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| Titel |
Origin of Pseudotachylites during slow creep experiments |
| VerfasserIn |
M. Peč, H. Stünitz, Renée Heilbronner, M. Drury, C. De Capitani |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068424
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| Zusammenfassung |
| Pseudotachylites are interpreted as solidified friction induced melts which form exclusively
during seismic or impact events and are thus accepted as ‘unequivocal evidence’
of paleo-earthquakes on exhumed faults. However, we found in experiments that
pseudotachylites can form under clearly aseismic conditions at confining pressures and
temperatures typical of mid crustal levels (Pc = 500 MPa, T = 300Ë C).
The starting material consists of granitoid powder crushed to a size of -¤ 200 μm in
diameter. This material (0.1 g), with 0.2 wt% water added, is placed between alumina forcing
blocks pre-cut at 45Ë , weld-sealed in platinum jackets with an inner nickel foil insert and
deformed in a solid medium deformation apparatus (modified Griggs rig). We applied
displacement rates of (10-8 ms-1 < Ḡ< 10-6 ms-1) which approximate typical tectonic
plate velocities of a few cm/a. In the ~0.7 mm thick layer of fault rock, this produces a bulk
shear strain rate of (~10-5s-1 < Î³Ë < ~10-3s-1). The samples reach a peak shear
strength of (~1200 MPa < Ï < ~1500 MPa) at bulk sample strains of (1.5
< γ < 2.3). Only at the highest displacement rates (~10-6ms-1), the samples
fail abruptly shortly after reaching peak strength, possibly due to fracturing of the
forcing blocks. However, at slower displacement rates (10-7ms-1 to 10-8ms-1)
the samples reach a peak strength of 1200 – 1400 MPa, then weaken slightly (by
~30 MPa), and continue to deform at approximately constant stress without any
abrupt stress drops. The weakening is accompanied by a transient increase of the
measured displacement rate of the forcing piston by ~25%. The friction coefficient, μ,
on the 45Ë pre-cut is ~0.6, which is in the range of values typical of intact rock
materials.
After the experiment, the fault rock consists of a S-C-C’ fabric with a percolating,
multiply connected layer of pseudotachylites decorating the C’- C shears. Microstructures
indicative for pseudotachylites are: injection veins, flow structures, bubbles, and bubble trains
following the local flow pattern, corroded clasts and amorphous glass identified by TEM. The
chemical composition of the pseudotachylites varies depending on the precursor material and
is in general more ferromagnesian and basic compared to the bulk rock indicating preferred
melting of biotite. The calculated temperature increase due to shear heating is at the most ~5Ë
C. High stresses cause pervasive comminution: the smallest crystalline fragments
within the bubbly melt have a grain diameter of ~10 nm. Nanomaterials exhibit a
‘melting point depression’ (dependence of melting point on grain size) which allows
melting well below bulk melting temperatures. Thus, it seems that melting is a
continuation of the comminution once the rock has reached small enough grain size.
We therefore suggest that pseudotachylites may also form as ‘mechanical melts’
at slow displacement rates without the necessity of reaching high temperatures. |
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