HR: 1340h
AN: T53C-1460    [Abstracts]
TI: Coulomb Fault Mechanics at Work in the Proterozoic: Strike-Slip Faults and Regional-Scale Veining in the Mt. Isa Inlier, Australia
AU: * Begbie, M J
EM: mike.begbie@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, New 9001 New Zealand
AU: Sibson, R H
EM: rick.sibson@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, New 9001 New Zealand
AU: Ghisetti, F C
EM: francesca.ghisetti@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, New 9001 New Zealand
AB: The Proterozoic Mt Isa inlier, comprising greenschist to amphibolite facies metamorphic assemblages intruded by granites during the Isan Orogeny (1590-1500 Ma), is disrupted by brittle, late- or post-orogenic strike-slip faults. The faults occur in two mutually cross-cutting sets; a set of NE-SW subvertical dextral strike-slip faults, and a conjugate set of NW-SE sinistral faults. These faults thus define a regional stress field with σ₁ oriented approximately E-W and σ₃ oriented approximately N-S. Locally, the faults outcrop as linear blade-like ridges of silicified microbreccias-cataclasites and quartz veining that extends for kilometres across the semi-arid terrain. The informally named Spinifex Fault is one of the dextral set of subvertical faults. This fault is a classic example of coulomb fault mechanics at work in the Proterozoic. The Spinifex Fault trends ~065° across an outcropping granitic pluton, the margins of which it offsets dextrally by ~0.75 km. Locally within the pluton, the fault refracts to ~075° across an amphibolite layer. In the surrounding granitic pluton the fault trace is comparatively inconspicuous and unmineralized but where it transects the amphibolite it is defined by an upstanding ridge of silicified microbreccia-cataclasite (~10 m thick). Associated with the Spinifex Fault is a swarm of predominantly extensional subvertical quartz veins (cm to m thick) trending 090-95° and a series of mineralised fault splays trending 070-080°. Extension veins define the σ₁-σ₂ plane, with the Spinifex fault lying at an angle of ~25-30° to the inferred σ₁. These veins are composed of colloform and crustiform banded quartz, brecciated fragments of quartz vein and wallrock that are typically rimmed with cockade overgrowths and bladed quartz after calcite pseudomorphs. Mineralised fault splays are < 50 m or so wide with a composite brittle fabric comprising: (1) bounding subvertical cataclastic `walls' <10 m or so thick made up of silicified microbreccias and cataclasites containing vein fragments; (2) innumerable subvertical quartz veins (cm to m thick) lying subparallel to the bounding shear zones with textures ranging from pure dilation to multiply recemented breccias of wallrock and quartz fragments; (3) irregular non-systematic veins; and (4) occasional minor faults from the complementary set. Mutual cross-cutting relationships between all the structural components indicate penecontemporaneous development within the inferred stress field. Slickenfibers and striations along fault components indicate predominantly strike slip motion on subvertical planes. Homogenisation temperatures from quartz hosted fluid inclusions cluster at ~210°C while vein textures record histories of incremental hydrothermal deposition under low effective stress, probably in the epizonal environment (<1-2 km depth). This regional study demonstrates the existence of a rather uniform stress province, corresponding to an Andersonian regime and initiation of faults in accord with the coulomb failure criterion.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8123 Dynamics: seismotectonics
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: Fall Meeting 2005