Palaeomagnetic dating of brittle tectonics structures: case studies on Ferques fault (Boulonnais, France) and two faults from the Holy Cross Mountains (Poland)

Calcite and sedimentary fills in fractures cutting the Upper Devonian carbo nates in the Holy Cross Mountains (HCM) and Boulonnais (B) were dated palae omagnetically by comparison with the Apparent Polar Wander Path (APWP) for Baltica in order to constrain age of faults. Here we present results obtain ed from calcite and sedimentary fills of two selected faults as case studie s. Hematite-bearing clastics and calcite possessed well-defined components of natural remanent magnetization (NRM) that were preserved up to thermal d emagnetization at ca. 500 degrees C. Two components in Rzepka calcite (HCM) and one component residing in clastics within Ferques Fault (B) could be d etermined. Comparison of directions of these components with reference dire ctions calculated from the APWP for Baltica constrains the upper age limit for the Ferques Fault to Middle Triassic, while Rzepka and Jazwica faults m ay be no younger than Late Permian. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena or minera lization processes. ((C) 1999 Academie des sciences / Editions scientifique s et medicales Elsevier SAS.).