Detecting of weakened zones in the geological basement of the Kola peninsula (north-eastern part of the Baltic Shield, Murmansk region)

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Geological Institute of the Kola Science Centre of the Russian Academy of Sciences

Detecting of weakened zones in the geological basement of the Kola peninsula (north-eastern part of the Baltic Shield, Murmansk region)

Dr. Sc.(Phys. & Math.), Lead Researcher

Filatova V.T.

Abstract

The Early Precambrian crust of the north-eastern part Baltic Shield formed during a long geological period, and the observed structure reflects the cumulative effect of multiple transformations. As a result, the geological basement of the region acquired lateral and mosaic heterogeneity. This work has been carried out using methods of numerical modeling to detect weakened zones in the basement of the region in regard to the construction and economic human activities. For this purpose, we have constructed quantitative models of the stressed-deformed state of the Earth's crust in the region considering its evolution. Determined for the first time are the structures that accumulated a tectonic-magmatic activity and produced mobile-permeable zones in the Precambrian mainly. For the first time, we have found an interrelation between localities of deep fractures (activation areas) and the stressed-deformed state of the Earth's crust caused by the impact of regional tangential stresses. The provided research indicates the necessity of studying the general geological structure of the region and identifying setting areas of ancient deep deformation structures in design and construction of the major industrial, road, hydraulic engineering and other objects.

Keywords: Kola Peninsula, Earth's crust, Early Precambrian, stress fields, numerical modeling, mobile-permeable zones, magma feeding channels, geological assistance of construction, geophysical assistance of construction.

The Murmansk region contains great sources of the most important types of mineral raw materials that predetermined the creation of a powerful mining complex. Currently, more than 100 mineral deposits in the region have a high investment and industrial value. At each step of designing and constructing industrial, power engineering (especially nuclear power plants), hydraulic engineering, road and other facilities, it becomes necessary to study the general geological structure of the basement in the region and obtain physical-geological estimates of its strength properties. Solving these tasks in mining areas, which location mostly correlates with the areas of long-lived deep faults, is particularly topical. baltic shield geological foundation

Different alternative geotectonic and geodynamic models for the development of the northeastern part of the Baltic Shield are suggested and elaborated [1,2,5,9,10], but up to the present time the processes that led to the specific structure and composition of the ancient crust remain unclear. Interpreting the conditions that cause tectonic deformations is a most essential part of reconstructing the geodynamic regimes predetermining features of the regional development and affecting its metallogenic specialization. One of the most essential features of the tectonosphere is the stress and strain state that controls tectonic and geodynamic processes in the crust. This work was carried out using methods of numerical modeling to explain the dynamic features of the formation of the magmatic structure system in the northeastern part of the Baltic Shield in the period of 3.0 - 1.6 Ga. With this in mind, we constructed quantitative models of the stressed deformed state of the Earth's crust in the region taking into account its evolutionary development. For the first time, we determined the structures that facilitate the ascent of mantle basic-ultrabasic magmas.

We assume that the study region represented a nonuniform elastic body subjected to the action of volume forces and specific stresses at its boundaries. The problem of stresses was solved. The search for weak zones in the basement, which predetermined the localization of magmatic processes, was performed by estimating the maximum shear stresses. For the first time, we found an interrelation of the localization of deep fractures (activation regions) and the stressed deformed state of the Earth's crust caused by the action of regional tangential stresses. Our results allow us to understand the causes of inheritance of the geodynamic activity regions in the Early Precambrian and give grounds to revise the existing concepts about the mechanism of the formation of the Earth's crust in the northeastern part of the Baltic Shield.

Figure 1. Geological structural map of the Kola region of the Baltic Shield (from [12]).

Domains: Mur - Murmansk, Kol - Kola, Bel - Belomorian, Ter - Tersky, Ke - Keivy, In - Inary. Belts: Jon - Yona, K-V - Kolmozero-Voronja (Archean greenstone belts); LGB - Lapland, KGB - Kandalaksha-Kolvitsa (granulate belts); Pe - Pechenga, Im-V - Imandra-Varzuga, S-K - North-Karelian (Early Proterozoic rocks); (1) contours of Paleozoic intrusions (a-nepheline syenite, b-ultramafic alkaline rocks); (2) Upper Proterozoic sedimentary rocks. Early Proterozoic: (3) granite, granodiorite and diorite; (4) charnokite and granite (a), alkalinegranite (b); (5) volcanic-sedimentary rocks; (6) anorthosite and gabbro-anorthosite (in Keivy - Archean), gabbro, pyroxenite, peridotite. Early Proterozoic (or Archean?): (7) basic and intermediate granulite; (8) acid granulite. Late Archean: (9) granodiorite, diorite and enderbite; (10) alumina and super-alumina gneiss and schist; (11) acid gneiss; (12) fragments of greenstone belts (gneiss, amphibolites and komatiite); (13) fragments of banded iron formation (gneiss, amphibolites and ferruginous quartzite); (14) gneiss and schist; (15) gneiss and amphibolites; (16) granodiorite and diorite; (17) plagiogranite and granite-gneiss; (18) kyanite-garnet-biotite gneiss; (19) granite-gneiss, gneiss, migmatites and, rarely, amphibolites. (20) strike and dip; (21) subvertical faults and gentry dipping thrusts that separate the Proterozoic domains; (22) subverticalfaults and thrusts. Encircled numbers indicate type-sections: 1 - Keivy, 2 - Kolmozero, 3 - Ura-Guba, 4 - Kaskama, 5 - Korva, 6 - Ar-Varench, 7 - Voche-Lambina, 8 - Iona, 9 - Kovdozero, 10 - Tersky.

Geological features of the region. The northeastern part of the Baltic Shield is characterized by a long and complex history of development, and the observed structure reflects the cumulative effect of multiple transformations [12,14,15,16,18]. A significant part of the study region is composed of Precambrian rocks [12,16]. These rocks, whose age is 3.2-2.65 Ga [3,8], form the Murmansk, Kola, Belomorian, and Karelian Archaean megablocks, which all together make up a collage structure of the Earth's crust in the region (Fig.1). The Archaean Keivy structure embedded in the structure of the Kola megablock is distinguished among the secondary structures owing to the peculiarities of its structure [12]. In addition, the following structural zones are significant: the Archaean greenstone belts, i.e., Kolmozero - Voronje, Yona, and Tersky- Allarechka, and the Early Proterozoic mobile belts, i.e., the Lapland granulate belt and Pechenga - Varguza rift belt, as well as the giant Paleozoic alkaline intrusions (Khibiny, Lovozero). The Early Proterozoic Pechenga - Varguza rift belt inherits the development region of the Archean Tersky - Allarechka belt. The Upper Archaean - Early Proterozoic period is characterized by linear tectonics and formation of belt structures, which were zones of repeated manifestation of mafic magmatism. All belt structures are divided by vast fields of infra - supracrustal, deep metamorphic sialic complexes. Magmatic formations including those of mafic - ultramafic composition play an important role in the development and formation of the crust in the region. The major part of the presently known mafic - ultramafic intrusions reveals spatial correlation with the zones of deep persistent fractures and rift genesis.

Statement of the problem and principal equations. In the Late Archaean, consolidation of the Earth's crust transformed the region into a relatively stable continental structure, and by that time, the study region was in a stable state [4]. Hence, we can admit that the region could have been subjected to overall uniform compression owing to remote forcing. In the Early Proterozoic, the main front of tangential stresses was directed to the northeast [16,18]. The Murmansk megablock was in a stable position. It is not improbable that it was subjected to stress directed from northeast to southwest. Hence, we admit that, in the Early Proterozoic, the region was subjected to monoaxialcompression by uniformly distributed forces from the southwest and northeast.

We assume that the northeastern part of the Baltic Shield over the entire period of the geological history of the region represented an inhomogeneous elastic body subjected to the impact of volume forces and specific stresses at its boundaries. We also admit that (a) the tectonic magmatic activity established in the Early Precambrian was of the intraplate type; (b) the configuration of contact boundaries between the Archaean megablocks did not change strongly over the entire geological history. The region considered here consists of a few finite subregions. Each of them is considered uniformly isotropic and linearly elastic with linear elastic constants (Poisson coefficient м and Young modulus E). Each Archaean megablock is a separate subregion. The zones of deep fractures dividing the Archaean megablocks are considered as subregions with a width of 15-30 km. We specify the condition that the region is in the equilibrium state and the components of the stress tensor in the case of the plane problem satisfy the equilibrium conditions [13]:

where and are volume forces. We used the method of boundary elements to numerically solve this boundary problem with respect to stresses. The numerical solution is constructed using previously obtained analytical solutions for simple singular problems so as to satisfy the specified boundary conditions at each element of the contour [6]. We consider the upper horizontal surface of the solid medium model.

Figure 2. Model block regions: approximation schemes of the Earth's crust block structure in the northeastern part of the Baltic Shield for the period of 3.0-1.6 Ga: (a) 3.0-2.8 Ga; (b) 2.8-2.5 Ga; (c) 2.5-1.6 Ga; (1) contours of the modern coastline; (2) contours of the study region; T is force.

Three time stages of the Kola region development were considered in the course of solving the formulated problem (3.0-2.8, 2.8-2.5, 2.5-1.6 Ga), and correspondingly, a certain basic model was specified at each of the stages that describes the study region with account for the geological structures formed by the corresponding time (Fig. 2). With this in mind, we defined a rectangular contour for simulations that envelope the study region including the Murmansk, Kola, and Belomorian megablocks and the marginal region of the Karelian megablock in the contact zone with the Belomorian megablock. The area of the specified contour significantly exceeds the study region in order to exclude the influence of the contour boundaries in the simulations. Stress T was specified along the entire boundary in the numerical experiment. Since we do not have reliable data on the absolute value of forces in the region, we assume that their intensity T is equal to unity and obtain stresses in the simulations in the units of T. Stresses , , were estimated for each basic model, which allowed us to calculate the main stresses , and the maximum absolute values of shear stresses [13]:

The main stresses were calculated using the following relations [13]:

where is the angle between the axis of the main stress with the OX-axis, .

Finally, the values of stresses were normalized and presented as percentage of the maximum value over the region. Thus, after normalizing, the domains with anomalous shear stresses were considered as weak zones in the basement of the region. All the works were carried out using a scale of 1 : 1 000 000 and the initial geological chart of the region with a scale of 1:500000 [11]. Simulations for several versions of the load applied to the region were performed. In the case of overall uniform compression of the region (Fig. 2a, 2b) and mono-axial compression along the southwest-northeast line (Fig. 2c), the structural peculiarities of the region caused by the development of the permeable zones of the Earth's crust were pronounced most clearly.

The values of linear elastic constants () for the rocks of the Archaean megablocks, greenstone belts, Keivy structure, and fracture zones were specified according to the data presented in [7]. The Poisson coefficient in the simulations for the Archaean megablocks (Karelian, Murmansk, Kola, and Belomorian) and the Keivy structure was assumed equal to = 0.25, while in the weak zones it was = 0.3. The zones of deep fractures dividing the Archaean megablocks were considered weak. The Young modulus in each of the subregions was determined as the weighted mean value: in the Karelian, Murmansk, Kola, and Belomorian megablocks, it was taken equal to E = 6.2 ? 10⁴ MPa, and in the Keivy structure it was E = 5.8 ? 10⁴ MPa. In the weak (fracture) zones, the Young modulus was taken as one order of magnitude smaller. The regions of the Kolmozero-Voronja and Tersky - Allarechka greenstone belts, respectively, and the Pechenga - Varzuga rift belt, may be considered as weak zones because they represent mobile permeable structures.

Basic model: age interval of 3.0-2.8 Ga. The basic model includes structural elements of the Archaean basement formed by the moment of termination of the Earth's crust accretion in the region. The following structures are considered as subregions: the Murmansk, Kola, Belomorian, and Karelian megablocks, the Keivy structure, and also the fracture zones dividing the megablocks (Fig. 2a). The region is subject to overall uniform compression. The boundary conditions at the contour enveloping the study region were specified conventionally: and . We assume that the region at its boundary is subject to equal normal stress , and the tangential stress is . At the contact surface between subregions and , at each of its points q, we specified the conditions of continuous force and .

Basic model: age interval of 2.8-2.5 Ga. The initial basic model is supplemented with the subregions marked by anomalous values of shear stresses at the first stage of investigation; they overlap the development territories of the Kolmozero-Voronja and Tersky-Allarechka greenstone belts (Fig. 2b). The region is also subject to overall uniform compression. The boundary conditions at the contour of the study region were specified in a manner similar to the first basic model (Fig. 2a). The conditions of continuous force were also specified at the contact surface between the subregions.

Basic model: age interval of 2.5-1.6 Ga. The configuration of the subregions in the initial basic model to a great extent resembles the second model (Fig. 2c). The region is subject to mono-axial compression by uniformly distributed force T from the southwest and northeast. The directions of compression were selected according to the direction of the gaping fault of the Pechenga-Varzuga rift system and with the direction of the general front of tectonic stresses in the region. If we take the condition that the Y-axis in the local coordinate system coincides with the direction of compression, we get

Let us make a transition to the unique coordinate system for all models and perform rotation of the coordinate axes by angle [13]:

We eventually get the boundary conditions along the contour of the region in the following form [13]:

The conditions of continuous forces were maintained at the contact surface between the subregions.

Figure 3. Weak zones in the Kola region basement formed in the Early Precambrian in the time interval of 3.0-1.6 Ga: (1) Archaean belts: Kolmozero-Voronje, Tersky-Allarechka, and Yona; (2) Keivy structure; (3) Highly alumina gneiss of the Keivy series (polar fox tundra column); (4) Anorthosite intrusions and gabbro-anorthosites (Archaean-Early Proterozoic); (5) Pechenga-Imandra-Varguza paleorift belt; (6) Lapland granulate belt; (7) Layered massifs of basic and ultrabasic rocks (Early Proterozoic); (8) Alkaline intrusions (Paleozoic); (9) Fractures (fracture zones) at the contact between megablocks; (10) (a) State border of Russia, (b) modern coastline. Magma conducting (weak) zones distinguished on the basis of anomalous values of simulated maximum shear stresses: (11) formed in the interval of 3.0-2.8 Ga; (12) in the interval of 2.8-2.5 Ga; (13) in the interval of 2.5-1.6 Ga. Numbers in circles indicate distinguished zones: (1) Kolmozero-Voronja belt; (2) Terskii-Allarechka belt; (3) Tsaga zone; (4) Shchuchieozero zone; (5) Tuloma zone; (6) Kolvitsa zone; (7) Liinakhamar zone; (8) Mt. Generalskaya; (9) Porjitash zone; (10) Salny Tundra zone-1; (11) Salny Tundra zone-2; (12) Moncha Tundra zone; (13) Khibiny zone; (14) East Kola zone; (15) North Kola zone (Kolmozero-Voronja); (16) Vaynospaa zone; (17) Pechenga zone; (18) Litsa-Araguba zone; (19) Kola-Imandra Lake zone; (20) Kontozero-Khibiny-Kovdor zone; (21) Continuation of the western slope of the East Barents Sea rift system; (22) Continuation of the eastern slope of the East Barents Sea rift system; (23) East Keivy-Panarechka zone; (24) Imandra-Varguza zone; (25) Kandalaksha zone.

Results and discussions. Figure 3 presents the weak zones in the region distinguished from the anomalous values of maximum shear stresses. They formed in the Late Archaean-Early Proterozoic. These regions have a shape of belt structures and correlate with the regions of mafic-ultramafic intrusions distribution and with zones of tectonic distortions in the Kola region. The simulations for the first basic model (Fig. 2a) allowed us to distinguish weak zones 1-7, zones 8-14 in the second basic model (Fig. 2b), and zones 15-25 in the third basic model (Fig. 2c). The distinguished zones 1-4 (Kolmozeo-Voronja (1), Terskii_Allarechka belt (2), Tsaga (3), and Shchuchieozero (4)) are known from the geological data as the Archaean magma conducting regions. This was confirmed by the results of modeling. Zones 5-6 (Tuloma (5) and Kolvitsa (6)) are known as Early Proterozoic ones. Zone 7 (Liinakhamar) is traced along the fracture of the northeastern direction that crosses the Murmansk and Kola megablocks. The regional geologists consider that the fractures of the northeastern extension are Archaean. The distinguished zones 8-12 (Mt. Generalskaya (8), Porjitash zone (9), Salny Tundra zone 1 (10), SalnyTundra zone 2 (11), Moncha Tundra zone (12)) are Early Proterozoic. Zone 13 (Khibiny) is known as Paleozoic.

Zone 14 (East Kola) spreading in the submeridional direction that crosses the eastern part of the Kola Peninsula and the White Sea basin is not considered as a magma conducting structure; its age and genesis are not known. It is worth noting that zone 14 crosses the system of fractures shown in tectonic schemes [4,16,18] at an angle of 10°. The admitted estimate of the origination time is Late Archaean. Zone 15 (North Kola) is known as Archaean (Kolmozero-Voronja), but magmatic activity was also later observed in this region. Zones 16-19 (Vaynospaa (16), Pechenga (17), Litsa-Araguba (18), and Kola-Imandra Lake (19)) are Early Proterozoic. Zone 20 (Kontozero-Khibiny-Kovgor) is known as Paleozoic. Zones 21 and 22 are continuations of the slopes of the Eastern Barents Sea rift system of Paleozoic age (continuation of the western slope (21), continuation of the eastern slope (22)). Zones 23 and 24 (Eastern Keivy-Panarechka (23), Imandra-Varzuga (24) are of Early Proterozoic age, and zone 25 (Kandalaksha) is known as Riphean.

Any critical situations in the origination of the ancient crust could have caused formation of deep fractures (activation regions) precisely in the weak zones that control the location of ore belts of sequential metallogenic epochs. Model simulations provide evidence of this viewpoint. These demonstrate that the main magma - releasing structures of the region are characterized by anomalous shear stresses. Identified weak zones in the basement of the Kola region are mostly superimposed and do not change the shape of enclosing megablocks and together form a frame, which unites the main structural elements (Archean megablocks) of the region (Fig 4). Figure 4 shows that the reconstructed ancient mobile permeable structures 1-3 extend from northwest to southeast, structures 4-8 - from the southwest to the northeast. Areas 1, 2, 3, and 5 formed in the Archean as weakened zones, areas 4, 6, 7, and 8 - in the early Proterozoic. It should be noted that the central part of structures 6, 7, and 8 (within the Kola megablock) began to form even in the Archean time.

Figure 4. Scheme of the location of the ancient deformation magmatic structures (frame tectonogens) in the northeastern part of the Baltic Shield. Numerals in circles indicate reconstructed mobile permeable zones in the Archean basement of the region.

Symbols are shown in fig. 3.

Mobile permeable zones formed in the crust of the region may be considered as frame tectonogens, which according to the terminology suggested by Sheinmann, Yu.M. [17] represent linear deformation igneous structures. These structures were the regions of stress relaxation in the crust, along which the crust was reconstructing during each of the tectonic cycles so that the crust qualitatively changed. At each stage of the Earth's crust development, the geotectonic contrasts were intensified and, correspondingly, the structure of tectonogens became more complex. Eventually, the ancient Archaean blocks of the region were crossed by a system of frame tectonogens controlling the local energy accumulation, which led to sharp activation of tectonic, thermal, and igneous processes.

Conclusions

The results of our research demonstrate that development of tectonogens (mobile permeable zones) is caused not only by the influence of the deep mobile zones whose roots penetrate into the mantle, but also by the stressed deformed state of the Earth's crust subjected to the influence of external tectonic forces. The investigations revealed the heredity of magma feeding channels in the region from the Archaean to the Early Proterozoic, which is confirmed by geological data. The analysis shows that the areas where tectonogens of different ages intersect are characterized by a wider range of multiple manifestations of the mafic - ultramafic magmatism within the Kola region. Thus, a lens-shaped belt structure of the crust was already formed in the Archaean, which along with the vertical layering acquires lateral mosaic inhomogeneity. Hence, the system of permeable zones in the basement of the Kola region formed in the Early Precambrian determined the course of development of geodynamic and mantle igneous processes in later epochs.

Notably, the seismicity in the Murmansk region has doubled in the last 30 years and is assumed to be capable of reaching 6 points on the MSK-64 scale. In addition, the powerful mining complex of the region has been generating a significant number of technogenic seismic events. The geological environment is a very mutable structure, and its detailed study shall be much expensive. The performed studies using numerical modeling and available data on geology and tectonic structure made it possible to identify the weakened zones (areas of activation) in the geological basement of the region. The obtained results allow choosing the most favorable areas for construction and conducting a special study within them. At the same time, it saves time and money, and the informative value of the results may be useful in solving a wide range of construction tasks. The methods that were developed in this research may be used as those of express diagnosis that make it possible to identify weak zones in the basement and estimate degree of their permeability with minimum time and cost.

References

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