We present new results from the MARADJA'03 cruise depicting the geological structures offshore central and western Algeria. Using swath bathymetry and seismic reflection data, we map and discuss the offshore limits of the Internal Zones corresponding to relics of the AlKaPeCa domain that drifted and collided the African plate during the Miocene. We identify large reverse faults and folds that reactivate part of these limits and are still active today. The morphology of the westernmost NE-SW margin suggests a former strike-slip activity accommodating a westward block translation responsible for the shift of the Internal Zones towards the Moroccan Rif.Résumé: Nous présentons les résultats récents de la campagne MARADJA'03, qui visent à mettre en évidence les structures géologiques dans le domaine marin au nord-ouest de l'Algérie. Grâce aux données de bathymétrie multifaisceau et de sismique réflexion, nous cartographions et discutons les limites en mer des Zones internes correspondant aux reliques du domaine AlKaPeCa qui a dérivé, puis est entré en collision avec la plaque africaine au Miocène. De grandes failles inverses et plis, actifs dans le champ de contrainte actuel, réactivent certaines de ces limites. La marge ouestalgérienne, orientée NE¿SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest.
Structural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz basin and post-5 Ma volcanic rocks of the Los Tuxtlas volcanic field (LTVF). The main volcanic alignment of the LTVF is located along this fault. Additional structural data collected in the Trans-Mexican volcanic belt (areas of Xalapa, Teziutlán and Huauchinango) show that the shear zone affects Pliocene Trans-Mexican volcanic rocks. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary strike-slip faulting in the Veracruz basin and in the eastern Trans-Mexican volcanic belt is important because it connects two important structural provinces: the left-lateral strike-slip faults province to the south and the left-lateral transtensive faulting that affects the central part of the Trans-Mexican volcanic belt. These three active deformation zones constitute the boundary between the southern Mexico block and the North American plate. It is generally assumed that strike-slip faulting along the Trans-Mexican and Central America volcanic arcs is the result of oblique subduction of the Cocos plate under the North American and Caribbean plates. However slip vectors along the Middle America trench are almost perpendicular to the trench. This Neogene sinistral strike-slip motion could be partially driven by the eastward motion of the Caribbean plate rather than by strain partitioning along the oblique Middle America trench subduction zone. La faille néogène de Veracruz : évidences d'un décrochement senestre sur la limite orientale du bloc Sud mexicainMots-clés. -Tectonique, Mexique, Faille de Veracruz, Néogène, Bloc crustal. Résumé. -L'analyse de profils sismiques ainsi que l'analyse d'images satellites et des observations de terrain montrent que le bassin de Veracruz et le volcanisme post 5 Ma du massif de Los Tuxtlas (Mexique) sont affectés par un grand dé-crochement senestre. Des données supplémentaires, collectées dans la partie orientale de l'axe volcanique Trans-Mexicain, montrent que ce décrochement affecte également des roches volcaniques pliocènes entre les villes de Xalapa et Huauchinango. Cette faille est toujours active comme en témoignent la sismicité modérée ainsi que les marqueurs quaternaires (cônes, terrasses alluviales et volcanisme) affectés par la déformation. Le décrochement plio-quaternaire de la région de Veracruz est important car il établit la jonction entre le système Tula-Chapala, transtensif senestre, qui affecte l'axe volcanique Trans-Mexicain et les décrochements également senestres qui affectent la région des Chiapas au sud. Ces trois zones de déformation (système Tula-Chapala, décrochements de Veracruz et des Chiapas) constituent la frontière active entre le bloc Sud Mexicain et la plaque Amérique du Nord. La déformation affectant l'axe volcanique Trans-Mexicain et l'arc volcanique d'Amérique Centrale est généralement interprétée comme...
Visual landmarks provide crucial information for human navigation. But what characteristics define a landmark? To be uniquely recognized, a landmark should be distinctive and salient, while providing precise and accurate positional information. It should also be permanent. For example, to find back to your car, a nearby church seems a better landmark compared with a distinct truck or bicycle, because you learned that there is a chance that these objects might move. To this end, we investigated human learning of landmark permanency for navigation while treating spatiotemporal permanency as a probabilistic property. We hypothesized that humans will be able to learn the probabilistic nature of landmark permanency and assign higher weight to more permanent landmarks. To test this hypothesis, we designed a homing task where participants had to return to a position that was surrounded by three landmarks. In the learning phase we manipulated the spatiotemporal permanency of one landmark by secretly repositioning it before participants returned home. In the test phase, we investigated the weight allocated to the nonpermanent landmark by analyzing its influence on the navigational performance during homing. We conducted four experiments: In the first two experiments we altered the statistics of permanency and accordingly found an influence on participants' behavior, nonpermanent objects were used less for finding home. In the last two experiments we investigated the role of short-term learning of novel statistics versus long-term knowledge about such statistics. No carry-over effects in Experiment 3 and very little influence of object identity with different long-term permanency characteristics in Experiment 4 revealed a dominance of short-term learning over the use of long-term a priori knowledge about object permanency. This indicates that long-term prior beliefs are quickly updated by the current permanency statistics. Taken together, consistent with a Bayesian account for navigation these results indicate that humans quickly learn and update the statistics of landmark permanency and use it in an effective way, assigning gradually more weight to the more permanent landmark and making it more important for navigation.
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Living in a complex and multisensory environment involves constant interaction between perception and action. There is evidence that multisensory integration is governed by temporal factors, such as physiological synchrony between cross-modal stimuli favouring multisensory benefit, and the existence of a range of asynchrony between the stimuli which affords their binding (the temporal window of integration). These factors were examined in this study in a bimanual sensorimotor synchronization task with cross-modal stimuli. Participants synchronized each hand to a pair of audio-tactile stimuli, in which the asynchrony between onsets of auditory and tactile stimuli was systematically manipulated. In cross-modal conditions, they were instructed to tap either to the auditory stimuli or to tactile stimuli. The results reported a temporal window of integration of 160 ms centred around 40 and 80 ms (tactile first). Moreover, the temporal interval between the auditory and tactile stimuli affected the stability of bimanual coordination and of synchronization exclusively when participants were instructed to synchronize with tactile stimuli. Overall, the results indicate that both physiological asynchrony and temporal window of integration apply to cross-modal integration in a bimanual synchronization task. In addition, it shows the effect of auditory dominance onto multisensory temporal processes. This study sheds light on the role of temporal factors in multisensory processes when perception and actions are rhythmic and coupled.
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