`thick` pseudotachylite

Rheological and structural constraints on the
accumulation of 'thick' pseudotachylites
biotite
Petrology
albite
Geochemistry & Petrogenesis
4000
‘Thick’ pseudotachylites have been interpreted in three ways:
• in situ melt generation during an single, enormous event,
• incremental in situ melt generation during multiple, smaller events,
• accumulation of allochthonous melt generated in a single event.
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2-Theta - Scale
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08-15e - File: 08-15e.raw
SB 8-15 - File: SB 8-15.raw
~5 cm
Rb Ba Th U
The pseudotachylite is crystalline and shows
no evidence of ever having been glassy:
• no devitrification or quench textures,
• no interstitial glass,
• an equicrystalline groundmass,
• no XRD ‘glass hump’.
mean "measured" (0.65% H2O)
mean "wet" (3.5% H2O, 1% F)
av. phonolite (1.57% H2O)
av. rhyolite (1.1% H2O)
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Tg
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Do the thermal conditions and viscosity of the
pseudotachylite melt allow it flow sufficiently far and fast to
accumulate to form ‘thick’ pseudotachylite sheets?
One of several listric, syn-emplacement ‘minidetachments’ displace the gneissic foliation below
the vein but not the thickened pseudotachylite itself.
Our preferred model (Ferre et al., 2012) of the
relationships between melt-generation and
accumulation of melt in an opening dilational jog.
maximum projected radius (m)
Formation of ‘thick’ pseudotachylite by melt accumulation is only
possible if the melt can feasibly flow from the generation site
(yellow) to the nucleation site (blue).
Extensional shear zones exhibit a strong displacement
gradient defined by the gneissic foliation. Note how
the pseudotachylite reaches its maximum thickness at
the ‘growth’ fault. Note white protolith porphyroclasts.
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Rb (ppm)
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Sc (ppm)
ABOVE: Selected bivariate trace element plots
comparing samples of pseudotachylite (blue) and the
surrounding wall-rocks (red). The pseudotachylite is
enriched in Ba and Rb (characteristic of melting of
biotite). Sr and Zr both behave compatibly
demonstrating that the pseudotachylite melt was
sourced from these wall-rocks.
Melt viscosity
Minimum melt viscosities
for three different volatile
contents were calculated in
the Giordano et al. (2008)
viscosity model using
liquidus temperatures
calculated with the MELTs
(Ghiorso & Sack, 1995;
Asimow & Ghiorso, 1998).
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1300
The ‘wet’ melt (blue) is the most appropriate to use for
this biotite-rich pseudotachylite.
This will allow us to constrain the 1D and 2D shear strain within the
melt and estimate how quickly and far the melt could flow before
being retarded by cooling and crystallization.
‘Thick’ pseudotachylite vein parallel to the gneissic
foliation, extensional ‘growth’ faults offset the subhorizontal foliation accentuating dilational jogs.
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melt temperature (°C)
To investigate this we have established a simple geometric model of
the pseudotachylite sheet that:
• provides a minimum estimate of the melt volume, and
• allows estimation of the original sheet radius for a given thickness.
Two ‘typical’ mm-thick pseudotachylite veins
parallel to the gneissic foliation - lower example
thickens in an extensional jog.
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ABOVE: Typical SEM image of the fine-equicrystalline
pseudotachylite groundmass [inset] of biotite and albite
(pale and medium grey ‘mossy’ texture) and rounded
porphyroclasts of quartz and orthoclase (dark grey).
Outcrop data
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70:30 melt-crystals
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pseudotachylite
wall rocks
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Sc (ppm)
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RIGHT: 80° W-dipping
normal fault and
parallel, penetrative,
spaced fracture cleavage
displacing the prominent
gneissic layering in the
Okanagan (para-)gneiss.
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mean "dry" (0% H2O)
av. basalt (0.95% H2O)
av. andesite (0.83% H2O)
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no injection veins
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liquidus - 0.65% H20, 0.1% F, 0.1 GPa
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microcrystalline quartz
ribbons
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K Nb La Ce Sr Nd P Hf Zr Sm Ti Tb Y
ABOVE: Trace element spidergram comparing
samples of pseudotachylite (black) and the
surrounding wall-rocks (grey). The pseudotachylite
is enriched in LILEs characteristic of melting of a
biotite-rich protolith, the absence of Srenrichment suggests minimal melting of feldspar.
This suggests crystallization from a melt
loaded with residual porphyroclasts (i.e. a
magma).
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large, rounded
mylonitized gneiss
porphyroclast
weak undulating
foliation defined by
groundmass-biotite
long axes and
porphyroclasts
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solidus - 3.5% H20, 1% F,
0.1 GPa
LEFT: LandSat7 image of
the southern Okanagan
Valley. Note the
prominent NNE-trending
grain in the footwall, the
surface expression of
high-angle normal faults
forming part of the
Okanagan Valley core
complex.
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no Sr enrichment 
no melting of feldspar
liquidus - 3.5% H20, 1% F, 0.1 GPa
This study examines the structural, petrological, and rheological evidence for melt
accumulation in ‘thick’ pseudotachylite using data from an exceptionally wellexposed, 15 cm-thick example in the Okanagan gneiss, Okanagan Valley shear
zone, southern British Columbia.
rotated porphyroclasts
and fringing quartz
ribbons - a hydrous melt?
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no ‘hump’ characteristic
of glass holocrystaline
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wall-rocks
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Zr (ppm)
Pseudotachylites are quenched silicate magmas resulting from frictional-melting
of the wall-rock during brittle deformation. Typically very thin, <5 mm, their
thickness can be related to seismic magnitude; however, this simple relationship is
challenged by the rare occurrence of anomalously thick (≥1 cm) pseudotachylite
sheets.
no injection veins
pseudotachylite
log η (Pa.s)
Geology, CSU Bakersfield, Bakersfield, CA, United States.
2 Earth & Ocean Sciences, University of British Columbia, Vancouver, BC, Canada.
3 Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.
Lin (Counts)
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1000
Ba (ppm)
3000
Rb, Ba, and K enrichment 
melting of biotite and/or orthoclase
Sr (ppm)
quartz
pseudotachylite prominent peaks for
biotite, albite, and quartz
sample / MORB
Sarah R. Brown1, Graham D.M. Andrews1, J. Kelly Russell2, H. Dan Gibson3
mylonitized felsic gneiss
with recrystallized
porphyroclasts of quartz
Future modeling will
account for progressive
crystallization and
increasing viscosity.
Summary
Anomalously thick pseudotachylites are
commonly inferred to have formed by
accumulation of allochthonous melt.
We have identified and described a 15 cm-thick,
crystalline pseudotachylite that is allowing us to
constrain melt and magma viscosity, original
geometry, and strain history to test the assertion
that it was able to flow from distal generation
sites and pool within dilational jogs in the shear
zone at its current location.
The melt probably formed by incongruent melting
of a biotite-rich layer within the Okanagan gneiss.
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planar disk
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oblate spheroid
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projected pseudotachylite
sheet thickness (mm)
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Asimow PD, Ghiorso MS, 1998. Am Min, 83, 1127-1131.
Brown SR, 2010. PhD thesis, Simon Fraser University.
Brown SR, Gibson HD, Thorkelson DJ, Andrews GDM,
Marshall DD, Vervoort JD, Rayner N, 2012.
Lithosphere, 4, 354-377.
Ferre E, Geissman JW, Zechmeister MS, 2012. JGR, 117,
B01106.
Ghiorso MS, Sack RO, 1995. Cont Min Pet, 119, 197-212.
Giordano D, Russell JK, Dingwell DB, 2008. EPSL, 271,
123-134.