Steady-State Failure Equilibrium and Deformation of Intraplate Lithosphere

Abstract

We present a simple conceptual model in which the entire lithosphere is in steady-state failure equilibrium-brittle failure in the upper crust and ductile creep in the lower crust and upper mantle-in response to finite, buoyancy-related plate tectonic forces. We demonstrate that, in the context of finite plate driving forces, high crustal strength provides a first-order constraint on the rate at which intraplate lithosphere deforms. For strike-slip stress states and moderate intraplate heat flow (~60 ± 6 mW m-2), average strain rates are less than 10-17 s-1, consistent with the upper bounds imposed by rigid-plate assumptions inherent in plate tectonic reconstructions as well as with average intraplate strain rates measured by very long baseline interferometry (VLBI). Because regions of higher heat flow are characterized by low effective viscosity in the lower crust and upper mantle, the available plate driving forces are sufficient to cause faster creep at depth (and higher seismicity rates in the overlying brittle crust) than in in regions of lower heat flow. We suggest that the current debate over whether intraplate deformation is best viewed in terms of a deforming continuum or as rigid crustal blocks separated by relatively narrow and weak fault zones may be a false dichotomy. We illustrate this for the Coast Ranges and Central Valley of western California. In the Coast Ranges, a region of high heat flow, high deformation rates are expected because of correspondingly high temperatures in the lower crust and upper mantle. The adjacent Central Valley is characterized by very low heat flow and deforms at such a slow rate that it appears to behave as a rigid block. Finally, in the context of steady-state lithospheric failure equilibrium, we demonstrate that the Holocene concentration of intraplate seismicity in the New Madrid seismic zone can be explained in terms of the stress perturbation caused by retreat of the Laurentide ice sheet and anomalous upper mantle structure beneath the Late Precambrian Reelfoot rift.