A Cautionary Tale About Vertical-Axis Rotations, or How to Recognize an Orocline When You See One?

The Ural-Mongol orogenic belt played an important role in the formation of Eurasia, and is considered to be key to understanding the processes of continental crust formation and super-continental amalgamation. The prominent feature of the central part of the belt is a series of concentric horse- shoe shaped volcanic arcs, with the youngest arc on the inside. Models suggested for the formation of this structure include 1) collision and amalgamation of arc segments without significant rotations, and 2) the oroclinal bending of an initially straighter volcanic arc. In theory, it is possible to distinguish between these two scenarios with a paleomagnetic study: if the variation in paleomagnetic directions is proportional to the regional change in structural trend - this is an indication of oroclinal bending, if there are no rotations, or rotations do not show any correlation with the structural trends, the orocline hypothesis could be excluded. To test the orocline hypothesis we collected samples from the Northeastern (the Chingiz Range) and Central (Akmola area) limbs of the curved belt. Studied rocks were found to contain magnetization of the Permian age. When compared with the coeval Baltica reference direction, paleomagnetic directions reveal a complex but consistent pattern of vertical-axis rotations, with rotations in the Northeastern arm being dominantly clockwise, and in the Southwestern and Central arm - counterclockwise. Correlation of the rotations with regional structural trends is statistically significant, with the correlation parameter R2 = 0.8, and the slope of the regression line ~ 0.4 (compared with the trend of the Devonian volcanic belt). Taken at face value, this suggests that 1) the curved structure is an orocline; 2) at least 40% of its curvature could be explained by bending in post-Permian time. However, available geological data preclude the possibility of lateral movements on the order of 400 km at this time. There is an alternative explanation for the observed pattern of rotations. Tectonic structure of Kazakhstan is dominated by two systems of large strike-slip faults: a system of NW-SE trending dextral faults and system of SW-NE sinistral faults. Numerous paleomagnetic studies and theoretical calculations indicate that significant block rotations occur in a strike-slip environment; blocks bounded by right-lateral faults rotate clockwise, those bounded by left-lateral faults rotate counterclockwise. It just happened that the NE arm of the ``orocline" is a dextral strike-slip zone causing clockwise block rotations in this area, whereas the Southern arm is dominated by a series of sinistral faults with the anticlockwise rotations. Theoretical predictions of the rotation angles (based on the total displacement along the shear zone and the size of the rotating blocks) are also in a good agreement with the observed deviations of the paleomagnetic directions. Thus, the observed deviations could be fully explained by block rotations within strike-slip fault zones, and the good correlation of the rotations with the regional structural trends is likely nothing more than a coincidence. In conclusion, the post-Permian shear-related rotations in Kazakhstan are shown to mimic the oroclinal bending, at least to some degree. As the study area has a long history of large scale strike-slip deformations, likely associated with block rotations, care should be taken when interpreting the declination data in terms of the oroclinal bending.