HR: 10:20h
AN: S32B-01    [Abstracts]
TI: Internal Structure of a Strike-Slip Dilational Fault Jog: Overlander Fault, Mt Isa Inlier, Australia
AU: * Sibson, R H
EM: rick.sibson@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, 9001 New Zealand
AU: Ghisetti, F
EM: francesca.ghisetti@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, 9001 New Zealand
AU: Begbie, M J
EM: mike.begbie@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, P.O. Box 56, Dunedin, 9001 New Zealand
AB: The Overlander Fault is one of a set of NE-SW subvertical dextral strike-slip faults which, together with a NW-SE conjugate sinistral set, disrupt the Mt Isa Proterozoic orogen (1590-1500 Ma) in NW Queensland, Australia. These late- to post-orogenic faults thus define a regional stress field with $\sigma$$_{1}$ oriented approximately E-W and $\sigma$$_{3}$ oriented approximately N-S. The Overlander Fault trends $\sim$$060\deg$ across the metamorphic assemblage except where it refracts to 070-$074\deg$ across an outcropping granitic pluton, the margins of which it offsets dextrally by $\sim$1.5 km. The stepover width of this dilational fault jog approaches 1 km, comparable to dilational stepovers within active strike-slip faults (e.g. the San Andreas fault at Parkfield). In the surrounding amphibolite facies metamorphic assemblage the fault trace is comparatively inconspicuous and unmineralized but where it crosses the granite it is defined by upstanding ridges of silicified microbreccia and associated quartz veining. The stepover region provides opportunities for studying incremental and finite dilatation associated with slip transfer across the jog, and associated influx of hydrothermal fluids. Shearing across the stepover region is accommodated by a mesh structure with principal components that include: (1) a series of silicified microbreccia-cataclasite `walls' $<$10 m or so thick with associated quartz veins $<$1 m or so thick trending $070\deg$ and defining a `main zone' about 100$\pm$20 m wide; (2) parallel subsidiary strike-slip cataclastic shear zones occurring $<$200 m laterally from the main zone; (3) a set of subvertical $<$1-2 m thick extension veins oriented 090-$100\deg$, some with evidence of marginal shearing (both sinistral and dextral); (4) a conspicuous sinistral extensional-shear curving eastwards for $\sim$250 m from the main fault core on a trend of 100-$115\deg$; and (5) a set of unmineralized faults with sinistral separations trending 120-$130\deg$. Slickenfibers and striations along the main fault-parallel components indicate predominantly strike-slip motion on subvertical planes but there is some local evidence for dip-slip. Mutual cross-cutting relationships between all the principal components indicate penecontemporaneous development. Orientations of the various components are broadly compatible with the inferred regional stress field but there is some evidence for fluctuating stress trajectories. Vein textures record histories of incremental growth and are generally consistent with hydrothermal deposition under low effective stresses, probably in the epizonal environment ($<$1-2 km depth). Recorded dextral separations along the major shear fracture components are commonly of the order of 1-10 cm, consistent with small-to-moderate seismic slip increments. It is common for shearing increments along the main strike-slip faults in the stepover to be accompanied by significant dilatation. Rigid-body analysis of the deflected fault trace across the granite suggests that total dilatation in the stepover should be of the order of 300 m but the major veins account for c. 10-20$%$ of this. Nonetheless, it is clear that progressive slip transfer across the jog involved substantial dilatation and massive fluid influx. The complex array of sub-structures within the stepover mesh invites comparison with structural complexity revealed by high resolution aftershock studies of dilational jog structures on seismically active strike-slip faults (e.g. Parkfield, Coyote Lake, Landers earthquakes) which are recognized sites of rupture perturbation or arrest.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8123 Dynamics, seismotectonics
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting