RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6002, doi:10.2205/2005ES000187, 2005
[56] In spite of the very scarce information on the lithologic and petrologic history of the rock masses for the Early Triassic to the Early Eocene, some assumptions can be proposed here. During the Late Permian to the Early-Middle Triassic the territory of the South Tien Shan region existed as a platform with the formation of a peneplain and areal weathering crust (see Figures 6 and 7). The time period, when the platform was highly elevated, witnessed the formation of numerous explosion pipes which brought the rocks of deep crustal origin to the ground surface, indicative of the significant heterogeneity of the deep-seated rocks of the region (The Earth Crust of Uzbekistan, 1974). The formation of explosion pipes and the high elevation of the territory might have been associated with the overstrain of the lower layers of the heterogeneous crustal layer, and also with the processes of gravitational, lithologic, and dynamic equilibration, which resulted in the redistribution and transformation of the rock material with the formation of a "granite-metamorphic" and a "basalt" layer.
[57] During the Late Triassic and Early and Middle Jurassic, the platform conditions continued to exist, this time period being marked by some tectonic reactivation, by the differentiation of the topography, by the folding of the pre-Mesozoic peneplain, and by the formation of a system of troughs and low highs. The troughs were filled mainly with alluvial and swamp-lake deposits. Some basins experienced local crumpling. This reactivation was partially associated with the tectonic events in the neighboring Pamir-Hindu Kush region [Nikonov, 1990; Shcherba, 1990]. The fact that the crust of this region was highly responsive to the tectonic events that had taken place in the Tethys region suggests its relative mobility and incomplete consolidation. However, these processes were not associated with the general compression between the "rigid" blocks, because the formation of some troughs was accompanied in some cases by some extension, which is proved by the presence of quartz porphyry flows and extrusions in the mountains surrounding the Fergana region.
[58] During the Late Jurassic and the Early and Late Cretaceous, the South Tien Shan platform continued to react to the phases of the Alpine tectogenesis operating in the more southern region [Chedia and Utkina, 1990], yet the effect of the latter diminished with time. During the second half of the Cretaceous to the end of the Eocene, the subsidence became insignificant, and the whole territory concerned experienced the accumulation of thin continental carbonate, argilaceous and gypsic rocks, as well as of the lagoonal and shallow-sea sediments of insignificant thickness. Some of the erosion zones remained, yet, judging by the lithology and grain size of the resoling rocks, the topography was relatively low, except for some benches and scarps [Shcherba, 1990]. The platform conditions with chemogenous carbonate clay sedimentation continued to exist to the end of the Eocene. During that time (Late Jurassic to Eocene) the territory concerned experienced the alternation of the epochs of relative isostatic and geodynamic equilibrium (for example, the epoch of pre-late Cretaceous peneplain development) and the epochs of insignificant tectonic reactivation. These periods were marked by the tectonic deformation of the pre-Mesozoic peneplain surface and by the formation of extensive narrow lows and highs which might have been associated with the lateral redistribution of the rock material in the basement, namely, with its flow from the descending areas to the elevated zones of the tectonic topography following the mechanism of bending instability [Lobkovskii, 1988]. The voluminous rock mass flow during that period of time is proved by the origin and development of a system of lows and highs, at least up to the middle of the Miocene, as plicated structural features deformed by faults only in some scarce areas complicated by faults. By that time the magnitude of the tectonic topography, measured using the surface of the pre-Mesozoic peneplain was as high as 3-6 km.
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Figure 16 |
[60] During the Oligocene-Early Miocene the conditions of this region were close to the platform ones, with the formation of some regional unconformities, but never intermitted by any folding events. This period was distinguished by the general low-magnitude uplifts and by the formation of the gentle folds of the basement with a large curvature radius.
Citation: 2005), The Post-Oceanic Geodynamics of the South Tien Shan Region, Russ. J. Earth Sci., 7, ES6002, doi:10.2205/2005ES000187.
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