and depleted relative to the enriched major element and trace element source composition envisioned beneath normal ultra-slow spreading ridges by Brown (2019). Thus, viscous forces imposed by mantle convection, potentially coupled with compositional buoyancy, result in the anomalously shallow depths of the central Red Sea axial valley. Basalts from the southern Red Sea are most likely sourced from this deep asthenospheric mantle, which along this segment has experienced small-scale mixing with the enriched Afar plume component. In addition, it is important to note that the thick lithosphere calculated for the central segment could represent the thickness of oceanic crust in this region if a mechanism for efficient melt delivery of lower-degree partial melts, such as rapid melt extraction via dikes that tap melt-rich areas (Cannat et al., 2003), is active. Thick crust (-15 km) in this section would play a dominant role in producing the anomalously high-standing topography of the central Red Sea ridge.

Lastly, the boundary between the central and southern Red Sea corresponds with the continuation of the Neoproterozoic Barka suture and Ad Damm fault zone, which separates the northern and southern domains of the Arabian-Nubian Shield (Johnson et al., 2011). South of this boundary, the Red Sea ridge is affected by plume-imposed dynamic topography as a result of viscous stresses associated with flow within the mantle (e.g. Braun, 2010; Flament et al., 2013;

Petrunin et al., 2017). However, as we will discuss, intersection of the Red Sea ridge by the Danakil rift has begun to largely inhibit migration of plume material north of I 7.050N.

83 7.3. Plume Influence Beneath Ramad Seamount and the Southern Islands

Furthermore, the topography of the southernmost Red Sea south of 19.75'N is characterized by an axial rise that shallows toward the Afar plume. Within this region of dynamically supported topography, swarm-type seismicity has been shown to be linked to volcanic activity (El-Isa and Shanti, 1989). It is thus not surprising that major, trace, and isotopic compositions of basalts from Ramad and the southern islands reflect large-scale interaction with the Afar plume. As the Ramad seamount is approached from the north, greater temperatures

(1 306'C) and depths of melting (-36 km) of a plagioclase-bearing lherzolite in accordance with higher mantle potential temperatures (1326'C) result in heightened melt production as well as thickened crust, below which the lithosphere has most likely been thermally eroded. Larger mantle potential temperatures and crustal thicknesses are consistent with the gravity structure of this region and active mantle flow inferred by Petrunin et al. (2017). Slow seismic velocity anomalies at mantle depths further indicate elevated potential temperatures (Chang and Van der Lee, 2011).

However, despite obvious plume-ridge interaction along this segment, calculated mantle potential temperatures (1326 0C) are not as high as we would expect. Rooney et al. (2012) uses PRIMELT2 software to calculate Tp for primitive basalts from Afar and conclude that mantle potential temperatures of volcanic rocks younger than 10 Ma from the East African Rift range from ambient mantle temperatures (-13500C) to 14900C, with the highest values in Djibouti (AT = 140'C). These values are also much lower compared to other hotspots. For example, at Iceland, Tp estimates range from 1480-15200C (MacLennan et al., 2001) to 1637'C. TP calculated using PRIMELT2 ranges from 1400-1500"C at the Canaries and Galipagos Islands (for reference, PRIMELT2 computes a

Tp for the East Pacific Rise of 13700 C; Herzberg and Asimow, 2008). Therefore, in the southernmost Red Sea, primary magmas are most likely derived from the cooler peripheries of the

84 Afar plume, while the greatly reduced shear velocities beneath this region have been interpreted to be derived from C02-assisted melting (Hertzberg and Gazel, 2009; Rooney et al., 2012).

7.4. Role of the Danakil Rift in Diverting Plume Flow

We agree with Volker et al. (1997) in that plume-derived mantle is concentrated near

Ramad seamount. Just north of this volcano, the intersection of the Danakil rift with the Red Sea at I 7.05'N serves to thermally and geochemically separate the southern Red Sea from the Afar plume focused beneath Ramad seamount and the southern islands. The Afar plume-like geochemical signature preserved in Sr, Nd, and Pb isotopic signatures of magmas from the southern Red Sea may indicate that, before the Danakil rift propagated northeast toward the Red

Sea rift at around 1 1 Ma or later (Eagles et al., 2002), plume material successfully migrated north along the incipient southern ridge and contaminated the mantle underlying this segment. The boundary between the southern and central segments formed by the remnant Barka-Ad Damm suture zone most likely serve to staunch northward flow of plume-derived mantle by completely deflecting plume migration. Although long-lived isotopic signatures of basalts from the southern segment preserve mixing between the Afar plume and the central Red Sea, the K/Ti, Nb/Zr, and

La/SmN ratios of the southern segment are relatively depleted and resemble those of the central

Red Sea. With the propagation of the Danakil rift around 10 Ma and subsequent isolation of the southern domain from the Afar plume. the trace element composition of the Afar plume within the southern Red Sea mantle experienced dilution. Diverted by the Danakil rift, channel flow along the Red Sea travels northeast beneath the Arabian plate. North-northeast mantle flow is enhanced by the channeling of plume material along preexisting N-S striking structures (such as the Bidah and Umm Farwah shear zones and the Al Junaynah fault zone) within the Asir terrane. Curiously, plume material continues north past the Ad Damm fault zone to form the young, N-S trending

85 alkaline Harrats along the MMN line. The trajectory of the Afar plume inferred from geochemistry and topography of the Red Sea region agrees with that indicated by seismic tomography-detected low-velocity anomalies (e.g. Chang and Van der Lee, 2011).

8. Conclusions

1. The Red Sea defines four tectonomagmatic segments (north, central, and southern Red Sea

and the southernmost Red Sea composed of Ramad seamount and the southern islands)

each with a distinct combination of structural, topographic, and geochemical characteristics

due to north-south changes in spreading rate, mantle source, mantle upwelling rate, and

proximity to the Afar plume along the ridge.

2. Basalts from the Red Sea north of 17.05'N record relatively constant mantle potential

temperatures within the range of ambient mantle and have geochemical signatures that

require higher degrees of partial melting and more vigorous mantle upwelling than normal

ultra-slow spreading ridges. Comparatively rapid mantle upwelling in the central and

southern Red Sea results in abnormally magma-rich spreading and, in the central Red Sea,

anomalously shallow axial ridge depths.

3. The northern Red Sea south of ~26.5-27'N is an ultra-slow spreading ridge characterized

by detachment faulting, a deep axial valley (-4700 m deep), and focused low-degree

magmatism derived from a plagioclase spinel-bearing enriched source. We agree with

Gaulier et al. (1988) in that the transition from seafloor spreading in the south to continental

rifting in the north begins around -26.5-27'N.

4. The central Red Sea extends from the Barka-Ad Damm suture zone at least to Nereus Deep

and is a magma-rich ultra-slow spreading center segmented by incipient transform faults

(intertrough zones) and non-transform discontinuities. Plagioclase spinel field melting of

86 a depleted asthenospheric source is associated with a 400-km low velocity anomaly

beneath this region.

5. Compared to the central segment, the southern Red Sea is a magmatic spreading segment

that displays shallower plagioclase field melting of a mantle source composed

predominantly of a central Red Sea depleted asthenosphere component with a minor Afar

plume component. Low Na8 indicates high degrees of partial melting as a result of higher

spreading rates and plume-induced mantle upwelling.

6. Plume-ridge interaction is sequestered south of the intersection between the Danakil rift

and the Red Sea at 17.05"N. Instead, the Danakil rift deflects flow of plume material

northeast beneath the Arabian shield.

7. Ramad seamount and the southern islands are defined by Afar-plume like geochemical

signatures (alkali, incompatible, and light rare-earth element enriched and heavy rare-earth

element depleted). Elevated mantle potential temperatures in this region compared to the

rest of the Red Sea are consistent with the presence of the Afar thermal anomaly in this

region. Deep pressures of fractional crystallization imply thickened crust as a result of

heightened melt production.

8. Viscous stresses created by mantle flow of plume material result in dynamically supported

topographic uplift of the Afar swell. Within the Red Sea, axial shoaling and associated

plume-imposed mantle convection occur south of 19.750 N. Uplift along the Red Sea (and

also the Nubian shield) is thus isolated to the south by the fossil Neoproterozoic Barka-Ad

Damm suture zone.

87 References

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