3D model of Svecofennian Accretionary Orogen and Karelia Craton based on
geology, reflection seismics, magnetotellurics and density modelling:
Geodynamic speculations
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Abstract
A 3D model of deep crustal structure of the Archaean Karelia Craton and late Palaeoproterozoic Svecofennian
Accretionary Orogen including the boundary zone is presented. The model is based on the combination of data
from geological mapping and reflection seismic studies, along profiles 1-EU, 4B, FIRE-1-2a-2 and FIRE-3-3a, and
uses results of magnetotelluric soundings in southern Finland and northern Karelia. A seismogeological model of
the crust and crust–mantle boundary is compared with a model of subhorizontal velocity-density layering of the
crust. The TTG-type crust of the Palaeoarchaean and Mesoarchaean microcontinents within the Karelia Craton
and the Belomorian Province are separated by gently dipping greenstone belts, at least some of which are
palaeosutures. The structure of the crust was determined mainly by Palaeoproterozoic tectonism in the intracontinental
settings modified by a strong collisional compression at the end of the Palaeoproterozoic. New insights
into structure, origin and evolution of the Svecofennian Orogen are provided. The accretionary complex is
characterized by inclined tectonic layering: the tectonic sheets, ~15 km thick, are composed of volcanic-sedimentary
rocks, including electro-conductive graphite-bearing sedimentary rocks, and electro-resistive granitoids,
which plunge monotonously and consecutively eastward. Upon reaching the level of the lower crust, the tectonic
sheets of the accretionary complex lose their distinct outlines. In the seismic reflection pattern they are replaced
by a uniform acoustically translucent medium, where separate sheets can only be traced fragmentarily. The
crust–mantle boundary bears a diffuse character: the transition from crust to mantle is recorded by the disappearance
of the vaguely drawn boundaries of the tectonic sheets and in the gradual transition of acoustically
homogeneous and translucent lower crust into transparent mantle. Under the effect of endogenic heat flow, the
accretionary complex underwent high-temperature metamorphism and partial melting. Blurring of the rock
contacts, which in the initial state created contrasts of acoustic impedance, was caused by partial melting and
mixing of melts. The 3D model is used as a starting point for the evolutionary model of the Svecofennian
Accretionary Orogen and for determination of its place in the history of the Palaeoproterozoic Lauro-Russian
intracontinental orogeny, which encompassed a predominant part of the territory of Lauroscandia, a palaeocontinent
combining North American and East European cratons. The model includes three stages in the evolution
of the Lauro-Russian Orogen (~2.5, 2.2–2.1 and 1.95–1.87 Ga). The main feature of the Palaeoproterozoic
evolution of the accretionary Svecofennian Orogen and Lauroscandia as a whole lay in the causal link with
evolution of a superplume, which initiated plate-tectonic events. The Svecofennian–Pre-Labradorian palaeo-ocean
originated in the superplume axial zone; the accretionary orogens were formed along both continental margins
due to closure of the palaeo-ocean.
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