Geophysical Evidence for Magmatism Southwest of the Brothers Islands, Northern Red Sea (Offshore Quseir, Egypt)

The Red Sea formed as a consequence of Cenozoic rifting between the African and Arabian plates. While continuous oceanic spreading is active in the southern and central sectors, in the northern Red Sea, exposure of oceanic crust is limited to a few along‐axis isolated deeps. However, several off‐axis magmatic edifices have been recognized in this sector, their description in terms of relative age and architectural features remains vague. One of these, offshore Quseir corresponding to a kilometer‐size structural high, was studied in detail using 3D industrial seismic data. This structure is interpreted as due to the coalescence of different volcanic edifices developed on the footwall of a major rift‐related normal fault. Magnetic and gravity data and forward modeling suggest a volcanic nature of the substratum underlying the morphological relief. A similar volcanic origin is proposed for other neighboring basement hills that show a similar magnetic signal. Relationships with the imaged seismo‐stratigraphic sequences point to a Pleistocene age for the magmatic event. Arabian alkali‐basalts, at the same latitude as Quseir, show ages comparable to those inferred for the volcano edifice studied, suggesting a similar geotectonic context. In western Arabian plate, large alkali‐lava fields (Harrats) developed along rift‐related faults since the early stages of the Red Sea rift (Late Oligocene). They display a significant northward rejuvenation trend that may be explained by the progressive propagation of the Afar mantle plume.

In this paper, different geophysical data sets and techniques are used to suggest the possible occurrence of volcanic edifices offshore Quseir (Egypt) better to define the size and geometry of such magmatic outpourings and possibly more tightly constrain their age of formation. The regional implications of these findings and their significance related to NRS rift evolution are also addressed.

Geological Setting of the NRS
Most of the Red Sea geology information comes from onshore stratigraphic and structural data and offshore geophysical data. Only 14 exploratory wells have been drilled in the Egyptian Red Sea margin for hydrocarbon prospecting in the last 50 years, and little associated data are in the public domain. In addition, offshore seismic interpretation is severely limited by a thick layer of syn-rift evaporites (e.g., Cochran, 1983;Le Magoarou et al., 2021;Rowan, 2014). As a result, the structure and the nature of the crust that floors the NRS is still a matter of debate (e.g., Augustin et al., 2021).
1 The crystalline basement of the Egyptian Red Sea rifted margin and the Eastern Desert formed during the Neoproterozoic and resulted from several magmatic and metamorphic phases developed during the Pan-African orogeny. Several studies on the basement composition and structures in the onshore outcrops have documented significant, complex Pre-Cambrian tectonic events (Johnson, 2014;Rogers et al., 1978;Stern et al., 1984). Conversely, only a few exploratory wells penetrated the Egyptian offshore basement, but the information has remained relatively poorly studied. Four boreholes north of Quseir (RSO-X 94-1; RSO-B 96-1; Figure 2a; Barakat & Miller, 1984;Tewfik & Ayyad, 1982) cut through granodiorite and granite, while the southern wells (Quseir A-1X; Quseir B-1X; Figure 2a; Bosworth et al., 2020;Ligi et al., 2018;Moustafa & Khalil, 2020) penetrated metamorphic rocks, including gabbros, suggesting a complex and still not entirely deciphered basement architecture. The results of the drilling program in the Egyptian Red Sea margin suggest that the Pan-African basement is present for several tens of kilometers offshore from the Western coastline. However, its composition and structure may be highly variable. Geochronological and geochemical analyses of gabbros drilled at the Quseir B-1X well and sampled from Brothers Islands revealed an oceanic-type basaltic melt composition and a cooling age of 25 ± 6 Ma ( 40 Ar/ 39 Ar), suggesting intrusion/ underplating of mid-ocean ridge basalts melt as a consequence of a thermal event during the initial phase of the Red Sea rifting (Ligi et al., 2018). Therefore, the Brothers gabbro was interpreted as a lower crust intrusion of asthenospheric melt that predated the onset of rift and then was exhumed above the sea level due to a significant uplift of an extensional fault footwall (Bonatti et al., 2015;Bonatti & Seyler, 1987;Guennoc et al., 1988;Ligi et al., 2018;Taviani et al., 1986). South far of the Brothers Islands, Zabargad Island represents an uplifted block of lithosphere, this block includes three distinct upper mantle peridotite bodies. The existence of peridotites provides an example of "preoceanic," rather undepleted mantle bodies (Bonatti et al., 2015).  (Bosworth, 2015). (b) The main syn-rift sequence, during which tectonically driven subsidence was most rapid, is composed of a lower succession of siliciclastic rocks and an upper section formed by thick halite and layered evaporite sequences ( Figure 1b). Carbonate units are volumetrically limited but represent regionally correlatable stratigraphic events. Several lines of evidence suggest that the entire Red Sea basin desiccated at the end of the Miocene age when it became subaerially exposed (Mitchell et al., 2017). This event resulted in widespread, major unconformity surface (post-Zeit), usually reported in the Red Sea literature as the "S-reflector" (Girdler & Whitmarsh, 1974;Mitchell et al., 2010;Ross & Schlee, 1973), which marks the base of post-evaporitic section. 3 During the Early Pliocene, most of the present-day NRS and the Gulf of Suez areas were flooded by open marine waters entering the Indian Ocean via Bab el Mandeb Strait (Said, 1990). Pliocene rocks include sandstone, claystone, limestone, and occasional gypsum, described with various formational names (Figure 1b). At the end of the Pliocene, an unconformity surface is observed in the Red Sea onshore. In contrast, the Pliocene-Recent sequence offshore is more complete and formed by sandstones, generally referred to as the Shukheir Formation (Bosworth et al., 2020).
The northwestern Red Sea and the Gulf of Suez display similar structural patterns characterized by zig-zagging faults that overall trend parallel to the rift strike (Jarrige et al., 1990;Montenat et al., 1988;Patton et al., 1994;Tewfik & Ayyad, 1982). Several km-scale tilted blocks bounded by major normal faults striking NW-SE have been described in the literature (Figure 1a; e.g., Khalil & McClay, 2001). At and south of Quseir, the Egyptian coast is dominated by a sizable onshore half-graben generated by a west-dipping master fault (the "Gebel Duwi fault") that runs sub-parallel to the shoreline ( Figure 1a). Offshore, the system is complicated by several irregular salt domes and many prominent salt walls that lead to complex structural patterns, especially at depth (Bosworth et al., 2020;Gordon et al., 2010;Mitchell et al., 2019). Faults cut the syn-kinematic Rudeis, Kareem, and Belayim Formations, as evidenced by interpretations on the published seismic profiles, but the presence of salt challenges the identification of deep structures on the seismic profiles. Therefore, the interpretation of the offshore fault system is mainly related to shallow structures at the interface with the seismic basement ( Figure 1a). A limited number of published seismic sections show similarities in the structural setting of the near-shore margins of Saudi Arabia and Egypt, while the conjugate distal margins have a different structural style (Stockli & Bosworth, 2019). Detachment fault systems have been observed in the Midyan area of Saudi Arabia and along the Egyptian shoreline (Bosworth & Burke, 2005;Mougenot & Al-Shakhis, 1999;Stockli & Bosworth, 2019). Stockli and Bosworth (2019), interpreted a NE-SW 2D seismic section in the distal Egyptian margin and observed two NE dipping low-angle normal faults rooted in the continental crust.
The setting of the NRS was also influenced by the formation of the Dead Sea transform that accommodated a northeastward displacement of the Arabia plate relative to the Sinai sub-plate. Phases of left-lateral shear along this fault system that presently runs from the Red Sea rift in the Aqaba Gulf to the Maras Triple Junction in SE Turkey may have influenced the NRS structural architecture. The system has been active since Mid-Late Miocene up to the present, with a prominent post-Pliocene pulse (Bosworth & Burke, 2005;Girdler, 1990;Quennell, 1951Quennell, , 1958. The Egyptian offshore fault system cuts entirely through the Late Miocene and Pliocene-Recent succession in the Hurghada offshore region. This is probably related to the young movement associated with the Dead Sea Transform activity in the NRS (Bosworth et al., 2020).

Data Sets and Methodology
Three different types of geophysical data were used to investigate and model the possible nature of a structural/ morphological high that characterizes the study area ( Figure 2). Gravity and magnetic public domain data sets covering a zone larger than the study area were used to integrate and extend interpretations of 3D seismic reflection data offshore Quseir. Geophysical data were processed at the Geophysics Laboratory of Khalifa University.

Seismic Data
3D seismic data were used to interpret the subsurface structural and seismo-stratigraphic elements of the study area. The database was acquired in 1999 by British Gas and covered 1,600 km 2 offshore central Egypt between Safaga and Quseir and west of the Brothers Islands ( Figure 2; Gordon et al., 2010). Time migrated, reflection seismic profiles are organized in NW-SE oriented inlines and related orthogonal crosslines, 25 and 12.5 m spaced, respectively. Seismic reflection data allow the imaging of subsurface structures down to 7 s two-way time (TWT). The seismic interpretation process includes the recognition and picking of the principal seismo-stratigraphic features and faults (carried out using the Schlumberger Petrel TM package) in order to define boundaries of the main seismo-stratigraphic units in the study area, and in particular, in the vicinity of the structural high (Ali, 2020;Ali et al., 2018;. Accordingly, six crosslines, three inlines, and three composite lines were interpreted to study the break out of the studied feature (Figures 2b, 3, 4, and 6). Several seismic attributes were applied and tested to resolve the surrounding evaporites and basement volume. We found that the root mean square amplitude (RMS) was the most effective ( Figure 5). Time slices were generated from the seismic volume to delineate the shape of the studied landform ( Figure 5). Two-way travel times were converted to depth, assuming an average interval velocity of 1.525 km/s for the seawater layer and 1.9 km/s for the Pliocene-Recent sediments (Table 1; Figure 7; Ligi et al., 2018;Mitchell et al., 2019). The obtained geological sections were used as an input for the 2D gravity/magnetic forward modeling and to delineate the 3D shape of the seamount.

Gravity Data
Free-air gravity data of the entire NRS were extracted from the 1-min satellite-derived global grid (version 31) of Sandwell et al. (2014) to analyze anomalies at the regional and local scale of about 175 × 175 km 2 ( Figure 8). Gravity-field accuracy derived from satellite altimetry depends on altimeter range precision, spatial track density, and diverse track orientation. The current version of the altimeter-derived gravity field has an accuracy of about 1 and 2 mGal with improvements mainly in the 12-40 km wavelength band that is of interest for studying structures up to 6 km. Bouguer gravity data were obtained from the WGM2012 global gravity model  provided by the Bureau Gravimétrique International; WGM2012 Bouguer gravity anomalies derive from the Earth global gravity models EGM2008 and DTU10 corrected (using a reference density of 2,670 kg/m 3 ) for  surfaces masses contributions based on the 1 arc-minute global relief model ETOPO1 . Five high-pass filters were applied to the Bouguer data to generate distinct maps from the study area with wavelengths lower than 5, 10, 25, 50, and 100 km ( Figure 9). A NE-SW Bouguer gravity profile along crossline 3,200 of the 3D seismic block was extrapolated to perform the forward model (see Subsection 3.4).

Magnetic Data
A magnetic anomaly map of 2 arc-minutes (about 3.7 km) resolution was generated from the Earth Magnetic Anomaly Grid (EMAG2 v3 ). EMAG2 v3 consists of a global grid compiled from satellite, ship, and airborne magnetic measurements (Meyer et al., 2017). Magnetic anomaly values are evaluated at the reference elevation of 4 km, and each grid value comes with an error estimate (Meyer et al., 2017). Regional and local maps were extrapolated to discuss the magnetic features at different scales ( Figure 10). The magnetic anomalies were reduced to the pole (RTP; Cooper & Cowan, 2005;Keating & Zerbo, 1996;Li, 2008) to remove the effect of induced magnetization and strike on the shape of magnetic anomalies ( Figure 10c). A high-pass filter was applied on the RTP grid with different wavelength cutoffs (10, 25, 50, and 100 km; Figure 11) to focus on the high-frequency content of the signal and enhance the resolution of the magnetic anomalies in the study area. A

Forward Modeling
A two-dimensional forward model of magnetic and gravity anomalies was generated to test the geophysical response of the interpreted geology based on 3D seismic data and stratigraphic information from the published exploration wells drilled in the area between 1970 and 2010. The 46 km-long crossline 3,200 extracted from the 3D seismic survey was used to set up the geological model. The GM-SYS profile tool of the Oasis Montaj ® software was used for the forward modeling. Different seismic horizons derived from the interpretation of the crossline 3,200 have been included in the model. In detail, they mark the top of the following units: the basement high (seamount), the Pliocene-Recent sediments, the layered evaporites, the massive halite, the pre-salt sediments, and the pre-rift seismic basement ( Figure 12). The horizons were depth converted using an average velocity for each stratigraphic unit (plus the water layer) as in Table 1. Densities and magnetic susceptibilities of the seismic layers are average values taken from the literature (Table 1). Parameters (densities and magnetic susceptibilities varying within the ranges indicated in Table 1) were adjusted to approach the best fit between the calculated magnetic and gravity values until observed magnetic and gravity anomalies were successfully achieved.

Results
Results from seismic interpretation, free-air and Bouguer gravity anomalies, and magnetic anomalies are discussed in detail in the following sections. Forward modeling results are reported in the Discussion.

Analysis of Seismic Data
Seismic data interpretation was focused on the region southwest of the Brothers Islands ( Figure 2). The area corresponds to a seismic basement rise between two prominent rift-related normal faults (Figures 3, 4, and 6). Picking of the seismic basement, pre-salt, massive halite, layered evaporites, and Pliocene-Recent sequences was carried out and displayed using the same seismo-stratigraphic criteria as Ligi et al. (2018Ligi et al. ( , 2019. Additional observations based on the reflection properties such as reflectivity, amplitude, frequency, and phase helped differentiate the deepest part of the pre-rift basement from that of the structural high to the SW of the Brothers Islands. show that the ridge elevation above the seafloor varies from the southeast to the northwest. The crossline 2,900 shows that the seamount is active through the entire Pliocene-Recent sequence, stopping its growth at the seafloor at about 0.9 s (TWT). The elevation above the surrounding seafloor significantly increases north-westward and reaches its maximum at about 0.33 s along crossline 3,250. Crossline 3,300 shows a drastic decrease in the height of the structural high down to 1 s, where it cuts the lower part of the Pliocene-Recent sequence without reaching the seafloor (Figure 4). Several sills and other intrusions within the sedimentary sequence have been recognized in crosslines 3,050 and 3,100 (Figure 4). At 2 s (TWT) depth, the structural high becomes sharper and forms a ridge, with borders easily distinguishable, in terms of RMS amplitudes, from the surrounding terrain. The shape of the ridge is oval in plan-view, elongated NW-SE, with a variable length from 17 to 13 km and width from 7 to 6 km at the depths of 2.5 and 2 s (TWT), respectively (Figures 5b and 5c). Two NW-SE elongate salt walls develop southwest of the ridge (Figure 5b). The 1.5 s time slice shows that the ridge is surrounded by halite (Figure 5a). The interpreted salt walls at 2 s continue in the 1.5 s time slice, where they become straighter and increase their width slightly. Figure 7h displays the top of the seismic basement emerging from the surrounding seafloor. The part of the ridge that emerges from recent-most sediments is characterized by a peculiar shape, composed of two distinct sub-circular cones (Figure 7h). This feature is evident from the cross-and inline analyses (Figures 3 and 4). The southwestern sector of the seamount is thinner than the northeastern one and forms a very sharp peak that disappears toward the northeast beneath the Pliocene-Recent deposits (composite lines 1 and 2 in Figure 3). Three composite lines image the top surface of the ridge, which is irregular. Some reflectors interpreted as recent sediments can be observed overlying the volcanic edifice, between seamount culminations and flanks. These sediments are interpreted as post-depositional growth of the volcano (Figure 3).
1. Dikes or minor magmatic bodies characterized by sub-vertical or high-angle reflectors with high amplitude, located below the PoErS and within syn-rift sediments ( Figure 6). 2. Sills or sub-horizontal igneous bodies showing low-angle, high-amplitude reflectors. They are common in the syn-intrusive sequence between 2.2 and 2.5 s, and are characterized by a lateral size up to 1 km and high amplitude reflectors. Figures 6a-6c display a complex network of dikes and sills that deform the pre-magmatic sedimentary layers. These intrusions probably derived from magma chambers. 3. Homogeneous sectors characterized by parallel, sub-horizontal reflectors that have been folded during magmatic activity and that display abrupt discontinuities that can be recognized in several seismic sections and have been interpreted as disrupted blocks ( Figure 6). These disrupted blocks are usually located beneath the eruptive centers, as discussed by Bischoff et al. (2019). Their presence confirms the migration of melts through the sedimentary succession producing deformations such as folds, normal, and reverse faults at hundreds-to-thousands meter scale plan (e.g., Holford et al., 2012;McLean et al., 2017;Muirhead et al., 2016;Planke et al., 2005;Senger et al., 2017). A significant number of normal faults, folds, and jacked-up strata, probably related to such a process, have been recognized and shown in Figure 6. 4. Jacked-up domes derived by the uplift and doming of the PrErS sequence reflectors usually associated with high amplitudes (Bischoff et al., 2019). The sub-circular mounds observed in 3D views, with variable diameters from 0.5 to 2 km, are associated with the interpreted underlying magmatic activity ( Figure 6). 5. Cone-shaped morphology of the seamount recalls that of a composite cone-type volcano (Jones et al., 2017;Lorenz & Kurszlaukis, 2007;Mitchell, 2001;White & Ross, 2011), characterized by a sequence of reflectors that accumulated around the vent zones and forms convex curvature. In the study area, the main volcanic body displays a single or multiple summit morphology, associated with a number of small cones, with a diameter ranging from 1 to 2 km. A small, isolated cone can also be recognized in the northeastern termination of the section (Figure 6c (Figure 6a). This is particularly evident and developed along the northeastern flank, where seismic reflectors locally have a convex shape ( Figure 6c) and down-lap on their substratum. This feature has been interpreted as a localized epiclastic debris accumulation ("epiclastic plume"), probably related to the erosion of the volcanic edifice.
Several 3D views were generated to illustrate the shape and physiographic features of the seamount, combining the 1.5 s time slice with four arbitrary lines in the NW-SE and NNW-SSE directions (Figures 7a-7d). Figures 7a  and 7b image the northeastern sector of the seamount, which consists of one prominent cone with steep flanks. Laterally, it grads to a more complex morphology consisting of two or three main culminations above the surrounding seafloor forming small and steep cones (Figures 7c and 7d). The TWT contour map of the seamount surface ( Figure 7e) has been used to produce a 3D view of the entire seamount (Figure 7g). Two-way travel times range from 0.28 to 3.2 s (Figures 7e and 7g). The TWT map has then been converted to depth using an average seismic velocity, given the complexity of the seismic unit geometries that probably lead to significant lateral and vertical velocity variations. The picked horizon was then gridded in depth-domain throughout the survey area to produce the structural map shown in Figure 7f (Ali et al., 2017a(Ali et al., , 2017b. The depth of the submarine portion of the volcanic edifice (i.e., the seamount itself) ranges from 215 m bsl at its summit to approximately 735 m bsl at the contact with the surrounding seafloor, while the roots of the volcanic apparatus, buried under sediments and halite, reaches the depth of about 2,800 m bsl (Figures 7f and 7h). The 3D view shows that the seamount rises above the surrounding seafloor on average between 300 and 520 m (Figure 7h). The seamount is elongated toward the NW-SE direction, and the exposed sector measures 6 × 1.6 km, while the buried apparatus measures approximately 7 × 17 km (Figures 7e-7h). The seamount is dissected by two major normal faults running NW-SE for 15 km parallel to the seamount elongation axis. Figures 7a and 7b display the northeastern sector of the seamount consisting of a single major cone with steep flanks and dip-angles from 50° to 65°. The north-western and south-eastern ends have gentle flanks with a dip angle varying from 15° to 30°. In the middle part of the seamount, two major conical summits rise from the surrounding seafloor (Figures 7c and 7d).

Analysis of Gravity Anomalies
The regional free-air gravity anomaly map for the entire NRS rift system is shown in Figure 8. The free-air gravity anomaly values range from −40 to 80 mGal. A sharp transition between the positive and negative free-air gravity values is observed along the Egyptian and Saudi coastal lines and the Sinai Peninsula. This contrast is due to onshore, parallel to the coast, rough topographic belts (Hijaz and Asir Mountains on the Saudi side and the Red Sea Hills on the Egyptian side), which generate positive anomalies, while the negative anomalies along both shorelines are probably due to elongated, subsistent basins. The regional free-air gravity map shows that most of the NRS offshore margin is characterized by low free-air gravity values <10 mGal, except for a few (c) A simplified cross-section was used to generate the best-fit model, where a volcanic edifice overlies the basement high. D and S indicate assumed density in g/cm 3 and magnetic susceptibility in cgs, respectively. positive anomalies in the north (north of Quseir city). All of these positive gravity anomalies were interpreted by Cochran (2005), Cochran and Karner (2007), and Duncan and Al-Amri (2013) as volcanic seamounts.
A free-air gravity anomaly map for the study area was extracted to interpret the origin of the gravity anomalies in the surroundings of the Brothers Islands (Figure 8b). The study area shows free-air gravity values of −40-40 mGal. Several minor positive anomalies are observed in the northern and northeastern parts of the study area, and three significant positive anomalies with values of more than 20 mGal can be identified. Most of them have a continuous elongated shape trending NW-SE parallel to the rift axis and likely represent volcanic seamounts. Furthermore, the Bouguer anomaly map shows several positive anomalies with high values varying between 160 and 200 mGal (Figure 9a). Accordingly, a high-pass filter with different wavelength cutoffs (5, 10, 25, 50, and 100 km) was applied to the Bouguer anomaly grid to enhance the high-frequency content of the signals (Figures 9b-9f), however the high-pass filtered maps at 50 and 100 km show the best results with several isolated high positive anomalies (Figures 9e and 9f). These anomalies are aligned in the NE-SW direction and are almost elongated in the NW-SE parallel to the rift orientation (Figures 9e and 9f). One of these areas, located southwest of the Brothers Islands, is covered by the 3D seismic data. The Bouguer gravity profile extracted along the seismic crossline 3,200 ( Figure 12b) shows that the peak anomaly is about 170 mGal. At this location, filtered Bouguer gravity anomalies at high frequencies show two local maxima (Figures 9d-9f), related to the interpreted volcanic edifice and Brothers Islands, with the higher positive anomaly in correspondence with Brothers Islands where gabbros are exposed (Ligi et al., 2018. The seamount anomaly is found as an elongated oval shape toward NW-SE, while the shape of the anomaly at the Brothers Islands is an elongated oval shape toward the NE-SW (Figure 9f).

Analysis of Magnetic Anomalies
Magnetic anomalies of the NRS show values ranging between −100 and 200 nT ( Figure 10). Most of the highest positive magnetic anomalies are found as spots along the axial trough and concentrated along the eastern coastline. The onshore areas show scattered and weak local positive anomalies (10-65 nT) corresponding to the Red Sea hills on the Egyptian side and Asir and Hijaz Mountains on the Arabian side and local occurrences of relatively lower anomalies (−5 to −80 nT). Nevertheless, the magnetic grid displays a low resolution, probably due to a lack of data in these areas (Figure 10a). A magnetic anomaly map for the study area was selected from the regional map of the NRS to highlight the magnetic pattern around the Brothers Islands (Figure 10b). This map, with contour lines 50 nT apart, displays values ranging from −120 to 170 nT. Five large-amplitude magnetic local maxima, aligned NE-SW, show values greater than 150 nT (Figure 10b). A RTP operator has been applied to minimize the polarity effects and center the anomalies directly above their source locations (Figure 10c). The RTP map shows that all the positive magnetic anomalies are shifted northward due to the removal of the dipolar effect of the Earth's magnetic field. The amplitude of the magnetic anomalies generally increases after RTP, and the magnetic gradients become steeper and more intensive.
Furthermore, after processing, a positive magnetic anomaly SW of the Brother Islands is observed, precisely in correspondence with a strong positive gravity anomaly. A magnetic profile along the track of crossline 3,200 shows values exceeding 170 nT in this location (Figure 12a). On the contrary, the basement high of the Brothers Islands itself shows no evidence for a local peak in the anomalies. Several magnetic highs can be observed in the RTP map, and they are characterized by high magnetic values of more than 150 nT and by a sub-circular shape. Four high-pass filtered maps with cutoffs of 10, 25, 50, and 100 km have been generated from the RTP map ( Figure 11). The constructed figures displayed the same magnetic positive anomalies approximately on the exact location and similar shapes. Seven intense magnetic highs have been identified, with N-S, NW-SE, and NE-SW trending alignment (Figure 11).

Evidences Supporting Recent Magmatism in Offshore Quseir
Combined observations from seismic, gravity, and magnetic data suggest that the seismic basement of the structural high southwest of the Brothers Islands displays some peculiarities regarding the nature of the underlying rocks. In particular (a) a positive magnetic anomaly precisely centered on the ridge summit and does not extend over the neighboring Brothers high, indicating a high magnetization that probably a different lithology of the basement underlying the two structures, (b) the presence of a strong positive gravity anomaly SW of the Brothers Islands with gravity values up to 40 mGal, and (c) a marked difference in RMS reflection amplitudes from the surrounding and underlying rocks.
This leads to the hypothesis that the southwestern ridge is more recent than the Brothers Islands and has been formed by rapid ascent and cooling of basaltic magma, constraining its magnetization higher than that of the underlying basement and of the Brothers Islands high, where gabbros are exposed beneath the Pleistocene reef carapace of the islands (Bosworth et al., 1996;Ligi et al., 2018;Shukri, 1944;Taviani & Rabbi, 1984). In addition, its morphological shape, as observed in seismic sections, recalls an elliptical cone that culminates with two conical summits. These observations suggest that this ridge is a complex volcanic edifice developed on top of the seismic basement at 26°15ꞌ25ꞌꞌ N; 34°46ꞌ53ꞌꞌ E (datum WGS84). The buried portion of the volcanic apparatus is partly rooted in the basement and partly surrounded by halite and sedimentary rocks. In contrast, the submarine portion reaches a depth of 215 m and affects largely the seafloor morphology, which is compatible with recent extrusive activity (Figure 7f). Furthermore, its elongated structure may have been driven by the large rift-related extensional fault affecting its southwestern flank, favoring the ascent of melt.
The hypothesis of a submerged volcano was tested by forward modeling. The assumed geological model is 46 km long, 20 km deep, and includes the southwestern high ( Figure 12c). The forward model was constructed using geometries deduced from the seismic data interpretation applying a narrow density value range of 2.7-2.82 g/cm 3 for the entire basement section and 2.65 g/cm 3 for the seamount above the basement with a high magnetic susceptibility value of 0.0034 cgs. The buried portion of the volcanic edifice is not precisely determined because of the difficulties in identifying its boundaries in contact with the basement. Nevertheless, the forward model shows a good match between the predicted and observed gravity and magnetic anomalies, with an error of 5.452 and 3.757 for the magnetic and gravity, respectively, supporting the volcanic origin of the seamount (Figures 12a and 12b).
Thus, the coexistence over the same area of physiographic-structural elements with high magnetic susceptibility and high density, inferred by magnetic and gravity anomalies, and associated with geological evidence from seismic data, strongly suggest the presence of a volcanic edifice. Adopting the same criteria, seven other similar volcanic seamounts outside of the 3D seismic coverage can be detected ( Figure 13). These seamounts are developed probably due to the rapid ascent and cooling of basaltic magmas along a rifting-related fault during late syn-rift times. However, without any information on their geometry and the nature of their basement, we can only affirm that they display similar geophysical signatures.
This interpretation provides new evidences for the recent magmatism in the western margin of the NRS from the interpretation of the 3D seismic survey as well supports and refines those of Cochran et al. (1986), Cochran (2005), Cochran and Karner (2007), Guennoc et al. (1988), and Martinez and Cochran (1988). They assumed that all the magnetic highs in the NRS are magnetic dipolar anomalies due to localized sources with normal magnetization, probably related to extrusive and intrusive bodies. Several authors suggest (Cochran, 2005;Cochran et al., 1986;Martinez & Cochran, 1988;Maury et al., 1985;Pautot et al., 1984) that they formed during the last 0.78 Ma (Pleistocene), after the last magnetic inversion, which is consistent with our observations. For the first time, this article provides a depth map and a 3D view for one of these recent volcanic edifices and discusses the changes in its shape and geometry laterally and vertically as well its relationships with the imaged seismic sequences.

Correlating Between Magmatism in Offshore Quseir and Western Arabia
Magmatism during the Cenozoic along the margins of the central and NRS rift systems has been abundant, particularly in western Arabia, where widespread volcanic fields form one of the largest alkali basalt provinces in the world (180,000 km 2 , Chorowicz, 2005;Coleman et al., 1983;Moucha & Forte, 2011). Magmatism in this region (30 Ma-Present) preceded, was concurrent with, and continued after the Red Sea rift. Lava compositions in this province include tholeiites, but are predominantly basanite to alkali olivine basalt to hawaiite, with minor more evolved compositions (Altherr et al., 2019;Camp & Roobol, 1989Camp et al., 1991;Coleman, 1993;Coleman et al., 1983;Duncan & Al-Amri, 2013;Moufti et al., 2012;Sanfilippo et al., 2019Sanfilippo et al., , 2021. Within several large centers, volcanic activity began (>12 Ma) with tholeiitic to transitional compositions, then became more alkaline for younger eruptions (<12 Ma) (Camp & Roobol, 1992). The older period of activity includes dikes and eruptive centers close to and aligned with the NW-SE Red Sea margin, while the younger volcanic centers are distributed over a 200 km swath and display N-S alignment. The most voluminous centers form a prominent N-S volcanic axis, the Makkah-Madinah-Nafud (MMN) line, where the volcanic activity occurred in pulses from 10 Ma up to the present day, with a progressive migration toward the north (Figure 14). The oldest MMN lava flows occurred in Harrat Rahat (about 10 Ma: Camp & Roobol, 1989). Further to the north, the oldest volcanic activity is dated at about 5 Ma in Harrat Khaybar, while in Harrat Ithnayn at 3 Ma (Camp et al., 1991).
Recently, 40 Ar/ 39 Ar age determinations provided younger ages for the Harrat Ithnayn lavas ranging from 500 to 120 ka (Alhejji et al., 2019). This scenario seems compatible with a progressive rejuvenation of the magmatism at higher latitudes along the MMN line ( Figure 14). Following the hypothesis of a northward rejuvenation of the volcanic products, we note that Harrat Ithnayn volcanic field rests at a comparable latitude with Quseir (26°N). We hypothesize that the magmatic events at similar latitudes along the Red Sea margins and with similar ages may have originated within the same geotectonic context. Most of the Cenozoic volcanic products of the harrats are related to alkaline and tholeiitic suites (e.g., Downs et al., 2019) and they have been interpreted to derive from different degrees of partial melting of a peridotitic source with poor crustal assimilation. The increased volcanic activity during the last 10 Ma has been interpreted as a response to a Western Arabian plate scale lithospheric up-doming, probably linked to a propagation of the Afar mantle plume toward the north (Camp & Roobol, 1992). In addition, the distribution of the volcanic vents along a NNW direction parallel to the Red Sea fault systems has been interpreted as a result of the reactivation of rift structures (Moufti et al., 2012(Moufti et al., , 2013.
The seamount object of the present study and the nearby possible equivalents can be considered in the framework of a regional-scale, late syn-rift magmatic reactivation of the structures along the margins. Although it is difficult, in the absence of geochemical and petrographic analyses from the studied sector, to propose a specific geodynamic scenario, a major change in the lithospheric setting must have occurred since 10 Ma. The studied example testifies that the effects of this event were not limited to the continental domain of the Arabian plate but extended further west, into the Red Sea and toward the African plate.

Conclusions
High magnetic susceptibility and high-density sub-circular areas centered on seafloor topographic highs characterize the NRS. One of these areas, to the southwest of Brothers Islands offshore Quseir, has been studied in detail using a 3D seismic data set that allows recognizing its architectural context, characterized by a rift-related normal fault and a prominent, shallow footwall-related basement high surrounded by syn-rift deposits. The basement high forms a pronounced physiographic body that rises from the surrounded seafloor by more than 520 m. The analysis of seismic amplitudes reveals that the structural high has a different seismic response when compared to the surrounding sedimentary sequence and the pre-rift basement. A forward model was calculated adopting the interpreted geological model and density and magnetization values compatible with a complex volcanic edifice on the summit of the basement high. The forward model shows good fitting between the measured and predicted Bouguer gravity and magnetic anomalies, suggesting a volcanic nature of the seamount located at 26°15ꞌ25ꞌꞌ N and 34°46ꞌ53ꞌꞌ E. Detailed observations based on 3D seismic interpretation allow us to define the shape and the volcanic edifice and the relationships with the imaged seismic sequences. Moreover, six classes of archi- tectural features have been observed; dikes, sills, disrupted blocks, jacked-up domes, cone shaped morphology, and epiclastic plume. Our interpretation suggests that volcanic activity occurred at different centers forming a complex and composite edifice at the footwall of a large tilted block during late syn-rift and likely during Pleistocene. The same volcanic origin has been postulated for several additional neighboring areas having similar gravity and magnetic pattern, suggesting the presence of a cluster of volcanic edifices offshore Quseir. In addition, it is also suggested that the studied volcanic edifices can be compared with the volcanic fields of the western Arabian onshore, which formed, at a similar latitude, during the Pleistocene and were probably generated by the progressive northward migration of the Afar plume.

Data Availability Statement
Gridded satellite-derived free-air gravity anomaly data are available from https://topex.ucsd.edu/pub/global_ grav_1min, the WGM2012 global gravity model (Bouguer anomalies) from https://bgi.obs-mip.fr/data-products/ grids-and-models/wgm2012-global-model/, and the EMAG2 v3 earth magnetic anomaly grid is available from EMAG2v3: Earth Magnetic Anomaly Grid (2-arc-minute resolution) -catalog (data.gov). The seismic data were provided courtesy of BG Egypt (now Royal Dutch Shell) and images are published with the permission of South Valley Egyptian Petroleum Holding Company (Ganope).