Quartz dislocation microstructure between 7000 m and 9100 m depth from the Continental Deep Drilling Program KTB

Abstract

We investigated the quartz microstructures from gneiss samples recovered from the German Continental Deep Drilling Program (KTB) main hole between 7000 m and the final depth of 9100 m. Optical microscopy and transmission 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 inclusions, and well‐developed low‐angle grain boundaries. Between 7000 and 9100 m depth, the mean dislocation density is reduced from 4×10 9 cm −2 to 1×10 9 cm −2 . Using dislocation density as a piezometer, the differential stress recorded in samples from 9100 m is estimated as approximately 140 MPa. Microstructures indicate that the drill hole reached 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 extrapolated down to 9.1 km range from 170 to 220 MPa. Fluid injection induced 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 entirely different techniques suggest that in situ differential stresses are not likely to increase with further depth. Stresses predicted from extrapolated quartz flow laws are mostly smaller for the low strain rates assumed for the KTB tectonic environment.