1. Shear heating in continental strike-slip shear zones:
model and field examples
Leloup, Ricard, Battaglia, Lacassin
Geophys. J. Int. (1999) 136, 19–40

2. Migmatites (partial melting)/Leucogranites (anatexy)
P/T paths for ASRRSZ:
Migmatites (partial melting)/Leucogranites (anatexy)
during deformation
T° : °C at 15km depth
High geotherm gradient
T° :> 700°C for melting

3. How to reach 700°C at 15km depth?
Mechanisms for temperature increase:
> rise of regional or local heat flow
> deep burial,
> thickening of radioactive crust
> Shear Heating
Can heat produced by friction raise temperature to generate in situ magmas?
Pb: shear heating > warmer/softener > buffers heat production
Objective: estimation of the maximum amount of shear heating/maximum increase of temperature if rheologically layered lithosphere.

4. 2 layers translithospheric strike-slip fault model
Investigated parameters:
K = thermal conductivity
Q = radioactive heat production
Hfb= basal heat flow
Mz= moho depth
F = friction
V = fault slip rate
Power flaw low
d=density,
F=friction,
A=pre-exponent coefficient
N= power flow law exponent,
E=activation energy,
Assumptions:
> constant fault rate and a thermal steady state reached
> all mechanical energy is dissipated in heat

5. From initial temperatures:
Strength profile calculated:
Brittle:
Ductile:

6. Add radiogenic heat and compute heat diffusion:
Once depth of brittle/ductile transitions zone
known:
Calculate heat produced by shear as
a function of depth:
Brittle: heat localized along the fault plane
Ductile: heat produced where
the deformation takes place
Add radiogenic heat and
compute heat diffusion:

7. Temperature for classical parameters:
Strain rate:

8. Normal conditions, after motion on the fault :
Width of the shear zone= 40 km
Max strain rate =
Velocity =
Surface heat flow = 109mWm-2
Moho at 735ºC
Increase of temperature:
at brittle/ductile transition =+176ºC at 10km depth
In the mantle =+208ºC at 35km depth
20km depth = 500ºC
F=0.6
Slip rate = 3cm/yr
Moho depth = 35km
Radiogenic heat = 1mW/m3 in the crust
Granitic crust/ Dunite mantle
Thermal conductivity = 2.5W/mK
Basal heat flow = 20mW/m2

9. Variations of thermal parameters: a) Thermal conductivity :
Very little variation on the temperature
b) Radiogenic heating :
Cool lithosphere : important shear heating,
but little increase of the temperature
compared to normal conditions
c) Basal heat flow :
High basal heat flow > low shear heating > buffers Tº
d) Moho depth:
No significant change
CCL:
700ºC not reached by thermal variations
because of buffering effect b) c) d)

10. Variations of mechanical parameters: a) Slip rate :
Important effect on the temperature
b) Coefficient of friction:
Greater effect in the upper crust?
On the surface heat flow
c) Power law coefficient :
'hard' crust + mantle > higher shear heating
CCL:
700ºC hardly reached with mechanical variations b) b) c)
Variations of thermal parameters:
Thermal conductivity :
Very little variation on the temperature
Radiogenic heating :
Cool lithosphere : important shear heating,
but little increase of the temperature
compared to normal conditions
Basal heat flow :
High basal heat flow > low shear heating > buffers T
Moho depth:
CCL:
700ºC not reached by thermal variations
because of buffering effect

11. Underestimation of the thermal anomaly: Fluids effects?
Comparison with shear zones data:
San Andreas fault
Parameters:
F=0.2
Slip rate = 4cmyr-1
Moho depth = 25km
Radiogenic heat = 1mWm-3 in the crust
Granitic crust/ Dunite mantle
Thermal conductivity = 2.5Wm-1K-1
Basal heat flow = 20mWm-2
Underestimation of the thermal anomaly:
Fluids effects?
Heat convection?

12. Alpine fault : Great slave SZ : NZ: V = 1-2.5cm/yr Width > 14 km
Offset = km
Width = 25km
ASRR SZ:
1000km long
Width = 20km
Offset = 7000 ± 200km
V= 4 ± 1cm/yr
Crust = 35km thick
Great slave SZ :

13. Underestimation of the geotherm, and the final temperature:
Comparison with high temperature metamorphism data:
New Zealand Alpine fault/ Ailo shan Red river SZ/ Great slave SZ
Min: F=0.1, soft crust, soft mantle
Max: F=0.6, hard crust, hard mantle
Slip rate = 4cmyr-1
Underestimation of the geotherm, and the final temperature:
Fluids effects?
Heat convection?

14. Ccl:
Shear heating : 40km width shear zone :
Others mechanisms of localization necessary to reach of 20km width
Shear heating not enough to explain high geotherm gradient observed
> Need convective heat transport : by fluids, extracting heat from where it is produced
> Fluids produced by dehydratation reaction > partial melting

15. Fast slip with inhibited temperature rise due to mineral dehydration:
Evidence from experiments on gypsum
Nicolas Brantut, Raehee Han, Toshihiko Shimamoto, Nathaniel Findling, and Alexandre Schubnel
Geology (2011), vol. 39; no. 1; p. 59–62

16. References:
Leloup et al., Geophy J. Int, 1999
Leloup et al., JGR 2001
Leloup et al., Tectonophysics 1995
Brantut et al., Geology, 2011