Pseudotachylite

Pseudotachylite (also spelled pseudotachylyte) is a dark, flinty, aphanitic to vitreous (glassy) rock that fills planar shear veins and irregular injection networks within brittlely deformed host rocks, containing angular-to-rounded survivor clasts of refractory minerals (chiefly quartz and feldspar) suspended in a quenched silicate melt matrix. The term was coined in 1916 by the British geologist Samuel James Shand while mapping the large dark breccia networks of the Vredefort Dome in South Africa, because the dark, glassy veins closely resembled volcanic basaltic glass (tachylite) in hand specimen despite having formed in situ from the melting of enclosing granitic gneiss without any external volcanic magma source.

Geologists recognize two distinct genetic classes of pseudotachylite:

1. Tectonic (Coseismic) Fault Pseudotachylite: Millimeter- to centimeter-thick veins formed along brittle fault surfaces in the upper and middle continental or oceanic lithosphere when rapid coseismic slip (at velocities of ~0.1 to 10 m/s during an earthquake) converts mechanical frictional work into intense localized heat (> 1,100–1,700 °C), melting the wall rock in seconds. Because it records ancient seismic slip directly in the rock record, fault pseudotachylite is widely referred to as a "fossil earthquake."
2. Impact Pseudotachylite (Pseudotachylitic Breccia): Centimeter- to decameter-wide dike networks formed during hypervelocity meteorite impact events (such as the Vredefort Dome in South Africa and the Sudbury Basin in Canada) through a combination of shock-wave decompression melting at mineral grain boundaries and high-velocity acoustic-fluidization slip during transient impact crater collapse.

Generation Veins vs. Injection Veins

In outcrop, tectonic fault pseudotachylites exhibit a diagnostic two-part structural architecture first systematized by R. H. Sibson in 1975:

• Fault (Generation) Veins: Planar, concordant seams (typically 1 mm to 5 cm thick) that lie directly along the primary seismic slip surface, bounded by sharp, planar wall-rock contacts showing measurable shear offset.
• Injection (Tensile) Veins: Wedge-shaped or branching offshoot dikes (ranging from millimeters to tens of centimeters long) that splay out at high angles (30°–90°) from the fault generation vein into dilational tension fractures in the adjacent damage zone, terminating blindly without shear offset.

Because ultracataclasites (extremely fine-grained crushed fault rocks produced purely by mechanical comminution without melting) can also form dark aphanitic fault seams and injection veins in the field, definitive identification of true pseudotachylite requires polarized-light and scanning electron microscopy (SEM) to demonstrate igneous quench textures:

Frictional Heating Rate and Disequilibrium Flash Melting

During an earthquake of slip displacement d (typically 0.1 to 5 m) at slip velocity v ≈ 1 m/s along a narrow slip zone of thickness w (1–10 mm) under effective normal stress σ_n (50–500 MPa) and sliding friction coefficient μ_f ≈ 0.6, the rate of frictional heat generation per unit fault area is q_f = τv = μ_f σ_n v ≈ 30 to 300 MW/m². Because the slip event lasts only t_slip = d/v ≈ 0.1–5 seconds, heat can diffuse outward into the wall rock over a thermal boundary layer thickness of only δ_th = √(κt_slip) ≈ 0.3 to 2 mm (where thermal diffusivity κ ≈ 10⁻⁶ m²/s).

Consequently, frictional heat is trapped almost adiabatically inside the millimeter-scale slip zone, raising local temperatures above 1,100–1,600 °C within fractions of a second. Unlike slow equilibrium crustal anatexis in migmatites—where quartz + K-feldspar + plagioclase + H₂O melt together at the minimum granitic eutectic (~650–700 °C)—pseudotachylite forms by disequilibrium flash melting of individual mineral phases according to their anhydrous or breakdown melting points and mechanical shear yields:

1. Phyllosilicates and Amphiboles Melt First (650–1,100 °C): Sheet silicates (muscovite, biotite, chlorite) and amphiboles (hornblende) dehydrate and melt completely within milliseconds, enriching the bulk melt in Fe, Mg, Ti, K, and Al (making the pseudotachylite melt more mafic / andesitic than the bulk granitic host rock).
2. Plagioclase and K-Feldspar Melt Second (1,150–1,450 °C): Feldspars undergo partial-to-complete fusion along grain margins.
3. Quartz Survives as Rounded Clasts (Melting Point ~1,713 °C): Refractory quartz resists fusion and persists as thermally rounded, embayed survivor porphyroclasts floating in the mafic-enriched melt.

Once seismic slip ceases, the thin sheet of superheated melt loses heat by conduction into the cold surrounding wall rock (typically 150–350 °C at 5–15 km depth), quenching below the glass transition temperature within seconds to minutes.

Dynamic Melt Lubrication vs. Viscous Braking

Laboratory high-velocity rotary-shear experiments (Hirose & Shimamoto, 2005; Di Toro et al., 2006) reveal a complex non-monotonic evolution of fault friction during melting: at the onset of melting, a thin discontinuous film of viscous silicate melt causes a transient spike in shear resistance (viscous braking), followed—once a continuous melt layer thicker than ~1 mm forms—by dramatic dynamic melt lubrication, dropping the effective friction coefficient μ_f from ~0.6–0.8 down to 0.05–0.20 and allowing large seismic stress drops.

Vredefort and Sudbury Super-Dikes

In Earth's two largest preserved multi-ring impact structures—the 2.02-Ga Vredefort Dome (original diameter ~250–300 km) and the 1.85-Ga Sudbury Structure (diameter ~200–250 km)—pseudotachylitic breccia bodies reach extraordinary dimensions, with individual dikes up to 10–100 meters wide and several kilometers long containing house-sized blocks of Archean gneiss alongside coesite and stishovite high-pressure polymorphs of SiO₂ and shatter cones. These giant bodies form within seconds during shock compression (which melts grain boundaries at 20–40 GPa) followed immediately by kilometers of rapid ring-fault displacement during central-uplift rebound and crater collapse.

Deep-Focus Subduction Earthquakes

In exhumed high-pressure eclogite and peridotite terranes (such as the Bergen Arcs in Norway and Corsica), eclogite-facies fault pseudotachylites formed at depths of 60–80 km (2.0–2.5 GPa) demonstrate that self-localizing thermal runaway instability and frictional melting work alongside dehydration embrittlement to nucleate intermediate-depth and deep-focus subduction zone earthquakes where high confining pressure would otherwise inhibit ordinary brittle fracture.

Why is a pseudotachylite melt typically more mafic (darker and richer in iron and magnesium) than the granite it melted from?

During an earthquake lasting only a few seconds, minerals melt individually by flash heating rather than reaching chemical equilibrium. Hydrous iron- and magnesium-rich minerals with lower melting points (biotite at ~850 °C and hornblende at ~1,050 °C) melt completely into the matrix, whereas pure silica quartz (melting point ~1,713 °C) remains solid as unmelted clasts, leaving the quenched melt enriched in Fe, Mg, and Al and depleted in SiO₂.

Why are fault pseudotachylites relatively rare in outcrop if earthquakes are so common?

Three factors limit their preservation: first, most shallow crustal faults contain pore water, where frictional heating pressurizes fluid (thermal pressurization) and reduces normal stress before rock temperatures can reach 1,100 °C; second, pseudotachylites require dry, crystalline host rocks at 5–15 km depth; and third, volcanic glass and microlites rapidly devitrify and alter into soft clay or chlorite fault gouge when exposed to post-seismic hydrothermal fluids.

How do geologists distinguish an impact pseudotachylite from a tectonic fault pseudotachylite?

Tectonic fault pseudotachylites form thin planar generation veins (usually < 1–5 cm thick) bounded by clear shear offset along a regional fault zone. Impact pseudotachylites form irregular, anastomosing three-dimensional networks and massive dikes (up to tens of meters wide) with little or no net offset, and are spatially associated with shock-metamorphic indicators such as shatter cones, planar deformation features (PDFs) in quartz, and high-pressure silica polymorphs (coesite and stishovite).

1. Shand, S. J. (1916). "The Pseudotachylyte of Parijs (Orange Free State), and Its Relation to 'Trap-Shotten Gneiss' and 'Flinty Crush-Rock'." Quarterly Journal of the Geological Society of London, 72(1–4), 198–221. https://doi.org/10.1144/GSL.JGS.1916.072.01-04.12
2. Sibson, R. H. (1975). "Generation of Pseudotachylyte by Ancient Seismic Faulting." Geophysical Journal of the Royal Astronomical Society, 43(3), 775–794. https://doi.org/10.1111/j.1365-246X.1975.tb06195.x
3. Spray, J. G. (1992). "A Physical Basis for the Frictional Melting of Some Rock-Forming Minerals." Tectonophysics, 204(3–4), 205–221. https://doi.org/10.1016/0040-1951(92)90308-S90308-S)
4. Di Toro, G., Hirose, T., Nielsen, S., Pennacchioni, G., & Shimamoto, T. (2006). "Natural and Experimental Evidence of Melt Lubrication of Faults During Earthquakes." Science, 311(5761), 647–649. https://doi.org/10.1126/science.1121012