Chapter 7: Transition from Brittle Fracture to Ductile Flow in Solenhofen Limestone as a Function of Temperature, Confining Pressure, and Interstitial Fluid Pressure

Abstract

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. 193

Structural Evolution of Upper Layer 2, Hole 896A

“…Figure 16. Longitudinal splitting of sample deformed under uniaxial compression (1 atmosphere confining pressure) from Heard (1960). Fragmentation is similar to that observed in jigsaw-puzzle breccias and associated vein networks in Hole 896A.…”

“…In laboratory experiments, compressive loading of rock specimens under conditions of no confining pressure (uniaxial compression) often results in longitudinal (axial) splitting ( Fig. 16; Heard, 1960;Brace, 1964;Twiss and Moores, 1992). Multiple anastomosing and branching extension fractures are formed, and much of the sample may be fragmented into small pieces (Fig.…”

Dilatancy, compaction, and failure mode in Solnhofen limestone

“…In the cataclastic flow regime the pore space undergoes significant inelastic compaction while the rock strain hardens [Edmond and Paterson, 1972]. The marbles, even the ones with very low porosity like the Carrara marble, undergo the brittle to plastic transition at room temperature for confining pressures accessible in the laboratory [Robertson, 1955;Paterson, 1958;Heard, 1960;Rutter, 1974]. This is probably due to the fact that calcite requires relatively low shear stresses to initiate mechanical twinning and dislocation slip even at room temperature [Turner et al, 1954;Griggs et al, 1960].…”

“…We also tlsed the saturation technique to determine the interconnected porosities in selected samples, which were found to be -1.5%, indicating that the limestone contains a large proportion of unconnected, vugular type of pores. We did not determine the average grain size, but it was reported to be -5 gm in previous studies [Robertson, 1955;Heard, 1960;Edmond and Paterson, 1972;Fredrich et al, 1990;Renner and Rumreel, 1996;Fisher and Paterson, 1989;Rutter, 1972].…”

“…Although many aspects of the brittle-ductile transition and creep behavior in this rock have been studied [Robertson, 1955;Heard, 1960 …”