BLOCK ROTATIONS ON VERTICAL AND HORIZONTAL AXIS IN THE W ANATOLIAN EXTENSIONAL PROVINCE, TURKEY: PALAEOMAGNETIC EVIDENCE

Abstract

Western Anatolia is one of the world's most rapidly extending regions. The N-S extension commenced in Late Oligocene in this region shortly after the collision of Eurasian (the Pontides) and Gondwanan (the Anatolide-Tauride Platform) continental blocks. This extension lead to exhumation of deeply buried footwall rocks in core complexes (i.e. the Menderes Massif) and to development of a series of E-W, NE-SW and NW-SE-trending graben system on the hanging walls of low-angle detachment fault system. Thus, compressional structures formed during the collisional stage were superimposed by low-angle detachment faults formed during the extensional stage in W Anatolia. One of the major shear zones exposed in N Menderes Massif, to the S of Simav Graben is marked by up to 150 m thick mylonitic zone, dipping southward at low angles (ca 20 degrees). Contrasting views (compression versus extension) on the nature of this shear zone were previously proposed. Detailed study of mesoscopic and microscopic structures in the mylonites revealed a top-to-the-northeast shear sense, supporting the thrust interpretation. During this study, paleomagnetic data were collected from the Miocene volcanics exposed both on the footwall and hanging wall of this shear zone in order to constrain post-Miocene tectonic rotations of the region. Previous K-Ar dating of these Miocene volcanic rocks on the footwall and hanging wall yielded a very similar age (15.3 +/- 0.3 Ma and 15.8 +/- 0.3 Ma, respectively). Our paleomagnetic data revealed a mean direction as D/I=22.6 degrees/31 degrees in the horst and D/I=212.4 degrees/-47.4 degrees in the graben. The difference of inclination between the Miocene volcanic rocks exposed in the graben and on the horst block is ca 16.4 degrees, indicating that significant block rotations of the study area on both horizontal and vertical axis occurred during the post-Early Miocene period. When the structural data are rotated back to their pre-Miocene position, the S-dipping mylonitic shear zone becomes a N-dipping, low angle zone, with a top to the north extensional sense of shear.