Imbricate thrust structure of the Northwestern Caucasus

Abstract

Many geologists suggest a determinative role of folding in the formation of the Northwestern Caucasus structure. Mapped or drilled thrusts are considered minor structures complicating steep fold limbs. Moreover, they are often described as reverse or even normal faults. In accordance with the generally accepted concept of overthrusts as subordinate structures relative to faulting, they are shown as short unlinked segments, which do not extend beyond anticlinal folds [1, 4, and others]. However, as will be shown below, this concept is erroneous and inconsistent with factual data. Actually, the genesis, morphology, and position of folds in plan view are completely controlled by overthrusts, which define the general tectonic style of the Northwestern Caucasus. The considered region adjacent to the Western Kuban marginal trough is divided into the Psebeps‐ Goitkh anticlinorium in the axial part, the Novorossiisk‐Lazarevsk synclinorium in the south, and Soberbash‐Gunai synclinorium in the north. The tectonic zones mentioned above are bordered by large regional overthrusts. The most important of them is the Akhtyr overthrust, which separates the Caucasus fold belt from the adjacent Western Kuban marginal trough in the north. The overthrust initiated at the orogenic stage separated terranes of different geodynamic settings during the subsequent evolution of the region. The Western Kuban marginal trough was significantly buried beneath allochthonous structures of the Soberbash‐ Gunai synclinorium as a result of the northward displacement and overthrusting of rock masses on its southern flank. Therefore, the trough has a distinct asymmetrical structure with the axis inclined to the thrust front. The complex imbricate Akhtyr thrust consists of smaller thrusts. Therefore, its main surface, which is marked by major tectonic displacements, can be taken as the northern boundary of the Soberbash‐Gunai synclinorium. Its front passes through the Dzhiginka‐UstChekup‐Adagum‐Kesler‐Kudak‐Kievsk areas. Smaller thrusts that splay off from the main thrust to the north control the North Varenikov, North Adagum, and North Krymsk anticlines among others. The Soberbash‐Gunai synclinorium and Psebeps‐ Goitkh anticlinorium are in tectonic contact along the thrust fault, the front of which marks high-amplitude local uplifts (South Krymsk, Damansk, Gladkov, Verkhnii Chekup, and others). The anticlines have a linear shape, with short northern (near-thrust) limbs but gentler and wider southern limbs. Tectonic doubling of sections, brecciation zones, and slickensides are recorded in boreholes. To the south-southwest, the thrust fault surface becomes gentler with depth. In plan view, its front represents a slightly curved rather than straight line, which consists of smaller arc-shaped thrusts concave to the north-northeast.