thermal structure of continental lithosphere from heat flow and seismic constraints: implications...
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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