PALAEOMAGNETIC ANALYSIS OF NEOTECTONIC ROTATION IN THE ERZINCAN BASIN, EAST TURKEY

The 50 km long Erzincan basin, which strikes WNW-ESE, is sited close to eastern end of the North Anatolian Fault Zone (NAFZ) and within ~150 km of the confluence of this fault with the East Anatolian transform. Although formerly interpreted as a simple pull-apart basin at a releasing step-over (e.g. ªengör 1979), it is now recognised as a more complex composite basin incorporating the ENE striking left-lateral Ovacik Fault which terminates in the southeast corner of the basin. The basin infill comprises Plio-Quaternary alluvial sediments and the total thickness may exceed 2-3 km. Morphologically-young basaltic, andesitic and locally rhyolitic cones emplaced along the NE margin of the basin provide the opportunity to monitor rotations during ongoing tectonic deformation. Radiometric dating by ourselves and others indicates that this volcanism is entirely younger than 305,000 years in age. Although sites suitable for palaeomagnetic investigation are difficult to locate because coherent lava units occur within morphologically young volcanic cones and within dominantly constructional surfaces mantled by volcanic debris, we were able to sample 43 sites. All were of normal polarity in conformity with the age evidence. Rock magnetic tests show that only titanomagnetite is a significant remanence carrier in these rocks. The mean palaeomagnetic direction is rotated clockwise with respect to the present field (D/I = 16.5/56.4°, R= 34.79, k = 6.4, α₉₅ = 9.6°). However, the group mean is dispersed for two reasons: (i) the volcanic cones lie within fault blocks which have rotated differentially with respect to each other and (ii) variable amounts of slumping, either down over-steepened primary slopes or during compaction of the volcanic edifices, has dispersed the magnetic inclinations. Although data are too few to draw positive conclusions, we find that two of the oldest cones show the largest rotations equivalent to average rates of ~4° per ten thousand years; comparable rates of rotation seem to be applicable to four of the cones with ages evidently younger than ~100 ka. We have also investigated Anisotropy of Magnetic Susceptibility in the lava flows and find that the majority of inferred flow directions are northerly or southerly; they appear to reflect the concentration of volcanic activity and the control of the lava conduits by the WNE-ESE fault system defining the north east margin of basin. We suggest that these all tap a single magma reservoir at depth.