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

Migmatites (partial melting)/Leucogranites (anatexy) P/T paths for ASRRSZ: Migmatites (partial melting)/Leucogranites (anatexy) during deformation T° : 650-750°C at 15km depth High geotherm gradient T° :> 700°C for melting

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.

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

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

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:

Temperature for classical parameters: Strain rate:

Normal conditions, after motion on the fault : Width of the shear zone= 40 km Max strain rate = 9. 10-14 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

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)

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

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?

Alpine fault : Great slave SZ : NZ: V = 1-2.5cm/yr Width > 14 km Offset = 300-700km Width = 25km ASRR SZ: 1000km long Width = 20km Offset = 7000 ± 200km V= 4 ± 1cm/yr Crust = 35km thick Great slave SZ :

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?

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

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

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