Tectonic structure and development of the “depression/ uplift” transition zones, Northern Tien Shan

Abstract

This article presents data on the structure of transition zones from areas of relative downwarping (intramountain depressions) to anticlinal uplifts dividing them. The geological and geophysical data obtained by the authors in recent years in different areas of Northern Tien Shan are considered and discussed in order to compare the structure and tectonic evolution of key objects. A comparative analysis of the depression/uplift tectonic zones in different regions indicates a fundamental similarity in their structure. These areas of gradient vertical movements are zones of concentrated deformation. We describe ensembles of structures formed at the same time in sedimentary cover and in basement rocks. Similar structural features are due to the common tectonic evolution of basins and ridges as parts of a unified activation structure of the Paleozoic folded belt. Over a long period of time, from the Oligocene to the Pliocene inclusive, the depressions of Northern Tien Shan developed under conditions of sedimentary subsidence, probably by the type of pull-apart structures in a latitudinally oriented region of plastic shear deformation. The relatively quiet tectonic setting of this stage is reflected in the lithological features of the sedimentary complexes. The next stage, which began about 3 Ma, was marked not only by the emergence of a high-altitude relief and the accumulation of molasse, but also by a change of tectonic regime to transpression. The generally flexible bending of the foundation surface in the steep sides of the depressions, to some extent complicated by uplifts, was accompanied by the formation of extensive detachments and thrust-folded structural ensembles in the sedimentary cover of the depressions. At the same time, contrary to traditional ideas, the volumes of disintegrated basement rocks demonstrate significant plasticity, while the lateral pressure of the side ledges was transmitted for many kilometers into the depressions inside the sedimentary cover.