Seminar presented by Professor Emeritus Rick Sibson
Fluid-overpressuring (pore-fluid pressure, Pf > hydrostatic and locally approaching lithostatic values) is quite widespread in the Earth’s crust, not only in sedimentary basins but also locally (on the evidence of extensional vein swarms) within crystalline basement assemblages. Because faults are fractally ‘rough’ they become highly permeable after seismic rupture and through the aftershock period, with permeability then diminishing by gouge compaction, hydrothermal sealing, etc.
If a fault ruptures across an overpressured fluid reservoir there is potential for postseismic discharge along the transitory permeability of the rupture zone before self-sealing – fault-valve’ behaviour. Crust under horizontal compression is better at both generating and containing overpressure with the most extreme valving discharges often associated with steep reverse faults. Such structures become frictionally ‘locked-up’ at dips of c. 60°: reshear of faults with dips beyond lock-up is possible only when the hydrofracture condition (Pf > σ3) is met, enhancing their ability to behave as fluid-pressure activated ‘valves’. Valving discharges appear responsible for a significant proportion of fault-hosted hydrothermal veins and, notably, orogenic gold-quartz lodes (< 75%? of historically mined gold).
Characteristics of deposits formed by fault-valve action include: (i) hydrothermal vein textures recording intermittent deposition and fluid-pressure cycling; (ii) mineralization from near-lithostatically overpressured fluids with 250 < T < 400 °C at depths of 7-15 km, consistent with lower portions of the upper crustal seismogenic zone; and (iii) mineralization extending over a considerable depth range (< 2-3 km). Tectonic settings especially favourable to extreme fault-valve action include areas of active compressional inversion, and collision zones where imbricate thrust stacks undergo domino steepening during continued shortening. Electrical and seismic velocity anomalies below such areas reveal near-lithostatic overpressures in the mid-crust. The Fault-Valve model evolved from observations of exhumed fault zones in Anglesey, Canada, California and Australia, and seismically active reverse fault systems in Japan and the northern South Island.
At level 2 there will be limited seating available in the Benson common room, so the talk will also be streamed via Zoom at the following link:
Professor Emeritus Rick Sibson, Department of Geology, University of Otago and Ruby Bay Associates
|Date||Thursday, 24 September 2020|
|Time||1:00pm - 1:55pm|
|Location||Benson Common Room (Gn9), Geology Building, Dunedin Campus, University of Otago|
|Contact Name||Dr Sophie Briggs|