The Greater Caucasus (GC) fold-and-thrust belt lies on the southern deformed edge of the Scythian Platform (SP) and results from the Cenozoic structural inversion of a deep marine Mesozoic basin in response to the northward displacement of the Transcaucasus (lying south of the GC) subsequent to the Arabia-Eurasia collision. A review of existing and newly acquired data has allowed a reconstruction of the GC history through the Mesozoic and Cenozoic eras. In Permo(?)-Triassic times, rifting developed along at least the northern part of the belt. Structural inversion of the basin occurred during the Late Triassic corresponding to the Eo-Cimmerian orogeny, documented SE of the GC and probably linked to the accretion of what are now Iranian terranes along the continental margin. Renewed development of extensional basin formation in the area of the present-day GC began in Sinemurian-Pliensbachian times with rift activity encompassing the Mid-Jurassic. Rifting led to extreme thinning of the underlying continental crust by the Aalenian and concomitant extrusion of mid-ocean ridge basalt lavas. A Bathonian unconformity is observed on both sides of the basin and may either correspond to the end of active rifting and the onset of post-rift basin development or be the record of collision further south along the former Mesozoic active margin. The post-rift phase began with deposition of Late Jurassic platform-type sediments onto the margins and a flysch-like unit in its deeper part, which has transgressed the basin during the Cretaceous and Early Cenozoic. An initial phase of shortening occurred in the Late Eocene under a NE-SW compressional stress regime. A second shortening event that began in the Mid-Miocene (Sarmatian), accompanied by significant uplift of the belt, continues at present. It is related to the final collision of Arabia with Eurasia and led to the development of the present-day south-vergent GC fold-and-thrust belt. Some north-vergent retrothrusts are present in the western GC and a few more in the eastern GC, where a fan-shaped belt can be observed. The mechanisms responsible for the large-scale structure of the belt remain a matter of debate because the deep crustal structure of the GC is not well known. Some (mainly Russian) geoscientists have argued that the GC is an inverted basin squeezed between deep (near)-vertical faults representing the boundaries between the GC and the SP to the north and the GC and the Transcaucasus to the south. Another model, supported in part by the distribution of earthquake hypocentres, proposes the existence of south-vergent thrusts flattening at depth, along which the Transcaucasus plunges beneath the GC and the SP. In this model, a thick-skinned mode of deformation prevailed in the central part of the GC whereas the western and eastern parts display the attributes of thin-skinned fold-and-thrust belts, although, in general, the two styles of deformation coexist along the belt. The present-day high elevation observed only in the central part of the belt would have resulted...
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 prefolded, 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 sedimentary cover for large blocks are in a range from 9 to 19 km. Steady combinations of these values forming certain structures 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 analysis (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.
<p>Talas Ridge forms the western part of the Tien Shan Caledonian structure. The sedimentary cover shows a thickness of about 10 km and consists of carbonate flysch and para-platform deposits metamorphosed under greenschist and lesser grade. This structure relates to the "hinterland" tectonic type, characterized by the abundance of many small and moderate-sized folds of the "similar" morphological type. Conventional cross-section balancing techniques developed for "foreland" structures, with large "parallel" folds cannot be applied correctly to such complicated structures. Thus, a special method based on the "geometry of folded domains" was developed for balancing of "hinterland" structures. To test the proposed method, we choose the westernmost Shilbilisaj profile of the Talas Ridge that consists of a large number of folds.</p><p>The proposed approach is based on the hierarchical system of hinterland fold structures, and on the accordance of the &#8220;folded domain&#8221; deformation to the strain ellipsoid, as described in detail in F. Yakovlev [2017]. On the first step the detailed structural profile is divided into a number of domains, 0.5-1 km wide; each domain consists of several folds of almost the same morphology. Consequently, a number of morphological parameters are measured, together with the axial surface dip angle and the interlimb angle that allow the construction of a strain ellipsoid for each domain. The core of the reconstruction method consists of three consecutive kinematic operations: 1) rotation, 2) horizontal simple shearing, and 3) horizontal stretching. As a result, a pre-folded form of a domain is produced, characterized by length and tilting of a domain segment that differ from the current profile parameter values. Sequential aggregation of all pre-folded domains leads to a complete pre-folded profile that allows the calculation of its shortening value. In the next step a few "structural cells" with a length approximately equal to the sedimentary cover thickness, are selected that combine several pre-folded domains. Taking into account the pre-folded and current lengths of such cells, their shortening values are determined. In the system of hierarchy of folded structures, folded domains and structural cells (and its strain parameters) belong to the third and fourth levels, respectively.</p><p>The first three project participants restored the structure of the section independently, starting with the domain selection procedure. The preliminary estimates of the shortening of the entire profile obtained by participants were close to each other and very high (K=L0/L1, where K &#8211; shortening value, L0, and L1 &#8211; pre-folded and current length in km, respectively): 4.49=118.5/26.4; 4.29=114.0/26.6; 4.67=119.1/25.5. The first participant allocated 63 domains and 12 structural cells, based on the thickness of the sedimentary cover. The shortening values for these cells varied along the profile from high in the southern cells to relatively small in the center and again to high in the northern parts (K=5.20, 4.47, 4.27, 3.79, 3.86, 3.93, 4.24, 4.91, 4.74, 5.53, 4.84, 4.9).</p><p>Yakovlev F.L. 2017. Reconstruction of folded and faulted structures in zones of the linear folding using structural cross-sections. Moscow, Published in IPE RAS, 60 p.</p>
Conventional cross-section balancing techniques based on layer length measuring can be applied only for foreland structures. To analyse complicated hinterland structure with numerous small-scale folds, this balancing technique requires the reliable and detailed tracing of the morphology of any layer throughout the cross-section, which is unattainable. We present a special kinematic method of balancing cross sections based "on the geometry of the folded domain" which enables the structural restoration of hinterland regions. We apply the method to restore the detailed structural section along the Shilbilisaj River, having a length of 26 km. We divided this section into 40-60 so-called "domains" each including 2-7 folds. Our method uses the fold's morphology to determine the strain ellipsoid, which describes the deformation of each domain and is used to restore its pre-folded state. By combining the pre-folded states of the domains, we reconstruct the entire profile, and calculate shortening values as K = L 0 /L 1 (initial to final length). The overall shortening value for the profile is 4.49, incrementally varying along the section from 3.79 to 5.53. The comparable results of two independently performed reconstructions emphasize the reliability of the applied balancing method.
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