Earth Planets Space, Vol. 56 (No. 12), pp. 1151-1161, 2004
Bruce E Hobbs1, Alison Ord1, Klaus Regenauer-Lieb1,3, and Barry Drummond2
1CSIRO Exploration and Mining, Perth, Australia
2Geoscience Australia, Canberra, Australia
3University of Mainz, Germany
(Received June 15, 2004; Revised November 29, 2004; Accepted December 15, 2004)
An important observation associated with seismic activity on the Nagamachi-Rifu Fault is the existence of tabular, fluid rich zones at mid-crustal levels. These zones resemble the "bright spots" seen in many seismic images of the crust worldwide. The aim of this paper is to develop the mechanical foundations for the formation of such zones. To do so requires an understanding of the distribution of pore fluid pressure in a deforming crust. In a hydrostatically stressed porous material, the pore fluid pressure should equal the mean stress in order to keep the pores from collapsing. Past discussions of this subject imply very high pore fluid pressures, two to three times lithostatic. Considerations of plastic yielding together with continuity arguments, particularly at the plastic/viscous transition, suggest that pressures closer to lithostatic are more the norm. Particularly just below the plastic/viscous transition in compressive regimes, this leads to collapse of porosity with an associated collapse in permeability resulting in an over-pressured region comprising that part of the lower crust that is characterised by high mean stress. The base of the plastic region is at a strong discontinuity in stress difference where localised deformation occurs. Tabular, dilatant fluid filled regions develop at and above this zone in close association with dilatant tensional zones in the hanging-walls of faults and diffuse shear zone development in the upper to mid crust. Some of these dilatant zones ultimately develop into listric transitions between steeply dipping, upper crustal faults and shear zones associated with the plastic/viscous transition. These zones are also the sites of strong mineral alteration that may, particularly in ancient examples, also contribute to the delineation of "bright spots" in seismic images. For high geothermal gradients another class of fluid filled layers, in the form of "stagnant fluid zones", develops below the region of high mean stress in the viscous lower crust. Mineral alteration associated with this second class of fluid rich layers is predicted to be asymmetric in distribution as opposed to the first class that would be homogeneous in the mode of alteration.
Key words: Fluid reservoirs, bright spots, plastic viscous transition.