The German Continental Deep Drilling Program KTB: Overview and major results

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Abstract
The German Continental Deep Drilling Program (KTB) was designed to study the properties and processes of the deeper continental crust by means of superdeep borehole. Major research themes were (1) the nature of geophysical structures and phenomena, (2) the crustal stress field and the brittle‐ductile transition, (3) the thermal structure of the crust, (4) crustal fluids and transport processes, and (5) structure and evolution of the central European Variscan basement. The project was conducted in distinct phases: a preparatory phase (1982–1984), a phase of site selection (1985–1986), and a pilot phase (1987–1990), which included sinking of a pilot borehole to 4000 m and a 1‐year experimentation program. The main phase (1990–1994) comprised drilling of a superdeep borehole which reached a final depth of 9101 m and a temperature of ∼265 °C, and three subsequent large‐scale experiments in the uncased‐bottom hole section. Among the outstanding results are the following (1) A continuous profile of the complete stress tensor was obtained. (2) Several lines of evidence indicate that KTB reached the present‐day brittle‐ductile transition. (3) The drilled crustal segment is distinguished by large amounts of free fluids down to midcrustal levels. (4) The role of postorogenic brittle deformation had been grossly underestimated. (5) Steep‐angle seismic reflection surveys depict the deformation pattern of the upper crust. (6) High‐resolution seismic images of the crust can be obtained with a newly developed technique of true‐amplitude, prestack depth migration. (7) The electrical behavior of the crust is determined by secondary graphite (±sulfides) in shear zones.

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