A PALEOMAGNETIC STUDY OF THE SIVAS BASIN, CENTRAL TURKEY - CRUSTAL DEFORMATION DURING LATERAL EXTRUSION OF THE ANATOLIAN BLOCK

The Sivas Basin is a complex collage of Eocene and younger rocks locat ed within the wedge-shaped eastern margin of the Anatolian Block betwe en the (dextral) North Anatolian Fault Zone and the (sinistral) Easter n Anatolian Fault Zone. It has been subject to ongoing deformation by movement of the Arabian Block into Eurasia and concomitant sideways ex pulsion of the Anatolian Block. Post-collisional deformation since mid -Miocene times has been dominated by N-S to NW-SE compression expresse d by thrusting and strike-slip faulting. Cretaceous and Eocene rocks w ere magnetically overprinted to variable degrees during the collisiona l phase although these overprints have since been rotated mostly antic lockwise. Rocks emplaced during the neotectonic history are high-fidel ity palaeomagnetic recorders of subsequent block movements. Regional a nticlockwise rotation is recognised across the basin with differential rotation of fault and thrust-bounded blocks. An absence of perceptibl e differences between group mean rotations identified from Miocene, Pl iocene and Quaternary units shows that most regional rotation has been concentrated within the latest phase of the neotectonic history durin g Quaternary times at an average rate of similar to 10 degrees/Ma. Com mencement of this rotation postdates initiation of the North Anatolian Fault Zone implying that compression following collision was accommod ated initially by crustal thickening during Late Miocene and Pliocene times. Subsequent anticlockwise rotations have resulted from sideways expulsion of blocks to the south of the Central Anatolian Thrust along major NE-SW sinistral faults to achieve the crustal shortening result ing from N-S compression. These fault orientations and their sense of motion are explained by a Prandtl model involving deformation of a tri angular plastic terrane (the Anatolian Block) between two rigid plates (Eurasia and Afro-Arabia). The variations in regional rotation identi fied by palaeomagnetism show that average contemporary anticlockwise r otation of Anatolia revealed by GPS data (similar to 1.2 degrees/Ma) i s achieved by variable, and locally large, block rotations between maj or thrusts and strike-slip faults.