Early Proterozoic nappe formation: an example from Sodankylä, Finland, Northern Baltic Shield

Abstract

The Central Lapland Greenstone belt comprises rift-related metavolcanic and metasedimentary rocks representing one of the largest supracrustal belts in the Baltic Shield. The Sodankylä area in the central part of the belt represents a complex thrust duplex within a nappe overlying Belomorian Archaean basement and autocthonous Luirojoki calc-silicate rocks. Here, an early D 1 schistosity is axial planar to at least three coaxial generations of southward-verging, subhorizontal, E-W-plunging D 1 folds associated with major southwards thrusting. D 2 is represented by broad, mapscale, upright, NE-trending folds in the south and crenulations in the north. Staurolite-grade metamorphism represented by post-tectonic andalusite + staurolite + kyanite assemblages occurred after D 2 folding. Later D 3 deformation was limited to local NW-trending folds and sinistral faults. The internal nappe-like structure of the Central Lapland Greenstone belt suggests that it represents the foreland of a large collisional complex cored by the Lapland Granulite belt.

Paleoproterozoic Metamorphism of the Archean Tuntsa Suite, Northern Fennoscandian Shield

“…In the northern Fennoscandian Shield collisional and subsequent igneous and exhumation processes between 1.92-1.77 Ga strongly reworked both the Archean bedrock and overlying Paleoproterozoic cover [50,51,56,[120][121][122], leading to similar metamorphic and structural style regardless of age. Gently dipping nappe and overthrust structures, like those in Tuntsa (Figure 3a,b), are common also in juxtaposed Proterozoic formations [123,124]. Kyanite is commonly found in the Proterozoic metasedimentary rocks west of the Tuntsa Suite (Figure 1) [124][125][126] and similar clockwise cooling paths as in Tuntsa have been described from the Proterozoic rocks in Central Lapland and the Lapland Granulite Complex and its southern contact zone [121,124,127].…”