Palaeomagnetic study of block rotations in the Niksar overlap region of the North Anatolian Fault Zone, central Turkey

Abstract

This palaeomagnetic study investigates crustal deformation within, and adjacent to, the Niksar overlap area of the North Anatolian Fault Zone (NAFZ) in central-east Turkey. The studied rock formations comprise: (1) red limestones of Late Cretaceous age (3 sites); (2) mafic lavas of Eocene age on the north side (13 sites) and south side (9 sites) of the NAFZ; and (3) volcanic rocks of Pliocene-Quaternary age from the Niksar pull-apart basin within the NAFZ (8 sites). Comparisons with reference palaeofield directions computed from apparent polar wander paths of the Eurasian and Afro-Arabian plates identify two scales of regional and local tectonic rotation: 1. (1) A pre-tilting remanence in the Eocene volcanic rocks south of the NAFZ ( D I = 144.1 -47.5°, β95 = 7.6°) is interpreted to reflect counterclockwise rotation by 30-40° from the reference palaeofields. Contemporaneous volcanic rocks from the north side of the NAFZ have the same reverse polarity recorded in pre-tilting magnetisations. The remanence is also rotated counterclockwise ( D I = 152.4 -42.5°, α95 = 11.3°), but by about 8° less than the volcanics on the south side of the NAFZ. Hence similar amounts of rotation are observed on both sides of the NAFZ and are interpreted to reflect motions during the pre-Middle Miocene collisional history in this sector of the Pontides. No distributed clockwise rotations anticipated from subsequent dextral motion along the NAFZ intracontinental transform are observed. The slightly larger anticlockwise rotation found on the south side of the NAFZ probably records relative rotation of en-echelon wedges by continental escape during post-Middle Miocene strike slip along the transform. 2. (2) Within the narrow zone of intense deformation along the NAFZ, Cretaceous limestones appear to be rotated clockwise by dextral strike-slip motion whilst Plio-Quaternary lavas within a fault-bounded block in the overlap region associated with the Niksar pull-apart basin, have magnetisations consistently directed 240-270°E. Magnetic inclinations are not diagnostic of polarity but both polarity solutions identify rapid clockwise rotation at rates in excess of 50°/m.y. A normal polarity solution is favoured and implies that a block (ca. 5 km across) has undergone a strike-slip displacement of around 12 km within the NAFZ during the last polarity chron. Cretaceous-Eocene palaeolatitudes are closer to those predicted from Eurasia than Afro-Arabia, but a study of older rocks is required to resolve affinities of this sector of the Anatolian block. Theoretical models of crustal deformation across intracontinental transforms obeying power-law behaviour and treating the lithosphere as a viscous medium predict that distributed clockwise rotations should be observed about as broad across the NAFZ. These rotations are not observed. Instead intense clockwise rotation is confined to a narrow zone close to the major fault break. The concentration of historic seismic activity here also implies that the bulk of the dextral motion between the Eurasian Plate and the Anatolian block is accommodated by slip along faults close and parallel to the main trace of the NAFZ. © 1995.