Chapter 5 Pseudotachylytes and Earthquake Source Mechanics

Frictional melting mechanisms of rocks during earthquake fault slip

Rapid slip, at rates in the order of 1 m/s or more, may induce frictional melting in rocks during earthquakes. The short-lived melting has been thought to be a disequilibrium process, for decades. We conducted frictional melting experiments on acidic, basic, and ultrabasic silicate rocks at a slip rate of 1.3 m/s. The experiments and microstructural observations reveal that all minerals in the rocks are melted at temperatures below their known melting temperatures (Tm); e.g., quartz is melted at ~ 1000-1200 °C, not ~ 1720 °C, while olivine at ~ 1300 °C, rather than ~ 1700 °C. The low-temperature melting is incompatible with the conventional disequilibrium melting, and may be caused predominantly by grain size reduction and phase boundary reactions during the early and later stages of slip, respectively. The newly estimated Tm and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.

The impact of melt versus mechanical wear on the formation of pseudotachylyte veins in accretionary complexes

Whether seismic rupture propagates over large distances to generate mega-earthquakes or is rapidly aborted mainly depends on the slip processes within the fault core, including particularly frictional melting or intense grain-size reduction and amorphization. The record of seismic slip in exhumed fault zones consists in many instances in Black Faults Rocks, dark and glass-like-filled aphanitic veins that have been interpreted as resulting from the quenching of frictional melts, i.e. pseudotachylytes. Such interpretation has nevertheless been questioned as similar macro to nano-microstructures have been observed either on intensely comminuted natural fault rocks or on slow creep experiments conducted on crustal rocks, where melting is absent. Here, we report a new dataset of Raman Spectroscopy of Carbonaceous Material analyses, aimed at discriminating the slip weakening processes operating in the fault core during slip. Using high spatial resolution profiles on natural Black Fault Rocks from exhumed accretionary complexes and an experimentally calibrated modelling of Raman intensity ratio evolution with temperature, we assessed different scenarios of temperature evolution during fault slip. None of them is able to account for the distribution of Raman signal, so that in the three studied Black Fault Rocks interpreted so far as natural pseudotachylytes, Raman Spectroscopy of Carbonaceous Material rather reflects the effect of intense and localized strain during fault slip. Furthermore, the absence of thermal imprint on Raman signal puts upper bounds on the temperature reached within the fault zone. If one cannot rule out the occurrence of high and short-lived temperature increase due to friction, the latter was not high enough as to melt the large quartz fraction of the fault zone rocks.

Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)

How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and ≤ 300°C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth.

Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity

In recent works on the determination of graphitization of carbonaceous materials (CM) within the principal slip zone (PSZ) of the Longmenshan fault (China), we demonstrated that the formation of graphite, resulted from strain and frictional heating, could be evidence of past seismic slip. Here we utilize Raman Spectroscopy of CM (RSCM) on the CM-bearing gouges in the fault zone of the Longmenshan fault belt, at the borehole depth of 760 m (FZ760) from the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1), to quantitatively characterize CM and further retrieve ancient fault deformation information in the active fault. RSCM shows that graphitization of CM is intense in the fault core with respect to the damage zone, with the graphitized carbon resembling those observed on experimentally formed graphite that was frictionally generated. Importantly, compared to the recognized active fault zone of the Longmenshan fault, the RSCM of measured CM-rich gouge shows a higher degree of graphitization, likely derived from high-temperature-perturbation faulting events. It implies that FZ760 accommodated numerous single-event displacement and/or at higher normal stresses and/or in the absence of pore fluid and/or along a more localized slip surface(s). Because graphite is a well-known lubricant, we surmise that the presence of the higher degree graphitized CM within FZ760 will reduce the fault strength and inefficiently accumulate tectonic stress during the seismic cycle at the current depth, and further infer a plausible mechanism for fault propagation at the borehole depth of 590 m during the Mw 7.9 Wenchuan earthquake.

From slow to fast faulting: recent challenges in earthquake fault mechanics

Faults-thin zones of highly localized shear deformation in the Earth-accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 10⁴ s^-1) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.