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

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)

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

Continental Heat Flow : example from Canadian Shield

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

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

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

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 ?

Variables of Continental Thermal Structure Problem

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

Archean Superior Province, Canada

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

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

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

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

Pn velocity

Crustal Thickness

Moho depth

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

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

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

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 ?

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

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

Example from Kaapvaal xenoliths

Model Geometry

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

Effect of Heat Production

Distribution of Heat Production

Δt = 0.25 Ga

Continental thickness from seismic tomography d d from Nettles (2004)

Continental thickness from seismic tomography d d d from Nettles (2004)

Continental Thermal Boundary Layer

Lateral Temperature Anomalies

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

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

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

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 × 106 RaH = 5 × 107

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

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

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

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