Geophysical support for aqueous fluids in the deep crust: seismic and electrical relationships

Abstract

S U M M A R YPrevious studies have shown that up to a few per cent porosity filled with saline fluid in the lower crust can explain many of the regions with: (1) low electrical resistivities, (2) velocities that appear to be too low for the otherwise inferred mafic composition, and (3) strong lower crustal reflectivity. Several predictions of the free porosity model are examined in this article. A compilation of approximately coincident magnetotelluric electrical resistivity and refraction seismic velocity data for the lower continental crustjs presented to test the predicted correlation. In spite of the limited geographically coincident data and the difficulties of ensuring accurate depth coincidence and of anisotropy effects, there is a general trend of decreasing velocity with decreasing resistivity. The data are scattered, but most fall between the reasonable bounds provided by pore geometry models with effective aspect ratio (for velocity) and Archie's Law pore tortuosity exponent (for resistivity) pairs of 0.03 : 2.0 and 0.1 : 1.5 respectively. As in previous compilations, shield areas tend to have both higher resistivities and higher velocities in the lower crust compared to Phanerozoic areas, although there is overlap for both parameters. A general correlation is also found between the top of low resistivity layers and the top of lower crustal reflective zones with the 400-450 "C isotherms. Possible explanations of this correlation with temperature include (1) an association with the brittleductile transition, below which pore geometries are such as to hold fluid in the required configuration, and (2) control provided by metamorphic reactions that restrict free fluid to below this depth. To constrain better the pore geometry, a compilation of the limited data on lower crustal Poisson's ratio shows most values -0.28. This is consistent with a mainly mafic composition with up to several per cent porosity. Reasonable pore geometry distributions predict a small decrease or constant Poisson's ratio with increasing porosity. While each of the three lower crustal geophysical data types have other reasonable explanations, the apparent correlations above provide support for the fluid-filled pores in the lower crust. The problems of the low permeability required to keep fluid in the lower crust, and of pore fluid consumption in retrograde metamorphic reactions during cooling are discussed briefly. Two mechanisms are suggested as means of producing a low-permeability cap in the middle to deep crust: one invokes deformation of textural equilibrium pore geometries by small deviatoric stresses, the other lower crustal shear processes. There remains some difficulty in reconciling free aqueous fluids in the lower crust with the expected retrograde metamorphism that should take up water into hydrated mineral assemblages.

Electrical structure of the Himalaya of central Nepal: High conductivity around the mid‐crustal ramp along the MHT

“…Our study consists of 12 soundings from the Gangetic plain to the ttigher Himalaya (Figure 1 An alternate explanation is that the conductive zone may be related to a concentration of deep fluids trapped under the brittle-ductile transition [Marquis and Hyndman, 1992]. Thermal modeling by Henry et al [1996] shows that the temperature at the ramp is between 300 and 500øC, so its upper and lower parts are respectively above and below the brittle-ductile transition (assumed here as 400øC).…”

Combined transient electromagnetic and magnetotelluric study of the southern Kenya rift valley

“…There is also evidence of a second transition around 700° to 750°C to a more resistive lower crust (Hyndman and Shearer, 1989;Marquis and Hyndman, 1992). These transitions can also be roughly correlated with metamorphic facies of upper greenschist at the first transition and upper amphibolite to granulite at the second (Nesbitt, 1993; granulite facies are presumed to have no free water and to be dry and resistive, the estimated temperature is about 730°C; e.g.…”

“…Nevertheless, it is apparent that the influence of the rifting processes extended beyond the limits of the rift; high temperatures and pressures may have altered the bulk composition and resistivity properties of the crust west of the rift. Rock porosity is progressively closed by the increase in effective pressure and the electrical resistivity increases (Marquis and Hyndman, 1992). The seismic data for the western flank of the rift have not recorded any dramatic change, but resistivity is the physical rock parameter that is more sensitive to various tectonic and composition factors (Haak and Hutton, 1986).…”

“…The presence of free aqueous fluids (Marquis and Hyndman, 1992) or mafic intrusions of partial melt derived from the upper mantle (Newton, 1990;Singh and McKenzie, 1993) may cause the reflectivity and the low resistivity. Birt (1996) explains the high velocity material at the base of the crust as magmatic additions, and its overall thickening towards the east as reflection of the change from Archaean to Proterozoic crust based on the model of Durrheim and Mooney (1991;.…”

“…Hyndman and Shearer, 1989;Marquis and Hyndman, 1992;Jones, 1992). Additionally, mantle devolatilization processes are a poorly defined but potentially large source of water for the lower crust (e.g.…”