The Brittle‐Plastic Transition, Earthquakes, Temperatures, and Strain Rates

Abstract

Maximum depths of earthquakes in different settings are commonly thought to lie within a transition, many kilometers in width, from brittle deformation at shallow depths to plastic deformation by high‐temperature creep at greater depth. A review of temperatures and strain rates, both of which are low in intraplate settings, shows faster deformation in warmer tectonically active regions and highest strain rates in regions where the movement of magma affects strain rates and temperatures are especially high. Although intraplate earthquakes appear to occur in oceanic lithosphere only where colder than ~600°C, in tectonically active regions, where strain rates are several orders of magnitude higher than intraplate settings, earthquakes occur where temperatures exceed 600°C and perhaps 800°C. The temperature cutoff increases by ~100°C as strain rates increase by ~2 to 2.5 orders of magnitude. Nowhere, however, do any two of average strain rates, background stresses, and temperatures seem to be determined well enough that, when combined with laboratory‐based flow laws for high‐temperature creep, they can place a tight constraint on the third of these quantities. At the same time, the pattern of higher temperatures at higher strain rates follows the general forms of power law and Peierls creep that are appropriate for lithospheric conditions.

A Model of Failure and Localization of Basalt at Temperature and Pressure Conditions Spanning the Brittle‐Ductile Transition

“…Fractures, in turn, are a fundamental control in the heat and mass transport in the crust (Ingebritsen et al., 2010), such that a strong link between rock rheology and poromechanical behavior is expected. Laboratory experiments have shown that permeable fractures are most likely present beyond what was previously considered as a limiting temperature of ∼400°C for permeability (Watanabe et al., 2017): Here we show that for basalt, it could be expected that fractures and shear zones form at temperatures ≥600°C (Adelinet et al., 2013; Molnar, 2020), especially at higher than tectonic strain rates (10 −5 s −1 in this case). Nonetheless, a complete rheology‐permeability relationship is still to be devised as, in general, are models that take into account the coupled effects of moving fluids (Benson et al., 2008), the reduction of strength in saturated conditions (Karato & Wong, 1995), and the decreased hydraulic transmissivity at high temperature (Watanabe et al., 2017).…”

“…As temperature rises with depth, it influences the solid and fluid rheology and their interaction, and as a result the environmental effects are even more relevant in the deep part of the crust and in regions where the geothermal gradient is above average, such as volcanic areas (Parisio et al., 2019). The deformation characteristics of rocks can determine the (potential) maximum depth at which earthquakes can be expected: While several have placed the limit at 600°C (references within Molnar, 2020), evidence suggests that such a limit is likely to be arbitrary and that earthquakes could be expected in the hot crust at ∼800°C (Molnar, 2020). Further support to this hypothesis seems to come from laboratory experiments that have shown that basaltic rocks exhibit acoustic emission in what is usually considered the ductile deformation regime (Adelinet et al., 2013).…”

Formation of Nanocrystalline and Amorphous Materials Causes Parallel Brittle‐Viscous Flow of Crustal Rocks: Experiments on Quartz‐Feldspar Aggregates

“…To incorporate frictional experiments performed at much lower stresses to much higher strains (Yund et al, 1990), we plot the volume of slip zones as a function of strain energy density (stress × strain) and PEC AND AL NASSER 10.1029/2020JB021262 19 of 26 Heilbronner (2012), Pec, Stünitz, Heilbronner, Drury, et al (2012), Pec et al (2016), Marti et al (2017, 2020), and Yund et al (1990 color coded for different conditions. Solid circles are for granitic rocks, empty circles are for diabase and squares for Qtz:Kfs aggregates.…”

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Relative motion of tectonic plates is accommodated along lithosphere-scale shear zones. The strength and stability of these shear zones control large-scale tectonics and the location of earthquakes (Bürgmann & Dresen, 2008;Molnar, 2020). Laboratory-derived strength profiles of the lithosphere postulate that the strength in the upper crust is controlled by frictional sliding along preexisting fractures, while the strength of the lower crust and upper mantle is controlled by viscous flow of rocks (Brace & Kohlstedt, 1980;Goetze & Evans, 1979;Kohlstedt et al, 1995).

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Seismic Strain Rate and Flexure at the Hawaiian Islands Constrain the Frictional Coefficient

“…An intuitive explanation for localized weakening in the brittle regime seems to be that increased temperatures due to magmatic intrusion weaken faults in this region. However, recent studies report that frictional sliding is unlike ductile deformation mechanisms which weaken with temperature, and that increased temperatures actually increase the effective strength of frictional sliding by increasing the areal contact of faults (Molnar, 2020). Therefore, increased temperatures would not weaken the brittle regime, but could shallow the depth of the brittle‐ductile transition to induce local weakening there and explain the region of low seismicity surrounding the stress center.…”