TS6.1

EDI
Continental Rift Evolution: from inception to break-up, with special attention to the The Afro-Arabian rifting system

Continental rifting is a complex process spanning from the inception of extension to continental rupture or the formation of a failed rift. This session aims at combining new data, concepts and techniques elucidating the structure and dynamics of rifts and rifted margins. We invite submissions highlighting the time-dependent evolution of processes such as: initiation and growth of faults and ductile shear zones, tectonic and sedimentary history, magma migration, storage and volcanism, lithospheric necking and rift strength loss, influence of the pre-rift lithospheric structure, rift kinematics and plate motion, mantle flow and dynamic topography, as well as break-up and the transition to sea-floor spreading.

We encourage contributions using multi-disciplinary and innovative methods from field geology, geochronology, geochemistry, petrology, seismology, geodesy, marine geophysics, plate reconstruction, or numerical or analogue modelling. Especially welcome are presentations that provide an integrated picture by combining results from active rifts, passive margins, failed rift arms or by bridging the temporal and spatial scales associated with rifting.

Withing this session, a specific segment will be dedicated to studies of rift tectonics in the The Afro-Arabian rift system (the basins of the Gulf of Suez, Gulf of Aqaba, Red Sea, Gulf of Aden, Afar depression and the surrounding regions or related areas). This system contains the world’s largest active continental rift and is the key locality for studying continental breakup processes. Natural phenomena such as basin formation, continental breakup, seismic and volcanic activity, and the formation of mineral resources in and around the three arms of the Afar triple junction highlights some of the key aspects of this complex rift system.

Public information:
Special issue alert:
This session is linked to three special issues in the gold open access journal "Frontiers in Earth Science".

(1) "Links between tectonics, fault evolution and surface processes in extensional systems", edited by Frank Zwaan, Alex Hughes, Laura Gregory, Joanna Faure Walker and Lisa McNeill. Manuscript submission deadline: 31 October 2021. Link: https://www.frontiersin.org/research-topics/20047
(2) "InSAR for Volcanoes and Tectonics", edited by Carolina Pagli, Hua Wang, Anne Socquet and Vincent Drouin. Manuscript submission deadline: 30 June 2021. Link: https://www.frontiersin.org/research-topics/18940
(3) "Geodynamics and Magmatism in the Afro-Arabian Rift System", edited by Nico Augustin, Froukje van der Zwan, Joël Ruch, Neil Mitchell, Daniele Trippanera. Manuscript submission deadline: 10 May 2021. Link: https://www.frontiersin.org/research-topics/16355

If you are interested in contributing a paper to one of these special issue, please contact us and/or register on the respective special issue website
Co-organized by GD5/GM9/GMPV2/SM4
Convener: Frank ZwaanECSECS | Co-conveners: Laura ParisiECSECS, Giacomo Corti, Daniele TrippaneraECSECS, Derek Keir, Froukje M. van der ZwanECSECS, Sylvie Leroy, Carolina Pagli
vPICO presentations
| Fri, 30 Apr, 09:00–12:30 (CEST)
Public information:
Special issue alert:
This session is linked to three special issues in the gold open access journal "Frontiers in Earth Science".

(1) "Links between tectonics, fault evolution and surface processes in extensional systems", edited by Frank Zwaan, Alex Hughes, Laura Gregory, Joanna Faure Walker and Lisa McNeill. Manuscript submission deadline: 31 October 2021. Link: https://www.frontiersin.org/research-topics/20047
(2) "InSAR for Volcanoes and Tectonics", edited by Carolina Pagli, Hua Wang, Anne Socquet and Vincent Drouin. Manuscript submission deadline: 30 June 2021. Link: https://www.frontiersin.org/research-topics/18940
(3) "Geodynamics and Magmatism in the Afro-Arabian Rift System", edited by Nico Augustin, Froukje van der Zwan, Joël Ruch, Neil Mitchell, Daniele Trippanera. Manuscript submission deadline: 10 May 2021. Link: https://www.frontiersin.org/research-topics/16355

If you are interested in contributing a paper to one of these special issue, please contact us and/or register on the respective special issue website

Session assets

Session materials

vPICO presentations: Fri, 30 Apr

Chairpersons: Frank Zwaan, Laura Parisi, Froukje M. van der Zwan
09:00–09:05
09:05–09:15
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EGU21-1580
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solicited
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Highlight
Ameha Muluneh
The northern Main Ethiopian Rift (MER), which forms the northern part of the East African Rift System, offers an excellent tectonic setting to study the transition from continental to oceanic crust and also from tectonic to magmatic rifting. Opening of the rift started at 11 Myr ago. Until about 7 Ma, deformation was mainly accommodated at the rift border faults. Between 7 and 3 Ma, deformation migrated from the border faults to 20-30 km wide, 60 km long  magmatic segments. Earlier geodetic and field geological observations suggest that more than 80% of the present day opening of the rift is accommodated beneath these magmatic segments. On the contrary, recent observations indicate that deformation is more widespread than previously thought, with only 40% of the present day deformation being accommodated at the rift centre. 
 
Detailed understanding on the depth and epicentral distribution of earthquakes provides an important constraint on how strain is partitioned between the rift floor and border faults. Here I use high resolution earthquake catalogue and thermo-rheological modeling to constrain the active deformation patterns in the northern MER by assuming that the long term properties of the lithosphere represent the short term earthquake cycle. The final result of this study has significant implications for the location and magnitude of seismic hazard in the rift. 

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.

09:15–09:17
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EGU21-10695
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ECS
Pauline Gayrin, Thilo Wrona, Sascha Brune, Simon Riedl, and Tim Hake

Continental rifts show surface expressions of deep crustal processes, such as faulting and volcanism. The East African Rift System (EARS) is one of the most prominent examples of an active continental rift driven by tectonics and magmatism. Nonetheless, we still struggle to quantify the amount of extension due to these processes on a kyr- to Myr-time-scale. In particular, the distribution of extension within low-offset normal fault networks within rift basin interiors is challenging to determine.

To address these issues, we develop a semi-automated workflow to extract normal faults from the TanDEM-X science DEM data (12 m horizontal resolution, 0.4 m average height error) of the Magadi-Natron Region of the Eastern branch of the EARS, limited to the north by the Suswa caldera (1.15°S) and to the south by Gelai and Oldoinyo Lengai volcanoes (2.75°S). This data allows us to quantify brittle surface deformation that occurred since the last deposition of widespread volcanic lavas in  these basins. Our workflow consists of five steps: (1) gradient calculation, (2) thresholding, (3) skeletonization, (4) Hough transformation, and (5) clustering. Because the surface faults appear as topographic discontinuities, we first calculate the gradient of the DEM to detect them. Then we use an adaptive threshold (Otsu) to distinguish faults from unfaulted areas. Next, we skeletonize the threshold to extract line segments and perform a Hough transformation to determine the orientation of these segments. Finally, we use a density-based clustering algorithm (DBSCAN) to group these segments into faults. This algorithm is considering proximity between the segment, similarity in dip and strike direction.

A strike analysis applied on the fault data of the whole basin shows four main directions from distinct fault populations. Each direction cluster corresponds to a geological layer and a time interval. For example, the azimuth N20°, corresponds to present and recent rift direction, mostly on the ~1Myr old Magadi trachyte. A direction of N170° is mostly represented in earlier,  Mio-Pliocene volcanic units of the rift. Moreover, we derive the fault displacement distribution throughout the basin.This allows us to calculate the total extension of each geological unit and to compute the overall amount of extension of the region during geologically recent times.

We provide a new high-resolution fault map that depicts strike direction and throw even of small-offset normal faults. This characterization helps us increase our understanding of recent brittle deformation within the Magadi-Natron region and thus the propagation of rifting in the eastern branch of the East African Rift System.

How to cite: Gayrin, P., Wrona, T., Brune, S., Riedl, S., and Hake, T.: Semi-automated fault extraction and structural analysis from DEM data of the Magadi and Natron basins, East African Rift System, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10695, https://doi.org/10.5194/egusphere-egu21-10695, 2021.

09:17–09:19
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EGU21-10761
Stéphanie Gautier, Adeline Clutier, Christel Tiberi, Fleurice Parat, Benoit Gibert, and Michel Gregoire

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.

09:19–09:21
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EGU21-1399
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ECS
Chiara Civiero, Sergei Lebedev, and Nicolas L. Celli

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.

09:21–09:23
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EGU21-7898
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ECS
Emma J. Watts, Thomas M. Gernon, Rex N. Taylor, Derek Keir, Melanie Siegburg, Jasmin Jarman, Carolina Pagli, and Anna Gioncada

The Danakil depression in the Afar region of Ethiopia marks the change from subaerial continental rifting to seafloor spreading further north in the Red Sea [1]. Extension and volcanism in this incipient spreading centre is localised to the ~70-km-long, 20-km-wide active Erta Ale volcanic segment (EAVS), with multiple volcanic centres consisting of a combination of fissures, shield volcanoes and stratovolcanoes [2]. This study aims to better understand the nature of interaction between three volcanoes with the EAVS (Alu, Dalafilla and Borale) while also investigating their evolution during the transition from continental to oceanic crustal production.

Here we combine results of mapping, using remote sensing, and geochemical analysis of Alu, Dalafilla and Borale in the northern half of the EAVS. Multispectral images were used to create a high-resolution map and establish a relative chronology of lava flows. Our results show that the majority of flows are sourced from a combination of scoria cones and fissures, representing in total 15 phases of volcanism within four major eruptive stages.

The first stage represents large-scale fissure volcanism comprising basaltic phases that erupted in a submarine environment. Stage two involves basaltic fissure volcanism centred around the Alu dome. The third stage is dominated by trachy-andesite to rhyolitic (SiO2 of 59-70%) volcanism sourced from the volcanic edifices of Alu, Dalafilla and Borale. The fourth and final stage is characterised by a resumption of small-scale basaltic/trachybasalt (SiO2 of 49-55%) fissure eruptions.

Geochemical modelling indicates a paucity of crustal assimilation and mixing within the sub-volcanic magmatic system. Spatial analysis of volcanic cones and fissures within the area indicate the presence of a cone sheet and ring faults. The fissures are likely fed by sills connecting the magma source with the volcanic edifices of Alu and Borale. Our results reveal the cyclic nature of both eruption style and composition of major volcanic complexes in rift environments, prior to the onset of seafloor spreading.

References

[1] Wolfenden et al. (2005) EPSL 224:213-228

[2] Barberi and Varet (1970) Bull Volcanologique 34:848-917

How to cite: Watts, E. J., Gernon, T. M., Taylor, R. N., Keir, D., Siegburg, M., Jarman, J., Pagli, C., and Gioncada, A.: Evolution of the Alu-Dalafilla and Borale Volcanoes, Afar, Ethiopia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7898, https://doi.org/10.5194/egusphere-egu21-7898, 2021.

09:23–09:25
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EGU21-10863
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ECS
Camilla Sani, Alessio Sanfilippo, Najeeb M.A. Rasul, Luigi Vigliotti, Nawaf Widinly, Abdulnasser S. AlQutub, Ahmed Osemi, and Marco Ligi

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.

09:25–09:27
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EGU21-7102
Ken McClay, Bill Bosworth, Samir Khalil, Marco Ligi, and Danny Stockli

The Gulf of Suez and the Northern Red Sea form the northwestern sector of the Afro-Arabian rift system.  Studies of outstanding outcrops of rift fault systems and syntectonic strata integrated with sub-surface data together with thermo-chronological studies indicate that the Gulf of Suez - Northern Red Sea rift system initiated at around the Oligocene to Miocene transition (24 to 23 Ma).  A regional NW-SE trending Oligocene-Miocene (~23 Ma) alkali basalt dike swarm and basalt flows near Cairo, appears to mark the onset of crustal-scale extension and continental rifting.  These dikes and scarce local flows, are interbedded with the oldest siliciclastic syn-rift strata (Aquitanian Nukhul Fm.), and are associated with the oldest recognized extensional faulting in the Red Sea.  Bedrock thermochronometric results from the Gulf of Suez and both margins of the Red Sea also point to a latest Oligocene onset of major normal faulting and rift flank exhumation and large-magnitude early Miocene extension along the entire length of the Red Sea rift.  The early phase of rifting produced complex, discontinuous fault patterns with very high rates of fault block rotation, distinct sub-basins with alternating regional dip domains separated by well-defined accommodation zones.  Sedimentary facies were laterally and vertically complex and dominated by marginal to shallow marine siliciclastics of the Abu Zenima, Nukhul and Nakheil Formations.  Neotethyan faunas appeared throughout all of the sub-basins at this time.  During the Early Burdigalian (~20 Ma) tectonically-driven subsidence accelerated and was accompanied by a concordant increase in denudation and uplift of the rift shoulders.  The intra-rift fault networks coalesced into through-going structures and extension became progressively more focused along the rift axis.  This reconfiguration resulted in more laterally continuous depositional facies and the moderate-to-deep marine deposits of the Rudeis, Kareem and Ranga Formations.
At the early Middle Miocene (~14 Ma) onset of the left-lateral Gulf of Aqaba transform fault system marked dramatic changes in rift kinematics and sedimentary depositional environments.  The Gulf of Suez became isolated from the active northern Red Sea rift, with a switch from orthogonal to oblique rifting and to hyperextension in the northern Red Sea.  The previous open marine seaway was replaced by an extensive evaporitic basin along the entire length of the rift from the central Gulf of Suez to Yemen/Eritrea.  In Egypt these evaporites are ascribed to the Belayim, South Gharib, Zeit and Abu Dabbab Formations.  Evaporite deposition continued to dominate until the end of the Miocene (~5 Ma) when a subaerial unconformity developed across most of the basins. With the onset of seafloor spreading in the southern Red Sea, Indian Ocean marine waters re-entered through the Bab el Mandab in the earliest Pliocene and re-established open marine conditions.  In the northern Red Sea well and seismic data demonstrate that continental crust extends at least several tens of kilometers offshore.  The northern Red Sea is a highly extended non-volcanic rift and true, laterally integrated sea-floor spreading has not yet developed.

How to cite: McClay, K., Bosworth, B., Khalil, S., Ligi, M., and Stockli, D.: The Northern Red Sea - a model for rifting leading to continental break-up, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7102, https://doi.org/10.5194/egusphere-egu21-7102, 2021.

09:27–09:29
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EGU21-4185
Sami El Khrepy, Ivan Koulakov, Nassir Al-Arifi, Mamdouh S. Alajmi, and Ayman N. Qadrouh

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.

09:29–09:31
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EGU21-9474
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ECS
Valentin Basch, Alessio Sanfilippo, Luigi Vigliotti, Antonio Langone, Najeeb Rasul, Salem AlNomani, Ali AlTharowi, Adel Jerais, and Marco Ligi

The Red Sea rift system represents the best case study of the rift-to-drift history, i.e., the transition from a continental to an oceanic rift and the formation of passive margins. Although the onset of seafloor spreading has been constrained by geophysical observations to 5 Ma in the southern Red Sea, recent studies have suggested that MORB-type melts were intruded within the extended continental crust already during the early stages of rifting. We present here a petro-geochemical investigation of gabbroic bodies and associated basaltic intrusions from the Tihama Asir igneous complex, which formed as part of the intense magmatism that occurred during early Red Sea continental rifting. The most primitive olivine gabbros present modal, bulk and mineral compositions consistent with formation from MORB-type parental melts, but more evolved gabbros and oxide gabbros show saturation of phlogopite and define a geochemical evolution that progressively diverges from that of lower oceanic crust at mid-ocean ridges. Indeed, the Tihama Asir evolved gabbros are characterized by enrichments in LREE and highly incompatible elements (Rb, Ba, U, Th, Nb, Sr, K), suggesting hybridization of a MORB-type parental melt through a process of progressive assimilation of continental crust during the emplacement of gabbroic bodies. Additionally, the gabbros are associated with basaltic dike swarms intruded into the extending continental crust. The basalts show enrichments in LREE and highly incompatible elements similar to the gabbros, suggesting that they formed from melts extracted from the hybridized gabbroic crystal mush. This indicates that the Red Sea oceanization started before the onset of seafloor spreading, and that the cold continental crust was partially assimilated and replaced by hot gabbroic bodies since the early stages of continental rifting.

How to cite: Basch, V., Sanfilippo, A., Vigliotti, L., Langone, A., Rasul, N., AlNomani, S., AlTharowi, A., Jerais, A., and Ligi, M.: Crustal contamination and hybridization of an embryonic oceanic crust during the Red Sea rifting: An example from the Tihama Asir igneous complex, Saudi Arabia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9474, https://doi.org/10.5194/egusphere-egu21-9474, 2021.

09:31–09:33
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EGU21-10872
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ECS
Ran Issachar, Jörg Ebbing, Dilixiati Yixiati, and Nils Holzrichter

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.

09:33–09:35
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EGU21-10905
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ECS
Matthieu Ribot, Yann Klinger, Edwige Pons-Branchu, Marthe Lefevre, and Sigurjón Jónsson

Initially described in the late 50’s, the Dead Sea Fault system connects at its southern end to the Red Sea extensive system, through a succession of left-stepping faults. In this region, the left-lateral differential displacement of the Arabian plate with respect to the Sinai micro-plate along the Dead Sea fault results in the formation of a depression corresponding to the Gulf Aqaba. We acquired new bathymetric data in the areas of the Gulf of Aqaba and Strait of Tiran during two marine campaigns (June 2018, September 2019) in order to investigate the location of the active faults, which structure and control the morphology of the area. The high-resolution datasets (10-m posting) allow us to present a new fault map of the gulf and to discuss the seismic potential of the main active faults.

We also investigated the eastern margin of the Gulf of Aqaba and Tiran island to assess the vertical uplift rate. To do so, we computed high-resolution topographic data and we processed new series of U-Th analyses on corals from the uplifted marine terraces.

Combining our results with previous studies, we determined the local and the regional uplift in the area of the Gulf of Aqaba and Strait of Tiran.

Eventually, we discussed the tectonic evolution of the gulf since the last major change of the tectonic regime and we propose a revised tectonic evolution model of the area.

 

How to cite: Ribot, M., Klinger, Y., Pons-Branchu, E., Lefevre, M., and Jónsson, S.: Bathymetry and uplift rate of the Gulf of Aqaba, Dead Sea Fault., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10905, https://doi.org/10.5194/egusphere-egu21-10905, 2021.

09:35–09:37
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EGU21-6227
Geraint Hughes and Osman Varol

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.

09:37–09:39
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EGU21-12263
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ECS
M. Clara Modenesi and J. Carlos Santamarina

The demand for metals and raw materials continues to increase as onshore deposits become more depleted. Our oceans contain large unexplored areas that may contain new resources in the form of Mn-nodules, Co-rich crusts, and massive sulfides. A complete characterization and assessment of these deposits are fundamental for the evaluation of resource extraction, separation, and disposal processes.

The Red Sea holds unique examples of sediment accumulations formed under distinctive environmental conditions. The Atlantis II deep is located in the central part of the Red Sea at 2 km depth and on top of the spreading axis. This deep accumulates sediments that result predominantly from the discharge of hydrothermal fluids into hot and stratified brine pools. The changes in environmental conditions and the hydro-chemical conditions in the brine pool control sediment formation. The accumulations are enriched with metals, such as Ag, Au, Cu, Co, and Zn. The sediments in this deep hold a record of the formation history and their brine pools tell a story about on-going processes.

On-going research at the Energy Geo-Engineering Laboratory EGEL, KAUST includes (1) Geotechnical index properties (liquid limit, grain size distribution, and specific surface) and consolidation tests to infer engineering properties, (2) Sediment classification based on the Revised Soil Classification System, (3) Geochemistry and mineralogy using XRD, ICP-OES and (4) Microstructure and texture with SEM imaging. An advanced sediment characterization of these fine-grained metalliferous deposits gives a comprehensive understanding of the soil behavior.

How to cite: Modenesi, M. C. and Santamarina, J. C.: The Red Sea Metalliferous Sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12263, https://doi.org/10.5194/egusphere-egu21-12263, 2021.

09:39–10:30
Break
Chairpersons: Giacomo Corti, Carolina Pagli, Sylvie Leroy
Hot spots and rift magmatism
11:00–11:02
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EGU21-13564
Carol Stein, Seth Stein, Molly Gallahue, and Reece Elling

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.

11:02–11:04
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EGU21-13631
Seth Stein, Molly Gallahue, Carol Stein, Tyrone Rooney, and Andie Gomez-Patron

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.

11:04–11:06
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EGU21-6683
Ronald M. Spelz, Néstor Ramírez-Zerpa, Juan Contreras, Ismael Yarbuh, Antonio González-Fernández, David Caress, David Clague, Robert Zierenberg, Jennifer B. Paduan, Raquel Negrete-Aranda, John M. Fletcher, Florian Neumann, and Brian Cousens

The Pescadero Basin Complex (PBC) in the southern Gulf of California comprises three distinctive stretched rhomboid pull-apart basins separated by several short transforms. Multibeam and Autonomous underwater vehicle (AUV) bathymetry data collected at 40-m and 1-m resolution, respectively, combined with the processing and interpretation of three 2-D high-resolution multichannel seismic reflection profiles, were used to characterize the architecture of the entire PBC, as well as the internal structure of the northern Pescadero basin. Detailed mapping and cross-sectional kinematic modeling based on multichannel seismic images of the northern Pescadero basin reveals a highly evolved pull-part geometry, characterized by a well-defined ~1.8 km wide axial graben stretching ~32 km in an NNE-SSW direction. Both finite and incremental strain analyses carried out in this study point out that the PBC developed under sustained transtensional deformation, where the relative motion of the crustal blocks is oblique and divergent to the transforms or principal displacement zones (PDZ's), and subsidence is likely being accommodated by one of more décollement layers located at the bottom of a broad negative flower structure. We also present new geochemical data of lava flows with a N-MORB composition outcropping on the NE segment of the northern Pescadero axial graben, and lava-flow samples of E-MORB composition from an uplifted sediment hill on the western margin of the southern Pescadero basin. MORB samples from the PBC represent the northernmost surface flows known in the Gulf of California, highlighting that the PBC has evolved beyond being a pull-apart complex to having initiated seafloor spreading with new oceanic crust formation in response to the opening of the Gulf of California.

How to cite: Spelz, R. M., Ramírez-Zerpa, N., Contreras, J., Yarbuh, I., González-Fernández, A., Caress, D., Clague, D., Zierenberg, R., Paduan, J. B., Negrete-Aranda, R., Fletcher, J. M., Neumann, F., and Cousens, B.: The Pescadero Basin Complex, southern Gulf of California: structure, tectono-stratigraphic evolution and magmatism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6683, https://doi.org/10.5194/egusphere-egu21-6683, 2021.

11:06–11:08
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EGU21-3187
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ECS
Reece Elling, Seth Stein, Carol Stein, and G. Randy Keller

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.

Extension tectonics around the globe
11:08–11:10
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EGU21-4543
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ECS
Tsung-Han Huang, Meng Wan Yeh, and Ching-Hua Lo

The continental crust of southeast Asia underwent from thickening, thinning to almost rifting during the Mesozoic era as the active continental margin transformed into a passive one. Such crustal thinning history is well-preserved in the Kinmen Island, as the lower crustal granitoids retrograded and rapidly exhumed to surface that were crosscutted by mafic dike swarm. Kinmen Island is situated on the SE coast of Asia, featured by the widespread Cretaceous magmatism as the Paleo-Pacific plate subducted and rollbacked underneath the South China block. Although these complex magmatism are well reported and studied, their associated structural evolution and plate kinematics have not been clearly deciphered. Detailed field mapping, structural measurement, and petrographic analysis of the Kinmen Island were conducted. Up to five deformation events accompanied with five relevant magmatic episodes as well as their corresponding kinematic setting are reconstructed. The ∼129 Ma Chenggong Tonalite (G1) preserved all deformation events identified in this study, which marks the lower bound timing of all reported events. D1 formed a gneiss dome with the Taiwushan Granite (∼139 Ma) at the core bounded by moderately dipping gneissic foliation (S1) as crust extended. D2 formed subhorizontal S-tectonite (S2) with further exhumation of D1 gneiss dome due to middle-to-lower crustal flow associated with further crustal thinning. D3 formed a sinistral ENE-WSW striking steeply S dipping shear belts with well-developed S/C/C’ fabrics. The moderately E-plunging lineation on C surface indicates its transtensional nature. Widespread garnet-bearing leucogranite (G2) associated with decompressional melting showed long lasting intrusion prior to D2 until post D3. D4 was the intrusion of biotite-bearing Tienpu Granite (∼100 Ma; G3) that truncated G1, G2, and all fabrics, which was followed by the intrusion of E-W striking, steeply dipping biotite-bearing pegmatite (G4) as the crust further extended. The youngest deformation event (D5) was NE-SW striking subvertical mafic dike swarm (G5; 90–76 Ma) due to mantle upwelling through significantly thinned crust. By integrating the structural evolution and the previously reported strain pattern, we delineate the slab rollback direction of the Paleo-Pacific plate, which changed from northeastward (129∼114 Ma) to southeastward (107∼76 Ma). This plate kinematic movement switched during 114–107 Ma.

How to cite: Huang, T.-H., Yeh, M. W., and Lo, C.-H.: Structural Evolution of Extended Continental Crust Deciphered From the Cretaceous Batholith in SE China, a Kinmen Island Perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4543, https://doi.org/10.5194/egusphere-egu21-4543, 2021.

11:10–11:12
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EGU21-1625
Cuimei Zhang, Zhen Sun, Gianreto Manatschal, Xiong Pang, Sanzhong Li, Daniel Sauter, and Gwenn Peron-Pinvidic

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.

11:12–11:14
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EGU21-2444
Penggao Fang, Weiwei Ding, and Yanghui Zhao

The hyper-extended continental crust in the passive margins, which recording the extensional processes in relation with the breakup of continental crust and lithosphere as well as the onset of seafloor spreading, have been widely recognized and studied at present-day rifted margins. The Baiyun Sag (BS) represents one of the hyper-extended continental marginal basins with a sharply thinned continental crust from 25 km to 7 km over a ~ 50 km distance along the Northern South China Sea, which experienced syn-rift to post-rift during the Cenozoic. Although the Cenozoic infill of the BS has been extensively described, newly acquired 3D seismic profiles revealed a thick succession (up to 10 km) with thicken syn-rift but relatively thin post-rift strata particularly well imaged in the central part. The imaged succession is controlled by the interaction between well-developed detachment systems and depth-dependent stretching, resulting in different and complex subsidence architecture. Attempts had been made to quantify the subsidence in the BS, while most studies were only carried out in a limit set with one or few 2D seismic sections and generally focused on the post-rift subsidence but ignoring that in the syn-rift stage. As result, we investigate the interaction between spatial-temporal distributions of tectonic subsidence from continent break-up to post-rift and the evolution of hyper-extended rift systems along the relatively young age passive margins.

In this presentation we analyze the vertical and horizontal motions of tectonic subsidence and sedimentary processes with integrated high-quality multi-channel seismic profile grid data (~30 seismic sections). This study enables us to 1) interpret the main unconformities and analyze the depth conversion of the BS, 2) reconstruct the tectonic subsidence from syn-rift to post-rift, 3) provide a 3D subsidence analysis unravelling the temporal and spatial architecture of Cenozoic infill of the BS. The main objectives of this contribution is to discuss the possible mechanisms accounting for the origin and subsidence at the BS, reveal its interrelationships with magmatic activities, and explore the style of rift to post-rift subsidence pattern at a hyper-extended continental margin.

How to cite: Fang, P., Ding, W., and Zhao, Y.: Cenozoic subsidence characteristics and evolution at a hyper-extended continental margin: Revealed by 3D high-resolution seismic data from the Baiyun Sag, northern South China Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2444, https://doi.org/10.5194/egusphere-egu21-2444, 2021.

11:14–11:16
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EGU21-8366
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ECS
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Highlight
Jiannan Meng, Ozan Sinoplu, Zhipeng Zhou, Bulent Tokay, Timothy Kusky, Erdin Bozkurt, and Lu Wang

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.

11:16–11:18
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EGU21-8632
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ECS
Nicolas Dall'asta, Guilhem Hoareau, Gianreto Manatschal, and Charlotte Ribes

 The external crystalline massifs of the Alps, which include the Mont-Blanc massif, are found in between the external and internal parts of the orogen. The external parts correspond to the proximal domain of the Alpine Tethys (Helvetic domain), whereas the internal part corresponds to the former distal domain of the margin (Penninic domain). Therefore, the Mont-Blanc massif is a key place for understanding the proximal-distal transition during Jurassic rifting of the Alpine Tethys. 

Despite numerous seismic observations at modern passive margins, the tectono-sedimentary and fluid evolution recorded in these domains called necking zone remain poorly understood. Many questions remain concerning the thermal evolution, the origin and composition of the fluids, their link to large-scale hydrothermal systems, and the impact of element transfer on the diagenesis of syn-rift sediments.

 

Here we focus on the Col du Bonhomme (southern Mont-Blanc massif near Bourg St-Maurice, France), where late Triassic / early Jurassic to late Jurassic sediments preserve pre-Alpine contacts between the sediment and the basement.  The syn-rift sedimentary tract is composed of Sinemurian to Pliensbachian sandstones called “Grès Singuliers”, lying unconformably above the pre-rift and over an exhumed fault plane corresponding to the top basement.

Characterization of the faults and overlying sediments requires a multi-scale and multi-disciplinary approach combining field observation, petrography, sedimentology, structural geology, and geochemistry. The protolith of the fault rocks is a Variscan migmatitic gneiss. The damaged zone consists of cataclasites and the core zone is made of black gouge. The gouge is overlaid conformably by Liassic sandstones that contain reworked clasts of cataclasite. The observations that the top basement fault is cut by a Pliensbachian high-angle normal fault and Triassic clasts occur in the gouge enables to date this fault as Early Jurassic. 

At the micro scale, the basement shows hydratation leading to chloritization of biotite and sericitisation of feldspaths (orthoclase and plagioclase). A strong hydration-assisted deformation with increase of deformation toward the fault core leads to the formation of cataclasites. They are composed of quartz, sericite with small remnants of orthoclase, chlorites with secondary pyrites and rutiles. The fault core is a black gouge with grain size comminuition and mineral neoformation.

Evidence for fluid flow is observed in the fault leading to the hydrothermal alteration of the basement (sericitisation of feldspath and corrosion of quartz)  and the formation of syn-gouge quartz and quartz-adularia veins in the black gouge (datation using the Rb-Sr an adularia and U-Pb on calcite method is in progress) . 

Based on our observations we interpret the fault observed at Col du Bonhomme as a Jurassic exhumation fault associated with the necking of the European crust during Jurassic rifting. This preliminary work shows that the fault acted as an important pathway for crustal fluids with important transfer of silica and at least K, Fe and Ti.  The Col du Bonhomme area gives an opportunity to study fluid circulation and basement alteration along a rift-related detachment fault in the necking domain and therefore to understand fluid-mediated element mobility during rifting.

Keywords : Detachment fault, Mont-Blanc massif, Fluid circulation , Alpine Tethys, Necking zone

How to cite: Dall'asta, N., Hoareau, G., Manatschal, G., and Ribes, C.: Fluid circulations associated with the necking of the crust: the example of the Mont-Blanc detachment fault, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8632, https://doi.org/10.5194/egusphere-egu21-8632, 2021.

11:18–11:20
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EGU21-2822
Anke Dannowski, Heidrun Kopp, Ingo Grevemeyer, Grazia Caielli, Roberto de Franco, Dietrich Lange, Martin Thorwart, Christian Filbrandt, Cruise participants msm71, and AlpArray Working Group

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.

11:20–11:22
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EGU21-1825
Egidio Armadillo, Daniele Rizzello, Pietro Balbi, Davide Scafidi, Andrea Zunino, Fausto Ferraccioli, Guy Paxman, Alessandro Ghirotto, and Martin Siegert

The Transantarctic Mountains (TAM) separate the Mesozoic to recent West Antarctic rift system (WARS) from a wide and depressed triangular sector of East Antarctica spanning from 100° E to 160° E in longitude and from the Oates, George V and Adelie coastlines to 85° S in latitude. The sub-ice bedrock of this sector shows a basin and range style topography comprising two major basins of continental proportions -the Wilkes Basin and the Aurora Basin complex- and many smaller basins such as the Adventure, Concordia, Aurora and Vostok trenches. Most of these basins and trenches exhibit a triangular shape with the acutest angle pointing approximatively to a single pole towards the South, giving a fan shaped pattern of significant dimensions. We name here this region as the East Antarctic Fan shaped Basin Province (EAFBP). To the West, this province is limited by the intraplate Gamburtsev Mountains (GM).

Origins and inter-relationships between these four fundamental Antarctic tectonic units (WARS, TAM, EAFBP, GM) are still poorly understood and strongly debated. In the EAFBP, very little is known about the mechanism generating the basins, their formation time, whether they are all coeval and if and how they relate to Australia basins before Antarctica-Australia rifting. Present genetic hypotheses for some of the basins span from continental rifting to a purely flexural origin or a combination of the two. Also, post-tectonic erosional and depositional processes may have had a significant impact on the present-day topographic configuration.

Here we investigate the possibility that the EAFBP is the result of a single genetic mechanism: a wide fan-shaped intra-continental extension around a pivot point at about 135° E, 85° S that occurred at the Mesozoic-Cenozoic transition. We discuss evidence from the sub-ice topography and potential field airborne and satellite data.

We have used international community-based Antarctic compilations in public domain, including BedMachine (Morlighem et al., 2020), AntGG (Scheinert et al., 2016) and ADMAP 2.0 (Golynsky et al., 2018). We have applied image segmentation techniques to the rebounded sub-ice topography to automatically trace the first order shape of the sub-ice basins. Then we have fitted the edges of the basins by maximum circles and we have estimated the best Euler pole identified by their intersection. Potential field anomalies have been taken into account in order to enlighten major discontinuities not revealed by the sub-ice topography.

Software simulations of the EAFBP opening in the frame of global plate tectonics reconstructions indicate that it may be inserted in the frame of the later phase of the Antarctica-Australia rifting, giving constraints on timing that allow us to date the EAFBP opening at the Mesozoic-Cenozoic transition.

The reconnaissance of the EAFBP as the result of a continental-scale fan-shaped extension may have deep implications on global and regional tectonics plate reconstructions, plate deformation assumptions and new tectonic evolutionary models of WARS, TAM and GM.

How to cite: Armadillo, E., Rizzello, D., Balbi, P., Scafidi, D., Zunino, A., Ferraccioli, F., Paxman, G., Ghirotto, A., and Siegert, M.: Some evidence for a wide fan-shaped extension of the East Antarctic plate at the Mesozoic-Cenozoic transition, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1825, https://doi.org/10.5194/egusphere-egu21-1825, 2021.

11:22–11:24
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EGU21-9481
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ECS
Julie Linnéa Sehested Gresseth, Per Terje Osmundsen, and Gwenn Péron-Pinvidic

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.

Modelling and tectonic restoration
11:24–11:26
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EGU21-3352
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ECS
Gianluca Frasca, Gianreto Manatschal, Patricia Cadenas, Jordi Mirò, and Rodolphe Lescoutre

Fossil remnants of rifted margins sampled in orogens enable to unravel the nature of rocks, structures and conditions controlling the formation of rifted margins and lithosphere breakup. However, a major problem in orogens is that disconnected remnants of only one margin are preserved, while the conjugate has often been subducted and/or obliterated during convergence. Thus, our understanding of rift processes leading to lithosphere breakup is hampered by the impossibility to direct access to well-preserved examples of conjugate rifted margins fossilised onshore. Here we focus our attention on the Mesozoic Alpine Tethys, bounded by the European and African plates and interleaving crustal blocks such Iberia and Adria. Two key points have to be resolved in order to reconstruct conjugate distal margins in the Alpine Tethys paleogeographic setting. First, a restoration of the European western side of the Alpine Tethys has to be performed. Second, the position of Iberia during the Mesozoic has to be restored taking into account the evolution and opening of the southern North Atlantic and the Bay of Biscay. Here we propose a new Mesozoic kinematic model for Iberia, which is compatible at a first order and large scale with recently published data and interpretations from the North Iberian margin and the Pyrenean domain. We discuss the impact of the results for the reconstruction of the Alpine Tethys.

How to cite: Frasca, G., Manatschal, G., Cadenas, P., Mirò, J., and Lescoutre, R.: The importance of Iberia for the restoration of the Mesozoic Alpine Tethys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3352, https://doi.org/10.5194/egusphere-egu21-3352, 2021.

11:26–11:28
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EGU21-5369
Sebastien Gac, Mansour M. Abdelmalak, Jan Inge Faleide, Daniel Schmid, and Dmitrii Zastrozhnov

The Vøring Margin offshore Norway is a typical example of volcanic passive margin. The evolution of the inner Vøring Margin is well explained by standard models of lithosphere extension (McKenzie, 1978). Basin modelling tools based on the assumption of lithosphere extension then satisfactorily simulate the tectonic and thermal evolution of the inner margin.

However, models of extension fail to reproduce key observations at the outer (volcanic) domain of the Vøring Margin. These specific observations include uplift at time of breakup, the presence of SDRs and magma additions at the base of the lower crust usually referred as the lower crustal body and interpreted as magma underplating or highly intruded lower crust. Additional, non-extensional processes are required to satisfy these observations.

Excess magmatism and uplift of the outer margin during the breakup time has been explained by the arrival of the hot Icelandic mantle “plume” (Skogseid et al., 2000) or by other sublithospheric processes such as small-scale convection (van Wijk et al., 2001). Melt retention in the asthenosphere has also been proposed to explain uplift at passive margins (Quirk & Rüpke, 2018). At last, mantle phase transitions caused by pressure and temperature changes in the mantle during extension may contribute to uplift (Simon & Podladchikov, 2008).

These processes must be included in the basin modelling procedure to reliably simulate the evolution of the volcanic margin.

We use the Tecmod2d modelling suite (Rüpke et al., 2008) to simulate the tectono-thermal evolution along two crustal transects crossing the Vøring Margin. Tecmod uses an automated inversion scheme approach. Processes such as magmatic underplating, melt retention, mantle phase transitions, and differential thinning can be taken into account.

We test various tectono-thermal models of the margin evolution that incorporate or not these processes. Models incorporating a plume emplaced at Eocene time and taking into account magmatic processes (melt retention and magmatic underplate) satisfactorily reproduce the specific observations of the outer (volcanic) margin. This result backs the contribution of the hot Iceland plume on the evolution of the Vøring Margin.

 

References

McKenzie, D. (1978) Some remarks on development of sedimentary basins. Earth Planet. Sci. Lett., 40, 25-32.

Quirk, D.G., Rüpke, L.H. Melt-induced buoyancy may explain the elevated rift-rapid sag paradox during breakup of continental plates. Sci Rep 8, 9985 (2018). https://doi.org/10.1038/s41598-018-27981-2

Rüpke, L.H., Schmalholz, S.M., Schmid, D.W. & Podladchikov, Y.Y. (2008) Automated Thermotectonostratigraphic basin reconstruction: Viking Graben case study. AAPG Bull., 92, 309^326.

Simon, N.S.C., Podladchikov, Y.Y., 2008. The effect of mantle composition on density in the extending lithosphere. Earth Planet. Sci. Lett.272, 148–157.

Skogseid, J., Planke, S., Faleide, J.I., Pedersen, T., Eldholm, O. & Neverdal, F. (2000)Ne Atlantic continental rifting and volcanic margin formation. In: Dynamics of the NorwegianMargin (Ed. by A.Nottvedt, B.T. Larsen, R.H.Gabrielsen, S. Olaussen, B.Torudbakken, J. Skogseid,H. Brekke & O. Birkeland), Geol. Soc. Spec. Publ., 167, 295^326.

van Wijk, J. W., Huismans, R. S., Ter Voorde, M., & Cloetingh, S. A. P. L. (2001). Melt generation at volcanic continental margins: No need for a mantle plume? Geophysical Research Letters, 28(20), 3995–3998. https://doi.org/10.1029/2000GL012848.

How to cite: Gac, S., Abdelmalak, M. M., Faleide, J. I., Schmid, D., and Zastrozhnov, D.: Automated reconstruction of the Vøring volcanic margin incorporating non-extensional processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5369, https://doi.org/10.5194/egusphere-egu21-5369, 2021.

11:28–11:30
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EGU21-4877
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ECS
Peter Haas, R. Dietmar Müller, Jörg Ebbing, Gregory A. Houseman, Nils-Peter Finger, and Mikhail K. Kaban

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.

11:30–11:32
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EGU21-688
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ECS
Frank Zwaan, Pauline Chenin, Duncan Erratt, Gianreto Manatschal, and Guido Schreurs

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.

11:32–12:30