Tectonically oriented paleomagnetic study of the Pag and Drniš-Sinj intramontane basins, External Dinarides

Márton, Emő and Pavelić, Davor and Vranjković, Alan and Ćosović, Vlasta (2016) Tectonically oriented paleomagnetic study of the Pag and Drniš-Sinj intramontane basins, External Dinarides. In: RCMNS Interim Colloquium 2016 and Croatian Croatian Geological Society Limnology Worksop, 20-24 May 2016, Zagreb, Croatia.

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Abstract

Magnetostratigraphy studies for a number of Miocene intramontane basins of the Dinarides documented that the lake sediments were good targets for paleomagnetism. Subsequent to the publication of magnetostratigraphy results from a series of sections (Mandic et al., 2008, Jiménez-Moreno et al., 2009, de Leeuw et al., 2010), we collected samples in 2011 from two of the basins, Pag and Drniš-Sinj for tectonic interpretation. In these basins lacustrine sedimentation took place, characterized by low sedimentation rate of predominantly carbonate material with tephra (Sinj basin) coal and bentonite intercalations (Pag Island). Post-depositional deformation affected the basin sediments in a non-uniform manner and intensity. In Pag Island, the Miocene basin sediments crop out in the 1200m long Crnika section. As several segments of it were obviously detached, we sampled the oldest and youngest segments, which seemed to be in situ, the former from the reversed, the latter from the normal polarity zone. The paleomagnetic directions obtained for the two parts were statistically different before tilt corrections, and remained so after restoring the strata to horizontal. A repeated visit to the section revealed that modern gravity–driven creeping can account for this, i.e. the results from the Pag basin should be rejected from regional tectonic interpretation. From the Drniš-Sinj basin eight geographically distributed Miocene localities and one Santonian yielded good paleomagnetic mean directions. Positive tilt test proved the pre-tilting age of the remanence for seven sedimentary localities. The overall-mean paleomagnetic direction implies CCW rotation of 13-20° with respect to Africa and 21-27° with respect to stable Europe, during the last 15 million years. A tephra intercalation with secondary remanence suggests similar rotation. The angle of the CCW rotation for the Santonian locality is somewhat larger than the one for the Miocene. The above conclusions are different from those by De Leeuw et al. (2012) for the Pag and Sinj basins, although the published paleomagnetic directions for the respective parts of the Crnika and the Lučane sections (in the latter case using simple tilt correction) are very similar. However, we had to come to the conclusion that the Crnika section must be excluded from regional tectonic interpretation, as it was discussed above. Concerning the Drniš-Sinj basin Lučane section belongs to a plunging structure and full tectonic correction has an effect on the declination. Even more important is that a paleomagnetic direction based on geographically distributed localities with different local tectonic positions, enhances the tectonic significance of a paleomagnetic result. As the External Dinarides are the loci of a complicated network of Miocene and even younger tectonic zones, we can not export the observed rotation for the whole unit, but consider our results as one more step for the kinematic constraints of the post-Oligocene tectonic history of the External Dinarides.

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