Making It Simple: Earthquake Thermometry and Rheology of the Continental Lithosphere Revisited

Rheology of the lithosphere depends on many factors, including temperature, pressure, strain rate, bulk composition and mineral assemblages, grain size, and the presence of fluids or volatiles. Meanwhile, a number of studies show that earthquake occurrences are limited by distinct temperatures (Tc): 300-400°C for crustal earthquakes and 600-700°C for earthquakes in the mantle (e.g., Chen & Molnar, 1983; Wiens & Stein, 1983; McKenzie et al., 2005). Laboratory experiments show that frictional instability can be conveniently described by a rate and state-variable (the "Dieterich-Ruina") relationship. In particular, the system is unstable (velocity weakening) when a combined variable, (a - b), is negative, where a and b are empirically determined material properties, and (a - b) represents how steady-stated coefficient of friction varies as a function of the natural logarithm of sliding velocity. For granite, (a - b) is negative at low temperatures but becomes positive above about 350°C (e.g., Scholz, 1998). This Tc for crustal seismicity is close to the onset of significant plasticity in quartz, an abundant rock-forming mineral in granite. Tc of about 600-700°C for mantle earthquakes is also consistent with where (a - b) changes sign for olivine, the most abundant mineral in the upper mantle (Boettcher et al., 2007). In other words, the limiting temperatures reflect the onset of substantial crystal plasticity in major minerals, which greatly influenced the combined parameter (a - b); so the seismic to aseismic transition is closely linked with brittle-ductile transitions, one in the crust and another in the uppermost mantle, where the strength profile of the lithosphere is expected to peak. In regions where the distribution of focal depths is bimodal (such as southern Tibet), temperature below the seismogenic upper crust should exceed about 350°C but that of the seismogenic uppermost mantle should remain below 700°C.