Extensional Reactivation of an Abandoned Thrust: A Bound on Shallowing in the Brittle Regime

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Shallow dip angles (⩽45°) suggested by field observations of continental extensional faults are not predicted by classical isotropic Mohr-Coulomb-Anderson theory. Earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30°. Some structural evidence suggests brittle-normal faulting with dip angles as shallow as 10°. One explanation of the apparent conflict between theory and structural/seismic observations is that intrinsically weak, shallow-dipping pre-existing faults are preferentially reactivated. Any reduction in frictional strength of the pre-existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated. Enhanced fluid pore pressure on the pre-existing fault reduces the effective strength and can further enhance shallow fault reactivation. An analytical treatment clarifies the roles of geometry, intact/pre-existing fault strengths and fluid pore pressures. Frictional strength ratios of 3 or greater could account for extremely shallow normal faults (dips 10–20°) without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector. Moderate reduction in friction (34) with respect to wall rock can reduce the dip to 30° and can account for shallow normal-slip earthquakes.

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Journal of Structural Geology, v. 12, issue 3, p. 303-314