The Deep Structure of the Early Precambrian Crust of the Karelian Craton, Southeastern Fennoscandian Shield: Results of Investigation Along CMP Profile 4B

摘要

A model of Early Precambrian crust and upper mantle structure down to a depth of 60 km beneath the Karelian Craton and Belomorian Province was developed on the basis of CMP seismic surveying along Profile 4B (FGUNNP Spetsgeofizika, 1999) coupled with results of geological, geochemical, and geochronological studies and examination of deep xenoliths. It is shown that the packets of seismic reflections within gneiss-amphibolite-migmatite sequences resulted from interference of reflections from contacts of thin amphibolite interlayers embedded into the granite-gneiss matrix. The Karelian Craton crust is composed of tectonic sheets consecutively plunging toward the eastern profile edge. The East Karelian imbricated thrust belt is made up of alternating sheets consisting of Neoarchean granite-greenstone and Paleoproterozoic volcanosedimentary sequences deformed with the formation of structural duplexes. The lower crust and uppermost mantle largely consist of mafic rocks (garnet granulite and eclogite resulting from the metamorphism of gabbro and gabbroan-orthosite), probably, with the participation of spinel lherzolite appearing below a certain depth. In the eastern segment of the profile, the Karelian Craton is overlapped by nappes and thrusts of the Belomorian Province. The spatial relationships between tectonic units of regional rank (Karelian Craton, Belomorian Province, and system of volcanosedimentary belts) are related to the Paleoproterozoic collisional events. The Paleoproterozoic Shombozero structure, which was traditionally regarded as a syncline, actually is an anticlinal bend of tectonic sheets, cut by erosion level.