Shear strain concentration mechanism in the lower crust below an intraplate strike-slip fault based on rheological laws of rocks

Abstract

We conduct a two-dimensional numerical experiment on the lower crust under an intraplate strike-slip fault based on laboratory-derived power-law rheologies considering the effects of grain size and water. To understand the effects of far-field loading and material properties on the deformation of the lower crust on a geological time scale, we assume steady fault sliding on the fault in the upper crust and ductile flow for the lower crust. To avoid the stress singularity, we introduce a yield threshold in the brittle-ductile transition near the down-dip edge of the fault. Regarding the physical mechanisms for shear strain concentration in the lower crust, we consider frictional and shear heating, grain size, and power-law creep. We evaluate the significance of these mechanisms in the formation of the shear zone under an intraplate strike-slip fault with slow deformation. The results show that in the lower crust, plastic deformation is possible only when the stress or temperature is sufficiently high. At a similar stress level, ∼100 MPa, dry anorthite begins to undergo plastic deformation at a depth around 28-29 km, which is about 8 km deeper than wet anorthite. As a result of dynamic recrystallization and grain growth, the grain size in the lower crust may vary laterally and as a function of depth. A comparison of the results with constant and nonconstant grain sizes reveals that the shear zone in the lower crust is created by power-law creep and is maintained by dynamically recrystallized material in the shear zone because grain growth occurs in a timescale much longer than the recurrence interval of intraplate earthquakes. Owing to the slow slip rate, shear and frictional heating have negligible effects on the deformation of the shear zone. The heat production rate depends weakly on the rock rheology; the maximum temperature increase over 3 Myr is only about several tens of degrees.

Crustal dynamics: unified understanding of geodynamic processes at different time and length scales

“…Results indicate that a combination of deformation in the upper crust as well as ductile deformation in the lower crust may contribute to the high strain rate in the northeastern NKTZ. Zhang and Sagiya (2017) modeled strain concentration in two dimensions within the lower crust, assuming steady fault sliding in the upper crust and ductile flow in the lower crust according to laboratoryderived power-law rheology with a yield threshold at the brittle-ductile transition. Possible physical mechanisms for strain concentration in the lower crust are investigated including frictional and shear heating, grain size, and power-law creep, taking account also of the role of water in promoting crystal plasticity.…”

Intraplate Strike‐Slip Faulting, Stress Accumulation, and Shear Localization of a Crust‐Upper Mantle System With Nonlinear Viscoelastic Material

“…Water fugacity is calculated with the van der Waals equation (Karato, 2012). The detailed calculation method can be found in Zhang and Sagiya (2017).…”

“…Previous investigations of the deformation of the lower crust and the upper mantle using mechanical models have shown that localized shear deformation can be developed in the lower crust under the interplate (e.g., Allison & Dunham, 2018;Moore & Parsons, 2015;Takeuchi & Fialko, 2012;Thatcher & England, 1998) and intraplate (e.g., Zhang & Sagiya, 2017) strike-slip faults. Some of these models considered earthquake cycles (e.g., Allison & Dunham, 2018;Erickson et al, 2017;Lambert & Barbot, 2016;Takeuchi & Fialko, 2012) and others considered a creeping fault (e.g., Moore & Parsons, 2015;Thatcher & England, 1998;Zhang & Sagiya, 2017) in the upper crust. The localized deformation in the lower crust under a fault appears in all these models.…”

“…Our simulation demonstrates that this spin-up corresponds to the tectonic stress buildup, and the fault behavior can be highly variable during this period. Without careful examination of the initial settings, unrealistic behavior appears, such as the negative shear strain rate (Takeuchi & Fialko, 2012) or extremely large shear stress right below the upper crust (e.g., Moore & Parsons, 2015;Zhang & Sagiya, 2017). Inappropriate model assumptions may have led to those unexpected results.…”

“…Various observations indicate shear localization occurs even under a very low slip rate condition of intraplate faults. Zhang and Sagiya () investigated physical mechanism of shear zone formation under a slow slip rate condition and concluded that the power law rheology plays the principal role; however, the formation of shear zone was not modeled in that study. As the stress propagates in the material with nonlinear (power law) rheology (Melosh, ), it is important to understand the evolution of shear stress and strain distribution in the crust.…”