RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6002, doi:10.2205/2005ES000187, 2005
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Figure 18 |
[65] The crustal layer and the upper mantle were found to contain heterogeneities of different sizes, which are reflected in gravity anomalies and also in the complex combinations of rock volumes with different seismic wave velocities and varying electric conductivity. A great number of wave guides have been recorded in the upper and lower crustal layers. In the upper crust they occur as broken, not extensive rock volumes. Also recorded were the mushroom-shaped rolls of high-velocity rocks over the low-velocity ones. An extensive thick waveguide was recorded between the upper "seismic" layer of the crust and the lower aseismic one in a depth interval of about 24-38 km. The crustal layer has a lower thickness under the Fergana and Afgan-Tajik basins, being thicker under the South Tien Shan Rise at the expense of the bulging of the lower crustal ("basite") layer and, to a lesser extent, of the upper ("granite") layer.
[66] The thinning of the crust at the transition from the mountains to the intermontane basins owed its origin to the rising of the subcrustal material and to some descending of all interface surfaces. The Conrad boundary is not recorded under the folded mountains. The upper boundary of the basement in the Fergana and Afgan-Tajik basins shows a gentle, subhorizontal position. Some segments of the Moho boundary show some regions of the "crust-mantle mixture" with seismic velocities of 7.4 km s-1 to 7.7 km s-1. The Moho surface shows narrow (15-20 km) and extensive flexure-type synform flexures with relatively steep angles. They are marked by negative gravity anomalies and by the suture zones of concentrated deformation and Alpine-type depressions, such as the Zeravshan, Karakul-Zidda, and other depressions. It is likely that these synforms are the traces of the Late Paleozoic zones of lock-joint subduction.
[67] The South Tien Shan area shows a regional gravity low situated inside of an extensive Central Asia low, the latter being associated with the low-density rocks of the mantle at depths of about 200 km. The varying density of the upper mantle is also marked by its varying electric conductivity. Magnetotelluric data suggest some volumes of some hot, low-density mantle material under the Fergana Basin (in its northern part) and under the Afgan-Tajik Basin, which has been proved by geothermal data. In the territory discussed in this paper, namely, under the large Afgan-Tajik and Fergana basins, the asthenosphere resides at the depths lower than under the Gissar-Alai Mountains. Seismic activity is restricted mainly to the upper crustal layers, being restricted to the depth interval of 10, 10-20, and 30-35 km.
[68] To sum up, the geophysical data correlate with the conceptual and model ones and confirm the layering of the crust and its lateral heterogeneity, suggesting some internal 3D mobility of its deep rock masses. The seismic and seismological data prove that the earthquake sources are concentrated along some critical depth levels, namely, along the tops and bottoms of the seismic wave guides, in the vicinity of the granite layer surface. In the Afgan-Tajik Basin, the earthquake foci coincide with the folded surface of the subsalt rock complex. Such levels are interpreted as the surface of lateral gliding and disharmonic breaking [Bekker et al., 1988]. The mobile state of the crustal material seems to be proved also by the mapping of "scintillation"-type boundaries. The analysis of the time sections showed that the recording and not recording of these boundaries in time and space are associated with changes in the state of the rock materials. Also established was the heterogeneity of the upper mantle and a change in the position of the asthenospheric layer in space. The uplifted volumes of the high-conductivity and low-velocity mantle are interpreted as the diapirs of the relatively heated, low-density mantle material. Changes in the mantle structure, as we pass from one large outcropping structural feature to another, prove the mantle control of the morphostructural change of the region. At the same time, the structural features of hercynian age are often traced using gravity data only to the depths of 10-15 km and are not directly reflected in the morphostructures of the surface of the underlying granite-gneiss basement and "basalt" layer, this reflecting the disharmony between the structural styles of the different crustal layers, on the one hand, and the "consumption" of the folded metamorphic rocks of the Paleozoic basement by the processes of volumetric metamorphism and granitization.
[70] The rocks of this region can be subdivided into two structural levels, the lower Paleozoic stage and the upper Mz-Kz stage, which are separated by the surface of angular (up to 90o) and stratigraphic unconformity. The consolidation of the Paleozoic rocks was associated with the hercynian tectogenesis. By the beginning of the Mesozoic Era, the South Tien Shan region was a slightly hilly continental plain. The Mesozoic-Paleogene time was marked by a qualitatively new, platform-type evolution period which was marked by the formation of the sedimentary cover composed of terrigenous-carbonate-gypsum shallow-sea marine, lagoonal, and continental deposits. From the end of the Eocene, or from the end of the Oligocene, as suggested by other geologists, the South Tien Shan region experienced its orogenic evolution with formation of mountains in the Neogene-Quaternary period of time. The surface of the pre-Mesozoic peneplane was deformed during its platform history and especially during the period of its recent reactivation. It should be noted that the surfaces of the pre-Mesozoic and younger (pre-Late Cretaceous and pre-Neogene) rocks are deformed almost conformably, therefore, below we will discuss merely the total deformation of the pre-Mesozoic peneplane.
[71] The South Tien Shan Mountains have the form of a megadome bordering the large negative structural features, such as, the Fergana and Afgan-Tajik basins. These structural features are treated here as the first-order basement folds or as megasynclines and megaanticlines. The basement surface is deformed differently in these large basins and in the Gissar-Alai Mountains. As follows from the results of the geophysical measurements, the basement surface producing the floors of the basins is fairly flat, being deformed merely by gentle bends with dip angles of 15o to 20o. Closer to the mountains, the pre-Mesozoic surface is more deformed, showing folds with the steep, overturned internal (mountain facing) limbs which are often cut by overthrusts. The vergence of the fore-mountain folds and overthrusts faces north in the southern side of the Fergana Basin, and south, in the northern side of the Afgan-Tajik Basin, the total structural pattern being divergent. The surface of the pre-Mesozoic peneplain shows a more complex deformation in these mountains. The background of this barrier-shaped (anticlinal) uplift shows a series of secondary-order bends of synclinal and anticlinal forms.
[72] The synclinal features are represented in the modern topography by basins with the preserved deposits of the Mz-Kz sedimentary cover. The floors of the synclines can be locally dislocated. The surface of the basement (the pre-Mesozoic peneplain), forming the floors of the basins, is usually poorly dislocated or undislocated at all. The marginal parts of the basins are more intensely deformed. The dip angles of the pre-Mesozoic peneplain are as high as 90o, the peneplain being often overturned or cut off by overthrusts with magnitudes of a few hundred meters to 2-3 km and larger in some areas.
[73] The basins have been classified as monovergent, divergent or convergent [Sadybakasov, 1990]. The monovergent basins are characteristic of the uplift limbs or of the zones of the contacts with the basins. The convergent forms are characteristic of the flanks and internal areas of the mountains. The divergent forms are restricted to the central part of the Gissar-Alai mountains, where they are uplifted highly and are almost undeformed (see Figure 13). To sum up, the form of the basins and their structural symmetry or asymmetry reflect the centrifugal movements of the rock masses from the axis of the mountains toward the depressions. In some of their segments the convergent basins are pressed between the opposite overthrusts with the formation of tectonic-suture structural features which were classified and mapped by Luk'yanov [1991]. However, as have been demonstrated above, these overthrusts are distinguished by the low magnitudes and decay rapidly with depth and along the strike, often grading to shear deformations or to the zones of longitudinal plastic flow.
[74] The modern mountain ridges are typical anticlinal structural features. The remnants of the pre-Mesozoic peneplanation surface reflect its significant plicated deformation. They show bends, including those of small curvature radii and dip angles, as high as 90o, and also the overturned positions of the peneplain surface. The forms of the pre-Mesozoic peneplain suggest the more intensive deformation in the uplifted areas compared to the basins. Along with the general "centrifugal" structural pattern, this suggests the relatively more "active behavior" of the rock masses in the anticlinal highs and bends in the course of the Alpine tectonic activity. Taking into account the rise of the rock masses to the elevations of more than 5000 m and the divergence structure of the region, it can be inferred that in the upper rocks of the elevation zones the vertical movements were transformed to horizontal ones, and the rock masses were "spread" from the middle of the fold belt toward the basins surrounding it, which was confirmed by the analysis of the stress state of the rocks [Nikolaev, 1992].
[75] The reconstructions of the pre-Mesozoic peneplain surface, based on the data reported by Lukina [1977] and Makarov [1990] and on my own observations, showed that the surface of the pre-Mesozoic peneplane was faulted and plicated in the course of the Alpine tectonic activity without any breaks. The curvatures of this surface are as high as many degrees and vary from subhorizontal to vertical and overturned positions. The example of these deformations is shown in Figure 14 for the area of the Fan Mountains.
[76] The style of the structural and lithological reworking of the rocks of the South Tien Shan folded metamorphic-rock basement, which provided their volumetric mobility, was highly variable. It is known that these variabilities of the mechanisms are associated, first of all, with the rheologic properties of the rocks and with their ability of tectonic volumetric flow or of rheid deformation [Beroush, 1991; King, 1967; Leonov, 1991, 1993, 1996, 1997; Patalakha, 1966, 1971]. In the Karakul, Zeravshan, and Kurganak zones, the central parts of which are composed of flysch deposits, the loss of coherence was caused by the process of melange formation; in the Ravat area, by the development of plastic deformation; in the area of the Gissar batholith granites, by volumetric cataclasis; in the carbonate massifs of the Fan mountains, by dynamic recrystallization. The loss of coherence resulted in rheid tectonics which, in its turn, permitted the bending of the basement surface (the top of the pre-Mesozoic peneplane) without its breaking.
[77] To sum up, the significant internal 3D mobility of the pre-Mesozoic folded metamorphic basement was recorded in the complex plicative form of its surface. The overthrusts complicating the sides of the basin are secondary and older than the pre-Mesozoic folds of the peneplane, which is confirmed by the attenuation of the faults along their strikes and dips, by their transformation to the zones of brittle-plastic flow, by the relatively poor deformation of the valley floors, and by the historical analysis of the development of negative structural features and of the uplifts surrounding them (for instance, V. D. Bosov, I. V. Koreshkov, V. N. Krestnikov, I. Sadybakasov, S. S. Shultz, to name but a few).
[78] The internal deformation of the rocks of the basement also confirms the conclusion of its 3D mobility. As follows from the data discussed above, three mechanisms might have been responsible for its mobility. These are the plastic deformation of the metamorphic schists of the Fan Mountains and Kurganak Zone; the melanging activity and the formation of the sedimentary melange protrusions in the Paleozoic rocks of the Zeravshan and Kurganak zones; the volumetric cataclasis of the significant volumes of the Central Gissar Batholith; the dynamic recrystallization of the carbonate massifs, and the formation and rejuvenation of the zones of vertical and horizontal plastic flow. In other words we have the real confirmation of the large-scale plastic, brittle-plastic, and cataclastic flow of the rocks in the area discussed. The internal deformation of the rocks of the pre-Mesozoic folded metamorphic basement agrees well with the folded surface of the pre-Mesozoic peneplain.
[79] In choosing a model for the structural shaping of some or other region, a question arises concerning the predominance of the vertical or horizontal crustal movements, and also concerning the stresses and forces that caused these movements. Many geoscientists (for instance, Makarov [1990], Zonenshain and Savostin [1979], to name but a few) supported the mechanism of general horizontal compression during the Alpine stage, which had been caused by the convergence of the Eurasian and Hindustan lithospheric plates. This view was based on the general warping of the Earth surface, the formation of folds, nappes, and tectonic sheets, as well as, on the existence of submeridional compression in the region.
[80] However, the space shrinking in the South Tien Shan region for the recent time was estimated by Chedia and Utkina [1990] to be merely 6 km, 12 km, and 14 km for the width of the zone measuring 80 km, 240 km, and 350 km, respectively. The tangential compression factor was found vary from 0.01 to 0.3, its average value being 0.04-0.05. Assuming that the South Tien Shan structure was shaped at the expense of the pressure from Hindustan and Pamir, the highest compression must have been concentrated at the meridian of the Pamir Arc curvature trend. However, this has not been proved, although the compression in the Alai Valley was found to be as high as 0.3, this suggesting that the Pamir pressure operated and simultaneously relaxed in the course of the accumulation of the rocks composing the Alai segment of the Afgan-Tajik depression. At the same time in the central and western parts of the Afgan-Tajik depression the surfaces of the "Paleozoic basement-sedimentary cover" boundaries and of the "Paleozoic basement-granite and metamorphic layer" boundaries are fairly flat and do not show any substantial bends or folds, suggesting that the tangential compression stress acting from the Pamir arcs and from the Hindustan Plate were not recorded structurally in the deformation of the boundaries mentioned above. If this assumption is correct, it is not clear how this pressure could cause the highly complex transformations of the rock masses and the deformation of the basement surface of the mountains located north of the region discussed.
[81] Some information of the space reduction might have been provided by the overthrusts and folds of Alpine age, yet, here, too, we deal with many problems. Many overthrusts are not of compression origin but reflect the extension conditions, including the formation of the subhorizontal tectonic flow zones. The potential operation of such processes has been demonstrated using models and geological examples [Luk'yanov, 1991; Ramberg, 1986]. As can be seen in Figures 9, 12 and 13 these overthrusts have insignificant magnitudes and attenuate along their strikes and dips or are transformed to the zones of brittle-plastic shear flow (in mechanical sense) of the overthrust kinematics. The deformation of the general compression associated with some large-scale reduction of the space is discarded by the relatively simple shapes of the internal syncline floors, compared to their sides. Combined with the overthrusting attenuation, this suggests that the synclines were compressed merely by the stresses arising in the basin sides. The formation of the structural pattern of the sedimentary cover in the Afgan-Tajik Basin, which is disharmonic relative to the basement surface, was controlled by another mechanism, namely, by the mechanism of lateral extrusion between the relatively rigid blocks of the Pamir and Tien Shan mountains [Kopp, 1997; Zakharov, 1970].
[82] As follows from the data reported by Nikolaev [1992], the crustal stress field, reconstructed for the lower layers of the crust in the Gissar-Alai mountains, was marked by submeridional compression and subvertical extension. The lower crust became shorter as it was forced toward the center of the mountains, and the rock masses were squeezed up. On the contrary, the regional surface field suggests the lateral subhorizontal extension and subvertical compression (flattening), which are reflected in the model offered in this paper. This view does not contradict the results of the light-type range finder measurements, which show that the Tajik Depression grows wider in spite its subhorizontal compression. This geodynamic interpretation agrees well with our interpretation [Guseva et al., 1993].
[83] The complex pattern of the pre-Mesozoic peneplain surface and the internal structural and material transformation of the basement rocks suggest that the foundation of the epi-Paleozoic platform, reactivated in recent time, behaved as a quasi-plastic body. The structural and lithologic transformations reflect the plastic, brittle-plastic, and cataclastic flow of huge rock masses. The degree of the Alpine structural and material transformations of the basement rocks was found to be higher than that of the sedimentary rocks, the structure of the basement rocks, associated with the Alpine tectonics, was found to be more stressed than that of the sedimentary rocks. Similar conditions have been discovered recently in the Central Tien Shan region, and an attempt was made to substantiate this phenomenon in physical and mathematical terms [Mikolaichuk et al., 2003].
[84] The studies of the general structural style, particular structural features, geophysical data, as well as of the physico-mathematical models available, suggest that the basic formation mechanism of the structural and morphologic styles of the South Tien Shan region is controlled at the present time by the volumetric redistribution of the crustal material, namely, by the flow of the rock masses from the basin regions and their displacement to the mountain region. This is confirmed by the geological and geophysical data available, by the deformation pattern of the pre-Mesozoic adjustment surface, by the operation of the mechanisms responsible for the flow and accumulation of rocks in elevated regions, by the geometry of earthquake sources, and other factors.
[85] The lateral tectonic flow operates at different levels of the upper and lower crustal layers, resulting in the origin of subhorizontal tectonic layering and in the formation of a disharmonic structure inside each layer. The origin of lateral flows, which are transformed to the zones of vertical rising and pumping, with subsequent spreading, seems to be associated with the development of heterogeneities (asthenospheric diapirs, convective and advective flows) in the reactivated low-density mantle, the existence of which is inferred from the results of geophysical measurements. The total subhorizontal compression stress, typical of the modern stress state of the region, were expressed in the mechanism of curvature instability, in the wave pattern of the distribution of the domes and basins of various sizes, and in the redistribution of the rock material in space. The compression stress controlled the spatial arrangement of the structural style and the formation (or regeneration) of some structural elements, such as diagonal strike-slip faults and longitudinal flattening zones (viscous faults and shear zones). It should be added that the recent time witnessed the operation of the mechanism of rock mass gravitational instability which was especially pronounced in the region of the Fan mountains. The gravitational instability of the highly elevated Fan rock masses (with the previous formation of their subhorizontal layering and the presence of low-viscosity rocks) resulted in the divergent subhorizontal "spreading" of the massif, the formation of overthrusts and sublayer breaks, the tectonic crushing of the young basins surrounding this rock massif, and the formation of the structural features of the "tectonic sewing" type.
[86] It follows that the formation of the Alpine structure of the South Tien Shan region was controlled by, at least, three different, relatively independent mechanisms, although associated with one another, namely, by the main compression-ejection mechanism, associated with the 3D lateral rock-mass redistribution [Leonov, 1993], and the accompanying mechanisms of bending and gravitational instability. The factors responsible for the mantle reactivation are not discussed here. This seems to have been a planetary event, unassociated with any regional mechanisms and environments, yet this event can be treated as the mantle response to the underplating of the Hindustan plate and its movement in the northern direction.
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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