Magmatic underplating and crustal intrusions accommodate extension during Red Sea continental rifting

Domain: earth_science
Published: 2025-07-14
Editorial outcome: minor_revision

Abstract
Magmatism’s role in continental rupture and ocean formation remains a critical question in Earth Sciences. The Red Sea, where Arabia is rifting from Nubia, offers an ideal setting to explore this process. This study analyses geochemical and isotopic data from gabbros and basaltic dikes in the Tihama Asir complex, formed during early Red Sea rifting (Late Oligocene). The results show that asthenospheric melts assimilated ancient lower crust before rising into shallow magma chambers with minimal upper crustal contamination. Rising asthenosphere driven by the Afar plume and by extensional stresses thermally weakened the lower crust, allowing it to decouple from the mantle, causing depth-dependent deformation. The generated asthenospheric melts underplated and intruded the thinning continental crust protracting continental rifting by re-thickening of the crust and accommodating extension. This study provides the evidence of tholeiitic underplating beneath thick continental crust during early rifting, substantially postponing the onset of seafloor spreading.

Introduction
Over billions of years, the Earth’s internal processes cyclically formed and broke apart supercontinents, leading to the birth of new oceans. The Wilson Cycle explains how continents split, forming passive margins1,2. Continental fragmentation takes place due to extensional stress, initiating rifts where the lithosphere is affected by progressive mechanical deformation and magmatic processes. This process continues until the continental lithosphere ultimately fractures, creating new oceanic lithosphere that separates two conjugate juvenile continental margins. The East African Rift System, including the Red Sea, represents various stages of this process, from continental rifts to an emerging ocean. This rift system began fragmenting the Afro-Arabian shield since the late Oligocene, influenced by a mantle plume impacting the Afar region around 34 Ma3–5. Magnetic anomalies in the Red Sea suggest an ongoing formation of oceanic crust, with the southern region reaching further in this process than the central or northern regions6–8. Variations in the amount of extension may reflect different stages of the rifting process, while differences in GNSS-derived relative velocities between the Arabian and Nubian plates, increasing from ~7 mm/yr in the northern Red Sea to ~16 mm/yr in the southern Red Sea9 suggest distinct dominant mechanisms for the formation of the northern and southern Red Sea. The onset of rifting in the Red Sea was preceded by flood basalts erupted in Ethiopia, NE Sudan, and SW Yemen5, also associated with rhyolitic volcanism10,11. Abundant melt was injected into the Arabian basement in the early stages of rifting (26–21 Ma)5,12–15 as testified by the occurrence of thousands of kilometers-long dike swarms running NW-SE parallel to the Arabian coast of the Red Sea12,16, along with the contemporaneous Tihama Asir12 (southern Red Sea), Al Lith16 (central Red Sea), and Quseir13,14 (northern Red Sea) magmatic complexes, which include gabbros, basaltic dikes, and granophyres. Widespread gabbro and dike injection was followed by successive phases of volcanism, leading to the formation of extensive alkali-basalt lava fields (harrats). Some of these phases had finite eruption periods, while others began at different times and continued into recent history12,16–20 (Fig. 1).

The formation of a magma-rich margin during rifting is driven by multiple geological factors21,22. A key factor is the magmatic supply, where sufficient magma generation from the mantle is essential for processes like dike injection and underplating (i.e., addition of mafic magma to the lower crust and uppermost mantle around the Moho in the form of batholiths and sill-like structures23). Efficient heat transfer from the mantle can promote localized melting and assimilation of crustal material, affecting the formation of transitional crust. The composition and thickness of the pre-existing continental crust affect how it interacts with ascending magmas, while the rheological properties of the lithosphere, such as its strength and viscosity, impact deformation behavior. The duration of rifting, the divergence rate and magmatic activity also play a critical role24. Understanding these constraints is fundamental for modeling magma-rich margin formation and predicting their evolution.

The Tihama Asir magmatic complex in southernmost Saudi Arabia serves as a key location to study the initial phase of syn-rift magmatism and its influence on the geodynamic evolution of the southern Red Sea rift system (Fig. 1), and, more broadly, of magma-rich rift systems. This region provides insights into the interplay between magmatic supply, tectonic setting, heat transfer, and crustal composition during the early stages of rifting12. In this study, we present geochemical data including bulk rock major and trace elements, as well as Sr, Nd, and Pb isotopic compositions of the Tihama Asir gabbros and dykes, in order to investigate the magma source and its migration and emplacement within the crust during rifting. We also measured the magnetic properties of these rocks to better constrain the geophysical signals they produce. Additionally, 40Ar/39Ar age determinations, combined with existing geophysical data, enable us to reconstruct the evolution of the magmatic complex within the broader framework of Red Sea rifting and ocean basin development.

Results
The Tihama Asir igneous complex
The Tihama Asir magmatic complex is located east of the Jizan coastal plain in southwestern Saudi Arabia (Fig. 1). It consists of various layered gabbroic bodies with compositional layering ranging from relatively primitive olivine gabbros to more evolved lithologies such as oxide gabbros, with layering occurring at meter- to decameter-scale and tending to diminish up-section, akin to typical Penrose-type ophiolite stratigraphy25–27. The gabbros are in primary contact with granophyre granitic intrusions, with preserved intrusive contacts to the strongly faulted Arabian Shield Neoproterozoic basement comprising Pan-African granitoids, low-grade metamorphic rocks, and gneisses26. Diffuse basaltic to rhyolitic dike swarms locally overlay the gabbroic lithologies and intrude the continental basement12,26,27. Our 40Ar/39Ar age determination of 26.03±0.54 Ma and recalibrated previous 40Ar/39Ar datings28 of the Tihama Asir intrusive rocks and the dike swarms constrain their age to within the range 26–21 Ma (Supplementary Fig. S1; Supplementary Data S1). Faulting in this area is suggested to have begun around 29 Ma15, just a few million years prior to the formation of the magmatic complex. This temporal relationship implies that the Tihama Asir magmatic complex formed due to extensive magmatism during the early stages of Red Sea continental rifting12,27–29.

Petrology and geochemistry
The most primitive Tihama Asir olivine gabbros show petrological and geochemical characteristics consistent with formation from tholeiitic parental melts, resembling modern MORBs (Supplementary Data S2). However, the evolved gabbros and oxide gabbros show saturation of phlogopite and selective enrichments in LREE and highly incompatible elements (Rb, Ba, U, Th, Nb, Sr, K). The geochemical evolution of the parental melts diverged progressively from that of oceanic tholeiites, suggesting a gradual assimilation of continental crust during the emplacement of the gabbroic bodies (Fig. 2). Similarly, the dike swarms overlying the gabbros (sheeted dike complex) exhibit transitional affinity, with selective enrichments in LREE and highly incompatible elements compared to Red Sea MORB and sub-chondritic Nb-Ta/LREE ratios. These dikes are in chemical equilibrium with the clinopyroxene in the gabbros and are interpreted as melts extracted from hybridized gabbroic crystal mush after high degrees of contamination from upper crustal material.

Furthermore, meter-scale anastomosed basaltic dikes crosscut the gabbros with irregular chilled boundaries, locally dissecting and embaying portions of the host rock (e.g., TA-02/Dy-1; see Fig. 2c). These basalts display a chemical composition indicative of limited fractionation in magma chambers and nearly flat PM-normalized incompatible trace element patterns, akin to present-day enriched MORBs31 (Fig. 2). Being close to chemical equilibrium with the clinopyroxene from the most primitive gabbros, these basalts are considered poorly fractionated mantle melts that escaped crustal contamination and were injected into the cooling gabbroic crystal mush soon after its emplacement30.

Radiogenic isotopes
The Tihama Asir gabbros exhibit higher initial (i.e., present-day values recalculated to their crystallization age; Supplementary Data S3) 87Sr/86Sr (0.7033–0.7037) and lower initial 143Nd/144Nd (0.51292–0.51296) ratios compared to those of Red Sea MORB32–34 and the Cenozoic alkaline magmatism in the northern Arabian plate19,20,35–40. Nonetheless, their initial 206Pb/204Pb and 207Pb/204Pb ratios plot amongst the most depleted samples from the Red Sea MORB and the Cenozoic volcanism from northern Arabia (Fig. 3). The gabbros isotopically resemble the inferred compositions of the Afar plume41–43 in terms of Nd and Sr isotopes, albeit they are less radiogenic in Pb. These compositions (Supplementary Data S3) suggest a source with relatively high time-integrated Rb/Sr and low Sm/Nd ratios compared to those of the northern Red Sea basalts, but with similarly low U–Th/Pb ratios. Interestingly, a basaltic dike (TA-02/Dy-1) cutting across the gabbros exhibits Sr–Nd–Pb isotopes nearly coincident with those of the inferred Afar plume (Fig. 3), supporting the idea that primary melts of the Tihama Asir formed from an asthenospheric source near the main impingement area of the Afar plume30. Upon closer examination, selected gabbroic samples gradually diverge from the crosscutting dike along trends of decreasing Sr–Nd and Pb isotope ratios. These isotopic signatures suggest contamination by material with ancient depletions in Rb/Sr and U/Th/Pb, coupled with relatively low Sm/Nd ratios, compositions commonly attributed to lower continental crustal material, such as the ancient lower continental crustal granulite xenoliths locally sampled within the Cenozoic volcanism of the Arabian plate25.

In contrast, samples from the sheeted dike complex have much higher initial 87Sr/86Sr (0.7038–0.7047) and lower initial 143Nd/144Nd (0.51284–0.51293) ratios compared to those of gabbros, yet they exhibit similar Pb isotope ratios. These samples cluster within the Nd–Sr and Pb isotopic space of the widespread volcanism in South Arabia and Yemen, well-known examples of Afar-type magma strongly contaminated by both upper and lower continental crust41,42. Given that these rocks display intrusive contacts with upper crustal lithologies and show selective enrichments in highly incompatible elements and LREE compared to the gabbros, we infer that their melts were generated within melt-rich zones of the gabbroic crystal mush and might have further assimilated Pan-African upper continental crust. Indeed, their Sr–Nd-Pb compositions fall between those of the Tihama Asir intrusives and the Pan-African granitoids (Fig. 3). Our hypothesis is corroborated by combined radiogenic Nd–Sr–Pb and δ18O isotopes of flood volcanics from Yemen42, which exhibit the same extent of isotopic variations observed in the Tihama Asir gabbros (Fig. 3). The less contaminated Yemenite basalts retain mantle-like δ18O isotopes and Sr–Nd and Pb isotopic ratios indicative of an Afar-like upper mantle. In contrast, those retaining evidence of crustal contamination have high δ18O isotopes42 and diverge along two different trends, aligned towards enriched and depleted Pb compositions, indicating contamination by upper and lower Pan-African crust, respectively.

Assimilation-fractional crystallization model
We conducted assimilation-fractional crystallization (AFC) modeling following the approach outlined by ref. 44. Specifically tailored to simulate the chemical evolution of a magmatic system undergoing fractional crystallization alongside assimilation of wall rocks (Figs. 4 and 5). We utilized the average compositions of lower crustal granulite xenoliths42 and Pan-African granitoids45 as the assimilated components, and the crosscutting basaltic dike displaying Afar-like compositions as the initial melt (for further details, see Methods). The isotopic trends observed in the Tihama Asir gabbros are faithfully reproduced across all different Nd–Sr–Pb isotopic spaces at low degrees of assimilation of lower crustal granulites, considering mass assimilated/mass crystallized ratios ranging from 0.7 to 0.9 (Fig. 4), and utilizing the same geochemical parameters used to replicate the trace element enrichments30.

Conversely, the high Sr and low Nd isotopes observed in the sheeted dikes necessitate additional assimilation of Pan-African granitoids, commencing from the hybrid melts that formed the gabbros. Notably, our model adeptly reproduces the distinct trends of the gabbros and dike swarms concerning Nd and Sr versus ΔPb (7/4) (Fig. 4b). The low 206Pb/204Pb ratios and comparatively high 207Pb/204Pb ratios of lower crustal granulites in the Arabian plate42 result in a gradual increase in ΔPb (7/4) as 143Nd/144Nd ratios decrease. Conversely, the upper crustal Pan-African granitoids have 206Pb/204Pb and 207Pb/204Pb lying on the NHRL45, thus producing a local decrease in ΔPb (7/4).

Methods
40Ar/39Ar dating
Age determinations were performed at the Laboratoire des Sciences du Climat et de l’Environnement facility (CNRS-CEA, Gif-sur-Yvette), France.

Sample preparation
40Ar/39Ar measurements were performed on the microcrystalline groundmass of the sample, considered representative of the phase that crystallized during lava solidification. Samples were crushed, sieved (250–125 mm size), and washed in 1N acetic acid at 60 °C for 40 minutes to remove secondary mineral phases. Phenocrysts and xenocrysts, carrying an excess of 40Ar, were removed using heavy liquids (Diiodomethane) and magnetic separations. Absence of alteration phases was verified using a binocular microscope.

Procedure
Splits of 120–130 mg were wrapped in 99.5% copper foil, loaded in aluminum disks, and irradiated in the CLICIT facility at Oregon State University’s TRIGA reactor for 10 h. After irradiation, samples were degassed at 550-650 °C to remove atmospheric argon. Neutron fluence (J) was monitored using FCs crystals, with J values calculated from 6 single crystal laser fusion analyses, reported in Supplementary Data S1. Age calculations used the decay constants from ref. 75 and an atmospheric 40Ar/36Ar ratio of 298.56 from ref. 76. Precision of mass discrimination correction was monitored by daily air argon measurements, achieving about 0.1–0.15% (1σ standard deviation).

Pure groundmass aliquots were incrementally heated in 10 steps between 650 °C and 1200 °C using a metal resistance furnace. Isotopic measurements were performed using a high-sensitivity GV5400 instrument in ion counting mode. An analytical run consisted of 20 peak scans of each argon isotope with integration times 2 s (40Ar, 39Ar), 10 s (37Ar, 38Ar, baseline) and 20 s (36Ar). This was preceded by a peak centering routine on the five Ar isotopes upon sample admission into the mass spectrometer. Raw argon isotope abundances were regressed back to inlet time using NG software (v. 2.90). System blanks measured prior to step-heating ranged from 6.8×10−16 to 1.2×10−15 moles of 40Ar and 2.5·10-18 to 6.2×10−18 moles of 36Ar, which were substantially lower than sample signals.

Results
Plateau ages, isochron regressions, probability of fit estimates and errors were calculated using ArArCalc77 following the criteria defined in ref. 78. Results of the step heating experiment are detailed in the Supplementary Data S1. The plateau age for sample TA-06/Dy-1, calculated from 66.4% of gas released, excluded the first three and last steps due to anomalously young apparent ages likely caused by 40Ar* loss from subtle alteration. The preferred age for this sample is the plateau age of 26.03±0.54 Ma (2σ uncertainty).

Re-evaluation of Ar–Ar literature dates using updated standards
40Ar/39Ar ages from ref. 28. were originally calibrated against an age of 17.25±0.20 Ma for the Bern 4B biotite standard, which corresponds to an age of 126.6±4.2 Ma for LP-679. Initially, Bern 4B’s age was recalibrated using the ArAR software80, considering an updated age of 127.5 Ma for LP-6, which equates to an age of 28.10±0.04 Ma for FCTs. This recalibration increased the age of Bern 4B to 17.38±0.20 Ma. This age was further recalculated using the FCTs age applied in this study (28.201±0.023 Ma81). Based on this calculation, the age of Bern 4B is adjusted to 17.44±0.20 Ma. This updated age was used to recalculate the ages reported by ref. 28. using the ArAR software. For consistency, we also applied the constant values from ref. 75. in the calculation (Supplementary Data S1).

Bulk rock major, trace elements, and Sr, Nd, and Pb isotope compositions
The bulk rock major and trace element compositions of the Tihama Asir gabbros and sheeted dikes are sourced from ref. 30. and were analyzed using XRF and ICP-mass spectrometry at Activation Laboratories Ltd., Ancaster, Ontario (Supplementary Data S2), following the analytical techniques detailed on the company’s website (www.actlabs.com).

Bulk rock Sr, Nd, and Pb isotope analyses were conducted at the Dipartimento di Scienze della Terra, University of Florence. Sample powders (approximately 250 mg each) were taken from the same aliquots used for major and trace element determinations. The samples were leached with 1M HCl in an ultrasonic bath and rinsed several times with MilliQ water to minimize potential contributions from secondary phases that could have altered the magma signature. The samples were dissolved through sequential HF–HNO3–HCl dissolution, and the Sr, Nd, and Pb fractions were purified through standard liquid chromatographic techniques.

Isotopic ratios were determined using thermal ionization mass spectrometry (Thermo-Fisher Scientific Triton Plus). Mass fractionation of Sr and Nd isotopes was exponentially corrected with 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. Interferences of 87Rb and 144Sm on 87Sr and 144Nd, respectively, were monitored by measuring 85Rb and 147Sm. Due to their extremely low abundances (when present) these masses were measured on 1013 Ohm amplifiers; such interferences resulted negligible for all the samples. All other masses were measured on 1011 Ohm amplifiers. The within-run 87Sr/86Sr average value for NBS 987 reference material was 0.710255±7 (2σ, n=4), and the 143Nd/144Nd average value for the internal standard NdFi was 0.511465±3 (2σ, n=3). Accuracy and precision were further checked by measuring the isotope ratios of international rock standards BHVO-1 (87Sr/86Sr=0.703484±6; 143Nd/144Nd=0.512986±9) and BCR-2 (87Sr/86Sr=0.705018±7; 143Nd/144Nd=0.512620±5).

Lead isotopic composition was measured by collecting the smaller 204Pb beam on 1013 Ohm amplifier and the other masses (206Pb, 207Pb, and 208Pb) on 1011 Ohm amplifiers. Instrumental mass bias was corrected using replicate analyses of NIST SRM 981 standard, yielding an average fractionation factor of 0.135% (±0.010%) per mass unit relative to the reference values, which was applied to all Pb isotopic ratios. The accuracy of the Pb isotope measurements was tested through replicate analyses of international reference material BHVO-1 (206Pb/204Pb=18.705±4, 207Pb/204Pb=15.577±5, 208Pb/204Pb=38.375±15). All the isotope ratios of the reference materials are reported in Supplementary Data S3 and are identical within error to previously published values82,83.

All the isotope ratios were recalculated to their initial values, assumed to correspond to the time of the rock formation. The formation ages of the studied samples were estimated based on the available Ar–Ar ages of the associated lithological units (this work and recalibrated ages of ref. 28. (Supplementary Data S1). For the age correction, we specifically re-measured the elemental Rb/Sr, Sm/Nd, U/Pb, and Th/Pb ratios on the leached samples using ICP-MS (7800 Agilent) at the University of Florence. A small aliquot of the same sample solution used for isotopic measurements was collected (after complete dissolution and before chemical separation), evaporated, and diluted in 2% HNO3 for ICP-MS analyses. Replicate analyses of international reference materials BHVO-1 and AGV-1 yielded Rb/Sr, U/Pb, Th/Pb, and Sm/Nd ratios consistent with reference values of ref. 84. and with reproducibility <4% (2σ). The internal errors (2σ) of the age-corrected Sr, Nd, and Pb isotope ratios for all samples were fully propagated to account for uncertainties in age, isotopic ratios, and elemental contents. Total procedure blanks for Sr, Nd, and Pb were 60 pg, 12 pg, and 43 pg, respectively. The data and associated uncertainties are reported in Supplementary Data S3.

Assimilation-fractional crystallization model
The enrichments in highly incompatible elements (e.g., Rb, Ba, Th, U, Zr, Hf) and the positive LREE/HREE fractionation observed in the melts in equilibrium with the Tihama Asir clinopyroxenes can be effectively reproduced by a process involving the assimilation of lower and upper continental crust, combined with fractional crystallization of a gabbroic assemblage (i.e., olivine, plagioclase, and clinopyroxene)30. A similar geochemical model is applied here to assess whether the assimilation of lower and upper continental crust can explain the isotopic compositions of the Tihama Asir gabbros and the associated sheeted dike.

The trace element (Sm, Nd, Rb, Sr, Th, U, and Pb) and initial Nd–Sr–Pb isotopic compositions of the melts generated during the two scenarios above (assimilation of lower and/or upper crust) were calculated using an assimilation-fractional crystallization (AFC) process, following the equations of ref. 44. For the assimilation of the lower crust, we used the basalts (TA-02/Dy-1) crosscutting the Tihama Asir gabbros at a high angle as the starting melt composition. These basalts, unmodified by any crustal contamination process, exhibit an EMORB-like trace element pattern and isotopic composition similar to that of the inferred Afar plume43. Conversely, the starting melt composition for the assimilation of upper crust has a Nd–Sr–Pb isotopic composition coinciding with the average isotopic composition of the Tihama Asir gabbros and is similar to melts extracted from the gabbros after 5–10% degrees of lower crustal contamination. The composition of the melt modified by the AFC process was calculated at 1% increments, representing the remaining melt mass (scaled from 100% to 0), corresponding to ‘F’ in the equation from ref. 44. Each of the two AFC scenarios was calculated using the mean and extreme (±1σ) compositions of the lower and upper crust assimilants42,45. The AFC scenarios were modeled with different Ma/Mc ratios of 0.9, 0.8, and 0.7, shown in Fig. 4 in red, blue, and gray, respectively. Parameters and results are provided in Supplementary Data S4.

Magnetic properties
Magnetic properties, including natural remanent magnetization (NRM) and volume-specific magnetic susceptibility (K), were measured on 14 rock samples at the paleomagnetic lab of ISMAR-CNR in Bologna, Italy. Four samples (marked AR) of basaltic dikes were hand-oriented in the field using a magnetic compass. In the laboratory, between 1 and 5 standard paleomagnetic cylinders (2.54×2.2 cm) were drilled from each rock sample, resulting in a total of 31 specimens. The NRM was measured with a PSM spinner magnetometer, while magnetic susceptibility (K) was determined using a Bartington MS2B meter. The Koenigsberger ratio (Q) was calculated as the ratio of remanent magnetization (NRM) to the induced magnetization produced by Earth’s field (26.506 A/m). Results are presented in Supplementary Fig. S3 and Supplementary Data S5.

Data availability
All the data generated in this study are provided in the Supplementary Data files (from S1 to S5) and have also been deposited in Figshare https://doi.org/10.6084/m9.figshare.29410169.

Code availability
The Assimilation-fractional crystallization (AFC) model code (alpha-MELTS), which can be used to re-produce the data presented here (Supplementary Data S4), is available on GitHub https://github.com/magmasource/alphaMELTS.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "I had the pleasure of reading \"Parting continents: lower-crustal intrusions delayed continental rupture in the Red Sea\" by\nAlessio Sanfilippo and others, and I thank both the editor and authors for the opportunity to contemplate on this manuscript\nand provide some feedback. In this manuscript, the authors present new argon dates, magnetic data, geochemical data\nincluding bulk rock elemental, Sr, Nd, and Pb isotopic data, and modelling of these data to investigate the evolution of the\nTihama Asir igneous complex in Saudi Arabia within the broader context of Red Sea formation and ocean basin\ndevelopment. These new data, and in particular the isotopic (Sr-Nd-Pb-Ar) data provide a valuable contribution to efforts in\nunderstanding the vast and complex petrogenesis of igneous complexes in Saudi Arabia and, as such, are of high value to\nthis journal. The authors take a unique and effective approach in this task by interpreting their varied data sources together\nwith published literature data, thus presenting a dynamic and multi-dimensional interpretation that is compelling and\nsupported by their data. The methods are detailed and are highly appropriate to this work. The authors outline an evolution\nof the Red Sea spanning the Oligocene-Miocene wherein the variable spreading rates of the Southern and Northern Red\nSea are explained by weakening of the lithosphere in the south due to impingement of the Afar Plume and collision in the\nnorth of the plate. Thermal weakening of lithosphere combined with asthenospheric upwelling initiated magmatism that\naccommodated extension and ultimately delayed oceanic crust formation by several million years. These results have a high\ndegree of significance for our understanding of ocean basin formation and are thus fitting for this journal.\nI would, however, make several suggestions for re-framing certain aspects of this manuscript before acceptance by the\njournal. Overall the manuscript is very well written and individual thoughts are easy to understand. I had the greatest degree\nof confusion with the organization of the manuscript. While it flows logically, the lack of clearly defined sections makes it\ndifficult to know when the authors are specifically referencing their own data and when they are still talking about published\ndata. For instance, the first paragraph on page 6 blends a discussion on both existing data in the literature and also the\nauthors new data, which is not at all clear. Upon first read it sounds like the authors are still discussing published data but\nwithout necessary references to support their observations, but then later on it becomes apparent that they are discussing\ntheir new data. The confusion is driven by two issues: 1) the lack of a clearly-defined results and discussion section, and 2)\nabundant mixing of introduction-type rhetoric with new observations (results-type rhetoric) and interpretations (discussion-\ntype rhetoric) by the authors. Again using this section as an example, on page 5 the authors walk through an introduction on\nthe Tihama Asir igneous complex but then later within the same section present new observations and interpretations which\nmake it difficult to clearly understand when something they are saying is novel or an accepted interpretation gleaned from\nliterature. The manuscript would thus benefit from a general re-organization to make it easier on the reader to follow the\nauthors' thoughts. For instance, a clear conclusion section would be greatly appreciated.\nAnother general criticism is the lack of a substantial discussion on the application of the authors' model on a global scale.\nThe authors briefly mention the names of other regions that may have experienced a similar geologic history in the very last\nparagraph of the manuscript, but they do not discuss any details on why they consider these regions to be worth mentioning. A\nsmall subsection within the discussion that highlights the broader implications of the findings in this paper, other regions in\nthe world that are or have experienced a similar geologic history, and details on how this model can be applied to those\nregions, would greatly enhance this manuscript and increase its suitability for this (or any) journal.", "reviewer_3": "DISCLAIMER\nI am not an igneous petrologist/geochemist/isotope specialist. As agreed with the editor, I therefore focused on the regional\naspect and the discussion about rifting processes but didn’t look in detail at the methods and results. However, I did\ncomment when a link between results and interpretation was not clear to me. As this journal has a broad audience (I also\nthink you are submitting your paper here to reach more than igneous petrologists), these comments might flag some parts of\nthe argumentation that might not be properly phrased or explained for non-experts.\nNote that I made a number of suggestions to increase clarity or completeness of the manuscript. Use them if you find them\nuseful, but forget them if you don’t agree with them.\nREVIEW SUMMARY\nSanfilippo et al. present a manuscript on a Cenozoic magmatic complex on the passive margin of the Red Sea. This\nstructure is one of the few rift-related igneous complexes cropping out and available to study magma-rich rifting processes\nand the rift-to-drift transition. They apply petrological, geochemical and isotopic methods to investigate the emplacement of\nthis igneous complex, including melt pathways and crustal contamination. They derive a model for the formation of the Red\nSea passive margin, where the magmatic intrusions increase the thickness of the crust, forming a proto-oceanic crust. They\nfinally propose that these magmatic intrusions delay seafloor spreading.\nOverall, the object of the study is rare, interesting, and its analysis is novel. The topic will appeal to a broad range of\ngeologists working on rift and passive margins and won’t be limited to igneous petrologists only.\nI am not qualified to judge the methodology. The links between the results and some of the interpretations are not clear to\nme. This does not necessarily mean that these interpretations are wrong, but maybe that this link is not explicitly phrased for\nnon-experts. I’ll leave the other reviewer(s) judge the validity of these aspects, but I encourage the authors to rephrase these\nargumentations for the broader audience.\nThe final interpretation of the authors is that this magmatic activity significantly delayed oceanic spreading. I question this\ninterpretation as the delayed nature of oceanic spreading in the southern Red Sea is far from clear and several arguments\nindicate a rather quick transition to oceanic spreading compared to other margins worldwide and other parts of the Red Sea.\nThe link between magmatism and delay is also not clear. I would advise the authors to either leave out this part of the\ninterpretation and focus on other interesting aspects, such as magma-assisted extension, or to present a robust\nargumentation.\nFinally, it is regrettable that the authors completely omit one of the prominent hypotheses for the extent and the age of\noceanic spreading in the Red Sea and I think this should be corrected.\nMore detailed comments are listed below.\nGENERAL COMMENTS\nGeneral message of the paper\nThis is certainly not the most important point of this review, but I think that one aspect that could be improved is to better\nexplain to the reader what your main message is. Nature Communication has a very broad audience, and you should make\nclear to them what your message is and why it does matter. You should be very explicit as to why your work is novel, and\nwhat it brings to the understanding of rifting processes. Sentences like “We show that...”, or “Our results confirm [for the first\ntime] that...” or “Our results indicate that previous hypotheses were probably incorrect because...”.\nThe main message of the paper was not clear to me after the first read.\nThis is particularly visible in the paragraph starting with “Although several questions remain, our work allows to outline the\nevolution of the southern Red Sea rift…” where you present the whole evolution of the margin but where it is not clear what\nyour contribution is to the story (detailed comment below).\nYou might also want to improve the end of your introduction. You correctly report the numerous factors that play a role in the\nrift to drift transition (that part might even be slightly shortened if you must save some characters for the total count), but you\ndon’t say specifically how this study site and your analyses are crucial for this understanding. I think it would better “sell”\nyour study and help the reader understand to purpose and value of your study if you put more emphasis on the uniqueness\nof your study area and the novelty of your analyses, e.g. what you present later in your manuscript “most constraints on the\nlocation and depth of magma intrusions during the initial stages of rifting have been derived from crustal-scale seismic\nimaging of relatively young (e.g., Ethiopian) and poorly evolved (e.g., Baikal) rifts.”. Then (still in the intro), I'd explain in more\ndetail what you did. Something like “We conduct XYZ analyses to unravel XYZ, the source of the magma and its migration\nand emplacement in the crust. Then we measured XYZ to better constrain the geophysical signal these rocks can produce”.\nSo that afterwards, the reader knows more precisely what exactly you investigated.\nJust a suggestion to make it more attractive.\nLink between results and interpretations\nI sometimes miss the link between your results and some of your interpretations. It might just be because I am not an expert\nin igneous geochemistry/isotopes, but the audience of the journal is also very broad and other readers might also have\ndifficulties following your reasoning.\nI have done detailed comments on these cases below under “Specific comments”. Some are important points for your\ninterpretation and should thus be addressed: E.g.: How do your results allow you to interpret that the lithospheric mantle was\nalready completely replaced by asthenospheric mantle? Which results allow you to say that the ascent of the magma\nthrough the crust was fast?\nWithout clarification and as a non-expert in these methods, I fail to completely understand how your results support some\nof your interpretations.\nAge of oceanic spreading in the Red Sea\nOne important paragraph of the paper deals with the age of oceanic spreading in the Red Sea. You conclude that oceanic\nspreading started around 5 Ma. The problem is that you omit one of the main hypotheses about the structure of the\n(southern) Red Sea. In the paper, you present two hypotheses, the 5 Ma oceanic crust hypothesis and the “two-stage”\noceanic crust hypothesis. I agree with you that the latter is not plausible from a timing and kinematic point of view (you could\neven shorten this argumentation if you’d need more space). But you completely omit the hypothesis of an onset of oceanic\nspreading between 10 and 14 Ma. This hypothesis has seen a lot of attention since the 80s and in the last decade. Many\nserious publications argue for such an interpretation\nThey estimate the initiation of seafloor spreading around 10 Ma (Okwokwo et al., 2022), between 10-12 Ma (Izzeldin,\n1987; Rihm & Henke, 1998), at ~13 Ma (Augustin et al., 2021; Le Pichon & Gaulier, 1988) and ~14 Ma (Delaunay et al., 2023;\nMakris & Rihm, 1991). This hypothesis is postulated based on the analysis of magnetic anomalies, gravimetry, bathymetry\nand seismic data (Augustin et al., 2016, 2021; Bohannon, 1986; Bohannon & Eittreim, 1991; Delaunay et al., 2023; Izzeldin,\n1987; Mitchell & Park, 2014; Mitchell et al., 2023; Okwokwo et al., 2022; Rihm & Henke, 1998).\nIt represents a very serious hypothesis (the most plausible to me). I don’t know whether there is a feud between you and\nthese authors, but I think it is not correct to not at least mention it. You don’t have to embrace it (the issue is not settled; these\nare still open hypotheses) but at least mention it and acknowledge that this is a possibility or argue why you reject it.\nThis also has implications for your main message of your paper: the delayed seafloor spreading (see below).\nDelayed seafloor spreading\nFor me, this represents the most important weakness of the manuscript. The delayed seafloor spreading seems to be one of\nthe most central points of your paper, it is mentioned in the title, in the abstract, and in the last sentences of the paper. But\nyou did not convince me that seafloor spreading was actually delayed in this part of the Red Sea. You actually never give\narguments to say it was delayed.\nConsidering that faulting started around 29 Ma (Bohannon, 1986), we have 24 to 15 My between start of rifting and seafloor\nspreading (for oceanic spreading starting at 5 and 14 Ma respectively, see above). Is that long? No. Compared to other\nmargins, this is relatively short (Sapin et al., 2021; Ziegler & Cloetingh, 2004). Was it delayed compared to the rest of the\nRed Sea? Not really. The northern Red Sea is thought be several authors to not having reached a steady-state oceanic\nspreading yet (e.g. Almalki et al., 2015; Coleman & McGuire, 1988), so the southern Red Sea is not delayed compared to it.\nEven more striking, the southernmost Red Sea in the Bab-el-Mandeb/Hanish Island region is delayed compared to your\nstudy area, not having reached oceanic spreading yet and likely not reaching it anytime soon (Rime et al., 2024).\nSo, if you want to say that seafloor spreading was “significantly delayed”, you’d have to argue for it. But for me, this is not\nclear that seafloor spreading was delayed in this area.\nFurthermore, I’m not sure magmatic intrusions are the best way to delay oceanic spreading (except for submarine ridges and\noceanic plateau). If you look at a few wide (and long-lived) rifts around the world, you could cite the Basin-and-Range, the\npre-Paleogene Vøring basin (which was the last part of the north Atlantic to break up; Gernigon et al., 2020; Zastrozhnov et\nal., 2020), the Santos basin (which was the last part of the south Atlantic to break up; Heine et al., 2013), the South China\nSea (e.g. Chang et al., 2022; Legeay et al., 2024). All these margins were magma-poor (or at least not magma-rich) margins.\nThe Vøring basin ended up breaking up as magma-rich but the significant pre-Paleogene extension was magma-poor. So,\nI’m not sure the mere fact that we observe significant magmatism indicates that the seafloor spreading was delayed.\nInstead, I’d say that what you observe here is a very efficient way to accommodate extension with magmatic intrusion\ninstead of faulting, rather than a way to delay oceanic spreading. So an excellent example of magma-assisted extension or\nmagmatic rifting, (e.g. Bastow & Keir, 2011; Buck, 2006; Kendall et al., 2005), and a case of magma-compensated-thinning.\nAnd this is still very interesting! Your study shows that these intrusions don’t have much problem ascending through the\ncrust, they do it fast. That’s interesting because it could imply that if magma is produced, then it is easy for it to accommodate\nthe extension. And that has important implications for the rifting process because it weakens the crust and allows lower\ntectonic forces to complete the breakup (Bialas et al., 2010; Brune et al., 2023; Buck, 2006). The timing is also interesting\nbecause 26 Ma is only 3 Ma younger than the 29 Ma mentioned by Bohannon (1986) for the start of faulting, meaning that\nthis mechanism can happen quite early in the rifting process. Another very interesting aspect is that your results show the\nmechanisms forming a transitional crust (or proto-oceanic as you name it). It might represent the deeper equivalent of some\nof what is observed in Greenland for example (Abdelmalak et al., 2015; Brooks, 2011; Hanghøj et al., 2003; Klausen, 2022).\nAll this to say that even if I am quite sceptical with the delay aspect, I think your results are still very interesting to understand\nrifting processes and rift-to-drift transition. So, one way to improve this aspect of the paper would be to step away from this\n“delayed” side and concentrate more on the magma-assisted extension side.\nSPECIFIC COMMENTS\nThis rift system began fragmenting the Afro-Arabian shield since the late Oligocene, influenced by a mantle plume impacting\nthe Afar region around 34 Ma10-12 [Hofmann et al. 1997; Storey 1995; Ebinger & Sleep 1998].\nStorey 1995 is definitely not an appropriate reference here as he doesn’t even mention, Ethiopia, Yemen or Afar. (Baker et\nal., 1996; Bosworth et al., 2005; Zumbo et al., 1995) would be more appropriate (or just keep the other references you have).\nVariations in GNSS-derived relative velocities between the Arabian and Nubian plates, increasing from ~7 mm/yr in the\nnorthern Red Sea to ~16 mm/yr in the southern Red Sea16 [Viltres et al., 2022], may indicate different stages of the rifting\nprocess, suggesting distinct dominant mechanisms for the formation of the northern and southern Red Sea.\nMaybe as important is the fact that the northern Red Sea experienced lower total extension. I’d argue that the lower\nextension is the cause of the different stages of rifting while the different mechanisms of formation are probably rather\ncaused by the lower velocity (among other effects such as plume influence). Detail though.\nThe onset of rifting in the Red Sea was preceded by flood basalts erupted in Ethiopia, NE Sudan and SW Yemen17\n[Chauvet et al., 2023],\nChauvet et al., 2023 is maybe not the best reference to cite here as they don’t deal much with the flood basalts or\nchronology. Maybe papers such as (Bosworth et al., 2005; Rime et al., 2023; Rooney, 2017) would be more appropriate.\nDetail though.\nalso associated by rhyolitic volcanism until ~26 Ma18,19 [Baker et al., 1996; Manetti et al., 1991].\nI’d maybe remove “until ~26 Ma” because rhyolitic volcanism has been recorded until much more recent time in Afar,\nincluding on the Red Sea coast (Lahitte et al., 2003; Varet, 2018). Detail though.\nSecond comment, I’d maybe add a small sentence around here to indicate the age of faulting along this margin. At the\nmoment, you only mention the age of magmatic activity, but the reader might also be interested in the age of faulting.\nBohannon (1986) for example mention an age of 23 to 29 Ma for faulting in the area.\nalong with the contemporaneous Tihama Asir20 [Coleman & McGuire, 1988] (southern Red Sea), Al Lith25 [Pallister, 1987]\n(central Red Sea) and Quseir21,22 [Ligi et al., 2018; 2019] (northern Red Sea) magmatic complexes, which include\ngabbros, basaltic dikes, and granophyres.\nMaybe you could also mention the granite batholiths found in Yemen (Capaldi et al., 1987; Davison et al., 1994) (if the length\nof the paper and references allows it)?\nMagmatic underplating and crustal contamination\nI’m not qualified to discuss the detail of this section, but the aspect of the underplating, cited in the title of the section and\nused again in the discussion, is not clearly presented in this section itself. If you mean underplating at the Moho, what proxy\nallows you to say there was underplating? If you mean underplating in sense of any ponding within the lower crust (sensu\nThybo & Artemieva, 2013), then what argument do you have for it? You say that there was only little assimilation from the\nlower curst (and use it as an argument to say the melts ascended rapidly, see below).\nIt might be straightforward for an expert, but it is not for me.\nEarlier studies proposed that seafloor spreading in the southern Red Sea began with the emplacement of the Tihama Asir\nand Al Lith magmatic complexes (26 – 21 Ma), where gabbros and dikes, with a strong geochemical affinity to melts\ngenerated by depleted mantle, could indicate emplacement of oceanic crust beneath much of the Red Sea's coastal plains,\nshelf regions and its central axis40,60-62 [Stern & Johnson, 2019; Girdler & Styles, 1974; Almalki et al., 2014; Augustin et\nal., 2021].\nNot very appropriate to cite Augustin et al. (2021) here as they don’t mention the Tihama Asir and Al Lith complexes at all\nand propose the formation of oceanic crust at 13 Ma, i.e. long after their emplacements.\nOur results show that syn-rift gabbros formed in relatively shallow magma chambers [...], fuelled by asthenospheric melts that\nincorporated ancient lower continental crust material. The minimal contamination from upper crustal material suggests these\nmelts ascended rapidly through the continental crust with limited interaction.\nAlso in the Abstract:\nthe formation of syn-rift gabbros in relatively shallow magma chambers, fed by asthenospheric melts that assimilated ancient\nlower continental crustal material. The limited interaction with the upper crust indicates a swift ascent through the continental\ncrust without significant contamination.\nIn both cases, you say that these melts incorporated lower continental crustal material and, right after, you say that they had\nlittle interaction with the continental crust. It sounds contradictory to me. If it incorporated lower continental crustal material,\nthen it did interact with the continental crust. Do you mean it ascended rapidly and had little/no interaction with the upper\ncrust only? Or you mean that it interacted with the lower crust, but not too much, and thus that it was still rapid (maybe not as\nfast as in the upper crust)?\nThis point of a rapid magma ascend is quite important to your paper. I think that the argumentation should be solid and not\nprone to confusion as it is now. You should probably rephrase these parts and/or argue a bit more to explain why you\ninterpret it ascended rapidly.\nI am not an expert, and I might miss something, but the audience of the journal is also very broad and other readers might also\nfind these statements confusing.\nThe above indicates a rapid replacement of the continental lithospheric mantle with upwelling asthenosphere,\nThis point is quite important to your paper, but the argumentation is not clear to me. What argument make you say the\nlithospheric mantle was already completely replaced? I miss a step to explain why the melt did not ascend through the\nlithospheric mantle before reaching the crust. It seems to me that you never explicitly explain this.\nAgain, it might be because I’m not an expert, but the reader might also not be one.\nOur new findings are crucial because...\nA few comments on this paragraph.\nI would maybe end the paragraph with a comparison of your results. You mention other examples, but I’d make a more direct\nlink with your results. “This is comparable with our results …” or “we confirm here these geophysical interpretations with\npetrological evidence”. Just a suggestion.\nAnother paper that you might perhaps want to compare to your work is the recent one of La Rosa et al. (2024), which image\nlower crustal intrusion in Afar with a completely different technique (and completely different timescale). Just a suggestion.\nBTW, good that you put here a real emphasis on the additional value of your work compared to other publications. This\nshould also come in the introduction.\nGeophysical data, including density, magnetic and seismic velocity anomalies, provide significant constraints...\nI would put this paragraph before the one comparing with other examples worldwide. So, you bring all arguments (samples\nanalyses + geophysics) before comparing with other areas. Now, you have a “lonely” sentence on the topic earlier “This\nprocess is also supported by gravity models explaining Bouguer anomaly patterns64 [Issachar et al., 2023].” and I would say\nthat the whole paragraph would be better placed replacing this lonely sentence.\nJust a suggestion.\nIn addition, at low extension rates, when migrating melt does not repeatedly intrude the same axial zone, dike intrusions may\nincrease crustal strength compositionally75,76 [Beutel et al., 2010; Daniels et al., 2014]. This process can accommodate\nextension in the upper crust, maintain its thickness, and delay crustal rupturing76 [Daniels et al., 2014].\nThe “in addition” makes the reader feel these sentences are connected to the previous point. But they are not really. Maybe\nbetter separate these two aspects.\nSecond comment: as far as I understand these papers, neither Beutel et al., nor Daniels et al. say that migrating intrusions\nwould increase crustal strength. Daniels et al. only mention that the crust would weaken less if the intrusions are distributed\nrather than on repeated at the same place. Overall, most models show that intrusion weaken the crust (Beutel et al., 2010;\nBialas et al., 2010; Brune et al., 2023; Buck, 2006; Daniels et al., 2014)\nour work allows to outline the evolution of the southern Red Sea rift and the transition from continental rifting to oceanic\nseafloor spreading, driven by interactions between thermal, structural and magmatic processes (Fig. 5)...\nThe paragraph following this sentence explains the process of rifting and drifting in the southern Red Sea citing multiple\npapers, but it would benefit if you would explain a bit more clearly which parts are coming from your analyses. I think you\nshould “sell” your work a bit better and make super clear for the reader what you bring to the story. At the moment, this\nparagraph sounds a bit like a review. I’d add sentences like “we show here that …” within the paragraph to really show what\naspects are new in this paper.\nAnother aspect of this paragraph is that you seem very (too) confident of your model. I’d tone down several statements which\nare not proved but suggested. E.g. “Plume-related thermal anomalies and underplating [probably] initially weakened the\nlower crust, allowing decoupling between the lower crust and the lithospheric mantle along the axial zone beneath the\nthinning upper continental lithosphere”, “The combination of thermal softening and ductile flow of the lower crust [possibly]\nled to depth-dependent, less localized and more distributed deformation, protracting crustal extension and favoring the\ndevelopment of a wide rifted margin”, “…within the slowly diverging Nubia-Arabia continental lithosphere [probably]\ncontributed significantly to rift initiation, accommodating plate separation by re-thickening…”, “…magmatic additions\n[probably] further widened the Red Sea basin and [possibly] caused rift axis migration…”\nThese comments also apply to the caption of Fig. 5.\ncaused rift axis migration\nDo we really have evidence of rift migration? Or do you infer this from modelling paper you cite? I’d see another possibility\nwhere, instead of rift migration, we have rift focusing.\naccommodating plate separation by re-thickening the thinned lower crust and compensating for extension through\nwidespread dike intrusions in the upper crust.\nThis phrasing implies that the crust thinned and then became thicker after the intrusions (in two phases). But everything\nmight happen at the same time and the thinning is only reduced by the intrusions. Maybe the notion of “magma-\ncompensated thinning” would be more appropriate?\nSeafloor spreading commenced approximately 5 Ma in the axial trough of the southern Red Sea\nAs mentioned in the general comments, that’s only one hypothesis. There are others.\nsuch as the eastern branch of the East African Rift84 [Bastow & Kier, 2011],\nI’d say “Afar Depression” rather than “eastern branch of the EAR” as most of the Afar is not part of the EAR sensu stricto.\nDetail though.\nMagmatic intrusions in the lower crust delay the localization of lithospheric extension and the final drifting stage by several\nmillion years,\nYou have not convinced me that these intrusions delay strain localization. As said above, it is far from certain that oceanic\nspreading started at 5 Ma in the Red Sea. If the age of onset of seafloor spreading is 14 Ma, then there is only 7-12 My\nbetween the intrusions and seafloor spreading. See general comment at the start of the review.\n(Abstract) Our study demonstrates that magmatic underplating during the early rifting phases significantly delayed the\ncontinental rupture and the onset of seafloor spreading in the southern Red Sea.\nA don’t really agree it demonstrates that it delayed the continental rupture. We are not even sure that the continental rupture\nis delayed (see above)."}, "author_response": "Response to Referees’ comments (21-MAY-2025) Nature\nCommunications - Manuscript # NCOMMS-24-76525-T\nWe thank Reviewers 2 and 3 for their valuable and constructive comments, which have been\nvery helpful in revising the manuscript. In response, we have changed the title, improved the clarity\nand structure of the text and added two figures as outlined below. The manuscript has been\nsubstantially revised in line with the reviewers’ suggestions, and we believe the changes have\nsignificantly improved the overall quality of the article.\nIn the following, reviewer comments are presented in italics, while our responses appear in\nnormal font.\nResponse to Referee 2\nWe thank this Reviewer for his/her suggestions. We did all the changes he/she suggested.\nRemark to the Author\nI had the pleasure of reading \"Parting continents: lower-crustal intrusions delayed continental\nrupture in the Red Sea\" by Alessio Sanfilippo and others, and I thank both the editor and authors for\nthe opportunity to contemplate on this manuscript and provide some feedback. In this manuscript,\nthe authors present new argon dates, magnetic data, geochemical data including bulk rock elemental,\nSr, Nd, and Pb isotopic data, and modelling of these data to investigate the evolution of the\nTihama Asir igneous complex in Saudi Arabia within the broader context of Red Sea formation and ocean\nbasin development. These new data, and in particular the isotopic (Sr-Nd-Pb-Ar) data provide a\nvaluable contribution to efforts in understanding the vast and complex petrogenesis of igneous\ncomplexes in Saudi Arabia and, as such, are of high value to this journal. The authors take a unique\nand effective approach in this task by interpreting their varied data sources together with published\nliterature data, thus presenting a dynamic and multi-dimensional interpretation that is compelling\nand supported by their data. The methods are detailed and are highly appropriate to this work. The\nauthors outline an evolution of the Red Sea spanning the Oligocene-Miocene wherein the variable\nspreading rates of the Southern and Northern Red Sea are explained by weakening of the lithosphere\nin the south due to impingement of the Afar Plume and collision in the north of the plate. Thermal\nweakening of lithosphere combined with asthenospheric upwelling initiated magmatism that\naccommodated extension and ultimately delayed oceanic crust formation by several million years.\nThese results have a high degree of significance for our understanding of ocean basin formation and\nare thus fitting for this journal.\nWe thank this Reviewer for his/her positive opening statements on this work.\nI would, however, make several suggestions for re-framing certain aspects of this manuscript\nbefore acceptance by the journal. Overall the manuscript is very well written and individual thoughts\nare easy to understand. I had the greatest degree of confusion with the organization of the manuscript.\nWhile it flows logically, the lack of clearly defined sections makes it difficult to know when the authors\nare specifically referencing their own data and when they are still talking about published data. For\ninstance, the first paragraph on page 6 blends a discussion on both existing data in the literature and\nalso the authors new data, which is not at all clear. Upon first read it\nsounds like the authors are still discussing published data but without necessary references to support\ntheir observations, but then later on it becomes apparent that they are discussing their new data. The\nconfusion is driven by two issues: 1) the lack of a clearly-defined results and discussion section, and\n2) abundant mixing of introduction-type rhetoric with new observations (results-type rhetoric) and\ninterpretations (discussion-type rhetoric) by the authors. Again using this section as an example, on\npage 5 the authors walk through an introduction on the Tihama Asir igneous complex but then later\nwithin the same section present new observations and interpretations which make it difficult to clearly\nunderstand when something they are saying is novel or an accepted interpretation gleaned from\nliterature. The manuscript would thus benefit from a general reorganization to make it easier on the\nreader to follow the authors' thoughts. For instance, a clear conclusion section would be greatly\nappreciated.\nWe sincerely thank the reviewer for the thoughtful and constructive feedback regarding the\nmanuscript’s structure. We fully agree that a clearer separation between the discussion of published\ndata and our new findings is essential for improving the manuscript's clarity and readability. In\nresponse to Referee’s suggestions, we have undertaken a general reorganization of the manuscript.\nSpecifically, we have:\ni) Introduced “Results” and “Discussion” sections, ensuring that our new observations are presented\ndistinctly from previously published work.\nii) Minimized the blending of introduction-type content with new results and interpretations, so the\nreader can easily distinguish between established knowledge and our novel contributions\nWe believe these changes greatly enhance the logical flow and overall clarity of the manuscript, and\nwe appreciate your guidance in helping us improve its structure.\nAnother general criticism is the lack of a substantial discussion on the application of the authors'\nmodel on a global scale. The authors briefly mention the names of other regions that may have\nexperienced a similar geologic history in the very last paragraph of the manuscript, but they do not\ndiscuss any details on why they consider these regions to be worth mentioning. A small subsection\nwithin the discussion that highlights the broader implications of the findings in this paper, other\nregions in the world that are or have experienced a similar geologic history, and details on how this\nmodel can be applied to those regions, would greatly enhance this manuscript and increase its\nsuitability for this (or any) journal.\nWe agree that discussing the broader implications of our model and its potential application to other\nregions would enhance the impact and relevance of the manuscript. In response, we have added a\ndedicated subsection in the Discussion where we elaborate on analogous geological settings\nworldwide. Specifically, we now provide supporting observations and reasoning for the regions briefly\nmentioned in the conclusion, explaining why we consider them to share key geological features with\nour study area. We also outline how the concepts developed in this work can be applied to these\nsettings, thereby reinforcing the global significance of our findings. Thank you again for encouraging\nus to expand this important aspect of the discussion.\nResponse to Referee 3\nReview Summary\nI am not an igneous petrologist/geochemist/isotope specialist. As agreed with the editor, I\ntherefore focused on the regional aspect and the discussion about rifting processes but didn’t look in\ndetail at the methods and results. However, I did comment when a link between results and\ninterpretation was not clear to me. As this journal has a broad audience (I also think you are\nsubmitting your paper here to reach more than igneous petrologists), these comments might flag some\nparts of the argumentation that might not be properly phrased or explained for non-experts. Note that\nI made a number of suggestions to increase clarity or completeness of the manuscript. Use them if you\nfind them useful, but forget them if you don’t agree with them.\nSanfilippo et al. present a manuscript on a Cenozoic magmatic complex on the passive margin of\nthe Red Sea. This structure is one of the few rift-related igneous complexes cropping out and available\nto study magma-rich rifting processes and the rift-to-drift transition. They apply petrological,\ngeochemical and isotopic methods to investigate the emplacement of this igneous complex, including\nmelt pathways and crustal contamination. They derive a model for the formation of the Red Sea passive\nmargin, where the magmatic intrusions increase the thickness of the crust, forming a proto-oceanic\ncrust. They finally propose that these magmatic intrusions delay seafloor spreading. Overall, the\nobject of the study is rare, interesting, and its analysis is novel. The topic will appeal to a broad range\nof geologists working on rift and passive margins and won’t be limited to igneous petrologists only.\nI am not qualified to judge the methodology. The links between the results and some of the\ninterpretations are not clear to me. This does not necessarily mean that these interpretations are\nwrong, but maybe that this link is not explicitly phrased for non-experts. I’ll leave the other reviewer(s)\njudge the validity of these aspects, but I encourage the authors to rephrase these argumentations for\nthe broader audience.\nThe final interpretation of the authors is that this magmatic activity significantly delayed oceanic\nspreading. I question this interpretation as the delayed nature of oceanic spreading in the southern\nRed Sea is far from clear and several arguments indicate a rather quick transition to oceanic\nspreading compared to other margins worldwide and other parts of the Red Sea. The link between\nmagmatism and delay is also not clear. I would advise the authors to either leave out this part of the\ninterpretation and focus on other interesting aspects, such as magma-assisted extension, or to present\na robust argumentation.\nFinally, it is regrettable that the authors completely omit one of the prominent hypotheses for\nthe extent and the age of oceanic spreading in the Red Sea and I think this should be corrected. More\ndetailed comments are listed below.\nWe sincerely thank the Reviewer for the thoughtful and constructive comments. We truly appreciate\nthe time and care taken to assess our manuscript, particularly from the perspective of a broader\ngeological audience, which aligns with the interdisciplinary readership of the journal. We have\nsubstantially revised the manuscript, closely following Referee 3’s general and specific suggestions\nas outlined below. In particular, we rephrased sections to improve clarity and completeness,\nincorporating the Reviewer’s suggestions. Regarding the interpretation that magmatic activity may\nhave delayed the onset of oceanic spreading, we acknowledge the Reviewer’s concerns and revised\nthe paper’s title, the abstract and the relevant sections. Additionally, we have now addressed the\ndifferent hypothesis regarding the extent and timing of oceanic spreading in the Red Sea, as rightly\npointed out. We are confident that these changes improve the manuscript’s clarity, balance, and overall\naccessibility, and we are grateful for the Reviewer’s valuable input in achieving this.\nGeneral comments\n“General message of the paper”\nThis is certainly not the most important point of this review, but I think that one aspect that could\nbe improved is to better explain to the reader what your main message is. Nature Communication has\na very broad audience, and you should make clear to them what your message is and why it does\nmatter. You should be very explicit as to why your work is novel, and what it brings to the\nunderstanding of rifting processes. Sentences like “We show that...”, or “Our results confirm [for the first\ntime] that...” or “Our results indicate that previous hypotheses were probably incorrect\nbecause...”. The main message of the paper was not clear to me after the first read. This is particularly\nvisible in the paragraph starting with “Although several questions remain, our work allows to outline\nthe evolution of the southern Red Sea rift...” where you present the whole evolution of the margin but\nwhere it is not clear what your contribution is to the story (detailed comment below). You might also\nwant to improve the end of your introduction. You correctly report the numerous factors that play a\nrole in the rift to drift transition (that part might even be slightly shortened if you must save some\ncharacters for the total count), but you don’t say specifically how this study site and your analyses are\ncrucial for this understanding. I think it would better “sell” your study and help the reader understand\nto purpose and value of your study if you put more emphasis on the uniqueness of your study area and\nthe novelty of your analyses, e.g. what you present later in your manuscript “most constraints on the\nlocation and depth of magma intrusions during the initial stages of rifting have been derived from\ncrustal-scale seismic imaging of relatively young (e.g., Ethiopian) and poorly evolved (e.g., Baikal)\nrifts.”. Then (still in the intro), I'd explain in more detail what you did. Something like “We conduct\nXYZ analyses to unravel XYZ, the source of the magma and its migration and emplacement in the\ncrust. Then we measured XYZ to better constrain the geophysical signal these rocks can produce”. So\nthat afterwards, the reader knows more precisely what exactly you investigated. Just a suggestion to\nmake it more attractive.\nWe fully agree that clearly highlighting the main message, novelty, and broader significance of our\nwork is essential, especially for a broad and multidisciplinary audience such as that of Nature\nCommunications. We have revised the manuscript clarifying the main message of the study in both\nthe Abstract and the Introduction. The Abstract has been rewritten with a focus on highlighting the\nstudy’s key contributions, and a new concluding paragraph has been added to the Introduction to\nbetter emphasize the uniqueness of our study area and the novelty of our approach. We now explicitly\nexplain why the Tihama Asir magmatic complex provides a rare and valuable opportunity to\ninvestigate the role of magmatism in continental rupture and ocean formation, and how our\nmultidisciplinary dataset including geochemistry, isotopic analyses, magnetic properties, and Ar/Ar\ngeochronology enables us to reconstruct critical aspects of this process.\n“Link between results and interpretations”\nI sometimes miss the link between your results and some of your interpretations. It might just\nbe because I am not an expert in igneous geochemistry/isotopes, but the audience of the journal is\nalso very broad and other readers might also have difficulties following your reasoning. I have done\ndetailed comments on these cases below under “Specific comments”. Some are important points for\nyour interpretation and should thus be addressed: E.g.: How do your results allow you to interpret\nthat the lithospheric mantle was already completely replaced by asthenospheric mantle? Which\nresults allow you to say that the ascent of the magma through the crust was fast? Without clarification\nand as a non-expert in these methods, I fail to completely understand how your results support some\nof your interpretations.\nWe respectfully disagree with the concern that key interpretations are not sufficiently supported by\nour results. We acknowledge that geochemical and isotopic inferences may be more familiar to\nspecialists, we have made a consistent effort throughout the manuscript to clearly explain the logical\nconnections between geochemical data and our interpretations. For example, the inference that the\nlithospheric mantle was largely replaced by asthenospheric mantle is supported by both\ngeochemical and radiogenic isotopic data. The major element compositions of the Tihama Asir\ngabbros and dikes cross-cutting the gabbro-bodies indicate a strong affinity with melts derived from\npartial melting of a mantle source, characteristic of mid-ocean ridge basalt (MORB) and typical of\nupwelling asthenosphere. Moreover, the trace element and isotopic signatures of the Tihama Asir\ngabbros suggest that these syn-rift intrusions from asthenospheric melts assimilated only ancient lower\ncontinental crust. The absence of signatures typical of continental lithospheric mantle, which is\ngeochemically and isotopically distinct from depleted asthenosphere, further supports the\ninterpretation of rapid continental lithospheric mantle replacement by upwelling asthenosphere.\nAdditionally, the evidence of limited lower crustal contamination and the absence of upper crustal\nassimilation imply short magma residence times within the upper continental crust, consistent with\nrapid ascent of melts. These interpretations are explicitly discussed and supported throughout the\nResults and Discussion sections of the manuscript.\n“Age of oceanic spreading in the Red Sea”\nOne important paragraph of the paper deals with the age of oceanic spreading in the Red Sea.\nYou conclude that oceanic spreading started around 5 Ma. The problem is that you omit one of the\nmain hypotheses about the structure of the (southern) Red Sea. In the paper, you present two\nhypotheses, the 5 Ma oceanic crust hypothesis and the “two-stage” oceanic crust hypothesis. I agree\nwith you that the latter is not plausible from a timing and kinematic point of view (you could even\nshorten this argumentation if you’d need more space). But you completely omit the hypothesis of an\nonset of oceanic spreading between 10 and 14 Ma. This hypothesis has seen a lot of attention since\nthe 80s and in the last decade. Many serious publications argue for such an interpretation. They\nestimate the initiation of seafloor spreading around 10 Ma (Okwokwo et al., 2022), between 10-12 Ma\n(Izzeldin, 1987; Rihm & Henke, 1998), at ~13 Ma (Augustin et al., 2021; Le Pichon & Gaulier, 1988)\nand ~14 Ma (Delaunay et al., 2023; Makris & Rihm, 1991). This hypothesis is postulated based on\nthe analysis of magnetic anomalies, gravimetry, bathymetry and seismic data (Augustin et al., 2016,\n2021; Bohannon, 1986; Bohannon & Eittreim, 1991; Delaunay et al., 2023; Izzeldin, 1987; Mitchell\n& Park, 2014; Mitchell et al., 2023; Okwokwo et al., 2022; Rihm & Henke, 1998). It represents a very\nserious hypothesis (the most plausible to me). I don’t know whether there is a feud between you and\nthese authors, but I think it is not correct to not at least mention it. You don’t have to embrace it (the\nissue is not settled; these are still open hypotheses) but at least mention it and acknowledge that this\nis a possibility or argue why you reject it. This also has implications for your main message of your\npaper: the delayed seafloor spreading (see below).\nWe fully agree with the Reviewer that this is an important issue that merits discussion in the\nmanuscript. Its omission was not intentional but rather a consequence of the strict space limitations\nof Nature Geoscience, where the manuscript was initially submitted before being transferred to\nNature Communications. Although we consider the second hypothesis (seafloor spreading initiating\naround 5–6 Ma, see Ligi et al., 2019 for further details) to be more consistent with the available\ngeophysical data, we acknowledge the validity of the alternative interpretation suggesting an earlier\nonset (10–14 Ma). We would also like to clarify that the corresponding author has co-authored several\npublications on the Red Sea with both Dr. Mitchell and Dr. Augustin, with whom we maintain a\ncollegial and collaborative relationship. Therefore, there were no other reasons (scientific or personal)\nfor not including this hypothesis in the discussion. In the revised version, we have now explicitly\naddressed this viewpoint and referenced the recent relevant studies to provide a more balanced and\ncomprehensive treatment of this ongoing debate adding the paragraph below at the beginning of the\nDiscussion section.\n“The evolution of the Red Sea rift has been debated for over 50 years, with no definitive evidence\nconfirming the nature of the underlying crust14,46 [Ligi et al., 2019; Augustin et al., 2021 and references\nherein]. Even in recent publications, significant uncertainty remains regarding the extent of oceanic\ncrust within the basin. Some researchers support a fully oceanic regime extending\nalong the entire Red Sea basin since 14–10 Ma46-48 [e.g., Augustin et al., 2021; Okwokwo et al., 2022;\nDelaunay et al., 2023], while others propose that attenuated continental crust persisted until 6–5 Ma,\nwhen seafloor spreading initiated in the southern and central Red Sea [e.g., Le Magoarou et al.,\n49-51\n2021; Ali et al., 2023; Issachar et al., 2023]. Both hypotheses are based on similar datasets including\nmagnetic, gravity, bathymetric, and seismic data, yet no borehole samples exist to conclusively\ndetermine the nature of the crust beneath the main trough. Furthermore, gravity and magnetic\nsignatures are ambiguous and can be attributed to either oceanic or continental sources. The thick,\nhigh-velocity evaporite sequence also limits the resolution of multichannel seismic reflection\nprofiles, obscuring the basement's tectonic architecture and hindering efforts to resolve its origin.”\n“Delayed seafloor spreading”\nFor me, this represents the most important weakness of the manuscript. The delayed seafloor\nspreading seems to be one of the most central points of your paper, it is mentioned in the title, in the\nabstract, and in the last sentences of the paper. But you did not convince me that seafloor spreading\nwas actually delayed in this part of the Red Sea. You actually never give arguments to say it was\ndelayed. Considering that faulting started around 29 Ma (Bohannon, 1986), we have 24 to 15 My\nbetween start of rifting and seafloor spreading (for oceanic spreading starting at 5 and 14 Ma\nrespectively, see above). Is that long? No. Compared to other margins, this is relatively short (Sapin et al., 2021; Ziegler & Cloetingh, 2004). Was it delayed compared to the rest of the Red Sea? Not\nreally. The northern Red Sea is thought be several authors to not having reached a steady-state\noceanic spreading yet (e.g. Almalki et al., 2015; Coleman & McGuire, 1988), so the southern Red Sea\nis not delayed compared to it. Even more striking, the southernmost Red Sea in the Bab-el-\nMandeb/Hanish Island region is delayed compared to your study area, not having reached oceanic\nspreading yet and likely not reaching it anytime soon (Rime et al., 2024). So, if you want to say that\nseafloor spreading was “significantly delayed”, you’d have to argue for it. But for me, this is not clear\nthat seafloor spreading was delayed in this area. Furthermore, I’m not sure magmatic intrusions are\nthe best way to delay oceanic spreading (except for submarine ridges and oceanic plateau). If you\nlook at a few wide (and long-lived) rifts around the world, you could cite the Basin-and-Range, the\npre-Paleogene Vøring basin (which was the last part of the north Atlantic to break up; Gernigon et\nal., 2020; Zastrozhnov et al., 2020), the Santos basin (which was the last part of the south Atlantic to\nbreak up; Heine et al., 2013), the South China Sea (e.g. Chang et al., 2022; Legeay et al., 2024). All\nthese margins were magma-poor (or at least not magma-rich) margins. The Vøring basin ended up\nbreaking up as magma-rich but the significant pre-Paleogene extension was magma-poor. So, I’m not\nsure the mere fact that we observe significant magmatism indicates that the seafloor spreading was\ndelayed.\nInstead, I’d say that what you observe here is a very efficient way to accommodate extension\nwith magmatic intrusion instead of faulting, rather than a way to delay oceanic spreading. So an\nexcellent example of magma-assisted extension or magmatic rifting, (e.g. Bastow & Keir, 2011; Buck,\n2006; Kendall et al., 2005), and a case of magma-compensated-thinning. And this is still very\ninteresting! Your study shows that these intrusions don’t have much problem ascending through the\ncrust, they do it fast. That’s interesting because it could imply that if magma is produced, then it is\neasy for it to accommodate the extension. And that has important implications for the rifting process\nbecause it weakens the crust and allows lower tectonic forces to complete the breakup (Bialas et al.,\n2010; Brune et al., 2023; Buck, 2006). The timing is also interesting because 26 Ma is only 3 Ma\nyounger than the 29 Ma mentioned by Bohannon (1986) for the start of faulting, meaning that this\nmechanism can happen quite early in the rifting process. Another very interesting aspect is that your\nresults show the mechanisms forming a transitional crust (or proto-oceanic as you name it). It might\nrepresent the deeper equivalent of some of what is observed in Greenland for example (Abdelmalak\net al., 2015; Brooks, 2011; Hanghøj et al., 2003; Klausen, 2022). All this to say that even if I am quite\nsceptical with the delay aspect, I think your results are still very\ninteresting to understand rifting processes and rift-to-drift transition. So, one way to improve this\naspect of the paper would be to step away from this “delayed” side and concentrate more on the\nmagma-assisted extension side.\nWe fully agree with the Reviewer’s argument that continental rupture in the southern Red Sea was\nindeed delayed (15-20 Ma later the beginning of faulting) when compared to other wide rifted margins\n(mostly magma-poor rifted basin), as well as to other segments of the Red Sea itself, such as the\nnorthern Red Sea, which remains in a stage of continental rifting, and the southernmost part near the\nBab-el-Mandeb/Hanish Island region. Our objective was to highlight a significant delay in continental\nbreakup in the Tihama Asir region, particularly when compared to interpretations that suggest\noceanization began contemporaneously with the emplacement of the magmatic complex.\nThat said, we have fully incorporated all of the Reviewer's suggestions by refocusing the central\nmessage of the paper on our key observations. The Tihama Asir region provides a clear example of\nmagma-compensated lithospheric thinning during magma-rich rifting, where extension is efficiently\naccommodated through magma underplating, re-thickening the thinning lower continental crust, and\nthrough magmatic intrusions into the extended upper crust, rather than by faulting. To reflect this, we\nhave changed the title of the paper, rewritten the Abstract, and revised the Discussion to emphasize\nthese main points. Additionally, we have introduced a new Figure 6 and Supplementary Figure 4.\nFigure 6 displays the distribution of magnetic anomalies in the region, together with a geological cross-\nsection derived from 3D commercial seismic data across the Tihama Asir coastal plain, which\nhighlights the lack of large faulted extensional blocks within the syn-rift sequence of the Jizan Group.\nSupplementary Figure 4 offers a present-day 3D view of the southern Red Sea, visually illustrating\nthe concept of magma-assisted rifting.\nSpecific comments\n“This rift system began fragmenting the Afro-Arabian shield since the late Oligocene,\ninfluenced by a mantle plume impacting the Afar region around 34 Ma10-12 [Hofmann et al. 1997;\nStorey 1995; Ebinger & Sleep 1998]“. Storey 1995 is definitely not an appropriate reference here as\nhe doesn’t even mention, Ethiopia, Yemen or Afar. (Baker et al., 1996; Bosworth et al., 2005; Zumbo\net al., 1995) would be more appropriate (or just keep the other references you have)\nWe agree with the Referee’s suggestion to remove the reference to Storey (1995) and to include\nBosworth et al. (2005) instead.\n“Variations in GNSS-derived relative velocities between the Arabian and Nubian plates,\nincreasing from ~7 mm/yr in the northern Red Sea to ~16 mm/yr in the southern Red Sea16 [Viltres\net al., 2022], may indicate different stages of the rifting process, suggesting distinct dominant\nmechanisms for the formation of the northern and southern Red Sea“. Maybe as important is the\nfact that the northern Red Sea experienced lower total extension. I’d argue that the lower\nextension is the cause of the different stages of rifting while the different mechanisms of formation are probably\nrather caused by the lower velocity (among other effects such as plume influence). Detail though.\nWe are not entirely certain we understand this comment, as the amount of extension and plate\nseparation velocity are closely linked during rifting. Nevertheless, we have revised the sentence in line\nwith the Reviewer’s suggestion.\n“The onset of rifting in the Red Sea was preceded by flood basalts erupted in Ethiopia, NE\nSudan and SW Yemen17 [Chauvet et al., 2023]“. Chauvet et al., 2023 is maybe not the best reference\nto cite here as they don’t deal much with the flood basalts or chronology. Maybe papers such as\n(Bosworth et al., 2005; Rime et al., 2023; Rooney, 2017) would be more appropriate. Detail though.\nWe have replaced the reference to Chavet et Al. (2023) with Bosworth et al. (2005).\n“also associated by rhyolitic volcanism until ~26 Ma18,19 [Baker et al., 1996; Manetti et al.,\n1991].“. I’d maybe remove “until ~26 Ma” because rhyolitic volcanism has been recorded until much\nmore recent time in Afar, including on the Red Sea coast (Lahitte et al., 2003; Varet, 2018). Detail\nthough.\nWe agree that rhyolitic volcanism in the Afar region has continued beyond ~26 Ma, as supported by\nthe cited studies (e.g., Lahitte et al., 2003; Varet, 2018). In response, we have removed the phrase\n“until ~26 Ma”.\nSecond comment, I’d maybe add a small sentence around here to indicate the age of faulting\nalong this margin. At the moment, you only mention the age of magmatic activity, but the reader might\nalso be interested in the age of faulting. Bohannon (1986) for example mention an age of 23 to 29 Ma\nfor faulting in the area.\nWe have added a sentence in the Results section that includes the initial age of faulting along this\nmargin suggested by Bohannon (1986), following the sentence on the emplacement age of the Tihama\nAsir volcanic complex.\n“along with the contemporaneous Tihama Asir [Coleman & McGuire, 1988] (southern Red\nSea), Al Lith [Pallister, 1987] (central Red Sea) and Quseir [Ligi et al., 2018; 2019] (northern Red\nSea) magmatic complexes, which include gabbros, basaltic dikes, and granophyres”. Maybe you could\nalso mention the granite batholiths found in Yemen (Capaldi et al., 1987; Davison et al., 1994) (if the\nlength of the paper and references allows it)?\nDue to the limited number of references allowed by Nature Communications, we have chosen not to\ninclude the references to the granite batholiths in Yemen by Capaldi et al. (1987) and Davison et al.\n(1994).\n“Magmatic underplating and crustal contamination”. I’m not qualified to discuss the detail\nof this section, but the aspect of the underplating, cited in the title of the section and used again in\nthe discussion, is not clearly presented in this section itself. If you mean underplating at the Moho,\nwhat proxy allows you to say there was underplating? If you mean underplating in sense of any\nponding within the lower crust (sensu Thybo & Artemieva, 2013), then what argument do you have\nfor it? You say that there was only little assimilation from the lower curst (and use it as an argument\nto say the melts ascended rapidly, see below). It might be straightforward for an expert, but it is not\nfor me.\nIn the Introduction section, we clarified that our use of the term underplating follows the definition of\nThybo & Artemieva (2013), referring to any significant ponding of mafic melts within the lower crust,\nnot necessarily restricted to the Moho level. In the Results section, we present geochemical and\nisotopic compositions of the Tihama Asir gabbros, which show minimal interaction with continental\ncrust but assimilation of lower crust material. This geochemical evidence supports the idea that the\nasthenospheric melts feeding the magmatic complex, were emplaced at lower crustal levels, consistent\nwith underplating sensu Thybo & Artemieva. We recognize that this interpretation may not be\nimmediately evident to non-specialists, and we have now revised the relevant paragraph in the\nDiscussion section to clarify this argument.\n“Earlier studies proposed that seafloor spreading in the southern Red Sea began with the\nemplacement of the Tihama Asir and Al Lith magmatic complexes (26 – 21 Ma), where gabbros and\ndikes, with a strong geochemical affinity to melts generated by depleted mantle, could indicate\nemplacement of oceanic crust beneath much of the Red Sea's coastal plains, shelf regions and its\ncentral axis [Stern & Johnson, 2019; Girdler & Styles, 1974; Almalki et al., 2014; Augustin et al.,\n2021].” Not very appropriate to cite Augustin et al. (2021) here as they don’t mention the Tihama\nAsir and Al Lith complexes at all and propose the formation of oceanic crust at 13 Ma, i.e. long after\ntheir emplacements”. A don’t really agree it demonstrates that it delayed the continental rupture. We\nare not even sure that the continental rupture is delayed (see above).\nWe agree that Augustin et al. (2021) is not directly relevant in this context, as they do not discuss the\nTihama Asir and Al Lith magmatic complexes and propose a later onset of oceanic crust formation.\nWe have therefore removed the citation to Augustin et al. (2021) from this sentence.\n“Our results show that syn-rift gabbros formed in relatively shallow magma chambers [...],\nfuelled by asthenospheric melts that incorporated ancient lower continental crust material. The\nminimal contamination from upper crustal material suggests these melts ascended rapidly through the\ncontinental crust with limited interaction.” Also in the Abstract:”the formation of syn-rift gabbros in\nrelatively shallow magma chambers, fed by asthenospheric melts that assimilated ancient lower\ncontinental crustal material. The limited interaction with the upper crust indicates a swift ascent\nthrough the continental crust without significant contamination”. In both cases, you say that these\nmelts incorporated lower continental crustal material and, right after, you say that they had little\ninteraction with the continental crust. It sounds contradictory to me. If it incorporated lower\ncontinental crustal material, then it did interact with the continental crust. Do you mean it ascended\nrapidly and had little/no interaction with the upper crust only? Or you mean that it interacted with the\nlower crust, but not too much, and thus that it was still rapid (maybe not as fast as in the upper crust)?\nThis point of a rapid magma ascend is quite important to your paper. I think that the argumentation\nshould be solid and not prone to confusion as it is now. You should probably rephrase these parts\nand/or argue a bit more to explain why you interpret it ascended rapidly. I am not an expert, and I\nmight miss something, but the audience of the journal is also very broad and other readers might also\nfind these statements confusing.\nWe disagree with the Referee’s observation, as we do not see a contradiction in our statement. Our\nresults show that the melts forming the most primitive gabbros and the cross-cutting dykes exhibit\nsome contamination from the lower crust and no evidence of interaction with the upper crust. In\ncontrast, the sheeted dikes (overlying the gabbros) display interaction with the upper crust. This overall\npattern of crustal contamination supports our interpretation that the asthenospheric melt ascended and\nwere emplaced relatively rapidly through the upper continental crust, reducing the time available for\nassimilation.\n“The above indicates a rapid replacement of the continental lithospheric mantle with upwelling\nasthnosphere” This point is quite important to your paper, but the argumentation is not clear to me.\nWhat argument make you say the lithospheric mantle was already completely replaced? I miss a step\nto explain why the melt did not ascend through the lithospheric mantle before reaching the crust. It\nseems to me that you never explicitly explain this. Again, it might be because I’m not an expert, but\nthe reader might also not be one.\nWe thank the Referee for this comment and agree that this point is central to our interpretation. To\naddress this, we have rewritten the relevant paragraph to explicitly clarify why we infer a complete\nreplacement of the continental lithospheric mantle by upwelling asthenosphere. The revised text now\nreads: “We show here that the rise of asthenosphere enabled the rapid replacement of the continental\nlithospheric mantle, as indicated by the geochemical and isotopic compositions of the Tihama Asir\nprimitive gabbros and dykes cross-cutting the gabbro bodies, which show no evidence of interaction\nwith continental mantle but only assimilation of lower crust material. The ascent of asthenosphere\nresulted in decompression partial melting, producing tholeiitic melts that underplated the lower\ncontinental crust and fed the gabbroic intrusions and basaltic dikes of the Tihama Asir and Al Lith\nmagmatic complexes, as well as the dike swarms extending along the entire Red Sea coast of Arabia.”\n“Our new findings are crucial because....“. A few comments on this paragraph. I would maybe\nend the paragraph with a comparison of your results. You mention other examples, but I’d make a\nmore direct link with your results. “This is comparable with our results ...” or “we confirm here these\ngeophysical interpretations with petrological evidence”. Just a suggestion.\nAnother paper that you might perhaps want to compare to your work is the recent one of La\nRosa et al. (2024), which image lower crustal intrusion in Afar with a completely different technique\n(and completely different timescale). Just a suggestion. BTW, good that you put here a real emphasis\non the additional value of your work compared to other publications. This should also come in the\nintroduction.\nWe revised the text according to the referee’s suggestions by adding a sentence at the end of the\nparagraph summarizing the main findings of La Rosa et al. (2025), and by placing more emphasis on\ncomparing our work with previous publications.\n“Geophysical data, including density, magnetic and seismic velocity anomalies, provide\nsignificant constraints... “. I would put this paragraph before the one comparing with other examples\nworldwide. So, you bring all arguments (samples analyses + geophysics) before comparing with\nother areas. Now, you have a “lonely” sentence on the topic earlier “This process is also supported\nby gravity models explaining Bouguer anomaly patterns64 [Issachar et al., 2023].” and I would say\nthat the whole paragraph would be better placed replacing this lonely sentence. Just a suggestion.\nIn agreement with the Referee’s suggestion, we have reorganized the manuscript by relocating the\nentire section on geophysical data analysis and interpretation before the discussion comparing our\nresults with global examples.\n“In addition, at low extension rates, when migrating melt does not repeatedly intrude the same\naxial zone, dike intrusions may increase crustal strength compositionally [Beutel et al., 2010; Daniels\net al., 2014]. This process can accommodate extension in the upper crust, maintain its thickness,\nand delay crustal rupturing [Daniels et al., 2014]”. The “in addition” makes the reader feel these\nsentences are connected to the previous point. But they are not really. Maybe better separate these\ntwo aspects. Second comment: as far as I understand these papers, neither Beutel et al., nor Daniels\net al. say that migrating intrusions would increase crustal strength. Daniels et al. only mention that\nthe crust would weaken less if the intrusions are distributed rather than on repeated at the same place.\nOverall, most models show that intrusion weaken the crust (Beutel et al., 2010; Bialas et al., 2010;\nBrune et al., 2023; Buck, 2006; Daniels et al., 2014).\nWe have moved this sentence to the following paragraph to improve clarity, and we have rephrased\nit and corrected the citations in accordance with the Referee’s comments.\n“our work allows to outline the evolution of the southern Red Sea rift and the transition from\ncontinental rifting to oceanic seafloor spreading, driven by interactions between thermal, structural\nand magmatic processes (Fig. 5)...”. The paragraph following this sentence explains the process of\nrifting and drifting in the southern Red Sea citing multiple papers, but it would benefit if you would\nexplain a bit more clearly which parts are coming from your analyses. I think you should “sell” your\nwork a bit better and make super clear for the reader what you bring to the story. At the moment, this\nparagraph sounds a bit like a review. I’d add sentences like “we show here that ...” within the\nparagraph to really show what aspects are new in this paper.\nWe have revised the paragraph as suggested by Referee.\nAnother aspect of this paragraph is that you seem very (too) confident of your model. I’d tone\ndown several statements which are not proved but suggested. E.g. “Plume-related thermal anomalies\nand underplating [probably] initially weakened the lower crust, allowing decoupling between the\nlower crust and the lithospheric mantle along the axial zone beneath the thinning upper\ncontinental lithosphere”, “The combination of thermal softening and ductile flow of the lower crust\n[possibly] led to depth-dependent, less localized and more distributed deformation, protracting crustal\nextension and favoring the development of a wide rifted margin”, “...within the slowly diverging\nNubia-Arabia continental lithosphere [probably] contributed significantly to rift initiation,\naccommodating plate separation by re-thickening...”, “...magmatic additions [probably] further\nwidened the Red Sea basin and [possibly] caused rift axis migration...” These comments also apply to\nthe caption of Fig. 5.\nWe agree that some statements in the paragraph, as well as in the caption of former Fig. 5 (Fig. 7 in\nthe revised manuscript), were overly assertive considering the interpretative nature of the model. In\nresponse, we have revised the text to adopt a more cautious and balanced tone to reflect the\nhypothetical nature of the proposed processes and to clearly distinguish interpretation from\nobservation. The caption of new Fig. 7 has also been adjusted accordingly.\n“caused rift axis migration”. Do we really have evidence of rift migration? Or do you infer this\nfrom modelling paper you cite? I’d see another possibility where, instead of rift migration, we have\nrift focusing.\nOk, rift focusing.\n“accommodating plate separation by re-thickening the thinned lower crust and compensating\nfor extension through widespread dike intrusions in the upper crust.”. This phrasing implies that the\ncrust thinned and then became thicker after the intrusions (in two phases). But everything might\nhappen at the same time and the thinning is only reduced by the intrusions. Maybe the notion of\n“magma-compensated thinning” would be more appropriate?\nWe have chosen to retain the original phrasing, as we believe it does not imply a specific temporal\nsequence. The processes described encompass two interconnected phases: an initial stage of\nlithospheric thinning and asthenospheric upwelling, followed by magmatic underplating and intrusion.\nThese magmatic additions may have occurred contemporaneously with crustal thinning and acted to\ncompensate for it.\n“Seafloor spreading commenced approximately 5 Ma in the axial trough of the southern Red\nSea”. As mentioned in the general comments, that’s only one hypothesis. There are others.\nWe have revised the text to include both current hypotheses for the timing of seafloor spreading\ninitiation: 6-5 Ma or 10-14 Ma.\n“such as the eastern branch of the East African Rift [Bastow & Kier, 2011]”. I’d say “Afar\nDepression” rather than “eastern branch of the EAR” as most of the Afar is not part of the EAR sensu\nstricto. Detail though.\nOk, Afar Depression.\n“Magmatic intrusions in the lower crust delay the localization of lithospheric extension and the\nfinal drifting stage by several million years”. You have not convinced me that these intrusions delay\nstrain localization. As said above, it is far from certain that oceanic spreading started at 5 Ma in the\nRed Sea. If the age of onset of seafloor spreading is 14 Ma, then there is only 7-12 Ma between the\nintrusions and seafloor spreading. See general comment at the start of the review.\nPlease, see our response to this point in the Referee’s General Comment section.\n“(Abstract) Our study demonstrates that magmatic underplating during the early rifting phases\nsignificantly delayed the continental rupture and the onset of seafloor spreading in the southern Red\nSea.”. A don’t really agree it demonstrates that it delayed the continental rupture. We are not even\nsure that the continental rupture is delayed (see above).\nThe Abstract has been completely rewritten to take into account the Referee’s suggestions. For details,\nplease see our response in the Referee’s General Comments section."}]

Decision letter