ISEL - Física
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Browsing ISEL - Física by Author "Afilhado, Alexandra"
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- From the lebombo monocline to the Mozambique deep basin, using combined wide-angle and reflection seismic dataPublication . Moulin, Maryline; Leprêtre, Angelique; Verrier, Fanny; Schnürle, Philippe; Evain, Mikael; Clarens, Philippe; Thompson, Joseph; Dias, Nuno; Afilhado, Alexandra; Loureiro, Afonso; Aslanian, DanielThe North Natal valley (NNV), South Mozambique margin, is a key area for the understanding of the SW Indian Ocean history since the Gondwana break-up as its crustal nature and geometry strongly impacted the recon-struction of the paleogeography before the rifting. It is also of considerable importance for the understanding of the evolution of a margin system as the NNV is situated at the transition between divergent and strike-slip segments and at the conjunction of Oxfordian-Kimmeridgian Indian Ocean and the Valanginian-Aptian Atlantic one. As one part of the PAMELA project (PAssive Margins Exploration Laboratories), the NNV and the East Limpopo margin have been investigated during the MOZ3/5 cruise (2016), through the acquisition of 7 intersecting wide-angle profiles and coincident marine multichannel (720 traces) seismic as well as potential field data. Simultaneously, land seismometers were deployed in the Mozambique coastal plain (MCP), extending six of those profiles on land for about 100 km in order to provide information on the onshore-offshore transition. Wide-angle seismic data are of major importance as they can highlight constraints on the crustal structure of the margin and the position of the continent-ocean boundary in an area where the crustal nature is poorly known and largely controversial. The MOZ3/5 data set therefore reveals new essential constraints for kinematic re-constructions. This work presents results on the crustal structure from P-waves velocity modeling along two E-W wide-angle profiles (MZ1 and MZ2) through the NNV, from the Lebombo Monocline to the Mozambique Basin (MB), and crossing the Mozambique Fracture Zone (MFZ). The new geophysical data reveals an upper sedimentary sequence characterized by low velocities generally not exceeding 3 km/s, and up to 3 km thick where a major contouritic structure was observed. This feature formes together with several other contouritic structures, a N-S alignment just west of the MFZ, which produces high positive gravity anomalies, previously thought to be related to the magmatism that built the Galathea and Dana Plateaus. High velocity lenses are locally identified through the sedimentary layers and interpreted as inter-bedded volcanic sills. Furthermore, from the NNV to the MFZ, the underlying sequence is formed of a 3.0-3.5 km thick volcano-sedimentary sequence presenting important lateral changes in its seismic signature and characterized by a large velocity range (4.4 to 5.8 km/s), which partly reflects variations in the volcanic/sedi-mentary ratio laterally and with depth. At depth, an initially smoother and reduced eastward thinning of the crust occurs to the West below the continental shelf, from 34 to 31 km thick. The crustal thickness remains relatively constant of about 28-29 km along the Central Domain (CD), whereas a second and major region of thinning (26 to 12 km thick) is imaged West of the MFZ, in the southward prolongation of the Limpopo Corridor (LC). By contrast, as the eastern extremity, the crust is <10 km thick when reaching the MB. Crustal velocities reveal low velocity gradients, with atypical high velocities, increasing to 7.3 to 7.6 km/s at the base of the crust, and globally in the whole crust in the LC, just West of the MFZ. We interpreted the velocity architecture com-bined with the evidence of volcanism at shallower depths as indicative of an intensively intruded continental
- Gondwana breakup: messages from the North Natal ValleyPublication . Moulin, Maryline; Aslanian, Daniel; Evain, Mikael; Lepretre, Angelique; Schnurle, Philippe; Verrier, Fanny; Thompson, Joseph; De Clarens, Philippe; LEROY, Sylvie; Dias, Nuno; Afilhado, Alexandra; Apprioul, R.; Bronner, A.; Castilla, R.; Corela, Carlos; Crozon, J.; Davy, C.; D'acremont, E.; Droz, Laurence; Duarte, J. L.; Fernagu, P.; Ferrant, A.; Fischer, M.; Franke, D.; Inguane, H.; Jorry, Stephan; Jouet, G.; Loureiro, Afonso; Le Bouteiller, P.; Le Bihan, C.; Mahanjane, S.; Moocroft, D.; Pelleau, P.; Picot, M.; Pierre, D.; Pitel, M.; Rabineau, M.; Rombe, C.; Roudaut, M.; Senkans, A.; Toucanne, SamuelThe Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide-angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30-km-thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo-reconstructions of the Gondwana. This 30-km-thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle-derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
- Imaging early oceanic crust spreading in the Equatorial Atlantic Ocean: insights from the MAGIC wide-angle experimentPublication . Moulin, Maryline; Schnurle, Philippe; Afilhado, Alexandra; Gallais, Flora; Dias, Nuno; Evain, Mikael; Soares, José; Fuck, Reinhardt; Neto, Otaviano da Cruz Pessoa; Viana, Adriano; Aslanian, Daniel; Team, MAGICDuring the MAGIC (Margins of brAzil, Ghana, and Ivory Coast) experiment, five combined wide-angle, and reflection seismic profiles were acquired in the Pará-Maranhão/Barreirinhas/Ceará basins northern Brazil. This is a pull-apart passive margin, with two strike-slip borders. The equipment deployed includes 143 sea-bottom seismometers (OBS), a 4.5-km seismic streamer, and a 7587-in3 airgun array. In this paper, we focus on the distal parts of three profiles, and one entire transverse NW-SE profile, located on the presumed Cretaceous oceanic crust. Forward modelling of these wide-angle data sets reveals an E-W lateral evolution of the oceanic crust spreading initiation with: 1) just after the so-called intermediate domain, 60 km-wide domain that consists of a 5-km-thick crystalline crust. The basement presents two layers characterized by high acoustic velocity. This domain is bounded to the NW by a NW-SE volcanic line (Volcano Alignment), and 2) a 5-km-thick oceanic crust consisting of two layers characterized by “normal velocities” spanning between the two main fracture zones that fringe the Pará-Maranhão-Barreirinhas-Ceará segment. Despite a similar thickness, these two sub-domains present different velocity distribution in their two layers. They are both overlain by 5.5 km of sedimentary deposits. Forward wide-angle modelling confirms that the seafloor spreading process was progressive, with firstly the emplacement of a proto-oceanic crust, and then a thin oceanic crust. The “proto-oceanic” crust presents a similar seismic velocity with the intermediate domain interpreted as exhumed lower continental crust except for the lower part where the intruded lower crust gives place to a very sharp Moho at the base of the proto-oceanic domain. By contrast, the thin oceanic crust domain has a lower velocity structure in its upper layer, that is interpreted as basalt and is absent in the proto-oceanic crust. This eastward evolution, as also observed in the Provençal Basin, and the Santos Basin, suggests the involvement of the lower continental crust in the first steps of seafloor spreading.
- Wide-angle seismic imaging of divergente and transform segments of the Pará-Maranhão-Barreirinhas-Ceará margin, NW BrazilPublication . Schnürle, Philippe; Gallais, Flora; Afilhado, Alexandra; Moulin, Maryline; Dias, Nuno; Soares, José; Loureiro, Afonso; Fuck, Reinhardt; Cupertino, José António; Viana, Adriano; Aslanian, DanielThe structure of the North-East equatorial Brazilian margin was investigated during the MAGIC (Margins of brAzil, Ghana and Ivory Coast) seismic wide-angle experiment. This study focuses on the MC5 profile, that spans NW-SE 720 km in length, from the São Paulo Double Fracture Zone to the Barreirinhas margin and continental Borborema province. Its main objective is to understand the fundamental processes which lead to the thinning and finally to the breakup of the continental crust in a specific context of a divergent pull-apart system with two strike-slip borders. The experiment was devised to obtain the 2D structure along this profiles from joint pre-stack depth migration of streamer data and travel-time inversion by forward modeling of 43 Ocean Bottom Seismometers and 21 Land Seismic Stations records. Along the MC5 wide-angle transect, 4 major sectors are identified: 1) the São Paulo Double Fracture Zone presenting a 4.5 km thick volcano-sedimentary Basin on top of a 5.5 km thick basement; 2) a volcanic alignment and intermediate domain SE-ward, formed by the 4.5 km thick Basin III; 3) the 7.5 km thick Basin II, and the 5.5 km thick Basin I composing the continental slope and shelf. While all the offshore basement remains about 6 km thick in the deep-sea domains, acoustic velocity evolves from two-layer 4.8–6 km/s and 6.1–6.8 km/s beneath Basin III to two-layer high velocity 6.1–6.8 km/s and 7.2–7.4 km/s beneath Basin II and I, The necking zone, forming the Parnaiba Platform and associated Piaui-Camocim and Ceará Basins, is 50 km wide; 4) the Médio Coreaú and Ceará Central thrust belt, where the un-thinned continental crust thickness reaches 32 km. Finally, a schematic kinematic reconstruction that satisfies these observation is argumented.