QUARTZ DISLOCATION MICROSTRUCTURE BETWEEN 7000-M-DEPTH AND 9100-M-DEPTH FROM THE CONTINENTAL DEEP DRILLING PROGRAM KTB

We investigated the quartz microstructures from gneiss samples recover ed from the German Continental Deep Drilling Program (KTB) main hole b etween 7000 m and the final depth of 9100 m. Optical microscopy and tr ansmission electron microscopy (TEM) show similar microstructures for most of the studied profile, At the final depth, enhanced recovery is indicated by fewer dislocation tangles, fewer submicroscopic fluid inc lusions, and well-developed low-angle grain boundaries, Between 7000 a nd 9100 m depth, the mean dislocation density is reduced from 4x10(9) cm(-2) to 1x10(9) cm(-2), Using dislocation density as a piezometer, t he differential stress recorded in samples from 9100 m is estimated as approximately 140 MPa, Microstructures indicate that the drill hole r eached the semibrittle transition zone and that strain is partitioned between brittle deformation, solution precipitation creep, and plastic flow. Differential stress estimates from in situ measurements extrapo lated down to 9.1 km range from 170 to 220 MPa. Fluid injection induce d microearthquakes do not seem to occur at a depth greater than 9 km, possibly indicating the absence of critically stressed brittle faults, Microstructural observations and differential stress estimates from e ntirely different techniques suggest that in situ differential stresse s are not likely to increase with further depth. Stresses predicted fr om extrapolated quartz flow laws are mostly smaller for the low strain rates assumed for the KTB tectonic environment.