Deformation induced melt
generation in rocks
Pseudotachylites
Sandra Piazolo
Stockholm University
Outline
What are they?
Where do they occur?
How do they
form?
What can they tell us?
Remaining questions
What are Pseudotachylites ?
•Dark massive rock, glassy
or/and very fine grained
•Crosscutting preexisting
rocks
What are Pseudotachylites ?
• Sharp
boundaries
• intrusive type
structures
What are Pseudotachylites ?
• Often several
generations
• different
colouration of
veins – rim /
centre
• thickness
varies from mm
to …
What are Pseudotachylites ?
• thickness varies
from mm to dm
What are Pseudotachylites ?
• cooling joints?
What are Pseudotachylites ?
• Often have
brecciated
’xenoliths’
• these are
rounded to
sharp edged
What are Pseudotachylites ?
• ’xenoliths’ are
rotated,
sometimes of
different origin
than immediate
border
What are Pseudotachylites - thinsection ?
What are Pseudotachylites - thinsection ?
What are Pseudotachylites ?
• Thick dyke like
occurrences
We know from the field
- intrusive characteristics
(sharp boundaries,
crosscutting, very fine
grained, glassy, ’xenoliths’,
difference in structure
middle to rim)
- associated with
deformation / breccia
- thicknesses generally
below 1 m
How do they form?
• (micro)structures -> melts
• local/externally derived?
• why different to other
intrusives ?
-> T of host rock, rheology
of host rock, melt amount
Back to basics
• Elastic deformation
– Strain dissapears when
force is taken away
Function of
1. Temperature
• Brittle deformation
– Discontinuous
deformation
• Ductile deformation
– Flowing movement
(continuous)
3. Pressure
5. Deformation rate
7. Composition
A) Depth: Faults and shear zones
• Near surface: Low P & T
– Brittle failure (Mohr-Coulomb
behaviour)
– Discrete narrow zones of
deformation
"Brittle-Ductile transition"
+P
+T
Passchier & Trouw, 2005
• Deeper: higher P & T
– Ductile deformation behaviour
– Wider zones of deformation:
Shear zones
The exception: Pseudotachylites
• Brittle deformation
– Discontinuous
deformation
– At very high
deformation
rates -> melt
generation
+P
+T
– can extend
deep down into
the crust (can
even occur
down to eclogite
facies (>60 km
depth) –
relatively cold
high pressure
rocks
Function of
• Temperature
• Pressure
• Deformation rate
• Composition
How do they form: Pseudotachylites
• Very fine grained cataclastite
• In-situ melting – frictional melting
Energy production Q = shear stress and slip distance d
Q = τ d = Qdyn + Q fric = Qdyn + Qs + Qh
Qdyn
Work done dynamically against resistance to
fracture propagation
Q fric
Work done against friction – non-thermal
component (overcome surface energy) & thermal
component (frictioanl heat generation)
How do they form: Pseudotachylites
At non-seismic slip rates: conductive and
advective heat transport can keep pace with rate
of frictional heat generation
Seismic slip (0.1-2m/s):
thermal component >>> dynamic and non-thermal
component
-> frictional heat = total energy production -> raise T
τ (dε / dt )t
∆T =
Cpρ
Phillpotts, 1990
Relate strain rate
to slip velocity v
and width w
τ vt
∆T =
wC p ρ
For example: at v= 0.5 m/s, w =1 cm
-> heat: 1800 C
How do they form: Pseudotachylites
τ vt
∆T =
wC p ρ
Phillpotts, 1990
For example: at v= 0.5 m/s, w =1 cm
-> heat: 1800 C
For example: at v= 0.1 m/s, heat: 1800 C
-> w = 0.2 cm
If you take heat conduction away from fault into
consideration -> mostly max. 1-2 mm thick
pseudotachylites can only been maintained, very
large seismic events up to 1 cm.
How do they form: Pseudotachylites
Further complications:
- energy sink: need for heat of fusion
- in middle/lower crust pseudotachylites in dry rocks
-> no fluids to help with melting
-> ”dry conditions” prerequisite? – fluid reduce friction
- with first drop of melt -> drop of friction -> no more melt
- volume increase with melt -> pore pressure increase ->
reduction of frictional contact
Theoretical: Self limiting process (negative feedback)
Field data: thick pseudotachylites are occurring
& several generations are common
Some positive feedback mechanism needed
How do they form: Pseudotachylites
Dilation during pseudotachylite generation -> melt
accummulation , reestablishement of contacts of host
rock
How do they form: Pseudotachylites
Dilation during pseudotachylite generation
How do they form: Pseudotachylites
Rheological contrasts: stress concentrations -> melt
generation
How do they form: Pseudotachylites
Rheological contrasts:
stress
concentrations ->
melt generation
How do they form: Pseudotachylites
Plattner et al. 2003
Different rocks
behaves
differently
Rheological
contrasts
What can they tell us?
Orientation of stress field – directivity of rupture
-> distribution and characteristics of
pseudotachylite
-> info about ’fossil’ seismic events
Age of
seismic
events
Di Toro et
al., 2005
What can they tell us?
Mechanics: e.g. rebound –
pressure release in impact
craters
What can they tell us?
Natural experiments:
Shock heating of
deformed/brecciated
rocks
Electron Backscatter
Diffraction (EBSD)
• SEM technique applicable to any
crystalline material (in
theory)
• Provides the absolute
crystal orientation with
sub-micron resolution
Example: outer
Hebrides Fault
zone
What can they tell us?
Natural experiments:
Shock heating of
deformed/brecciated
rocks
Electron Backscatter
Diffraction (EBSD)
• SEM technique applicable to any
crystalline material (in
theory)
• Provides the absolute
crystal orientation with
sub-micron resolution
What can they tell us?
Natural experiments: Shock heating of
deformed/brecciated rocks
What can they tell us?
What can they tell us?
What can they tell us?
Within 20 degrees of main grains
• few larger relicts remain &
related grains
• many small (micron sized)
grains with very different
orientation and low local
lattice distortion
-> shock heated and cooled
’nuclei’?
Summary & Outlook
Pseudotachylites: friction heat induced melts
Greenland /
Archean
-Already good ”helpers” in several fields
BUT
- mechansims of feed backs still under discussion
- positive & negative feedbacks
Importance of rheological contrast
Importance of low pressure areas – melt accumulations
-> 3D field work & check local chemical variations (melt
from other areas)
Further use as natural laboratories