Formation history of the Cheka pluton of alkaline granitoids (Southern Urals): fracture analysis

Introduction. Fracture analysis of rock formations allows us to reconstruct formation history and structural development of magmatic blocks. This study investigates the Cheka block of alkaline granitoids (Southern Urals, Chelyabinsk Oblast). The objective of this study was to evaluate the main deformation characteristics of the sincollisional Cheka block (pluton). For this purpose, stress fields were reconstructed.

The Cheka pluton is composed of the Cheka Mountain and has a meridional strike and dimensions of 6.5 km long and 1-2 km wide. The pluton is composed of alkaline rocks of three intrusion phases: first – monzodiorites, second – alkaline syenites, third – alkaline granites and granosyenites. The pluton is Triassic and intrudes Carboniferous volcanics. The western contact of the Cheka pluton is limited by a dextral fault. The pluton is situated in the Magnitogorsk zone.

During the formation of the pluton, extension changed to compression. This led to formation of a right-lateral transpression setting with a system of meridional strike-slip and near-slip extension zones.

Materials and methods. Space images show several fracture systems with approximate strike lengths: -20° and 310°. During the field work more than 180 fracture orientations were measured Samples were taken for petro- and paleomagnetic, geochemical investigations, and isotope dating at five locations.

The Stereonet v.11.3.0 software was used to analyse the fractures. Schematics (Mohr circles) with fracture poles were created for each location. From these, five swarms of poles with Kamb contours were extracted, showing the statistical concentration of the poles. At locations 701 and 702, three swarms of sub-perpendicular poles were most clearly observed and interpreted as a system of tectonic fractures. The S, Q, and L fractures were identified among the prototectonic fractures based on the relation to linearity and pluton contacts. All the poles that fell within these three swarms were treated as prototectonic, while remaining locations outside these zones were treated as a system of fractures of tectonic origin. Numerous fractures, which are not part of the described systems, are most likely random and require more detailed research.

Results and discussion. A series of vertical fractures, arranged in a pattern relative to each other, were considered. Based on these swarms, a deformation model was built, and the directions of tension and compression were determined. Sub-horizontal compression was oriented northeastward, resulting in the formation of sub-meridional right-lateral shear and a general right-lateral transpression setting. The predominant fractures were synthetic P (according to Riedel), and they are also the most pronounced geomorphologically and on satellite images. Less pronounced are synthetic R and antithetic R' fractures.

This study of the Cheka pluton made it has possible to separate two fracture systems. These systems point to right-lateral transpression, which confirms the model of the massif formation as a shear magmatic duplex.

Financial support. The reported study was funded by RFBR and Czech Science Foundation according to the research project № 19-55-26009. Centre of collective usage ‘Geoportal’, Lomonosov Moscow State University (MSU), provided access to remote sensing data.