UPPER-CRUSTAL STRENGTH INFERRED FROM STRESS MEASUREMENTS TO 6 KM DEPTH IN THE KTB BOREHOLE

IT has been suggested1-6 that in many cases the average strength of th e continental crust is quite low (tens of megapascals), so that the cr ust has little effect on the large-scale deformation of the lithospher e. But laboratory friction studies7,8, combined with simple faulting t heory9,10 (as well as extrapolation of in situ stress measurements fro m the upper 3 km of the crust11), imply that if pore pressure is appro ximately hydrostatic at mid-crustal depth, crustal strength is appreci able (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12-15. Here we report estimates of the mag nitude of in situ stresses to 6 km depth in the KTB borehole in southe rn Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. W e suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the up per crust, and that this may also be the case in other continental are as of moderate to elevated heat flow.