Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.