ARCHEAN CRUSTAL PETROFABRICS IN THE MINNESOTA RIVER VALLEY COMPLEX, SUPERIOR PROVINCE AND IMPLICATIONS FOR SEISMIC ANISOTROPY

Archean cratons expose rock assemblages of high-grade gneisses and low-grade mafic-ultramafic-sedimentary series. These cratons are intruded by large volumes of granitoids and are commonly referred to as the granite-greenstone terranes. The tectonic evolution of Archean granite-greenstone terrains remains controversial. While the majority of researchers agree that Archean assemblages were formed by plate tectonic processes, others argue against this idea stating that the Archean assemblages do not fit in plate-tectonic models. The Archean high-grade gneiss terrane of the Minnesota River Valley (MRV) forms the southernmost part of the Superior Province of the Canadian Shield. The Superior Province on its own accounts for about one fourth of the Earth’s exposed Archean continental crust. The MRV subprovince is unique in that it mainly contains migmatites, gneisses, granites and minor amphibolites whereas other subprovinces of the Superior Province also host greenstone belts. Moreover, the gneisses of the MRV subprovince are older than 2.7 Ga in age while the greenstone belts are relatively younger. The MRV subprovince is also characterized by shear wave splitting (SWS) delay times as large as 1.38 s. The source of this seismic anisotropy partly resides in the layered lithospheric mantle of the North American craton but the possibility of a significant contribution from the Archean continental crust cannot be ruled out. Migmatites are one of the major rock units of the MRV subprovince and are considered to be representative of the lower to middle continental crust. Hence these rocks are ideal candidates in the study of the development of crustal anisotropy in Archean rocks. Classic structural field methods and kinematic analysis typically fails in these high-grades gneisses because they have undergone pervasive and significant recrystallisation after the peak of metamorphism. However, the Anisotropy of Magnetic Susceptibility (AMS) has recently proven to be a valuable tool in the petrofabric analysis of high-grade gneisses despite their structural complexity. Our objective is to test the AMS method to a large region of Archean rocks, and to evaluate the relationships between magnetic and seismic anisotropy on the same rocks.