High‐temperature fracturing and subsequent grain‐size‐sensitive creep in lower crustal gabbros: Evidence for coseismic loading followed by creep during decaying stress in the lower crust?

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) which commences by fracturing. The 10–20 µm wide fractures show little displacement. The fine‐grained plagioclase and orthopyroxene in the fractures lack a crystallographic preferred orientation (CPO) or a systematic crystallographic orientation with respect to the host grains. Fractures grade into narrow shear zones, which are composed of fine (10–20 µm), equant grains of recrystallized plagioclase, amphibole, and pyroxene. Recrystallized plagioclase and pyroxene have compositions different from the magmatic grains, suggesting that they have formed by nucleation and growth. Based on conventional plagioclase‐amphibole thermobarometry, the shear zones have formed at temperatures and pressures of 700–750°C and 0.5–0.6 GPa. The observed primary minerals cut by fractures suggest high‐temperature fracturing in the absence of high pore pressures, which implies a high strength of the lower crustal gabbros and high stresses at fracturing. The shear zones are characterized by the lack of CPO and a small grain size, suggesting that the mechanism of deformation of the fine‐grained plagioclase and orthopyroxene has been grain boundary sliding accommodated by diffusive mass transfer. The amphibole grains have strong CPOs, which most likely result from oriented growth and/or rigid body rotations during deformation. The process that initiated the fracturing and subsequent viscous creep in the Hasvik gabbro may have resulted from a process of coseismic loading followed by creep during decaying stress in the lower crust.