HR: 16:45h
AN: T54A-04    [Abstracts]
TI: In-Situ Resolution of Internal Fault Zone Properties: Structure, Rock Mechanics and Rupture of the Pretorius Fault, South Africa (NELSAM Project)
AU: * Heesakkers, V
EM: heesakkers@ou.edu
AF: School of Geology and Geophysics, U of Okalhoma, Norman, OK 73019, United States
AU: Lockner, D
EM: dlockner@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Reches, Z
EM: reches@ou.edu
AF: School of Geology and Geophysics, U of Okalhoma, Norman, OK 73019, United States
AB: We analyze the reactivation mechanism of the Pretorius fault, TauTona mine, South Africa. The analysis is based on our mapping of the fault-zone structure at a depth of 3.6 km, observations of its rupture zone during an M2.2 earthquake, and mechanical testing of the fault and host rocks in the laboratory. The slip localization mechanisms of the earthquake are modeled with the finite element method. The Pretorius fault is a 10 km long, 25-30 m wide, Archean fault with dextral displacement of 200 m and vertical displacement of 30-60 m. The fault zone consists of a network of tens of anastomosing fault segments, some of which include massive, well-cemented cataclasite. We mapped the rupture zone of the M2.2 earthquake of December 12, 2004, exposed in mining tunnels for at least 25 m horizontally and 5 m vertically. The rupture reactivated four main, quasi-planar, crosscutting segments within the complex network of the ancient Pretorius fault. The slip during this event generated 1 to 5 zones (each 0.5-1.0 mm thick) of fresh, fine-grained rock powder. This powder was located predominantly along the contacts of the quartzitic host rock and the ancient massive cataclasite and indicates slip localization during the rupture. Rock mechanics experiments were conducted on samples of the fault-rock (quartzitic cataclasite) and host rock (quartzite), collected from boreholes drilled across the fault zone. The elastic properties of the host quartzite (E = 81 GPa, v = 0.17) are similar to those of the cataclasite (E = 71 GPa, v = 0.15), but the former is twice as strong as the later (uniaxial strength of 200 MPa vs. 100 MPa). On the other hand, the host quartzite is severely damaged and shows significant strain hardening with inelastic deformation starting at ~25% of the total axial strain (brittle- plastic behavior). The cataclasite is undamaged, with only minor inelastic deformation occurring at 85% of the axial strain (brittle-elastic behavior). We developed a 2D finite element model in which a simplified fault (elliptical inclusion), composed of cataclasite- like rock, is embedded within a quartzite-like medium, using the material properties obtained from the lab tests. The model shows an abrupt increase of the shear stress at the contact between the inclusion and the host rock. A similar trend in the plastic shear strain suggests that the shear stress gradient is a result of the plastic behavior of the host rock. We propose that the mechanical contrast between the plasticity of the damaged host quartzite and the brittle cataclasite results in a steep shear stress gradient across the contact, leading to the slip localization. We will include the observed rupture complexity into our model and explore its effect on slip localization mechanisms within network fault-zones.
DE: 7209 Earthquake dynamics (1242)
DE: 8010 Fractures and faults
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting