HR: 0800h
AN: S41A-0928    [Abstracts]
TI: Fault-Segment Rupture and Mineralization During Aftershock Localized Fluid Flow
AU: * Micklethwaite, S
EM: steven.micklethwaite@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University,, Canberra, ACT 0200 Australia
AB: Contemporary seismogenic fault systems can be used as an analogue for understanding paleo-fault systems. We show the fault-hosted goldfields of the Kalgoorlie terrane, Western Australia, are best understood in this framework. Lode gold deposits are the products of focused fluid flow through faulted crust. Gold deposits tend to be localized within low displacement faults adjacent to high displacement faults. These low displacement faults have been interpreted as aftershock structures, repeatedly activated after slip events on the high displacement faults. Deposits in the Mount Pleasant area are clustered on small-displacement structures over $<$10km of the $>$50km-long Black Flag fault. Field relationships and net slip distribution along the fault indicate that the deposits are adjacent to, but not within, a large dilatant jog, where two segments of the fault link together. On this basis we infer the jog was a long-term rupture-arrest site. By analogy with active seismogenic fault systems, rupture on segments of the Black Flag fault changed Coulomb failure stress in the surrounding crust and brought specific zones closer to failure, generating regions of preferential aftershock activity. Stress-transfer modeling of the system helps explain the location of mineralized small-displacement structures around the Black Flag fault and indicates that the gold deposits are hosted on structures that became transiently permeable and localized fluid flow during repeated aftershock sequences. Taking the analysis further, we calculated the cumulative stress change after ruptures on all the large faults in mineralized regions. This effectively identifies those domains that most regularly hosted aftershocks following rupturing or stress triggering of the larger faults. We found that areas of positve stress change closely match the distribution of gold mineralization. Firstly, we conclude long-term rupture arrest sites led to repeated aftershocks and transiently high permeability in particular zones. Thus the permeability of a fault system will vary significantly along strike and with time (equally likely at contractional or dilational fault jogs). Secondly, effects of earthquake dynamics around long-term rupture arrest sites, such as stress triggering and secondary aftershocks, can be identified in the rock record. Thirdly, stress transfer modeling, previously used for earthquake hazard prediction, has potential as a target prediction tool for the mineral industry.
DE: 8045 Role of fluids
DE: 8135 Hydrothermal systems (8424)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting