Viscous roots of active seismogenic faults revealed by geologic slip rate variations
Résumé
Viscous flow at depth contributes to elastic strain accumulation along seismogenic faults during both post-seismic and inter-seismic phases of the earthquake cycle. Evaluating the importance of this contribution is hampered by uncertainties regarding (i) the extent to which viscous deformation occurs in shear zones or by distributed flow within the crust and/or upper mantle, and (ii) the value of the exponent, n, in the flow law that relates strain rate to applied stress. Geodetic data, rock deformation experiments, and field observations of exhumed (inactive) faults provide strong evidence for non-linear viscous flow but may not fully capture the long term, in situ behaviour of active fault zones. Here we demonstrate that strain rates derived from Holocene offsets on seismogenic normal faults in the actively uplifting and extending central and southern Italian Apennines may be used to address this issue. The measured strain rates, averaged over a time scale of 104 years, exhibit a well-defined power-law dependence on topographic elevation with a power-law exponent ≈ 3.0 (2.7 - 3.4 at 95% CI; 2.3 - 4.0 at 99% CI). Contemporary seismicity indicates that the upper crust in this area is at the threshold for frictional failure within an extensional stress field and therefore differential stress is directly proportional to elevation. Our data thus imply a relationship between strain rate and stress that is consistent with non-linear viscous flow, with n ≈ 3, but because the measurements are derived from slip along major crustal faults they do not represent deformation of a continuum. We know that, down-dip of the seismogenic part of active faults, cataclasis, hydrous alteration, and shear heating all contribute to grain size reduction and material weakening. These processes initiate localisation at the frictional-viscous transition and the development of mylonitic shear zones within the viscous regime. Furthermore, in quartzo-feldspathic crust, mylonites form a fabric of mineral segregated layers parallel to shear with their strength controlled by the weakest phase: quartz. Using a published flow law for wet quartz calibrated for mylonitic rocks to fit the strain rates across individual fault zones (~5 km wide), we estimate a lower bound on the temperature of the deforming material using our data. This temperature is reached at or just below the base of the seismogenic zone, as constrained by regional surface heat flow data and the depth distribution of crustal seismicity. We conclude that it is the rate of viscous flow in quartz-rich mylonitic shear zones, not distributed flow within the lower crust and/or upper mantle, which modulates the Holocene slip rates on the up-dip seismogenic part of the faults in this area. Our observations support the idea that the irregular, stick-slip movement of brittle faults, and hence earthquake recurrence, are ultimately modulated by down-dip viscous flow over multiple earthquake cycles.