ALONG-STRIKE STRUCTURAL VARIATIONS IN THE FORELAND THRUST AND FOLD BELT OF THE SOUTHERN URALS

The foreland thrust and fold belt of the southern Urals has a number o f along-strike structural changes in which regional-scale structures t erminate abruptly. One important along-strike structural change is the southern termination of the Precambrian-cored Bashkirian Anticlinoriu m, which plunges southward beneath the synformal Zilair Nappe. Balance d and restored dip cross-sections, a N-S-oriented strike section, toge ther with a geological interpretation of a CDP reflection seismic prof ile have been used to determine the 3D structure and the amount of sho rtening in the foreland thrust and fold belt in the area of the Bashki rian Anticlinorium and the Zilair Nappe. The geometry of the foreland thrust and fold belt is that of a west-vergent, basement-involved thru st stack with shortening on the order of 13% to 17% (approximately 17 km). The small amount of shortening allows a correlation to be made be tween the along-strike structural changes and pre-existing basement st ructures. In particular, the southern termination of the Bashkirian An ticlinorium coincides with a Precambrian-age aulacogen, the Sernovodsk -Abdulino Aulacogen, developed in the basement. An E-W-oriented tear f ault, 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 forelan d thrust and fold belt, and exerted strong control on its structural a nd tectonosedimentary development. Because of the small amount of shor tening involved, the foreland thrust and fold belt of the southern Ura ls provides an important example for understanding the effects of pre- existing basement structures on the development of foreland thrust and fold belt's.