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

<p> Shallow dip angles (&les;45&deg;) 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&deg;. Some structural evidence suggests brittle-normal faulting with dip angles as shallow as 10&deg;. 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&ndash;20&deg;) <em> without </em> consideration of pore pressures in excess of the least principal stress <em> or </em> 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&deg; and can account for shallow normal-slip earthquakes.</p>