THE CONUNDRUM OF CONTINENTAL INTRAPLATE FAULTS REACTIVATION (Invited Presentation)

Several independent lines of evidence indicate that intraplate continental brittle crust rests in a state of failure equilibrium (i.e., shear stresses are near the strength limit of the crust), and that faults that are critically stressed are also hydraulically conductive (i.e., have elevated permeability). It has been proposed that elevated values of permeability are maintained through brecciation associated with repeated slip through time, which counteract fault-sealing mechanisms. Due to low strain rates, intraplate seismogenic faults are also characterized by long recurrence intervals, on the order of 1,000 to 100,000 years, and stress can reactivate faults not associated with historic seismicity. However not all currently active intraplate faults are created equal. Evidence from faults in the central US shows that those that are naturally active today exhibit distinctive deformation histories, indicating long and sustained activity through time, with displacements breaching through Quaternary (Late Pleistocene and Holocene) sediments. On the other hand, those faults rupturing today in response to anthropogenic industry practices (e.g., deep-well injection of wastewater or hydrofracking operations) display resolvable displacements limited to Early to Middle Paleozoic sedimentary successions, suggesting that these faults have not moved for hundreds of millions of years until now. While this evidence further supports the causal factor for the reactivation of these latter faults, it does raise a question: how do we reconcile faults at or near failure over long periods (~200-300 Ma) with no evidence for failure? In this talk I will explore some of the alternative approaches to the conundrum, and observations that can help further clarify and answer this question.