Chapter 7: Transition from Brittle Fracture to Ductile Flow in Solenhofen Limestone as a Function of Temperature, Confining Pressure, and Interstitial Fluid Pressure
More than 115 triaxial compression and extension experiments have been performed on the mechanically isotropic, homogeneous Solenhofen limestone to determine the transition from brittle fracture to ductile flow as a function of temperature, confining pressure, and interstitial fluid pressure. Temperature and confining pressures ranged from 25° to 700°C. and from 1 to 7500 atmospheres; interstitial fluids included water and carbon dioxide. Strain rates remained constant at 10–4/second. Transitional behavior was arbitrarily defined as that point at which 3–5 per cent strain may be induced without notable loss in cohesion. In dry extension tests, the confining pressure required to induce transitional behavior decreased from 7300 atmospheres at 25°C. to about 700 atmospheres at 700°C. In dry compression tests, the pressure required for the transition was 1000 atmospheres at 25°C., decreasing to 1 atmosphere at 480°C. For transitional behavior in compression tests with interstitial fluids, with increasing confining pressure, the difference between confining pressure and interstitial pressure decreased almost exponentially from 1000 atmospheres at 25°C. and 850 atmospheres at 150°C. to nearly zero at 5000 atmospheres. Data for all stress-strain curves are summarized; most stress-strain curves are plotted and compared for a wide variety of test conditions. From these, plots of transition stress or ultimate stress at transition conditions, maximum and minimum principal stresses, and mean stress are derived and reported as a function of temperature. None of the various theories of strength examined were able to correlate results in compression and extension. The Mohr criterion predicted shear fracture angles within 4° at the brittle-ductile transition for dry compression tests and within 7° for extension tests at temperatures ranging from 25° to 400°C. By applying the Mohr theory, similar angles were predicted within 3° for transitional compression tests with interstitial water at both 25° and 150°C. The geological implications of these experiments are briefly discussed. Even though the experimental strain rate is vastly greater than tectonic strain rates, the qualitative difference observed between compression and extension would probably apply to naturally deformed limestones. The analogy of compression experiments to reverse faulting and of extension experiments to normal faulting is discussed. In the event that strain rate does not affect the brittle-ductile transition, these experiments predict normal faulting of dry limestone to a depth of 15 km and reverse faulting to a depth of 3.5 km. Interstitial fluid pressure would increase the depth to which faulting could occur.