Coupled stress and pore fluid pressure changes in the middle crust:: Vein record of coseismic loading and postseismic stress relaxation

The structural, microstructural, and fluid inclusion record of late-stage quartz veins in metamorphic rocks of southern Evia ( Greece) is analyzed and a mechanical model is proposed. The veins are monogenetic. They developed in a single stage from tensile cracks at temperatures similar to 300 degrees C. The orientation of the veins is uniform on the kilometer scale and indicates an extensional tectonic regime, with sigma(1) = sigma(v) (sigma(v) denoting the overburden stress). The length to aperture ratio of the veins is incompatible with an elastic distortion and requires ductile deformation of the host rock after fracture arrest, with crack parallel shortening by less than 2%. The fluid inclusion densities as a function of position reveal an increase in pore fluid pressure P-f during progressive sealing. Fracture mechanics suggests that Pf in an open fissure should be similar to the least principal stress, P-f approximate to sigma(3). For the given crustal stress regime, sigma(1) = sigma(v) can be taken as constant, as vein formation takes place within a sufficiently short time span, for which the overburden does not change significantly. If so, the observed increase in Pf during progressive opening and sealing of the veins reflects a decrease in differential stress, as expected for postseismic deformation in the earthquake cycle. We propose that the parental fractures formed in consequence to coseismic loading of the middle crust near the tip of an active fault in the upper crust. Each vein, which developed from such a fracture during postseismic stress relaxation, records the stress and P-f history of a single seismic cycle and the strain accumulated during postseismic creep.