Deep drilling and induced seismicity experiments at several locations worldwide indicate that, in general, the brittle crust in intraplate regions is critically stressed, pore pressures are close to hydrostatic, and in situ bulk permeability is ∼10 −17 to 10 −16 m 2 . This high permeability, three or four orders of magnitude higher than that measured on core samples, appears to be maintained by critically stressed faults and greatly facilitates fluid movement through the brittle crust. We demonstrate that such high permeabilities can maintain approximately hydrostatic fluid pressures at depths comparable to the thickness of the seismogenic crust. This leads to the counterintuitive result that faulting keeps intraplate crust inherently strong by preventing pore pressures greater than hydrostatic from persisting at depth.

How faulting keeps the crust strong

[1] High-pressure metamorphic rocks provide evidence that in subduction zones material can return from depths of more than 100 km to the surface. The pressure-temperature paths recorded by these rocks are variable, mostly revealing cooling during decompression, while the time constraints are generally narrow and indicate that the exhumation rates can be on the order of plate velocities. As such, subduction cannot be considered as a single pass process; instead, return flow of a considerable portion of crustal and upper mantle material must be accounted for. Our numerical simulations provide insight into the self-organizing large-scale flow patterns and temperature field of subduction zones, primarily controlled by rheology, phase transformations, fluid budget, and heat transfer, which are all interrelated. They show the development of a subduction channel with forced return flow of low-viscosity material and progressive widening by hydration of the mantle wedge. The large-scale structures and the array of pressure-temperature paths obtained by these simulations favorably compare to the record of natural rocks and the structure of high-pressure metamorphic areas.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2002TC001406

Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation

In the upper crust, where hydraulic gradients are typically 10 MPa km−1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ∼10−16 m² to ∼10−18 m² between 5- and 12-km depth Below ∼12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ∼10−185±1 m² Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ≈ −32 log z - 14, where k is in meters squared and z is in kilometers At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling

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Permeability of the continental crust: Implications of geothermal data and metamorphic systems

The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains

SUMMARY

During the hydraulic-fracturing experiment in the German Continental Deep Drilling Borehole (KTB) in December 1994, microseismic activity was induced. Here we develop a technique for estimating permeability using the spatio-temporal distribution of the fluid-injection-induced seismic emission. The values we have obtained for the KTB experiment (0.25times10-16 to 1.0times10-16 1.0times10-16 m2) are in a very good agreement with the previous hydraulic-type permeability estimates from KTB deep-observatory studies. In addition, our estimates of the hydraulic diffusivity support the previously calculated value for the upper crust, which is of the order of 1 m2 s-1. However, this estimate now relates to the depth range 7.5-9 km.

Estimating the crust permeability from fluid-injection-induced seismic emission at the KTB site

Both boreholes of the German Continental Deep Drilling Program encountered a fluid-conducting crust over a 9101 m vertical profile. Low matrix permeability was observed on the laboratory scale (geometric mean value 7×10−20 m2 with something more than one decade standard deviation) and in situ higher values of the order of 5×10−18 to 3×10−16 m2. With respect to the scatter of measured matrix permeability there was no lithology dependence observed, whereas permeability parallel to the foliation (geometric mean value 3×10−19 m2) is significant higher than those perpendicular to the foliation (2×10−20 m2). With increasing depth the in situ permeability varies within a few orders of magnitude, showing, however, no clear depth dependence. The in situ permeability increases over 3 orders of magnitude while the effective pressure decreases by 50 MPa, whereas in contrast, the permeability of laboratory core specimens changes by 1 order of magnitude. The pilot drill hole “Vorboh-rung” and the main drill hole “Hauptbohrung” communicate through a network of conductive fractures at the bottom hole level of the Vorbohrung as well as between both bottom hole sections. The formation pressure increases with a mean gradient of 11.5 MPa km−1 from surface to 103 ± 3 MPa to 9101 m depth. Overhydrostatic increasing pore pressure and so a strong stress reduction can be excluded as a reason for the low permeability decrease observed at the bottom of the hole. Formation pressure remains hydrostatic with respect to probable increasing salinity.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB03442

J. Kück

Papers

The permeable crust: Geohydraulic properties down to 9101 m depth

KTB and the electrical conductivity of the crust