Along-strike structural variations in the foreland thrust and fold belt of the southern Urals

Abstract

The foreland thrust and fold belt of the southern Urals has a number of along-strike structural changes in which regional-scale structures terminate abruptly. One important along-strike structural change is the southern termination of the Precambrian-cored Bashkirian Anticlinorium, which plunges southward beneath the synformal Zilair Nappe. Balanced and restored dip cross-sections, a N-S-oriented strike section, together with a geological interpretation of a CDP reflection seismic profile have been used to determine the 3D structure and the amount of shortening in the foreland thrust and fold belt in the area of the Bashkirian Anticlinorium and the Zilair Nappe. The geometry of the foreland thrust and fold belt is that of a west-vergent, basement-involved thrust stack with shortening on the order of 13% to 17% (approximately 17 km). The small amount of shortening allows a correlation to be made between the along-strike structural changes and pre-existing basement structures. In particular, the southern termination of the Bashkirian Anticlinorium coincides with a Precambrian-age aulacogen, the Sernovodsk-Abdulino Aulacogen, developed in the basement. An E-W-oriented tear fault, the Belaya tear fault, appears to have developed in the basement, along the northern margin of the Sernovodsk-Abdulino Aulacogen. The Belaya tear fault was active during much of the history of the foreland thrust and fold belt, and exerted strong control on its structural and tectonosedimentary development. Because of the small amount of shortening involved, the foreland thrust and fold belt of the southern Urals provides an important example for understanding the effects of pre-existing basement structures on the development of foreland thrust and fold belt's.