3D MODEL OF THE DEEP STRUCTURE OF THE SVECOFENNIAN ACCRETIONARY OROGEN BASED ON DATA FROM CDP SEISMIC REFLECTION METHOD, MT SOUNDING AND DENSITY MODELING

Abstract

A 3D model of the deep structure of the Late Paleoproterozoic Svecofennian accretionary orogen is presented. The model is based on coordinated data of geological mapping, seismic reflection surveys along the profiles FIRE-2-2a-1 and FIRE-3-3a, incorporates and utilizes cross-sections of the 3D density crustal model, the results of magnetotelluric surveys along the Vyborg-Suojarvi profile in the Northern Ladoga area and materials from MT surveys previously carried out in Southern Finland. The main structural features of the Svecofennian accretionary orogen were described relying on the 3D model. The accretionary complex is characterized by dipping tectonic layering: tectonic sheets some 15 km thick, made up of volcanic-sedimentary associations with electrically conductive graphite-bearing sediments and of granitoid rocks, plunge successively eastwards in a monotonous manner. At the lower crustal level the tectonic sheets of the accretionary complex become fuzzy, appearing in seismic reflection images as a homogenous acoustically semi-transparent medium where only fragments of the margins of some sheets can be discerned. The crust-mantle boundary is diffuse: the crust to mantle interface is determined by the gradual vanishing of the fuzzy boundaries of tectonic sheets and a gradational transition from the acoustically homogenized and semi-transparent lower crust to the transparent mantle. The ‘blurring’ of the images of rock contacts, which had originally produced acoustic impedance contrasts, is caused by melting and partial fusion. The analysis of the 3D model of the deep structure has brought about some new conclusions about the principal features and sources of the evolution of the Svecofennian accretionary orogen. A model of the onset and evolution of Svecofennian orogeny, which is considered as one of the components of the supercontinent Lauroscandia, is described in the paper by M. V. Mints “3D model of the deep structure of the Svecofennian accretionary orogen: a geodynamic interpretation”, which constitutes the second part of this study.

Keywords

• svecofennian orogen
• seismic reflection surveying
• magnetotelluric sensing
• 3d crustal model
• vyborg-suojarvi profile
• fire-2-2a-1
• fire-3-3a
• fennoscandian shield
• paleoproterozoic era.