Geology

Strengthening the Crust by Breaking It

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Science  19 May 2000:
Vol. 288, Issue 5469, pp. 1137-1139
DOI: 10.1126/science.288.5469.1137e

An active fault seems the archetypal expression of upper-crustal weakness, but could such weak structures actually contribute to the overall strength of the crust? Townend and Zoback suggest that, in intraplate settings, faults actually may help keep the crust strong.

Reviewing bulk-permeability data from a variety of sources, they note that permeability measured from core samples invariably is three to four orders of magnitude lower than larger-scale crustal permeability measured in situ through borehole studies, induced seismicity, and other techniques. This scale-dependent permeability difference is best explained by networks of hydraulically conductive faults; moreover, a large suite of data from high-resolution borehole logs suggests that only critically stressed faults (fractures at or near the point of Coulomb frictional failure) are hydraulically conductive. The relatively high permeability of the crust facilitated by these conductive faults prevents subsurface pore fluid pressures from building up to above-hydrostatic levels. That, in turn, keeps the intraplate crust relatively strong, as high pore fluid pressures at depth tend to reduce the effective failure strength of rock.—SW

Geology28, 399 (2000).

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