Lower crustal earthquakes in the East African Rift System: Insights from frictional properties of rock samples from the Malawi rift

Abstract

Earthquakes in the southern part of the East African Rift System (EARS) occur at depths up to 45 km in the lower crust, unusually deep for an extensional regime. Typically, earthquakes in continental crust nucleate at temperatures less than 350°C, the temperature at which crystal plastic creep in quartz becomes efficient, corresponding to a depth of ~15 km with an average continental geothermal gradient. Several hypotheses have been proposed to explain the deep seismicity in the EARS, including the presence of high-viscosity mafic material, an abnormally low geotherm, locally elevated strain rates, or high fluid pressures. Here, we provide frictional properties of simulated fault gouges to address the seismic behavior of felsic and mafic crystalline basement rocks from the amagmatic Malawi Rift at the southern, propagating end of the EARS. Rotary shear experiments are conducted at 100, 200, and 250 MPa effective normal stress, a fluid pressure of 100 MPa and from room temperature to 700 °C. The friction of the felsic sample (58% plagioclase) is fairly constant up until 400 °C, after which it decreases. At temperatures of 600 °C and above, the relation between shear stress and effective normal stress deviates from linearity. The friction coefficient of the mafic sample is nearly constant in the considered range of temperature and normal stress and the sample shows a linear relation between normal stress and shear stress. The mafic sample also has negative, and decreasing, (a-b) values at temperatures > 400°C. These results are consistent with earthquake nucleation in mafic material, if present, at lower crustal P-T conditions. On the other hand, the data indicate that earthquake nucleation is unlikely in felsic material at temperatures above 500 °C, unless effective normal stress is low and strain rate is elevated by several orders of magnitude.

How Do Variably Striking Faults Reactivate During Rifting? Insights From Southern Malawi

“…This is significant as faults that are frictionally weak (fault static coefficient of friction [ μ s ] < 0.4) typically contain interconnected phyllosilicates phases that constitute >30%–40% of the fault rock (Massironi et al, ; Moore & Lockner, ). Thus, we infer that these faults exhibit “Byerlee” frictional strengths ( μ s ~ 0.6–0.8; Byerlee, ), which is consistent with the results of deformation experiments on a suite of basement lithologies from the Malawi Rift ( μ s = 0.55–0.80; Hellebrekers et al, ). Differences in composition between country rock and fault rock samples are observed, such as for the Chingale Step fault, where calcite is dominant in the fault rock but is not detected in the country rock sample (Table ).…”

“…Principal stress magnitudes can be derived as μ s ′ is being equated to the stresses acting on an optimally oriented fault (Figure ), thus under Mohr‐Coulomb theory (Jaeger et al, ): where ϕ i = tan −1 μ i and μ i is the frictional strength of intact rock. Given the results of Hellebrekers et al (), μ i = 0.7; thus, ϕ i = 35°. Since σ 1 can be derived from equation , it is thus also possible to calculate σ 3 and σ 2 by rearranging equation and the equation for Φ (equation (S1)), respectively.…”

“…However, if it is cohesionless at 10-to 35-km depth, in Stress State 1 μ s ′ is between 0.50 and 0.60 (Figures 9b and S9a). This is at the lower end of frictional strengths inferred from its composition (Table 2) and deformation experiments on basement rocks in Malawi (Hellebrekers et al, 2019). In the cohesive fault case, μ s ′ = 0.35-0.45 (Figure 7a, 8a, and 8d), and so below its likely frictional strength.…”

“…Contour plots for effective coefficient of friction ( μ s ′) needed for reactivation of a cohesive fault ( c = 40 MPa) in intact rock frictional strength‐stress shape ratio ( μ i ‐ Φ ) space for the given fault orientations and Stress States 1–3 at 20‐km depth. Black circle represents point where Φ = 0.43 and μ i = 0.70, as used in Figure , with error bars representing range of μ i reported by Hellebrekers et al (; μ i = 0.55–0.80), and 1 standard deviation uncertainty in Φ (0.43 ± 0.24). For similar analysis for cohesionless fault, see Figure S10.…”

How do variably striking faults reactivate during rifting? Insights from southern Malawi

“…Geochemistry, Geophysics, Geosystems phyllosilicates phases that constitute >30%-40% of the fault rock (Massironi et al, 2011; Moore & Lockner, 2004). Thus, we infer that these faults exhibit "Byerlee" frictional strengths (μ s~0 .6-0.8; Byerlee, 1978), which is consistent with the results of deformation experiments on a suite of basement lithologies from the Malawi Rift (μ s = 0.55-0.80; Hellebrekers et al, 2019). Differences in composition between country rock and fault rock samples are observed, such as for the Chingale Step fault, where calcite is dominant in the fault rock but is not detected in the country rock sample (Table 2).…”