Seismicity and regional tectonics of the Estremadura, Southwestern Portugal

Abstract

The RESTE Project was an integrated geophysical-geological study of the Estremadura, southwestern Portugal. The core of the programme consisted of the acquisition and analysis of microearthquake data. This was complemented by an investigation of the structural evolution of the sedimentary basins of the Estremadura. The geological evolution of the Lusitanian Basin was strongly marked by the reactivation of Palaeozoic basement faults, in response to a sequence of tectonic events: opening of the Central Atlantic, opening of the North Atlantic and Alpine convergence between Africa and Eurasia. The current tectonics are regarded as a subdued continuation of the Miocene deformation (Betic Orogeny), and the "tectonic memory" revealed by the Lusitanian and Lower Tagus Basins is explored to characterize the current tectonic processes. Strike-slip tectonics are identified as a dominant feature of several stages of the evolution of the basins, with particular relevance during the Miocene. The technique of "backstripping" is applied to well data, to constrain the history of vertical movement in the basins. This analysis highlighted the pre mature truncation, in the Late Jurassic, of a normal passive-margin evolution. Tectonic unstability caused the structural inversion of areas within the basins, and seems to have inhibited the predictable thermal subsidence. The rifting process, initially taking place at the Lusitanian Basin, jumped westwards in the Late Jurassic. Crustal underplating and the activity of transfer faults are in-voked as possible explanations for the subsequent deformation of the aborted rift. An upper-plate margin configuration is in good agreement with several observations. The tendency for structural inversion continued throughout theCretaceous, and with the onset of the Alpine convergence in the Turonian the control of the tectonic activity seems to have switched from the Atlantic to the Mediterranean. This reinforced the tectonic unstability, marked by magmatic activity and by a regional upwarp that was to last until the Eocene. Of particular interest was the behaviour, during the Late Cretaceous, of the Setúbal Peninsula sub-basin, which seems to have tilted towards the NW as a block, with a hinge line along the present Lower Tagus Valley. When sedimentation was resumed in the Eocene, a pattern of differential vertical movement was established, with some areas continuing to undergo inversion while nearby areas subsided. This pattern characterized the Cainozoic evolution of the basins, and probably still applies to the neotectonic deformation. The activity of strike-slip basement faults, reactivated under the compressive regime caused by the Afro-Eurasian conver-gence, is proposed as the best explanation for the Miocene deformation, with particular relevance for the Lower Tagus Valley. The RESTE Microearthquake Survey is described, and the data acquired with the RESTE network are analysed. The local earthquakes are accurately located, and focal mechanism solutions are obtained for some of them. This information is used to discuss a neotectonic model for the Lower Tagus Valley. In view of their small magnitudes (l.1< M(_L) < 3.8), the focal mechanisms of these events cannot be interpreted directly in terms of the current tectonics. Such small events are usually local readjustments to previous episodes of deformation. However, such features as the along-strike reversal of the polarity of vertical motion or the coexistence at the same region of different types of source mechanism are diagnostic of strike-slip deformation. This model was supported by the occurrence of a macroearthquake (M(_D)=3.8) with an interpreted source mechanism of sinistral strike-slip. The alignment of four hypocentres along the direction of the Lower Tagus Valley, with a compatible orientation of the interpreted nodal planes, supports the existence of a crustal fracture associated with the Valley. The hypocentral depths of the recorded events reach 20 km, showing that the basement faults responsible for the seismicity affect at least the entire upper crust. Since the limited existing data suggest a high level of heat flow in the Lusitanian Basin, the depths reached by the microearthquakes may indicate an abnormally thick seismogenic layer. An investigation of the broad velocity structure of the lithosphere underneath the RESTE Network using the technique of teleseismic tomographic inversion suggested a correlation between Moho undulations and the inversion of areas of the Lusitanian Basin, and this may indicate that the controlling faults cut the entire crust. In order to provide a rationale for the intraplate seismicity of western Portugal, the neotectonics of Iberia are discussed, and a new kinematic model, centred on the idea of continental extrusion, is proposed. According to the model, a continental block formed by Iberia and northern Morocco is being pushed west wards by the convergence between Africa and Eurasia. The resistance offered by the oceanic parts of the plates varies across the East Azores Transform, leading to dextral shear in the Betic Range. The regional stress field induced by the continental convergence can explain the reactivation, in a simple-shear regime, of basement faults of Hercynian orientation, in particular that proposed for the Lower Tagus Valley.