Geophysical studies of saline fluids in the deep crust




Marquis, Guy

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Geophysical studies have shown that the lower continental crust, especially in Phanerozoic areas, is commonly more conductive and reflective and has a lower seismic velocity than what is expected from the composition of xenoliths and exposed lower crustal terrains. These anomalous in situ properties can be explained by the presence of small amounts of free aqueous fluids in the intergranular space. A compilation of lower crustal geophysical data shows a correlation between electrical resistivity and seismic velocity, in agreement with physical properties models of porous rocks, as well as a general decrease of inferred porosity with geological age. Correlations with geothermal data also show that the reflective and conductive layers usually have their tops near the 400-450°C isotherms suggesting an association with the brittle-ductile transition. The rheology at depth might have an effect on the trapping of the fluids that are in textural equilibrium pores in the ductile crust. Alternatively metamorphic reactions may constrain free fluids to below this depth. Model for the effects of porosity in textural equilibrium pores on seismic and electrical properties of rocks have been developed, and are also in good agreement with the data compilation. Re-processed LITHOPROBE South Cordillera magnetotelluric and seismic reflection data in the Intermontane Belt support a coincidence between the top of low resistivity and high reflectivity of the crust at depths of about 20 km in the west, and about 15 km in the east of the Belt, corresponding to temperatures around 450°C. Two models for reconciling the low vertical permeability required for maintaining the porosity at depth with the interconnection required to reduce the electrical resistivity are presented: one involves the deformation of equilibrium pores by small deviatoric stresses that pinch off the vertical interconnection, the other the flattening and alignment of pores by lower crustal shear processes. A difficulty is recognized in reconciling free aqueous fluids in the lower crust with the expected retrograde metamorphism that should take up any free water. This processes can be avoided if the fluids are of high salinity. High-salinity fluids are in liquid phase in the lower crust, not in supercritical phase as often thought.



Salinity, Geophysics