PUBLICATIONS Journal of Geophysical Research: Solid Earth RESEARCH ARTICLE High-temperature fracturing and subsequent grain-size- 10.1002/2014JB011708 sensitive creep in lower crustal gabbros: Evidence Key Points: for coseismic loading followed by creep during • Shear zones formed by fracturing and subsequent creep are found in gabbro decaying stress in the lower crust? • Deformation occurred at the lower crustal conditions Takamoto Okudaira1, Petr Jeřábek2, Holger Stünitz3, and Florian Fusseis4 • Shear zones are related to a process of coseismic loading followed by creep 1Department of Geosciences, Osaka City University, Osaka, Japan, 2Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republic, 3Department of Geology, University of Tromsø, Tromsø, Norway, 4School of Geosciences, University of Edinburgh, Edinburgh, UK Correspondence to: T. Okudaira,
[email protected] Abstract The mechanism of shear zone formation in lower crustal, relatively “dry” rocks is still poorly understood. We have studied the high-temperature deformation of the Hasvik gabbro (northern Norway) – μ fi Citation: which commences by fracturing. The 10 20 m wide fractures show little displacement. The ne-grained Okudaira, T., P. Jeřábek, H. Stünitz, and plagioclase and orthopyroxene in the fractures lack a crystallographic preferred orientation (CPO) or a F. Fusseis (2015), High-temperature systematic crystallographic orientation with respect to the host grains. Fractures grade into narrow shear fracturing and subsequent grain-size- zones, which are composed of fine (10–20 μm), equant grains of recrystallized plagioclase, amphibole, and sensitive creep in lower crustal gabbros: Evidence for coseismic loading followed pyroxene. Recrystallized plagioclase and pyroxene have compositions different from the magmatic grains, by creep during decaying stress in the suggesting that they have formed by nucleation and growth.