Kinematics of Paleotectonic movements in the collision zone of the East-European platform Печать E-mail

FGUNPP «Geologorazvedka», Russia, FGUNPP «Sevmorgeo», Russia VNIIOkeangeoiogia, Russia

In the articulation system of the Atlantic and Arctic Oceans basins, a nearly identical structural-kinematics pattern of the Scandinavia and Kola Peninsulas can be noted:

1) at the rear of Scandinavia, which is bulging up at present, there is a subrectangular and knee-shaped orthogonal extension system - the Baltic Sea (the sublatitudinal link) with the Gulf of Bothnia (the submeridional link); 2) at the rear of the Kola Peninsula, which together with Sredny and Rybachy peninsulas, rose for 100 m and more in post-glacial time, there is a similar, but diagonal, system of the Kandalaksha Gulf (the west-north-west link) - the White Sea (the north-north-east link), that occupied the keel part of Epibaikal avlakogene. Thus, in the two cases, the following kinematics scheme is involved: an extension (subsidence) in the rear south and east parts and a compensating compression (uplift) in the frontal north and west parts, perpendicularly terminated into adjacent water areas.

In the geological structure of the region, of common knowledge are: 1) tectonic contacts of ancient (Katarchean with Riphean. Riphean with Paleozoic-Mesozoic) complexes in high-amplitude (up to 10km) faults at the similar marks of the relief sinking to the NE; 2) a step-like sinking of basement's blocks toward the sea; 3) a submonoclinal structure of the cover in tectonic steps; 4) the largest faults striking along the coast control the composition and thickness of Late Pre-Cambrian and Phanerozoic rock masses. The main tectonic feature of Sredny and Rybachy peninsulas and Kildin island is the prebaikalian (White Sea-Karelia) horst-graben (riftogenetic) structure of north-west (along the coast) strike, formed by a Proterozoic sedimentary complex (thick up to 8 km and greater), sinking submonoclinally to the north-east, and clearly reflected in the territory and water area relief This structure, being part of the riftogenetic system at the northern end of the East-European platform, is featured by:

1) a tangential location, "reflected" at an acute angle from the rigid Baltic Shield;

2) the neotectonic and recent activation, perhaps, leading to the rift closing:

3) dissection by transversely cross-cutting (transfer) faults;

4) the presence of intrarift horsts emerged above sea level in the form of Sredny and Rybachy peninsulas and Kildin island, formed by two series of Late Proterozoic suites: the Kildinskaya (Sredny island) coastal suite and the Rybachinskaya deep-water sea suite, which, in formation terms, are fragments of the hyperborean platform and pericratonnal sinking (foredeep) with the sedimentary cover practically undisturbed by folding:

5) the seismic sections diagnose clearly enough a sharp sinking of the basement in the areas of the Karpinsky and Trollfjord- Rybachy- Kildin (TRK) faults amounting to a few kilometers as well as the progradation complexes adjacent to the paleocontinental slopes;

6) the absence of any striking diagnostic signatures of thrust spreading dislocations - an intense folding in allochthonous overthrust plates and suture zones, accompanied with ophiolites (Ural, Timan, the Yenisei ridge, Kamchatka).

We think that a thrust (on the Barents Sea side) model for the structure of these land areas should be for discussion. As to the development of differently oriented thrusts, a more attractive and promising one seems to be the divergence-fan structure model for Sredny and Rybachy peninsulas [12] and, apparently, all the near-continental (near Kola) part of the Kola- Kaninskaya monocline. Such a model seems to be strikingly confirmed by tomographic sections. However, the kinematics of divergence-fan tectonic parageneses is developed insufficiently due to their exclusively complicated structure: they are essentially manifestations of regional zonal diapirism in which, as is known, after longitudinal contraction stresses, as a reaction to their relieving an intense caving, i.e. faulting takes place. It is precisely such structural parageneses are formed in the region in the near-surface frontal parts of allochthones, often veiling their originally thrust nature. The thrust nature of structural patterns of the Scandinavia and Kola Peninsulas can be substantiated by a scalloped form (the presence of fjords and bays) of their northern (frontal) parts limited by ruptures. This is typical of thrusts, the hypsometrically uplifted allochthones of which are subject, by their origin, to greater mechanical destruction in displacement than autochthones in contrast to rectilinear faults and shifts, whose uplifted walls are autochthones, i.e. stable blocks, formed by relatively more lithified rocks.

The analysis of the remote space photo and airborne geophysical materials suggests the following conclusions about the region's tectonic features:

1. The potential fields clearly reflect a divergence-fan morphology of block dislocations, and, therefore, a wide expansion of subparallelly extending (NW-SE) thrusts, whose shifters are oppositely directed. The direction of the contraction stress is the southwest - northeast. One can determine the superposition of thrust dislocations on the previous (for the same shifters) shearing tectonic movements, which resulted in big shift displacements. The latter were likely to give rise to the pericratonnal Middle-Upper Riphean sedimentation basin, mainly formed by terrigenous deep-water "Rybachinsky" complexes, which came into contact later on with some fragments of the hyperborean platform (Sredny peninsula), "soldered" to the shield's prebaikalian complexes.

2. The tomographic distribution sections of potential fields sources and densities of these sources according to the profiles, which are transverse to the generalized strike of the Riphean complex, reflect clearly enough several boundaries (layers) sinking southwards, the upper of which begins at the edge (beach scaip) of the Kola Peninsula and coincides with the Karpinsky lineament outcrop. Taking into account the Archean (for the Kola Peninsula) and the Proterozoic (for Sredny peninsula and Kildin island) ages of the rocks contacting in the Karpinsky lineament, as well as their general regional dip to the north-east, a thrust kinematics can be supposed for this fault. Worthy of attention and further investigation is the information contained in these sections (earlier nowhere made public) about the deeper layers sinking southwards in the zone covering the shelf Kildin island, Sredny and Rybachy peninsulas.

3. The comparative analysis materials on the spatial position of the main (limiting the megablocks) discontinuities in the region, mapped on the day surface (in satellite photographs) and reflected in the potential fields (at depth), allows to reasonably suppose the dip directions of then shifters and. knowing the age (stratigraphic level) of the rocks contacting in the rupture - their kinematics. Judging by the southward shift of the Karpinsky fault trace with depth of projections on the horizontal surface, one can determine a thrust of Archean complexes of the Kola Peninsula over Riphean Sredny peninsula and the littoral shelf. As very strong indirect indications for a thrust kinematics of this fault can be considered the "expansion" of the potential fields of Sredny peninsula (autochthone) into the territory of the Kola Peninsula (allochthone) and the Proterozoic complex spread under the Archean one.

4. A vertical layering in the form of tectonic plates of the earth's crust upper part (basement and cover) and a divergence-fan structure of the upper plate are clearly reflected in the longitudinal sections of the region's formation complexes.

5. According to regional gravity and magnetic data (Zhdanova, 1997; Glebovsky et aL 2002), it should be more justified to relate the Rybachinsky-Murmansk (Kildinsky) shelf not to the Kola-Kaninskaya monocline, limited in me west and pressed against tne continent (perhaps, going under it) by the Tulomskaya-Murmansk zone of magnetic maxima and by the transverse Riphean-Devonian rift (see Khain et al.. 2002), but to the western flank of the Mumiansk-Kurentsovskaya step, characterized by relatively elevated values of the magnetic and gravity fields. In such a situation, the Kola-Kaninskaya monocline is buried under the Murmansk block (allochthone).

6. In some land-sea areas a "telescopic" stepwise character, reflecting old riftogenetic structures, can be clearly noted for the zones delimiting the Rybachy, Sredny and Kola peninsulas, the Kola Peninsula and Kildin island.

Unfavorable conditions for preserving the syngenetic HC accumulations and forming the epigenetic ones in the region, characterized by an intense fragmentation of rigid, mainly terrigenous complexes, are the absence of regional fluid traps such a Vendian-Cambrian carbonate-halogen complex for Riphean HC deposits in the Siberian platform and, in some places, multilayer traps. It is necessary to look there for a reliable tectonic screen, dipping gently and unconformably with respect to the position of rocks. Such screens may be only thrusts and separations, dipping toward the continent, taking into account a monoclinal sinking of rocks into the sea.

Thus, the Sredny and Rybachy peninsulas, the Kildin island and the Murmansk shelf are a Riphean (4-8 km thick) graben-horst structure of the edge part of the East-European platform, perhaps, tectonically covered with baikalites on the sea side and Archean complexes on the continent side. All other conditions being equal (the most important of which are weak indications of a potential oil and gas formation of Late Proterozoic complexes in the Sredny peninsula), under the western or eastern limbs of the transverse Titovsky-Bolshemotkovsky graben the formation is possible of underthrust traps (and deposits) in the areas where the shift displacements are transformed into the thrust ones in the Karpinsky lineament zone, but only if the Kola Peninsula is thrusted over the Sredny peninsula and the Barents Sea water area. Gravity and magnetic data give strong grounds for development of a thrust tend in the "continent-Hie Barents Sea" direction in this region. This assumption may be checked by carrying out the mutually perpendicular marine seismic profiles: Titovka bay -B.Motka bay and Sredny peninsula's isthmus - traverse of Pikshuev Cape. It should be kept in mind, that in the folded systems, where the reconstruction of Riphean oil-and-gas-bearing basins during Baikal tectonomagmatic events was accompanied with an intense thrust formation (Timan, the Yenisei ridge, the Predyeniseisky trough), the HC fields are destroyed.



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