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http://hdl.handle.net/10174/550
2024-03-29T04:42:04ZThe endorheic – Exorheic transition and later stage of fluvial incision in a wet tropical margin setting: The Atlantic draining Paraíba do Sul River basin (Brazil)
http://hdl.handle.net/10174/33443
Title: The endorheic – Exorheic transition and later stage of fluvial incision in a wet tropical margin setting: The Atlantic draining Paraíba do Sul River basin (Brazil)
Authors: Freitas, Marcelo; Paixão, Rodrigo; Salgado, André; Silva, Luis; Cunha, Pedro; Gomes, Alberto; Martins, António; Almeida, Júlio; Tupinamba, Miguel; Dantas, Marcelo
Abstract: Present-day endorheic drainage systems are rare in tropical humid regions and/or close to the coast. During the late Cenozoic, under a humid tropical climate, the Paraíba do Sul River basin (SE Brazil) has developed along the South America passive margin. This basin currently drains into the South Atlantic Ocean, but it preserves landforms that are indicative of previous endorheic paleodrainage. This study examines the possibility that this region was endorheic for most of the Neogene, prior to the establishment of the present-day drainage to the Atlantic and discusses the transition from an endorheic to an exorheic system. Data was obtained through analysis of geomorphological features identified by remote-sensing techniques and verified by fieldwork, as well as the interpretation of landscape evolution models elaborated by the Seppômen method. Five drainage convergence areas and possible endorheic paleobasins, previous to the Quaternary (or to the Pliocene) have been identified within the present-day Paraíba do Sul River basin. Each area is associated with a Cenozoic graben and is separated by structural highs which would have formed paleodrainage divides. The mechanism for the transition endorheic-exorheic is the overspill, the inland regressive erosion or, more probable, a combination between these two processes. In fact, these two processes often occur concomitantly and both contribute to the same result: the expansion of an exorheic basin by the incision of a permanent channel into the endorheic basin infill. The geological evolution of the ancestral Paraíba do Sul River, draining to the Atlantic Ocean, was later strongly controlled by the very low sea levels during the Quaternary which determined the stage of fluvial incision. No numerical dating has been yet obtained for the proposed endorheic-exorheic transition; nonetheless, published regional denudation rates suggest that this transition occurred sometime in the interval between 21 and 5 Ma (Miocene to Pliocene). This transition was controlled by a previous decrease in subsidence within the aforementioned grabens and by a much wetter climate that promoted overspill and connection to the Atlantic.2022-03-31T23:00:00ZSimulations of strong ground motion in SW Iberia for the 1969 February 28 (Ms = 8.0) and the 1755 November 1 (M~ 8.5) earthquakes - II. Strong ground motion simulations
http://hdl.handle.net/10174/3673
Title: Simulations of strong ground motion in SW Iberia for the 1969 February 28 (Ms = 8.0) and the 1755 November 1 (M~ 8.5) earthquakes - II. Strong ground motion simulations
Authors: Grandin, Raphaël; Borges, José Fernando; Bezzeghoud, Mourad; Caldeira, Bento; Carrilho, Fernando
Abstract: This is the second paper of a series of two concerning strong ground motion in SW Iberia due to earthquakes originating from the adjacent Atlantic area. The aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 (Ms = 8.0) and 1755 (M = 8.5–8.7) earthquakes.
First, we propose a regression to convert simulated values of Peak Ground Velocity (PGV) into Modified Mercalli Intensity (MMI) in SW Iberia, and using this regression, we build synthetic isoseismal maps for a large (Ms = 8.0) earthquake that occurred in 1969. Based on information on the seismic source provided by various authors, we show that the velocity model effectively reproduces macroseismic observations in the whole region.We also confirm that seismic intensity distribution is very sensitive to a small number of source parameters: rupture directivity, fault strike and fault dimensions. Then, we extrapolate the method to the case of the great (M = 8.5–8.7) 1755 earthquake, for a series of hypotheses recently proposed by three authors about the location of the epicentral region. The model involving a subductionrelated rupture in the Gulf of C´adiz results in excessive ground motion in northern Morocco, suggesting that the source of the 1755 earthquake should be located further west. A rupture along thewestern coast of Portugal, compatible with an activation of the passivewestern Iberian margin, would imply a relatively low average slip, which, alone, would could not account for the large tsunami observed in the whole northern Atlantic ocean. A seismic source located below the Gorringe Bank seems the most likely since it is more efficient in reproducing the distribution of high intensities in SW Iberia due to the 1755 earthquake.2007-01-01T00:00:00ZOfiolitos variscos e o metamorfismo de alta pressão associado, no ramo sul da Cadeia Varisca Ibérica / Variscan ophiolites and related high-pressure metamorphism in Southern Iberian Variscan foldbelt
http://hdl.handle.net/10174/2376
Title: Ofiolitos variscos e o metamorfismo de alta pressão associado, no ramo sul da Cadeia Varisca Ibérica / Variscan ophiolites and related high-pressure metamorphism in Southern Iberian Variscan foldbelt
Authors: Fonseca, Paulo; Araújo, Alexandre; Munhá, José; Pedro, Jorge
Abstract: The southern Iberia, Variscan ophiolites occur both as a thin belt along the boundary between the Ossa-Morena and South Portuguese Zones — Beja- Acebuches ophiolite — and as dismembered, scattered allochthonous klippen on top of lower Palaeozoic sequences within the internal areas of the Ossa-
Morena Zone.
The Beja-Acebuches ophiolite corresponds to a thin amphibolite-serpentinite belt displaying internal lithological organisation including, from bottom to top: metaperidotites (harzburgitic/dunitic) and cumulate pyroxenites, flaser gabbros with trondhjemitic intrusions, amphibolites (locally derived from a sheeted dike complex) and fine grain greenschists (locally preserving pillowed structures). Deformation structures result from three main deformation phases: D1 (early Devonian) corresponds to high-temperature ophiolite obduction towards N-NE, D2 (middle Devonian) is related to retrogression during transpressive sinistral shearing to WNW, and finally, D3 is a more brittle event, and involved sinistral south-westwards thrusting reactivating D2 structures. The ophiolite is bounded to the north by a thrust that brought Ossa-Morena
Zone infra-crustal rocks over the ophiolitic sequence; towards the south the ophiolitic complex has been thrust over the South Portuguese Zone units and is unconformably overlained by a late Devonian flysch sequence. The Beja-Acebuches amphibolites were originally tholeiitic gabbros/dolerites/basalts
displaying considerable geochemical variations that range from MORB-type to those transitional to arc tholeiites, suggesting derivation from a back-arc basin oceanic crust. The internal ophiolitic klippen were emplaced contemporaneously with the obduction of the Beja-Acebuches ophiolite. They comprise small, dismembered tectonic slices that were imbricated within a high-pressure (eclogite/blueschist), early Palaeozoic passive continental margin sequence, and then thrust onto the Ossa-Morena Zone. The high-pressure metamorphism was polyphase; early (pre- to syn-D1) eclogite recrystallization is interpreted as reflecting type-A subduction and initial D1-thrusting; late blueschist facies overprinting corresponds to tectonic imbrication related to the nappe emplacement. Ophiolite geochemistry display wide variations in incompatible element fractionation, ranging from N-MORB type LREE-depleted to LREE-enriched T/P-MORB; contrasting with similar lithologies from the Beja-Acebuches ophiolite, the orogenic (island arc-like) characteristics were not detected in these internal ophiolitic occurrences. The contrasting characteristics of the Ossa-Morena ophiolite types are reminiscent of those already described from other ophiolite belts and suggest that they probably represent different oceanic basins.2004-01-01T00:00:00ZSequências Ofiolíticas Internas da Zona de Ossa-Morena: implicações geodinâmicas na evolução da Cadeia Varisca Ibérica
http://hdl.handle.net/10174/2373
Title: Sequências Ofiolíticas Internas da Zona de Ossa-Morena: implicações geodinâmicas na evolução da Cadeia Varisca Ibérica
Authors: Pedro, Jorge; Araújo, Alexandre; Fonseca, Paulo; Munhá, José
Abstract: The Internal Ophiolitic Sequences correspond to allochthonous oceanic crust fragments that outcrop near the SW boundary of the Ossa-Morena Zone (Evora-Beja Domain), in an internal position relative to the Beja-Acebuches Ophiolitic Complex. The Internal Ophiolitic Sequences occur as tectonic imbrications or klippen, in the Moura Phyllonitic Complex. Despite deformation and metamorphism (greenschist/amphibolite facies), they still preserve a typical ophiolitic internal stratigraphy identical to the one described for many LOT (“Lherzolitic Ophiolite Type”) ophiolites. Geochemical data indicates that the Internal Ophiolitic Sequences have variable tholeiitic chemistry, transitional between N-MORB and E-MORB. Petrogenetic modelling suggests that the observed geochemical
variations reflect both mantle source heterogeneity and partial melting processes. The Internal Ophiolitic Sequences geochemistry indicates that their igneous protholites formed in an anorogenic tectonomagmatic environment, like the ocean ridge basins, without any influence of orogenic components related to subduction mechanisms. These features contrast markedly with those reported for the Beja-Acebuches Ophiolitic Complex, which have been interpreted as representing oceanic crust generated in a “back-arc” setting,.
Thus, supporting the existence of two distinct oceanic basins (ocean ridge and back-arc) during the geodynamical evolution of the SW Iberian Variscan Chain. Therefore, the Internal Ophiolitic Sequences in the Ossa-Morena Zone reflect the early oceanization events during the Variscan Orogeny.2005-01-01T00:00:00Z