Position of the Blyb Complex in the Pre-Mesozoic Crustal Structure of the Front Range Zone in the Northern Caucasus

摘要

A number of Variscan nappe complexes were recognized in the Late Mesozoic structure of the Front Range Zone of the Greater Caucasus in the 1970s. They consist predominantly of greenstone units and override one another in a consecutive order. The only exception is the upper, Atsgara Nappe, which is composed of crystalline schists, amphibolites, and microgneisses. Crystalline schists, gneisses, amphibolites, and other rocks of the so-called Blyb Complex occur at the base of the nappe packet. The affinity of crystalline rocks of the Blyb Complex to one of the upper Variscan nappes is substantiated in this paper. The Middle Paleozoic rocks, which originally were located below the Blyb Complex in the Front Range structure, overrode its rocks along the surface of the Blyb Thrust Fault in the Early Triassic. Since that time, the crystalline rocks of the Blyb Complex have occupied the lowermost position in the structure of the Front Range. The absence of Upper Paleozoic rocks in the footwall of the thrust fault is due to the fact that, in the Late Paleozoic, the area underlain by the Blyb Complex was an inlier and a source of clastic material. The hanging wall of the Blyb Thrust Fault may be traced farther southward into the Main Range Zone, where it most likely consists of the Laba Group and other rocks. As has been established previously, the lower portion of the Laba Group consists of analogues of the Middle Paleozoic successions of the Front Range Zone, while its upper portion consists of crystalline schists of the Lashtrak Nappe, which occupy a position similar to that of the Atsgara Nappe metamorphic rocks. These relationships suggest that the rock complexes of the Front Range Zone could have undergone repeated displacements due to post-Variscan (Indosinian) tectonic events and overrode crystalline rocks in the Main Range Zone and more easterly areas. Owing to the uplift of the Central Caucasus, they are now eroding. The difference in the metamorphic grade of the Blyb Complex and the rocks of the Atsgara and Marukha nappes is due to the fact the Blyb Complex lies close to the root zones of nappes or belongs to different nappe sheets. The Blyb Thrust Fault pertains to the Indosinian faults that played the main role in the formation of the Front Range structure.