Thermal localization and rheological control on the steady-state width of 1D ductile shear zones

N. M. Beeler, H.S Shabtian, & Greg Hirth

In Preparation 2026, SCEC Contribution #15047

Over geologic time, the initiation and growth of upper crustal plate boundary faults that host present Earth's largest earthquakes and tsunamis are influenced by thermally localized ductile shear zones (DSZs) in the lower crust and upper mantle. Current DSZs influence elastic loading over the earthquake cycle, and the down dip extent of earthquakes ~M≥6.8 that rupture into and beyond the brittle ductile transition (BDT). Despite their mechanical importance to tectonics and on-going crustal deformation, there is relatively little observational data on DSZ properties due to their distant proximity to surface instrumentation. As in numerous previous 1 & 2D DSZ simulations, the 1D numerical calculations of this study couple ductile strength, shear heating and conduction, producing spontaneous shear localization from thermal weakening due to self-heating. These are at steady-state within a fixed length region, L=30 km, at a background temperature of 400°C. The combination of a 1D geometry, fixed length and background temperature, and the assumption of steady-state results in highly thermally constrained DSZs. When a DSZ is fully contained within the region: 1) the DSZ hosts a fixed fraction, 87%, of the deformation of the entire region; 2) temperature change within the region is proportional to shear heat production via a geometric constant; and 3) the shape of the resulting strain rate distribution is scale-independent. All three properties are independent of the rheology used. The strain rate distribution is approximately Gaussian. For a Gaussian shear zone at steady-state the velocity and temperature distributions, and rate that thermal energy is generated from shearing are also analytic, allowing for a perturbation analysis of DSZ properties. The analysis yeilds a characteristic width where strength is minimized and shear is localized at the lowest possible energy release rate. This characteristic scale of localization is determined by a balance between the rheology's opposing temperature weakening and rate strengthening. Thus, despite being highly constrained by geometry and assumptions, DSZ width and localization depends entirely on the choice of rheology. Contrasting with quartz (dislocation creep), rather than localizing talc (glide) tends to deform via broadly distributed shear even at highly elevated loading rates. Distributed shear in talc is also expected at natural conditions; it is too weak to produce a thermal width smaller than the strike normal dimension of the largest natural occurrences.

Citation
Beeler, N. M., Shabtian, H., & Hirth, G. (2026). Thermal localization and rheological control on the steady-state width of 1D ductile shear zones. Earth and Planetary Science Letters, (in preparation).

Related Projects & Working Groups
Community rheology model, Fault and Rupture Mechanics (FARM)