14. 3D deep structure of the Early Precambrian crust in the East European craton: A synthesis
Michael V. Mints, 2015. "14. 3D deep structure of the Early Precambrian crust in the East European craton: A synthesis", East European Craton: Early Precambrian History and 3D Models of Deep Crustal Structure, Michael V. Mints, Ksenia A. Dokukina, Alexander N. Konilov, Irina B. Philippova, Valery L. Zlobin, Pavel S. Babayants, Elena A. Belousova, Yury I. Blokh, Maria M. Bogina, William A. Bush, Peter A. Dokukin, Tatiana V. Kaulina, Lev M. Natapov, Valentina B. Piip, Vladimir M. Stupak, Arsen K. Suleimanov, Alexey A. Trusov, Konstantin V. Van, Nadezhda G. Zamozhniaya
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In this chapter, the available seismic and geological data are integrated and applied to the East European craton as a whole.
The deep structure and seismic characteristics of the granite-greenstone crust in the Archean microcontinents vary significantly. The unevenly distributed and vaguely oriented short reflections are occasionally gathered into packets. The intensely reflecting bodies with high acoustic impedance correspond to greenstone belts reaching many tens of kilometers in extent. The oval acoustically transparent domains in the middle-crustal level are composed of moderately dense rocks that correspond to granitoid plutons. The total thickness of the crust in granite-greenstone domains can vary from 40 km in the Kola-Karelia continent to 50 km in the Kursk microcontinent.
The Archean granulite-gneiss complexes form the delineated belts localized in the upper crust. These are synformal tectonic nappes, the vertical thickness of which reaches 15 km. A special case is the Volgo-Uralia continent, the crust of which is completely composed of granulite-gneiss rocks partly replaced by retrograde metamorphic assemblages. The granulite-gneiss crust is distinguished by significant thickness (~60 km; maximum 65–70 km). The lower-crustal “layer,” ~35 km thick, consists of inclined tectonic sheets plunging in a northwestern direction and penetrating the upper mantle. The interface between upper and lower crust is replaced by acoustically transparent granitoid crust 10–20 km in thickness.
The Middle Paleoproterozoic East Voronezh intracontinental collisional orogen between the Archean Kursk and the Khopior microcontinents is represented by a “crocodile-jaw” structure. The structural pattern in the seismic image of the crust clearly indicates a Paleoproterozoic age of lower-crustal layer and shows the absence of the Paleoproterozoic lower-crustal complex at the base of the Kursk craton proper.
The Late Paleoproterozoic intracontinental Lapland–Mid-Russia–South Baltia orogen surrounds in a wide arc the Karelian craton in the northeast, east, south, and southwest. The upper crust in the inner zone of the orogen in the Mid-Russia sector is composed of alternating granulite-gneiss and gneiss-migmatite-amphibolite tectonic sheets 5–10 km thick, deformed in gentle synformal folds. The marginal zones consist of south-plunging tectonic sheets of the Totma belt in the north and the Aprelevka belt in the south. The Totma sheet, reaching 10 km in thickness and dipping at a mean angle of 5°–10°, is traced by reflectors from the basement surface (interval 1700–1800 km) to the crust-mantle interface (interval 2000–2200 km). These parameters, along with composition of the rocks, allow interpretation of the Totma belt as a suture zone, separating the synformal structural assemblage from the lower crust.
In the Paleoproterozoic Svecofennian accretionary orogen, interpretation of the FIRE-1 profile shows that the Central Finland granitoid massif is a nearly horizontal, sheetlike intrusive body that conceals an accretionary complex—a succession of tectonic sheets, 10–20 km thick, which plunges northeast at angles of 10°–12° down to the crust-mantle interface at a depth of 65 km and can be traced beneath the margin of the Karelian craton for more than 150 km.
The lower-crustal layer, often called a reflectivity zone, is always present at the base of the Lapland–Mid-Russia–South Baltia orogen and the Archean cratons surrounded by this arcuate orogen. This layer was formed in the Early Paleoproterozoic as a result of under- and interplating of mantle melts accompanied by granulite-facies metamorphism. The increase in thickness of the lower-crustal layer is related to hummocking (mutual over- and underthrusting and wedging) of tectonic sheets at the base of the crust. The lower-crustal layer of the Kola-Karelia continent was formed before the main collisional events, which took place in the Late Paleoproterozoic.
The structure of the crust depicted by seismic reflectors indicates in some cases that the crust-mantle interface has remained unchanged since the time of crust formation, whereas in other situations, this boundary is younger than the bulk crust. The crust-mantle interface beneath the East European craton reveals manifold deviations from its persistent near-horizontal outline due to bending, plunging, and apparent dissolution of lower-crustal sheets in the mantle. The underlying upper mantle reveals a number of indistinct reflectors imaged as dashed lines, which trace lower-crustal structural elements incorporated into the mantle. These domains are regarded as crust-mantle mixtures. The crust images along the seismic lines exhibit widely varying structural features and degrees of contrast (sharpness) of the crust-mantle interface. The following structural and morphological types of the crust-mantle interface are distinguished beneath the East European craton: (1) a smooth, generally flat or horizontal or slightly sloping boundary with an abrupt decrease in number of reflectors at the lower edge of intensely reflecting layered lower crust; (2) a boundary similar to the previous type but periodically interrupted at sites where the sheetlike fragments of the lower crust sharply bend and sink into the mantle and acoustically as if they dissolve therein; (3) a serrated boundary in the regions of consecutive plunging lower-crustal tectonic sheets into the mantle; these domains are commonly conjugated: reverse-thrust assemblages in the upper crust rise in the same direction as the lower-crustal sheets plunge; (4) a serrated boundary confining from below the ensemble of inclined tectonic sheets that form the crust completely or partly; (5) a diffuse crust-mantle interface that is observed where a distinct lower-crustal reflectivity zone is absent; and (6) a phantom (disappearing) crust-mantle interface that separates acoustically transparent crustal and mantle domains that can be detected by seismic-refraction exploration.
Integration of the entire body of information allowed us to simulate the 3D deep structure of the Early Precambrian crust of the East European craton as a whole using sections along the 1-EU geotraverse, cross-traverse 4B, TATSEIS regional profile, and FIRE-1 and FIRE-4 profiles. The 3D deep structure of the East European craton is represented by the tectonically delaminated Early Precambrian crust with a predominance of gently dipping boundaries between the main tectonic subdivisions and a complexly built crust-mantle interface. The integral model includes Archean granite-greenstone domains and granulite-gneiss areas and Paleoproterozoic accretionary and intracontinental collisional orogens.
Figures & Tables
- continental crust
- crust-mantle boundary
- geophysical methods
- geophysical profiles
- geophysical surveys
- greenstone belts
- igneous rocks
- lower crust
- metamorphic belts
- metamorphic rocks
- orogenic belts
- Russian Platform
- seismic methods
- seismic profiles
- Svecofennian Orogeny
- three-dimensional models
- upper mantle
- upper Precambrian