STRAIN LOCALIZATION INDUCED BY LATE-MAGMATIC CO2 FLUX AND REACTION-DRIVEN STRAIN SOFTENING

Strain localization in association in association with hydration processes is well documented in all types of tectonic settings and has major implications for the rheological behavior in active plate margin processes. The implications of CO₂-bearing fluids is less well studied though experiments have shown how CO₂ can influence the flow laws of olivine by imposing a brittle and more localized type of deformation (Rowettta and Blasic, 1987).

This study involves narrow shearzones in ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>50,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25-30 km) between 570 and 560 Ma under an extensional regime. The shearzones are (mm-cm) thick extensional shearzones, containing extremely fine-grained material with a distinct shape-preferred orientation, but weak to absent crystallographic preferred orientation. These shearzones offset dykes across numerous micro-faults are documented in areas close to the major fault zone cutting through the area. Within the shearzones olivine and clinopyroxene reacts to form orthopyroxene and dolomite at approximately 11 kb and 850°C according to the reaction:

Olivine + Clinopyroxene + CO2= Dolomite + Orthopyroxene

As evidenced by coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shearzones. In addition large olivine grains proximal to the shearzones show a microstructure with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. With clinopyroxene at least hundreds of microns away this suggested at least some material transport within the shearzone. The shearzones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. The CO2cracking and mineral reaction also serves to reduce grain-size , making grain-boundary sliding an efficient process, further enhancing the rheological contrast between the shearzone and the host rock. The sudden decrease in rock strength lead to sudden fast deformation and we suggest a relation to pseudotachylites and hence also earth quakes in near proximity of the micro-shearzones.

Rowettta and Blasic (1987): JGR, VOL. 92, NO. B12, pages 12,902-12,910