Brittle strength of basaltic rock masses with applications to Venus

Abstract

Laboratory measurements of rock deformation in the brittle regime provide constraints on the response of rocks to stress. These values are essential parameters in tectonic models of near‐surface deformation because they influence both the stress state and the conditions for predicting the types and occurrences of brittle structures such as joints and faults. However, additional parameters must be included before the laboratory values can be used to construct brittle strength envelopes for near‐surface materials. The properties of basaltic rock masses provide a more precise estimate of the strengths of basaltic lava flows on the terrestrial planets than other, more widely used approaches (intact rock or frictional strength of a through‐going surface). Rock mass strength is defined by three parameters including unconfined compressive strength of intact basalt and two others related to the degree of fracturing of the material. Experimental results for elevated temperature extend the applicability of these parameters to the near‐surface environment of Venus. Representative values of strength parameters for intact basalt at ambient temperature (20°C)and negligible confining pressure are: Young's modulus, 73 GPa; Poisson's ratio, 0.25; tensile strength, −14 MPa; unconfined compressive strength, 262 MPa; fracture toughness, 1–3 MPa m½ cohesion, 66 MPa; and coefficient of friction, 0.6. At elevated temperature (∼450°C) and zero confining pressure, reference values for the strength of intact basalt are: Young's modulus, 57 GPa; Poisson's ratio, 0.25; unconfined compressive strength, 210 MPa; and fracture toughness, 2–2.8 MPa m½. Corresponding values for a basaltic rock mass that incorporate the weakening effects of scale (but not elevated temperature) are: Deformation modulus, 5–50 GPa; Poisson's ratio, 0.3; tensile strength, −0.2 to −2 MPa; uniaxial compressive strength, 12–63 MPa; cohesion, 0.5–6 MPa. Values of tensile and cohesive strength for the basaltic rock mass are approximately one to two orders of magnitude lower than corresponding values for intact basalt. Temperatures comparable to those at the Venus surface may slightly increase the deformation modulus but decrease the compressive strength of the rock mass. Brittle strength envelopes for the rock mass as a function of depth are typically stronger in both extension and compression than conventional envelopes that assume a simple frictional strength. These results indicate that the strengths of basaltic rocks on planetary surfaces and in the shallow subsurface are significantly different from strength values commonly used in tectonic modeling studies which assume properties of either intact rock samples or single planar shear surfaces.