HR: 0830h
AN: T41D-0250    [PDF]
TI: Experimental Study of Hybrid Fractures and the Transition From Joints to Faults
AU: * Ramsey, J M
EM: jake_ramsey@anadarko.com
AF: Center for Tectonophysics, Department of Geology and Geophysics, Texas A \& M University, College Station, TX 77843 United States
AU: * Ramsey, J M
EM: jake_ramsey@anadarko.com
AF: Anadarko Petroleum Corp., 1201 Lake Robbins Drive, The Woodlands, TX 77380 United States
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Center for Tectonophysics, Department of Geology and Geophysics, Texas A \& M University, College Station, TX 77843 United States
AB: Joints and faults are end members of a continuous spectrum of brittle fractures including the hybrid fractures, hypothesized to form under mixed compressive and tensile stress. However, unequivocal evidence for the existence of hybrid fractures has not been presented. To investigate this transition, we have conducted triaxial extension experiments on dog-bone shaped cylindrical samples of Carrara marble at room temperature, an axial extension rate of 2x10$^{-2}$ mm s$^{-1}$, and confining pressures between 7.5 and 170 MPa. Two parallel suites of experiments were completed, one using very weak, latex jacketing to obtain accurate failure strength, and another using copper foil jacketing to preserve fracture surfaces. The combined data set provides strong evidence for the existence of hybrid fractures on the basis of the progressive change in failure strength, fracture orientation, and fracture surface morphology from joints to faults. At the lowest confining pressures (7.5 to 60 MPa), fractures are oriented approximately parallel to the maximum principal compressive stress, form at a tensile axial stress of approximately -7.75 MPa (i.e. the uniaxial tensile strength), and display fracture surfaces characterized by many reflective grain-scale 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 the maximum principal compressive stress, form under completely compressive stress states where the axial stress is between 0 and 4.3 MPa, and are characterized by short slip lineations and powdery, finely comminuted grains consistent with faulting. At intermediate confining pressures (70 to 120 MPa), fractures are oriented from 3.7 to 12.4 degrees to the maximum principal compressive stress, form under mixed stress conditions with the axial stress ranging from -10.6 to -3.0 MPa, and display both reflective cleavage faces and short slip lineations with comminuted grains, consistent with hybrid fracturing.
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting