Thermal structure of continental Thermal structure of continental lithosphere from heat flow and lithosphere from heat flow and

seismic constraints: seismic constraints: Implications for upper mantle Implications for upper mantle composition and geodynamic composition and geodynamic

modelsmodels

Claire PerryGEOTOP-UQAM-McGill, Montreal, Canada

Stability of continental lithosphereStability of continental lithosphere

• equilibrium between chemical and thermal buoyancy (e.g., Jordan 1979) ?

δTδFe#

Perry et al. GJI (2003); Forte & Perry Science (2000)

Accurate lithospheric thermal models required (heat flow, crustal heat production)

150 km

Introduction : Global Terrestrial Heat Introduction : Global Terrestrial Heat LossLoss

Pollack et al. (1993)

Continental Heat Flow : example Continental Heat Flow : example from Canadian Shieldfrom Canadian Shield

Heteogenity of Continents …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 Thermal Structure of the Continental LithosphereLithosphere

Gung et al. (2003)• variable seismic thickness• d3 detected by tomography

Presentation OutlinePresentation Outline

1.1. Lithospheric thermal structure, upper mantle Lithospheric thermal structure, upper mantle temperatures, and Pn velocity-temperature temperatures, and Pn velocity-temperature conversions from heat flow and seismic conversions from heat flow and seismic refraction studiesrefraction studies

2.2. The thermal boundary layer of continental The thermal boundary layer of continental lithosphere and average mantle lithosphere and average mantle temperatures from a geodynamic flow modeltemperatures from a geodynamic flow model

How does continental heat production affect How does continental heat production affect lithospheric and mantle temperatures ?lithospheric and mantle temperatures ?

Variables of Continental Thermal Variables of Continental Thermal Structure ProblemStructure Problem

Variables of Continental Thermal Variables of Continental Thermal Structure ProblemStructure Problem

(Aavg~0.7 µWm-3) : distribution of radiogenic elements ?

small(~0.02µWm-3)

Archean Superior Province, CanadaArchean Superior Province, Canada

Heat Flow Data …Heat Flow Data …

• Qs Tmoho

• correlation VP – T

• mechanical resistance of lithosphere

Distribution of Radiogenic elements_____________

Differentiation Index:

DI = <Asurf> Ac

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

Province DI

Perry et al. JGR 2006a

Distribution of Radiogenic elements_____________

Differentiation Index:

DI = <Asurf> Ac

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

Province DI

Perry et al. JGR 2006a

Crustal ModelCrustal Model

distribution of Adistribution of ACRCR in crustal columns in crustal columns Moho temperature estimated using using k(T)Moho temperature estimated using using k(T)

LITH5.0 (LITH5.0 (Perry et al. GJI, 2002) + more recent data) + more recent data Hc, Pn

Principal unknown QmPrincipal unknown Qm

Fixed ParametersFixed Parameters :: Qs, A0, k(T), Hc Qs, A0, k(T), HcFree Parameter : Free Parameter : Qm Qm (constrained by (constrained by

xenolith + heat flow, xenolith + heat flow, A(z) constrained by A(z) constrained by QmQm, , QsQs, , HcHc

Pn velocityPn velocity

Crustal ThicknessCrustal Thickness

Moho depth

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

Average Cratonic Mantle CompositionAverage Cratonic Mantle Composition

• on-craton VP-T ≠ off-craton VP-T• predicted/measured VP Qm≥ 12 mWm-2

Perry et al. JGR 2006b

Preferred Mineralogical Composition :Preferred Mineralogical Composition :Superior upper-mantleSuperior upper-mantle

joint Qs + Pn

lithospheric mantle

composition + Qm

Perry et al. JGR 2006b

Conclusions – Part IConclusions – Part I Comparison of large-scale empirical geophysical Comparison of large-scale empirical geophysical

data and in-situ experiments of mantle data and in-situ experiments of mantle composition provide further confidence in mantle composition provide further confidence in mantle temperatures from seismic studies and heat flowtemperatures from seismic studies and heat flow

Joint inversions of heat flow and seismic Pn Joint inversions of heat flow and seismic Pn velocity constrain :velocity constrain : mantle mineralogical compositionmantle mineralogical composition effects of water ?effects of water ?

Average composition of cratonic mantle in Average composition of cratonic mantle in southern Superior Province : ‘Proton’ or ‘Archon’ ?southern Superior Province : ‘Proton’ or ‘Archon’ ? Superior crust was rejuvenated by Superior crust was rejuvenated by

Keweenawan rifting at 1.1 Ga – metasomatism Keweenawan rifting at 1.1 Ga – metasomatism ??

Refine thermo-chem

structure

subcontinental mantle dynamics :

Thermo-chemical structure of cratonic roots

+ upper mantle temperature from heat flow ++ crustal models(test tomographic model)

Using V-T conversio

ns

Thermal Boundary Layer at the base Thermal Boundary Layer at the base of Continentsof Continents

‘rheological’ thicknessof continent

Example from Kaapvaal xenolithsExample from Kaapvaal xenoliths

Model GeometryModel Geometry

Oceanic vs. Continental GeothermsOceanic vs. Continental Geotherms

• δc»δo

• δc depends on A

• (dT/dz)cond =

O(dT/dz)a

Effect of Heat ProductionEffect of Heat Production

Distribution of Heat ProductionDistribution of Heat Production

Δt = 0.25 Ga

Continental thickness from Continental thickness from seismic tomographyseismic tomography

d

from Nettles (2004)

Continental thickness from seismic Continental thickness from seismic tomographytomography

d

from Nettles (2004)

Continental Thermal Boundary Continental Thermal Boundary

LayerLayer

Lateral Temperature AnomaliesLateral Temperature Anomalies

Scaling Law for Average Mantle Scaling Law for Average Mantle Temperature Temperature ΘΘ

232.0

724.0

5.0Ra

HC

s

4/1232.0

724.0

5.0

FRa

HC

s

Sotin & Labrosse (1999)

Total oceanic area, F

C = 1.02

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 ?

A

H

d

wTo

To+ΔT

D

Model Setup :

Example Present-day Model : Example Archean Model : Htotal = 5 pW/kg Htotal = 10 pW/kgA = 300 pW/kg (~0.9μWm-3) A = 300 pW/kgRaH = 5 × 106 RaH = 5 × 107

Hm + Vo + A × Vc = Ct = Htotal × Vtotal

1.0

0.5

0.0

Po

ten

tial te

mp

erature

Archean

Today

Same mean mantle temperaturefrom two models after 1Ga

1.0

0.5

0.0

Po

ten

tial te

mp

erature

Archean

TodayV

rms

con

tin

ent/

Vrm

s m

ax

RaH

1.0

0.5

0.0

Po

ten

tial te

mp

erature

Archean

Today

RaH

A/H

Tmanto~Tmant(t)

Tmanto>>Tmant(t)

Lateral temperature anomalies between Lateral temperature anomalies between ocean/continent diminished as A ocean/continent diminished as A increasesincreases

Thickness of the thermal b.l. below Thickness of the thermal b.l. below continents depends strongly on A (Acontinents depends strongly on A (A+ + δδ--))

Average mantle temperature may be Average mantle temperature may be scaled as a function of the total oceanic scaled as a function of the total oceanic areaarea Implications for time evolution of mantle Implications for time evolution of mantle

temperaturetemperature Average mantle temperature (and heat flow) may Average mantle temperature (and heat flow) may

not be have been significantly higher than today :not be have been significantly higher than today :Feedback between mantle & continents : Ra, AcontFeedback between mantle & continents : Ra, Acont

Conclusions - IIConclusions - II