Current geologic thinking is that there are two, and only two distinct types of brittle fractures, joints and shear fractures (faults). For over a half century, it has been debated that a third type of fracture, referred to as hybrid fractures, could exist and that joints and shear fractures may be end members of a continuous spectrum of brittle fractures. Hybrid fractures are hypothesized to form under mixed compressive and tensile stress states and have structural characteristics intermediate to those of joints and shear fractures. While this hypothesis is accepted in many modern structural geology textbooks used at the college and graduate level, no unchallenged evidence exists for the existence of hybrid fractures.
Following the general methodology of a previously performed study by W.F. Brace (1964), but incorporating several key modifications to the experimental methods, a series of dog-bone triaxial experiments were performed on Carrara marble at room temperature, an axial extension rate of 2x10-2 mm s-1, and confining pressures between 7.5 and 170 MPa. The experiments provide strong evidence for the existence of hybrid fractures on the basis of the progressive change in fracture orientation, surface morphology, and failure strength between end-member joints and shear fractures. At the lowest confining pressures (7.5 to 60 MPa), fractures are oriented approximately parallel to the maximum principal stress, s1, form at an axial stress s3, of approximately -7.75 MPa (i.e. the uniaxial tensile strength), and display fracture surfaces characterized by many reflective cleavage faces, consistent with jointing. At the highest confining pressures (130 to 170 MPa), fractures are oriented from 13.4 to 21.6 degrees to s1, form under completely compressive stress states with s3 between 0 and 4.3 MPa, and are characterized by powdery white surfaces with short slip lineations, consistent with shear fracturing. At intermediate confining pressures (70 to 120 MPa), fractures are oriented from 3.7 to 12.4 degrees to s1, form under mixed stress conditions with s3 ranging from -10.6 to -3.0 MPa, and display both reflective cleavage faces and powdery white surfaces with short slip lineations, consistent with hybrid fracturing.