Christine SOUQUE*, Dominique FRIZON DE LAMOTTE*, Philippe ROBION* & Hugues FEINBERG**
Cergy-Pontoise, département des Sciences de la Terre (CNRS, ESA 7072),
F95031 Cergy-Pontoise, France.
and paleomagnetic evidence for vertical axis
Identification of vertical axis rotations within fold-thrust belts is crucial to constrain 3D kinematic models. In order to progress in this way we propose an integrated work combining structural and paleomagnetic studies.
The " la Cagalière " duplex (Averbuch et al., 1992; Laumonier et al., 1995; Frizon de Lamotte et al., 1997) is a well exposed hectometric scale structure situated at the front of the Lagrasse fold (NE Pyrenees, France) (figure 1). It is made up of four sheets of eocene limestone sandwiched between a floor thrust which is the upper flat of the Lagrasse ramp related fold and a roof thrust flooring its allochtonous forelimb.
At regional scale, two successive tectonic events are documented (Cluzel, 1977; Genna, 1989). The first one characterized by a NNW tectonic transport is responsible for the development of the Lagrasse fold. The second one, characterized by a WNW tectonic transport is responsible for the development of out of sequence thrust faults cutting through the previously emplaced fold. The " la Cagalière " duplex has been built between the folding and the out of sequence thrusting perhaps during the change in the tectonic transport direction (Frizon de Lamotte et al., 1997). In order to test and constrain the tectonic scenario we have performed microtectonic, anisotropy of magnetic susceptibility (ASM) and paleomagnetic studies in each sheet of the duplex as well as in the hangingwall and the footwall of the fold.
Magnetic fabrics record the internal deformation linked to the layer parallel shortening developed before the emplacement of one sheet and microfaults record shearing occurring during its emplacement. It appears that each sheet exhibits its own magnetic fabric and microtectonic signature. Within a given sheet, we observe a small (lesser than 10°) but systematic counterclockwise apparent rotation between the direction of layer parallel shortening given by AMS and the tectonic transport direction given by microfaults analysis (figure2). A counterclockwise rotation of the same order is also observed between a sheet and the sheet emplaced immediately after. Such data can be interpreted as the result of a progressive counterclockwise rotation of the tectonic transport direction (from N160 to N110) during the duplex building.
However, primilarly paleomagnetic results indicate a 50° clockwise vertical axis rotation between the first and uppermost sheet and the last and lowermost one. This suggests that the duplex has been built during the second (N110) tectonic event and that the sheets have experienced a clockwise rotation during their emplacement. Each sheet underwent the sum of its own emplacement rotation and the rotation linked to the sheets emplaced later. So that the coincidence between the apparent direction of shortening observed in the uppermost sheet and the N160 of the first regional shortening must be consider as only circumstantial.
Based on these results which are consistent with the local sequence of structures development within the Lagrasse fold, a 3D kinematic of the " la Cagalière " duplex will be proposed. We will emphasise the role of oblique reactivation of the ramp that we consider, in this case, as the leading process of vertical axis rotations.
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