TRANSALP SEISMIC TRAVERS - Magnetics & Petrophysics

Research project P 14422 TRANSALP SEISMIC TRAVERSE - Magnetics & Petrophysics Wolfgang SEIBERL 08.05.2000 The TRANSALP East-Alpine Reflection Seismic Traverse is an international Earth Science research program to investigate the crustal and lithospheric structure of the East-Alpine collisional orogen along a profile starting in Bavaria and ending north of Venice. In this project, geophysical investigations will play the most important role. The University of Leoben will carry out the petrophysical research program, the University of Vienna the magnetic investigations. Since there is already a good coverage of aeromagnetic data available in the area under consideration, only qualitative and quantitative procedures have to be applied to the data. The researcher of the University of Vienna will therefore apply several 2D and 3D computer inversion program, which have been developed at the Institute of Meteorologie and Geophysics to contribute to a complex model of the crust.

In the framework of this project, petrophysical parameters were investigated at numerous geological outcrops and rock-samples in the vicinity of the Transalp Seismic Travers (TST), a N-S running seismic profile. 3D models of the magnetic susceptibility were calculated for selected anomalies of the measured total magnetic field in the survey area. Besides the magnetic parameters (magnetic susceptibility and natural remanent magnetization - NRM), the specific density, the seismic wave-velocities, the heat conductivity as well as the complex electrical resistivity were measured. Also thin films were prepared and interpreted. It was found out, that the lithological unit can not be seperated by their magnetic properties because the values are in a range typical for Para- and Orthometamorphites. Only areas containing rocks with high susceptibility are identified as anomalies of the total magnetic field, primarily if the occurrence shows a corresponding size. The NRM has no significance for the 3D modeling of airborne magnetics carried out in this project. The development of the 3D models was based on the measured petrophysical data and the results of two airborne geophysical surveys: the survey of southern Germany and western Austria (1977) and the survey west of Zell am See respectively southeast of Wörgl (1980). For the 3D modeling the subsurface of the survey area was dissected into a large number of equally big cubes. The modeling of the total magnetic field was done by variation of the magnetic susceptibilities of the individual cubes. Survey area southeast of Wörgl: It was already known that the basic and ultrabasic rocks of the Marchbachjoch are the reason for the measured magnetic anomalies. The 3D modeling shows a tunnel shaped anomalous body subducting in a flat angle to the northeast. Survey area west of Zell am See: The magnetic anomalies within the Grauwackenzone are partly caused by a number of basic metavulcanites. Another substantial element of the magnetic structure is the entire trend of the Uttendorfer Schuppenzone. Based on the magnetic data it can be traced down to 2km underneath the surface with a steep dip of about 70 against north. Besides Metabasites the magnetization also should be explained by the precipiation of ore minerals from fluid phases. Survey area rear Zillertal: The magnetic anomalies are caused by Serpentinites and partly associated Amphibolites in the old Paleozoic Schieferhülle. The largest anomalous body is formed by the Serpentinites of the Ochsner and Rotkopf. Sepentinites and Amphibolites at the Pfitscher Joch form another large anomalous body. Some partly hitherto not yet known nearsurface anomalous bodies with a diameter of approx. 400 m have also beeen detected.