Original paper

Microfabric of folded quartz veins in HP-LT metamorphic greywackes from Pacheco Pass, California - progressive deformation and subgrain rotation recrystallization at quasi-stationary conditions

Trepmann, Claudia A.; Stockhert, Bernhard

Abstract

The microfabric of folded quartz veins in HP-LT metamorphic greywackes from Pacheco Pass, California, indicates progressive deformation by dislocation creep accompanied by subgrain rotation recrystallization. In contrast, the host greywacke is deformed by dissolution precipitation creep. Folding of a quartz layer in a host rock undergoing deformationby dissolution precipitation creep is expected to occur in a continuous process at quasi-stationarystrain rate. The high viscosity contrast between the host greywacke and the quartz vein results in stress concentrations. The recrystallized grain size of the vein quartz (3-20 11m)indicates high differential stresses in the range of 100-300 MPa. The temperature of 300 ±50°C corresponds to that at the base of the seismogenic layer in the continental crust. Therefore this example can be used for the identificationof the microstructuralrecord of quasi-stationarycreep and for a discrimination to the microstructural record of episodic deformation in the middle crust caused by systematic stress variations during the seismic cycle. The homogenous microfabric in quartz veins from Pacheco Pass, with small isometric recrystallized grains that can show a high dislocation density is taken to reflect dynamic subgrain rotation recrystallizationduring progressive deformation. In contrast, episodic deformation at initially very high and then rapidly decaying stresses has been found to result in a microfabric characterizedby small-scale shear zones and relict grains with a high density of dislocations, deformation lamellae and elongate subgrains in assocation with small recrystallizedgrains devoid of dislocations(Trepmann &StOckhert2003, Trepmann et al. 2007).