Intraplate paleostresses reconstructed with calcite twinning and faulting: improved method and application to the eastern Paris Basin (Lorraine, France)

Abstract

This study deals with the reconstruction of intraplate paleostresses. Calcite twinning inversion provides paleostress directions and differential values, even in poorly outcropping areas. However, existing methods are not well adapted to multi-phase deformation cases, and the calculation of stress values provides unreliable estimates. Modifications were brought to a calcite twinning inverse method and to the processing of results. In a new twin data separation process, the whole set of data is used to calculate each tensor. The variability of yield stress value for twinning with strain hardening is taken into consideration. A first order yield stress-calcite grain size dependence function has been established with a sample showing various grain sizes. This modified method of calcite twinning inversion was applied in the eastern Paris Basin (Lorraine, France). In this platform area, measured fractures are preferentially trending NE–SW and NW–SE at all scales. The Meso-Cenozoic paleostress succession reconstructed with fault slips is complex. Jurassic sedimentation occurred during the Tethys rifting. A mainly strike–slip phase with σ 1 successively striking NNW and NNE is attributed to the Tertiary Pyrenean orogenesis, and is followed by WNW extension in the Oligocene. After local ENE compression, WNW compression was probably due to the Alpine Late Miocene collision. The calcite twinning inversion has confirmed these tectonic events and their succession order, and the directions of stress axes were specified. Using grain size- and strain rate-sensitive yield stress values, the differential stresses can be estimated with an uncertainty of few MPa. They range between 20 and 30 MPa, depending on the tectonic event. The Oligocene paleodepths of the sampled calcite, estimated using differential stress values, are structurally sensible. Thus, the original calcite twinning inverse method has been improved. Complex paleostress successions such as in platform areas may be deciphered by combination with fault slip inversion. Even if additional improvement is possible, this calcite twinning inverse method opens new ways to determine stress distributions in tectonic plates.