Séminaire Romain Augier

Dater l'activité des failles par la méthode U/Pb sur Calcite

Absolute dating of an increment of deformation remains extremely challenging as the result of the complex behavior of the isotopic systems in geological environments. This prevents estimating duration and rates of geological processes. Rates are however crucial to unravel the rheology of the continental crust, particularly away from plate boundaries, i.e. in low-strain portions of the continental crust. The unpreceded development of U/Pb LA-ICP-MS analyses on carbonates opened the way to unambiguously constrain the timing of deformation by dating syn-tectonic calcite from fault planes (whatever their size or finite displacement) or closely associated microstructures. It appears therefore possible to date increments of slip on a fault plane, to estimate duration of fault activity or the field stress responsible of deformation and ultimately to propose rates of deformation.

In order to explore the potential of this structurally-constrained geochronological approach, a wealky deformed portion of the Golfe du Lion passive margin was selected. Providing exceptional outcrop conditions, the Corbières area displays the most proximal meso-scale normal faults associated with a dense microstructure array of the rifted area.

In a first step, meso-scale structures that often characterized by thick breccia bodies, were carefully mapped, while their local kinematics were determined as much as possible. In parallel, microstructures were used to derive robust palaeostress field reconstructions. Results show a quite homogeneous paleostress field in which meso-scale structure integrate well despite a potentially strong tectonic inheritance and a subsequent reactivation. This detailed structural analysis was used as the base for the calcite sampling strategy allowing to select calcite samples from both microstructures and meso-scale fault breccias. LA-ICP-MS analyses were conducted on polished 2mm-thick oriented rock-sections often preserving calcite growth history during deformation. U/Pb dating on calcite by LA-ICP-MS, together with new improvements for the evaluation of calcite samples will be presented.

Geochronological results show a quite large and common age span from ca. 28 Ma to 14 Ma for both meso-scale structures and microstructures in two age clusters, two periods between ca. 28 to 21 Ma and from 17 to 14 Ma. Taking into account errors, it seems that extensional field stress appears immediately recorded by the activity of meso-scale structures. Both types of structures even show unambiguous evidences of tectonic reactivation of first period structures formed during the second period.

It appears that proximal portions of the margin far away from the hinge line do record the whole thinning history of the continental crust. The whole volume of the extended domain therefore remained active with dratically different bulk strain rates and fault slip rates.

Regional implications including the discrimination of the spreading period in the Ligurian Basin are also presented.

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