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A tectono-sedimentological study of the Upper Visean–Lower Westphalian Fourhal basin (northwestern Moroccan Meseta) has shown that deposition and depocentre migration were controlled by the north-westward propagation of thrust-related folds in the wedge-top depozone of a flexural foreland basin system. These results and a comparison of the ages of regional contraction in the Moroccan Meseta and the French Massif Central favour the interpretation of the Moroccan Meseta as a southern continuation of the south-European Variscan belt.

A new geotectonic framework of the Taiwan orogen is presented in accordance with the hypothesis of an oblique arc- arc collision. The colliding Luzon arc is physically connected to the eastern Coastal Range in which a subduction complex remnant is preserved and backthrust with intra-arc sediments in a small retroforeland basin. A southern and extinct extension of the Ryukyu arc is characterized in western Taiwan. It displays a duplex structure (Tananao and Backbone horses and Lishan triangle zone) between a buried floor thrust located in the arc crust and a roof thrust developed in the arc cover (Hsfiehshan Range and South Backbone Range). Westward the basal thrust climbs in the sedimentary series of the western proforeland (Foothills and Hengchun Peninsula) and dies out in a buried tip line. The northern part of the orogen, including all the Tananao arc core, is shown as an intra-oceanic-continental arc-arc collision belt characterized by an unroofed duplex culmination above a leading floor thrust and both proforeland and retroforeland basins. The southern part, which displays a roof thrust sequence above a buried duplex, is shown as an accretionary prism built in a transition zone between continent and oceanic subduction (transition from the Asian continental crust, including the former Ryukyu arc, to the oceanic Old Philippine Sea crust). The partition is believed to be induced by a deep intracontinental transcurrent fault zone able to influence the difference in shortening, duplex pattern, and leading thrust depth. The evolution was controlled by the Ryukyu subduction (backarc extension, arc magmatism extinction, and cooling and intra-arc collapse) until the early middle Miocene (around 15 Ma) and then it was controlled by the Luzon arc progression (continental subduction, collision, indentation, and hinterland uplift and frontal thrust propagation).

The Pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault-related folds. The Pakuashan ridge development arises from the late Quaternary uplift of the most external thrust zone of the western foothills of Taiwan. From Kaoshiung to Taichung, recent and active westward thrusting occurs at the front of the foothills. The Pakuashan anticline, trending N 150°E in the northern part to N 000° in the southern part, has been active throughout the Quaternary period. This activity is marked by geological structures, tectonic geomorphology and seismicity. A multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics, sedimentology and geomorphology. Studies of fracture patterns allow identification of two main features of stress orientations: a WNW/ESE compression direction, and E–W and N–S extension directions. Quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front. Geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline. Quaternary terraces are uplifted, tilted and folded over the Pakuashan ridge. Drainage systems in areas of active compression give information on the thrust zone structures and their development. Steep drainage and high local relief indicate that the Pakuashan anticline forms a well-defined zone of high uplift, especially in the southern part. The two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south.

Resume L'analyse combinee des structures tectoniques et des facies conduit a interpreter le flysch Cretace superieur de Saint-Jean-de-Luz comme faisant partie du bassin flexural nord-pyreneen, actif a partir du Turonien. Cet exemple illustre l'evolution des zones de chevauchement dans le stade d'initiation du raccourcissement en milieu profond et a taux de sedimentation reduit. La formation et la migration des depocentres sont reliees a la propagation d'un systeme de chevauchement frontal aveugle. Le flysch est lui-meme subdivise en sequences de depot. Ces sequences enregistrent les stades de propagation des chevauchements a travers des corteges de bas niveau relatif, relies a l'acceleration de l'activite tectonique et des corteges de haut niveau, relies au ralentissement de cette activite.

The major structures of the Western Foothills of Taiwan mainly consist of NNE-SSW-trending folds and imbricated west-vergent thrust systems. The additional occurrence of N140°E-striking oblique structures was revealed through a multisource approach involving a Digital Elevation Model (DEM), a study of drainage network anomalies, aerial photographs, Side-Looking Airborne Radar (SLAR) images and SPOT-P and Landsat images. These structures are described from north to south based on new field analyses (including stratigraphy and tectonics studies). They are also compared to seismic data and geodetic reconstruction, in order to evaluate their present-day activity. These N140°E major morphostructures are interpreted as left-lateral transfer fault zones, either inherited from the Eurasian passive margin and/or newly formed in the cover in response to the presence of basement highs within the foreland basin (Peikang and Kuanyin highs). The Sanyi and the Chishan transfer fault zones display a high seismic activity; the distribution of earthquakes and the related focal mechanisms confirm the left-lateral movement along N140°E directions. The Chiayi, Chishan, and Fengshan fault zones act presently as transfer fault zones, as indicated by GPS data. The associated N70°E- to N100°E-trending faults result from the reactivation of normal faults of the Eurasian passive margin as right-lateral strike-slip faults in the Foothills during the Plio-Quaternary collision in Taiwan. We conclude that multisource and multiscale geomorphic studies combined with tectonic analysis in the field yield a significant contribution to the understanding of the structural and kinematic development of the Western Foothills at the front of the Taiwan collision belt.

[1] The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep-seated decollements within the basement related to thick-skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin-skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross-section balancing shows an along-strike homogenous horizontal shortening of ∼56 km (∼30%) across the Ene−southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N-S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generated major decollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene−southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region.

International audience; The relationship between tectonics and sedimentary fill has been studied in two syncontractional basins of the western Corbieres (eastern North Pyrenean retro-foreland basin). The Late Cretaceous basin formed during c.10-12 Ma as a result of left-lateral transpressional deformation, and is composed of forward-younging sub-basins characterized by reworking of the forelimbs of growing fold-propagation folds. Thrust-wedge advance and cratonward migration of the platform are recorded by a deepening-upward stacking pattern indicating increased regional subsidence with a limited contribution of the submarine orogen. Tectonic quiescence and erosional unloading lasting 29-30 Ma are recorded by a shallowingupward stacking pattern, and fluvial sedimentation issued from widespread sources in the emerging inner orogen. The Early to Middle Eocene basin formed as a result of pure shortening normal to the range. The marine Early Eocene basin developed during c.2 Ma by widening of a single basin provoked by the two-step propagation of a basement duplex. This is recorded by growth-stratal patterns and coarsening-upward depositional sequences indicating the increasing contribution of the emerged orogen. The Middle Eocene continental deposits infilled two sub-basins working synchronously and were transported by alluvial fans with a provenance in the inner orogen, during decreasing thrust-wedge advance and increasing erosional unloading.

La nappe de Gavarnie est constituee de terrains d'âge paleozoIque, cretace superieur et eocene. Elle repose sur un ensemble autochtone constitue par des formations metamorphiques (migmatites, micaschistes) et leur couverture cretacee. La mise en place de la nappe s'est faite lors de l'orogenese pyreneenne. La geometrie des diverses deformations qui affectent cet ensemble complexe est envisagee. La chronologie de ces deformations est etablie ainsi que la distinction entre les deformations pyreneennes et varisques. On compare ensuite cette nappe avec d'autres structures chevauchantes particulierement etudiees du point de vue mecanique.

The overthrust model developed by Hubbert and Rubey, and Hsu is based on the concept of a rigid-plastic block, lightened by a fluid overpressure whereas the model developed by Kehle consists of a number of viscous layers with high viscosity contrasts. The mechanical features of these two different models are summarized and a third model is proposed. This model consists of a thick rigid-plastic block sliding on a thin visco-plastic layer. This model seems more realistic to the geometric problems posed by the Gavarnie overthrust in the central Pyrenees and we have obtained relationships between geometrical and mechanical parameters of this overthrust mass.

The Cammazes orthogneissic massif in the western Montagne Noire (France) has been affected by numerous tectonic phases during the Hercynian orogeny. The principal deformation, synchronous with a mesozonal metamorphism, is represented by different structural features such as foliation, ‘augen’ structures and deformation of megacrystals. These structural features are essentially varied with the concentration of feldspar megacrystals in the gneisses. The structural studies of these gneisses have allowed the recognition of the nature and the orientation of the principal deformation. To determine the intensity of this deformation, large undeformed feldspar crystals have been used as markers. These crystals have been assimilated to rigid ellipsoidal elements disposed in a random fashion in the initial stage of the mesostase which is considered as a viscous Newtonian fluid. The statistical studies of the orientation of these crystals with reference to the plane of flattening (foliation) has caused the appearance of an anisotropy in the final deformed state. This anisotropy, when compared to that obtained theoretically from a population of rigid elements included in a Newtonian fluid flowing at a slow speed, could evaluate the rate of the finite strain.

The structural history of the Hercynian Pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations, the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes, the migmatization and the intrusions of plutonic and basic and ultrabasic rocks. The metamorphism can be shown to have progressed upwards with time, attaining the presently exposed levels just before, during and a short time after the major penetrative regional deformation. The intrusion of major plutonic, anatectic and gneissic massifs, which has determined the metamorphic domes in country rocks, was roughly synchronous with the progression of the metamorphism, but seems to have continued later in the uppermost levels. The basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation, thus showing that their intrusion predated this deformation in the lowermost levels. These results, together with the presently available sedimentological data, lead us to propose a dynamic model of the tectonic evolution of the Hercynian Pyrenees. The first step was a crustal extension and thinning, attaining its maximum during Frasnian time, marked in the upper levels by a “ horst and graben” controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low-pressure metamorphism and anatexis characteristic of the Hercynian Pyrenees. The second step was a crustal shortening marked by large-scale crustal thrusting followed by widespread regional penetrative deformation. The flysch troughs formed in front of the progressing and emerging thrusts. In lower levels, the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels.