DEFINING THE EASTERN BOUNDARY OF THE SIBERIAN CRATON FROM STRUCTURAL AND SEDIMENTOLOGICAL STUDIES OF THE VERKHOYANSK RANGE

The Verkhoyansk Range and the thrust-and-fold belt of the same name is located to the east of the Siberian craton. Sedimentary rocks dominate the succession and display multistage evolution with several rifting events and final thrusting and folding in the Cretaceous. Imbricate thrust fans define the predominant structural style of the Verkhoyansk thrust-and-fold belt. In the frontal ranges, where thick carbonate units locally predominate in the succession, the structure is very similar to that of the Rocky Mountains in southern Canada but with lesser amounts of shortening that do not exceed 35%. However, in contrast to the Cordillera, structural equivalents of the Front Ranges occupy an area that is up to 500-km in width. Small-scale potential field studies suggest that the entire Verkhoyansk Range is underlain by continental crust that forms the eastward continuation of the Siberian craton basement. Application of the thrust-wedge model to the frontal ranges of the study area shows good agreement between observations and theory. However, application of this model to the entire belt meets problems related to very a small wedge angle and impossibly high amounts of erosion required in the internal part of the wedge. Salt and evaporite units widespread in the Devonian section can not be an important factor to reduce the thrust wedge angle because the basal detachment is typically located much lower in the succession and the structural setting of these weak units is within the thrust wedge, not at its base. Preliminary estimations of the thrust-wedge size assume that its length should not be more than 200 km. This is in general agreement with sedimentologic and tectonic subsidence studies that assume that the continent-oceanic transition lies fairly close to the frontal thrusts separating the Verkhoyansk belt and the Siberian craton. Two interpretations are possible: (1) a significant part of the Verkhoyansk Range is underlain by oceanic-like crust, or (2) the Verkhoyansk Range is underlain by different continental blocks, in part, probably, of non-Siberian origin. In both cases, the eastern boundary of the Siberian craton basement is located not farther than 200 km eastward of its modern structural boundary.