HR: 1340h
AN: GP43C-1493    [Abstracts]
TI: Anisotropy of magnetic susceptibility and quantification of the internal deformation by means of deformed echinoids (Upper Cretaceous, Southern Pyrenees)
AU: * Pueyo Anchuela, O
EM: opueyo@gmail.com
AF: Area de Geodinamica Interna. Departamento de Ciencias de la Tierra. Universidad de Zaragoza, C/Pedro Cerbuna, n 12., Zaragoza, 50003, Spain
AU: Pocovi Juan, A
EM: apocovi@unizar.es
AF: Area de Geodinamica Interna. Departamento de Ciencias de la Tierra. Universidad de Zaragoza, C/Pedro Cerbuna, n 12., Zaragoza, 50003, Spain
AU: Gil Imaz, A
EM: agil@unizar.es
AF: Area de Geodinamica Interna. Departamento de Ciencias de la Tierra. Universidad de Zaragoza, C/Pedro Cerbuna, n 12., Zaragoza, 50003, Spain
AB: The AMS technique has been used as a very sensitive technique to determine the orientation of the strain ellipsoid in rocks. In this work it is presented a study by means of AMS of a NE-SW anticline in the western part of Central Pyrenees (Sierra de Alaiz thrust sheet). The obtained results show deformed sedimentary magnetic fabrics, where the magnetic foliation is parallel to the layering. The stereographic plotting of the K3 axes shows three groups of data, that can be related with the northern limb, the anticline hinge zone and the southern limb. When the magnetic results are bedding corrected the grouping of the K3 axes happens, and the magnetic lineation shows a horizontal NE-SW trend. These results allow us to establish a pre-folding acquisition magnetic fabric. The studied outcrops, in the northern limb, show deformed fossils, mainly echinoids. These fossils show an apparent main flattening plane parallel to the layering. In this outcrop 14 echinoids were orientated and collected. In the laboratory these elements were restored to the field situation and photographed. The photograph was made normal to the layering, the echinoid section was adjusted to an ellipse, and the results were plotted. The plotting of the ratio between long vs. short axes and the orientation of the long axes (Rf/phy method), show a maximum of axes ratio that represent the symmetrical plane of the obtained results. These results show a vertical plane that contains the more elongated section and show an orientation of N055 (D plane). From these results, a photograph in the pole of the D plane was taken. The same methodology described before was applied, the obtained results show an orientation of the higher ratio of 45º respect to the horizontal. This value is the same of the dipping of the level, then the assumption of the more flattening plane was parallel to the layering has been confirmed. The results show a stretching direction that is contained in the layering and it has an orientation of N055, 45. The magnetic lineation obtained in this site is parallel to this direction, then there are a parallelism between the magnetic lineation axis and the more stretching direction obtained by means of the deformed echinoids. If we apply the same conclusions obtained in the regional AMS study, we can infer that the deformation of the echinoids took place during a pre-folding stage. If we know that the flattening plane is parallel to the layering, and also we know the orientation of the long axis (X strain axis), the study in two different sections show that the intermedium axis (Y-axis) is also parallel to the layering. We have two sections that contain the three axes of the deformation, because off it is possible to prove that the flattening plane is parallel to the layering. With these results we can apply the Lisle method to quantify the internal deformation. If we assume that not volume change has happened, there are a shortening (in z axes) of 15.78 per cent and a stretching in the x direction of the 15.11 per cent. These results show that a very important shortening happened before folding, while the restored sections show shortenings of 6 to 8 per cent, the shortening related with pre-folding processes is even higher than this. In this case a tectonic shortening in these materials can be expected during pre-folding and also pre-detachment moments, while when the thrust sheet starts to move to the south all of the internal deformation goes to the detachment level, folding and faulting the upper materials in a passive manner.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8025 Mesoscopic fabrics
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting