1. Thermal structure of continental lithosphere from heat flow and seismic constraints: Implications for upper mantle composition and geodynamic models
Claire Perry
GEOTOP-UQAM-McGill, Montreal, Canada

2. Stability of continental lithosphere
equilibrium between chemical and
thermal buoyancy (e.g., Jordan 1979) ?
δFe# δT
150 km
Perry et al. GJI (2003); Forte & Perry Science (2000)
Accurate lithospheric thermal models required (heat flow, crustal heat production)

3. Introduction : Global Terrestrial Heat Loss
Pollack et al. (1993)

4. Continental Heat Flow : example from Canadian Shield

5. Heteogenity of Continents …
geological
compositional
link between
surface geology
and lateral
variations in Qs
Canadian Shield

6. generic thermal model for all cratons ?
influence of temperature + composition on
seismic velocity precise thermal model

7. Thermal Structure of the Continental Lithosphere
Gung et al. (2003)
variable seismic thickness
d3 detected by tomography

8. Presentation Outline
Lithospheric thermal structure, upper mantle temperatures, and Pn velocity-temperature conversions from heat flow and seismic refraction studies
The thermal boundary layer of continental lithosphere and average mantle temperatures from a geodynamic flow model
How does continental heat production affect lithospheric and mantle temperatures ?

9. Variables of Continental Thermal Structure Problem

10. Variables of Continental Thermal Structure Problem
(Aavg~0.7 µWm-3) : distribution of radiogenic elements ?
small
(~0.02µWm-3)

12. Archean Superior Province, Canada

13. mechanical resistance of lithosphere
Heat Flow Data …
Qs Tmoho
correlation VP – T
mechanical resistance of lithosphere

14. Distribution of Radiogenic elements _____________
Differentiation Index:
DI = <Asurf> Ac
Province DI
Slave Province
2.1±0.5
Superior Province
1.2±0.1
Trans-Hudson Orogen
1.1±0.2
Wopmay Orogen
2.3±0.1
Grenville Province
1.3±0.2
Appalachians
2.5±0.2
Perry et al. JGR 2006a

15. Distribution of Radiogenic elements _____________ Differentiation
Index:
DI = <Asurf> Ac
Province DI
Slave Province
2.1±0.5
Superior Province
1.2±0.1
Trans-Hudson Orogen
1.1±0.2
Wopmay Orogen
2.3±0.1
Grenville Province
1.3±0.2
Appalachians
2.5±0.2
Perry et al. JGR 2006a

16. Crustal Model distribution of ACR in crustal columns
Moho temperature estimated using using k(T)
LITH5.0 (Perry et al. GJI, 2002) + more recent data
Hc, Pn
Fixed Parameters : Qs, A0, k(T), Hc
Free Parameter : Qm (constrained by xenolith + heat flow,
A(z) constrained by Qm, Qs, Hc
Principal unknown Qm

17. Pn velocity

18. Crustal Thickness

19. Moho depth

21. dV(Pn)/dT=-0.60x10-3 ± 10% kms-1K-1 (close to mineral physics
estimates)

22. Average Cratonic Mantle Composition
Perry et al. JGR 2006b
on-craton VP-T ≠ off-craton VP-T
predicted/measured VP Qm≥ 12 mWm-2

23. Preferred Mineralogical Composition : Superior upper-mantle
joint
Qs + Pn
lithospheric mantle composition + Qm
Perry et al. JGR 2006b

24. Conclusions – Part I
Comparison of large-scale empirical geophysical data and in-situ experiments of mantle composition provide further confidence in mantle temperatures from seismic studies and heat flow
Joint inversions of heat flow and seismic Pn velocity constrain :
mantle mineralogical composition
effects of water ?
Average composition of cratonic mantle in southern Superior Province : ‘Proton’ or ‘Archon’ ?
Superior crust was rejuvenated by Keweenawan rifting at 1.1 Ga – metasomatism ?

25. Refine thermo-chem structure
Using V-T conversions
+ upper mantle
temperature from
heat flow ++
crustal models
(test tomographic model)
subcontinental mantle dynamics :
Thermo-chemical structure of cratonic roots
Refine thermo-chem structure

26. Thermal Boundary Layer at the base of Continents
‘rheological’ thickness
of continent

27. Example from Kaapvaal xenoliths

28. Model Geometry

29. Oceanic vs. Continental Geotherms
δc depends on A
(dT/dz)cond =
O(dT/dz)a

30. Effect of Heat Production

31. Distribution of Heat Production

32. Δt = 0.25 Ga

34. Continental thickness from seismic tomographyd d
from Nettles (2004)

35. Continental thickness from seismic tomographyd d d
from Nettles (2004)

36. Continental Thermal Boundary Layer

37. Lateral Temperature Anomalies

38. Scaling Law for Average Mantle Temperature Θ
Sotin & Labrosse (1999)
Total oceanic area, F

39. Continental geometry and average mantle temperature
Perry, Jaupart & Tackley, in prep.

40. Continent thermal structure and average mantle temperature
Perry, Jaupart & Tackley, in prep.

41. Effect of crustal accretion on the mantle’s thermal history ?
Model Setup : To w d A D H
To+ΔT
Hm + Vo + A × Vc = Ct = Htotal × Vtotal
Example Present-day Model : Example Archean Model :
Htotal = 5 pW/kg Htotal = 10 pW/kg
A = 300 pW/kg (~0.9μWm-3) A = 300 pW/kg
RaH = 5 × RaH = 5 × 107

42. Potential temperature
Today
1.0
0.5
0.0
Potential temperature
Archean
Same mean mantle temperature
from two models after 1Ga

43. Potential temperature
Today
1.0
0.5
0.0
Potential temperature
Archean
Vrms continent/Vrms max
RaH

44. Potential temperature
Today
1.0
0.5
0.0
Potential temperature
Archean
Tmanto~Tmant(t)
A/H
Tmanto>>Tmant(t)
RaH

45. Conclusions - II
Lateral temperature anomalies between ocean/continent diminished as A increases
Thickness of the thermal b.l. below continents depends strongly on A (A δ-)
Average mantle temperature may be scaled as a function of the total oceanic area
Implications for time evolution of mantle temperature
Average mantle temperature (and heat flow) may not be have been significantly higher than today :
Feedback between mantle & continents : Ra, Acont