ALBERT

Description: The initial opening of the Africa-Antarctica Corridor, in the heart of Gondwana, is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the Jurassic. Therefore, in 2014, new ship-borne magnetic data were systematically acquired in the northern Mozambique Basin and across Beira High, which we merged with all accessible magnetic data in the Mozambique Basin. Herein, distinct magnetic lineations are observed, which allow a refined identification of a whole set of Jurassic magnetic spreading anomalies, constraining the timing of the onset of oceanization, beginning at M38n.2n (164.1 Ma). In combination with high-resolution potential field data from the conjugate Antarctic margin, well-expressed fracture zones can be traced throughout the Africa-Antarctica Corridor and allow the precise rotation of Antarctica back to Africa. The initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account. Within a tight Gondwana fit, the Beira High can be restored along the major sinistral Namama-Orvin Shear Zone of the East African-Antarctic Orogen. The Beira High represents a continental block, which was detached from Antarctica, by 157 Ma at the latest. Simultaneously, the Antarctic plate cleared the area of the MCP. However, the crustal nature of the southern MCP remains ambiguous. The Northern Natal Valley and the Mozambique Ridge consist of thick oceanic crust, being emplaced between M26r-M18n (157.1–144 Ma) and M18n-M6n (144–131.7 Ma), respectively. About the half of this crust was won from the Antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the Explora Wedge. A refined kinematic break-up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the Africa-Antarctica Corridor and the Somali Basin.

Description: SUMMARY As 3-D geological models become more numerous and widely available, the opportunity arises to combine them into large regional compilations. One of the biggest challenges facing these compilations is the connection and alignment of individual models, especially in less explored areas or across political borders. In this regard, gravity modelling is suitable for revealing additional subsurface information that can support a harmonization of structural models. Here, we present an integrated geological and gravity modelling approach to support the harmonization process of two geological 3-D models of the North German Basin in the cross-border region between the federal states of Saxony-Anhalt and Brandenburg. Gravity gradient calculation, filtering and Euler deconvolution are utilized to reveal new insights into the local fault system and gravity anomaly sources. The independent models are merged and harmonized during 3-D forward and inverse gravity modelling. Herein, density gradients for individual layers are incorporated in the framework of model parametrization. The resulting geological 3-D model consists of harmonized interfaces and is consistent with the observed gravity field. To demonstrate the plausibility of the derived model, we discuss the new geophysical findings on the sedimentary and crustal structures of the cross-border region in the context of the regional geological setting. The cross-border region is dominated by an NW–SE oriented fault system that coincides with the Elbe Fault System. We interpret a low-density zone within the basement of the Mid-German Crystalline Rise as a northward continuation of the Pretzsch–Prettin Crystalline Complex into the basement of the North German Basin. Additionally, we observe two types of anticlines within the basin, which we link to provinces of contrasting basement rigidity. Our gravity modelling implies that the Zechstein salt has mostly migrated into the deeper parts of the basin west of the Seyda Fault. Finally, we identify a pronounced syncline that accommodates a narrow and up to 800 m deep Cenozoic basin.

Description: The initial opening of the Africa-Antarctica Corridor, in the heart of Gondwana, is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the Jurassic. Therefore, in 2014, new ship-borne magnetic data were systematically acquired in the northern Mozambique Basin and across Beira High, which we merged with all accessible magnetic data in the Mozambique Basin. Herein, distinct magnetic lineations are observed, which allow a refined identification of a whole set of Jurassic magnetic spreading anomalies, constraining the timing of the onset of oceanization, beginning at M38n.2n (164.1 Ma). In combination with high-resolution potential field data from the conjugate Antarctic margin, well-expressed fracture zones can be traced throughout the Africa-Antarctica Corridor and allow the precise rotation of Antarctica back to Africa. The initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account. Within a tight Gondwana fit, the Beira High can be restored along the major sinistral Namama-Orvin Shear Zone of the East African-Antarctic Orogen. The Beira High represents a continental block, which was detached from Antarctica, by 157 Ma at the latest. Simultaneously, the Antarctic plate cleared the area of the MCP. However, the crustal nature of the southern MCP remains ambiguous. The Northern Natal Valley and the Mozambique Ridge consist of thick oceanic crust, being emplaced between M26r-M18n (157.1–144 Ma) and M18n-M6n (144–131.7 Ma), respectively. About the half of this crust was won from the Antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the Explora Wedge. A refined kinematic break-up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the Africa-Antarctica Corridor and the Somali Basin.

Description: The timing and geometry of the initial Gondwana break-up between Africa and East Antarctica is still poorly known due to missing information about the continent-ocean boundaries along the rifted margins. In this context, the Beira High off central Mozambique forms a critical geological feature of uncertain crustal fabric. Based on new wide-angle seismic and potential field data across Beira High a P-wave velocity model, supported by amplitude and gravity modelling, provides constraints on the crustal composition of this area. In the Mozambique Basin mainly normal oceanic crust of 5.5–7 km thickness with velocities of 6.5–7.0 km/s in the lower crust is present. A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23 km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0 km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsicmaterial in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 12 km of sediments cover the underlying 7 km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4 km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta and consequently the landward position of the continentocean boundary remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage.

Description: A consolidated knowledge of the formation and dispersal of the former supercontinents reveals important evidence for the earth’s climate and biosphere in the past and contribute to the prediction of their future evolution. Nowadays, a main objective is the investigation of the initial break-up of the continental assembly of Gondwana that serves as a constraint for its subsequent dispersal and the evolution of all oceans and seas in the southern hemisphere. Evidence of the early rifting stages are expected at the margins of Southeast Africa and East Antarctica, whereas the latter one is difficult to access, due to its remote position and ice coverage. To understand the driving forces and the chronology of the break-up and of the massive volcanism additional detailed knowledge of the crustal setting along the margins of Southeast Africa is required. Therefor, a new geophysical dataset was acquired with the RV Sonne in the northern Mozambique Basin at the beginning of year 2014. This comprises a deep seismic sounding profile across a so-far unknown structural high, the Beira High. Additional gravity and magnetic data were systematically recorded across the entire northern Mozambique Basin. Based on velocity, amplitude, density and magnetic modelling, a geological model of the continental margin of Central Mozambique was prepared. A new compilation of all available magnetic data in the Mozambique Basin reveals information about the age of the sea floor, which serves as constraint for the reconstruction of the initial Gondwana break-up. The study depicts a continental origin of up to 23 km thick and partly highly intruded crust at Beira High. In the adjacent coastal areas of the south-western part of Central Mozambique, 7 km thin crust is observed, which is covered by more than 11 km thick sediments and implies the continuation of the continent-ocean transition towards onshore Mozambique. This is in clear contrast to the narrow transition observed in the north-eastern part of the margin and reveals a clear asymmetric crustal setting, as supposed for the conjugate margin in the Riiser-Larsen Sea in Antarctica and consequently suggests a complex break-up scenario. The presence of a pronounced high-velocity lower crustal body is interpreted as magmatic material, which underplates the crust and extends about 200 km from the Central Mozambican margin towards the Mozambique Basin and testifies for the massive volcanism during the break-up. The distribution of further volcanics along the entire margin clearly depicts the continuation of the north-eastern branch of the Karoo large igneous province and are mainly emplaced between 177-157 Ma. The magmatism in Southeast Africa seemed to be continuous throughout the initial break-up, which points to the presence of either a mantle plume or a thermal anomaly as source of the giant magmatism. An additional late stage of rift-volcanism mainly affected the margin of Dronning Maud Land and causes the difference in the magnetic signature of the conjugate margins. The tracing of continuous fractures throughout the Africa-Antarctica Corridor leads to the reconstruction of a tight Gondwana fit prior rifting, which reveals several geological links between the plates. A main structure of the East African-Antarctic Orogen extends from the Namama Shear Zone in Central Mozambique across the Orvin Shear Zone towards the Forster magnetic anomaly in Dronning Maud Land. During the initial Gondwana break-up at 182 Ma, Beira High started to separate from West Gondwana along this suture until it demerged as well from East Gondwana by a rift jump. The investigation of further partly unknown tectonic structures along the western and southern coast of Mozambique revealed a possible oceanic origin of the southern part of the Mozambique Coastal Plains, due to similarities of the magnetic signature to the oceanic crust south of Beira High as well as the tentative identification of magnetic spreading anomalies. The subsequent emplaced Mozambique Ridge moved southwards as part of a micro plate during an additional active spreading centre in the Northern Natal Valley. The resulting reconstruction of the initial Gondwana break-up in the Africa-Antarctica Corridor accounts for all present-day available geological, geophysical and geodynamic constraints and might serve as a basis for the investigation of the subsequent dispersal of Gondwana.

Description: Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Since some 185 Ma the rifting in our research area caused the dispersal of Gondwana and Eastern Africa. The timing and geometry of the break-up as well as the amount of volcanism connected to the Jurassic rifting are still controversial. In the southern part of the Mozambique channel a prominent basement high, the Beira High, forms a specific crustal anomaly along the margin. It is still controversial if this high is a continental fragment or was formed during a period of enhanced magmatism. Therefore a deep seismic profile with 37 OBS/H was acquired from the deep Mozambique Channel, across the Beira High and terminating on the shelf. The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the continent-ocean transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data was forward modelled by means of 2D-Raytracing. Furthermore, potential field data acquired in parallel to the seismic data were used to calculate a 2D gravity model. Preliminary results indicate a 20-24 km thick crust for the Beira High. In good agreement to the adjacent oceanic crust in the Mozambique Channel the upper crust has velocities between 5.5-5.9 km/s. The middle crust is characterised by velocities between 6.2-6.7 km/s and the lower crust higher than 6.7 km/s and a density of 3.0 g/cm3. However, these velocities are only constrained by Moho reflections, since no diving waves are observed for the lower crust. In the area of the Zambezi Delta Depression the top of the acoustic basement is at 11.5 km depth and the crust thickness thins to 7 km. The basement here is overlain by a 2 km thick layer of 4.9-5.1 km/s, which we interpret as pre-rift sediments (Karoo-Belo-Group, including Lava Flows on top). Furthermore, evidence for the presence of a high velocity body (HVB) at below the western part of Beira High with a velocity of 7.2-7.4 km/s and 3 km thickness is found. Below the shelf our results indicate evidences for an increased volcanism during the initial break-up. The location of the continent-ocean boundary as well as the geometry of the break-up depend strongly on the tectonic classification of Beira High. Future work will provide further constraints by amplitude modelling, a 3D gravity model of Beira High and by means of interpretation of the magnetic anomalies.

Description: Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Some 185 Ma the onset of rifting caused the dispersal of this vast continent into several minor plates. The timing and geometry of the initial break-up between Africa and Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still controversial. In the southern part of the Mozambique Channel a prominent basement high, the Beira High, forms a distinct crustal anomaly along the Mozambican margin. It is still controversial if this area of shallow basement is a continental fragment or was formed during a period of enhanced magmatism and is of oceanic origin. Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi River. The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the position of the continent-ocean transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data were forward modeled by means of the 2D-Raytracing method. Preliminary results indicate a clear thickening of the crust below the Beira High up to 20-24 km. Evidences for a high velocity body are found in the area below the Zambezi shelf with velocities of 7.2-7.4 km/s and up to 5 km thickness. Oceanic basement velocities at the very eastern part of the line start with values of 5.5 km/s, and increase to 6.9 km/s at lower crustal levels, that are typical for Jurassic oceanic crust. Across the Beira High the starting velocity and its gradient slightly change, presenting typical values for continental fragments. However, due to a sparse ray coverage of diving waves for the Beira High lower crust, these velocities still have to be proved. Thus, we will introduce the final results of a Finite Difference amplitude modeling, which will constrain the lowermost velocity gradients to allow a sound interpretation of the Beira High origin. The acquired shipborne, magnetic data show a complex magnetic pattern and strong influences by the presence of lava flows and intrusions and require further investigations. We will introduce the latest results of the joint interpretation of seismic and potential field data sets.

Description: Main objective of the project is the investigation of the crustal structure of the margin of Mozambique. This will improve our understanding of the driving forces and processes leading to the initial Gondwana break-up. Some 185 Ma the onset of rifting caused of the opening of the Mozambique and Somali Basin and the dispersal of this vast continent into several minor plates. The timing and geometry of the initial break-up between Africa and Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still controversial. However, the conjugated margin in the Riiser-Larsen Sea is covered by an up to 400 m thick ice cap, precluding the set-up of a deep seismic experiment in this area. Consequently, the investigations focus on the continental margin of central Mozambique. Here, a prominent basement high, the Beira High, forms a critical geological feature of uncertain crustal fabric. It is still controversial if this area of shallow basement is a continental fragment or was formed during a period of enhanced magmatism and is of oceanic origin. Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi River (Fig. 1). The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the position of the continent-ocean transition along the margin of central Mozambique. To obtain a P-wave velocity model of this area the data were forward modelled by means of the 2D-Raytracing method, supported by an amplitude and gravity modelling. In the Mozambique Basin mainly normal oceanic crust of 5.5–7 km thickness with velocities of 6.5–7.0 km/s in the lower crust is present (Fig. 2). A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23 km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0 km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsic material in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 11 km of sediments cover the underlying 7 km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4 km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage. The acquired shipborne magnetic data complement our dataset in the Mozambique Basin and reveal clear evidence for the presence of lava flows and intrusions, pointing to an increased break-up related magmatism.