2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

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Symposium 1: Structural Geology, Tectonics and Geodynamics
1.11
Slow Pseudotachylites
Matej Pec 1 , Holger Stünitz 2 and Renée Heilbronner 1
1 Geological Institute, Basel University, Basel, Switzerland (matej.pec@unibas.ch)
2 Department of Geology, Tromsø University, Tromsø, Norway
Tectonic pseudotachylites as solidified, friction induced melts are believed to be the only unequivocal evidence for paleoearthquakes.
Earthquakes occur when fast slip (1 – 3 ms -1 ) propagates on a localized failure plane and are always related
with stress drops. The mechanical work expended, together with the rock composition and the efficiency of thermal
dissipation, controls whether the temperature increase on a localized slip plane will be sufficient to induce fusion.
We report the formation of pseudotachylites during steady-state plastic flow at slow bulk shear strain rates (~10 -3 s -1 to
~10 -5 s -1 corresponding to slip rates of ~10 -6 ms -1 to ~10 -8 ms -1 ) in experiments performed at high confining pressures (500
MPa) and temperatures (300°C) corresponding to a depth of ~15 km.
Crushed granitioid rock (Verzasca gneiss), grain size ≤ 200 µm, with 0.2 wt% water added was placed between alumina
forcing blocks pre-cut at 45°, weld-sealed in platinum jackets and deformed with a constant displacement rate in a solid
medium deformation apparatus (modified Griggs rig).
Microstructural observations show the development of an S-C-C’ fabric with C’ slip zones being the dominant feature.
Strain hardening in the beginning of the experiment is accompanied with compaction which is achieved by closely spaced
R1 shears pervasively cutting the whole gouge zone and containing fine-grained material (d < 100 nm). The peak strength
is achieved at γ ~ 2 at shear stress levels of 1350-1450 MPa when compaction ceases.
During further deformation, large local displacements (γ > 10) are localized in less densely spaced, ~10 µm thick C’-C slip
zones which develop predominantly in feldspars. In TEM, they appear to have no porosity consisting of partly amorphous
material and small crystalline fragments with the average grain size of 20 nm. After the peak strength, the samples weaken
by ~20 MPa and continue deforming up to γ ~ 4 without any stress drops. Strain localization progresses in the C’-C slip
zones and leads to the formation of pseudotachylites. Rough estimates of slip rates in the deforming slip zones are 2 to 4
orders of magnitude higher (~10 -2 ms -1 to ~10 -6 ms -1 ) than the bulk induced slip rate but clearly slower than seismic.
The composition of the pseudotachylites is usually more ferro-magnesian and less silicic than that of the bulk rock.
Microstructural observations show the presence of corroded clasts of especially quartz, injection veins and bubbles with a
strong shape preferred orientation within the molten material following the local flow pattern. The pseudotachylites are
locally folded; their thickness varies between < 1 µm to 10 µm. Pseudotachylites have a distinct CL signal compared to any
other material present in the less deformed experiments.
Our results indicate that pseudotachylite formation at the bottom of the seismogenic layer may not necessarily be connected
with stress drops and thus with earthquakes.
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT