TS6.1

How to cite: Muluneh, A.: The Main Ethiopian Rift: Ongoing deformation inferred from earthquake mechanism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1580, https://doi.org/10.5194/egusphere-egu21-1580, 2021.

The North Tanzanian Divergence (NTD) is a zone of rift initiation. Its surface expression results from interactions between deep-mantle (mantle plume), lithospheric (inherited rheology and stratification, melting...) and crustal (dyke propagation, fault activation...) processes. However, the role of each process on the observed surface activity is still debated, because highly difficult to decorrelate.

We recently carried out a study to obtain enhanced P and S-wave tomography, from the surface down to 150-200 km depth. The particularity of our method consists in its initial velocity model. It is composed of a 1D IASP91 regional velocity model in which we inserted an a priori 3D crustal velocity model with a fine grid. This crustal model was deduced from an independent local tomography inversion.

The P and S images obtained, resulting from the teleseismic inversion of this hybrid method, show strong contrasted velocity anomalies: from 10 % of P (Vp) and S velocity (Vs) variation on the craton, to -17 % below the rift axis. The anomalies locations are consistent with the surface geology (rifting basin, border faults, volcanoes). At a regional scale, the strongest velocity contrasts correspond to the lithospheric inherited structure (Tanzanian craton and Proterozoic belts) boundaries, which control the propagation of the rift. In particular, the Masai cratonic block, south of the NTD, is inferred to have a strong influence in the rift evolution. The transition from the North-South axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani) is likely due to the change in rheology and to the presence of magma along inherited sutures between the craton and the mobile belts.

However, interrogations about the role of the thermal changes, the melt/fluid presence and the mantle composition in the NTD on these velocity anomalies still remain. To distinguish which parameters are acting in the rift, we realize a Vp/Vs ratio map. With this new data, and in the light of parallel petrological studies, we interpret the Vp/Vs anomalies in term of gas and/or melt concentration zones.

How to cite: Gautier, S., Clutier, A., Tiberi, C., Parat, F., Gibert, B., and Gregoire, M.: Role of inherited structures and magmatism in North Tanzania from high resolution teleseismic P and S body-wave tomographies., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10761, https://doi.org/10.5194/egusphere-egu21-10761, 2021.

Hot plumes rising from Earth’s deep mantle are thought to form broad plume heads beneath lithospheric plates. In continents, mantle plumes cause uplift, rifting and volcanism, often dispersed over surprisingly broad areas. Using seismic waveform tomography, we image a star-shaped, low-velocity anomaly centered at Afar and composed of three narrow branches: beneath East Africa, beneath the Gulf of Aden, and beneath the Red Sea and West Arabia, extending north to Levant. We interpret this anomaly as the seismic expression of interconnected corridors of hot, partially molten rock beneath the East Africa-Arabia region. The corridors underlie areas of uplift, rifting and volcanism and accommodate an integral, active plume head. Eruption ages and plate reconstructions indicate that it developed south-to-north, and tomography shows it being fed by three deep upwellings beneath Kenya, Afar and Levant. These results demonstrate the complex feedbacks between the continental-lithosphere heterogeneity and plume-head evolution. Star-shaped plume heads sprawling within thin-lithosphere valleys can account for the enigmatic dispersed volcanism in large igneous provinces and are likely to be a basic mechanism of plume-continent interaction.

How to cite: Civiero, C., Lebedev, S., and Celli, N. L.: Evolution of the East Africa-Arabia plume head, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1399, https://doi.org/10.5194/egusphere-egu21-1399, 2021.

The extensive volcanism in the western part of the Arabian plate forms one of the largest Cenozoic alkali basalt provinces in the world where large lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen (Coleman et al. 1983). Most of volcanism took place after the emplacement of the Afar plume in Yemen (~30 Ma) and progressively propagated northward due to the lithospheric thinning related to the Red Sea rifting starting from 27-25 Ma (Bosworth and Stockli, 2016). However, few lava fields were emplaced during the Mesozoic, with the oldest volcanic activity as old as 200 Ma in the north Israel (Atlit- 1 and Haifa-1 drillholes) (Khon et al., 1993). Here, we report new results from volcanic pipes in the Marthoum area immediately to the east of Harrat Uwayrid where over a hundred pipes are aligned along NW-SE fractures in the Ordovician sandstone of the Saq Formation. The chilled vitric nature of these basalts suggests that the pipes are the result of phreatomagmatic explosions which occurred when the rising magma columns met the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions that plot out of the field of the Cenozoic Arabian alkaline volcanism, being far more enriched in Nd-Hf and Pb isotopes than any lava ever reported in the Arabian plate. New K-Ar dating limits their age to 80 and 50 Ma, thus predating the emplacement of the Afar plume and the rifting in the Red Sea. Our findings indicate that these volcanic eruptions formed from melts generated by a low-degree partial melting of an enriched lithospheric source triggered by local variations in the asthenosphere-lithospheric boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in eastern Africa Rift (Rooney et al., 2014) and considered to represent the lowermost lithospheric mantle of the Nubian shield. Although apparently hidden, this enriched deep lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the Arabian and African lithospheric mantle, but variously mixed with melts derived from a depleted asthenosphere to produce a HIMU-like flavour dispersed in the Cenozoic Arabian alkaline volcanism.

Bosworth, W. and Stockli, D. Early magmatism in the greater Red Sea rift: timing and significance. Can. J. Earth. Sci., 53, 1158–1176, 2016.

Coleman, R. G., Gregory, R. T., Brown, G. F. Cenozoic volcanic rocks of Saudi Arabia. Saudi Arabian Deputy Minist. Miner. Resour., Open File Report, USGS-OF-03-93, pp. 82, 1983.

Khon, B. P., Lang, B. and Steinitz, G. ⁴⁰Ar/³⁹Ar dating of the Atlit-1 volcanic sequence, northern Israel, Israel J. Earth-Sci., 42, 17–28, 1993.

Rooney, T. O., Nelson, W. R., Dosso, L., Furman, T., Hanan, B. The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates. Geology, 42, 419–422, 2014.

How to cite: Sani, C., Sanfilippo, A., Rasul, N. M. A., Vigliotti, L., Widinly, N., AlQutub, A. S., Osemi, A., and Ligi, M.: Hidden but ubiquitous: the pre-rift continental mantle in the Red Sea region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10863, https://doi.org/10.5194/egusphere-egu21-10863, 2021.

Lithosphere extension, which plays an essential role in plate tectonics, occurs both in continents (as rift systems) and oceans (spreading along mid-oceanic ridges). The northern Red Sea area is a unique natural geodynamic laboratory, where the ongoing transition from continental rifting to oceanic spreading can be observed. Here, we analyze travel time data from a merged catalogue provided by the Egyptian and Saudi Arabian seismic networks to build a three-dimensional model of seismic velocities in the crust and uppermost mantle beneath the northern Red Sea and surroundings. The derived structures clearly reveal a high-velocity anomaly coinciding with the Red Sea basin and a narrow low-velocity anomaly centered along the rift axis. We interpret these structures as a transition of lithospheric extension from continental rifting to oceanic spreading. The transitional lithosphere is manifested by a dominantly positive seismic anomaly indicating the presence of a 50–70-km-thick and 200–300-km-wide cold lithosphere. Along the forming oceanic ridge axis, an elongated low-velocity anomaly marks a narrow localized nascent spreading zone that disrupts the transitional lithosphere. Along the eastern margins of the Red Sea, the lithosphere is disturbed by the lower-velocity anomalies coinciding with areas of basaltic magmatism.

How to cite: El Khrepy, S., Koulakov, I., Al-Arifi, N., S. Alajmi, M., and N. Qadrouh, A.: Transition from continental rifting to oceanic spreading in the northern Red Sea area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4185, https://doi.org/10.5194/egusphere-egu21-4185, 2021.

We explore the lithosphere structure of the Red Sea using gravity and magnetic data.

We re-processed marine data form past surveys conducted during the 70’s and the 80’s, available at the NGDC database. By correcting the magnetic measurements according to the DGRF (definitive magnetic reference field), leveling and replacing the long wavelengths with satellite data (LCS1 model) we managed to generate a consistent magnetic anomaly map for the entire length of the Red Sea that is composed of 10 different surveys and contain overs 100,000 measuring points. The magnetic anomaly map highlights structural differences between the southern, central and northern parts of the Red Sea.

Using forward gravity approach, constrains from seismic, wells and petrophysical data, and by integrating insights from magnetic analysis, we define the lithospheric model of the Red Sea to address key questions regarding rifting, sea floor spreading and transition processes.  For example, the southern parts of the Red Sea are characterized by shallow and wide asthenosphere upwelling, while in the axial trough lithosphere is thin with thicknesses of less than 15 km. The lithosphere thickness increase asymmetrically towards the rift shoulders. In general, the lithosphere is thicker on the eastern sides than on the western sides. In the central parts of the Red Sea, the lithosphere structure is not significantly different from the southern parts, however, asthenosphere upwelling is slightly narrower. In northern parts of the Red Sea asthenosphere upwelling significantly narrows and focused mainly beneath the axial trough and the lithosphere is thicker. This architecture reflects the currently transition from continental rifting (in the north) to oceanic seafloor spreading (in the south) in the Red Sea.

How to cite: Issachar, R., Ebbing, J., Yixiati, D., and Holzrichter, N.: Insights into the Red Sea area from magnetic and gravity analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10872, https://doi.org/10.5194/egusphere-egu21-10872, 2021.

Marine sediments deposited in response to the Neogene opening of the Red Sea during divergence of the African-Arabian plate margin provide micropalaeontological chronological evidence to calibrate synchronous palaeoenvironmental events from the Gulf of Suez to the Gulf of Aden. This facility provides insights to the timing and relative rates of tectonic subsidence associated with the rifting episodes of the region. Biostratigraphic index forms include planktonic and benthonic foraminifera and calcareous nannofossils. These, combined with various associated microfossils and macrofossil fragments, permit interpretation of a range of depositional environments that span intertidal to bathyal regimes. Onset and recovery from various hypersaline events are similarly interpreted by integrating microfossils and lithology. Following an episode of emergence and sporadic volcanicity, subsidence and the first Neogene marine transgression created brackish to shallow marine lagoons during the Early Miocene (Foraminiferal Letter Stage Upper Te). Rapid subsidence and accumulation of deep marine mudstones, of local hydrocarbon source-rock quality, with thinly interbedded siliciclastic and calciclastic debris flows commenced in the Early Miocene (Planktonic foraminiferal zones N5-N8; Nannofossil zones NN3-NN5). The debris flows increased in abundance and provide good hydrocarbon reservoirs. The Gulf of Suez and Red Sea experienced episodic isolation from the Indian Ocean during the latest Early Miocene and earliest Middle Miocene (Planktonic foraminiferal zones N8-N9; Nannofossil zone NN5 Foraminiferal Letter Stage Middle-Upper Tf1), resulting in hypersaline events with precipitation of submarine gypsum and halite. The isolation is attributed to constriction of the southern Red Sea, in the vicinity of the Bab El Mandab Straits, by eustatic sea level fall as well as probable tectonic activity; the synchronous Gulf of Aden succession does not display evidence for such hypersaline events. A prolonged hypersaline phase extended over most of the Middle Miocene, for which absence of biostratigraphic data precludes age control. During the latest Middle Miocene to Late Miocene, rejuvenation of the hinterland cause rapid deposition of terrestrial and fluviatile coarse and fine siliciclastics, with similar biostratigraphic paucity except for rare diatoms and palynomorphs. Renewed subsidence, associated with opening of the Aqaba Fault, combined with eustatic sea level rise caused marine deposition to recommence in the Pliocene.

How to cite: Hughes, G. and Varol, O.: Biostratigraphically constrained Neogene palaeoenvironments of the Red Sea rift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6227, https://doi.org/10.5194/egusphere-egu21-6227, 2021.

In two classic papers, Burke and Dewey (1973) and Dewey and Burke (1974) proposed that continental rifting begins at hotspots - domal uplifts with associated magmatism - from which three rift arms extend. Rift arms from different hotspots link up to form new plate boundaries along which the continent breaks up, generating a new ocean basin and leaving failed arms termed aulacogens within the continent.  In subsequent studies, hotspots became increasingly viewed as manifestations of deeper upwellings or plumes, which were the primary cause of continental rifting. We revisit this conceptual model and find that it remains useful, though some aspects require updates based on subsequent results.  Many three-arm systems identified by Burke and Dewey (1973) are now recognized to be or have been boundaries of transient microplates accommodating motion between diverging major plates. Present-day examples include the East African Rift system and the Sinai microplate.  Older examples include rifts associated with the opening of the South Atlantic in the Mesozoic and the North Atlantic Ocean over the last 200 Ma,  rifts in the southern U.S associated with the breakup of Rodinia, and intracontinental rifts formed within India during the breakup of Gondwanaland. The microplates form as continents break up, and are kinematically distinct from the neighboring plates, in that they move separately. Ultimately, the microplates are incorporated into one of the major plates, leaving identifiable fossil features on land and/or offshore. In many cases the boundaries of microplates during continental breakup are located on preexisting zones of weakness and influenced by pre-existing fabric, including older collisional zones. Hotspots play at most a secondary role in continental breakup, in that most of the associated volcanism reflects plate divergence, so three-arm junction points may not reflect localized upwelling of a deep  mantle plume.

How to cite: Stein, C., Stein, S., Gallahue, M., and Elling, R.: Revisiting Hotspots and Continental Breakup – Updating the Classical Three-arm Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13564, https://doi.org/10.5194/egusphere-egu21-13564, 2021.

The rifting of continents can lead to the initiation of seafloor spreading and the formation of passive margins. Magma-rich passive margins, which are defined as being underlain by enormous volumes of igneous rocks, are often associated with large igneous provinces (LIPs). However, the relationship between the igneous units found along these magma-rich passive margins, rifting processes, and LIPs is poorly understood.

We have developed the VOLMIR (VOLcanic passive Margin Igneous Rocks) dataset to investigate these relationships. VOLMIR is based on seismic reflection profiles in which the volumes and geometries of both shallow seaward dipping reflector (SDR) and deeper high velocity lower crustal (HVLC) units can be measured. We find a relatively consistent ratio of SDR to HVLC volumes, with SDR volumes about one third that of HVLC. This consistency suggests that the units are related during formation and may be used to provide insight into how such units form during continental rifting and breakup. Presumably, as magmas rise and erupt to the surface to form SDRs, the remaining high-density residuum or cumulate becomes the HVLC. The volumes of SDR units are moderately positively correlated with distance from the Euler pole, and weakly negatively correlated with distance from the nearest hotspot, suggesting that lithospheric processes play more of a role in late-stage continental rifting and breakup than hotspot/mantle plume processes.

The Mid- and South Atlantic Ocean breakups, and the resulting offshore volcanic passive margins, are temporally and spatially associated with the Central Atlantic Magmatic Province (CAMP) and Paraná-Etendeka LIP. Using VOLMIR, we estimate the amount of igneous material in the offshore volcanic passive and compare it to estimates for the adjacent on-land LIPs. The results indicate that a significant volume of volcanics exist in the offshore passive margins, increasing the inferred amount of volcanic output associated with the LIPs. Further studies will provide insight into the relationship between offshore passive margins and on-land LIPs, and perhaps provide further understanding on the role of magmatism in rifting processes.

How to cite: Stein, S., Gallahue, M., Stein, C., Rooney, T., and Gomez-Patron, A.: Comparing onshore and offshore volumes of large igneous provinces associated with passive continental margins, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13631, https://doi.org/10.5194/egusphere-egu21-13631, 2021.

Comparative study of North America’s failed continental rifts allows investigation of the effects of extension, magmatism, magmatic underplating and rift inversion in the evolution of rifting. We explore this issue by examining the gravity signatures of the Midcontinent Rift (MCR), Reelfoot Rift (RR), and Southern Oklahoman Aulacogen (SOA). The ~1.1 Ga MCR records aspects of the complex assembly of Rodina, while the structures related to the ~560 Ma RR and SOA formed during the later breakup of Rodinia and subsequent assembly of Pangea. Combining average gravity anomalies along each rift with seismic data, we examine whether these data support the existence of high-density residual melt underplates (“rift pillows”), reflect the possible amounts of inversion, and whether these rifts should be considered analogs of one another at different stages in rift evolution. The MCR and SOA have strong gravity highs along much of their length. Furthermore, the west and east arms of the MCR have different gravity signatures. The west arm of the MCR has a positive gravity anomaly of 80-100 mgals, while the east arm and SOA have positive anomalies of only 40-50 mgals. The RR does not exhibit a high positive anomaly along much of its length. The positive anomalies of both arms of the MCR and SOA reflect 10-20 km thick underplates at the base of the crust. These gravity anomalies also reflect greater amounts of inversion, during which the rift-bounding normal faults are reactivated by later compression, bringing the high-density igneous rocks closer to the surface. By averaging gravity data along the length of each failed rift, we can more easily distinguish between the history of individual rifts and general features of rifting that apply to other failed or active rifts around the world.

How to cite: Elling, R., Stein, S., Stein, C., and Keller, G. R.: Exploring rift magmatism and evolution through gravity analysis of North America's failed rifts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3187, https://doi.org/10.5194/egusphere-egu21-3187, 2021.

Ocean Continent Transition (OCT) located between the edge of the continental and unequivocal oceanic crust is an ideal laboratory to understand one of the most fundamental processes of Plate Tectonics, namely the mechanism of formation of a new plate boundary, also referred to as lithospheric breakup. However, the location and architecture of the OCT and the processes governing the rupture of continental lithosphere and creation of new oceanic crust remain debated. In this paper, we present newly released high-resolution seismic reflection profiles that image the complete transition from unambiguous continental to oceanic crust in the mid-northern South China Sea (SCS), accompanied with IODP drill hole and gravity data, with the aim to map the OCT and explore where, when and how lithospheric breakup occur.

Based on observations and interpretations we define the limits of OCT. The results show that OCT corresponds to hybrid crust resulting from the complex interaction between crustal thinning along detachment systems and accretion of new syn-tectonic igneous materials. The observations suggest a sharp along strike transition in the OCT from a lower to an upper plate setting over a lateral distance of 25 km. The breakup in the northern SCS and the conjugate margin occurred asymmetrically and was accomplished by core-complex type structures related to a successive oceanward transition from tectonic to magma-controlled processes during plate separation. The along-strike variability in the basement architecture and the abrupt flip in detachment polarity in the OCT imply a sharp transfer fault to explain the segmentation of the margin. Such segmentation results from inherited pre-rift crustal and/or lithospheric heterogeneities. It is important to note that the segmentation did not control breakup and subsequent oceanic accretion.

How to cite: Zhang, C., Sun, Z., Manatschal, G., Pang, X., Li, S., Sauter, D., and Peron-Pinvidic, G.: The continent-ocean transition architecture and breakup mechanism at the mid-northern South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1625, https://doi.org/10.5194/egusphere-egu21-1625, 2021.

A devastating M 7.0 earthquake on October 30, 2020, offshore Samos Island, Greece, was a consequence of the Aegean and Anatolian upper crust being pulled apart by north-south extensional stresses resulting from slab rollback, where the African plate is subducting northwards beneath Eurasia, while the slab is sinking by gravitational forces, causing it to retreat southwards. Since the retreating African slab is coupled with the overriding plate, it tears the upper plate apart as it retreats, breaking it into numerous small plates with frequent earthquakes along their boundaries.  The earthquake happened offshore of the extensional Büyük Menderes Graben, where a 150 km long, 10 km wide, incipient upper plate rift system formed in the Anatolian plate, showing that the entire Aegean-Western Anatolian region is being pulled apart by extensional stresses related to the slab rollback. Earthquake solutions and fault plane studies around western Anatolia support this spectacular extension, and show that the modern extension was preceded on many faults by oblique extension and strike-slip motions, perhaps reflecting a change in tectonic setting from sideways escape from the Africa-Arabia collision with Eurasia,  to the pure extension related to slab rollback of the African plate, and the retreat of the Hellenic trench. Historical earthquake swarms and deformation of the upper plate in the Aegean have been associated with massive volcanism and cataclysmic devastation, such as the M 7.7 Amorgos earthquake in July 1956 between the islands of Naxos and Santorini (Thera). Even more notable was the eruption of Santorini 3650 years ago, which contributed to the fall of the Minoan civilization. The Samos earthquake highlights the long historical lack of appreciation of links between deep tectonic processes and upper crustal deformation and geological hazards, and is a harbinger of future earthquakes and volcanic eruptions, establishing a basis for studies to institute better protection of infrastructure and upper plate cultures in the region. Further detailed studies are needed in this area to better understand and predict earthquake frequency, possible locations, and to establish better building codes to protect people's lives and property.

How to cite: Meng, J., Sinoplu, O., Zhou, Z., Tokay, B., Kusky, T., Bozkurt, E., and Wang, L.: Unusual Mw 7.0 Extensional Aegean Earthquake Related to African Slab Rollback and Formation of Extensional Plate Boundary in Anatolia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8366, https://doi.org/10.5194/egusphere-egu21-8366, 2021.

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, seismicity indicates the closure of this back-arc basin. In the basin, earthquake clusters occur in the lower crust and uppermost mantle and are related to re-activated, inverted, normal faults created during rifting.

To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile comprises 35 OBS and three land stations at Corsica to give a complete image of the continental thinning including the necking zone.

The majority of the refraction seismic data show mantle phases with offsets up to 70 km. The arrivals of seismic phases were picked and used to generate a 2-D P-wave velocity model. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The P-wave velocities in the crust reach 6.6 km/s at the base. The uppermost mantle shows velocities >7.8 km/s. The crust-mantle boundary becomes shallower from ~18 km to ~12 km depth within 30 km from Corsica towards the basin centre. The velocity model does not reveal an axial valley as expected for oceanic spreading. Further, it is difficult to interpret the seismic data whether the continental lithosphere was thinned until the mantle was exposed to the seafloor. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. The distribution of earthquakes and their fault plane solutions, projected along our seismic velocity model, is in-line with the counter-clockwise opening of the Ligurian Basin.

How to cite: Dannowski, A., Kopp, H., Grevemeyer, I., Caielli, G., de Franco, R., Lange, D., Thorwart, M., Filbrandt, C., msm71, C. P., and Working Group, A.: Lithospheric architecture of the Ligurian Basin from seismic travel time tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2822, https://doi.org/10.5194/egusphere-egu21-2822, 2021.

Detachment fault systems recording displacements in the order of 10s to 100s of km remain poorly understood compared to smaller scale normal faults. The evolutionary models developed for the growth and interaction of Andersonian-type faults are not fully applicable to these large-magnitude systems. Consequently, the associated basins - the so-called supradetachment basins - are still poorly understood compared to extensional half-graben basins.

Numerical and analogue 2D modelling have shed light on the mechanisms of footwall back-rotation during progressive extension (rolling hinge model; e.g. Lavier et al., 1999) but the along-strike evolution of such large-scale detachment systems remain poorly understood. It has been proposed that with increasing amounts of extension, detachment faulting favors formation of isostatically induced, longitudinal and transverse folds and consequently basin inversion in the area of maximum displacement (e.g. Kapp et al., 2008; Osmundsen & Péron-Pinvidic, 2018). The 4D configuration of the associated supradetachment basins is then controlled by the growth and (potential) lateral linkage of such faults - which may result in complex geometries.

In this study, we use interpretation of 3D- and 2D seismic reflection data from the necking domain of the Mid-Norwegian rifted margin to discuss the effects of lateral interaction and linkage of extensional detachment faults. The study area demonstrates how successive incision of such master faults may induce a complex structural relief in response to extensional detachment faulting and folding. In the inner parts of the south Vøring and northeastern Møre basins, the Klakk and Main Møre Fault Complexes form the outer necking breakaway complex and the western boundary of the Frøya High. The central Frøya High contains remnants of a metamorphic core complex, which we interpret as an extension parallel turtleback-structure. The turtleback is flanked two main synclinal depocenters constituting a supradetachment basin, whose location corresponds to the crustal taper break associated with the outer necking domain. We attribute the turtleback exhumation to Late Jurassic-Early Cretaceous detachment faulting along the Klakk and Main Møre Fault Complexes. Southwest of the Frøya High, the supradetachment basin links the Frøya High Turtleback with the core complex previously interpreted for the Gossa High, near where the Main Møre Fault Complex incises the Slørebotn detachment. The Slørebotn Subbasin consequently forms a synclinal keel basin with rafted blocks, a structural configuration which is recognizable also north of the ‘Frøya High Turtleback’ towards the Halten Terrace. We find that the pre-rift structural template and crustal heterogeneity facilitated differential supradetachment basin configuration during and after Late Jurassic-Early Cretaceous rifting, and that the supradetachment basin architecture was likely controlled by localized isostatic uplift, lateral linkage and successive incision of large-magnitude normal faults.

How to cite: Gresseth, J. L. S., Osmundsen, P. T., and Péron-Pinvidic, G.: Detachment Faulting, Successive Incision and Controls on Supradetachment Basin Formation at the Mid-Norwegian Rifted Margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9481, https://doi.org/10.5194/egusphere-egu21-9481, 2021.

In this contribution, we examine the evolution of the South Atlantic passive margins, based on a new thermal lithosphere-asthenosphere-boundary (LAB) model. Our model is calculated by 1D advection and diffusion with rifting time, crustal thickness and stretching factors as input parameters. The initial lithospheric thickness is defined by isostatic equilibrium with laterally variable crustal and mantle density. We simulate the different rifting stages that caused the opening of the South Atlantic Ocean and pick the LAB as the T=1330° C isotherm. The modelled LAB shows a heterogeneous structure with deeper values at equatorial latitudes, as well as a more variable lithosphere along the southern part. This division reflects different stages of the South Atlantic opening: Initial opening of the southern South Atlantic caused substantial lithospheric thinning, followed by the rather oblique-oriented opening of the equatorial South Atlantic accompanied by severe thinning. Compared to global models, our LAB reflects a higher variability associated with tectonic features on a smaller scale. As an example, we identify anomalously high lithospheric thickness in the South American Santos Basin that is only poorly observed in global LAB models. Comparing the LAB of the conjugate South American and African passive margins in a Gondwana framework reveals a variable lithospheric architecture for the southern parts. Strong differences up to 80 km for selected margin segments correlate with strong gradients in margin width for conjugate pairs. This mutual asymmetry suggests highly asymmetric melting and lithospheric thinning prior to rifting.

How to cite: Haas, P., Müller, R. D., Ebbing, J., Houseman, G. A., Finger, N.-P., and Kaban, M. K.: Modelling thermal lithospheric thickness along the conjugate South Atlantic passive margins implies asymmetric rift initiation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4877, https://doi.org/10.5194/egusphere-egu21-4877, 2021.

During extension of the continental lithosphere, deformation often localizes along pre-existing weaknesses originating from previous tectonic phases. When simulating such structures with analogue or numerical methods, modellers often focus on either crustal or mantle heterogeneities. By contrast, here we present results from 3D analogue models to test the combined effect and relative impact of (differently oriented) mantle and crustal weaknesses on rift systems.

Our model set-up involves a rigid base plate fixed to a mobile sidewall. When this sidewall moves outward, the edge of the base plate induces a “velocity discontinuity” (VD) that acts as an upper mantle fault/shear zone in a strong upper mantle. The VD is either parallel to the model axis, or 30˚ oblique. On top of this base plate, we apply a viscous layer representing the ductile lower crust, followed by a sand cover that simulates the brittle upper crust. Crustal weaknesses were either imposed by implementing “seeds” (i.e. ridges of viscous material at the base of the sand layer), or by pre-cutting the sand. Similar to the basal plate edge, we apply different crustal weakness orientations as well.

Without weaknesses in the model crust, an axis-parallel VD forms an axis-parallel rift basin above along the VD. When adding oblique seeds, they strongly localize deformation, creating a series of obliquely oriented graben. Yet the VD still induces faulting along the model axis, leading to the development of offset axial graben as well. Pre-cut faults also localize deformation but are less dominant than the seeds. As a result, the VD has more control and the axial rift structures are much more pronounced. In the oblique VD case, the reference model develops a series of en echelon graben along the VD. Axis-parallel seeds strongly localize faulting, to such a degree that the effect of the VD is very much overruled. Pre-cut faults allow more influence from the VD, but still dominate the system. Doubling the extension rate increases the strength of the viscous layer, enhancing coupling between the VD and sand cover, so that a series of en echelon graben crosscutting the seed-induced structures develop.

We find that the orientation and relative weakness of inherited weaknesses in the mantle and crust, as well as extension rates control subsequent rift structures. These structures and their relative evolution can be complex due to the interplay of the above factors, and importantly, all develop under the same pure shear extensional boundary condition. Our results show that very differently oriented rift structures can form during one phase of extension without the need to invoke multiple rift phases. Furthermore, coupling can change over time due to changes in extension velocity or gradual thinning of the lower crust, thus affecting rift evolution. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

How to cite: Zwaan, F., Chenin, P., Erratt, D., Manatschal, G., and Schreurs, G.: Are complex rift patterns the result of interacting crustal and mantle weaknesses, or of multiphase rifting? An analogue modelling study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-688, https://doi.org/10.5194/egusphere-egu21-688, 2021.