Pierre Tricart

Widespread post-nappe normal faulting in the Internal Western Alps:a new constraint on arc dynamics

Abstract: Field data collected in the Briançonnais and Piémont zones at the centre of the western Alpine arc demonstrate a high density of normal faults. Normal displacements are pervasive from the millimetric to the kilometric scales. They occur both parallel and at high angles to the arc, overprinting all the Alpine folds and thrusts and the associated schistosities. The orientation of fault striae implies that the pile of metamorphic nappes SE of the Pelvoux massif has undergone a general extension. Extension tends to be multidirectional in the Briançonnais zone. Orogen-parallel extension prevails in most of the Piémont Schistes lustrés. To the west, in Briançonnais zone, brittle extension is associated with a reversal of movement along the Briançonnais Frontal Thrust that started before the end of the Oligocene. In the Piémont Schistes lustrés, the onset of normal faulting occurred early in the Miocene and continued the ductile extension that accompanied the exhumation of HP–LT metamorphic thrust sheets during the Palaeogene. In both zones, extension remains currently active, resulting in the regional seismicity. Since more than 20u2009Ma ago, this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc. We discuss its possible significance in the context to the dynamic evolution of the Alps.

The Guinea continental margin: an example of a structurally complex transform margin

Abstract The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E-W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). Local tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.

Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs

We present new apatite fission-track (AFT) data from the central western Alps that confirm the synchronicity and high cooling rates during Latest Miocene–Pliocene final exhumation of the External Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump in AFT ages (c. 22 Ma) and in final cooling rates is significantly larger than further north. This difference results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western Briançonnais Zone provides a mean AFT age of c. 27 Ma, which is older than further north. Early cooling in the southern Briançonnais Zone would result from rapid erosion of the compressional fan structure built during the Oligocene. Across the entire Briançonnais and Piémont nappe stack, with the exception of the Dora-Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this structure underwent extension. Further north a reverse gradient with ages younging northwestward has been described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage tectonic and morphological evolution.

Source tracing of detrital serpentinite in the Oligocene molasse deposits from the western Alps (Barrême basin): implications for relief formation in the internal zone

We present the first contribution of tracing the source area of ophiolitic detritus in the Alpine molasse deposits by Raman spectroscopy. The lower Oligocene molasse deposits preserved in the Barreme basin, in the SW foreland of the western Alpine arc, are known for the sudden arrival of the first ‘exotic’ detritus coming from the internal Alpine zones. Among them, the pebbles of serpentinized peridotites have so far not been studied. We show that they only consist of antigorite serpentinite, implying that they originate from erosion of high temperature blueschists. In contrast, the upper Oligocene/lower Miocene molasse shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile. This suggests that they were derived from a less metamorphosed unit such as the low temperature blueschist unit. Taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean-derived detritus, we constrain the lithologic nature of the source zones and the location of the relief zones, identified as the internal Alps, SE of the Pelvoux external crystalline massif. Available structural data and in situ thermochronological data allow the reconstruction of the Oligocene to early Miocene collisional geometry of the Palaeogene subduction wedge. This phase corresponds to two major phases of uplift evolving from a single relief zone located above the Ivrea body during early Oligocene times and persisting up to early Miocene times; then during late Oligocene/early Miocene times a second relief zone developed above the Brianconnais zone. At that time, the internal western Alps acquired its double vergency.

La zone d'Acceglio (Alpes cottiennes) :un nouvel exemple de croûte continentale éclogitisée dans les Alpes occidentales

Abstract Petrological study on andesitic schists sampled in the Acceglio zone, allows, for the first time, the Alpine P – T path of this metamorphosed continental unit to be quantified. Three successive metamorphic stages are distinguished, the first one under eclogite facies conditions, with a pressure of 13.5xa0±xa01.5xa0kbar and a temperature of 450xa0±xa025xa0°C. A second stage, characterized by a moderate temperature increase at the beginning of the decompression, corresponds to a reequilibration near the boundary between blueschist and eclogite facies conditions. The third stage, characterized by the costability of lawsonite and albite, documents a reequilibration at the boundary between blueschist and greenschist facies conditions. These new data confirm the heterogeneity of the metamorphic conditions observed in the Piemont zone of the western Alps. They are in agreement with structural data and allow us to interpret the Acceglio domain as a tectonic extrusion of eclogitized continental crust within the less metamorphic calcschists of the Piemont zone.