Disentangling magnetic subfabrics and their link to deformation processes in cleaved sedimentary rocks from the Internal Sierras (west central Pyrenees, Spain)

摘要

Here we present a detailed study of the magnetic fabrics and subfabrics of remagnetized Upper Cretaceous limolites that crop out in the Internal Sierras (west central Pyrenees) affected by a penetrative pressure-solution cleavage. The bulk magnetic fabrics of these rocks (RT-AMS) show variable orientations that do not conform to what is typically reported for cleaved sedimentary rocks. In contrast, the paramagnetic subfabrics (LT-AMS) show remarkably constant directional properties, so that their K_(max) and K_(min) axes cluster parallel to the intersection lineation and to the poles to bedding, respectively. These LT-AMS subfabrics indicate a preferred orientation of phyllosillicates that is consistent with a syn-sedimentary (Late Cretaceous) period of NNE-oriented layer-parallel shortening. Noticeably, these phyllosillicate subfabrics are not further altered by the subsequent formation of cleavage in the Late Eocene-Early Oligocene. The ferrimagnetic subfabrics (AARM) also show remarkably constant orientation, so that their K_(max) axes are strikingly parallel to the shortening direction in the area. We interpret this preferred orientation of ferrimagnetic grains as being caused by subhorizontal shear associated to cleavage formation, which is consistent with the age and mechanisms (authigenic growth and rotation of pre-existing magnetite grains) proposed for the pervasive remagnetization that affects the studied rocks.