GEOLOGY AND TECTONIC CHARACTERISTICS OF THE NORTH-NURATA FAULT

The North Nurata Fault is a part of the system of Bukantau-South Ferghana deep faults, which extended for a distance of more, than 1000km from the South Ferghana through the Nurata Mts. in Kyzylkum desert. In plan it is a stepped echelon-like system of parallel displacements. We regard zone of this fault as a major geotectonic and paleogeographical border, which was retained high mobility from the Early Paleozoic up today.

For full length the fault is a compound constructed zone, which is located on junction of two major Paleozoic geostructural elements: the Middle and the South Tien Shan and classified as a sutural zone, generated on the place of closure of paleoocean basin in the Middle Carboniferous.

Structural design of the North-Nurata Fault is rather complicated. Direct near displacements are observed numerous sharply angular blocks and wedges of different size, which are limited by fractures of some orders. In vertical section they are manifested like horsts, grabens, monoclines, graben-synclines and horst-anticlines. Within the zone of the Fault are wide spreaded small fractures, which are formed along displacements monotonous systems. There are two systems of shear fractures and one system of ruptures. For whole zone is characterized numerous dikes of medium composition, as usual within rupture fractures. All above named formations are determined complicated design of the North-Nurata Fault zone and underlined its feather-like character.

Plicative dislocations within fault zone are of very complicate structure as well. Usually are observed asymmetrical, strong and compressed folds of some orders, overturned into counter overlap of southern direction. Often are marked lying folds, which are in overturned bedding. These structures are accompanied by numerous longitudinal overlaps and upthrusts and formed systems of inclinated and deformed wedge scales.

Granitoid intrusives spatially are connected with overlap zones. Intrusives are located direct along overlap zones or at a distance from them. Intrusives are more or less parallel to overlap sutures and, apparently, are submerged together with them.

Specific feature of geological structure of the North Nurata Fault is a thickening of subparallel dislocations, among which are wide spreaded overlaps, which often are complicated with recent srike-slip faults. Strike of subparallel dislocations’ system coincides with deep structure’ strike.

This system of dislocation breaks up rocks into numerous blocks, lenses and plates of different dimensions (boudinage). Inner structures of plates and blocks are fragments of sections of different formations of various ages. Often they are of different zones and sub-zones and have very mixed lithological composition. It is characteristic, that almost complete absence of normal stratigraphical interrelations between stratified formations, manifestations of products of basaltoid magmatism, development of overthrust structures.

The structure of the North Nurata deep Fault was formed after the packet of covers was overthrusted during Moscovian age. At the same time accumulation of Chormagyz molasses were ended and forming of molasses of Farish suite (Middle-Upper Carboniferous) was begun in back-arc trough.

The North Nurata Suture is a result of prolonged being of subduction zone, which was subsided under the Middle Tien Shan and fixed zone of collision of two continents: Kyrgyz-Kazakh continent (the Middle and the North Tien Shan) and Alai microcontinent (the South Tien Shan. Paleogeodynamic reconstructions (Yu.S.Savchuk) show, that collision of these continental masses was not perpendicular, but slantwise. As a result the suture zone was turned into left-side system of displacements. On this stage (Late Hercinian stage) covers were deformated into synforms and antiforrms – large brachy-structures, often with slipped left wing (Kattaich anticline, Sentyab and Andreev synforms). On the next stage were formed vertical sygmoids. The rocks of covers were dynamic metamorphosed: they were crushed, partly schist-formed, rumpled into small isoclinic folds. In final stage of the Hercinian age intrusives of gabbro-granodiorite composition (Kattaich complex) and then mainly granitoides (Shurak complex) were injected within the southern horst-anticlinorium block of the North-Nurata Deep Fault. As a result neogenetic continental crust was consolidated still more and subsequently it was underwent mainly splitting strike-slip displacements. Hydrothermal fluids, which were injected from below, from relic chambers through channels, provoked metamorphism of rocks (ferruginization, silicification, metasomatism, etc.). In favorable conditions within lithologo-stuctural traps there were ores formation.

In Alpen stage of neotectonical activity the North-Nurata Deep Fault was an active fault again. At present the North-Nurata Deep Fault is the system of overlaps, zones of dislocations, steeply dipping upthrusts-displacements, secondary tectonically dislocations and transversal structures.