PRELIMINARY PALEOMAGNETIC RESULTS FROM THE ISTANBUL ZONE: EASTERN EXTENT OF THE HERCYNIAN OROGENIC BELT?

The Istanbul Zone, northwestern Turkey, consists of a Neoproterozoic middle to high-grade crystalline basement with relicts of oceanic lithosphere, volcanic arc and continental crust and it is overlain by a continuous, well-developed transgressive sedimentary sequence extending from the medial-late Ordovician to the early Carboniferous. The basin, which was tectonically stable from the Ordovician to the end of the Devonian, became a site of turbiditic flysch deposition and tectonically active during the early Carboniferous. The Carboniferous flysch marks the progress of a collision. That collision created a dominantly (now) west vergent marginal fold and thrust belt on the eastern side of the Bosphorous and what now seems an east vergent (but with many inconsistencies) on the western side as a retrocharriage. The region of Istanbul shows essentially no metamorphism and only a weak cleavage development. The Palaeozoic sequence is unconformably overlain by Permian and younger sedimentary strata.

In this study we present new paleomagnetic data from late Silurian-early Devonian limestones. Our results show that, despite the alteration, rocks preserved their primary magnetic mineralogy. Alternating field and thermal demagnetizations yield a characteristic remanent magnetization direction; D=328.9⁰, I=12.2⁰ (a95=4.7⁰, N=75) after tilt correction for the 7 best sites. The dispersion parameter is higher for tilt corrected directions (K=12.9) than for in-situ directions (K=24.5) suggesting a pre-tectonic origin for the magnetization. Since the inclination shallowing effect has not studied, paleolatitude information is not reliable. Assuming a normal polarity, results imply 31⁰, anticlockwise rotation of the Istanbul Zone. This supports the idea that Istanbul Zone was a north facing Atlantic type continental margin, which was located at the northeastern margin of Gondwana-Land.

Recent paleontological studies place the İstanbul Zone to about 30⁰–40⁰S for early Ordovician, however paleomagnetic studies indicate 16.4⁰S for the same time interval. Devonian fauna shows similarities with Thuringia, Rhenish Massif, Cantabrian Mts., Pyrenees, Holy Cross Mts. and N. Africa. This ongoing debate requires a detailed structural analysis and more geological- geophysical sampling for the Istanbul Zone.