Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Basement Sucks Bruce Yardley, Sarah Gleeson School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Ingrid-Anne Munz IFE, PO Box 40, N-2007 Kjeller, Norway That groundwaters can penetrate crystalline rocks to mid-crustal depths was first appreciated from stable isotope studies of altered granite batholiths, and reinforced by examples of water inflows encountered during deep crustal drilling. Nevertheless, fluid flow modelling of sedimentary basins has often treated underlying crystalline basement rocks as impermeable. The purpose of this contribution is to explore examples of fluid influxes into crystalline basement from overlying sedimentary basins or the surface itself, and to explore their potential significance, if any. Basement rocks are potentially large sinks for water because they are dehydrated and consume water in incipient retrograde reactions. Such reactions mean that fluid pressure can be below hydrostatic values throughout the stable crust. However basement rocks are also generally highly impermeable so the effectiveness of water infiltration may be minimal. There are well-documented examples of free convection of surface waters resulting in temperatures at the base of the cell of the order of 250°C, but higher temperature examples unrelated to magmatism often preserve extensive fluid inclusion evidence of overpressuring. This is surprising since there must have been an open flowing system for the initial infiltration to take place. We have suggested previously that fluid-filled fractures became sealed, isolated and pressurised, possibly providing an opportunity for further downward injection of fluid in favourable circumstances. In the Modum area of southern Norway, basinal brines penetrated high grade basement rocks and reacted with wall rocks to hydrate them; in the process sufficient water was removed from the overpressured fluid to double its salinity, reaching salt saturation at temperatures around 300°C. At the same time, infiltrating hydrocarbons were cracked to methane and bitumen. In contrast to geothermal systems, the basement here has been able to destroy much of the fluid that has penetrated it, and remains only locally altered. Many crustal shear zones are also sites of fluid infiltration, but here it is coupled to deformation. Retrograde hydration causes weakening of many rock types, and if the volume of rock affected is sufficiently large, deformation focussed on sites of initial hydration results in the generation of sites of low fluid pressure into which further fluid is drawn. Thus in a particular stress regime, there may be a threshold beyond which fluid infiltration becomes self-accelerating through the promotion of deformation, subject to the restraint that most retrograde reactions result in an increase in solid volume and hence a reduction in permeability. This contribution will explore the evidence for deep penetration of surface fluids into basement, and the consequences for crustal rheology.