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Abstracts 14th Workshop of the International Lithosphere Program Task Force Sedimentary Basins conference dedicated to the memory of Frank Horváth Tentative results of the analysis of the main geodynamic processes of formation of the crust and the upper mantle structure of the Greater Caucasus; their spatial variability
Abstract
The results of restoration of the balanced structures of the sedimentary cover for Greater Caucasus in the rank of "structural cells" are used as a source material for the analysis of geodynamic processes of formation of the entire structure using factor analysis. The data set for 78 structural cells includes 6 parameters, which have relation to the development of the Greater Caucasus: the depth of the basement top at the pre-folded stage (stage 1), at the post-folded and modern stages (stages 2 and 3), the shortening value of the structure, the amplitude of the uplift and the difference in the depths of the basement top for stages 1 and 3. Factor analysis showed that the dispersion of all signs is provided the process of "Isostasy" (weight 47%) and "Shortening" (40%). Both processes are related to the processes of changing the density of the crust and mantle rocks. The combination of factor analysis of the two major regions and due to the addition of materials on Dagestan to them revealed that a good combination of the two mechanisms exists in the form of two processes only in the main structure of the Greater Caucasus, and especially reliably – in its western part.
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The article describes the factor analysis procedure ensuring its correct usage for identifying the processes that cause formation of fold structures and the main layers of the continental crust in mobile belts. The proposed approach to this problem of geodynamics is specific: it aims at solving the inverse (rather than direct, which is common) problem of identifying the processes that led to the occurrence of a natural structure characterized by quantitative indicators varying within a certain range of values. The objectives of the study were to specify the number of main processes/factors, describe their nature and calculate their relative ‘loading’ values. The database included detailed structural profiles across the fold structure of the Greater Caucasus. A special method was applied to construct a balanced model of the sedimentary cover, considering ‘structural cells’ which are 5–7 km long along the profile. Each of the 78 ‘cells’ studied was characterized by six parameters: the depth of the basement top at three stages of development (pre-folded, post-folded, and post-mountain-building), the amount of shortening, the amplitude of neotectonic uplifting, and the difference between the depths of the basement at the first and final stages. The parameters, that are directly related to the evolution of the blocks of the continental crust in the study area, constituted the initial data array for the factor analysis. In the first step, the Kaiser criterion was used to determine the number of factors, and it was equal to two. This number was specified for the main study using the methods of principal components with rotation. Factor 1 (Isostasy) amounted to 46 % of loading value, with high loads of the parameter of the depth of the basement top at stages 1 and 3. Factor 2 (Shortening) amounted to 40 %, with high loads of the indicators of shortening values and the amplitude of neotectonic uplifting. Factor 1 is related to the process of ‘isostasy’: after folding and orogeny is complete, the basement top of the ‘structural cells’ tends to return to its depth which was obtained on the pre-folded stage. Factor 2 is related to the process of shortening of the structure. The Chiaur zone was chosen as an example to analyze the Alpine-type development of the structures using the isostatically balanced model. The analysis shows that this zone formed as the density of the crystalline crust gradually increased to the ‘mantle’ values. Geodynamic modeling still fails to properly take such transformations into account. In the discussion of the results, attention is drawn to the fact that the established process of ‘isostasy’ is natural, i.e. not pertaining only to a theoretical model. It is noted that a geodynamic model can be correctly constructed if it considers the impacts of both processes revealed in this study. The obtained results can be used for improving the geodynamic modeling of fold-thrust mobile belts.
This article is focused on identifying geodynamic mechanisms leading to formation of large crustal blocks in nature. A specific feature of our study is statistical analysis of the data obtained by the methods of tectonophysics and structural geology. The analyzed material included 24 detailed structural sections (almost 500 km in total length) of Greater Caucasus. The Meso-Cenozoic sedimentary cover, that was intensely folded in the Oligocene and Early Miocene, is 10–15 km thick. A structure balanced in strain amounts and sediments volumes was reconstructed for three stages in the development of the studied area: 1 – pre-folded, 2 – post-folded, 3 – modern post-orogenic. The ‘geometry of folded domains’ method was used. For this purpose, 505 structural domains were identified in the detailed structural sections, the pre-fold state for every domain were reconstructed, and all the domains were aggregated into 78 structural cells. The reconstructions were based on structural indicators measurable in the folds forming the folded domains. Each structural cell was characterized by six parameters: an amount of shortening; depths of the basement top in the pre-folded, post-folded, and modern stages (i.e. stages 1, 2, and 3, respectively); a calculated position of the eroded top of the sedimentary cover (i.e. amplitude of orogenic uplifting); and a difference between the basement depths in stages 1 and 3. For 78 structural cells, shortening is about 50 % on average (from 2–10 % to 67 %). An average modern depth of the basement top is 13 km (from 2.2 to 31.7 km). The amplitudes of uplifting and of the erosion of top of the sedimen-tary cover for large blocks are in a range from 9 to 19 km. Steady combinations of these values forming certain struc-tures have been detected on the studied areas. It was found that the depth of the basement top in stage 3 (modern) has tendency to keep the value similar to the depth acquired in stage 1 (pre-folded) generally. This effect may be caused by an isostasy.. A number of estimated high values of the pair correlations have a genetic meaning. Using the factor analy-sis (as generalization of pairs correlations), we detected two factors related to the geodynamic mechanisms leading to formation of the structures larger than the cells – of the crust, and the upper mantle. Factor F1 (shortening, 60 % weight) depends on the amount of shortening and is responsible for amplitudes of uplifting. Factor F2 (isostasy, 27 % weight) is related to the initial thickness of the cover; it is responsible for the stability of the depth of the basement top. Isostasy assumes significant changes in the density of rocks in the crust and mantle, including the obtaining of mantle density volumes by the large volumes of the crust rocks. The factor “isostasy” in such kind was not taken into account in geodynamic models earlier.
(in Russian, English captions)
The compact disk is an addition to the book. The disk contains computer programs, files of data to the program of balancing of sections, images of initial structural sections, files of EXCEL tables for calculations of parameters and files of other materials, mentioned in the book.
Abstract of Doctoral thesis. Manuscript official. 2015. In Russian
