Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera massifs (Western Alps, France)

Chapter 1: Apparent divergence of thrusting directions horth and south of the Pelvoux massif: implications for the evolution of the Western Alpine Arc: In the Western Alps, the Penninic Basal Contact (PBC, Ceriani et al. 2001) defines the tectonic boundary between the external Dauphinois domain and the interanl Penninic units.
During the Eocene, the PBC corresponds to the suture between Dauphinois and Subriançonnais/Briançonnais domains related to the subduction of the Valais ocean (Ceriani et al. 2001). In the Oligo-Miocene, i.e. during a second and post-collisional stage, Eocene structures were passively transported towards the WNE along a major out-of-sequence thrust, the Roselend thrust (RT), partially coinciding with and reactivating the PBC. Structural analysis of key outcrops combined with microfabric investigations of calcyte ultramylonites allowed to trace the southern continuation of the RT in a broad shear zone at the rim of the Pelvoux massif and further south along the Briançonnais front (Tricart 1980) at the rear of the Embrunais Ubaye nappe stack. The RT finally branches into a sinistral strike slip zone NE of the Argentera massif.
The area south of the Pelvoux massif is additionally characterized by SW-directed thrusting, a feature totally absent north of it. Detailed investigations of outcrops found along strike of the RT, the basal decollement of both Dauphinois para-autochthonous Tertiary cover and Helmenthoid Flysch of the Embrunais Ubaye nappe stack reveal that top-SW thrusting is a younger (i.e. D2) event, overprinting earlier features linked to D1 WNW-directed displacements.
The youngest deformation event found in the area (D3) is consistent with normal faulting along the Durance system.
On a larger scale, it is concluded that the whole Adriatic indenter was thrusted onto the European plate along the RT during Oligo-Miocene times. Activity along this structure is linked to dextral movements along the Rhone-Simplon line and sinistral movements at the rear of the Argentera massif. Additionally, it is proposed that the onset of D3 SW-directed movements finds its origin in the opening of the western Mediterranean basin and the foramtion of the Appenine chain in middle Miocene times.
Chapter 2: Microfabrics of calcite ultramylonites recording coaxial and non-coaxial deformation kinematics: examples from the Rocher de l'Yret shear zone (Western Alps):
Microfabrics were analysed in calcite mylonites from the rim of the Pelvoux massif (Western Alps, France). WNW-directed emplacement of the internal Penninic units onto the Dauphinois domain has lead to intense deformation of an Eocene-age nummulitic limestone under lower anchizone metamorphic conditions (slightly below 300°C). Two types of microfabrics developed during deformation under coaxial or non-coaxial kinematics, respectively, primarily by dislocation creep accompanied by dynamic recrystallization in the absence of twinning. Coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry. Their texture is characterized by two c-axis maxima at the periphery of the pole figure, symmetrically oriented at 15° from the normal to the macroscopic foliation. Non-coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterized by a single oblique c-axis maximum tilted with the sense of shear by about 15°. From the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures developed in coaxial kinematics fabour activity of <10-11> and <02-21> slip along the f- and r-planes, respectively, with minor contributions of basal <a> slip. In contrast, the monoclinic textures developed during simple shear are most suited for duplex <a> slip along the basal plane. The transition between orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallization upon texture development. Calcite textures alone should not be used as unequivocal shear sense indicators, but need to be complemented by microstructural criteria such as shape preferred orientations, grain size estimates and amount of twinning.
Chatper 3: Microfabric analysis of calcitic marbles from the area investigated in the Western Alps and the Gavernie basal contact (Central Pyrenees):
In the previous chapter, a model of microfabric evolution of calcite ultramylonites, plastically deformed at temperatures lower than 350°C, has been presented. The very specific setting of the outcrop investigated in great details at the "Rocher de l'Yret" shear zone (RYSZ) allowed for the recognition of both coaxial and non-coaxial deformation kinematics. The samples deformed with a dominant component of rotational deformation show a c-axis distribution tilted synthetically with the sense of shear. In the absence of twinning, this feature, commonly observed for quartz tectonites, can be used as a reliable shear sense indicator. Additionally, the model allows for the recognition of incipient rotational deformation in the bulk flow accommodated in the rock.
The structural data presented in chapter 1 will now be completed by means of a microfabric analysis of a series of calcitic marbles, found in abundance in the investigated area.
The results of investigations on calcite ultramylonites found along the Roselend Thrust (RT) are presented first. four localities along the RT have been investigated between the eastern rim of the Pelvoux massif in the North and the rear of the Argntera massif in the South (sites BFY, Laux, Reot and Arg in fig 3.1). This study aims to 1) independently confirm the overall WNW-directed sense of transport along the RT, 2) deduce the type of strain along this structure and its possible variations using the model of microfabric evolution presented in chapter 2.
In a second step, the basal decollement of the Dauphinois parautochthonous cover from the southeastern rim of the Pelvoux massif will be investigated. Such decollement occurred within the nummulitic limestone layer intensively mylonitised in the Dormillouse valley (site Dorm, see fig. 3.1 and 3.2). This study confirms the overall top-SW overprinting deformation recorded in this domain and documents important bulk flow partionning within the basal decollement level of the Eocene stratigrahpic series.
Finally the restults of investigations on samples from the Gavernie thrust from the central Pyrenees (courtesy J.H.P. de Bresser) will be presented (see fig. 3.37). The restults are in accordance with the overall top-S directed emplacement of the Gavernie nappe during Pyrenean orogeny and give evidence for strain variations within the shear zone developed along the Gavarnie basal thrust.