Localized Shearing along a Gabbro Intrusion: Evidence for Strain Partitioning in the Lower Crust
Noah John Phillips, McGill University
Christie Rowe, McGill University
Erik Young, Simon Fraser University
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Localization of strain has been invoked to explain earthquakes in the lower crust, changes in grain size within lower crustal units, and the creation of permeable networks in the lower crust through brittle fracture. However, recent numerical studies have indicated that the dissipation of heat in lower crustal shear zones does not lead to localized structures, and a compilation of stress measurements based on quartz piezometers from lower crustal units are consistent with distributed deformation. This is in conflict with mapped lower crustal shear zones which show stress and strain rate variations due to material heterogeneities from scales of 10's of m to km. The Lower Fish River Onseepkans Shear Zone in Namibia provides a unique field locality to explore stress distributions in the lower crust due to unparalleled exposure. The shear zone originated as a thrust zone during continental collision at ~1200 Ma, with peak metamorphism at 1160 Ma, and was reworked during an extensional phase at ~1040 – 1010 Ma. A mixture of quartz-rich orthogneisses, metagranites, gabbroic intrusions, metapsammites, and metapelites led to stress concentrations during deformation along the margins of strong units. We examine a pelitic unit that became mylonitized along the margin of a gabbroic intrusion. The pelite is coarse grained away from the intrusion and is composed of biotite-garnet-sillimanite-cordierite -plagioclase-quartz. These phases are distributed in the coarse grained unit, while segregation into ~2 cm thick, finely recrystallized layers occurs within the mylonites. The mineral phases do not change between coarse grained and mylonitized sections, and corresponding pseudosections show phase equilibria at ~700 ºC and 500-600 MPa. Ti in quartz geothermometry of recrystallized quartz shows deformation temperatures of ~730 ºC in both units, indicating that they deformed contemporaneously. Models of stress distributions during deformation show elevated stresses around the strong gabbroic intrusion, consistent with the observed smaller recrystallized grain size. These observations are evidence of stress heterogeneity in the lower crust. The margins of strong lithologies (e.g. cooled mafic plutons), where stress concentrations are high, may be the nucleation sites for intermediate depth earthquakes where strain localization may lead to increased heat production and thermal runaway.