Using the examples of Eastern Ustyurt and Southwest Aral region, it is shown that consolidated crust is distinguished as a stratified and multi-layered stratum with no regionally defined boundaries, which is apparently due to the fluctuations of heat flow and deformation stress fields in time and space, as well as magmatic “stratum inserting” from below. There are three types of consolidated crust distinguished in the mentioned regions based on the degree of tectonic stress and crust reflectivity which demonstrate considerable variation in the levels of geodynamic stress in space and time and varying degrees involved by folded-disjunctive dislocations caused with nesh macro shear deformation and shock metamorphism in the fault zones.Significant concentrations of differentiated tectonic stress fields cause large-scale disruption, shears and crustal substrate displacement on the different levels of depths. In the intermediate structural stage and sedimentary cover, these processes generate the overthrust structure and directly determine particular features of geological evolution, the structural and formation appearance, and oil and gas bearing potential. Consequently, this diagnostic feature can be used as a basis for petroleum zonation, the results of which are recommended for consideration while planning oil and gas exploration.
«…When doing oil & gas geological research, the most important thing is to distance oneself from modern topography and orography totally and at once, and understand that they are not a decisive factor of oil distribution. The key factors are found in deep-lying tectonics».
(A.A. Bakirov, 1986)
1. Introduction
A great attention should be given to consolidated crust and its structure, composition and tectonic mobility whilst studying the structural dislocations, broken conditions of sedimentary cover and intermediate
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structural stage (ISS), their genesis and determining their sedimentation characteristic, lithological composition and petroleum potential This component establishes the character and structural dislocation style of the sedimentary cover and
ISS, as well as determines the closest and causal, spatial and temporal correlations and interactions between them.Under these conditions, it is quite remarkable that the methodology of the study of these relations practically
has never been developed due to lack of attention and remains poorly studied issue.Hence is the purpose of this article: to study geodynamic processes taking place in the consolidated crust,
and to evaluate its tectonic stability and the influence these factors on the structural and rock formation properties of ISS sets using the examples of Eastern Ustyurt and South Aral region. Retain the status of weak study of this problem determines the resulting conclusions of this article in the form of forward-looking, high-quality models.Referring to the paradigm of the problem, we should say that in 80-90s of the last century in the periodical
press publications appeared, which indicated that the nature of geodynamic processes at various levels in the consolidated crust largely depends on the physical, and more accurate rheological properties making up its individual shells. The most important indicator of these properties is the viscosity of rock, which depends on their petrographic composition and hydration, temperature and pressure (Babadjanov et al. 1996; Leonov M. 1993; Leonov Y. 1993, Podurushin 2010).Direct correlations change in viscosity of rocks from the petrographic composition and hydration with depth
contrast does not appear, while the temperature and pressure are critical factors for viscosity changes with depth.Velocity of elastic waves and seismic activity are indirect indicators of rock viscosity: higher seismic activity
means more viscous and nesh rocks, while no seismic activity means that the rock pathing from brittle to ductile state.
2. Historical data
A small historical excursion shows that until the mid 70-s of the structure of the crust of the Eastern Ustyurt and Southwest Priaralye based solely on the interpretation of geophysical potential field. Since the mid 70’s and 80’s the consolidated crust in these regions actively has been studied with DSS (Deep Seismic Sounding) – ECWM (Earthquake Converted-Wave Method) and RCM (Refraction Correlation Method) - CDP (Common Deep Point Method) seismic survey. The total length of seismic lines exceeded 3000 kilometers. However, despite the relative informativeness these methods, the final model of the consolidated crust in these regions have not gone beyond the flat parallel structures (Babadjanov et al. 1989; Babadjanov et al. 1991). These ideas certainly played a positive role at a certain stage of development of the geological models.However, the solution of current problems of fundamental and applied geology led to the need to identify
more subtle structural and physical features of the structure of the consolidated crust or the magma-
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Deep structure, seismic models, rheology and geodynamics of consolidated crust
metamorphic earth crust (MMEC) (Kunin 1975).For those purposes, in 1986-1989 eight DSS – CDP lines with a total length over 800 km, crossed
the main tectonic elements of Eastern Ustyurt and Southern Aral region – Central Ustyurt dislocation system, Barsakelmes depression, Kuanysh-Koskalin arch and Sudochiy depression (Figure 1). Results of interpretation of RCM – CDP lines (Babadjanov et al., 1987) were added to the outcomes of interpretation of the seismic lines mentioned above.
Fig. 1. Schematic map showing the location of DSS-CDP lines.
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3. Research outcomes
Results of interpretation and the study carried out on their basis shows that the consolidated crust of Eastern Ustyurt and Southern Aral region is clearly stratified with no regionally determined boundaries (including seismic ones). This situation can be explained by fluctuations of heat flow and geodeformation stress fields in time and space, as well as magmatic “underplating” from below (Hamrabayev 1993; Babadjanov et al. 1996).By analogies with the other regions of the world, we will see that in the zones of high tectonic stress
the consolidated crust has increased reflective capacity (Kunin, Sheikh-Zade 1993). Using this diagnostic principle, three consolidated crust types can be distinguished in Eastern Ustyurt and Souther Aral region.First type of consolidated crust stands apart in Barsakelmes, Samskiy, Kosbulakskiy and Central - Aral
Sea depressions which were formed upon the rigid massifs of ancient consolidation (Abidov et al. 1994; Babadjanov et al. 1984; Kunin 1975). It is characterized by a clear two-stage structure (Figure 2).
Fig. 2. Subsurface seismic cross-section 05390488 (type I crust) (symbols are given in Figure 3).
The upper stage (with the thickness 8 - 14 kilometers) is “transparent” in seismic sections. He is expected to slightly affected by folded-disjunctive dislocations caused by brittle macro shear deformation and shock metamorphism in some fault zones. These zones are marked by diffraction points situated at depth to 15 - 20 kilometers and narrow subvertical zones lack of layering within the consolidated crust as well. It is assumed that these deformations occur under the influence of tectonic compression from the lower
Type II consolidated crustType I consolidated crust
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stage, which is seismically stratified. There are no significant changes in shape and size of the upper stage and tectonic stress here significantly are relaxed. The upper, seismically “transparent” layer (with V fv –formation velocity = 5.8-6.3 км/с) has a tendency to reduce the thickness in the edge areas of the stable blocks (Vol’vovskiy 1991).The lower stage (Vvif= 6.45-7.12 км/с) is marked by high concentration of reflectors, starting from the
depths of 16-24 km. In the interior of the stable blocks its thickness reduced to 7-10km (Vol’vovskiy 1991), preferably the type of bedding is subhorizontal (Figure 2), which indicates the existence of a threshold of tectonic deformation, that has not yet been overcome by tectonic stresses accumulated in it.In stable rock massifs, heat flow density is the lowest (30 to 45 MW/m2). According to various researchers,
only 15-25% of it is accumulated in the sedimentary cover (Babadjanov et al. 1984; Volozh et al. 1981). The asthenosphere level here is very deep (up to 400 kilometers) and, according to Zavgorodnyaya (1988), has a low melting grade. Consequently, the isolation properties of the lithosphere these blocks prevent the normal circulation of the heat flows. In the outlying zones of stable massifs, the thickness of the lower stage highly is increased. Here, it is distinguished with high concentration of reflectors which form pockets/swarms/lenses/systems.
Their appearance is obliged to significant concentrations of differentiated tectonic stress fields, which cause large-scale stripping, shears and streams of lower crust substrate under the influence of geodynamic pressure from tectonically mobile geological structurals - Central Ustyurt and Aktumsuk dislocation systems and Kuanysh-Koskala swell (Babadjanov et al. 2000). As result - in the marginal areas of the stable blocks extensively developed overthrust tectonic elements , listric faults, faults and cross-crust fault systems (Figure 2, 3).Summarizing the points mentioned above it can be assumed that the upper structural stage within the
blocks with I type of consolidated crust until the depth of 15 - 20 km has a high viscosity and friability; and lack of seismic activity tells us about stagnation of active tectonic processes.Lower structural stage from depth of 15-20 km is mostly ductile. Viscosity reduction occurs under the
influence of heat flow and falls on the Conrad discontinuity (K) level. Thus, in the blocks with I type МMЕС lowering viscosity determines its tectonic bedding and the different behavior of the lower stage of consolidated crust during movements and deformations of the crustal substrate.Second type of consolidated crust is characterized by layering on all it thickness by ensembles (wave
packets) of sub-horizontal, sloping, hummocky, divergent/convergent, deformed and split boundaries (with large polygonal inclusions of seismically “transparent” structure, interpreted as intrusions (Figure 3) - consequence of the lateral flow and increased mobility crustal substrate at all levels.This type of MMEC is confined to the blocks have been or are in a state of intra- platform tectogenesis and
undergo active stretching or compression in the Phanerozoic era. Heat flow density here is increased (up to 70 MW/m2) according to Y. Zuyev (1986).The stretching process (the northern part of the Central Ustyurt system of dislocations) is identified by the
appearance of divergent systems of listric faults and thickness reduction of consolidated crust. Listric faults start to flatten or traceable terminated close to Moho discontinuity (Figure 2).In addition, on Moho discontinuity the viscosity reduction is noted, which creates conditions for stripping of
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some ophiolite sheets directly along that discontinuity. The intense serpentinization usually is observed at the level of Moho discontinuity (Hawkesworth 2010; Hamrabayev 1993).Compression processes (eastern periphery of Kuanysh - Koskala swell, Aktumsuk system of dislocations)
is marked by a thickening of the MMEC up to 45km and by appearance of wedge-shape structures (Figure 3).
Fig. 3. Subsurface seismic cross-section 02890188 (type II crust)
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Thus, in the blocks with type II of MMEC continuous stratification is observed i.e. it can be assumed for the stripping and flowing of crustal substrate at all levels of the consolidated crust take place.Third type of consolidated crust is allocated in Sudochiy depression and in the southwestern part of Aral
Sea depression. A large number of branched, intersecting faults, sloping borders and ACFS (across crust faults system) create a cell-block image for highly fragmented consolidated crust.
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Within each cell the lamination is arcuate shape or divergent. It is hypothesized that type III MMEC in pre- Jurassic time, and later, in the Neogene -Quaternary epoch of tectonic regime activation was swept by active rifting process, when it widely is appeared dynamical processes of reconsolidation and disintegration the consolidated crust (Aplonov et al. 2000; Babadjanov et al. 1995).Widespread the seismically transparent “windows” because of the heating and the repeated displacement
of the mantle mass (ophiolite) (fig. 4). In the cells situated between those “windows” deformation processes at all levels take place under the conditions of increased crustal substrate ductility.Extremely hot heat flows have been detected within blocks with type III of the consolidated crust. Heat
flow density exceeds 90 mW / m2 in places, with increased values (60-90 mW / m2) according to Y. Zuyev (1986).This goes to prove a significant heating of the earth’s crust and its possible partial thermal processing.
Those zones are situated near the Ural-Oman lineament, which is considered to be a conductor for heat and mass transfer.Thus, convective heat flows stream demonstrate selectiveness and generally determine the tectonic
mobility of the MMEC.At the conjunction of large tectonic elements of Eastern Ustyurt and Southwestern Aral region there are
systems of dipping reflectors and deep diffraction points, which can be traced from the upper consolidated crust levels to TMZ (Moho Transition Zone) (Figure 3). Those systems are identified with listric faults on planes which the deformation and strength properties of rocks are experience rapid fluctuation.The length of such faults can reach 20-30 kilometers, and its dips varies over a wide range (from several
degrees to 40-55 degrees). The mechanism of deformation and movement of large volumes of rock along listric faults is realized under the influence of tectonic and hydro-geodeformation stress.Analogous objects have been well studied in Rhine and Pripyat grabens and in the conjunction zone of
Eastern - European platform with Ural Mountains, as well as various other regions (Kunin, Sheikh-Zade 1993).In some parts of the territory under consideration the consolidated crust is broken at full thickness by
across crust faults system (ACFS), which are rooted deep in the mantle. ACFS width can exceed 10 kilometers. No correlation between ACFS occurrence and particular structure type has been established.ACFS are interpreted as either mylonitization zones or channels saturated with products of magmatism.
For these «overpasses» products of magmatism can invade into the upper crust and sedimentosphere, a kind of geodynamic “asylum”. In this case, a mechanism of viscous inversion takes place; local mélange appears with accompanying hydrothermal and deformation processing of rocks.At the base of the consolidated crust there is specific and complicated construction Moho transition zone
(MTZ). Its top (with VRV(refractor velocity)=7,7-7,8 km/s) can be clearly allocated only in stable blocks with type I of consolidated crust, where it occurs at depths of 34-38 kilometers [12]. In blocks with types II and III of consolidated crust it is established along angular displacement and high formation velocities (7.2-7.8 km/s). MTZ behavior is nonconformable to intra-crustal boundaries, and is associated with the genesis of magmatic stratum from below.However, Y.Volozh and Y.Akishev have a different opinion – they believe that MTZ origin is associated
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with petrographic heterogeneity at the base of the consolidated crust.MTZ thickness significantly varies depending on the type of the consolidated crust. In blocks of type I it is
rather stable (4-5 km). In type III of MMEC abnormally large MTZ thickness (10-12 km) was established.Generally, MTZ thickening matches by thinning of the consolidated crust and vice versa. Due to the
thinning of MTZ in the consolidated crust there are additional heating and partial recrystallization of rocks under the influence coming from the mantle heated material. This situation can be observed in the northern part of Central Ustyurt dislocation system and in the western part of Kuanysh-Koska swell. Consequently, MTZ can be viewed as a screen blocking deep-lying thermal flows.Thus, the consolidated crust of Eastern Ustyurt and Southwestern Aral region is characterized by a complex
combination of three its types, widespread listric faults and ACFS. All of those tell us about significant variation of geodynamic stress over time and space. In the context of the above mentioned quite reasonable looks assumption that the rheological layering
of the crust is a prerequisite for its tectonic stratification. In this case, the formation of mass overthrust at different depths with their peeling in the consolidated crust, sedimentary cover and ISS get a logical explanation.Therefore, one can argue about the causal and spatiotemporal relations rheology of the consolidated crust
and structural-rock formation composition of the overlying its sedimentary cover and ISS sets.Within geoblocks with MMEC type I the minimal disruption of ISS and the sedimentary cover caused by
tectonics-magmatic dislocations is observed; there are weakly differentiated and non-intensive gravitational and geomagnetic anomalies; small amplitude of neotectonics movements, oil and gas shows, and hydrocarbons deposits explored in sedimentary sets of sedimentosphere (Figure 5).
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Fig. 5. Correlation between a type of consolidated crust and amount of oil and gas shows 1 - Type I consolidated crust. 2 - Type I with decreased thickness of a seismically “transparent” layer 3 - Type II consolidated crust. 4 - Type II consolidated crust. 5 - Regions of listric faults concentration. 6 - Oil and gas shows in pre-Jurassic rock complexes. I-VI: Depressions: I. Sam. II. Kosbulak. III. Barsakelmes. IV. Sudochiy. V. Assakeaudan. VI. Aktumsuk. VII. Central Ustyurt dislocation system. VIII. Kuanysh-Koskalin arch.
In geoblocks with MMEC type II strength of the gravitational and magnetic anomalies reaches anomalously high values. Increased positive neotectonics movements here are accompanied by deformation and reduction of ISS and the sedimentary cover by tectonic - magmatic and fault dislocations. No oil and gas shows have been explored here.The blocks with type III of consolidated crust characterized by minimums in gravitational and magnetic
fields (complicated by insignificant maximum), intensive subsidence in the Neogene-Quaternary time and increasing thickness of the Mesozoic sediments. Here are also fixed increase in thickness of all ISS stratons
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up to 8 km, characterized by sloping bedding [9]. Apparently, in these geoblocks inherited subsidence occurred and they should be treated as historical
depocenter.Numerous oil and gas shows, large and medium-sized gas and oil fields have been explored in the Jurassic
deposits of those geoblocks. It is hypothesized that MMEC with type III had undergone active rift genesis during the pre-Jurassic, when the processes of dynamic reconsolidation were widespread. Oil bearing potential of ISS sediments remains to be seen.
4. Conclusions
1. Consolidated crust of Eastern Ustyurt and Southwest Aral region stands out as multi-layered stratum, within which there is no stable regional borders.
2. The structure of the consolidated crust in these regions determine and generate by the heat flows and fluctuations of geodeformation stress fields in time and space.
3. There are three types of consolidated crust to varying degrees affected by fold- disjunctive dislocations, which are realized through macro shear deformation and shock metamorphism in the fault zones.
4. Rheological and tectonic layering of the crust cause large-scale disruptions, shears and crustal substrate displacement on the different levels of depths.under the influence of geodynamic stress and convective heat fluxes streams from tectonically mobile geostructures.
5. Deformation-stress state of the consolidated crust defines and controls the characteristics of geological evolution, the structural and formation appearance of the sedimentary cover and the intermediate structural stage, as well as the potential of its oil and gas bearing.
6. Deformation-stress state of the consolidated crust is recommended to be considered as the decisive criterion for petroleum-zonation, the results of which should be regarded when planning oil and gas exploration.
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