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. Special emphasis will be given to 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.
Dear participants of EGU session GD6.3 on rifting
We will start the discussion at 10:45 CET on Friday 8 May, and it will last until 12:30 CET, although the chat will remain active for 30 min more.
This is how we plan to carry on the session:
· Every contribution will get about 5 minutes of discussion
· The conveners will introduce the contribution (title, authors,..)
· The presenting authors will give a short summary/introduction (2-3 sentences) of their work and contact details for potential further discussion (@ authors, please prepare these in advance to ensure a smooth transition).
· Discussion with participants
If time permits, we will have a more general discussion after all contributions have been presented.
Here’s the order of the presentations:
· Tortelli et al.
· Welford & Geng
· Phillips & McCaffrey
· Pan et al.
· Glerum & Brune
· Braschi et al.
· Bauer et al.
· Pagli et al.
· La Rosa et al.
· Keir et al.
· King et al.
· Lymer et al.
· Yang & Welford
· Chenin et al.
· Forzese et al.
· Frasca et al.
Files for download
Chat time: Friday, 8 May 2020, 10:45–12:30
In Afar, an active mature rift, close to break-up have been described for many decades. However, no data were provided so far to precisely constrain the long-term structure of the rift, its evolution, and the rift maturity, i.e. the documentation of ongoing break-up processes. Here, we synthetize structural, geophysical, and geochemical data that show that break-up is indeed ongoing in Central Afar.
Geological mapping and dating of volcanic units provided data to build a detailed surface cross-section, showing a dense network of continentward dipping normal faults and two main unconformities. The field cross-section allows to estimate the amount of crustal thinning and stretching (beta factor > 2.5 in the rift center). The crust first stretched quite uniformly during late Oligocene-early Miocene times and then thinned is a more localized manner during late Miocene times. Subsequently, during Plio-Quaternary times the volcanic Stratoïd series emplaced in the thinned area. Eventually, the present-day magmatic axes have been active for about 500 kyr in the thinned continental crust.
The crust decreases in thickness from 40 km beneath the Ethiopian plateau to about 20-25 km in Central Afar, from receiver functions (RF). New RF data and tomographic models confirm that thickness and show that a necking zone formed at depth most likely during late Miocene times (thinning phase).
The combination of geological and geophysical data allows to attempt for balancing the cross-section. This balancing suggests that, beneath Central Afar, the crust is much thicker than it should be. This crust in excess is interpreted as representing magma intrusions and/or underplating that occurred in the course of the Cenozoic rifting.
Trace element analyses suggest that the depth of melting has strongly decreased just after the plume impingement, suggesting an early thermal erosion of the lithosphere base. Geochemical (isotopic) data show that in Central Afar most of the magma sources were in the plume head/tail since its impingement. These data also show that the recent lavas (< 500 kyr) were almost not contaminated by continental crustal rocks, attesting for a transitional crust beneath Afar and suggesting a recent switch from crustal thinning to dyke-related divergence.
To sum up, we propose that (1) the Afar plume was channelized in the lithosphere, probably since late Oligocene times, (2) a late Miocene thinning event (necking) predated a Quaternary SDR-like volcanic wedge emplacement, and (3) divergence is now accommodated by magmatic accretion.
How to cite: Bellahsen, N., Pik, R., Leroy, S., Ayalew, D., and Doubre, C.: Rifting, magma, and break-up in Central Afar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13122, https://doi.org/10.5194/egusphere-egu2020-13122, 2020.
Growth of rift segments and development of crustal magmatic systems in continental rifts remain debated issues. We integrate volcanological, geochemical, petrological and seismic data from the Ma’alalta stratovolcano near the western rift margin of Afar to show that active magmatic rifting occurs there. Growth of Ma’alalta started around 0.55 ± 0.05 Ma (Barberi et al. 1972) with the age of the youngest flows unknown. Ma’alalta produced lava flows but also large-volume, caldera-forming ignimbrites, as well as silicic intracaldera domes. The products are mainly trachytic and some are slightly peralkaline. The most recent magmatic activity of Ma’alalta consists of mafic lava fields, scoria cones and peralkaline obsidianaceous silicic domes produced along the ~40 km long magmatic segment and erupted from several vents aligned NNW-SSE rather than from central volcanic activity. Local seismicity (2005-2009 and 2011-2013) also shows a NNW-SSE-trending alignment of earthquakes with good correlation to where the recent magmatic products were erupted. The geochemical features of the mafic rocks (e.g., Ba/La, Rb/Ta and Zr/Ta) as well as the petrogenesis of the recent NNW-SSE-trending silicic domes are similar to the nearby on-rift Dabbahu and Durrie volcanoes. Inferred magma storage depth from mineral geobarometry show that a shallow, silicic chamber existed at ~4-5 km depth below the stratovolcano, while a stacked plumbing system with at least two magma storage levels at ~14 and ~24 km of depth fed the recent basalts. We interpret the wide set of observations from Ma’alalta as evidences that the area is an active rift segment, showing that localised axial extension can be heavily offset towards the rift margin.
How to cite: Tortelli, G., Gioncada, A., Pagli, C., Rosi, M., Keir, D., and De Dosso, L.: Evidence of active magmatic rifting in Ma’alalta marginal volcano (Afar, Ethiopia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6976, https://doi.org/10.5194/egusphere-egu2020-6976, 2020.
In magmatically active continental rifts, crustal deformation is often accompanied by caldera volcanism along the rift axis. These caldera volcanoes help to characterize the spatiotemporal relationship between regional tectonic extension, the development of normal faults, and the role of magmatism during the long-term evolution of continental rifts. In the Kenya Rift, magmatic activity has been focused at regularly spaced Quaternary volcanoes, each located within an extensional sub-basin of the rift. We document the structural characteristics of the c. 36-ka-old Menengai Caldera and adjacent regions located within such a young zone of extension, to gain insight into the role of regional-scale structures and volcanism in a rift zone subjected to oblique extension, and discuss the role of magmatic centers in the context of advanced stages of rift-basin differentiation.
Our field mapping and high-resolution digital surface models in the greater Menengai area located in the Central Kenya Rift show that the interior rift sectors are dominated by NNE-striking Holocene normal faults perpendicular to the regional ESE-WNW extension direction. Inside the caldera, these structures continue, but are overprinted by post-collapse doming and faulting of the magmatic center, resulting in obliquely slipping normal faults bounding a resurgence horst. Radiocarbon dating of faulted units as young as 5 ka cal BP and the paleo-shorelines of a lake formed during the African Humid Period in the Nakuru Basin that we use as strain markers indicate that volcanism and faulting inside and in the vicinity of Menengai must have been sustained during the Holocene.
Our analysis confirms that the caldera is located at the center of an extending rift segment that is kinematically linked with adjacent zones of extension; similar volcano-tectonic relationships apply to virtually all larger volcanic centers in the Kenya Rift. These zones of extension in the inner sectors of the rift are arranged in en échelon patterns and are linked by transfer zones. In contrast to punctiform spreading centers in much more advanced extensional regions (e.g., the Red Sea) normal faulting in the Kenya Rift is not focused at these volcanic centers. We suggest that the magmatic centers in the segmented Kenya Rift are precursors of a more evolved rifting stage, where magmatic centers may constitute nucleation points of faulting in future magma-assisted rifting that will ultimately lead to the final stages of continental break-up.
How to cite: Riedl, S., Melnick, D., Mibei, G. K., Njue, L., and Strecker, M. R.: Magmatic centers and rift segmentation: insights from the Late Quaternary Menengai Caldera, Central Kenya Rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7003, https://doi.org/10.5194/egusphere-egu2020-7003, 2020.
The Gondwana and particularly its south-central part encompassing Africa and Antarctica has recorded a complex rift to drift history during the mid-Jurassic. Large discrepancies between the many kinematic reconstruction models that attempted to reconstruct such an history emphasise the urgent need for a critical reappraisal. Here we combine high-resolution seismic data sets and oil company wells to propose a deformation history and an evolution scenario for the oblique Mozambique margins system from the Davie ridge area to the Agulhas fracture zone through Angoche, Beira High, Limpopo and Natal segments.
Our results indicate large differences in rifting style and magmatism, ranging from wide to narrow rifting associated with restricted and/or widespread magmatic activities (synchronous or post-rift) combined with asymmetric to more symmetrical structures. At first glance, such differences seem to be related to the African mantle plume melting triggering thermal perturbations, but the importance and the influence of inherited lithospheric structures and thick layer of sediments enhancing mantle melt extraction cannot be excluded. We propose a geodynamic model in three main stages for the evolution of the Mozambique margins, from the extension initiation to the seafloor spreading. Stage T1, representing the first extensional event inducing crustal thinning before the Gondwana’s breakup. It is characterized by an E-W extension trend responsible for the formation of a large fault-controlled basin during the Permo-Trias (e.g. Limpopo). Stage T2 is marked by the onset of a mantle plume activity responsible for the Karoo Large Igneous Province formation, from about 186 Ma in a cratonic and belt lithosphere. Stage T2 is defined by NW-SE trending extension leading to mid-Jurassic basins infilling, and to a first onset of oceanic crust from Chron M33 (160 Ma) to Chron M25 (156 Ma), depending on the area. Stage T3 corresponds to the rift continuation with a stress field rotation ranging from NW-SE to N-S, suggesting that Antarctica moved in a SSE direction with respect to Africa after 156 Ma. This change of kinematics is defined for instance, by flower structures occurrences along the Limpopo margin (i.e. Limpopo transform fault zone), and allowed for the deposition of several seaward dipping reflectors. While the Angoche and Beira High margins recorded a period of quiescence, the Limpopo and Astrid ridge areas experienced an episode of uplift and erosion probably related to mantle dynamics (e.g. mantle plume, small-convection due to the difference of thickness of the lithosphere).
How to cite: Roche, V., Leroy, S., Guillocheau, F., Revillon, S., Dietrich, P., Watremez, L., Lepretre, A., Nonn, C., Despinois, F., and Vetel, W.: Deformation and evolution history of oblique Mozambique margins system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11077, https://doi.org/10.5194/egusphere-egu2020-11077, 2020.
The rifted continental margins of Newfoundland represent one of the best-studied examples of non-volcanic/magma-poor margins in the world. In addition to hosting proven prolific resources within the rift basins on the continental shelf, the rifted margins also host many promising frontier regions for oil and gas exploration in both the Flemish Cap and Orphan basins. Prior to rifting and opening of the North Atlantic Ocean during the breakup of Pangaea, the Newfoundland margins lay conjugate to the Iberian margin to the southeast and the Irish Atlantic margin to the northeast. Rifting and breakup evolved from south to north during three rift phases of varying orientation: NW-SE oriented Late Triassic-Early Jurassic rifting between Iberia/Eurasia and North America, W-E oriented Late Jurassic to Early Cretaceous rifting between Eurasia (Ireland) and North America, and SW-NE oriented Late Cretaceous rifting in the Labrador Sea. While the first phase of rifting exploited pre-existing Caledonian-Appalachian basement structures and tectonic fabrics, later rifting reactivated and crosscut these same inherited structures.
While multichannel seismic reflection imaging has been extensively undertaken across the Newfoundland shelf and rifted margins, deep crustal structure from seismic refraction profiling has been more sparsely constrained. To interpolate between existing crustal-scale seismic refraction profiles, constrained 3-D gravity inversion has previously been undertaken, providing regional constraints on Moho depth, crustal thickness, and beta factors. However, these early inversion attempts suffered from coarse parameterizations of densities within the sedimentary column and an inability to incorporate sparse deep seismic constraints. In this work, we present 3-D density anomaly models for the crust and upper mantle across the Newfoundland margin using constrained 3-D gravity inversions performed using two independent inversion methodologies (minimum structure and probabilistic). Common features to both inversions are deemed robust and provide an improved regional view of the crustal architecture of the offshore margins. In particular, crustal thinning is observed to align with earlier projections of ancient terrane boundaries such as the boundary between the Avalonian terrane and the Meguma terrane at the southeastern limit of the Grand Banks. Furthermore, the derived crustal thicknesses also provide a clear means of delimiting rafted continental fragments, revealing rift trends and the resulting crustal scars. This is particularly evident for the Orphan Basin where the southeastward rotation and displacement of the Flemish Cap has left a trail of orphaned continental pieces. These form crucial components for future deformable plate reconstructions in GPlates and, until then, provide a detailed regional view of the segmentation of the margin during rifting.
How to cite: Welford, J. K. and Geng, M.: The continental shelf and rifted continental margins of offshore Newfoundland revisited using constrained 3-D gravity inversions: tracking inheritance trends and rift scars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5048, https://doi.org/10.5194/egusphere-egu2020-5048, 2020.
Distinct crustal terranes and intruded igneous plutonic material are repeatedly brought together and deformed throughout multiple tectonic events. Accordingly, the resulting continental crust is highly heterogeneous and exhibits widespread lateral lithological and rheological variability that exerts fundamental controls on the structural style of rifting and eventual continental breakup across multiple scales of observation.
Lateral variations in crustal rheology and strength, such as those posed by igneous intrusions or distinct crustal terranes, may cause certain areas to be less prone to rifting and localise activity in adjacent areas, influencing rift structural style and the geometry of individual faults. Furthermore, the boundaries between different terranes often represent crustal-to-lithospheric-scale heterogeneities that may localise strain and reactivate during subsequent tectonic events, acting to partition the rift into distinct structural domains.
Using borehole-constrained 2D and 3D seismic reflection data, we showcase a number of styles of structural inheritance in the Great South Basin, offshore of the South Island of New Zealand. Pre-rift basement in this area comprises terranes corresponding to a relict Island arc system, including the dominantly plutonic Median Batholith and the dominantly sedimentary Murihiku Terrane, a former forearc basin. We find the spacing, structural style and geometry of faults varies greatly between the relative ‘strong’ and ‘weak’ crustal terranes, whilst the boundaries between individual terranes are often associated with complex plan-view fault geometries. In particular, the southern margin of the Median Batholith is reactivated as a large-scale shear zone and upper crustal fault system that is oriented at a high angle to, and thereby segments, the Great South Basin. Furthermore, these terrane boundaries also appear to exhibit some controls over the locations of Cenozoic intraplate volcanic systems and fracture zones associated with breakup along the Pacific-Antarctic ridge.
Within the rift itself we identify a series of granitic laccoliths along, and potentially exploiting, the southern boundary of the Median Batholith. The presence of this ‘strong’ granitic material inhibits fault nucleation and retards the propagation of approaching rift-related faults, causing them to splay and eventually terminate as they approach the granitic material. Through quantitative fault analyses, we find that individual fault segments maintain kinematic, and to some degree, geometric coherence across the system before terminating at the margin of the stronger crustal material.
The multi-scale complexity and variability of continental crust exerts a strong influence over multiple aspects of rift geometry from inception to its eventual breakup. We showcase a range of different styles of structural inheritance that are applicable to other rift systems worldwide. We show how different styles of structural inheritance, ultimately related to lateral and vertical variations in crustal strength and rheology, may dictate the location, geometry and structural style of different aspects of rift physiography from the whole rift scale to that of individual faults and fractures.
How to cite: Phillips, T. and McCaffrey, K.: Styles and scales of structural inheritance throughout continental rifting; Examples from the Great South Basin, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4815, https://doi.org/10.5194/egusphere-egu2020-4815, 2020.
Rifting of the continental lithosphere is accommodated by the development of large, linked, normal fault arrays. However, the timescales over which fault arrays develop - from the interaction of small, isolated faults towards localisation of through-going fault systems, has not been well constrained from observations in natural systems. Our limited knowledge of timescales over which fault arrays develop has also resulted in the development of different and debated fault growth models. While scaling relationships between fault displacement and length have been extensively used to understand fault evolution, the scaling exponent value is still not resolved due to significant scatter in global displacement-length profiles.
Here we use 3D seismic reflection and borehole data from the Exmouth Plateau, NW Shelf of Australia to investigate the timescales of faults growth within an array. The excellent quality seismic data allows for the entire Jurassic to Early Cretaceous fault array to be analysed over a large areal extent (~1200 km2), and the fault activity can be dated using biostratigraphy from wells. Our study is novel in that we reconstruct and quantify the length and throw on faults back through time to investigate how fault populations evolve. We find that the early stage of rifting was characterised by distributed faulting, where fault trace lengths were established early within the first 7.2 Myrs of rifting (out of a total rifting duration of 85.5 Myrs). By 28.5 Myrs of rifting (33% of the total rifting duration), strain localises on major west dipping faults as a fully linked system. Localisation continues on major faults until the cessation of rifting where strain is accommodated with maximum throw in the centre of faults decreasing towards its tips. Our results suggest that fault displacement and length may scale linearly, but grow in alternations of fault lengthening and fault displacement phases. The growth of active fault systems and death of inactive faults located in stress shadow zones is responsible for the scatter of data points frequently observed in global displacement-length profiles.
How to cite: Pan, S., Bell, R., and Jackson, C.: Displacement-length scaling relationships for a fault array reveal that faults grow via alternating phases of lengthening and localisation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20528, https://doi.org/10.5194/egusphere-egu2020-20528, 2020.
The Victoria plate in the East African Rift System (EARS) is one of the largest continental microplates on Earth. The partly overlapping eastern and western EARS branches encompassing Victoria follow the inherited lithospheric weaknesses of the Proterozoic mobile belts. Multiple lines of evidence show that Victoria rotates counter-clockwise with respect to Nubia, in striking contrast to its neighboring plates. Previous numerical modeling (Glerum et al., under review) has shown that this rotation is induced through the ‘edge-driven’ mechanism (Schouten et al., 1993), where stronger lithospheric zones transmit the drag of the major plates along the edges of the microplate, while weaker regions facilitate the rotation.
The current work enhances the previous 3D box models with a spherical domain, detailed data-driven lateral thickness variations and the inclusion of mantle structure in terms of temperature and density. Crustal and lithospheric thickness variations are taken from recent geophysical datasets of the present-day African continent (Tugume et al., 2013; Globig et al., 2016). Mantle structure is either scaled from seismic tomography models or generated through the addition of thermal upwellings mimicking the East African Superplume (e.g. Ebinger and Sleep, 1998). Preliminary results show that the counterclockwise rotation of Victoria, its rotation pole and its angular velocity as observed through GPS are consistently reproduced through the data-driven lithospheric strength distribution. With subsequent models we will demonstrate the effect of mantle structure on dynamic topography, strain localization and stress distribution in the EARS.
Ebinger, C.J. and Sleep, N.H. (1998), Cenozoic magmatism throughout East Africa resulting from impact of a single plume. Nature 395 (6704), 788–791.
Glerum, A., Brune, S., Stamps, D. S. and Strecker, M. (under review), Why does Victoria rotate? Continental microplate dynamics in numerical models of the East African Rift System.
Globig, J., Fernàndez, M., Torne, M., Vergés, J., Robert, A. and Faccenna, C. (2016), New insights into the crust and lithospheric mantle structure of Africa from elevation, geoid, and thermal analysis, J. Geophys. Res. Solid Earth, 121, 5389–5424.
Schouten, H., Klitgord, K. D. and Gallo, D. G. (1993), Edge-driven microplate kinematics. J. Geophys. Res. 98, B4, 6689–6701.
Tugume, F., Nyblade, A., Julià, J. and van der Meijde, M. (2013), Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation. Tectonophysics 609, 250–266.
How to cite: Glerum, A. and Brune, S.: How preexisting lithospheric heterogeneities and mantle upwellings affect Victoria’s rotation in the East African Rift System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6999, https://doi.org/10.5194/egusphere-egu2020-6999, 2020.
The Ririba rift represents the southern termination of the Main Ethiopian Rift and formed from the southward propagation of this latter during, or shortly after, the emplacement of subalkaline basalts that produced a widespread basaltic lava basement, at ~3.7 Ma.
The activity of the Ririba rift was short-lived and ceased between 2.8 and 2.3 Ma, when deformation migrated westward into an oblique, throughgoing rift zone directly connecting the Ethiopian and Kenyan rifts. Rifting was followed by the eruption of limited volumes of Late Pleistocene-Holocene alkaline basalts, associated to several, monogenetic volcanic centres, forming the Dilo-Dukana and Mega volcanic fields.
We provide new petrological, geochemical and isotopic data on the still poorly studied magmatic products emplaced by the volcanic activity of the Ririba rift with the aim of investigating the source of both the older Pliocene lava basement and the Late Pleistocene-Holocene alkaline basalts as well as their pathways to the surface.
Major and trace elements, besides discriminating the Pliocene lavas from the other younger alkaline products, reveal that the Dilo-Dukana and Mega samples always overlap in composition. On the whole they display variable major and trace element contents compared to a limited variation in silica (43-46wt.%) describing slightly defined trends. Regular and evident trends are observed comparing some incompatible trace elements (e.g., Rb, Ba, Zr, Nb) suggesting a prominent role of fractional crystallization for their differentiation.
Isotopes reveal that the products of the two volcanic fields have small but significantly different behaviour: the Dilo-Dukana products are isotopically homogeneous and clustered around 87Sr/86Sr values of 0.70303 and 143Nd/144Nd values of 0.51292, whereas the Mega lavas and pyroclastics display a small but wider variability partially overlapping the Dilo-Dukana samples, with 87Sr/86Sr ranging from 0.70300 to 0.70334 and 143Nd/144Nd from 0.51293 to 0.51290. Conversely, isotopes data corroborates the evidence that the younger Dilo-Dukana and Mega products are well distinct from the Pliocene basaltic lava basement (as well as with respect to all the older magmatic rocks of the area) and are characterised by a more prominent mantle signature.
Moreover, the Sr vs Nd isotopes variation among the younger Holocenic lavas of Mega describe a negative well-defined trend allowing to make inferences about the possible role of crustal contamination during the Ririba rift magmatic activity.
All these evidences are consistent with the interpretation that the two young volcanic fields of Dilo-Dukana and Mega are fed by deep structures directly transferring mantle melts up to the surface, as also suggested by the large abundance of mantle xenoliths in the different products. As a consequence, this strongly corroborates the interpretation that the two volcanic fields are not related to the major faults of the Ririba rift, but are associated to different, deep, NE-SW-trending inherited structures which cut the roughly N-S boundary faults of the rift.
How to cite: Braschi, E., Franceschini, Z., Cioni, R., Corti, G., Sani, F., and Muluneh, A.: Geochemical and isotopic characterization of the recent magmatic activity of the Dilo-Dukana and Mega volcanic fields (Ririba rift, southern Ethiopian Rift), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8118, https://doi.org/10.5194/egusphere-egu2020-8118, 2020.
The crystalline basement of central Madagascar is composed of the Neoarchaean, high-grade metamorphic Antananarivo Domain, made up of granulite to upper-amphibolite orthogneisses and paragneisses, and intruded by Tonian igneous rocks of the Imorona-Itsindro suite (Archibald et al. 2016). Along its southern, western and northern margins several terranes were accreted between the Paleoproterozoic and the Neoproterozoic (Tucker et al. 2014) before Madagascar was affected by the collision of East- and West-Gondwana at the end of the Ediacaran.
Within the Antananarivo Domain, a more than 700 km long and up to 80 km wide belt of supracrustal amphibolite-facies rocks forms te Ambatolampy Group. It is characterized by abundant monotonous biotite schists and gneisses that are locally migmatised. The schists contain biotite, sillimanite, garnet and locally thick graphite-rich layers. Associated paragneisses are also biotite-rich and commonly carry sillimanite or hornblende. White quartzites ranging from thick-bedded ridge-forming units to fine, cm-scale interbeds are coarse-grained and contain often sillimanite. Dark quartzites rich in magnetite and heavy minerals occur as cm-thin layers throughout the whole group. Small bodies of pyroxenite, pyroxene-amphibolite, amphibolite ±garnet, and pyroxene gneiss are common, especially close to the base of the group.
The age of the Ambatolampy Group is highly controversial. A group of researchers from BGS and USGS reported a youngest detrital zircon age of 1054 Ma, whereas Archibald et al. (2016) assumed a Mesoproterozoic age, based on their youngest zircons of roughly 1.8 Ga. We present new near-concordant U-Pb detrital zircons ages as young as 800 Ma, indicating a sedimentary input from igneous rocks of the Imorona-Itsindro suite. Sedimentation must have ceased before 630 Ma which is constrained by the U-Pb zircon age of an intruding leucogabbro.
About half of Madagascar’s known 1050 gold occurrences are lying within the Ambatolampy Group. Fine-grained disseminated gold appears to be concentrated within relatively narrow stratigraphic intervals of the Ambatolampy Group, defined by the occurrence of boudinaged or fractured magnetite quartzite. In general, the gold grades in fresh rocks are below economic interest, the highest gold tenors were recorded in an up to 30 meter thick laterite zone above the basement. Another important commodity related to the Ambatolampy Group is graphite which had seen a mining boom in the 1910s and 1920s. The graphite is flaky with crystal diameters between 0.5 and 5 mm and contents of graphitic carbon between 6 and 15 %. Individual seams are up to 12 m wide and can be tracked for several kilometers.
We interpret the Ambatolampy Group as a mainly siliciclastic fill of a continental rift basin during a phase of crustal extension occurring contemporaneously with the intrusion of the Imorona-Itsindro Suite. The gold mineralization is most likely related to fluvial deposits from surrounding gold-bearing Archean basement.
Archibald, D.B. et al. 2015. Tectonophysics 662, pp. 167-182.
Archibald, D.B. et al. 2016. Precambr. Res. 281, pp. 312–337.
Tucker, R.D. et al. 2014. J. African Earth Sci. 94, pp. 9-30.
How to cite: Bauer, W., Rasaona, I. T., Tucker, R. D., and Horton, F.: The Ambatolampy Group, central Madagascar, a Neoproterozoic rift-basin sequence?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1413, https://doi.org/10.5194/egusphere-egu2020-1413, 2020.
We present a joint analysis of seismological images and petrophysical data in the North Tanzanian Divergence, where the lithospheric break-up is at its earliest stage. In this part of the East African Rift, the current surface deformation is related to complex interaction between tectonic (active fault, pre-rift lithospheric structure) and magmatic processes within the mantle and the crust. We present here the compilation of seismological results such as receiver function, local tomography, regional tomography on datasets collected during CRAFTI-CoLiBrEA and HaTARi projects, in a region with clearly opposite seismological and magmatic behaviours: near Natron the seismicity is well located within the upper crust and linked to present day magmatism (Lengai edifice), whereas Manyara area is characterized by a deep seismicity and no evidence of present magmatic activity at the surface. First, these different approaches deliver Vp, Vs and deduced Vp/Vs images with both different resolution and different depth investigation. The combined images of crustal and lithospheric structure provide the appropriate scale to point out the interactions between melt, gas, faults, and inherited fabrics in specific areas. We then compare those geophysical observations with magma composition, magma storage (depth of reservoir, magma volume) and ascent as well as partial melts content at depth obtained from petrophysical and geochemical analysis of lava samples. We will analyze if this combination of seismological approaches constrained with petrological and geochemical data produce accurate images of the entire current magma plumbing system. Finally, we will discuss the results in terms of magmatic processes and how they interact with the rifting in a cratonic lithosphere.
How to cite: Gautier, S., Clutier, A., Parat, F., and Tiberi, C.: Crustal and lithospheric complexity beneath the North Tanzanian Divergence from seismological and geochemical analyses., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3554, https://doi.org/10.5194/egusphere-egu2020-3554, 2020.
Mid-ocean ridges are segmented and offset along their length. However, the kinematics of rift linkage and the initiation of oceanic transform faults in magmatic rifts remain debated. Crustal deformation patterns from the Afar continental rift provide evidences of how rifts grow to link in an area of incipient seafloor spreading. Here we present examples of rift linkage processes in Afar integrating seismicity and geodetic (InSAR and GPS) measurements, and explained by numerical and analytical models. We show that in central Afar overlapping spreading rifts link through zones of rift-perpendicular strike-slip faulting at the tips of the spreading rifts, demonstrating that distributed extension drives rift-perpendicular shearing. Conversely, in northern Afar we identify a linkage zone between the Erta Ale and Tat Ali segments where shear is accommodated by a conjugate set of oblique slip faults. There, InSAR modelling of a ML 5.1 earthquake in 2007 show that overall right-lateral shear is accommodated primarily by oblique left-lateral slip along faults subparallel to the rift segments but an active conjugate fault system with right-lateral slip is also highlighted by low-to-moderate seismicity during 2011-2013. Thermomechanical models of transform fault formation are consistent with the presence of a proto-transform fault that may develop into a throughgoing transform in the future. Our results provide evidences that offset rift segments during continental breakup can be linked by a wide variety of strain types and proto-transform zones can form before the onset of seafloor spreading.
How to cite: Pagli, C., La Rosa, A., and Illsley-Kemp, F.: Rift linkage processes in areas of incipient oceanic spreading: examples from Afar , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6828, https://doi.org/10.5194/egusphere-egu2020-6828, 2020.
During the evolution of continental rift systems, extension focuses along on-axis magmatic segments while extensional structures along the rift margins seem to progressively become inactive. However, how strain is partitioned between rift axes and rift margins is still poorly understood. The Afar Rift is the locus of extension between Nubia, Arabia and Somalia and is believed to record the latest stages of rifting and incipient continental break-up. The Afar rift axis is bounded at its western margin by a seismically active system of normal faults separating the Afar depression from the Ethiopian Plateau through a series of large bounding faults and marginal grabens. Although most of the extension in Afar is currently accommodated on-axis, several earthquakes with Mw > 5.0 occurred in the past decades on the Western Afar Margin (WAM). Here we analysed the most recent Mw 5.2 earthquake on the WAM on 24 March 2018 and the following seismic sequence using data recorded by a temporary seismic network, set up between 2017 and 2018. We located 800 events from the 20 March to the 30 April 2018 using twenty-three local seismic stations and a new velocity model for the WAM based on a new receiver function study. Preliminary results show that seismicity during the 2018 event focused at mid-to-low crustal depths (from ~15 km to ~35 km) along west-dipping fault planes. Shallower upper crustal earthquakes also occurred on west-dipping fault planes.
The hypocentral location of the mainshock has also been investigated using InSAR. We processed four independent interferograms using Sentinel-1 data acquired from a descending track. None of them shows any significant surface deformation, confirming the large depth of the hypocenters. Furthermore, we tested possible ranges of depth by producing a series of forward models assuming fault located at progressively increasing depths and corresponding to a Mw 5.2 earthquake. Our models show that surface deformations are < 1 cm at depths greater than 15 km, in agreement with our hypocentral depth of 18 km for the main shock estimated from seismic data.
Our seismicity observations of slip along west-dipping faults show that deformation across the WAM is currently accommodated by antithetic faulting, as suggested by structural geology studies. Lower crustal earthquakes might occur in a strong lower crust due to the presence of mafic lower crust and/or be induced by migrating fluids such as magma or CO2.
How to cite: La Rosa, A., Doubre, C., Pagli, C., Sani, F., Corti, G., Leroy, S., Ahmed, A., Ayele, A., and Keir, D.: Anomalously deep earthquakes in the March 2018 swarm along the Western Margin of Afar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3084, https://doi.org/10.5194/egusphere-egu2020-3084, 2020.
The geological evolution of the Mozambique Channel sensu largowas controlled by three major transfer zones (Davie, Mozambique and Agulhas/Falklands), (1) related to the migration of four continents (Africa, Madagascar, Antarctica, South America), (2) recording five major volcanic episodes from 186 Ma to today and (3) contemporaneous of the uplift of several plateaus (e.g. Southern African Plateau), affecting a quite heterogeneous lithosphere of Archean to Neoproterozoic ages. This is therefore a unique area for a better understanding of (1) the evolution of transform margins in a volcanic setting and (2) the relationships between deformation, relief growth and sediment routing evolution. We established in the frame of the PAMELA (Passive Margin Experiment LAboratory) project (TOTAL, IFREMER, CNRS) a chart and nine paleogeographic maps (with tectonic structures, magmatism, catchments and sediment routing system) to better constrain the timing of evolution of this domain. The main results are as follows:
(1) 255-240 Ma: a first E-W extension (Karoo “Rifts”) with no ocean opening;
(2) 185-160 Ma: a second NW-SE extension between Antarctica/Madagascar and Africa coeval of the Karoo Large Igneous Province and initiation of volcanic margins along the future Somali Ocean;
(3) 160-145 Ma: major change of the plate migration toward a N-S extensional initiation of very oblique margins along the Mozambique Fracture Zone (FZ), indicating an Antarctica motion toward SSE;
(4) 134 Ma: onset of the migration of the Falkland continental domain along the Agulhas FZ;
(5) 115 Ma: major deformation of the four plates with (i) end of the southward migration of Madagascar and (ii) major inversion along the Davie FZ (initiated around 135 Ma) and uplift of Madagascar;
(6) 92-70 Ma: uplift of the Southern Africa Plateau first eastward (92 Ma) and second westward (81-70 Ma);
(7) 40 Ma: onset of the uplift of the Zimbabwe/Zambia/Malawi Plateaus, East African Dome and Madagascar Plateau – last uplift of the Southern African Plateau;
(6) 11-5 Ma: acceleration of the uplift of the Zimbabwe/Zambia/Malawi Plateaus and East African Dome – growth of a dome crossing the Mozambique Channel from Madagascar to southern offshore of the Limpopo Plain.
How to cite: Robin, C., Guillocheau, F., Baby, G., Ponte, J.-P., Delaunay, A., Dietrich, P., Roche, V., Leroy, S., Revillon, S., and Dall'Asta, M.: Tectonic, magmatic and sedimentary evolution of the strike-slip margins of eastern Austral Africa (Mozambique Channel): palaeogeographic constrains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11056, https://doi.org/10.5194/egusphere-egu2020-11056, 2020.
Corbetti is currently one of the fastest uplifting volcanoes globally, with strong evidence from geodetic and gravity data for a subsurface inflating magma body. A dense network of 18 stations has been deployed around Corbetti and Hawassa calderas between February 2016 and October 2017, to place seismic constraints on the magmatic, hydrothermal and fault slip processes occurring around this deforming volcano. We locate 122 events of magnitudes between 0.4 and 4.2 were located using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the east shore of Lake Hawassa. The shallower 0-5km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in NW-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids from a cross-rift pre-existing fault. Source mechanisms are predominantly strike-slip and different to the normal faulting away from the volcano, suggesting a local rotation of the stress-field. These observations, along with a low Vp/Vs ratio, are consistent with the inflation of a gas-rich sill, likely of silicic composition, beneath Urji. In contrast, the seismicity beneath the east shore of Lake Hawassa extends to greater depth (16 km b.s.l.). These earthquakes are focused on 8-10 km long segmented faults, which are active in seismic swarms. One of these swarms, in August 2016, is focused between 5 and 16 km depth b.s.l. along a steep normal fault beneath the city of Hawassa, highlighting the tectonic hazard for the local population.
How to cite: Keir, D., Lavayssiere, A., Greenfield, T., Kendall, M., and Ayele, A.: Local seismicity near the actively deforming Corbetti volcano in the Main Ethiopian Rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21585, https://doi.org/10.5194/egusphere-egu2020-21585, 2020.
Low-temperature thermochronology has long been utilised in the Afro-Arabian Rift System (AARS) to examine exhumation cooling histories of normal fault footwalls and elucidate rifting chronologies where datable syn-rift strata and/or markers are absent. In particular, apatite fission track (AFT) and (U-Th)/He (AHe) analyses have constrained the timing and rate of rift-related, upper crustal thermal perturbations between ~30 and 120 °C (up to ~5 km depth). In turn, these provide insights into the spatio-temporal evolution of individual rift basins, morphotectonic rift shoulder development, normal fault system growth and, in some cases, the thermal influence of igneous intrusions and circulation of hot fluids. However, the relatively limited number of samples and confined areas generally involved in individual case studies have precluded insights into longer wavelength tectonic and geodynamic phenomena, such as regional denudation trends and the growth of topography due to plume impingement.
Here, we present a synthesis of >2000 apatite fission track (AFT) and ~1000 (U-Th)/He (AHe) analyses from the Eocene-Recent AARS collated using LithoSurfer, a new cloud-based geoscience data platform. This continental-scale low-temperature thermochronology synthesis, the first of its kind in Africa, provides novel insights into the upper crustal evolution of the AARS that were previously difficult to decipher from an otherwise cumbersome and intractably large dataset. The data record a series of pronounced episodes of upper crustal cooling related to the development of the Red Sea, Gulf of Aden and East African Rift System (EARS). They also provide insights into the inherited tectono-thermal histories of these regions which controlled the spatial and temporal distribution of subsequent extensional strain.
Thermochronology data trends along the AARS reflect a combination of rift maturity, structural geometry and geothermal regime, intrinsically linked to lithospheric architecture and magmatic activity. These relationships are best illustrated by contrasting the upper crustal thermal evolution of different AARS segments of varying age and complexity: for example, between the nascent Okavango, mature Ethiopian and evolved Red Sea rifts, wide (e.g. Turkana Depression) versus narrow (e.g. Main Ethiopian Rift) zones of deformation, between areas of transtensional (Dead Sea Transform), oblique (e.g. Gulf of Aden) and sub-orthogonal rifting (e.g. Malawi Rift), and the magmatic eastern versus amagmatic western branches of the EARS.
A regional interpolation of standardised thermal history models generated from the mined AFT, AHe and, in some cases, vitrinite reflectance data yield Mesozoic-recent heat maps, extrapolated to produce paleo-denudation and burial histories for eastern Africa and Arabia. Integrating these thermotectonic images with other regional datasets allows for the interrelationship between tectonic and dynamic topography development, the denudation history of the land surface, and sediment transport and deposition to be explored in new ways.
How to cite: Boone, S., Kohlmann, F., Balestrieri, M.-L., McMillan, M., Kohn, B., Noble, W., Mackintosh, V., and Gleadow, A.: Thermo-tectonic imaging of the Afro-Arabian Rift System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12452, https://doi.org/10.5194/egusphere-egu2020-12452, 2020.
The tectonic evolution of the southern North Atlantic is a subject of increasing interest due to its continental margins playing host to several world-class frontier regions for oil and gas exploration. The Newfoundland-Iberia conjugate margin pair serves as one of the best studied non-volcanic rifted conjugate margin pairs in the world, and is a topic of constant scientific debate due to its complex plate kinematic history and geological evolution. Recent adaptability of the GPlates freely available plate tectonic reconstruction software provides an excellent tool for gaining insight into complex plate kinematic problems. The ability to account for regions of deformation, integration of various geological and geophysical datasets, and the ability to calculate temporal variations in crustal thickness, strain rates, and velocity vectors provide an optimal environment for solving crustal-scale geological and geophysical problems. Building upon previous rigid and deformable plate tectonic modelling studies, the aim of this work is to create deformable plate tectonic models of Iberia with emphasis on the West Iberian margin and the Pyrenees to assess Iberia’s evolution during the formation of the southern North Atlantic from 200 Ma to present day. A comparison of crustal thickness results calculated from GPlates models with those obtained from gravity inversion, passive and controlled source seismology, and geological field mapping, provided a good metric for investigating the plate kinematics of Iberia and assessing previous discrepancies when considering the crustal evolution of the West Iberian margin and the Pyrenees as an integrated plate kinematic system. Results from the GPlates models produced in this study also demonstrate the significance of continental fragments and their independent motion during rifting. In particular, we investigate the independent motion of the Galicia Bank and its role with respect to the deformation experienced within the Galicia Interior Basin and the role of the Ebro Block and Landes High during deformation prior to the Pyrenean Orogeny. In addition, this study highlights the importance of inherited structures with respect to the styles of deformation experienced during rifting of continental crust. Preliminary deformable plate modeling results of the West Iberian margin indicate that the independent motion of the Galicia Bank and its interplay with inherited structures is crucial for deriving the amount of deformation inferred by gravity inversion and regional seismic studies within the Galicia Interior Basin.
How to cite: King, M., Welford, K., and Peace, A.: Investigating the role of Iberia and its interplay with the Newfoundland and Irish offshore margins using plate reconstructions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3701, https://doi.org/10.5194/egusphere-egu2020-3701, 2020.
The hyper-extended Porcupine Basin, offshore southwest Ireland, is a component of the Eastern North Atlantic rifted continental margin. The basin developed following multiple rifting phases with different extension directions between the Late-Palaeozoic and the Cenozoic. The present-day north-south trend of the Porcupine Basin developed during the main Middle-Jurassic to Lower-Cretaceous rifting phase, which is interpreted to have overprinted earlier extension directions. In this study, we outline the tectono-stratigraphic architecture and the kinematics of the Porcupine Basin derived from seismic interpretation and fault analysis of multiple 2D and 3D seismic datasets.
Our ongoing work identifies different fault networks with distinctive orientations and ages, confirming multiple rifting phases of the basin. The older faults identified strike NE/SW and offset the top of the Jurassic basement and the oldest syn-tectonic sequences. The younger faults strike N/S, offset the whole syn-tectonic stratigraphic sequence and bound the present-day tilted blocks of thinned continental crust. Interactions between these two main generations of faults created strong lateral variability in the geometry of the fault-bounded blocks. In addition, our interpretations highlight strong segmentation along the axis of the basin, evidenced by changes in the structural architecture of the faults along the flanks of the basin, and by rapid changes in the depth to the Jurassic basement from one segment to another. This segmentation occurs across several lineaments that are orthogonal to the main N-S direction of the tilted blocks observed along the flanks of the basin, and that are also observed in the central parts of the basin with gravity data and by the compartmentalisation of sedimentary depocenters. We interpret these lineaments as transfer zones that can be related to the kinematics of the basin. These zones may have accommodated either temporal or spatial changes in the directions of extension, or a stepped variation in E-W extension along the axis of the basin.
Our results help to better understand the controls on the geometry and kinematics of fault systems within the Porcupine Basin, and to better evaluate the structural evolution of the Porcupine Basin and its significance in the broader context of the North Atlantic rifting.
How to cite: Lymer, G., Roche, V., Saqab, M., Childs, C., and Walsh, J.: Multi-phase development of the Porcupine Basin during the rifting of the North-Atlantic , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5065, https://doi.org/10.5194/egusphere-egu2020-5065, 2020.
In past years, a good understanding of the structure and tectonics of the Flemish Cap and the Goban Spur margin has been obtained based on seismic data, potential field data, and borehole data. However, due to limited data coverage and quality, the rift-related domains along the margin pair have remained poorly defined and their architecture has been primarily delineated on the basis of a small number of co-located 2-D seismic profiles. In addition, according to previous studies, the geophysical characteristics (e.g. velocity structure, crustal thickness, seismic patterns, etc.) across both the margins are strikingly different. Furthermore, from restored models of the southern North Atlantic, some scholars argue against the linkage of the Goban Spur and the Flemish Cap, questioning the widely-accepted “conjugate” relationship of the two margins. However, these restored models are mainly dependent on potential field data analysis, lacking seismic constraints, particularly for the Irish Atlantic Margin.
In this study, new long offset 2D multichannel seismic data, acquired in 2013 and 2014 by Eni Ireland for the Department of Communications, Climate Action & Environment of Ireland, cover the shelf, slope, and deepwater regions of the offshore Irish Altlantic margin. Combining these with seismic reflection data at the NE Flemish Cap, seismic refraction data, DSDP drilling sites, gravity and magnetic maps, crustal thickness maps, and oceanic isochrones, we integrate all constraints together to characterize the structure and evolution of both margins. These geophysical data reveal significant along-strike structural variations along both margins, and aid to delimit five distinct crustal zones related to different rifting stages and their regional extents. The geometries of each crustal domain are variable along the margin strike, probably suggestive of different extension rates during the evolution of the margin and/or inherited variations in crustal composition and rheology. Particularly, the along-strike exhumed serpentinized mantle domain of the Goban Spur margin spans a much wider (~ 42 - 60 km) area while it is much narrower (~25 km) at the NE Flemish Cap margin. In the exhumed domain, only peridotite ridges are observed at the Flemish Cap, while both peridotite ridges and a wide region of exhumed mantle with deeper basement are observed at the Goban Spur, indicative of a more complex evolutionary model than previously thought for both margins. Plate reconstruction of the Goban Spur and the Flemish Cap using GPlates reveals asymmetry in their crustal architectures, likely due to rift evolution involving more 3-D complexity than can be explained by simple 2-D extensional kinematics. In spite of uncertainties, the crustal architecture comparison between the two margins provides 3D seismic evidence related to the temporal and spatial rifting evolution on both sides.
How to cite: Yang, P. and Welford, J. K.: Revealing tectonic evolution across the Northeastern Flemish Cap-Goban Spur margin , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5614, https://doi.org/10.5194/egusphere-egu2020-5614, 2020.
The rifting of the Eastern Sardinian margin is considered to have occurred during the Neogene, by back-arc extension related to the eastward migration of the Apennine subduction system, leading to the development of the Tyrrhenian Basin, Western Mediterranean Sea. The locus of the extension during the rifting is interpreted to have occurred first in the East-Sardinia Basin (proximal margin), and then in the Cornaglia Terrace (distal margin). However, the dynamics of the Western Tyrrhenian Basin during the rifting is still largely undated, and the kinematics of the development of the different domains of the Eastern Sardinian margin remains poorly understood. This is due to the sparsity of dated rock samples within the basin, mainly recovered locally during ODP Leg 103, and because the timing of activity of the structures of the basin has been addressed so far using regional data, whose resolution is insufficient to observe the timing of the extension at the scale of a fault plane.
In this study, we use a 2400 km-long high-resolution seismic-reflection dataset acquired along the Eastern Sardinian margin during the “METYSS” research cruises in 2009 and 2011, and specifically designed to observe the block-bounding faults and the syn-rift and post-rift units. We interpret the syn-tectonic markers of the crustal deformation across the Eastern Sardinian margin, and we use the seismic markers of the Messinian Salinity Crisis (MSC) that provide exceptional and accurate natural time markers, to estimate the age of faults development and understand their evolution within the Western Tyrrhenian Basin.
Our observations demonstrate that the rifting was polyphased across the Western Tyrrhenian Basin, and that syn-rift extension was active on the proximal margin long before the MSC, whereas the distal Cornaglia Terrace developed only a short time before the MSC. Surprisingly, our interpretations also evidence significant post-rift reactivation of some structures of the Eastern Sardinian margin. These reactivations, formerly considered to be very minor or absent in the Western Tyrrhenian Basin, started in the Pliocene and occurred up to very-recent times along local fault-planes, as shown by the deformation of the shallowest Pleistocene layers and the seafloor.
Our results permit to precise the timing of the syn-rift and post-rift evolution of the Eastern Sardinian margin. Given the rarity of natural time makers in offshore sedimentary basins to address the dynamics of basins development, we expect our results to provide useful comparisons to study the kinematics of extension at other back-arc basins worldwide.
How to cite: Gaullier, V., Lymer, G., Chanier, F., Maillard, A., Thinon, I., and Sage, F.: Back-arc extension dated from natural time markers across the Western Tyrrhenian Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9739, https://doi.org/10.5194/egusphere-egu2020-9739, 2020.
Although so-called "magma-poor" rifted margins display a large variability on a local scale, they are characterized by a number of common primary features worldwide such as their first-order architecture (proximal, necking, hyperextended, exhumation and oceanic domains), their lithological evolution along dip and the deformation processes associated with their different rifting stages. In this contribution, we first emphasize the primary morphological and lithological architecture of magma-poor rifted margins and how they relate to specific deformation modes (pure shear thinning, mechanical necking, frictional extensional wedge, detachment faulting and seafloor spreading). Second, we focus on the necking stage of rifting, which corresponds to the first major thinning event (when the crust is thinned from its initial thickness to ~ 10 km). We display the range of possible topographic and thermal evolutions of "magma-poor" and "sedimentary starved" rift systems depending on their lithosphere rheology. Our two-dimensional thermo-mechanical numerical models suggest that extension of lithospheres where the crust and the mantle are mechanically decoupled by a weak lower crust results in a complex morphotectonic evolution of rift systems, with formation of temporary restricted sub-basins framed by uplifted parts of the future distal margin. Mechanical decoupling between the crust and the mantle controls also largely the thermal evolution of rift systems during the necking phase since for equivalent extension rates and initial geotherms: (i) weak/decoupled lithospheres have a higher geothermal gradient at the end of the necking phase than strong/coupled lithospheres; and (ii) weak/decoupled lithospheres show intense heating of the lower crust at the rift center and intense cooling of the crust on either side of the rift center, unlike strong/coupled lithospheres. These behaviors contrast with the continuous subsidence and cooling predicted by the commonly used depth-uniform thinning model.
How to cite: Chenin, P., Manatschal, G., Schmalholz, S. M., and Duretz, T.: Primary deformation phases during "magma-poor" rifting with special focus on the tectono-thermal evolution during the necking process, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7136, https://doi.org/10.5194/egusphere-egu2020-7136, 2020.
The structural inheritance of the basement plays an important role controlling rift formation and evolution. Here we investigate tectonic and rheological inheritance on brittle reactivation of the Precambrian basement and shear zones in the formation and evolution of the Cretaceous Araripe Basin. The basin is a part of the Northeast Brazilian Rift System, associated with the junction of the Southern and Equatorial branches of the Atlantic Rift. Its basement is part of Neoproterozoic Transversal Zone (Borborema Province), a crustal scale transpressional duplex system, related to the Brasiliano escape tectonic events.
We present here a synthesis of field observations from the Araripe Basin and its adjacent basement, combined with topographic, aeromagnetic and seismic data to propose a general overview on the tectonic framework and evaluate how it influenced the basin initiation and evolution. Our integrated analysis shows that there are three main structural trends for the basin and its surroundings: NE-SW, E-W and NW-SE. The NE-SW and E-W trends are the most expressive sets of lineaments in the topographic and aeromagnetic data, directly related to the basement framework. Integration of seismic data and filtered aeromagnetic maps confirms that NE-SW and E-W trends represent oblique fault systems.
Archean, Paleoproterozoic, Mesoproterozoic and Neoproterozoic terranes are arranged side by side in NE-SW mega sigmoid, bounded by the E-W Pernambuco (to the south) and Patos (to the north) dextral shear zones. The Araripe basin units are distributed mostly in two sub-basins, Cariri and Feira Nova, separated by a structural high, controlled by NE-SW and ESE-WNW faults. Analyzing these terranes and their link to the distribution of the depocenters and structures, we find that the NE trending Archean terrane coincides partially with the Feira Nova NE-SW single graben. On the eastern portion of the basin, the graben system is much wider and controlled by NE-SW and ESE-WNW trending fault systems. This wide graben overlies a Neoproterozoic basement terrane constituted by a supracrustal unit (Cachoeirinha Group) of phyllites, metasandstones, metavolcanics with low to medium metamorphic grade.
This evidence corroborates with the hypothesis that the rheology of the upper crust might be partially influenced by distinct lithotectonic terranes. The older Archean block sustained the narrow sub-basin, indicating a more localizing behavior, while the younger Neoproterozoic terrane, controlled a less localizing graben system with a wider sub-basin in the eastern Araripe basin.
The authors gratefully acknowledge support from Shell Brasil Petroleo Ltda. and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation (Technical Cooperation #20.219-2).
How to cite: Richetti, P., da Silva Schmitt, R., César Araújo, B., Filipa da Gama, M., and Teixeira da Costa Soares, M.: Basement inheritance affecting initiation and evolution of intracontinental rifts: Araripe Basin, northeast Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22477, https://doi.org/10.5194/egusphere-egu2020-22477, 2020.
During the Mesozoic, the relative movement of African and Eurasian plates caused the opening of the Tethys Ocean. The rifting phase is well charted by the stratigraphic sequence of Western Alps, which provide an exceptional record of continental margin evolution. The Briançonnais domain occupies a pivotal place for testing various rifting models. This domain contains a remarkably uniform succession of very shallow-water carbonates of Triassic age, capped by Middle-Jurassic shallow-water carbonates or by non-deposition before passing abruptly up into deep-water facies. Here we show that the back-stripped Mesozoic tectonic evolution of the Briançonnais block can be applied to investigate models of lithospheric stretching. Applying the Airy correction, we found that the Triassic is characterised by a constant tectonic subsidence rate of 17 m/Ma. If this is the result of “post-rift” thermal re-equilibration of upper mantle after late Palaeozoic rifting, this rift phase occurred with a stretching factor of c 1.4. That this thermal subsidence was modulated by differential uplift and erosion of the Briançonnais in the early Jurassic implies significant mantle thinning, reducing net density of the Briançonnais lithosphere. The subsidence of more than 3000m during Bathonian-Callovian stages are too rapid to be explained by thermal re-equilibration: it suggests substantial crustal thinning. Our results demonstrate that a uniform stretching model is not able to explain the Jurassic isostatic movement of the Briançonnais domain. It is consistent with two-stage, depth-variable stretching of the Briançonnais lithosphere during the Jurassic. Our study represents a starting point for more sophisticated and developed numerical models, to explain rapid vertical movements in hyper-extended continental margins.
How to cite: Forzese, M., Butler, R. W. H., Stephenson, R., and Maniscalco, R.: Backstripping the Briançonnais (Western Alps) to test Alpine rifting models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-409, https://doi.org/10.5194/egusphere-egu2020-409, 2020.
The Influence of lithosphere and basement properties on the stretching factor and the development of extensional faults across the Otway Basin and eastern Bight Basin
- KHARAZIZADEH*, W.P. SCHELLART, J.C. DUARTE
School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia
Department of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, Netherlands
Instituto Dom Luiz (ILD) and Geology Department, Faculty of Sciences of the University of Lisbon, Campo Grande, Lisbon, Portugal
The large southern continental margin of Australia, with a wide variety of sedimentary basins, formed during Mesozoic rifting. The evolution of sedimentary basins is mainly controlled by plate tectonic activity and the mechanism of continental extension. This work presents a comparative study between two main depocentres of the Bight Basin (Ceduna, Duntroon sub-basins) and the Otway Basin. Here, the total amount of extension (∆L) and stretching factor (β) have been measured across the Otway Basin and eastern Bight Basin. The results show significant variation in extensional stretching along the basins, with the smallest stretching factors in the Ceduna and Duntroon sub-basins (1.2<β<1.4), and the largest amount of extension (~ 177 km) and the largest stretching factor (β=1.85) in the eastern part of the passive margin. The regions with the lowest β factor are underlain mostly by thicker lithosphere, while the regions with the largest β factor and amount of extension are related to younger and thinner lithosphere. The main basement structures have been mapped throughout South Australia and Victoria to examine the possible relationships between the new pattern of extensional faults and old basement fabrics. The distribution pattern of normal faults varies considerably along onshore and offshore components of basins. It is proposed that in some regions fault strike varies due to changes in orientation of pre-existing structures in the basement. For example, the north-south Coorong Shear Zone seems to affect the geometry of normal faults by changing their strike from E-W to NW-SE and also, in the easternmost part of the basin, the Bambra Fault changes the strike of normal faults to the NE-SW. Also, the NE-SW basement structures in the western part of the Gawler Craton have some control on normal faults in the western Ceduna sub-basin. Normal faults in the easternmost and westernmost parts of the Otway Basin have a similar orientation to the basement faults. However, in most regions basement faults are perpendicular to the normal faults and there is a minor influence on the new pattern of faulting. Our results imply that the properties of the continental lithosphere (age, thickness and strength of lithosphere) exert a major influence on the β factor and amount of crustal extension but only a minor influence on the geometry of extensional faults.
Keywords: Otway Basin, Ceduna and Duntroon sub-basins, rifting, total amount of extension, β factor, normal faults, lithosphere properties
How to cite: kharazizadeh, N.: The Influence of lithosphere and basement properties on the stretching factor and the development of extensional faults across the Otway Basin and eastern Bight Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1976, https://doi.org/10.5194/egusphere-egu2020-1976, 2020.
Continental rifting preceding stable seafloor spreading is characterized by a multistage evolution during lithosphere extension. Wide regions of exhumed mantle contain linear magnetic anomalies with a strongly debated nature and origin. Contrasting information used to set up dynamic plate models has resulted in a plethora of alternative interpretations. Structural and stratigraphic records at plate boundaries show indeed variable degree of discrepancies with what expected from computed plate motions during rifting stages. The definition of robust spatial and temporal kinematic constraints using combined offshore and onshore approaches represents a major challenge to unravel rifted margins evolution.
In this study, we address the problem outlined above using the Mesozoic southern North Atlantic and the Alpine Tethys, west and east of the Iberian plate, as a natural laboratory. The two systems are part of the same Africa-Europe kinematic framework and record distinctive Mesozoic rift events and a subsequent Tertiary compression. While in the southern North Atlantic the kinematic framework is still preserved, in the Alpine Tethys, subsequent subduction/collision erased the paleogeographic framework. The study area is among the best investigated but also most debated geological domains on the globe.
In our analysis we (1) integrate rift domains in plate kinematic models and re-consider the nature of the magnetic anomaly J in the southern N-Atlantic; (2) discuss the results of recent studies in the northern part of the Iberian plate; and (3) show new data from the Alpine Tethys realm (Central European Alps and Southern Apennines). We discuss the implications of these observations for the geometry of the rift systems developed around Iberia.
Our robust data network radically reduces the range of possible kinematic solutions. We reconstruct thus the position of Iberia and Adria relative to Europe and Africa and we evaluate the kinematic evolution and the width of the southern North Atlantic and the Alpine Tethys domains during the Mesozoic. The analysis emphasizes (1) the stepping geometry of the plate boundary for the Atlantic-Tethys interaction, (2) the strong partitioning of deformation in time and space, and (3) the large-scale pattern of coeval compression and extension along the Africa-Europe diffuse plate boundary region.
How to cite: Frasca, G., Manatschal, G., and Cadenas Martínez, P.: Kinematic restoration of the southern North Atlantic and the Alpine Tethys during the Mesozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9977, https://doi.org/10.5194/egusphere-egu2020-9977, 2020.