Magnetic fabrics in deformed metaperidotites of the Outokumpu Deep Drill Core, Finland: Implications for a major crustal shear zone

Abstract

The Outokumpu (OKU) assemblage was studied in drill cores from the 2516m deep OKU Deep Drill Hole, Finland, and we observed that the strongest magnetic anomalies found in borehole measurements are related to three ferrimagnetic metaperidotite units with magnetic susceptibilities of up to 93×10−3SI separated by paramagnetic rock units. The main ferrimagnetic minerals are magnetite and minor pyrrhotite. Magnetic fabric studies were done in order to examine deformation within the OKU assemblage during the collisional deformation event 1.9Ga ago. Well-defined magnetic axes (kmax, kmin) and nearly horizontal magnetic foliation subparallel to the macroscopic foliation occur at the top and the bottom of the OKU formation. Low- and high-field AMS measurements showed that ferri- and paramagnetic subfabrics are coaxial. Paramagnetic units intercalated between the ferrimagnetic units show mostly triaxial magnetic fabrics, and low degrees of magnetic anisotropy values (P′<1.3) with a high variation of the shape factor (T) from prolate to oblate shapes. P′ of the ferrimagnetic metaperidotite is high and range between 1.3 and 3.6 and T tends to more oblate shapes (T>0). Ferrimagnetic unit 2 shows the highest magnetic susceptibility and P′ up to 5.6, which is related to a high amount of magnetite and pyrrhotite mostly concentrated in huge aggregates and veins.Alignment of magnetic minerals parallel with the structural foliation and the high P′ values in the ferrimagnetic metaperidotite indicate that magnetic fabric was acquired in a high strain zone. The magnetic fabrics represent shear zone (SC) fabrics, which have been formed during early obduction-related deformation of the Svecofennian orogeny. The stacked sequence of the ferri- and paramagnetic metaperidotite bodies can be interpreted as a thrust system with an imbricate fan, in which three individual listric thrust sheets occur. This interpretation is in accordance with previous tectonic models of the Outokumpu area.