Thermal modelling of crustal stacking and exhumation during the Paleoproterozoic orogenic growth of the Fennosscandian Shield I.T. Kukkonen 1, A. Korja 2, R. Lahtinen 1, P. Heikkinen 2 & FIRE Working Group 1 Geological Survey of Finland, Espoo 2 Institute of Seismology, University of Helsinki Presentation at ”New and Classical Applications of Heat Flow Studies”, Aachen, Germany October 7, 2004 _____________________________________________________________ ______

The Central Fennoscandian Shield Geological characteristics Evolution Archaean craton (ca. 2.7 Ga) Break-up of the Craton at Ga, formation of the Svecofennian Sea Closure of the Svecofennian Sea at Ga Collisision of the Craton with Proterozoic arc (island and continental) complexes Svecofennian collisional orogen: Accretion of arc complexes to the craton Significant period of stacking, and crustal growth Exhumation of the crust close to present erosion level by ca 1.6 Ga Today: Archean crust: Proterozoic (Svecofennian) metamorphic overprint (4-5 kb, °C) Anomalously thick crust (ca. 52 km) and lithosphere (250 km) Thick high velocity lower crust ( km/s, km depth) Huge granitoid areas outcrop, e.g. the Central Finland Grantitoid Complex (ca Ga) about 300 km x 300 km in area

Overthrusting or Proterozoic metasediments Underthrusting of island arc lower crust ProterozoicArchaean Reflection seismic data FIRE-1 Tectonic model of the Svecofennian collision at A/P boundary

_____________________________________________________________ ______ Motivation of the present study Thermal evolution of the Precambrian lithosphere in the central Fennoscandian Shield Thermal effects of plate collisions during the Svecofennian orogeny (1.9 –1.8 Ga) Interpretation of the metamorphic overprint on the Archaean craton Thermal conditions during formation of the Central Finland granitoid complex Thermal modelling of evolutionary models derived from reflection seismic data

Situation before collision Crust 1Crust 2 Z T Collision and stacking Stack t = 0 Ma T Mantle Principles of conductive thermal modelling of crustal stacking Z Geotherm Mantle

After stacking : Warming of the stacked crust Warming controlled by heat production, conductivity and mantle heat flow Exhumation begins and starts to remove heat production Exhumation leads to a decrease of crustal temperatures Decrease of temperature leads to stabilization of lithosphere Secular decrease of radiogenic heat production also stabilizes lithosphere Warmed stack t = Ma Exhumated, cooling stack t = Ma T Z Z T New erosion level Z1Z1 Z2Z2 Z 1,new Z 2,new Initial erosion level Principles of conductive thermal modelling of crustal stacking (cont.)

Stacking models can be used for simulating the T-Z-t evolution of the lithosphere Modern times Inital stack Warming stack Hottest geotherm T Z Principles of thermal modelling of crustal stacking _____________________________________________________________ ______

Modelling: Methods, parameter estimation and boundary conditions Numerical 1-D transient models of conductive heat transfer Vertical discretization of model 1 km, depth range 150 km A finite difference code was applied (code SHEMAT, Clauser, 1988) Thermal parameters estimated from from measured conductivity and heat production data and lithospheric geothermal models Exhumation included in 1-5 km slices according to a pre-determined rate Secular decrease of radiogenic heat production included Geophysical boundary conditions: Constant surface temperature (5 deg. C) Mantle heat flow decreases with time according to heat production decrease Present-day geotherm derived from a lithospheric model calibrated with kimberlite-hosted mantle xenoliths

_____________________________________________________________ ______ 1.93 Ga: Collision of island arc and craton Overthrusting of about 20 km of metasediments on the craton Underthrusting of hot arc-type lower crust at Moho depth 1.85 Ga: thermal overprinting of the craton at present erosion level ( ºC, 4-5 kb, U-Pb age from xenotime) 1.81 Ga: Temperature at present erosion level still > 500 ºC (K-Ar –age dating) Ga: Temperatures at present erosion level >150 ºC (Ar-Ar ages from feldspar) 0.0 Ga: Exhumation totals km, stabilized thick lithosphere with low geotherm Sources for geological and geothermal data: Korsman et al, 1999 Pajunen and Poutiainen, 1999 Kontinen et al., 1992 Murrell, 2004 Kukkonen et al., 2003 Geological boundary conditions for the Archaean craton in eastern Finland

Overthrusting of Proterozoic metasediments Underthrusting of island arc lower crust ProterozoicArchaean Reflection seismic data FIRE-1 Tectonic model of the Svecofennian collision at A/P boundary

Proterozoic collision and over/under-thrusting at A/P boundary Over and under-thrusting at 1.93 Ga Stacking of - Proterozoic (metasediments) on top of Archaean crust - Underthrusting of arc lower crust Initial temperatures determined from: Metasediments (0-20 km) Lower crustal arc piece (60-70 km) Elsewhere Archaean stabilized lithosphere Initial-T 1.93 Ga 1.85 Ga 1.81 Ga Ga _____________________________________________________________ ______

1.93 Ga 1.85 Ga 1.81 Ga 1.60 Ga 0 Ga 1.93 Ga: Thrusting, stacking, warming starts Tectonic underplating of hot arc lower crust 1.85 Ga: T-peak in upper crust, no exhumation yet 1.81 Ga: Warming cont., total exhumation 3 km 1.60 Ga: Cooling/stabilization, total exhumation 15 km 0.0 Ga: Cooling cont., total exhumation 20 km ECL facies _____________________________________________________________ ______ Present lower crust (at 50 km): Modelling thermal evolution (Z-T-t path) 20 km of Prot. metasediments on Archaean craton 10 km of tectonic underplating at Moho level

What is the high-velocity lower crust? Comparison of wide-angle model velocities from SVEKA transect and lab data on rock type velocities 1 Mafic granulite 2 Mafic garnet-granulite 3 Mafic eclogite 4 Anorthosite 5 Hornblendite 6 Pyroxenite 7 Dunite 8 Gabbro-norite-troctolite Rock type velocities corrected for crustal PT-conditions of eastern Finland with a xenolith calibrated geotherm Note: Downward increasing velocity trend in the HVLC Lab data on velocities of different rock types suggest a decrease of VP with depth HVLC not explained with homogeneous composition Composition becomes more mafic downward A plausible lithological composition: 70-85% mafic garnet granulite (rock type 2) 15-30% mafic eclogite (rock type 3) HVLC = Mafic garnet granulite with eclogitic inclusions Model (in situ) velocities Solid circles: upper boundary of HVLC Open circles: lower boundary of HVLC

1.93 Ga: Thrusting, stacking, warming starts 1.85 Ga: T-peak at present erosion surface, no exhumation 1.81 Ga: Warming cont., exhumation 3 km 1.60 Ga: Cooling/stabilization, exhumation 15 km 0.0 Ga: Cooling cont., exhumation 20 km Initial-T Archaean craton: Modelling the Z-T-t path of the present erosion surface

>1.92 Ga: Several micro-continents 1.92 Ga: Mutual collisions of micro-continents, stacking 1.88 Ga: Crystallization of the Central Finland Granitoid Complex 1.50 Ga: Temperature at present erosion level still > 150 ºC (Ar-Ar age from feldspar) 0.0 Ga: Exhumation has removed a total of ca. 15 km, geotherm is low Geological boundary conditions for the Central Finland Arc Complex _____________________________________________________________ ______

FIRE 3 FIRE 1 FIRE 2

Comparison of FIRE-1 and FIRE 3A: 3-d character of reflectors under CFGC NE SW S-Finland volcanic-sedim. complex Central Finland Granitoid Complex Raahe-Ladoga Shear Belt FIRE-1FIRE-2 FIRE-3A Bothnian coast Central Finland Granitoid Complex SE NW

Central Finland Arc Complex Z-T-t models 0 Ga >1.92 Ga: Micro continent stage 1.92 Ga: Collision and stacking of two micro cont. blocks 1.90 Ga: Warming of crust, exhumation begins 1.88 Ga: Warming continues, exhumation totals 5 km; partial melting of lower crust (35-50 km) -> redistribution of of heat producing elements in the crust Ga: Stabilization and cooling of crust, exhumation totals 10 km Ga: Slow cooling to present conditions, exhumation totals 15 km Green arrows: Present erosion level Red arrows: Upper surface of high-velocity lower crust Black arrows: Present Moho

And-das Basaltic And-das Basaltic UM Stack + 20 Ma 1.90 Ga A = 2.25 A = 0.3 A = 2.25 A = 0.3 A = And-bas Basaltic And-das Basaltic UM 65 km Stack + 40 Ma 1.88 Ga Q m = km: Granitoid solidus exceeded Partial melt transported to upper crust Central Finland Arc Complex: Layer thicknesses and heat production values of the stack Heat production values given in units of W m -3

And-basaltic Basaltic comp.+ felsic additions Basaltic UM 60 km 1.88 Ga Q m = 16 mW m -2 Gr.dior. Restitic A = 2.25  W m -3 A = 2.4 A = 1.2 A = 0.38 A = 0.3 A = Ga A =  W m -3 A = 2.16 A = 1.08 A = 0.34 A = 0.27 A = Q m = 18 mW m Ga: Intrusion of granitoid magmas in the upper crust Ga: Crustal cooling and exhumation continues Crustal heat production decreases according to decay of U, Th and K Mantle heat flow decreases accordingly 0.0 Ga A = 1.6  W m -3 A = 0.8 Gr.dior. Bas + felsic additions Restitic A = 0.25 Basaltic A = 0.2 UM A = Q m = 12 mW m -2 Central Finland Arc Complex: Layer thicknesses and heat production values of the stack Melt transport

A = 1.3 A = 0.3 A = A = 0.6 A = Effect of re-distributing crustal heat production Q (Moho) = 12 mW m -2 Q (surface) = 42 mW m -2 T (surface) = 0ºC Homogeneous heat production Re-distributed heat production Steady-state geotherms 50 km Moho Heat production concentrated in the upper crust leads to a colder geotherm! Redistributed AHomogeneous A

Conclusions: Eastern Finland Archaean craton The thermal overprint of the craton is due to overthrust-related conductive heating at later stages of the Scecofennian orogen The Svecofennian overthrust sheet was about 20 km thick Present high velocity lower crust is partly eclogite facies conditions _____________________________________________________________ ______

Conclusions: Central Finland Arc Complex Thermal evolution well modelled by stacking arc-type (micro-continental) crustal blocks Model suggests that the Central Finland Granitoid Complex was formed by partial melting of middle crust Redistribution of heat producing elements influences essentially the geotherm and thermal stabilization lithosphere _____________________________________________________________ ______

Conclusions: General Collision, stacking and exhumation provide a feasible interpretation of the crustal evolution (at least in the Fennoscandian Shield) Warming of crust in a collisional orogen can be attributed to conductive heat transfer, radiogenic heat production of the rocks, accumulation of heat production by stacking and subsequent heating Stabilization of lithosphere can be attributed to removal of heat production by exhumation and conductive coolling Convective and magmatic heating is important locally, but not required in the lithospheric scale Heat production needs not to be anomalously high, values represent normal rock types An anomalously high mantle heat flow is not required _____________________________________________________________ ______

Conclusions: General (cont.) Conduction of heat is a sufficient heat transfer mechanism for generating major thermal and metamorphic events Thickness of thermal lithosphere during the Svecofennian orogeny was of the order of 100 km Redistribution of U, Th and K is a major process leading to stabilization of lithosphere Abundance of granitoids indicates cold, stabilized lithosphere beneath (?) _____________________________________________________________ ______

The talk is over. Thank you! _____________________________________________________________ ______

Evolutionary history

Palaeoproterozoic evolution of the Fennoscandian Shield Archaean craton (ca. 2.7 Ga) Break-up of the Craton at Ga, formation of the Svecofennian Sea Closure of the Svecofennian Sea at Ga Collision of the Craton with Proterozoic arc (island and continental) complexes Svecofennian collisional orogen: Accretion of arc complexes to the craton Significant period of stacking, and crustal formation Exhumation of the crust close to present erosion level by ca 1.6 Ga 1.93 Ga 1.91 Ga 1.90 Ga 1.89 Ga 1.88 Ga Today: Archean crust: Proterozoic (Svecofennian) metamorphic overprint (4-5 kb, °C) Anomalously thick crust (ca. 52 km) and lithosphere (250 km) Thick high velocity lower crust ( km/s, km depth) Huge granitoid areas outcrop, e.g. the Central Finland Grantitoid Complex (ca Ga) about 300 km x 300 km in area

FIRE 3 FIRE 1 FIRE 2 Location of the FIRE reflection seismic transects

1.93 Ga 1.85 Ga 1.81 Ga 1.60 Ga 0 Ga 1.93 Ga: Thrusting, stacking, warming starts Tectonic underplating of hot arc lower crust 1.85 Ga: T-peak in upper crust, no exhumation yet 1.81 Ga: Warming cont., total exhumation 3 km 1.60 Ga: Cooling/stabilization, total exhumation 15 km 0.0 Ga: Cooling cont., total exhumation 20 km Archaean/Prot. boundary zone: Modelling the Z-T-t paths of the modern lower crust ECL facies _____________________________________________________________ ______

1.93 Ga 1.85 Ga 1.81 Ga 1.60 Ga 0 Ga 1.93 Ga: Thrusting, stacking, warming starts Tectonic underplating of hot arc lower crust 1.85 Ga: T-peak in upper crust, no exhumation yet 1.81 Ga: Warming cont., total exhumation 3 km 1.60 Ga: Cooling/stabilization, total exhumation 15 km 0.0 Ga: Cooling cont., total exhumation 20 km ECL facies _____________________________________________________________ ______ Numerical modelling of collisional stacking on the A/P boundary zone: 20 km of Prot. metasediments on Archaean craton 10 km of tectonic underplating at Moho level